U.S. patent number 3,597,690 [Application Number 04/666,645] was granted by the patent office on 1971-08-03 for tone control circuit having a frequency-controllable filter.
Invention is credited to Keith H. Wycoff.
United States Patent |
3,597,690 |
Wycoff |
August 3, 1971 |
**Please see images for:
( Certificate of Correction ) ** |
TONE CONTROL CIRCUIT HAVING A FREQUENCY-CONTROLLABLE FILTER
Abstract
A tone control circuit is in a receiver adapted to respond to a
sequence of control tones alternately selected from a first group
of tones in a first band of frequencies and a second group of tones
in a second band of frequencies, wherein the two bands are
separated by an intermediate band and wherein the time duration of
the gap between adjacent tones in the sequence of tones is
substantially zero, the tone control circuit including a filter
device which is tuned to the frequencies of the control tones as
they are received, a plurality of AND circuits corresponding in
number to the control tones and respectively having one input
coupled to the filter device and another input coupled to the
preceding AND circuit so that each AND circuit is operative to
produce an output only in the presence of a tone being passed by
the filter and an output signal from the preceding AND circuit, the
filter device being tuneable either manually or electronically via
the outlet signals from the AND circuits.
Inventors: |
Wycoff; Keith H. (Lexington,
NB) |
Family
ID: |
24674869 |
Appl.
No.: |
04/666,645 |
Filed: |
September 11, 1967 |
Current U.S.
Class: |
340/7.49 |
Current CPC
Class: |
H03G
3/005 (20130101); H04W 88/027 (20130101) |
Current International
Class: |
H03G
3/00 (20060101); H04Q 7/16 (20060101); H04b
001/04 (); H04b 001/26 () |
Field of
Search: |
;325/64,133,302,306,392,408 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Griffin; Robert L.
Assistant Examiner: Weinstein; Kenneth W.
Claims
What I claim is:
1. A communication system comprising a transmitter including a
tone-generating circuit for generating tones selected from a first
group of tones in a first band of frequencies and for generating
tones selected from a second group of tones in a second band of
frequencies separate and distinct from said first band of
frequencies, said first band of frequencies being separated from
second band of frequencies by a substantial intermediate band of
frequencies, said tone-generating circuit generating a sequence of
tones alternately selected from said first and second groups of
tones, the time duration of the gap between adjacent tones being
substantially zero, a transmitter output circuit coupled to said
tone-generating circuit for transmitting signals corresponding to
the sequence of tones a receiver including an input circuit for
receiving the signals from said transmitter, a first tone control
circuit, a second tone control circuit, said first tone control
circuit being coupled to said input circuit and to said second tone
control circuit and responsive to the application of tones from
said first group of tones to said input circuit to provide first
control signals to said second tone control circuit, said second
tone control circuit being coupled to said input circuit and to
said first tone control circuit and responsive to the application
of the tones from said second group of tones to said input circuit
to provide second control signals to said first tone control
circuit, one of said tone control circuits furnishing an output
control signal in response to the application of the last tone in
the sequence of tones, and an output signal utilization circuit
coupled to said one tone control circuit for utilizing said output
control signal therefrom.
2. The communication system, set forth in claim 1, wherein certain
of the tones in said second band of frequencies are harmonics of
certain tones in said first band of frequencies.
3. The communication system set forth in claim 1, wherein said
first band of frequencies and said second band of frequencies are
in the audio range of frequencies.
4. The communication system set forth in claim 1, wherein each of
said band of frequencies contains at least ten separate tones with
each tone being separated from adjacent tones by about 31/3 percent
of the frequency thereof.
5. The communication system set forth in claim 1, wherein the
frequency of a tone is slightly greater than 1.035 times the
frequency of the next lower tone.
6. The communication system set forth in claim 1, wherein said
transmitter includes a generator for generating a carrier signal
that is modulated by the control tones.
7. The communication system set forth in claim 6, wherein the
control tones are applied to the carrier signal by frequency
modulation.
8. The communication system set forth in claim 1, wherein said
sequence of tones comprises one tone selected from said first group
of tones and having a time duration not substantially greater than
40 milliseconds and one tone selected from said second group of
tones and having a time duration of at least not substantially
greater than 40 milliseconds.
9. The communication system set forth in claim 1, wherein said
sequence of tones comprises two tones selected from said first
group of tones each having a time duration not substantially
greater than 40 milliseconds and two tones selected from said
second group of tones each having a time duration of not
substantially greater than 40 milliseconds.
10. The communication system set forth in claim 1, wherein said
sequence of tones comprises four tones selected from said first
group of tones and each having a time duration not substantially
greater than 40 milliseconds and three tones selected from said
second group of tones each having a time duration of not
substantially greater than 40 milliseconds.
11. In a communication system for selectively transmitting signals
including a sequence of control tones alternately selected from a
first group of tones in a first band of frequencies and from a
second group of tones in a second band of frequencies separate and
distinct from said first band of frequencies, a receiver comprising
an input circuit for receiving the signals from an associated
transmitter, a first tone control circuit, a second tone control
circuit, means coupling said first tone control circuit to said
input circuit and to said second tone control circuit, said first
tone control circuit having a first tapped filter therein
adjustable to a selected one of a plurality of positions
respectively corresponding to a selected tone in said first group
of tones so that upon the application of the selected tone to said
input circuit there is produced first control signals that are
applied to said second tone control circuit to said input circuit
and to said first tone control circuit, said second tone control
circuit having a second tapped filter therein adjustable to a
selected one of a plurality of positions respectively corresponding
to a selected tone in said second group of tones so that upon the
application of the selected tone to said input circuit there is
produced second control signals that are applied to said first tone
control circuit, one of said tone control circuits furnishing an
output control signal in response to the application of the last
tone in the sequence of tones, an output signal utilization circuit
coupled to said one tone control circuit for utilizing said output
control signal therefrom.
12. The receiver set forth in claim 11, wherein said first tapped
filter is adjustable to pass a selected tone in a first band of
frequencies and said second tapped filter is adjustable to pass a
selected tone in a second band of frequencies, said first band of
frequencies being separated from said second band of frequencies by
a substantial intermediate band of frequencies.
13. The receiver set forth in claim 11, wherein said first and
second tapped filters are adjustable to pass tones that are in an
audio range of frequencies.
14. The receiver set forth in claim 11, wherein each of said tapped
filters is adjustable to pass a selected one of at least ten
separate tones, each tone being separated from the adjacent tones
by about 31/3 percent of the frequency thereof.
15. The receiver set forth in claim 11, and further comprising a
holding circuit interconnecting said first and second tone control
circuits so as to accommodate tone duration of any length longer
than a delay period provided the time gap between adjacent tones in
the sequence of tones is substantially zero.
16. The receiver set forth in claim 11, wherein said first tapped
filter has a connection to only one of the taps thereon and said
second tapped filter has a connection to only a signal tap thereon,
whereby said receiver is responsive to a sequence of two selected
tones.
17. The receiver set forth in claim 11, wherein said first tapped
filter has connections to two of the taps thereon corresponding to
two tones in said first group of tones, and said second tapped
filter has connections to two taps thereon corresponding to two
tones selected from said second group of tones, whereby said
receiver is responsive to a sequence of four control tones.
18. The receiver set forth in claim 11, wherein said first tapped
filter has connections to four of the taps thereon corresponding to
four tones selected from said first group of tones, and said second
tapped filter has connections to three taps thereon corresponding
to three tones selected from said second group of tones, whereby
said receiver is responsive to a sequence of seven selected
tones.
19. The receiver set forth in claim 11, wherein the selection of
connections to the taps on said tapped filters is made
mechanically.
20. The receiver set forth in claim 11, wherein the connection to
the selected tap on said tapped filter is made electronically.
21. The receiver set forth in claim 11, wherein the selection of
the tones to which the receiver is responsive is made by the
connectors which connect said tapped filters to the tone receiver
components.
22. A communication receiver for responding to a sequence of first
and second control tones, said receiver comprising an input circuit
for receiving the control tones, a first filter coupled to said
input circuit and tuned to the frequency of the first control tone
and providing a signal which has an amplitude dependent upon the
frequency of the control tone applied thereto, a first electronic
switching device coupled to said first filter and responsive to a
signal from said first filter exceeding the threshold of said first
switching device to provide a DC voltage of fixed value independent
of the amount by which the amplitude of the signal from said first
filter exceeds said threshold level, a first delay circuit coupled
to said first electronic switching device and operative to produce
at its output a first control signal only when the first control
tone has persisted for a predetermined fixed duration, a second
filter coupled to said input circuit and tuned to the frequency of
the second control tone and providing a signal which has an
amplitude dependent upon the frequency of the control tone applied
thereto, a second electronic switching device coupled to said first
delay circuit and to said second filter and responsive to both said
first control signal and to a signal exceeding the threshold level
of said second switching device to provide a DC voltage of fixed
value and independent of the amount by which the amplitude of the
signal from said second filter exceeds said threshold level, a
second delay circuit coupled to said second electronic switching
device and operative to produce at its output a second control
signal only when the second control tone has persisted for a
predetermined fixed duration, and a control signal utilization
circuit coupled to said second delay circuit for utilizing said
second control signal.
23. The communication receiver set forth in claim 22, wherein each
of said electronic switching devices is a transistor means having
its base coupled to the associated filter and having its collector
and emitter coupled to the associated delay circuit.
24. The communication receiver set forth in claim 22, wherein each
of said delay circuits includes a capacitor and resistor coupled in
series across the output of the associated electronic switching
device, and the output of each delay circuit is the junction of the
resistor and capacitor.
25. A tone control circuit for use in a communication receiver
including an input circuit which provides a sequence of control
tones and including an output signal utilization circuit, said tone
control circuit comprising filter means coupled to the input
circuit and having a plurality of control inputs for receiving one
at a time signals selectively to tune said filter means to
predetermined frequencies, a plurality of AND circuits
corresponding in number to the number of control tones in the
sequence of control tones and each having first and second inputs
and an output, control means for providing at the output thereof a
control signal during reception of the first control tone in the
sequence of control tones, the output of said control means being
coupled to one of the control inputs of said filter means and to
the first input of the first of said AND circuits, said filter
means being responsive to the application of the control signal
from said control means to be tuned to the frequency of said first
control tone, means coupling the output of said filter means to the
second input of each of said AND circuits, said first AND circuit
being responsive to the application thereto of both the control
signal from said control means and said first control tone for
producing at the output thereof an output signal, the output of
said first AND circuit and the output of each succeeding AND
circuit being coupled respectively to the first input of the next
succeeding AND circuit, means coupling the outputs of said AND
circuits respectively to the control inputs of said filter means,
means coupled to said control means for removing the control signal
therefrom after termination of said first control tone, said second
AND circuit and each succeeding AND circuit being responsive to the
application thereto of both the output signal from the next
preceding AND circuit and the associated control tone in the
sequence of control tones from said filter means for producing at
the output thereof an output signal, said filter means sequentially
being tuned to the frequencies of the control tones as they are
received by the input circuit in accordance with the sequential
application of said output signals to said control inputs, and
means coupling the output of the last of said AND circuits to the
output signal utilization circuit of the receiver.
26. The tone control circuit set forth in 25, and further
comprising a plurality of delay circuits corresponding in number to
the number of control tones in the sequence of control tones and
coupled respectively between said AND circuits, the last of said
delay circuits being coupled between the last of said AND circuits
and said output signal utilization circuit, each of said delay
circuits being operative to produce an output signal only when the
associated control tone has persisted for a predetermined
duration.
27. A tone control circuit for use in a communication receiver
including an input circuit which provides a sequence of control
tones and including an output signal utilization circuit, said tone
control circuit comprising first filter means coupled to the input
circuit and having a plurality of control inputs for receiving one
at a time signals selectively to tune said first filter means to
predetermined frequencies, second filter means coupled to the input
circuit and having a plurality of control inputs for receiving one
at a time signals selectively to tune said second filter means to
predetermined frequencies, a plurality of AND circuits
corresponding in number to the number of control tones in the
sequence of control tones and each having first and second inputs
and an output, control means for providing at the output thereof a
control signal during reception of the first control tone in the
sequence of control tones, the output of said control means being
coupled to one of the control inputs of said first filter means and
to the first input of the first of said AND circuits, said first
filter means being responsive to the application of the control
signal from said control means to be tuned to the frequency of said
first control tone, means coupling the output of said first filter
means to the second input of said first AND circuit and to the
second input of each succeeding odd-numbered ANd circuit, means
coupling the output of said second filter means to the second input
of said second AND circuit and to the second input of each
succeeding even-numbered AND circuit, said first AND circuit being
responsive to the application thereto of both the control signal
from said control means and said first control tone for producing
at the output thereof an output signal, the output of said first
AND circuit and the output of each succeeding AND circuit being
coupled respectively to the first input of the next succeeding AND
circuit, means coupling the outputs respectively of said first AND
circuit and of each succeeding odd-numbered AND circuit
respectively to the control inputs of said second filter means,
means coupling the outputs respectively of said second AND circuit
and of each succeeding even-numbered AND circuit respectively to
the control inputs of said first filter means, means coupled to
said control means for removing the control signal therefrom after
termination of said first control tone, said second AND circuit and
each succeeding AND circuit being responsive to the application
thereto of both the output signal from the next preceding AND
circuit and the associated control tone in the sequence of control
tones from the associated filter means for producing at the output
thereof an output signal, said first and second filter means
alternately being tuned to the frequencies of the control tones as
they are received by said input circuit by the sequential
application to said control inputs of said output signals, and
means coupling the output of the last of said AND circuits to the
output signal utilization circuit of the receiver.
