U.S. patent number 3,783,384 [Application Number 05/222,146] was granted by the patent office on 1974-01-01 for high speed selective calling communication system having low drain receiver.
Invention is credited to Keith H. Wycoff.
United States Patent |
3,783,384 |
Wycoff |
January 1, 1974 |
**Please see images for:
( Certificate of Correction ) ** |
HIGH SPEED SELECTIVE CALLING COMMUNICATION SYSTEM HAVING LOW DRAIN
RECEIVER
Abstract
The system includes a transmitter which sequentially transmits a
multiplicity of code signals each having a cue tone and a
predetermined number of control tones, the code signals being
arranged into a plurality of signal groups for sequential
transmission thereof, the number of signal groups corresponding to
the number of cue tone frequencies, each signal group containing
all those code signals which include the corresponding cue tone of
that group. The system also comprises at least one receiver
including a pulser circuit for producing a series of pulses
intermittently to render a signal processing circuit operative, a
decoder responsive to a predetermined cue tone for generating a
first control signal and responsive to a predetermined code signal
for generating a second control signal, the pulser circuit being
responsive to the first control signal to furnish a continuous
supply voltage to the processing circuit, a utilization circuit
responsive to the second control signal to cause an annunciator to
provide an alerting signal.
Inventors: |
Wycoff; Keith H. (Lexington,
NB) |
Family
ID: |
26916486 |
Appl.
No.: |
05/222,146 |
Filed: |
January 31, 1972 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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861719 |
Sep 29, 1969 |
3651413 |
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Current U.S.
Class: |
340/7.28;
340/309.16; 340/7.49 |
Current CPC
Class: |
H04W
88/188 (20130101); H04W 88/027 (20130101) |
Current International
Class: |
H04Q
7/18 (20060101); H04Q 7/10 (20060101); H04Q
7/06 (20060101); H04b 001/00 () |
Field of
Search: |
;325/55,64,391,392,395,478,492 ;340/311,309.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Mayer; Albert J.
Attorney, Agent or Firm: Vogel; David A.
Parent Case Text
This application is a continuation in part of the co-pending
application of Keith H. Wycoff, for COMMUNICATION RECEIVER
INCORPORATING TONE OPERATED PULSER CIRCUIT AND ELECTRONIC SWITCH,
Serial No. 861,719, filed Sept. 29, 1969, now U.S. Pat. No.
3,651,413.
Claims
What is claimed is:
1. A selective calling communication system comprising a
transmitter including means for generating a multiplicity of code
signals each having a cue tone and at least one control tone, means
for arranging the code signals into a plurality of signal groups
for sequential transmission thereof, the number of signal groups
corresponding to the number of cue tone frequencies, each signal
group containing all these code signals which include the
corresponding cue tone of that group, and means for sequentially
transmitting the code signals in accordance with the arrangement
thereof into said signal groups; and a receiver including a
processing circuit for processing the code signals, pulser circuit
means coupled to said processing circuit for producing a series of
pulses intermittently to render said processing circuit operative,
decoding means coupled to said processing circuit and having first
and second outputs, said decoding means being responsive to a
predetermined cue tone for generating at said first output a first
control signal and responsive to a predetermined code signal for
generating at said second output a second control signal, said
pulser circuit means having an input coupled to said first output
and responsive to the application thereto of said first control
signal to furnish a continuous supply voltage to said processing
circuit, a utilization circuit having an input coupled to said
second output and responsive to the second control signal to
provide an output signal, and an annunciator coupled to said
utilization circuit for converting the output signal into usable
form.
2. The selective calling communication system set forth in claim 1,
wherein each code signal includes a plurality of control tones.
3. The selective calling communication system set forth in claim 1,
wherein the cue tone of each code signal is transmitted prior to
the control tone thereof.
4. The selective calling communication system set forth in claim 1,
wherein each code signal includes a plurality of control tones
sequentially transmitted.
5. The selective calling communication system set forth in claim 1,
wherein the cue tone of the first code signal of each signal group
is substantially longer in duration than the cue tone of the rest
of the code signals of each signal group.
6. A selective calling communication system comprising a
transmitter including means for generating a multiplicity of code
signals each having a cue tone and a sequence of a predetermined
number of control tones, means for arranging the code signals into
a plurality of signal groups for sequential transmission thereof,
the number of signal groups corresponding to the number of cue tone
frequencies, each signal group containing all these code signals
which include the corresponding cue tone of that group, the
commencement of one cue tone being separated from the termination
of the preceding cue tone by a predetermined time interval, the cue
tone of the first code signal in each group having a predetermined
duration that is substantially greater than the duration of the cue
tone of the rest of the code signals in each group, and means for
sequentially transmitting the code signals in accordance with the
arrangement thereof into said signal groups; and a receiver
including a processing circuit for processing the code signals,
pulser circuit means coupled to said processing circuit for
producing a series of pulses intermittently to render said
processing circuit operative, said pulses being spaced from each
other by an amount no greater than said predetermined duration,
decoding means coupled to said processing circuit and having first
and second outputs, said decoding means being responsive to a
predetermined cue tone for generating at said first output a first
control signal and responsive to a predetermined code signal for
generating at said second output a second control signal, timing
means coupled to said first output for providing an extended
control signal persisting beyond termination of said predetermined
cue tone for at least said predetermined time interval, said pulser
circuit means having an input coupled to the output of said timing
means and responsive to the application thereto of said extended
control signal to furnish a continuous supply voltage to said
processing circuit at least until termination of said predetermined
time interval, a utilization circuit having an input coupled to
said second output and responsive to the second control signal to
provide an output signal, and an annunciator coupled to said
utilization circuit for converting the output signal into usable
form.
7. A selective calling communication system comprising a
transmitter including means for generating a multiplicity of code
signals each having a cue tone and a sequence of a predetermined
number of control tones, means for arranging the code signals into
a plurality of signal groups for sequential transmission thereof,
the number of signal groups corresponding to the number of cue tone
frequencies, each signal group containing all these code signals
which include the corresponding cue tone of that group, the
commencement of one cue tone being separated from the termination
of the preceding cue tone by a predetermined time interval, the cue
tone of the first code signal in each group having a predetermined
duration that is substantially greater than the duration of the cue
tone of the rest of the code signals in each group, and means for
sequentially transmitting the code signals in accordance with the
arrangement thereof into said signal groups; and a receiver
including a processing circuit for processing the code signals,
pulser circuit means coupled to said processing circuit for
producing a series of pulses intermittently to render said
processing circuit operative, said pulses being spaced from each
other by an amount no greater than said predetermined duration,
decoding means coupled to said processing circuit and having first
and second outputs, said decoding means being responsive to a
predetermined cue tone for generating at said first output a first
control signal and responsive to a predetermined code signal for
generating at said second output a second control signal, timing
means coupled to said first output for providing an extended
control signal persisting beyond termination of said predetermined
cue tone for at least said predetermined time interval, said pulser
circuit means having an input coupled to the output of said timing
means and responsive to the application thereto of said extended
control signal to furnish a continuous supply voltage to said
processing circuit at least until termination of said predetermined
time interval, electronic switching means coupled to said second
output and responsive to said second control signal to provide an
enabling signal, a utilization circuit having an input coupled to
said switching means and responsive to the enabling signal to
provide an output signal, and an annunciator coupled to said
utilization circuit for converting the output signal into usable
form.
8. The selective calling communication system set forth in claim 7,
wherein the predetermined duration of the first cue tone in each
signal group is at least 30 times the duration of the rest of the
cue tones in each signal group.
9. The selective calling communication system set forth in claim 7,
wherein the predetermined duration of the first cue tone of the
first code signal in each signal group is at least 30 times the
duration of the rest of the control tones of the first code signal
in each signal group.
10. The selective calling communication system set forth in claim
7, wherein the cue tones and the control tones in the second code
signal and in all succeeding code signals in each group are of the
same order of magnitude.
11. The selective calling communication system set forth in claim
7, wherein the predetermined duration of the first cue tone in each
signal group is at least one second in duration.
12. The selective calling communication system set forth in claim
7, wherein the second tone in each signal group and each succeeding
tone therein have durations no greater than about 50
milliseconds.
13. The selective calling communication system set forth in claim
7, wherein the second tone in each signal group and each succeeding
tone therein and the gap between succeeding code signals have
durations no greater than 50 milliseconds.
14. The selective calling communication system set forth in claim
7, wherein the duty cycle of said pulser circuit means in the
absence of said control signals is no greater than about 5
percent.
15. A selective calling communication system comprising a
transmitter including means for sequentially generating a
multiplicity of code signals each having a cue tone and a sequence
of a predetermined number of control tones, and means for arranging
the code signals into a plurality of signal groups for sequential
transmission thereof, the number of signal groups corresponding to
the number of cue tone frequencies, each signal group containing
all those code signals which include the corresponding cue tone of
that group, the commencement of one cue tone being separated from
the termination of the preceding cue tone by a predetermined time
interval, the cue tone of the first code signal in each group
having a predetermined duration that is substantially greater than
the duration of the cue tone of the rest of the code signals in
each group, and means for modulating the code signals onto a
carrier wave; and a receiver including a processing circuit for
receiving the modulated carrier wave and detecting the code signals
therein, pulser circuit means coupled to said processing circuit
for producing a series of pulses intermittently to render said
processing circuit operative, said pulses being spaced from each
other by an amount no greater than said predetermined duration,
decoding means coupled to said processing circuit and having first
and second outputs, said decoding means being responsive to a
predetermined cue tone for generating at said first output a first
control signal and responsive to a predetermined code signal for
generating at said second output a second control signal, timing
means coupled to said first output for providing an extended
control signal persisting beyond termination of said predetermined
cue tone for at least said predetermined time interval, said pulser
circuit means having an input coupled to the output of said timing
means and responsive to the application thereto of said extended
control signal to furnish a continuous supply voltage to said
processing circuit at least until termination of said predetermined
time interval, a utilization circuit having an input coupled to
said second output and responsive to the second control signal to
provide an output signal, and an annunciator coupled to said
utilization circuit for converting the output signal into usable
form.
16. A selective calling communication transmitter comprising means
for generating a multiplicity of code signals each having a cue
tone and at least one control tone, means for arranging the code
signals into a plurality of signal groups for sequential
transmission thereof, the number of signal groups corresponding to
the number of cue tone frequencies, each signal group containing
all those code signals which include the corresponding cue tone of
that group, and means for sequentially transmitting the code
signals in accordance with the arrangement thereof into said signal
groups.
17. The selective calling communication system set forth in claim
16, wherein each code signal includes a plurality of control
tones.
18. The selective calling communication system set forth in claim
16, wherein the cue tone of each code signal is transmitted prior
to the control tone thereof.
19. The selective calling communication system set forth in claim
16, wherein each code signal includes a plurality of control tones
sequentially transmitted.