28. The tone control circuit set forth in 27 and further comprising
a first rectifier circuit coupled between said first filter means
and the second input of each odd-numbered AND circuit, and a second
rectifier circuit coupled between said second filter means and the
second input of each even-numbered AND circuit.
29. The tone control circuit set forth in claim 27, wherein the
output of said third AND circuit and the output of each succeeding
odd-numbered AND circuit is coupled to said control means for
removing the control signal therefrom at least during the presence
of said third control tone and during the presence of each
succeeding odd-numbered control tone.
30. A tone control circuit for use in a communication receiver
including an input circuit which provides a sequence of control
tones and including a DC voltage utilization circuit, said tone
control circuit comprising filter means coupled to the input
circuit and having a plurality of control inputs for receiving one
at a time signals selectively to tune said filter means to
predetermined frequencies, a plurality of AND circuits
corresponding in number to the number of control tones in the
sequence of control tones and each having first and second inputs
and an output, control means for providing at the output thereof a
control signal during reception of the first control tone in the
sequence of control tones, the output of said control means being
coupled to one of the control inputs of said filter means and to
the first input of the first of said AND circuits, said filter
means being responsive to the application of the control signal
from said control means to be tuned to the frequency of said first
control tone, means coupling the output of said filter means to the
second input of each of said AND circuits, said first AND circuit
being responsive to the application thereto of both the control
signal from said control means and said first control tone for
producing at the output thereof an output signal, a plurality of
electronic switching circuits corresponding in number to the number
of control tones in the sequence of control tones and each having
an input and output, the output of said first AND circuit being
coupled to the input of the first of said electronic switching
circuits and the output of each succeeding one of said AND circuits
being respectively coupled to the input of the next succeeding one
of said electronic switching circuits, the output of said first
electronic switching circuit being coupled to the first input of
said first AND circuit and the output of each succeeding one of
said electronic switching circuits being coupled respectively to
the first input of the next succeeding one of said AND circuits
respectively to the control inputs of said filter means, said first
electronic switching circuit being responsive to the application
thereto of the output signal from said first AND circuit for
producing at the output thereof a DC voltage, means coupled to said
control means for removing the control signal therefrom after
termination of said first control tone, said second AND circuit and
each succeeding AND circuit being responsive to the application
thereto of both the DC voltage from the next preceding electronic
switching circuit and the associated control tone in the sequence
of control tones from said filter means for producing at the output
thereof an output signal, said second electronic switching circuit
and each succeeding electronic switching circuit being responsive
to the application thereto of the output signal from the next
preceding AND circuit for producing at the output thereof a DC
voltage, said filter means sequentially being tuned to the
frequencies of the control tones as they are received by the input
circuit in accordance with the sequential application of said DC
voltages to said control inputs, and means coupling the output of
the last of said electronic switching circuits to the DC voltage
utilization of the receiver.
31. A tone control circuit for use in a communication receiver
including an input circuit which provides a sequence of control
tones and including a DC voltage utilization circuit, said tone
control circuit comprising filter means coupled to the input
circuit and having a plurality of control inputs for receiving one
at a time signals selectively to tune said filter means
respectively to predetermined frequencies, a plurality of AND
circuits corresponding in number to the number of control tones in
the sequence of control tones and each having first and second
inputs and an output, control means for providing at the output
thereof a control signal during reception of the first control tone
in the sequence of control tones, the output of said control means
being coupled to one of the control inputs of said filter means and
to the first input of the first of said AND circuits, said filter
means being responsive to the application of the control signal
from said control means to be tuned to the frequency of said first
control tones, means coupling the output of said filter means to
the second input of each of said first AND circuits, said first AND
circuit being responsive to the applications thereto of both the
control signal from said control means and said first control tone
for producing at the output thereof an output signal, a plurality
of electronic switching circuits corresponding in number to the
number of control tones in the sequence of control tones and each
having an input and output, the output of said first AND circuit
being coupled to the input of the first of said electronic
switching circuits and the output of each succeeding one of said
AND circuits being respectively coupled to the input of the next
succeeding one of said electronic switching circuits, the output of
said first electronic switching circuit being coupled to the first
input of said first AND circuit and the output of each succeeding
one of said electronic switching circuits being coupled
respectively to the first input of the next succeeding one of said
AND circuits, means coupling the outputs of said electronic
switching circuits respectively to the control inputs of said
filter means, said first electronic switching circuit being
responsive to the application thereto of the output signal from
said first AND circuit for producing at the output thereof a DC
voltage, means coupled to said control means for removing the
control signal therefrom after termination of said first control
tone, said second AND circuit and each succeeding AND circuit being
responsive to the application thereto of both the DC voltage from
the next preceding electronic switching circuit and the associated
control tone in the sequence of control tones from said filter
means for producing at the output thereof and output signal, said
second electronic switching circuit and each succeeding electronic
switching circuit being responsive to the application thereto of
the output signal from the next preceding AND circuit for producing
at the output thereof a DC voltage, the output of said second AND
circuit and the output of each succeeding one of said AND circuits
being coupled respectively to an input of the next preceding one of
said electronic switching circuits to enable each of said
electronic switching circuits to continue to produce said DC
voltage as long as the associated control tone is being applied to
the next succeeding one of said AND circuits, said filter means
sequentially being tuned to the frequencies of the control tones as
they are received by said input circuit in accordance with the
sequential application of said output signals to said control
inputs, and means coupling the output of the last of said
electronic switching circuits to the DC voltage utilization circuit
of the receiver.
32. A tone control circuit for use in a communication receiver
including an input circuit which provides a sequence of control
tones and including an output signal utilization circuit, said tone
control circuit comprising first filter means coupled to the input
circuit and including a first frequency determining element having
a plurality of inputs thereto, a plurality of electronic switching
devices coupled respectively to the inputs of said first
frequency-determining element, second filter means coupled to the
input circuit and including a second frequency-determining element
having a plurality of inputs thereto, a plurality of electronic
switching devices coupled respectively to the inputs of said second
frequency-determining element, each of said electronic switching
devices being closed by the application thereto of a signal to
couple in circuit a portion of the associated frequency-determining
element and thereby tune the associated filter means to a
predetermined frequency, a plurality of AND circuits corresponding
in number to the number of control tones in the sequence of control
tones and each having first and second inputs and an output,
control means for providing at the output thereof a control signal
during reception of the first control tone in the sequence of
control tones, the output of said control means being coupled to
the control inputs of said first filter means and to the first
input of the first of said AND circuits, said first filter means
being responsive to the application of the control signal from said
control means to be tuned to the frequency of said first control
tone, means coupling the output of said first filter means to the
second input of said first AND circuit and to the second input of
each succeeding odd-numbered AND circuit, means coupling the output
of said second filter means to the second input of said second AND
circuit and to the second input of each succeeding even-numbered
AND circuit, said first AND circuit being responsive to the
application thereto of both the control signal from said control
means and said first control tone for producing at the output
thereof an output signal, the output of said first AND circuit and
the output of each succeeding AND circuit being coupled
respectively to the first input of the next succeeding AND
circuits, means coupling the outputs respectively of said first AND
circuit and of each succeeding odd-numbered AND circuit
respectively to the control inputs of said second filter means,
means coupling the outputs respectively of said second AND circuit
and of each succeeding even-numbered AND circuit respectively to
the control inputs of said first filter means, means coupled to
said control means for removing the control signal therefrom after
termination of said first control tone, said second AND circuit and
each succeeding AND circuit being responsive to the application
thereto of both the output signal from the next preceding AND
circuit and the associated control tone in the sequence of control
tones from the associated filter means for producing at the output
thereof an output signal, said first and second filter means
alternately being tuned to the frequencies of the control tones as
they are received by said input circuit by the sequential
application to said control inputs of said output signals, and
means coupling the output of the last of said AND circuits to the
output signal utilization circuit of the receiver.
33. The tone control circuit set forth in claim 32, wherein each of
said frequency-determining elements is a tapped inductor.
34. In a communication receiver for receiving signals including
intelligence and a sequence of control tones, a receiver comprising
an input circuit which provides the signals including the
intelligence and the sequence of control tones, a translating
circuit coupled to said input circuit for translating the
intelligence into useful form, an output control circuit coupled to
said translating circuit and effective in a first condition thereof
to render said translating circuit inoperative and effective in a
second condition thereof to render said translating circuit
operative, filter means coupled to the input circuit and having a
plurality of control inputs for receiving one at a time signals
selectively to tune said filter means respectively to predetermined
frequencies a plurality of AND circuits corresponding in number to
the number of control tones in the sequence of control tones and
each having first and second inputs and an output, control means
for providing at the output thereof a control signal during
reception first control tone in the sequence of control tones, the
output of said control means being coupled to one of the control
inputs of said filter means and to the first input of the first of
said AND circuits, said filter means being responsive to the
application of the control signal from said control means to be
tuned to the frequency of said first control tone, means coupling
the output of said filter means to the second input of each of said
AND circuits, said first AND circuit being responsive to the
application thereto of both the control signal from said control
means and said first control tone for producing at the output
thereof an output signal, the output of said first AND circuit and
the output of each succeeding AND circuit being coupled
respectively to the first input of the next succeeding AND circuit,
means coupling the outputs of said AND circuits respectively to the
control inputs of said filter means, means coupled to said control
means for removing the control signal therefrom after termination
of said first control tone, said second AND circuit and each
succeeding AND circuit being responsive to the application thereto
of both the output signal from the next preceding AND circuit and
the associated control tone in the sequence of control tones from
said filter means for producing at the output thereof an output
signal, said filter means sequentially being tuned to the
frequencies of the control tones as they are received by said input
circuit in accordance with the sequential application of said
output signals to said control inputs, and means for applying the
output signal from the last of said AND circuits to said output
control circuit from the first condition thereof to the second
condition thereof so as to render said translating circuit
operative.
35. A communication system comprising a transmitter including a
tone-generating circuit for generating tones selected from a first
group of tones in a first band of frequencies and for generating
tones selected from a second group of tones in a second band of
frequencies separate and distinct from said first band of
frequencies, said tone-generating circuit generating a sequence of
control tones alternately selected from said first and second
groups of tones, and a transmitter output circuit coupled to said
tone-generating circuit for transmitting signals corresponding to
the sequence of control tones to be transmitted; and a receiver
including an input circuit for receiving the signals from said
transmitter, filter means coupled to the input circuit and having a
plurality of control inputs for receiving one at a time signals
selectively to tune said filter means to predetermined frequencies,
a plurality of AND circuits corresponding in number to the number
of control tones in the sequence of control tones and each having
first and second inputs and an output, control means for providing
at the output thereof a control signal during reception of the
first control tone in the sequence of control tones, the output of
said control means being coupled to one of the control inputs of
said filter means and to the first input of the first of said AND
circuits, said filter means being responsive to the application of
the control signal from said control means to be tuned to the
frequency of said first control tone, means coupling the output of
said filter means to the second input of each of said AND circuits,
said first AND circuit being responsive to the application thereto
of both the control signal from said control means and said first
control tone for producing at the output thereof an input signal,
the output of said first AND circuit and the output of each
succeeding AND circuit being coupled respectively to the first
input of the next succeeding AND circuit, means coupling the
outputs of said AND circuits respectively to the control inputs of
said filter means, means coupled to said control means for removing
the control signal therefrom after termination of said first
control tone, said second AND circuit and each succeeding AND
circuit being responsive to the application thereto of both the
output signal from the next preceding AND circuit and the
associated control tone in the sequence of control tones from said
filter means for producing at the output thereof an output signal,
said filter means sequentially being tuned to the frequencies of
the control tones as they are received by said input circuit in
accordance with the sequential application of said output signals
to said control inputs, and an output signal utilization circuit
coupled to the output of the last of said AND circuit for utilizing
the output signal therefrom.
36. A tone control circuit for use in a communication receiver
including an input circuit which provides a control tone and
including an output signal utilization circuit, said tone control
circuit comprising filter means coupled to the input circuit and
including a first frequency-determining element having a plurality
of taps thereon and a second frequency-determining element, a
connector plug including first and second and third portions, said
first portion having a plurality of terminals thereon fixedly
coupled respectively to the taps on said first
frequency-determining element, said second portion having a
terminal fixedly coupled to said second frequency-determining
element, said third portion having a plurality of first terminals
respectively matable with the terminals in said first portion and
having a second terminal matable with the terminal in said second
portion, means associated with said third portion connecting said
first terminal to a selected one of said second terminals, whereby
mating said third portions with said first and second portions
operates to connect a selected section of said first
frequency-determining element in circuit with said second
frequency-determining element and thereby tune said filter means to
the frequency of the control tone, an output circuit coupled to
said filter means and responsive to the control tone being coupled
therethrough to provide an output signal, and means coupling the
output of said output circuit to the output signal utilization
circuit of the receiver.
37. The tone control circuit set forth in claim 36, wherein said
first frequency-determining element is an inductor and said second
frequency-determining element is a capacitor.
38. The tone control circuit set forth in claim 36, wherein said
first frequency-determining element is an inductor and said second
frequency-determining element is a capacitor, one terminal of said
capacitor being fixedly coupled to one terminal of said inductor
and said connector plug being operative to connect the other
terminal of said capacitor to a tap on said inductor so as to
define a parallel resonant circuit.
39. A tone control circuit for use in a communication receiver
including an input circuit which provides a sequence of control
tones and including an output signal utilization circuit, said tone
control circuit comprising filter means coupled to the input
circuit and including a first frequency-determining element having
a plurality of taps thereon and a second frequency-determining
element, a set of electronic switching devices each having a
control input and one terminal coupled to said second
frequency-determining element, a connector plug including first and
second and third portions, said first section having a plurality of
terminals thereon fixedly coupled respectively to the taps on said
first frequency-determining element said second portion having a
corresponding set of terminals respectively fixedly coupled to the
other terminals of said electronic switching devices, said third
portion having a plurality of first terminals respectively matable
with the terminals in said first portion and a set of second
terminals respectively matable with the terminals in said first
portion, means associated with said third portion for connecting
selected ones of said first terminals respectively to selected ones
of said second terminals, whereby mating said third portion with
said first and second portions enables said electronic switching
devices respectively to couple selected sections of said second
frequency-determining element in circuit with said first
frequency-determining element, an output circuit having an input
coupled to said filter means and having a plurality of outputs
respectively coupled to the control inputs of said electronic
switching devices, said electronic switching devices sequentially
being closed by the application of signals from said output circuit
to said control inputs sequentially to tune said filter means to
the frequencies of the control tones as they are received by the
input circuit, and means coupling the output which is responsive to
the last tone in the sequence of control tones to the output signal
utilization circuit of the receiver.
40. The tone control circuit set forth in claim 39, wherein each of
said electronic switching devices is a transistor means having a
base which is said control input and a pair of output electrodes in
the form of a collector and an emitter, one of said output
electrodes being coupled to the associated tap and the other of
said output electrodes being coupled to said second frequency
determining element.
Description
The present invention relates to communication systems, and
particularly to communication systems for selectively transmitting
intelligence from a transmitter to at least one selected
receiver.
It is an important object of the present invention to provide in a
communication system for selectively transmitting intelligence from
a transmitter to at least one selected receiver, the combination
comprising a transmitter including a tone-generating circuit for
generating tones selected from a first group of tones in a first
band of frequencies and for generating tones selected from a second
group of tones in a second band of frequencies separate and
distinct from the first band of frequencies, the tone-generating
circuit generating a sequence of tones alternately selected from
from the first and second groups of tones, a transmitter output
circuit coupled to the tone-generating circuit for transmitting
signals corresponding to the sequence of tones and corresponding to
the intelligence to be transmitted, a receiver including an input
circuit for receiving the signals from the transmitter, a detecting
circuit coupled to the input circuit for detecting the sequence of
tones and the intelligence of the signals, a translating circuit
coupled to the detecting circuit for translating the intelligence
into a useable form, an output control circuit coupled to the
translating circuit and effective in a first condition thereof to
render the translating circuit inoperative and effective in a
second condition thereof to render the translating circuit
operative, first and second tone control circuits, the first tone
control circuit being coupled to the detecting circuit and to the
second tone control circuit and responsive to the application of
tones from the first group of tones to provide first control
signals to the second control circuit, the second tone control
circuit being coupled to the detecting circuit and to the first
tone control circuit and responsive to the application of the tones
from the second group of tones to provide second control signals to
the first tone control circuit, and means for applying an output
control signal from the tone control circuits to the output control
circuit upon the application of the last tone in the sequence of
tones for actuating the output control circuit from the first
condition thereof to the second condition thereof, thereby to
render the translating circuit operative.
In connection with the foregoing object, it is another object of
the invention to provide a communication system of the type set
forth wherein the control tones are in the audio range of
frequency, each band of frequencies containing at least 10 separate
tones with each tone separated from the adjacent tones by about
31/3 percent of the frequency thereof with instantaneous switching
between adjacent tones in the sequence.
Another object of the invention is to provide a communication
system of the type set forth wherein the transmitter produces a
carrier signal upon which the intelligence and the control tones
are applied by modulation, the preferred type of modulation being
frequency modulation.
Yet another object of the invention is to provide a communication
system of the type set forth wherein each tone in the sequence of
tones has a time duration substantially not greater than about 40
milliseconds, and the spacing between adjacent tones in the
sequence of tones each has a time duration as short as possible,
whereby to provide for rapid receiver turn-on with freedom from
false operation.
Still another object of the invention is to provide an improved
receiver for use in a communication system of the type set
forth.
In connection with the foregoing object, another object of the
invention is to provide an improved receiver of the type set forth
having therein two tone control circuits each having a tapped
filter therein adjustable to a selected one of a plurality of
positions respectively corresponding to a selected tone in one of
the groups of tones so that upon the application of the selected
tone thereto, an output is derived therefrom.
Yet a further object of the invention is to provide an improved
receiver of the type set forth wherein the tapped filter has a
plurality of connections thereon, the appropriate connection being
electronically established thereby to select the response frequency
of the tapped filter.
Still another object is to provide an improved communication system
compatible with a numbering system (or alphanumeric designation)
wherein there will be a large shift in frequency between any pair
of successive digits or letters regardless of the number or letter
to be selected, all while requiring only a relatively narrow total
frequency spectrum.
A still further object of the invention is to provide a receiver of
the type set forth wherein the taps selected on the tapped filters,
and therefore the frequency of response of the filter, are selected
and determined by the connectors interconnecting the tapped filter
and the other circuit components of the receiver.
A further object of the invention is to provide an improved topped
filter for use in the receiver of the present invention.
Further features of the invention pertain to the particular
arrangement of the elements of the communication system, the
receiver therefor, and the components circuits and elements
thereof, whereby the above-outlined and additional operating
features thereof are attained.
The invention, both as to its organization and method of operation,
together with further objects and advantages thereof will best be
understood by reference to the following specification taken in
connection with the accompanying drawings, in which:
FIG. 1 is a schematic diagram of a communication system made in
accordance with and embodying the principles of the present
invention, the transmitter and the receiver embodying the system
being illustrated in block form;
FIG. 2 is a more detailed schematic and block diagram of the
receiver diagram of the receiver forming a part of the
communication system of FIG. 1;
FIGS. 3, 4 and 5, taken together, comprise a schematic electrical
diagram of the tone control circuits forming a part of the receiver
of FIG. 2;
FIG. 6 is a schematic block diagram of a second form of a receiver
made in accordance with and embodying the principles of the present
invention;
FIG. 7 is a schematic electrical diagram of the filters and
inverter forming a part of the receiver illustrated in FIG. 6;
FIG. 8 is a schematic block diagram of a third form of a receiver
made in accordance with and embodying the principles of the present
invention; and
FIG. 9 is a schematic electrical diagram of the filters and the NOR
circuit forming a part of the receiver of FIG. 8.
The principles of the present invention are equally applicable to
communication systems utilizing wire lines, modulated supersonic
signals, AM radio signals, and FM radio signals. For illustrative
purposes, there is shown in the drawings a communication system
employing FM radio signals. Those skilled in the art will readily
understand that the various principles to be describer hereinafter
in conjunction with the system employing FM radio signals can be
readily adjusted to the other types of communication systems using
other forms of transmission such as those set forth above.
Referring to FIG. 1 of the drawings, there is shown a mobile FM
radio communication system made in accordance with and embodying
the principles of the present invention, the system being generally
designated by the numeral 20. The system 20 includes an FM
transmitter 30 and an FM receiver 100, it being understood that the
transmitter 30 and/or the receiver 100 may be either fixed or
mobile, each operating system typically containing both a
transmitter 30 and a receiver 100.
The transmitter 100 includes the usual RF oscillator 31, the output
of which is applied via the conductor 32 to a modulator 40, the
modulator 40 serving to impress audio signals upon the RF carrier
provided by the oscillator 31 by means of frequency modulation. One
of the audio inputs to the modulator 40 is derived from a voice
input appearing on a conductor 51 which is applied through an audio
amplifier 50 to a conductor 52 serving as an audio input to the
modulator 40. There also is provided a tone sequence generator 60,
the output of which is connected to the conductor 51 and is applied
through the audio amplifier 50 and through the conductor 52 as a
second input to the modulator 40. The output of the modulator 40
appears on the conductor 41 which is coupled to the output circuits
70 where power amplification is effected, the output from the
circuits appearing on the conductor 71 that is coupled to a
transmitting antenna 80, all in the usual construction and
arrangement.
The tone sequence generator 60 is of the type that can generate a
sequence of tones, for example, a sequence of two tones, three
tones, ...seven tones, etc., the tone sequence being preferably
generated automatically after being encoded therein. Furthermore,
alternate tones in the tone sequence are selected from two
different groups of tones in two different bands of audio
frequencies. For example, the first, third, fifth, etc. tones would
be selected from a first group of tones in a first band of
frequencies, while the second, fourth, sixth, etc. tones in the
sequence of tones would be selected from a second group of tones in
a second band of frequencies separate and distinct from the first
band of frequencies. Preferably the two bands of frequencies are
separated by a substantial frequency gap from each other. In an
illustrative example of a typical system providing 10 tones in each
group of tones, the first group of tones would have the following
frequencies providing 10 different channels: 1177, 1219, 1261,
1306, 1352, 1400, 1449, 1500, 1553 and 1608; while the second group
of tones would have the following frequencies providing 10
different channels: 1980, 2049, 2121, 2196, 2274, 2354, 2437, 2523,
2612 and 2704. It will be noted that each tone is spaced from the
adjacent tones in an amount equal to approximately 31/3 percent of
the frequency thereof, while there are five tones missing between
the two groups of tones, thus constituting the frequency gap
between the nearest adjacent tones in the two groups of tones. In a
typical illustration, the odd-numbered tones in the sequence of
tones would be selected from the first group of frequencies, while
the even-numbered tones in the sequence of tones would be selected
from the second group of frequencies. Selecting one tone from each
group of tones to provide a two-tone sequence permits 100 different
codes, while choosing four tones from one group of tones and three
tones from the other group of tones to provide seven tones can
provide 10 million different codes. If the frequency groups are
reversed in sequence an additional 100, or 10 million codes can
similarly be provided for 227-tone sequences. In the tone sequence
generator 60, each tone may, for example, have a time duration of
15 to 100 milliseconds, a typical time duration being 30
milliseconds, for a seven-tone sequence or 75 milliseconds for a
two-tone sequence, while the time gap between adjacent tones in the
sequence should be as short as possible; as a consequence, for a
seven-tone sequence, response times as low as 100 milliseconds are
available, all while providing greater security against false
activation, as will be explained more fully hereinafter.
The transmissions from the transmitter 30 are adapted to be
received by the receiver 100 and more particularly by the antenna
101 thereof which is connected by a conductor 102 to the usual RF
and IF and detecting circuits 103--113. The output from the
circuits 103--113 appears on a conductor 114 which applies an input
to the audio output circuit 115--117 that in turn have the output
connected as the input to audio speaker circuits 120--133. In
accordance with the present invention, no output is obtained from
the audio speaker circuits 120--133 unless the tone decoder
circuits 200--300 are first operated by a suitable tone sequence,
the input to the tone decoder circuits being on a conductor 124 and
the output therefrom being on a conductor 126.