20. The selective calling communication system set forth in claim
16, wherein the cue tone of the first code signal of each signal
group is substantially longer in duration than the cue tone of the
rest of the code signals of each signal group.
21. A selective calling communication transmitter comprising means
for generating a multiplicity of code signals each having a cue
tone and a sequence of a predetermined number of control tones,
means for arranging the code signals into a plurality of signal
groups for sequential transmission thereof, the number of signal
groups corresponding to the number of cue tone frequencies, each
signal group containing all these code signals which include the
corresponding cue tone of that group, the commencement of one cue
tone being separated from the termination of the preceding cue tone
by a predetermined time interval, the cue tone of the first code
signal in each group having a predetermined duration that is
substantially greater than the duration of the cue tone of the rest
of the code signals in each group, and means for sequentially
transmitting the code signals in accordance with the arrangement
thereof into said signal groups.
22. The selective calling communication system set forth in claim
21, wherein the predetermined duration of the first cue tone in
each signal group is at least 30 times the duration of the rest of
the cue tones in each signal group.
23. The selective calling communication system set forth in claim
21, wherein the predetermined duration of the first cue tone of the
first code signal in each signal group is at least 30 times the
duration of the rest of the control tones of the first code signal
in each signal group.
24. The selective calling communication system set forth in claim
21, wherein the cue tones and the control tones in the second code
signal and in all succeeding code signals in each group are of the
same order of magnitude.
25. The selective calling communication system set forth in claim
21, wherein the predetermined duration of the first cue tone in
each signal group is at least one second in duration.
26. The selective calling communication system set forth in claim
21, wherein the second tone in each signal group and each
succeeding tone therein have durations no greater than about 50
milliseconds.
27. The selective calling communication system set forth in claim
21, wherein the second tone in each signal group and each
succeeding tone therein and the gap between succeeding code signals
have durations no greater than 50 milliseconds.
28. The selective calling communication system set forth in claim
21, wherein the duty cycle of said pulser circuit means in the
absence of said control signals is no greater than about 5
percent.
29. A selective calling communication generator comprising input
means for receiving a multiplicity of sets of coded instructions
respectively corresponding to a multiplicity of code signals each
having one cue tone and at least one control tone, means for
arranging the sets of coded instructions into a plurality of groups
respectively corresponding in number to the number of cue tone
frequencies, each group continuing all those sets of coded
instructions corresponding to the code signal which include the
corresponding cue tone of that group, oscillator means for
generating code signals in response to the sets of coded
instructions, means for sequentially applying to said oscillator
means all of the sets of coded instructions in one group followed
by successively releasing all of the sets of coded instructions in
each succeeding group, thereby sequentially to generate signal
groups respectively containing all those code signals which include
the corresponding cue tone of the associated signal group.
30. A selective calling communication receiver for receiving
carrier waves modulated by a code signal including a cue tone and
sequence of a predetermined number of control tones, said receiver
comprising a processing circuit for processing the modulated
carrier wave and detecting the code signal therein, pulser circuit
means coupled to said processing circuit for producing a series of
pulses intermittently to render said processing circuit operative,
decoding means coupled to said processing circuit and having first
and second outputs, said decoding means being responsive to a
predetermined cue tone for generating at said first output a first
control signal and responsive to a predetermined code signal for
generating at said second output a second control signal, timing
means coupled to said first output for providing an extended
control signal persisting beyond termination of said predetermined
tone for a predetermined time interval following termination of the
code signal, said pulser circuit means having an input coupled to
the output of said timing means and responsive to the application
thereto of said extended control signal to furnish a continuous
supply voltage to said processing circuit at least until
termination of said predetermined time interval, a utilization
circuit having an input coupled to said second output and
responsive to said second control signal to provide an output
signal, and an annunciator coupled to said utilization circuit for
converting the output signal into usable form.
Description
This invention relates to selective calling communications systems
in general and more particularly to a system capable of high-speed
code transmission utilizing low drain receivers.
It is an important object of the present invention to increase the
number of code signals which can be transmitted and received in a
selective calling communication system and at the same time
increase the useful lives of batteries used in receivers in that
system.
Another object is to provide a transmitter which is capable of
sending a multiplicity of code signals such that all code signals
having a first cue tone are sent sequentially followed by all those
code signals having a second cue tone, etc.
Still another object is to provide an improved selective calling
communication receiver which is intermittently energized by voltage
pulses.
In connection with the foregoing object, it is yet another object
to increase the space between adjacent pulses so as to improve the
life of the battery in that receiver.
A further object is to provide in a selective calling communication
system, a transmitter which is coupled to the telephone system and
is operative to store calls as they are received and to transmit
corresponding code signal seriatum.
A yet further object is to provide a transmitter capable of
substituting a predetermined code signal for a predetermined number
applied to the transmitter.
In summary, there is provided a selective calling communication
system comprising a transmitter including means for sequentially
transmitting a multiplicity of code signals each having a cue tone
and a predetermined number of control tones, and means for
arranging the code signals into a plurality of signal groups for
sequential transmission thereof the number of signal groups
corresponding to the number of cue tone frequencies, each signal
group containing all those code signals which include the
corresponding cue tone of that group, a receiver including a
processing circuit for processing the code signals, pulser circuit
means coupled to the processing circuit for producing a series of
pulses intermittently to render the processing circuit operative,
decoding means coupled to the processing circuit and having first
and second outputs, the decoding means being responsive to a
predetermined cue tone for generating at the first output a first
control signal and responsive to a predetermined code signal for
generating at the second output a second control signal, the pulser
circuit means having an input coupled to the first output and
responsive to the application thereto of the first control signal
to furnish a continuous supply voltage to the processing circuit, a
utilization circuit having an input coupled to the second output
and responsive to the second control signal to provide an output
signal, and an annunciator coupled to the utilization circuit for
converting the output signal into usable form.
In a preferred form, each code signal includes a cue tone followed
by a sequence of a predetermined number of control tones.
With the foregoing and other objects in view, which will appear as
the description proceeds, the invention consists of certain novel
features and a combination of parts hereinafter fully described,
illustrated in the accompanying drawings, and particularly pointed
out in the appended claims, it being understood that various
changes in the details of the circuitry may be made without
departing from the spirit or sacrificing any of the advantages of
the invention.
For the purpose of facilitating an understanding of the invention,
there is illustrated in the accompanying drawings a preferred
embodiment thereof, from an inspection of which, when considered in
connection with the following description, the invention, its mode
of construction, assembly and operation, and many of its advantages
should be readily understood and appreciated.
FIG. 1 illustrates a transmitter in block form, including therein
an encoder which incorporates the features of the present
invention;
FIG. 2A is a more detailed block diagram of a portion of the
encoder of FIG. 1;
FIG. 2B is a detailed block diagram of the remainder of the encoder
of FIG. 1;
FIG. 3 illustrates a circuit diagram of an oscillator which may be
used in the encoder;
FIG. 4 illustrates the sequence of code signals derived from the
encoder;
FIG. 5 is a block diagram of a receiver used in the selective
calling communications system incorporating the features of the
present invention;
FIG. 6 is a detailed block diagram of the decoder circuit of FIG.
5;
FIGS. 7, 8 and 9 are detailed circuit diagrams of certain portions
of the receiver of FIG. 5;
FIG. 10 is a graph depicting signals at various points in the
circuitry shown in FIGS. 8 and 9;
FIG. 11 is a graph depicting certain additional details of
operation of the receiver.
There is described herein a selective calling communication system
comprising a transmitter and a number of receivers. Each receiver
is responsive to the same carrier wave frequency but is responsive
to a predetermined set of tones. A given receiver will emit an
alerting signal such as light and/or sound in response to a carrier
wave modulated with a particular set of tones. The possessor of
such receiver will then perform some previously agreed action, such
as, calling his office. A different set of tones modulated on that
carrier wave will activate another receiver, etc.
The transmitter is coupled to the telephone system so that by
picking up ones telephone and dialing a predetermined number, the
caller can be connected into the transmitter. Thereafter, the
caller dials the number corresponding to the receiver with which he
wishes to get in touch. The transmitter will modulate the carrier
wave with the selected set of tones which will then activate the
receiver in question.
As different calls come in to the transmitter, the numbers are
processed to transmit the associated code signals, each consisting
of a cue tone and a set of control tones. The processing is so
effected that the first group of code signals transmitted includes
all those having a first cue tone, followed by all those having a
second cue tone, etc., until all the stored code signals have been
transmitted.
The receivers are intermittently energized so as to minimize
battery consumption. Once the proper cue tone is received in any
given receiver it is continuously energized until all the code
signals having that cue tone have been transmitted. That receiver
will be activated to alert the possessor thereof upon receipt of
the proper control tones.
Turning now to the drawings, and more particularly to FIG. 1
thereof, there is shown a selective communication transmitter 20
which is part of a communication system also including selective
calling receivers to be described hereinafter. The transmitter 20
includes an oscillator 21 which develops a relatively low frequency
oscillatory signal for application to one input of the modulator
22. The modulator 22 also has a second input coupled to an encoder
30 which provides a code signal in the form of a tone sequence for
modulation onto the oscillatory signal from the oscillator 21. A
plurality of telephone trunks 31 is coupled to the inputs of the
encoder 30. The modulated oscillatory signal is applied to a
frequency multiplier 25 which increases the frequency of the
modulated oscillatory signal. The relatively high frequency signal
from the frequency multiplier 25 is applied to a power output
amplifier 26 where the signal strength is increased to provide a
high level, frequency modulated carrier wave which is radiated by
an antenna 27.
Turning now to FIG. 2A, the details of the encoder 30 will be
described. Coupled to each of the telephone trunks 31 is a number
register 32. Each register 32 has one output conductor 33 and four
sets of output conductors 34a, 34b, 34c and 34d, each set having
four conductors. The output conductor 33 of each register 32 is
coupled back to the associated telephone trunk 31 and also is
coupled as a first input to an AND circuit 35. The input on each
telephone trunk 31 will consist of a four digit number such as
"8491," each digit corresponding to a predetermined tone which is
to be sent. Each register 32 converts each digit of the incoming
number into binary information. Thus, on the set of conductors 34a
a binary output will appear corresponding to the first digit, a
similar binary output will appear on the set of conductors 34b to
correspond to the second digit, etc. As is well known to those
skilled in the art, four bits in each set will be required to
provide a binary output in response to a decimal input. As soon as
a complete number has been applied to a register 32, it will
furnish on the associated output conductor 33 a signal which is
sent back to the associated telephone trunk 31 to prevent further
entry to that register 32. That signal also provides a first input
to the associated AND circuit 35.