There is illustrated in FIGS. 2 of the drawings, a more complete
diagram of the radio receiver 100 forming a part of the
communication system of the present invention. The carrier signal
from the transmitter 30 is picked up on the antenna 101 and is
conveyed by the conductor 102 to the input of a radiofrequency
amplifier 103. The output of the radiofrequency amplifier 103 is
supplied by a conductor 104 as one of the inputs to the mixer 105,
the usual local oscillator 106 being provided and having the output
thereof connected by a conductor 107 as a second input to the mixer
105. The intermediate frequency which is the output of the mixer
105 is applied by a conductor 108 as the input to the IF amplifier
109, the output of which is transmitted by the conductor 110 to the
input of a limiter 111. The output of the limiter 111 appears on a
conductor 112 and is the input to the discriminator 113, the output
of the discriminator 113 being an audiofrequency signal appearing
on the conductor 114. The audio signal on the conductor 114 is
amplified by an audio amplifier 115 and is then conveyed by a
conductor 116 to an audio output amplifier 117. The output from the
amplifier 117 is applied to an output transformer 120, and
specifically to the primary winding 121 thereof, a secondary
winding 122 being provided having one terminal connected by a
conductor 123 to one of the input terminals of a loudspeaker 125,
and the other terminal being connected to a conductor 124. The
conductors 123 and 124 are also connected as an input to the
limiter 150 which provides a signal for a first tone control
circuit 200 and a second tone control circuit 300. Before there is
any output derived from the loud speaker 125, the tone control
circuits 200--300 must be activated by a proper sequence of control
tones, the output then being applied to a control relay 130
including a coil 131 to which are connected the conductors 134 and
139, respectively. The armature of the relay 130 controls a movable
switch contact 132 which cooperates with a fixed switch contact 133
connected to the conductor 124. A conductor 126 interconnects the
other input terminal of the loud speaker 125 and the switch contact
132. A proper output from the tone control circuits 200--300
operates the relay 130 to close the contacts 132--133 thereby to
connect the loudspeaker 125 across the output terminals of the
transformer secondary winding 122, thus to provide an audio output
from the receiver 100.
The first tone control circuit 200 has as an input thereto a first
control tone as the output of the limiter 150 that appears on the
conductor 165, the input being a series of pulses of essentially
square waveform that are applied as an input to a tapped filter
201, the output of the tapped filter 201 being a sinusoidal
waveform appearing on a conductor 209, provided that the frequency
of the input pulse is that to which the tapped filter 201 is tuned.
The conductor 209 connects to a rectifier 220 which serves to
rectify the input and to provide a DC output voltage on a conductor
227 that is connected as one input to a Schmidt trigger circuit
230. Another input to the Schmidt trigger 230 is a suitable bias
from a DC voltage that is applied on a conductor 155. The output of
the Schmidt trigger 230 is connected by a conductor 244 to a delay
circuit 260, and if the DC voltage on the conductor 244 persists
for a predetermined period of time, an output is derived from the
delay circuit 260 upon cessation of the first control tone. The
output of the circuit 260 appears on a conductor 262 that is
connected to the input of the monostable multivibrator 270. The
multivibrator 260 is effective to produce an output pulse on a
conductor 296 which is applied as one of the inputs to the second
tone control circuit 300.
The second tone control circuit 300 has as one of the inputs
thereto a second control tone as the output of the limiter 150 that
appears on the conductor 165, this input being a series of pulses
of essentially square waveform that are applied as an input to a
tapped filter 301, the output of the tapped filter 301 being a
sinusoidal waveform appearing on a conductor 309, provided that the
frequency of the input pulses is that to which the tapped filter
301 is tuned. The conductor 309 connects to a rectifier 320 which
serves to rectify the input and to provide a DC voltage on a
conductor 327 that is connected as one input to a Schmidt trigger
circuit 330. Another input to the Schmidt trigger circuit 330 is
the pulse on the conductor 296 from the monostable multivibrator
270 described above. The output of the Schmidt trigger 330 is
connected by a conductor 344 to a delay circuit 360, and if the
control voltage on the conductor 344 persists for a predetermined
period of time, an output is derived from the delay circuit 360 on
a conductor 366 that is connected as the input to the control
circuits 370 that is sufficient to control 370. The output of the
control circuits 370 appears on the conductors 134 and 139 that
apply an energizing potential to the relay 130 to connect the loud
speaker 125 to the audio output of the receiver 100 after the
proper sequence of tones has been received. There further is
provided a holding circuit 335 that has the input connected to the
conductor 344 and has the output connected by a conductor 298 as
one of the inputs to the monostable multivibrator 270.
Referring to FIG. 3 of the drawings, there is illustrated in detail
a portion of the tone control circuits, and specifically the
limiter 150 and the two tapped filters 201 and 301. The output from
the audio output amplifier 117 is coupled via the output
transformer 120 and the conductors 123--124 to the input of the
limiter 150, and specifically to the input terminals of a
transformer 151. More specifically the transformer 151 has a
primary winding 152 and a secondary winding 153, the terminal of
the primary winding 152 being connected respectively to the
conductors 123--124. The secondary winding 153 is center tapped and
has connected to the center-tap one terminal of a limiting resistor
154, the other terminal of the resistor 154 being connected to the
conductor 155. It will be understood that the +DC source circuit of
which the conductor 155 is the output terminal provides a fixed DC
voltage of for example 12 volts positive, the value of the voltage
being regulated to a stable value as by a Zener diode, for
example.
The upper terminal of the secondary winding 153 is connected as an
input to a transistor 160, and particularly to the base 161
thereof, the collector 162 being connected as an output to a
conductor 165 and through a resistor 167 to the +12-volt DC
conductor 155; and the emitter 163 is connected to a conductor 166.
The lower terminal of the secondary winding 153 is connected by a
conductor 168 has an input to a second transistor 170, and
specifically to the base 171 thereof. The collector 172 is
connected by a conductor 174 to the +12-volt DC conductor 155,
while the emitter 173 is connected to the conductor 166. There also
is provided in the limiter 150 a bias circuit in the form of a
transistor 180, the base 181 of the transistor 180 being connected
by a conductor 184 to one terminal of a potentiometer 185, the
other terminal of the potentiometer 185 being connected to the
+12-volt conductor 155, the potentiometer being provided with the
usual arm 186 having one end connected to the conductor 184 and the
other end contacting an adjusted point on the resistive member of
the potentiometer 185. The collector 182 is connected to the
conductor 166, while the emitter 183 is connected by a conductor
187 to one terminal of a resistor 188, the other terminal of the
resistor 188 being grounded as at N.
The output from the limiter 150 is a train of essentially square
waves appearing on the conductor 165, the train of square waves
being coupled to the filters 201 and 301 by the conductor 165.
Considering now the construction of the tapped filter 201, there is
provided an input capacitor 208 and an inductor in the form of a
coil 202 having associated therewith a magnetic core 203, at least
a portion of the core 203 being movable and adjustable, whereby the
inductor 202 can be slug tuned. The inductor 202 has an input
terminal 204 that is connected by a conductor 209 to one terminal
of a capacitor 205, the other terminal of the capacitor 205 being
grounded as at N, the output being on the conductor 209. As
illustrated, the inductor 202 has a plurality of taps thereon, and
specifically 10 taps that are connected to contacts numbered 210
through 219. Associated with the contacts is a movable contact arm
206 which is also grounded via the conductor N. The series of
resonance impedance of the filter 201 can be readily changed by
moving the contact arm 206 from one contact to another, whereby to
change the frequency at which the parallel circuit consisting of
the tapped coil 202 and the capacitor 205 become series resonant
with the capacitor 208. At resonance the output from the filter 201
will be a maximum and the output will be sinusoidal.
The construction of the tapped filter 301 is identical to that of
the tapped filter 201, whereby like reference numerals in the 300
series have been applied to like parts thereof, and in the interest
of brevity, no further description of the tapped filter 301 will be
given, except to point out that the output therefrom is a
sinusoidal wave on the conductor 309, and that the output will be a
maximum when the audio input pulse rate is at the frequency to
which the output filter 301 is tuned.
Referring to FIG. 4 of the drawings, there is illustrated the
details of the construction of the remainder of the first tone
control circuit 200. The sine wave on the conductor 209 is applied
as an input to the rectifier 220, the rectifier 220 more
specifically being a transistor having a base 221 to which the
conductor 209 is connected. The collector 222 of the transistor 220
is connected to the +12-volt DC conductor 155, while the emitter
223 is connected by a conductor 225 to a voltage divider network
including resistors 226 and 228 connected in series to ground
potential as at N, the adjacent terminals of the resistors 226--228
being connected by a conductor 227. A filtering capacitor 229 is
connected between the conductor 225 and the ground potential and in
parallel with the series resistors 226--228.
The output from the rectifier 220 is a DC voltage appearing on the
conductor 227 which is applied as one of the inputs to the Schmidt
trigger circuit 230, and specifically to a transistor 240 therein,
the base 241 of the transistor 240 being connected to the conductor
227. The collector 242 of the transistor 240 is connected via a
conductor 244 and a resistor 245 to the + DC conductor 155, a
capacitor 246 being provided in parallel with the resistor 245;
while the emitter 243 is connected by a conductor 239 to a bias
transistor 231 and to another transistor 250.
The bias transistor 231 has the base 232 thereof connected by a
potentiometer 235 to the + DC conductor 155, the potentiometer 235
being provided with a movable contact 237 having one terminal
thereof connected by a conductor 236 to the base 232 and having the
other terminal thereof in sliding adjusted engagement with the
resistive element of the potentiometer 235. The collector 233 is
connected to the conductor 239, while the emitter 234 is connected
through a resistor 238 to ground potential as at N.
The transistor 250 has a base 251 that is connected by a conductor
256 to one terminal of a resistor 257 and to one terminal of a
resistor 258, the other terminal of the resistor 257 being
connected to the conductor 244, while the other terminal of the
resistor 258 is connected to ground potential as at N. The
collector 252 of the transistor 250 is connected via a conductor
254 and a resistor 255 to the +DC conductor 155, while the emitter
253 is connected to the conductor 239.
The output from the Schmidt trigger circuit 230 is a DC voltage
appearing on the conductor 244 that provides an input to the delay
circuit 260 which comprises a capacitor 261 and a resistor 263
interconnected by a conductor 262. More specifically, one terminal
of the capacitor 261 is connected to the conductor 244 and the
other terminal thereof is connected to the conductor 262, and one
terminal of the resistor 263 is connected to the conductor 262 and
the other terminal thereof is connected to ground potential as at
N.
The output from the delay circuit 260 is a voltage pulse appearing
on the conductor 262 that is applied as one of the inputs to the
monostable multivibrator 270, and specifically to the input
transistor 280 thereof. The transistor 280 has the base 281 thereof
connected to the conductor 262, has the collector 282 thereof
connected via a conductor 284 and a resistor 285 to the +DC
conductor 155, and has the emitter 283 thereof connected to a
conductor.
A bias transistor 271 is provided to apply suitable bias to the
transistor 280 and a second transistor 290 in the monostable
multivibrator 270. The transistor 172 has a base 272 that is
connected through a potentiometer 275 to the DC conductor 155, the
potentiometer 275 having a movable contact 277 bearing against the
resistive element thereof, which contact 277 is connected by a
conductor 276 to the base of 272. The transistor 271 also has a
collector 273 connected to the conductor 279 and an emitter 274
connected to the ground conductor N.
The transistor 290 has the base 291 thereof connected to a
conductor 298 that is in turn connected to the adjacent terminals
of resistors 286 and 299, the other terminal of the resistor 286
being connected to the conductor 284 and the other terminal of the
resistor 299 being connected to the grounded conductor N; the
transistor 290 also has a collector 292 connected via a conductor
296 and a potentiometer 295 to the +DC conductor 155, and an
emitter 293 connected to the conductor 279. The potentiometer 295
is provided with the usual contact arm 297 in sliding contact with
the resistive element of the potentiometer 295, the contact arm 297
being connected to the conductor 296. The output from the
monostable multivibrator 270 appears on the conductor 296 as a
positive going voltage pulse upon the removal of the first control
tone from the receiver 100 provided the first tone has been present
for a period longer than a minimum delay period determined by the
time constants of delay circuit 260.
Referring next to FIG. 5 of the drawings, there is illustrated in
detail the remaining portions of the tone control circuit 300. The
sinusoidal waveform corresponding to the selected second tone
appears on the conductor 309 and is applied as an input to the
rectifier 320, the rectifier 320 being in the form of a transistor
having a base 321 to which the conductor 309 is connected. The
transistor 320 further has a collector 322 being connected to the
+DC conductor 155, and an emitter 323 connected via a conductor
325, a resistor 328 to the grounded conductor N, a capacitor 329
being connected in parallel with the resistors 326 and 328.
The output from the rectifier 320 appearing on the conductor 327 as
+DC voltage that is applied a s an input to the Schmidt trigger
circuit 330 including a bias transistor 331 and a pair of
transistors 340 and 350. The bias transistor 331 has a base 332 to
which is connected the conductor 296 carrying the output from the
monostable multivibrator 270. The transistor 331 has a collector
333 connected to a conductor 339 and an emitter 334 connected by a
resistor 338 to the grounded conductor N.