As is usual in telephone systems, when the first trunk is in use,
the next incoming call automatically will be rerouted onto the
second telephone trunk 31 so as to be applied to the second
register 32. As soon as such number is completed, a signal will be
developed on the output conductor 33 of the second register 32 to
prevent further access thereto. If two trunks are in use, the next
call will automatically be rerouted to the third trunk 31 and to
the third register 32, which would cause a signal to be developed
on the associated output conductor 33 to prevent further access to
the third register 32. If only three trunks are coupled to the
encoder 30, then the next caller would receive a busy signal, until
one of the registers 32 again becomes available. Any number of
additional telephone trunks may be provided depending upon the
amount of use the encoder 30 will be put to.
Each register 32 is coupled to an associated readout circuit 36 by
way of the conductor sets 34a-d. The readout circuits 36
respectively have further inputs on conductors 37 respectively
derived from the AND circuits 35. Each readout circuit 36 also has
four sets of output conductors 38a, 38b, 38c and 38d which sets of
output conductors are shown schematically to merge into a cable
38.
The encoder 30 also includes a sequential switch 40 driven by a
clock 41. The clock 41 generates pulses at a predetermined rate
such as for example a pulse being produced at a one millisecond
rate. The sequential switch 40 has three output conductors 42
respectively coupled as second inputs to the AND circuits 35. It
should be understood that although three such conductors are shown,
the number will correspond to the number of telephone trunks 31
coupled to the encoder 30. The sequential switch 40, generates on
the conductors 42, one after another, pulses respectively initiated
from the pulses of the clock 41. Thus, when a complete number has
been furnished to the first register 32 so that one input along the
conductor 33 is applied to the first AND circuit 35 and a pulse
from the sequential switch 40 provides a second input to the first
AND circuit 35, a signal will be coupled to the first readout
circuit 36 which causes the four binary codes from the first
register 32 to be coupled to the conductors 38a-d. Thus if the
number "8491" is applied to the first register, the binary code
representing the digit 8 will be read out on the conductors 38a,
the binary code representing the digit 4 being read out on the
conductors 38b, etc. As soon as the signal in the first register
has been read out, the signal on the conductor 33 is terminated,
whereupon the first telephone truck 31 is available to receive
another number. The sequential switch 40 then provides a signal on
a conductor 42 to the second AND circuit 35. If at this time, the
second register 32 has a number stored therein so as to provide a
signal on the conductor 33 to the second AND circuit, a signal on
the conductor 37 will be generated to cause the second readout
circuit 36 to read out onto the conductors 38a-d, the signals
stored in the second register 32. Thereupon, the signal on the
associated conductor 33 will terminate to permit access to the
second register 32. The sequential switch 40 will then provide a
signal on a conductor 42 to cause the third register 32 to read out
the associated signals stored therein.
The encoder 30 is provided with a random access memory unit 45
which has four sets of input conductors 46a, 46b, 46c and 46d, each
of which sets has four conductors. The corresponding conductors
38a-d from the readout circuits 36 are connected together and to
the corresponding one of the conductors 46a-d. For example, the
first conductor in the set 46a is coupled to the first conductor of
each of the sets 38a. The memory unit 45 has 10 groups of output
conductor sets 47a, 47b, 47c and 47d, there being four sets of four
conductors each in each group.
The random access memory unit 45 uses any type of alterable memory
element, which preferably is non-volatile. The four parallel BCD
(binary code decimal) inputs select a location in the memory which
is then read out to one of ten digital storage buffers (silo
stores) as is subsequently described.
The encoder 30 also includes ten silo stores 48, each being the
same in construction and being capable of storing codes applied
thereto. These stores may be made of devices supplied by Texas
Instruments, Incorporated of Dallas, Texas, which it calls a
Digital Storage Buffer under the identification TNS 4006 JC, NC.
The first silo store 48 is coupled, by way of one of the groups of
output conductor sets 47a-d, to the memory unit 45; the second silo
store 48 is coupled, by way of the second group of output conductor
sets 47a-d, to the memory unit 45, and so forth, the tenth silo
store 48 being similarly coupled to the memory unit 45. A silo
store is a memory device wherein the first word stored in the
buffer is the first word released. Thus, if ten words are
sequentially entered into storage, readout signals coupled to the
silo store read out these words in the same order.
The four BCD inputs to the memory unit 45 select a location in the
memory and cause read out of the four digit number stored therein.
Those numbers in storage having a predetermined first digit are
routed to the first silo store 48; all those numbers having their
first digit as a predetermined second number being routed to the
second silo store 48, and so on, those numbers having their first
digit as a predetermined tenth number being routed to the tenth
silo store 48. For example, all numbers having a common first digit
of 2 (the digits applied to the memory unit 45 are not in decimal
form so that, the digit 2 would be represented by a binary code on
the conductors 46a) would be coupled to the second silo store 48.
Presumably, when a system is first shipped, all numbers in memory
locations would be the same numbers by which the locations are
identified.
There is provided a manual entry console 50 having a plurality of
entry buttons 51 and being coupled via a conductor 52 to an input
of the memory unit 45. The console 50 is operative selectively to
modify the individual elements of the memory unit 45 so that a
number out of the memory unit 45 does not necessarily match the
input number thereto. For example, the console 50 can modify the
memory unit 45 so that an input number of "8765" selecting location
8765 in the memory will read out an output number of "9413," for
example. This enables complete substitution of receivers. If a
receiver assigned the number "8765" becomes damaged, the user may
obtain any replacement receiver which happens to be available such
as the receiver responsive to the number "9413." The console 50 is
then manipulated to modify the memory unit 48 to cause an input
number of "8765" to yield an output number of "9413." Thus,
would-be callers need never know of the substitution and can simply
call the same number "8765" and still alert the user in question.
In changing the number for "8765" to "9413," a different silo store
will be selected for transmission of the paging codes.
Each silo store 48 has an output conductor 55 coupled thereto,
which conductors are schematically shown to merge into a cable 55'.
Input conductors 56 (schematically shown to merge into a cable 56')
are respectively coupled to the silo stores 48 and cause the codes
stored therein to be released upon receipt of a suitable signal.
Each silo store 48 has four output conductor sets 57a, 57b, 57c and
57d, there being four conductors in each set, thus providing for
parallel outputs of four BCD digits.
Turning now to FIG. 2B, there is provided a set of four
binary-to-decimal converters 60, respectively having sets of input
conductors 61a, 61b, 61c, and 61d coupled thereto. The first
conductors respectively in the conductor sets 57a from the silo
stores 48 are "OR" coupled together and to the first conductor in
the conductor set 61a; the second conductors in the conductor sets
57a are similarly connected together and to the second conductor in
the conductor set 61a, and so forth, the first conductors in the
conductor sets 57c being connected together and to the first
conductor in the conductor set 61c, and so on, the last conductors
in the conductor sets 57d being connected together and to the last
conductor in the conductor set 61d.
The converters 60 respectively have sets of output conductors 62a,
63b, 62c and 62d, each set containing 10 conductors. Each of the
converters 68 is capable of responding to a binary number applied
thereto and coupling a voltage to the corresponding one of the
output conductors 62a. Thus, for example, if the binary digits 0110
appeared respectively on the conductors in the conductor set 61a,
the first converter 60a would provide a voltage on, for example,
the sixth conductor of the conductor set 62a.
There is provided a first oscillator 65 having a set of ten input
conductors 64 connected respectively to the corresponding
conductors in the conductor sets 62a and 62c. A second oscillator
70 has a set of input conductors 69 connected respectively to the
corresponding conductors in the conductor sets 62b and 62d. The
conductor sets 64 and 69 are respectively coupled to different
frequency determining elements in the oscillators 65 and 68. Thus,
a voltage on the first conductor in the conductor set 64 causes the
first oscillator 65 to produce a first predetermined tone, a
voltage on the second conductor in the conductor set 64 will cause
the first oscillator 65 to produce a second predetermined tone,
etc. Similarly, voltages on different ones of the conductors in the
conductor set 69 cause the second oscillator 68 to produce
corresponding tones.
FIG. 3 illustrates the details of an exemplary oscillator 70 (the
oscillator 65 may be similarly constructed) which includes an
oscillatory portion 71, the oscillatory circuit 71 being of
standard construction and including an NPN transistor 72 having its
base coupled to a resistor 73 to ground reference potential and its
emitter coupled through a resistor 74 to ground reference
potential. A resistor 75 coupled from the B+ operating voltage to
the base of the transistor 72 provides a biasing voltage thereon. A
pair of capacitors 76 and 77 is coupled in series between the
collector of the transistor 72 and the B+ operating voltage. There
is provided a connection between the emitter of the transistor 72
and the junction of the capacitors 76 and 77. A coil 78 coupled to
the collector of the transistor 72 has 10 taps thereon. The
oscillator 70 also includes an amplifier 80 having an NPN
transistor 81 with its base coupled, by way of a capacitor 82, to
the emitter of the transistor 72, the emitter of the transistor 81
being coupled through a resistor 83 to ground reference potential.
A resistor 84 is coupled between the collector and the base of the
transistor 81.
In operation, the oscillatory portion 71 produces a tone the
frequency of which is determined by the values of the capacitors 76
and 77 and the portion of the coil 78 in circuit in the oscillatory
portion 71. Thus, if a voltage is applied to the third conductor
(by connecting it to B+) in the conductor set 69, approximately 30
percent of the coil 78 will be coupled in the oscillatory portion
71, thereby causing a given tone to be produced thereby.
Accordingly, the tone produced by the oscillators 65 and 70 will be
dependent upon which of the input conductors thereto receives an
energizing voltage.
The first oscillator 65 is coupled to a first gated amplifier 90
and the second oscillator 70 has its output coupled to a second
gated amplifier 91, the outputs of the amplifiers 90 and 91 being
coupled to a conductor 95. The conductor 95 is coupled to the input
of the modulator 22 (FIG. 1).
The encoder 30 also includes a sequential switch 100 driven by a
clock 101. The sequential switch 100 has four output conductors,
102a, 102b, 102c and 102d respectively coupled to further inputs of
the converters 60. The sequential switch 100 has a fifth output
conductor 102e. The sequential switch 100 generates a pulse on the
conductor 102a which pulse is initiated by the first pulse from the
clock 101. The sequential switch 100 then produces a pulse on the
conductor 102b; followed by a pulse on the conductor 102c and then
a pulse on the conductor 102d and finally, a pulse on the conductor
102e. Such a sequence of pulses on the conductors 102a-e is
continuously repeated. The parameters of the sequential switch 100
may be adjusted to cause the pulses on the conductors 102a-e to
have any desired duration. The conductors 102a and 102c are also
coupled to an OR circuit 105 and the conductors 102b and 102d are
coupled to an OR circuit 106. The output of the OR circuit 105 is
coupled to the gate of the first gated amplifier 90, and the second
OR circuit 106 is coupled to the gate of the second gated amplifier
91. Thus, the first gated amplifier 90 is gated on for the duration
of the first and third pulses from the sequential switch 100 and is
gated off for the duration of the second, fourth and fifth pulses
from the sequential switch 100. On the other hand, the gated
amplifier 91 is gated on for the duration of the second and fourth
pulses on the conductors 102a and 102d from the sequential switch
100 and is gated off for the duration of the first, third and fifth
pulses from the sequential switch 100.
In order sequentially to activate the silo stores 48, there is
provided a silo store scanner 110 driven by a clock 111 and having
10 output conductors 112. The output conductors 112 are coupled
respectively to ten AND circuits 113. A second input for each of
the AND circuits 113 is provided by the fifth output from the
sequential switch 100 on the conductor 102e via an inverter 122.
The outputs of the AND circuits 113 are respectively coupled along
the conductors 56 to the silo stores 48.
The clock 111 generates pulses at a predetermined rate such as, for
example, one every ten seconds. The sequential switch 110 produces
sequentially on the conductors 112, pulses of substantial duration,
for example, 10 seconds. An AND circuit 113 will produce an output
on its associated conductor 56 whenever there is no output from the
conductor 102e and a pulse is present on the associated conductor
112. Thus, there will appear on the first conductor 56 (from left
to right) a sequence of pulses, each having a duration equal to the
combined duration of the pulses on the four conductors 102a-d and
will be spaced by an amount equal to the duration of the pulse on
the conductor 102e. Such pulses on the conductor 56 will
continuously repeat as long as the pulse on the first conductor 112
is present. Thus, if each pulse from the scanner 110 is 10 seconds,
and each of the pulses on the conductors 102a-e is 25 milliseconds,
the sequence of pulses on the first conductor 56 will last for 10
seconds, each pulse being 100 milliseconds in duration and spaced
from an adjacent pulse by 25 milliseconds. Following termination of
the first pulse by the scanner 100, a second pulse is produced on
the second conductor 112, and so forth, until a pulse is produced
on the last conductor 112, whereupon the scanner 110 again begins
the cycle producing a pulse on the first conductor 112. Thus there
will appear on the second conductor 56 a sequence of pulses similar
to the sequence on the first conductor 56. Similarly such sequences
will appear successively on the conductors 56.
A silo store 48 is operative to read out or release information
stored therein only upon application of a pulse thereto along the
associated conductor 56. Thus, the first group of pulses on the
first conductor 56 will permit the first silo store 48 to release
the codes stored therein but because none of the other silo stores
48 have signals on their associated conductors 56, none of the
codes stored in the latter stores will be released. The first pulse
from the first AND circuit 113 will cause the first silo store 48
to release the codes corresponding to the four digits respectively
on the conductor sets 57a, 57b, 57c and 57d. corresponding to the
last digit. The four codes thus released are applied to the
converters 60. At the end of the first pulse from the scanner 110,
a pulse will appear on the conductor 102e so that for the duration
thereof no output is furnished by the first AND circuit 113 and
thus no information is released from the first silo store 48. The
next pulse from the first AND circuit 113 will cause the first silo
store 48 sequentially to release the next four-digit code in
storage, which code is coupled to the converters 60. The first silo
store 48 will sequentially release the codes in this fashion, only
so long as the first AND circuit 113 receives an input from the
scanner 110. If the input is 10 seconds long, then the first silo
store 48 will continue to release stored codes for 10 seconds or
until empty. If after that interval there is still information
stored in the first silo store 48, such information will be stored
until the scanner 110 completes a cycle. The clock 111 is
synchronized with the clock 101 so that the scanner 110 will not
stop during the middle of a readout cycle. That is, the sequential
scanner 110 should stop only when there is an output pulse on the
connector 102e.
The scanner 110 then produces a voltage for the second AND circuit
113, whereby in a similar fashion, a sequence of pulses is produced
on the output conductor 56 of the second AND circuit 113, each of
which pulses has a duration equal to the combined duration of the
pulses produced by the sequential switch 100 on the conductors
102a-d and are spaced by an amount equal to the duration of the
pulse on the conductor 102e. The sequence of pulses produced by the
second AND circuit 113 will cause the second silo store 48
sequentially to release the codes stored therein to the converters
60 in the same manner as that described in respect to the first
silo store 48. The scanner 110 produces voltages in succession for
the AND circuits 113 to cause the silo stores 48 to release in
succession the codes stored therein. After the scanner 110 has
produced an operating voltage for the tenth AND circuit 113 to
cause the tenth silo store 48 sequentially to release the codes
stored therein, the scanner 110 commences a new cycle to release
codes from the first silo store either not released the first time
or subsequently applied thereto by additional calls on the trunks
31.
It will be noted that each silo store 48 has an output conductor 55
coupled back to the scanner 110. When a silo store 48 is empty, it
will couple back a signal along the associated conductor 55 to
instruct the scanner 110 immediately to terminate the voltage for
the associated AND circuit 113 and to provide a voltage for the
next AND circuit. For example, if the scanner 110 is set to provide
voltages for each AND circuit 113 for 10 seconds and the first AND
circuit 113 has only five seconds of codes stored therein, the
first silo store 48 will have been emptied after 5 seconds, which
causes a signal to be applied along the associated conductor 55 to
the scanner 110 and instruct it to step to the second AND circuit
113 and commence providing a voltage therefor.
The encoder 30 includes a first OR circuit 115 having five inputs
respectively coupled to the first, third, fifth, seventh and ninth
output conductors 112 of the silo store scanner 110; and a second
OR circuit 116 having five inputs respectively coupled to the
second, fourth, sixth, eighth and tenth output conductors 112 of
the scanner 110. The outputs of the OR circuits 115 and 116 are
respectively coupled to a pair of monostable multivibrators 117 and
118, the outputs of the latter being coupled to an OR circuit 119.
There is also provided an AND circuit 120 having its first input
coupled to the output of the OR circuit 119 and having its second
input coupled to the output conductor 102a of the sequential switch
100. The output of the AND circuit 120 is coupled to a pulse
extender 121, the output of which is coupled to the clock 101.
The pulse extender 121 causes the first pulse on the conductor 102a
to be substantially longer than the ensuing pulses on the
conductors 102b-e. Since the pulse extender 121 will not be
operative for the rest of the duration of the voltage produced by
the scanner 110 on the first conductor 112, subsequent pulses on
the conductors 102a-e have equal durations. However, the pulse
extender 121 will again be operative for the first pulse produced
by the sequential switch 100 on the conductor 102a during the time
that the scanner 110 is applying a voltage to the second AND
circuit 113. Thus, the pulse first produced by the sequential
switch 100 on the conductor 102a after the sequential switch 110
steps to the second AND circuit 113 will have a long duration,
whereas all of the other pulses produced by the sequential switch
100 while voltage is applied to the second AND circuit 113 are
equal, but shorter, in duration. Thus, the output of the sequential
switch 100 will consist of a long first pulse on the conductor 102a
followed sequentially by four short pulses successively on the
conductors 102b-e, followed by a sequence of five short pulses
sequentially on the conductors 102a-e, etc., until the scanner 110
steps to the next AND circuit 113, whereupon the first pulse on the
conductor 102a will again have an extended duration, followed
sequentially by four short pulses successively on the conductors
102b-e, followed by a sequence of five short pulses sequentially on
the conductors 102a-e, followed one after the other by such
sequences of five short pulses, etc. The scanner 110 will
successively step to the AND circuits 113 one after the other,
performing the above operation until voltage is provided to the
tenth AND circuit 113 whereupon the first pulse on the conductor
102a will again have an extended duration, followed sequentially by
four short pulses successively on the conductors 102b-e. The last
set of five pulses produced on the conductors 102a-e while the
scanner 110 is providing voltage to the tenth AND circuit 113, will
be equal, but shorter, in duration. Following the last pulse, the
scanner 110 will again step back to the first AND circuit 113 and
the sequential switch 100 will commence the cycle again.
The first long pulse on the conductor 102a gates the first
converter 60 to enable the first binary code from the first silo
store 48 to couple the associated portion of the coil in the
oscillator 65 in circuit therein and thereby produce a first tone
or cue tone lasting for a duration determined by the duration of
the pulse on the conductor 102a. The OR circuit 105 also responds
to the pulse on the conductor 102a to gate on the first amplifier
90 for the duration of the cue tone to enable it to be coupled to
the conductor 95.
The next pulse, which is present on the second conductor 102b,
gates on the second converter 60 so that the second binary code
from the first silo store 48 will cause the corresponding portion
of the coil in the second oscillator 70 to be coupled in circuit
therein and thereby produce a second tone persisting for the
duration of the pulse on the conductor 102b. The OR circuit 106
also responds to the pulse appearing on the conductor 102b to gate
on the second amplifier 91, so as to couple to the conductor 95 the
second tone. The third pulse, which appears on the conductor 102c,
gates on the third converter 60 to enable the third binary code to
couple the associated portion of the coil in circuit in the
oscillator 65 to produce a third tone. Since the gated amplifier 90
is gated on by the pulse on the conductor 102c, the third tone will
similarly be coupled to the conductor 95. The next pulse appears on
the conductor 102d and is operative to gate on the fourth converter
60 to enable the binary code on the input thereof to couple the
associated portion of the coil in the oscillator 70 in circuit
therein, thereby producing a fourth tone having a duration equal to
the duration of the pulse on the conductor 102d. The second
amplifier 91 being gated at this time by virtue of the pulse on the
conductor 102d, couples the fourth tone to the conductor 95. During
the occurrence of the fifth pulse on the conductor 102e, none of
the converters 60 is operative, whereby no tone is produced on the
conductor 95 for the duration of that pulse. If the first pulse on
the conductor 102a has a duration of 1.5 seconds, for example, and
each of the pulses on the conductor 102b-e has a duration of 25
milliseconds, then the code signal produced on the conductor 95
will consist of a first tone or cue tone 1.5 seconds in duration
followed by a sequence of three control tones each 25 milliseconds
in duration, with no time between successive tones, followed by a
gap of 25 milliseconds.
After the termination of the pulse 102e, the sequential switch 100
again produces a pulse on the conductor 102a followed sequentially
by pulses on the conductors 102b-e. As previously described, the
pulse extender 121 is not operative for the remainder of the time
that a voltage is coupled to the first AND circuit 113, and,
therefor, all five pulses have the same duration of, for example,
25 milliseconds. Thus, after the gap following the first code
signal, the first of these five pulses will gate the first
converter 60 and the first amplifier 90 to provide a first tone of
a second code sequence. The second of the five pulses gates the
second converter 60 and the second amplifier 91 to provide a second
tone on the conductor 95. The third pulse gates the third converter
60 and the first amplifier 90 to provide a third tone on the
conductor 95. The fourth of these five pulses gates the fourth
converter the second amplifier 91 to provide a fourth tone on the
conductor 95. During the occurrence of the fifth of these five
pulses on the conductor 102e, none of the converters 60 is
operative. Accordingly, no tone is produced on the conductor 95 for
the duration of that fifth pulse. If each of the pulses on the
conductors 102a-e has a duration of 25 milliseconds, then the
second code signal produced on the conductor 95 will consist of a
first tone or cue tone, 25 milliseconds in duration, followed by a
sequence of three controlled tones, each 25 milliseconds in
duration, followed by a gap of 25 milliseconds. Because the code
signals are derived from codes stored in the first silo store 48,
the cue tone (first tone) of each will be the same in frequency but
not in duration.