The transistor 340 has a base 341 to which is connected the
conductor 327 carrying the DC tone rectifier output as an input to
the Schmidt trigger circuit 330. The transistor 340 has a collector
342 that is connected via a conductor 344 and a resistor 345 to the
+DC conductor 155, a capacitor 346 being connected in parallel with
the resistor 345; and an emitter 343 that is connected to the
conductor 339.
The transistor 350 has a base 351 that is connected by a conductor
356 to adjacent terminals of two resistors 357 and 358, the other
terminal of the resistor 357 being connected to the conductor 344
and the other terminal of the resistor 358 being connected to the
grounded conductor N. The transistor 350 also has a collector 352
connected via a conductor 354 and a resistor 355 to the +DC
conductor 155 and an emitter 353 connected to the conductor
339.
The output from the Schmidt trigger circuit 330 appears on the
conductor 344 as a negative going voltage, the output being applied
to the HOLD ON circuit 355 and the delay circuit 360. As
illustrated, the HOLD ON circuit 355 is in the form of a diode
having one terminal connected to the conductor 344 and the other
terminal connected to the conductor 348 to provide a holding
potential for the monostable multivibrator 270.
The input to the delay circuit 360 is applied to one terminal of
the diode 361. The other terminal of the diode 361 is connected by
a conductor 362 to one terminal of a charging capacitor 363, and
one terminal of a resistor 364, the other terminal of resistor 364
being connected by a conductor 366 to one terminal of a capacitor
365, the other terminal of the capacitor 365 being connected to the
+DC conductor 155.
The control circuit 370 contains two transistors 371 and 380, the
transistor 371 being of the PNP type, whereas all of the other
transistors described heretofore, and including transistor 380, are
of the NPN type. The transistor 371 has a base 372 to which is
connected the conductor 366, the base 372 also being connected
through a resistor 375 to the +DC conductor 155. The collector of
the transistor 371 is connected by a conductor 368 to the base 381
of the transistor 380, and the emitter 374 of the transistor 372 is
connected by a conductor 377 to the adjacent terminals of a pair of
resistors 376 and 378, the other terminal of the resistor 376 being
connected to the +DC conductor 155 and the other terminal of the
resistor 378 being connected to the grounded conductor N. The
collector 382 of the transistor 380 is also connected to the +DC
conductor 155 through a resistor 379 and is connected by the
conductor 367 to one terminal of the capacitor 363. The emitter 383
of the transistor 381 is connected by a conductor 384 as the input
to a transistor 391.
The conductor 384 more specifically connects to the base 392 of the
transistor 391, the transistor 391 controlling the conduction of
current through the control relay 130 that has been described
above, the conductor 139 being connected to the collector 393 of
the transistor 391. The emitter 394 of the transistor 391 is
connected to ground potential via a resistor 399, a conductor 387,
and a resistor 385, a capacitor 386 being connected in parallel to
the resistor 385. The other connection for the relay 130, namely
the connector 134, is connected to the +DC conductor 155. Arranged
in parallel with the relay 130 is a "push to reset light" 137
having one terminal connected by a conductor 136 through a switch
135 to the +DC conductor 155, and having the other terminal
connected via a conductor 138 to one of the power terminals of a
silicon-controlled rectifier (SCR) 395, specifically to the power
terminal 396 thereof, the other power terminal 397 being grounded.
A control terminal 398 is provided for the SCR 395 and is connected
to the conductor 397.
Considering now the operation of the receiver 100, in order to
obtain an output from the loudspeaker 125, a two-tone sequence of
control tones must be applied to the receiver 100, each tone
preferably having a time duration of at least about 15 milliseconds
without any gap therebetween. Furthermore, the first tone received
must be that to which the tapped filter 201 is tuned, and the
second tone received must be that to which the tapped filter 301 is
tuned. Upon the reception of the first tone, an audio signal
corresponding to the first tone is applied along the conductors
123--124 as an input to the limiter 150. The transistors 160--170
provide in essence a differential amplifier, the total current that
can flow through the transistors 160--170 being that which flows
through the transistor 180 which is in turn controlled by the
setting of the potentiometer 185. The potentiometer 185 is adjusted
so that the current through the transistor 180 is not sufficient to
allow one of the transistor 160--170 to be driven to saturation
when the other is nonconducting. The incoming control tone on the
conductors 123--124 alternately increases the conduction of one of
the transistors 160--170 and decreases to cut off the conduction of
the other, the conduction through the conducting transistor
160--170 being limited by the conduction through the transistor
180. Therefore the output on the conductor 165 is a square wave
whenever the amplitude of the incoming audio signal is more than a
certain minimum determined by the gain of the limiter 150 and the
adjustment of limiting level by the potentiometer 175. Assuming
that the amplitude of the first control tone appearing at the input
of the limiter 150 is sufficient to cause limiting a square wave
output is provided on the conductor 165. It is pointed out that
once the limiter 150 operates, the filter control band width will
be constant if the limiter is symmetrical, i.e., if the circuits
for the transistors 160 and 170 are balanced and symmetrical. An
important feature of the limiter 150 resides in the fact that with
a carefully controlled DC bias potential on the conductor 155 such
as that provided by a Zener diode supply, there is no change of the
band width as a result of a change in the amplitude of the incoming
tone control signal, since above limiting, the voltage level to the
output tone filters will be constant. When the amplitude of the
incoming signal is below the limiting level, the band width will
narrow; however this narrowing portion will be very small. By
adjusting the limiting level of the limiting amplifier 150 by means
of the slider 186, the maximum band width can be readily
adjusted.
The series of square wave pulses on the conductor 165 are then
applied to the tapped filters 201 and 301 and assuming that the
contact 206 is positioned so as to be at resonance for the first
tone voltage, the output at the conductor 209 will be a sinusoidal
wave having a frequency equal to that of the first control tone. It
is pointed out that the tapped filter 201 responds to the selected
one of the 10 tones in the first group of tones that are all
disposed in a first band of frequencies extending from 1,177 cycles
per second to 1,608 cycles per second, while the tapped filter 301
responds to the elected one of the ten tones in the second group of
tones that are disposed in a second band of frequencies extending
from 1,980 cycles per second to 2,704 cycles per second. It is
further pointed out that when the band width for a single tone is
adjusted as a percentage of its resonant frequency by slug tuning
of the inductors 202 or 302, as the case may be, the percentage of
band width remains constant for all other taps. This results from
the fact that the frequency of an LC circuit varies directly with
the number of turns on the coil and because the Q also varies in a
fashion such as to hold the band width as a constant percentage of
the particular frequency selected. This results in substantial
savings in manufacturing costs since a single frequency or band
width adjustment adjusts the band width for all frequencies.
Stating the matter in another way, in the resonant circuit such as
the tapped filter 201, when the capacitance of the capacitors 208
and 205 are held constant and the inductance of the inductor 202 is
varied, by the tap selection, the Q varies proportionately to the
number of effective turns used in the inductor 202, the resonant
frequency varies inversely as the number of effective turns in the
inductor 202, and the Q varies proportionately to the frequency. If
the resonant frequency is varied by changing the number of
effective turns on the inductor 202, the circuit Q will remain
constant at resonance for each selected frequency, i.e., if the
turns are doubled, the resonant frequency will be halved while the
Q will be doubled by the turns increase at the same frequency but
halved by the frequency change, thereby to leave the Q unchanged.
The voltage as a percentage of the peak voltage gives a band width
which is a predetermined percentage of the resonant frequency for
any given Q. If Q is held constant, as explained above, the band
width will remain a constant percentage of the resonant frequency
as the resonant frequency is varied, for example +2percent
preferred channel separation is that wherein the spacing between
adjacent frequencies is a uniform percentage of resonance, the 31/3
percent spacing, for example referred to previously.
The sinusoidal wave corresponding in frequency to the first control
tone is applied along the conductor 209 as the input to the
rectifier 220, and there appears as the output from the rectifier
220 a positive going voltage on the conductor 227 that is applied
as an input to the Schmidt trigger circuit 230.
Before the application of the input along the conductor 227 to the
Schmidt trigger circuit 230, the bias transistor 231 is conducting
to establish a bias on the emitters of the transistor 240 and 250,
the amount of bias being adjustable by means of the potentiometer
235, whereby the potentiometer 235 serves as the band width
adjustment for the first control tone. Initially the transistor 240
is nonconducting and the transistor 250 is conducting. Upon the
application of the positive voltage along the conductor 227 to the
base of the transistor 240, the transistor 240 begins to conduct
and quickly drives the transistor 250 to a nonconducting condition,
the potential on the conductor 244 that is applied to the base 251
of the transistor 250 rapidly dropping. After a suitable time delay
as determined by the time constant characteristics provided by the
capacitor 261 and the resistor 263 and upon cessation of the first
tone, an input is applied to the monostable multivibrator 270 along
the conductor 262.
Prior to the application of the first control tone to the first
control circuit 200, the biasing transistor 271 is conducting in an
amount determined by the adjustment of the potentiometer 275 and
applies a bias along the conductor 279 which serves to render the
transistor 280 nonconducting and the transistor 290 conducting.
Upon the application of the first control tone to the receiver 100
the voltage on the conductor 244 drops as explained above and the
capacitor 261 begins to charge at a rate determined by the value of
the capacitor 261 and the resistor 263. If the charge time of the
capacitor 261 is sufficient, when the potential on the transistor
244 returns to the high potential of the capacitor 261 it will
supply a voltage to the base 281 of the transistor 280 that will
exceed the voltage at the collector 273 of the transistor 271. In
other words, the transistor 281 is rendered conducting by a
positive pulse from the delay circuit 260 which exceeds the
potential on the conductor 279 that is established by the
adjustment of the potentiometer 275, and at the same time the
transistor 290 is rendered nonconducting, thereby to provide a
positive pulse output on the conductor 296, the potential on the
conductor 296 rising to the DC voltage on the conductor 155 (less
any voltage drop in the potentiometer 295 caused by conduction of
the second tone circuit 300).
The initial low bias on the conductor 296 is applied to the base
332 of the transistor 331 of the Schmidt trigger circuit 330 of the
second control circuit 300, thereby to render the transistor 331
nonconducting which in turn renders the transistor 340
nonconducting and the transistor 350 nonconducting, whereby the
output of the second tone control circuit is blocked prior to the
reception of the first control tone and for so long as the first
control tone persists. Upon the removal of the first control tone
from the input to the first tone control circuit 200, the
conditions described above in the first control circuit are
essentially reversed, whereby the high positive potential on the
conductor 296 rapidly rises as the transistor 290 is blocked and
the adjustment of the potentiometer 295 in effect adjusts the band
width of the second tone control circuit 300.
Upon the removal of the first control tone and the accompanying
rise in potential on the conductor 296, the transistor 331 begins
to conduct, the transistor 340 is continued in a nonconducting
condition, while the transistor 350 begins to conduct. Now the
second tone control circuit 300 is ready to receive the second
control tone.
Assuming now that a proper second control tone is applied to the
receiver 100 and passes through the limiter 150 and the tapped
filter 301, there is provided on the conductor 309 a sinusoidal
wave form having the frequency of the second control tone. This
input is applied to the rectifier 320 thereby to produce a positive
going output on the conductor 327 that is applied to the base of
the transistor 340 in the Schmidt trigger circuit 330.
As has been described heretofore, at this point in the operation of
the system, the transistor 340 is nonconducting while the
transistor 350 is conducting, whereby upon the application of the
positive going voltage along the conductor 327 to the base 341 of
the transistor 340, the transistor 340 is rendered conductive. Upon
heavy conduction of the transistor 340, the heretofore relatively
high potential on the conductor 344 rapidly drops and this drop in
potential in the form of a negative going voltage is applied to the
base of the transistor 350 to render the transistor 350
nonconducting, and the signal is also applied as an input to the
delay circuit 360. The capacitors 363 and 365 now begin to charge
and after a predetermined time as determined by the time constant
characteristics of the delay circuit 360 (for example 40
milliseconds), a potential is reached on the conductor 366 which
will render the transistor 371 conductive to provide a positive
going pulse on the conductor 368. The transistor 380 acts as a
current amplifier and amplifies the positive going pulse applied to
the base thereof and in turn applies a positive going pulse to the
base of the transistor 391. The output from the transistor 391 is a
positive going pulse that is applied to the gate 398 of the SCR
395, thereby to cause heavy conduction thereof to light the
indicating light 137. Heavy conduction of the transistor 391 also
operates the relay 130 to close the switch contacts 132--133, thus
to apply the output of the transformer 120 to the loud speaker 125
via the conductors 123 and 126. The relay 130 will be energized for
a short period determined by the time which the transistor 391 is
conducting. The relay 130 and the lamp 137 could be interchanged in
their connection in the circuit if desired, depending upon the
control requirements. The relay 130 could be used for any control
purpose desired. Once the SCR 395 is conducting, it will remain
conducting until the switch 135 is opened, and such conduction will
maintain the light 137 in the energized condition.