In similar fashion, additional code signals, each having a common
cue tone, will be provided on the conductor 95 until the scanner
110 steps to the second AND circuit 113. Thereafter, the first
pulse produced on the conductor 102a will again have an extended
duration which gates the first converter 60 to enable the first
binary code from the second silo store 48 to couple the associate
proportion from the coil in the oscillator 65 in circuit therein
and thereby produce a first long tone. The OR circut 105 gates the
first amplifier 90 to enable the cue tone to be coupled to the
conductor 95. The succeeding pulses appearing on the conductors
102a-d respectively enable subsequent binary codes from the second
silo store 48 to cause the oscillators 65 and 70 to produce the
second, third and fourth tones. The fifth pulse on the conductor
102e produces a gap following the fourth tone.
The next set of pulses on the conductors 102a-e will all be equal
in duration and will respectively operate the converter 60, the
oscillators 65 and 70, and the amplifiers 90 and 91 to produce a
second code signal including a sequence of four tones equal in
duration. Again, the cue tone in the first code signal is many
times longer than the cue tone in the second code signal but the
frequencies are the same.
Similarly, additional code signals, each having a common cue tone
will be provided on the conductor 95, until the scanner 110 steps
to the third AND circuit 113. Then appearing on the conductor 95
will be the third group of code signals each having the common cue
tone, the cue tone for the first code signal being substantially
longer than the cue tone for succeeding code signals. In this
manner, the groups of code signals will be successively transmitted
until the tenth silo store 48 is emptied, whereupon those code
signals having the common first cue tone will again be
transmitted.
Although not necessary, the oscillators 65 and 70 may continuously
oscillate, with the frequency of the tones therefrom being shifted
as the subsequent code is applied to the next converter 60. Thus,
for example, the oscillator 65 is continuously producing a tone but
because the gated amplifier 90 is gated on only during the first
and third tones, only these tones will appear on the output
conductor 95.
FIG. 4 schematically illustrates the output on the conductor 95. In
this example, it will be assumed that the first cue tone in a group
has a duration of 1.5 seconds and the duration of each of the rest
of the cue tones, each of the control tones, and each of the gaps
between code signals is 25 milliseconds. In that case, 67 code
signals could be transmitted in a 10 second period. There will be
provided ten signal groups, although only the first signal group
130, the second signal group 140 and the tenth signal group 150 are
shown. Further, it will be assumed that each group includes 67 code
signals. The first signal group 130, includes a first code signal
131, a second code signal 132, a third code signal 133, and so
forth and a 67th code signal 134. The first code signal 141 and a
second code signal 142 and the 67th code signal 143 of the signal
group 140 are illustrated. The tenth signal group 150 includes 67
code signals, there being illustrated the first code signal 151,
the second code signal 152, and the 67th code signal 153. The tones
in each of the code signals are represented respectively by the
letters a, b, c and d, following the number of the associated code
signal. The first cue tones 131a, 141a, and 151a, respectively of
the signal groups 130, 140, and 150, each having a duration of
about 1.5 seconds. All the rest of the cue tones 132a, 133a, 134a,
142a, 143a, 152a and 153 a, each have a duration of about 25
milliseconds. The three control tones 131b-d which follow in
sequence after the cue tone 131a, each have a duration of about 25
milliseconds as does the gap 131e. The rest of the control tones
132b-d, 133b-d, and 134b-d, each have a duration of 25
milliseconds, in the code signals in the first signal group 130.
The rest of the code signals have a similar composition.
It should be understood that although each signal group is shown to
have 67 code signals, a lesser number would be provided if the
associated silo store 48 had less than 67 binary code sets stored
therein.
Although FIGS. 2A and 2B illustrate a particular form of the
encoder 30, it is to be understood that any number of different
constructions may be utilized to provide a multipliicty of code
signals each having a cue tone followed by a sequence of three
control tones, and arrange the code signals into a plurality of
signal groups for sequential transmission thereof, the number of
signal groups corresponding to the number of cue tone frequencies,
with each signal group containing all those code signals which
include corresponding cue tone of that group. It is further to be
understood that although the system described is one capable of
delivering the cue tone followed by three control tones in
sequence, additional or fewer control tones may be provided. Also,
the control tones may be sent either simultaneously or
sequentially. The cue tone in each code signal may be sent the same
time as the control tones or prior thereto.
There is illustrated in FIG. 5 of the drawings, a diagram of a
communication receiver 220 having a processing circuit 229. The
carrier signal is picked up by an antenna 230 and is conveyed to
the input of a radio frequency amplifier 231. The output of the
radio frequency amplifier 231 is applied as one of the inputs to a
mixer 232, the usual focal oscillator 233 being provided and having
the output thereof connected as a second input to the mixer 232.
The intermediate frequency (IF) signal which is the output of the
mixer 232 is applied as the input to an IF amplifier 234, the
output of which is transmitted to the input of a limiter 235. The
output of the limiter 235 is coupled to a discriminator 236, which
provides a demodulated signal on the conductor 237. The processing
circuit 229 includes the elements 231-236.
It is to be assumed that the receiver 220 is responsive to a code
signal including a predetermined sequence of four tones. The
demodulated signal on the conductor 237 includes a code signal in
the form of a cue tone followed by sequence of three control tones.
There is provided a pulser circuit 500 which produces a series of
pulses which are coupled back to the processing circuit 229 to
provide the supply voltage therefor. During the presence of the
pulses, the processing circuit 229 is operative to process and
detect RF signals impressed on the antenna 230; whereas between
successive pulses, the processing circuit 229 is inoperative and
any signals on the antenna 230 will not pass to the conductor
237.
A decoder 240 is coupled to the discriminator 236. If the first
tone of the code signal on the conductor 237 corresponds to the
first tone to which the decoder 240 is tuned, a first control
signal will be developed on the conductor 285 for application to a
timing circuit 470. The first control signal on the conductor 285
commences in the presence of both the first control tone in the
code control and a pulse from the pulser circuit 500. The timing
circuit 470 extends the control signal, which extended control
signal causes the pulser circuit 500 to furnish on the conductor
516 a continuous supply voltage for a predetermined interval. Such
supply voltage renders the processing circuit 229 continuously
operative for that interval to process and detect RF signals on the
antenna 230. Upon termination of the last control tone in the code
signal, the control signal on the conductor 285 is removed and
after a period of at lesat 50 milliseconds during which no cue tone
is received, the pulser circuit 500 again produces a series of
pulses for intermittent operation of the processing circuit 229. A
second control signal from the decoder 240 appears on the conductor
465, which control signal commences essentially with the reception
of the last control tone in the code signal, assuming the previous
tones have been received in the proper order. The control signal on
the conductor 465 terminates with the termination of the last
control tone.
The control signal on the conductor 465 is applied to a timer
switch circuit 370 which, in turn, energizes a utilization circuit
such as the oscillator 540. The series of pulses from the pulser
circuit 500 is also applied to the oscillator 540, and, in the
presence of both signals, a pulsating oscillatory signal is applied
to an annunciator such as a speaker 545 which generates a series of
bursts of alerting tones.
The control signal on the conductor 465 is also applied to a
latching switch circuit 570 which, in turn, energizes a utilization
circuit such as the lamp control circuit 590. Also applied to the
lamp control circuit 590 is the series of pulses on the conductor
516. In the presence of both the series of pulses and the enabling
signal from the latching switch circuit 570, an annunciator such as
the lamp 600 blinks on and off at a rate determined by the series
of pulses. Another output from the latch switching circuit on the
conductor 582 is applied to a pulse extender circuit 610 which,
upon termination of the last control tone, lengthens the pulses
developed by the pulser circuit 500 to increase the duration of the
bursts from the speaker 545 and to increase the on-time of the lamp
600. After expiration of a predetermined time, the timer switch
circuit 370 ceases to provide the enabling signal on the conductor
530 and the bursts of audio cease. When the user operates a manual
switch in the latching switch circuit 570, the lamp 600 becomes
extinguished and the pulser circuit 500 reverts to producing pulses
of shorter duration.
The output from the discriminator 236 is applied to the deocder 240
which is shown in block form in FIG. 6. The decoder 240 is adapted
to respond to a code signal including a cue tone followed by a
sequence of three control tones in predetermined order.
Alternatively, the code signal may be received as a sequence of
four tones. The code signal from the discriminator 236 is applied
to a pair of tone control channels, the lower tone control channel
including a special tapped filter 241 of a construction to be
described hereinafter. If the filter 241 is tuned to the frequency
of the first tone (cue tone) from the discriminator 236 it will
pass to the conductor 246 and be applied to a rectifier 260. The
tones and any noise on the conductor 237 are also applied to a
reference circuit 270 which provides a reference voltage on the
conductor 275. If the first tone on the conductor 246 exceeds the
reference voltage on the conductor 275, the rectifier 260 will
operate to rectify the tone and provide a filtered DC voltage on
the conductor 266. The DC voltage is applied to an electronic
switch 280 so as to power amplify the voltage and apply it on a
conductor 285 as one input to an AND circuit 290. A second input
for the AND circuit, on a conductor 426, is derived from an
inverter 420. If both inputs are present, an output voltage on the
conductor 294 will result, which voltage is applied to a timer 300.
Upon termination of the first tone, a DC voltage appears on the
conductor 303 and persists for a duration dependent on the setting
of the timer 300. The voltage on the conductor 303 is coupled to an
electronic switch 310 which provides a DC voltage pulse on its
output conductor 314. The voltage on the conductor 314 is coupled
to a filter 341 in the second tone channel and tunes the same to
receive the second tone (first control tone) present on the
conductor 237. If the second tone on the conductor 231 appears
immediately and is at the frequency to which the filter 341 is now
briefly tuned, it will pass to the conductor 346 and will be
applied to a rectifier 360. If the second tone on the conductor 346
exceeds the reference voltage on the conductor 275, the rectifier
360 will operate to rectify the tone and provide a filtered DC
voltage on the conductor 366. The DC voltage is applied to an
electronic switch 380 so as to power amplify the voltage and apply
it on a conductor 385 as one input to an AND circuit 390. The
second input for the AND circuit 390 is the voltage on the
conductor 314. Thus, if the first tone (cue tone) was received and
has terminated so as to provide a voltage pulse on the conductor
314, and the second tone is being received while that pulse is
present to provide a DC voltage on the conductor 385, the AND
circuit 390 will operate to produce a DC output voltage on the
conductor 394. This voltage is applied to a timer 400 which
provides a DC voltage on the conductor 403 upon termination of the
first tone, persisting for a duration dependent on the setting of
the timer 400. This voltage is applied to an electronic switch 410
which produces a DC voltage on the conductor 414 for application to
the inverter 420 so as to place the same in its other stable
condition. The resulting output from the inverter 420 on the
conductor 426 is coupled back to one input of the filter 241 which
causes the filter 241 no longer to be tuned to the first tone.
Simultaneously, the voltage on the conductor 414 is applied to
another input of the filter 241 to retune the same to respond to
the third tone (second control tone). Finally, the voltage on the
conductor 414 is applied as a first input to an AND circuit
430.
If the proper third tone is received on the conductor 237, the
filter 241 will pass the tone to the rectifier 260. If the third
tone exceeds the reference voltage on the conductor 275, it
actuates the electronic switch 280 to provide one input to the AND
circuit 290. However, the inverter is in its second stable
condition so that a second input to energize the AND circuit 290 is
lacking. The third tone, in addition, provides a second input to
the AND circuit 430. With both inputs to the AND circuit 430, a
potential is developed on the conductor 438 which is applied back
to the input of the electronic switch 410 to hold it in the active
condition as long as the third tone is received. A second output of
the AND circuit 430 on the conductor 437 is applied to a timer 440.
Upon termination of the third tone, a DC voltage appears on the
conductor 443 and persists for a duration dependent on the setting
of the timer 440. This voltage is applied to an electronic switch
450 which produces a DC voltage on the conductor 454. This output
voltage is fed back to the filter 341 to retune the same so as to
be operative to receive the fourth tone (third control tone). The
signal on the conductor 454 is also applied as one of the inputs to
an AND circuit 460.
Assuming that the proper fourth tone in the sequence of tones is
now received, there will be an output from the filter 341 which
will be rectified in the rectifier 360 to provide a DC voltage.
This voltage operates the electronic switch 380 and provides a
second input, on the conductor 385, to the AND circuit 460. In the
presence of both inputs, the AND circuit 460 provides a control
signal on the conductor 465. A hold-on potential is applied from
the AND circuit 460 on the conductor 466 to the input of the
electronic switch 450 to hold the latter in its active condition as
long as the fourth tone is being received.
Coupled to the conductor 285 of the decoder 240 is a timing circuit
470. The timing circuit 470 is operative to provide on its output
conductor 473 a control signal commencing with the concurrence of
the first control tone and a pulse from the pulser circuit 500, it
being pointed out that a voltage appears on the conductor 285
throughout the third tone also. The timing circuit 470 serves to
extend the duration of the control signal beyond termination of the
first tone for a predetermined time interval determined as
explained hereinafter.
Referring now to FIGS. 7 and 8 of the drawings, there are
illustrated further details of the decoder 240. The filter 241
includes an inductor 242 having associated therewith a magnetic
core 243, at least a portion of the core 243 being movable and
adjustable, whereby the inductor 242 can be slug tuned. The
inductor 242 is connected through a capacitor 245 to the conductor
237, and a capacitor 244 is coupled from the top of the inductor
242 to ground. The output from the filter 241 appears on a
conductor 246. The inductor 242 has a plurality of taps thereon,
two of which are identified by the numerals 247 and 248. Associated
with selected ones of the taps are two NPN transistors 250 and 253.
A resistor 249 is coupled between the base of the transistor 250
and the conductor 426. The transistor 250 has a collector connected
to the tap 248 on the inductor 242, while the emitter is connected
to ground potential. A resistor 254 is coupled between the base of
the transistor 253 and the conductor 414. The transistor 253 has a
collector connected to the tap 247 on the inductor 242, while the
emitter is connected to ground potential.
The decoder 240 also includes an inverter 420 including a PNP
transistor 421, the base of which is coupled through a resistor 422
to ground and through a diode 423 and a resistor 424 to the
conductor 414. A source of B.sup.+ supply voltage is coupled to the
emitter of the transistor 421 through a diode 425. In its quiescent
condition, the transistor 421 is heavily conductive so that the
supply voltage appears on the conductor 426 to render the
transistor 250 in the filter 241 heavily conductive, thereby
effectively to ground the tap 248 on the inductor 242. In this
condition, there is defined a parallel resonant circuit in the
filter 241, composed of the capacitor 244 coupled across the top
half of the inductor 242. If the first tone (cue tone) on the
conductor 237 is at the frequency to which the filter 241 is now
tuned, the control tone, at an increased amplitude, will appear on
the conductor 246. It should be noted that, at this time, the
transistor 253 is nonconductive.
The first tone, together with the noise on the conductor 237, is
applied to a reference circuit 270 which provides a reference
voltage on the conductor 275 proportional in amplitude to the tones
and the noise on the conductor 237. The first tone on the conductor
246 is applied to a rectifier 260 which provides a DC voltage on
the conductor 266 if the tone on the conductor 246 exceeds the
reference voltage on the conductor 275.
The next stage is an electronic switch 280 consisting of a pair of
cascaded NPN transistors 281 and 283, having their collectors
coupled to a DC voltage supply respectively via resistors 282 and
284. The DC voltage on the conductor 266 will cause the transistors
281 and 283 to conduct heavily, so as effectively to ground the
collector of the transistor 283.
The next stage is an AND circuit 290 including a PNP transistor 292
having a base coupled by way of a resistor 291 to the conductor
285. The emitter of the transistor 292 is coupled by way of a diode
293 to the conductor 426, and the collector is coupled to ground
through a resistor 301. There are two inputs to the AND circuit 290
from the conductors 285 and 426. If the conductor 285 is
effectively grounded, which occurs through the transistor 283 when
the first tone is present, and if the positive voltage appears on
the conductor 426, which occurs when the inverter 420 is in its
quiescent condition, the transistor 292 becomes heavily conductive
to place a positive voltage on the conductor 294. A timer 300,
consisting of the resistor 301 and a capacitor 302 produces a
negative DC voltage on the conductor 303 upon termination of the
positive voltage on the conductor 294 which occurs upon termination
of the first control tone. The next stage is an electronic switch
310 which includes a PNP transistor 311 having its emitter coupled
to the source of supply voltage, having its base coupled thereto
through a resistor 312 and a diode 313, and having its collector
coupled to the conductor 314. While the first tone is being
received, the capacitor 302 is being charged through the diode 313
and the transistor 311 is not conductive. However, upon termination
of the first tone, the capacitor 302 discharges through the
resistor 301 to render the transistor 311 heavily conductive to
place the supply voltage on the conductor 314. This voltage
persists for a duration determined by the RC time constant of the
timer 300. The positive voltage on the conductor 314 is applied as
one input to the AND circuit 390 and an input to the second filter
341.
The filter 341 includes an inductor 342 having associated therewith
a magnetic core 343, at least a portion of the core 343 being
movable and adjustable, whereby the inductor 342 can be slug tuned.
The inductor 342 is connected through a capacitor 345 to the
conductor 237, and a capacitor 344 is coupled from the bottom of
the inductor 342 to ground. The output from the filter 341 appears
on a conductor 346. The inductor 342 has a plurality of taps
thereon, two of which are identified by the numerals 347 and 348.
Associated with selected ones of the taps are two NPN transistors
350 and 353. A resistor 349 is coupled between the base of the
transistor 350 and the conductor 314. The transistor 350 has a
collector connected to the tap 348 on the inductor 342, while the
emitter is connected to ground potential. A resistor 354 is coupled
between the base of the transistor 353 and the conductor 454. The
transistor 353 has a collector connected to the tap 347 on the
inductor 342 while the emitter is connected to ground
potential.
The positive supply voltage on the conductor 314 developed during
the presence of the first tone renders the transistor 350 in the
filter 341 heavily conductive thereby effectively to ground the tap
348 on the inductor 342. In this condition, there is defined a
parallel resonant circuit composed of the capacitor 344 coupled
across the bottom portion of the inductor 342. If the second tone
(first control tone) in the series of tones on the conductor 231 is
at the frequency to which the filter 341 is then tuned, the second
tone, at an increased amplitude, will appear on the conductor 346.
It should be noted that at this time the transistor 353 is
nonconductive.
The second tone on the conductor 346 is applied to a rectifier 360
which is constructed like the rectifier 260. A rectified DC voltage
will appear on the conductor 366 if the second tone exceeds the
reference voltage on the conductor 275.
The next stage is an electronic switch 380 consisting of a pair of
NPN transistors 381 and 383 coupled in cascade, and respectively
having their collectors coupled to the source of supply voltage by
way of resistors 382 and 384. The rectified DC voltage on the
conductor 366 causes the transistors 381 and 383 to conduct
heavily, thereby effectively grounding the collector of the
transistor 383.
The next stage is an AND circuit 390 comprised of an PNP transistor
391 having its base coupled to the conductor 385 by the resistor
392. The collector of the transistor 391 is coupled to ground
through a resistor 402. There is further provided an NPN transistor
393 having its base coupled to the emitter of the transistor 391,
and its emitter coupled to the conductor 385 by a resistor 396. The
junction of the base of the transistor 393 and the emitter of the
transistor 391 is coupled to the conductor 314. The two inputs for
the AND circuit 390 are on the conductors 385 and 314. It will be
remembered, that a positive voltage appeared on the conductor 314
after termination of the first tone, which positive voltage, in
conjunction with the grounding of the conductor 385, by immediate
reception of the second tone causes both transistors 391 and 393 to
conduct heavily. The collector of the transistor 393 is coupled by
way of a conductor 395 back to the conductor 303. The heavy
conduction of the transistor 393 permits current to flow from
B.sup.+ through the base-emitter junction of the transistor 311,
through the transistor 393 from collector to emitter, the
transistor 383 from collector to emitter, to maintain the
transistor 311 conductive for the duration of the second control
tone. As long as the transistor 311 is conductive, one input to the
AND circuit 390 is provided and, as long as the second control tone
is present, the second input to the AND circuit 390 is provided.
Thus a DC voltage will be present on the conductor 385 for the
duration of the second control tone. A second output from the AND
circuit 390 on the conductor 394 is derived from the collector of
the transistor 391. A timer 400, consisting of the resistor 402 and
a capacitor 401 produces a negative DC voltage on the conductor 403
upon termination of the positive voltage on the conductor 394 which
occurs upon termination of the second control tone. The next stage
is an electronic switch 410, which includes a PNP transistor 411
having its emitter coupled to the source of supply voltage and
having its base coupled to said source by way of a resistor 412 and
a diode 413. While the second tone is being received, the capacitor
401 is being rapidly charged through the diode 413 and the
transistor 411 is not conductive. However, upon termination of the
second control tone, the capacitor 401 discharges through the
resistor 402 to render the transistor 411 heavily conductive to
place the supply voltage on the conductor 414. This voltage
persists for a duration determined by the RC time constant of the
timer 400.