It is further pointed out that upon receiving the second control
tone so as to provide the negative going voltage on the conductor
344, a pulse is transmitted via the holding circuit 395 and the
conductor 298 to the monostable vibrator 270 to hold the
multivibrator 270 in a condition such that the Schmidt trigger
circuit 330 will continue to be held in a condition to transmit the
signals generated by the second control tone. In this manner it
will be appreciated that the Schmidt trigger 330 also serves as an
AND circuit wherein the output from the first tone control circuit
200 must be applied thereto before the application of the second
control tone output from the rectifier 320 is effective.
Further the feedback of the HOLD ON voltage permits the second tone
to continue for as long as desired, so long as it appears soon
enough after the first tone period to cause the hold on voltage to
be generated, thereby controlling the multivibrator 270 in the "on"
condition. The multivibrator 270 may, for example, be on for only
15 or 20 milliseconds in the absence of a HOLD ON voltage. During
this period, control tone two must be applied, and the filter
output must rise to a voltage sufficient to produce the HOLD ON
voltage.
An important feature of the tone control circuit of the receiver
100 resides in the freedom from falsing, i.e., the freedom from
operation by false and spurious signals that may be applied as an
input thereto. The freedom from falsing results from several
factors. First, by utilizing the tone filter circuitry (including
the limiter 150, the tapped filters 201-301, the rectifiers 220-330
having the characteristics as described; there positively can be no
response to signals that lie out of the filter band pass,
regardless of the false signal duration or intensity. Second, by
requiring that subsequent tones in a tone sequence be spaced apart
in frequency by several tone channels, together with requiring
substantially instantaneous switching between successive tones, a
signal of smoothly varying pitch such as might be produced by a
heterodyne circuit, a siren near a microphone or like source,
cannot actuate the tone control circuits, this resulting from the
fact that the tone control circuits require a sudden jump or change
in frequency between successive tones.
It is further pointed out that the described tone control circuit
is readily adaptable to and compatible with a decimal numbering
system, at 10 different frequencies in each group of tones
corresponding to and representing the 10 different digits, with
successive tones being selected alternately from the two groups of
tones. If only 10 frequencies were utilized (rather than the 20
frequencies of the present system), to represent the 10 digits,
i.e., with no change or jumping between bands of frequencies on
successive digits, it would not be possible to utilize codes such
as 22, 55, etc. in two-tone sequence systems. Although it has been
suggested that an 11th tone be utilized as a repeat tone in such
sequence, the frequency spectrum utilized if five channels are to
separate adjacent tones would be substantially greater than that
required for the present system, and the circuitry would be
substantially more complicated. It is pointed out that utilizing
this present system wherein alternate tones are selected from two
groups of tones wherein the groups are separate at a minimum gap of
five tones, only a 25-channel spectrum is required to produce any
pair of digits in sequence while still maintaining a minimum gap of
five channels therebetween.
Referring now to FIG. 6 of the drawings, there is illustrated a
second embodiment of the present invention wherein a sequence of
four tones is required to actuate the receiver, the receiver being
designated by the numeral 400 and being illustrated in schematic
and block diagram form. The receiver 400 utilizes the input
circuits and components from the antenna 101 through the limiter
150 and onto the conductor 165, from the receiver 100, whereby the
same reference numerals have been applied to these components and
the description thereof will not be here repeated in the interest
of brevity.
The output of the limiter 150 on the conductor 165 is fed to
two-tone control circuits, the lower tone control circuit including
a special tapped filter 401 identified by the legend FILTER -1 of a
construction to be described hereinafter, the input to the filter
401 being a series of square waves and the output thereof appearing
on a conductor 409 in the form of a sinusoidal wave having a
frequency of the selected control tone. The conductor 409 connects
to a rectifier 440 which serves to rectify the input thereto and
which has the same construction as the rectifier 220 described
above. The output from the rectifier 440 is a positive voltage that
is supplied along a conductor 441 as one of the inputs to an AND
circuit 445. The output from the AND circuit 445 appears on a
conductor 446 that is connected to the input of a Schmidt trigger
circuit 450, the AND circuit 445 and the Schmidt trigger circuit
450 having the construction of the Schmidt trigger circuit 330
described above. The output from the Schmidt trigger circuit 450 is
connected by a conductor 451 to a delay circuit 452 of the same
construction as the delay circuit 260, and the output from the
delay circuit 452 is connected by a conductor 453 as one input to a
monostable multivibrator 455 of the construction described above
with respect to the monostable multivibrator 270. The output from
the multivibrator 455 appears on a conductor 525 that connects into
the upper tone control channel. Also associated with the lower tone
control channel is an inverter 460 having an input thereto from a
conductor 435 and having an output on a conductor 425 that connects
as an input to the filter 401, and to provide the second input to
the AND circuit 445.
The upper tone control channel includes a special tapped filter 501
identified as FILTER -2 also to be described more fully
hereinafter, one of the inputs to the filter 501 being from the
conductor 525 and another being from the conductor 165. The output
from the filter 501 appears on a conductor 509 connected to a
rectifier 540 of the same construction as the rectifier 320
described above. The output from the rectifier 540 is on a
conductor 541 that supplies one input to an AND circuit 545, the
output of which appears on a conductor 546 connected as an input to
a Schmidt trigger circuit 550; the AND circuit 545 and the Schmidt
trigger circuit 550 together having the construction of the Schmidt
trigger circuit 330 described above. The output from the Schmidt
trigger circuit 550 appears on a conductor 551 and is supplied as
an input to a delay circuit 552 having the construction of the
delay circuit 260 described above. The output from the delay
circuit 550 appears on a conductor 553 and is an input to a
monostable multivibrator 555 having a construction like the
monostable multivibrator 270.
The outputs on the conductors 435 and 441 are applied as inputs to
an AND circuit 560, the output from the AND circuit 560 being on
the conductor 561 that is connected to a Schmidt trigger circuit
565 of the construction of the Schmidt trigger circuit 330
described above. The conductor 561 also connects to the monostable
multivibrator 555, and the conductor 475 also connects to the
Schmidt trigger circuit 565. The output from the Schmidt trigger
circuit 565 appears on the conductor 566 and is applied to a delay
circuit 570 having the construction of the delay circuit 260
described above, the output from the delay circuit 570 being on the
conductor 571 connected to a monostable multivibrator 575. The
monostable multivibrator 575 is of the same construction as the
monostable multivibrator 270 and is connected by a conductor 576 to
a combination Schmidt trigger and AND circuit 580, of the
construction of the circuit 330, the output of which is connected
to a delay circuit 585 of the construction of the delay circuit
260, the output of the delay circuit 585 being on a conductor 586
connected to an output control circuit 590 having the same
construction and arrangement as the control circuits 370 described
above. The conductor 576 also connects via a conductor 535 as an
input to the filter 501, the conductor 541 is also connected as an
input to the AND circuit 580 and a HOLD ON circuit is provided
interconnecting the conductor 581 and a monostable multivibrator
575.
Referring now to FIG. 7 of the drawings, there are illustrated the
further details of the filters 401 and 501 and the inverter 460. As
illustrated, the filter 401 includes an inductor in the form of a
coil 402 having associated therewith a magnetic core 403, at least
a portion of the core 403 being movable and adjustable, whereby the
inductor 402 can be slug tuned. The inductor 402 has an input
terminal 404 that is connected by a conductor 409 to one terminal
of a capacitor 405, and through a capacitor 408 to the conductor
165, the other terminal of the capacitor 405 being grounded as at
N, the output from the filter 401 appearing on the conductor 409.
The inductor 402 has a plurality of taps thereon, and specifically
10 taps that are identified by the numerals 410 through 419.
Associated with selected ones of the taps are two transistors 420
and 430. More specifically, the transistor 420 has a base 421 that
is connected to one terminal of a resistor 426 by means of a
conductor 404, the other terminal of the resistor 426 being
connected to the conductor 425. The transistor 420 has a collector
422 that is connected to the tap 419 on the inductor 402, while the
emitter 423 is connected to ground potential.
The transistor 430 has a base 431 that is connected to one terminal
of a resistor 436, the other terminal of the resistor 436 being
connected to the conductor 435. The transistor 430 has a collector
432 that is connected to the tap 412 on the inductor 402, while the
emitter 433 is connected to ground potential.
The tapped filter 501 includes an inductor in the form of a coil
502 having associated therewith a magnetic core 503, at least a
portion of the core 503 being movable and adjustable whereby the
inductor 502 can be slug tuned. The inductor 502 has an input
terminal 504 that is connected by a conductor 509 to one terminal
of a capacitor 505 and through a capacitor 508 to the conductor
165, the other terminal of the capacitor 505 being grounded as at
N, the output from the filter 501 appearing on the conductor 509.
The inductor 502 has a plurality of taps thereon, and specifically
10 taps that are identified by the numerals 510 through 519.
Associated with the selected ones of the taps are two transistors
520 and 530. More specifically, the transistor 520 has a base 521
that is connected by a conductor 524 to one terminal of a resistor
526, the other terminal of the resistor 526 being connected to the
conductor 525. The transistor 520 has a collector 522 that is
connected to the tap 511 on the inductor 502, while the emitter 523
is connected to ground potential.
The transistor 530 has a base 531 that is connected to one terminal
of the resistor 536, the other terminal of the resistor 536 being
connected to the conductor 535. The transistor 530 has a collector
532 that is connected to the tap 518 on the inductor 502, while the
emitter 533 is connected to ground potential.
Associated with the tone control circuits is the inverter 460 that
is also illustrated in detail in FIG. 7, the inverter being in the
form of a transistor 470 having a base 471 connected through a
resistor 462 to the conductor 435. The transistor 470 further has
its emitter 473 grounded and the collector 472 is connected to the
conductor 425 and through a resistor 461 to the +DC conductor
155.
Considering now the operation of the receiver 400, in order to
obtain an output from the loudspeaker 125, a four tone sequence of
the proper selected control tones must be applied to the receiver
400, each tone preferably having a time duration of at least about
40 milliseconds and adjacent tones having substantially no gaps
therebetween. Furthermore, the first and third tones received must
be those to which the filter 401 is tuned when the transistors 420
and 430, respectively, are conducting, and the second and fourth
tones received must be those to which the filter 501 is tuned when
the transistors 520 and 530, respectively, are conducting. Upon the
reception of the first tone, an audio signal corresponding to the
first tone is applied along the conductors 123--124 as an input to
the limiter 150, and there appears on the conductor 165 a series of
square waveforms. The transistor 470 in the inverter 460 is off at
this time so as to allow the transistor 420 to be conducting and
thus to tune the filter 401 for reception of the first control
tone. Accordingly, the output on the conductor 165 is developed
across the resonance circuit and the output of filter 401 as a
sinusoidal waveform appears on the conductor 409. The sinusoidal
waveform on the conductor 409 is rectified by the rectifier 440 and
is applied as one of the positive inputs to the AND CIRCUIT 445,
the other required positive input being applied along the conductor
425.
The output of the AND circuit 445 is a positive voltage which
triggers the Schmidt trigger 450, thus to cause a negative going
voltage to be applied along the conductor 451 to the delay circuit
452 which after a predetermined time interval cause a trigger pulse
to be applied on removal of tone to the monostable multivibrator
455. Upon the removal of the first tone from the receiver 400, a
positive output is derived from the multivibrator 455 that is
applied along the conductor 525 to select the second tone in the
filter 501 by rendering the transistor 520 conductive in the filter
501 and also to provide one of the necessary positive inputs to the
AND circuit 545.
Assuming that the second control tone in the sequence is now
received, the filter 501 is resonant to the input frequency and
thereby develops a sinusoidal waveform at conductor 509 of the
second control tone frequency thereon which is applied to the
rectifier 540. The output from the rectifier 540 is a positive
voltage that provides the second required positive input to the AND
circuit 545, thereby to cause an output therefrom that is applied
to the Schmidt trigger circuit 550. The output from the AND circuit
545 is fed back along the conductor 546 to the multivibrator 455 to
hold it in the active condition for the duration of the second
tone, regardless of the time duration of the second tone. The
output from the AND circuit 545 also trips the Schmidt trigger
circuit 550 and the output is applied to the delay circuit 552,
whereby if the output from the Schmidt trigger circuit persists for
the predetermined delay period, the multivibrator 555 will provide
an output on the interruption of the second control tone. The
output of the multivibrator 555 is applied along the conductor 435
to select the proper tone No. 3 in the filter 401 by rendering the
transistor 430 conductive, and the output for the multivibrator 555
is also applied along the conductor 435 as a positive input to the
AND circuit 560 and to the inverter 460. The inverter 460 now
conducts so as to cause the transistor 420 to cease conduction and
thus effectively to remove the connection thereof in the filter
401, and at the same time positive voltage is removed from the
conductor 425 to the AND circuit 445 preventing it from
operating.
If the proper third control tone is now received on the conductor
165, the filter 401 will pass the third control tone to the
rectifier 440 which will now apply the third control tone as a
positive input to the AND circuit 445 (there being no output from
circuit 445 since there is now no input on conductor 425) and also
to provide a second input to the AND circuit 560, whereby a
potential is applied along the conductor 561 to the multivibrator
555 to hold it in the active condition so long as the third tone is
received.
Output of the AND circuit 560 is also applied to the Schmidt
trigger circuit 565, the output of which is supplied along the
conductor 566 through the delay circuit 570 and the conductor 571
to the multivibrator 575. Provided that the third tone persists for
a time interval, determined by the delay circuit 570, upon the
release or removal of the third tone persists for a time interval,
determined by the delay circuit 570, upon the release or removal of
the third tone, the multivibrator 575 causes an output pulse to
appear on the output conductor 576 which is fed back to select the
proper filter for tone No. 4 along the conductor 535, and this
signal is also applied as one of the positive inputs to the AND
circuit 580.
Assuming that the proper fourth tone in the sequence of tones is
now received, there will be an output from the filter 501 and the
rectifier 540 which is applied along the conductor 541 as a second
positive input to the AND circuit 580, thereby to provide an output
therefrom that is fed via the conductor 581, the delay circuit 585
and the conductor 586 to the output control circuits 590, whereby
the circuits 590 are energized if the time duration of the fourth
tone is longer than the delay provided by the delay circuit 585. It
is noted that a HOLD ON potential is applied from the output of the
AND circuit of the conductor 580 along the conductor 581 to the
multivibrator 575 to hold it in the active condition so long as the
fourth tone is received. The output signal on the conductor 586 is
operative to cause operation of the circuits 590 in the same manner
as the control circuits 370 described above, thereby to energize
the relay 130 and to connect the loudspeaker 125 to the output of
the transformer 120.
It is further pointed out that in the receiver 100 of FIG. 2 a
fixed bias has been applied to the first Schmidt trigger circuit,
which circuit is so designed that a full output is obtained
therefrom whenever the fixed voltage bias is exceeded. If the fixed
voltage bias is not exceeded, the circuit is completely
inoperative. The limiter 150 provides a square wave as an input to
the filters, which square wave as an input to the filters, which
square wave is of constant amplitude, whereby there is a constant
output voltage from the filters for any given tone frequency. These
features provide a constant frequency bandwidth for the limited
signal, i.e., the upper and lower frequency that will trip the
Schmidt trigger circuit is predetermined by the interrelationship
between the filter curve, the limiting level, and the DC bias
level. As explained above, the limiting or bias level can be varied
to alter the response bandwidth. On all other Schmidt triggers the
bias is supplied by the monostable multivibrator feeding its AND
input when it is triggered to the "on" condition. These are each
shown with a potentiometer similar to 295 in FIG. 4.
Referring now to FIGS. 8 and 9 of the drawings, there is
illustrated a third embodiment of the present invention wherein a
sequence of 7 tones is required to actuate the receiver, the
receiver being designated by the numeral 600 and being illustrated
in schematic and block diagram form in FIG. 8. The receiver 600
utilizes the input circuits and components from the antenna 101
through the limiter 150 and onto the conductor 165 from the
receiver 100, whereby the same reference numerals have been applied
to these components and the description thereof will not be here
repeated in the interest of brevity.
The output of the limiter 150 on the conductor 165 is fed to
two-tone control circuits, the lower tone control circuit including
a special tapped filter 601 identified by the legend FILTER -1 of a
construction to be described hereinafter, the input to the filter
601 being a series of square waves and the output thereof appearing
on a conductor 609 in the form of a sinusoidal wave having the
frequency of the selected control tone. The conductor 609 connects
to a rectifier 670 which serves to rectify the input thereto and
which has the same construction as the rectifier 220 above. The
output from the rectifier 670 is a positive voltage that is
supplied along a conductor 671 as one of the inputs to an AND
circuit 672. The output from the AND circuit 672 appears on a
conductor 673 that is connected to the input of a Schmidt trigger
circuit 675, the AND circuit 672 and the Schmidt trigger circuit
675 together having the construction of the Schmidt trigger circuit
330 described above. The output from the Schmidt trigger circuit
675 is connected by a conductor 676 to a delay circuit 680 of the
same construction as the delay circuit 260, and the output from the
delay circuit 680 is connected by a conductor 681 as one input to a
monostable multivibrator 685 of the construction described above
with respect to the monostable multivibrator 270. The output from
the multivibrator 675 appears on a conductor 725 that connects into
the upper tone control channel.
The upper tone control channel includes a special tapped filter 701
identified as FILTER -2, also to be described more fully
hereinafter; one of the inputs to the filter being from the
conductor 165 and other inputs being from the conductors 725, 735
and 745, to be described more fully hereinafter. The output from
the filter 701 appears on the conductor 709 connected to a
rectifier 770 of the same construction as the rectifier 320
described above. The output from the rectifier 770 is on a
conductor 771 that supplies one input to an AND circuit 772, the
output of which appears on a conductor 773 connected as an input to
a Schmidt trigger circuit 775; the AND circuit 772 and the Schmidt
trigger circuit 775 together having the construction of the Schmidt
trigger circuit 330 described above. The output from the Schmidt
trigger circuit 775 appears on a conductor 776 and is supplied as
an input to a delay circuit 780 having the construction of the
delay circuit 260 described above. The output from the delay
circuit 780 appears on a conductor 781 and is an input to a
monostable multivibrator 775 having a construction like the
monostable vibrator 270.
The output from the monostable multivibrator 785 appears on a
conductor 635 and is applied as an input to the filter 601 and as
an input to a NOR circuit 800. Other inputs to the NOR circuit 800
are applied by the conductors 645 and 655, while the output from
the NOR circuit 800 appears on the conductor 625.
The outputs on the conductors 635 and 671 are applied as inputs to
the first of four tone selecting and responding circuit 870, each
of which includes an AND circuit 872, a Schmidt trigger circuit
875, a delay circuit 880 and a monostable multivibrator 885, the
AND circuit 872 in combination with the Schmidt trigger circuit 875
having the construction of the Schmidt trigger 330, the delay
circuit 880 having the construction of the delay circuit 260 and
the monostable multivibrator 885 having the construction of the
multivibrator 270, all described heretofore.
More specifically, the inputs on the conductors 635 and 671 are
applied as inputs to such a tone selecting and responding circuit
870A including an AND circuit 872A. The output from the AND circuit
872A appears on a conductor 873A which is connected to the input of
a Schmidt trigger circuit 875A, and also as an input to the
monostable multivibrator 785. The output from the Schmidt trigger
circuit 875A is connected by a conductor 876A to a delay circuit
880A, the output of which is applied along a conductor 881A as an
input to a monostable multivibrator 885A. The output of the
monostable multivibrator 885A, which is also the output of the tone
selecting and responding circuit 870A, is applied along the
conductor 735 as a tone-selecting signal to the filter 701 and as
an input to the next tone selecting and responding circuit
870B.
The tone selecting and responding circuit 870B has the same
essential construction and arrangement as the tone selecting and
responding circuit 870A, and therefore in the interest of brevity
will not here be repeated, it being pointed out that one of the
outputs therefrom appears on a conductor 873B which is a HOLD ON
circuit for the multivibrator 885A, and the output from the circuit
870B appears on the conductor 645 and is applied as a
tone-selecting signal to the filter 601 and as the input to the
next tone selecting and responding circuit 870C.
The tone selecting and responding circuit 870C likewise has the
same essential construction and arrangement as the circuit 870A,
and therefore will not be described in detail, except to point out
that one of the outputs therefrom is along the conductor 873C to
the multivibrator 885B to serve as a HOLD ON signal therefor, and
the output from the circuit 870C appears on the conductor 745 and
is applied as the signal to the filter 701 to select control tone
No. 6.
The output from the circuit 870C is applied also as an input to the
circuit 870D which has the same essential construction and
arrangement as the circuit 870A, it merely being pointed out that
one of the outputs thereof is a HOLD ON signal appearing on the
conductor 873D which is applied to the multivibrator 885C, the
output of the circuit 870D appearing on the conductor 655 and being
applied as a signal to the filter 601 to select the control tone
No. 7.
The output of the circuit 870D is also applied as one of inputs to
a combination Schmidt trigger and AND circuit 980 of the
construction of the circuit 330 described above, the signal on the
conductor 671 also being applied thereto. The output from the
circuit 980 is applied along a conductor 981 to a delay circuit 985
of the same construction as the delay circuit 260, the output of
the circuit 980 also being applied to the multivibrator 885D as a
HOLD ON signal therefor. The output of the delay circuit 985 is
applied along a conductor 986 to an output control circuit 990
having a same construction and arrangement as the control circuits
370 described above.
Referring now to FIG. 9 of the drawings, there are illustrated the
further details of the filter 601 and 701 and of the NOR circuit
800. As illustrated, the filter 601 includes an inductor in the
form of a coil 602 having associated therewith a magnetic core 603,
at least a portion of the core 603 being movable and adjustable,
whereby the inductor 602 can be slug tuned. The inductor 602 has an
input terminal 604 that is connected by a conductor 609 to an
output terminal 607 and through a capacitor 608 to the conductor
165, the other terminal of the capacitor 605 being grounded as at
N, the output from the filter 601 appearing on the conductor 609.
The inductor 602 has a plurality of taps thereon, and specifically
10 taps that are identified by the numerals 610 through 619, each
of the taps as illustrated being connected by a suitable conductor
to a male terminous in a plug 660. Associated with selected ones of
the taps are four transistors 620, 630, 640 and 650. More
specifically, the transistor 620 has a base 621 that is connected
to one terminal of the resistor 626 by means of a conductor 624,
the other terminal of the resistor 626 being connected to the
conductor 625. The transistor 620 has a collector 622 which is
connected by a conductor 627 to a male terminous in a connector
669. The emitter 623 is connected to ground potential.
The transistors 630, 640 and 650 each have the same construction
and arrangement as the transistor 620 whereby the various parts
thereof and the circuit elements connected thereto have had applied
thereto like reference numerals in the corresponding series of
numerals. In order to connect the several collectors to selected
ones of the taps on the inductor 602, a connector 665 has been
provided having selected connections 666, 667 and 668 which
selectively interconnect certain of the male termini on the
connector 660 to selected male termini on the connector 669. As
illustrated, the conductor 666 connects the collector 632 to the
tap 610; the conductor 667 connects the connector 662 to the tap
616; and the conductor 668 connects both the collector 622 and the
collector 652 to the same tap 616.
The tapped filter 701 includes an inductor in the form of a coil
702 having associated therewith a magnetic core 703, at least a
portion of the core 703 being movable and adjustable, whereby the
conductor 702 can be slug tuned. The conductor 702 has an input
terminal 704 that is connected by a conductor 709 to one terminal
of a capacitor 705 and through a capacitor 708 to the conductor
165, the other terminal of the capacitor 705 being grounded as at
N, the output from the filter 701 appearing on the conductor 709.
The inductor 702 has a plurality of taps thereon, and specifically
10 taps that are identified by the numerals 710 through 719, each
of the taps being connected to a male terminous in a connector 750.
Associated with selected ones of the taps are three transistors
720, 730 and 740. The transistor 720 has a base 721 that is
connected by a conductor 724 to one terminal of a resistor 726, the
other terminal of the resistor 726 being connected to the conductor
725. The transistor 720 has a collector 722 that is connected by a
conductor 727 to a male terminous in a connector 759 and has an
emitter 723 connected to ground potential.
The transistors 730 and 740 have the same construction and
essentially the same circuit connections as the transistor 720, and
accordingly, in the interest of brevity, like reference numerals
have been applied to like parts thereof in the appropriate number
series. It is pointed out that each of the collectors terminates in
a male terminous in the connector 759. In order to connect the
several collectors to the required tap on the inductor 702, a
connector 755 has been provided having conductors 756, 757 and 758
therein arranged to connect selected ones of the male termini in
the connector 750 and 759. More specifically, the connector 756
connects the collector 732 to the tap 711; the conductor 757
connects the collector 740 to the tap 715; and the conductor 758
connects the collector 722 to the tap 719.
From the above, it will be seen that by suitable selection of
connections within the conductors 665 and 755, any one of the taps
on the inductors 602 and 702, respectively, can be connected to any
one of the tone-selecting transistors associated in the respective
filters 601 or 701. In other words, selection of the desired tap is
made by a simple wire connection, whereby the selection of the
desired tone frequency in a tone sequence is effected entirely by
the conductors or jumper wires 666, 667, etc. which connect the two
connectors 660--669 and 750--759, respectively. Thus the provision
of two tapped coils and the connectors suitably wired as explained
above will permit the selection of any one of 20 million tone
combinations in a seven-digit tone control circuit of the type
illustrated by the receiver 600. It will be appreciated that the
connection arrangement may be provided in the tapped filters in the
receivers 100 and 400, thereby to permit like simple selection of
the desired code by means of replacing only the connector such as
the connectors 665 and 755 described above.