The positive voltage on the conductor 414 is coupled to the
inverter circuit 420 to render the transistor 421 nonconductive,
which in turn renders nonconductive the transistor 250 of the
filter 241. Also, the conductor 414 applies a positive voltage to
the base of the transistor 253 to render it heavily conductive,
thereby effectively placing the capacitor 244 across the top
portion of the coil 242. If the third tone (second control tone) on
the conductor 237 has a frequency to which that resonant circuit is
tuned, the resonant circuit will develop a voltage on the conductor
246 which will be rectified by the rectifier 260 to provide a DC
voltage to operate the switch 280, the output of which is applied
as one input to the AND circuit 290. Since the voltage on the
conductor 426 is no longer positive, the AND circuit 290 will not
operate in spite of the presence of the voltage on the conductor
285. The DC voltage is also applied to an AND circuit 430, the AND
circuit 430 including an NPN transistor 431 having its base coupled
to the emitter of a PNP transistor 432. The base of the transistor
432 is coupled to the conductor 285 by means of a resistor 434, and
the emitter of the transistor 431 is coupled by a resistor 435 to
the conductor 285. The conductor 414 is coupled to the junction of
the base of the transistor 431 and the emitter of the transistor
432. The grounded condition of the conductor 285, resulting from
the presence of the third tone, and the plus voltage on the
conductor 414, resulting from the cessation of the second tone,
cause the transistors 431 and 432 to conduct heavily. The collector
of the transistor 431 is effectively grounded which provides a path
for current flow through the base-emitter junction of the
transistor 411 to cause the transistor to continue to conduct
heavily despite interruption of the second tone. Thus, the
conductor 438 is a feedback path to maintain conductive the
transistor 411 for the duration of the third tone. The heavy
conduction of the transistor 432 effectively places a positive
voltage on the conductor 437 which is applied to a timer 440
consisting of a resistor 441 to ground and a series capacitor 442.
The timer 440 produces a negative DC voltage on the conductor 443
upon termination of the positive voltage on the conductor 437 which
occurs upon termination of the third tone. The next stage is the
electronic switch 450 comprised of a PNP transistor 451 having its
base coupled to the conductor 443, and its emitter coupled to
B.sup.+. There is also provided a resistor 452 and a diode 453
coupled in parallel between the base of the transistor 451 and the
voltage supply source. While the third tone is being received, the
capacitor 442 is being rapidly charged through the diode 453 and
the transistor 451 is not conductive. However, upon termination of
the third tone, the capacitor 442 discharges through the resistor
441 to render the transistor 451 heavily conductive to place the
supply voltage on the conductor 454. This voltage persists for a
duration determined by the RC time constant of the timer 440. The
next stage is an AND circuit 460 including an NPN transistor 462
having its base coupled to the emitter of a PNP transistor 467. The
base of the transistor 467 is coupled by way of a resistor 463 to
the conductor 385. The base of the transistor 462 and the emitter
of the transistor 467 are connected together and to the conductor
454.
When the transistor 451 conducts heavily in response to the
termination of the third tone, the supply voltage is effectively on
the conductor 454 which is coupled back to the filter 341 to cause
heavy conduction of the transistor 353 thus effectively to place
the capacitor 344 across a different, greater portion of the
inductor 342. It should be noted that the transistor 350 is now
nonconductive since the transistor 311 became nonconductive upon
termination of the third tone. If the fourth tone (third control
tone) on the conductor 237 has a frequency corresponding to the
resonant frequency of that resonant circuit, it will be rectified
by the rectifier 360 and switch the electronic switch 380, thereby
grounding the conductor 385 which results in the grounding of the
junction between the resistors 463 and 464 in the AND circuit 460.
The concurrent grounding of the conductor 385 and the presence of
the supply voltage on the conductor 454 cause the transistor 467 to
conduct heavily and provide on its collector and thus the conductor
465, a positive control voltage substantially equal to the B.sup.+
supply voltage. Also, the heavy conduction of the transistor 462
effectively grounds its collector, and thus the conductor 466, so
that the transistor 451 remains conductive until the fourth tone
terminates thereby removing the ground from 385.
Accordingly, the decoder 240 provides two control signals: a
control signal on the conductor 465 that does not appear until the
occurrence of the fourth tone in the code signal to which the
decoder 240 is set; and a control signal, equal to the supply
voltage, which is derived on the conductor 285, but appears upon
the appearance of the cue tone or first tone.
The output of the decoder 240 on the conductor 285 is applied to a
timing circuit 470 which includes a PNP transistor 474 having its
emitter coupled to the supply voltage and its base coupled to the
input conductor 285 by a resistor 475. The collector of the
transistor 474 is coupled through a resistor 477 and a diode 478 to
the conductor 473. There is also provided an integrating network
479 connected to the collector of the transistor 474 and including
a parallel resistor and capacitor arrangement. It will be
remembered that the conductor 285 is effectively grounded upon
concurrence of the first tone (cue tone) and a pulse from the
pulser circuit 500 and stays grounded until termination of the
first tone and also throughout the third tone (second control
tone). Accordingly, the transistor 474 is biased on during the
first and third tones and is effectively saturated so as to provide
on the conductor 473 a second control signal equal to the supply
voltage. The integrating circuit 479 serves to extend the duration
of the control signal, so that it persists for a time beyond
termination of the first tone. Specifically, the values of the
resistor and the capacitor in the circuit 479 and the resistor 477
are so selected that the extended control signal will extend beyond
termination of the first tone and beyond termination of the gap
which occurs between the code signal containing that first tone and
the very next code signal transmitted by the transmitter. For
example, if each control tone and the gap between code signals has
a duration of 25 milliseconds then the values of components of the
timing circuit 470 would be such as to extend the termination of
the control signal at least 100 milliseconds so as to terminate
after the commencement of the next code signal.
Referring now to FIG. 9, the signal on the conductor 473 is applied
to a pulser circuit 500 which includes an astable multivibrator 501
in which there is an NPN transistor 502 having its emitter on
ground its collector coupled through a resistor 503 to a supply
voltage, and its base coupled to the cathode of a diode 504, the
anode of which is on ground. The multivibrator 501 also has a
second NPN transistor 505 with its emitter grounded and having its
base coupled through a capacitor 506 to the collector of the
transistor 502. The collector of the transistor 505 is coupled to
the source of supply voltage by way of the resistor 507. There is
also provided a diode 508 coupled from ground to the base of the
transistor 505. Lastly, the multivibrator 501 includes a feedback
capacitor 509 coupled from the collector of the transistor 505 back
to the base of the transistor 502.
The pulser circuit 500 also includes an electronic switch 510
having an NPN transistor 511 with its emitter grounded and its base
coupled to the resistor 512 and its collector coupled by way of a
resistor 513 to the source of supply voltage. The switch 510 also
includes a PNP transistor 516 having its emitter coupled to the
source of supply voltage, its base coupled to the collector of the
transistor 511 by way of a resistor 515 and its collector coupled
to the conductor 516. Also coupled to the base of the transistor
511 is the conductor 473.
In operation, the multivibrator 501 serves to produce a series of
pulses having a peak-to-peak value equal to the value of the supply
voltage. The duration of the pulses is determined primarily by the
values of the resistor 503 and the capacitor 506, and the interval
between successive pulses is determined primarily by the values of
the resistor 507 and the capacitor 509. The series of pulses is
applied to the switch 510 through the transistors 511 and 514 to
provide a series of pulses on the conductor 516 having a
peak-to-peak value equal to the value of the supply voltage. The
series of pulses are translated along the conductor 516 to the
various elements of the processing circuit 229 (see FIG. 5). It
should be clear that these pulses of supply voltage render
operative each element in the processing circuit 229 to process RF
signals appearing at the antenna 230. Of course, if an RF signal
appears at the antenna 230 between pulses, the processing circuit
229 will not be operative and that signal will not be
processed.
If an RF signal is received at an instant when a pulse is present,
the signal will be processed in the processing circuit 229. If the
demodulated signal on the conductor 237 contains the sequence of
tones to which the decoder 240 is tuned, a control signal will
appear on the conductor 473 as previously described. This control
signal on the conductor 473 is applied (FIG. 9) to the base of the
first transistor 511 in the switch 510 to render the transistor 511
conductive, which, in turn, renders conductive the transistor 514
to place on the conductor 516 a constant DC voltage equal to the
B.sup.+ supply voltage, which is applied back to each element in
the receiver circuits 220. Now the receiver circuits 220 are in
condition to receive and process any RF signals impressed on the
antenna 230 for the duration of the control signal on the conductor
473. It should be apparent that, once the control signal is
removed, the pulser circuit 500 reverts back to its original state
and produces the series of pulses for intermittently energizing the
receiver circuits 220. The control signal on the conductor 473
terminates after the time a subsequent code signal is sent by the
transmitter.
There is also provided a switch circuit 520 which may either be
timed to maintain the pulser circuit 500 operative to generate a
continuous supply voltage for a longer duration, or may be of the
latching variety, in which case, the pulser circuit 500 will
produce a continuous supply voltage until some positive act is
effected by the user to interrupt its operation. In the embodiment
shown, the switch 520 is a monostable multivibrator and functions
as a timer.
The switch 520 includes an NPN transistor 521 having its emitter
coupled to ground via a resistor 522 and having its base coupled to
ground by way of a resistor 523 and a diode 524 coupled in
parallel. There is also provided a PNP transistor 525 having its
base connected directly to the collector of the transistor 521, its
collector connected through a resistor 527 to ground and its
emitter connected to a source of supply voltage, a resistor 526
being connected between the base and the emitter of the transistor
525. The collector of the transistor 525 is coupled by way of a
capacitor 528 to the base of the transistor 521. A conductor 530 is
coupled to the collector of the transistor 521. The base of the
transistor 521 is coupled to the conductor 465 by way of a diode
532.
In operation, the appearance of the control signal on the conductor
465 during occurrence of the last tone causes conduction of the
transistor 521 which provides a path for current flow from the
source of supply voltage through the base-emitter junction of the
transistor 525 and the collector and the emitter of the transistor
521. This renders the transistor 525 highly conductive so as to
provide current flow through the collector and the emitter of the
transistor 525 and the resistor 527 and thereby to place an
enabling signal on the conductor 530.
During the conduction periods of the transistors 521 and 525,
current flows from B.sup.+, through the collector and the emitter
of the transistor 525, through the capacitor 528, and through the
base-emitter junction of the transistor 521 to charge the capacitor
528. Accordingly, when the control signal on the conductor 465 is
removed by virtue of the last control tone terminating, the
transistor 521 remains conductive because the capacitor 528 is
still being charged through the base-emitter junction of the
transistor 521 and the resistors 522 and 523. Of course, the
conduction of the transistor 521 maintains the transistor 525
conductive to maintain the enabling voltage on the conductor 530
for a time interval determined by the RC time constant of the
switch circuit 520, that is, the resistors 522 and 523 and the
capacitor 528. By selecting the value of those parts, the time
period that the enabling signal remains on the conductor 530 may be
controlled. If desired, the switch 531 may be closed to maintain
the transistors 521 and 525 conductive which causes the enabling
signal to be present on the conductor 530 as long as the switch is
closed. The positive voltage on the base of the transistor 521 when
the time switch 520 is closed is, by virtue of the diode 532,
isolated from the latching switch 570 so as to prevent undesired
operation thereof.