Associated with the tone control circuits, and useful in selecting
the proper tone to be passed by the filters 601 and 701, is the NOR
circuit 800, the NOR circuit as illustrated including three
transistors 810, 820 and 830. The transistor 810 has a base 811
that is connected by a conductor 814 to one terminal of a resistor
815, the other terminal of the resistor 815 being connected to the
conductor 635 which is the third tone selector conductor. The
transistor 810 also has a collector 812 connected to the conductor
625 which is the first tone selector conductor, and has an emitter
813 that is grounded as at N. The transistor 820 has a base 821
connected by a conductor 824 to one terminal of the resistor 825,
the other terminal of the resistor 825 being connected to the
resistor 815 which is the fifth tone selector conductor. The
transistor 820 also has a collector 822 connected to the conductor
625 and an emitter 823 connected to ground potential. The
transistor 830 has a base 831 connected by a conductor 834 to one
terminal of a resistor 835, the other terminal of the resistor 835
being connected to the resistor 655 which is the seventh tone
selector conductor. The transistor 830 also has a collector 822
connected to the conductor 625 and an emitter 833 connected to
ground potential. It also is pointed out that the conductor 625 is
connected by a resistor 836 to the +12-volt DC conductor 155.
Considering now the operation of the receiver 600, in order to
obtain an output from the loud speaker 125, a seven-tone sequence
of the proper selected control tones must be applied to the
receiver 600, each tone preferably having a time duration of at
least about 25 milliseconds and adjacent tones having substantially
no gaps therebetween. Furthermore, the first and third and fifth
and seventh tones received must be those to which the filter 601 is
tuned when the transistors 620, 630, 640 and 650, respectively, are
conducting; and the second and fourth and sixth tones received must
be those to which the filter 701 is tuned when the transistors 720,
730 and 740, respectively, are conducting. Upon the reception of
the first tone, an audio signal corresponding to the first tone is
applied along the conductors 123--124 as an input to the limiter
150, and there appears on the conductor 165 a series of square
waveforms. The transistors in the NOR circuit 800 are all
nonconducting, whereby a high potential is applied from the
conductor 155 via the resistor 836 and the conductor 625 to the
base 621 of the transistor 620 to the base 621 of the transistor
620. As a result, the transistor 620 is conducting thereby to
connect the associated tap 619 as the effective tap on the inductor
602 in the filter 601. Assuming that the first control tone is of
the proper frequency as selected by the transistor 620, an output
is developed on the conductor 620, an output is developed on the
conductor 609 in a form of sinusoidal wave of the frequency of the
first control tone. The sinusoidal wave on the conductor 609 is
rectified by the rectifier 670 and is applied as one of the
positive inputs to the AND circuit 672, the other required positive
input being applied from the conductor 625 as explained above.
The output of the AND circuit 672 is a positive voltage which
triggers the Schmidt trigger circuit 675, thus to cause a negative
going voltage to be applied along the conductor 676 to the delay
circuit 680, which after a predetermined time interval is in
condition such that the cessation of the first control tone causes
a trigger pulse to be applied to the monostable multivibrator 685.
Upon the removal or cessation of the first tone, positive output is
thus derived from the multivibrator 685 that is applied along the
conductor 725 to select the second tone in the filter 701 by
rendering the transistor 720 conductive, and also to provide one of
the necessary positive inputs to the AND circuit 772.
Assuming that the second control tone in the sequence is now
received and the filter 701 is resonant thereat, there is developed
a sinusoidal waveform on the conductor 709 having a frequency
corresponding to that of the second control tone, which output is
then applied to the rectifier 770. The output from the rectifier
770 is a positive voltage that is applied along the conductor 771
as the second required input to the AND circuit 772, thereby to
provide an output from the AND circuit 772. The output of the AND
circuit 772 is applied along the conductor 773 to the multivibrator
685 to hold it in the active condition for the duration of the
second tone, regardless of the time duration of the second tone,
and the output is also applied to the Schmidt trigger circuit 775
to cause an output therefrom to appear on the conductor 776 that is
applied to the delay circuit 780. Assuming that the second control
tone persists for a time that exceeds the delay time of the circuit
780, then upon cessation of the second control tone, the
multivibrator 785 is triggered to provide an output on the
conductor 635. The positive going output on the conductor 635 is
applied to the base of the transistor 630 in the filter 601, thus
to select the filtering frequency for the third control tone, and
the output is also applied as one of the two required inputs to the
AND circuit 872A in the tone selecting and responding circuit 870A.
Furthermore, the output from the multivibrator 785 is applied to
the NOR circuit 800, and specifically to the base of the transistor
810 therein, thus to cause the heavy conduction thereof and to drop
the potential on the conductor 625, thereby effectively to remove
the transistor 620 by causing the transistor to become
nonconductive, thus leaving only the transistor 630 and that
portion of the inductor 602 associated therewith as active elements
in the filter 601.
The circuit is now in condition for the reception of the third
control tone, which if applied at this time and at the proper
frequency causes a sinusoidal output of a corresponding frequency
on the conductor 609. The sinusoidal wave for the third tone on the
conductor 609 is supplied to the rectifier 670 where it is
rectified to provide a positive going voltage on the conductor 671.
Although the positive going voltage on the conductor 671 is applied
to the AND circuit 672, there is no output from the ANd circuit 672
at this time because the second required positive potential thereto
has been removed from the conductor 625 by the operation of the NOR
circuit as described above. The positive going voltage on the
conductor 671 is however applied as an input to the AND circuit
872A, thus to provide a second positive input thereto which causes
an output therefrom to appear on the conductor 873A. The output on
the conductor 873A is applied to the multivibrator 785 as a HOLD ON
potential to hold the multivibrator 785 active so long as the third
control tone persists. The output on the conductor 873A is also
applied to the Schmidt trigger 875A which applies the output
thereof to the delay circuit 880A. Assuming that the third control
tone persists for a time interval greater than the time duration of
the delay from 880A, then upon cessation of the third control tone,
the multivibrator 885A is tripped to provide an output on the
conductor 735A. The output on the conductor 735A is a positive
going voltage that is applied as an input to the filter 701, and
specifically to the base of the transistor 730 to ready the filter
701 for reception of the fourth control tone. It is further noted
that the multivibrator 685 at this time has assumed its normal
condition, whereby to remove the positive potential from the base
of the transistor 720, thus to remove this connection to the
inductor 702, thus leaving the selection by the transistor 730 as
the only connection to the inductor 702.
The circuit is now in condition for reception of the fourth control
tone which is applied from the limiter 150 as a square wave on the
conductor 165 to the filter 701. Assuming that the fourth tone is
of the frequency selected by rendering the transistor 730
conducting, an output will be obtained on the conductor 709 that
will be a sinusoidal wave having the frequency of the fourth
control tone. This sinusoidal wave will be applied along the
conductor 709 to the rectifier 770 and the output therefrom will be
a positive going voltage applied to the conductor 771. Although
this positive going voltage is applied to the AND circuit 772,
there will be no output therefrom since the second required output
from the multivibrator 685 is now missing. The positive going
voltage on the conductor 771 will be applied as a second input to
the AND circuit 872B, the first positive input being from the
multivibrator 885A as described above. There now is an output from
the AND circuit 872B which appears on the conductor 873B, this
output being applied back to the multivibrator 885A as a HOLD ON
potential therefor in order to render this circuit active so long
as the fourth tone is being received. The conduction from the AND
circuit 872B also is applied to the Schmidt trigger circuit 875B,
the output of which is applied to the delay circuit 880B. If the
fourth tone persists for a time duration that exceeds the delay
time of the circuit 880B, then upon the cessation of the fourth
tone, the multivibrator 885B is tripped to provide a positive going
output therefrom on the conductor 645. The output on the conductor
645 is applied to the filter 601, and specifically to the base of
the transistor 640 to select tone No. 5, and is applied to the NOR
circuit 800, and specifically to the base of the transistor 820 to
cause conduction thereof so as to be sure that the transistor 620
is nonconducting, the transistor 630 being rendered nonconducting
by the return of the multivibrator 785 to the normal condition
thereof, whereby the only active connection in the filter 602 is
that provided by the transistor 640 which selects the frequency of
the fifth control tone. The output on the conductor 645 is also
applied as one of the positive inputs to the circuit 872C forming a
part of the tone selecting and responding circuit 870C.
Assuming that the fifth control tone of proper frequency is now
received, it will be applied along the conductor 165 to the filter
601 that will provide an output on the conductor 609 in the form of
a sinusoidal wave having a frequency of the fifth control tone.
This output is rectified by the rectifier 670 to provide a positive
voltage on the conductor 671, the AND circuits 672 and 872A not
responding at this time since neither has the second positive
voltage applied thereto. The positive going output is also applied
as the second required input to the AND circuit 872C so as to
provide an output therefrom on the conductor 873C. This output is
applied to the multivibrator 885B to hold it in the active
condition so long as the fifth tone is received, and is also
applied as the input to the Schmidt trigger circuit 875C, which
circuit operates to provide an input to the delay circuit 880C.
Assuming that the fifth tone persists for a time duration greater
than the time delay of the circuit 880C, then upon cessation of the
fifth control tone, the multivibrator 885C operates to provide a
positive going voltage on the output conductor 745. The positive
going voltage on the conductor 745 is applied to the filter 701,
and specifically, to the base of the transistor 740 to render the
transistor 740 conductive, and thus to select the frequency for the
sixth control tone. It is noted at this time that the multivibrator
885A has returned to its normal condition whereby to remove the
potential from the conductor 735 so as to render the transistor 730
nonconducting. The output on the conductor 745 is also applied as
one of the positive inputs to the AND circuit 872D forming a part
of the tone selecting and responding circuit 870D.
Assuming that the sixth control tone of the proper frequency is now
received, an output is applied along the conductor 165 to the
filter 701 and there is derived therefrom a sine wave on the
conductor 709 having a frequency corresponding to that of the sixth
control tone. This sine wave is rectified by the rectifier 770 and
applied to the conductor 771, the only AND circuit now in condition
to act therefrom being the AND circuit 872D that now provides an
output on the conductor 873D that is applied to the Schmidt trigger
circuit 875D, the output from the Schmidt trigger circuit 875D
being applied to the delay circuit 880D. It also is pointed out
that the output on the conductor 873D is applied as a HOLD ON
potential to the multivibrator 885C so that it is held in the
active condition so long as the sixth control tone is received.
Assuming that the sixth control tone persists for a time duration
that exceeds the time delay of the circuit 880D, upon cessation of
the sixth control tone the multivibrator 885D operates to provide a
positive voltage on the conductor 655. The positive voltage on the
conductor 655 is applied to the filter 601, specifically to the
base of the transistor 650 to render it conducting, and is also
applied to the NOR circuit 800 to ensure that the transistor 620 is
rendered nonconducting by causing the transistor 830 to conduct. As
a consequence, the only active connection in the filter 601 is that
provided by the conducting transistor 650. The positive going
signal on the conductor 655 is also applied as one of the positive
required inputs to the combined Schmidt trigger and the AND circuit
980.
Assuming that the seventh control tone of the proper frequency is
now received, an output is provided on the conductor 165 that is
applied to the filter 601. Since the transistor 650 is now
conducting to select the proper seventh control tone frequency, a
sine wave output is obtained on the conductor 609 having a
frequency corresponding to that of the seventh control tone. This
output is rectified by the rectifier 670 to provide a positive
voltage on the conductor 671. The only AND circuit now in condition
to be activated by the application of the positive voltage from the
conductor 671 is that in the combination Schmidt trigger and AND
circuit 690 which is now operated to provide an output therefrom on
the conductor 981. The output on the conductor 981 is applied as
the HOLD ON potential for the multivibrator 885D, thereby to hold
this circuit in the active condition so long as the seventh tone
persists. The output on the conductor 981 is also applied to the
delay circuit 985. If the seventh tone persists for a time interval
that exceeds the time delay of the circuit 985, the delay circuit
985 operates to provide an output signal on the conductor 986 which
will cause operation of the output control circuit 990. The output
signal on the conductor 986 is operative to cause operation of the
circuit 990 in the same manner as the control circuit 370 described
above, thereby to energize the relay 130 and to connect the loud
speaker 125 to the output of the transformer 120.
From the above, it will be seen that there has been provided an
improved selective calling communication system and improved
components therefor which will fulfill all of the objects and
advantages set forth above.
Although there have been illustrated and described certain
preferred embodiments of the invention, it is to be understood that
various changes and modifications can be made therein without
departing from the spirit and scope of the invention, and it is
intended that all such changes and modifications be covered as fall
within the scope of the appended claims.
* * * * *