There is also provided an oscillator circuit 540 including a free
running oscillator 541. The oscillator 541 includes an NPN
transistor 542 and a feedback network 543 the components of which
are adjusted to cause the free running oscillator 541 to oscillate
at an audio frequency, such as, for example, 1,000 hertz. A speaker
545 is coupled between the collector and emitter of the transistor
542 through a DC isolation capacitor 546, and the emitter of the
transistor 542 is connected to ground through a normally closed
switch 547.
There is also provided a PNP transistor 550 which functions as an
AND device, its base being coupled to the conductor 530 and its
emitter being coupled to the conductor 516. The collector of the
transistor 550 is direct current coupled to the base of an NPN
transistor 551, the emitter of which is coupled through a resistor
552 to the base of the transistor 542, and the collector of which
is coupled to B.sup.+. In operation, the transistor 521 will become
saturated when the four control tones have been received, thereby
to ground the base of the transistor 550 through the resistor 522.
The series of pulses on the conductor 516 cause the transistor 550
to be conductive for the duration of each pulse and to be
nonconductive between successive pulses. The intermittent
conduction of the transistor 550 causes similar intermittent
conduction of the transistor 551 which in turn intermittently
energizes the transistor 542. When energized, the transistor 542 is
able to oscillate at the frequency determined by the feedback
network 543 to form an oscillatory signal which is converted into
single, spaced bursts of an alerting tone by the speaker 545.
Between pulses, when the transistor 542 has no base bias, the
oscillator portion 541 does not oscillate and no alerting tone is
developed. It can be seen, therefore, that the output of the
speaker 545 will be a series of intermittent tones or beeps.
Of course, the pulses on the conductor 516 are continually
developed as long as the receiver is on and the conductor 530 is
grounded through the resistor 522 in accordance with the time
constant of the timer switch 520. There is provided a switch 547
from the emitter of the transistor 542 to ground which interrupts
operation of the oscillator 541. If desired, a manual switch may be
provided on the conductor 530, so that the user can open the same
to disable the audio channel.
Summarizing, prior to receiving the code signal consisting of a cue
tone followed by a sequence of three control tones, the pulser
circuit 500 is producing a series of pulses on the conductor 516
which is applied to the processing circuit 229 intermittently to
energize it. If an RF signal is impressed on the antenna 230 while
the processing circuit 229 is energized, it will be processed and
detected, and, if it contains the cue tone (first tone) to which
the decoder 240 is to respond, a first control signal will be
developed on the conductor 285 and an extended control signal on
the conductor 473, the latter causing the pulser circuit 500 to
produce a continuous supply voltage for the processing circuit 229.
This control signal persists after termination of the cue tone for
a time interval determined by the values of the parts in the
network 479 and the resistor 477. These parts are selected such
that the extended control signal will persist beyond termination of
the code signal containing that cue tone and beyond the time the
next code signal commences. The pulses and the subsequent continual
supply voltage are also, of course, coupled to the emitter of the
transistor 550 in the oscillator circuit 540. However, without
more, no alerting tone is emitted by the speaker 545 since one of
the inputs to the AND transistor 550 is not present.
If the code signal is that to which the decoder 240 is to respond,
a second control signal will be developed on the conductor 465,
commencing during the fourth tone (third control tone). The second
control signal operates the timer switch 520 to provide on the
conductor 530 an enabling signal. This enabling signal provides the
requisite second input for the AND transistor 550 and thereby
renders same conductive. As previously explained, the signal on the
conductor 516 is coupled to the oscillator 541 to cause same to
produce a pulsating signal for the speaker 545. It can be seen
that, during the last control tone when the supply voltage on the
conductor 516 is continuous, the alerting tone generated by the
speaker 545 would be continuous. After termination of the fourth
tone, when the signal on the conductor 516 reverts to a series of
pulses again, the output of the speaker 545 becomes a series of
intermittent alerting tones.
There is also provided a second electronic switch 570, but instead
of being of the timing variety, it is a latching switch, that is,
it develops an enabling signal which will last indefinitely until
interrupted. The electronic switch 570 includes an NPN transistor
571 having its emitter grounded and having its base coupled to
ground through a resistor 572. The base is also coupled through a
diode 573 and a resistor 574 to the conductor 465. A resistor 575
and a capacitor 576 are coupled in parallel between the base of the
transistor 571 and a switch 576a. There is provided a PNP
transistor 577 having its emitter coupled to the supply voltage and
having its base coupled through a resistor 578 to the collector of
the transistor 571. The collector of the transistor 577 is coupled
back to the base of the transistor 571 through a resistor 579. In
addition, there is a biasing resistor 580 between the emitter and
the base of the transistor 577. The electronic switch 570 provides
an enabling signal on the conductor 581, as will be explained, to
operate a lamp control circuit 590.
In operation, both transistors 571 and 577 are nonconductive in the
absence of the second control signal on the conductor 465. If the
receiver receives a code signal containing the sequence of the
proper four tones, the control signal will appear on the connector
465 during the fourth tone. That control signal is coupled to the
transistor 571 to render it conductive which, in turn, renders the
transistor 577 conductive, to place a positive voltage on the
collector of the transistor 577. Part of this voltage is fed back
through the resistor 579 to the base of the transistor 571 in a
regenerative fashion to provide an enabling signal on the conductor
582 equal to the supply voltage, and an enabling signal on the
conductor 581 essentially equal to ground reference potential. The
enabling signals on the conductors 581 and 582 will persist, even
though the fourth control tone has terminated and no control signal
is being applied to the electronic switch 570, this being due to
the regenerative switching action. To "unlatch" the electronic
switch 570 and remove the enabling signals from the conductors 581
and 582, the switch 576a is closed momentarily grounding the
feedback resistor 579. The positive voltage on the base of the
transistor 571 when the latching switch 570 is "closed" is isolated
from the timer switch 520 so as to prevent undesired operation
thereof, by virtue of the diode 573.
The enabling signal on the conductor 581 is applied to a lamp
control circuit 590, the lamp control circuit including a PNP
transistor 591 having its emitter coupled to the conductor 516, and
its base coupled to the conductor 581. The collector is direct
current coupled to the base of an NPN transistor 592, the collector
of which is coupled to the supply voltage and the emitter of which
is coupled through a resistor 593 to the base of another NPN
transistor 594. The emitter of the transistor 594 is grounded and
the collector is coupled through a resistor 595 to a lamp 600.
Without the control signal on the conductor 465, no enabling signal
appears on the conductor 581 (i.e., it is not grounded), so that
the lamp control circuit 590 is not operative. However,
commencement of the fourth tone causes a control signal to appear
on the conductor 465 which will result in an enabling signal on the
conductor 581 (i.e., it is grounded) to ground the base of the
transistor 591. The series of pulses on the conductor 516 will
intermittently energize the transistor 591 which will, in turn,
cause conduction of the transistor 592 so as to provide current
flow through the collector and the emitter thereof, through the
resistor 593 and into the base-emitter junction of the transistor
594. This will cause current to flow through the lamp, the resistor
595 and the collector and the emitter of the transistor 594. The
lamp 600 will be lit for a duration equal to the width of the pulse
and will be extinguished between pulses. Thus, the lamp provides a
blinking effect so as more easily to attract the attention of the
user. The light will blink on and off indefinitely since the series
of pulses on the conductor 516 occur indefinitely and since the
enabling signal on the conductor 581 is latched in its grounded
condition. If the user wishes to turn off the lamp, he closes the
switch 576 which removes the enabling signal on the conductor 581
as previously set forth. Accordingly, the oscillator 540 may be
viewed as a first oscillator for applying a first oscillatory
signal to the speaker 545. The pulser circuit 500, together with
the control circuit 590, may be viewed as a second oscillator
which, in the presence of the enabling signal on the conductor 581,
will cause a second oscillatory signal to be produced for
energizing the lamp 600. Also, in this particular form, the
oscillator circuit 540 may be viewed as a utilization circuit for
the enabling signal on the conductor 530, and the lamp control
circuit 590 may be viewed as a utilization circuit for the enabling
signal on the conductor 581.
The conductor 582 is coupled back to the base of the transistor 474
in the timing circuit 470. The positive enabling signal on the
conductor 582 interrupts conduction of the transistor 474, whereby
the control signal on the conductor 473 ceases upon reception of
the fourth tone. Thus, the pulser circuit 500 reverts to producing
pulses for intermittent operation of the processing circuit
229.
In order to minimize the current drain of the communication
receiver 220 in its standby condition, the pulse width should be
many times shorter in duration than the time between pulses. On the
other hand, when the user is being alerted, it may be desirable to
increase the duty cycle or pulse width with respect to the interval
between pulses so that the alerting tone from the speaker 545
persists, and or the lamp 600 is on, for a greater percentage of
the time.
To this end, there is provided a pulse extender circuit 610 having
an NPN transistor 611 with its base coupled by a resistor 612 to
the conductor 582. The collector of the transistor 611 is coupled
to one side of the capacitor 509 in the pulser circuit 500, and the
emitter of the transistor 611 is coupled through a capacitor 613 to
the other side of the capacitor 509.
When the last tone begins, there is provided a positive enabling
signal on the conductor 582 which causes the transistor 611 to
conduct and thereby place the additional capacitance of the
capacitor 613 in parallel with the capacitor 509, thereby to
increase the on-time of the multivibrator circuit 501. This results
in an increased duty cycle, which is reflected on the conductor 516
as pulses of increased width and decreased time between successive
pulses.
Summarizing, the pulser circuit 500 produces a series of pulses on
the conductor 516, which are used intermittently to provide a
supply voltage for the various elements in the processing circuit
229. In a particular embodiment, the pulse width was 30
milliseconds and the time between pulses was 1.4 seconds or about a
2 per cent duty cycle. This means that during 98 per cent of the
time the communication receiver 220 is drawing essentially no
current, and that during the other two per cent of the time the
receiver 220 is drawing "standby" current. Accordingly, the useful
life of the battery in the communication receiver 220 may be
increased theoretically by a factor of 50. However, in the
embodiment described, the pulser circuit 500 may reduce this
theoretical increase by about 10 per cent. Also, the lamp control
circuit 590 and the oscillator circuit 540, when energized, draw
additional current and contribute perhaps 5 per cent additional
battery drain. Finally, the cue tone, if the proper frequency, will
cause the continuous supply voltage to be developed for a time
sufficient to examine the first tone in the next code signal. If
the first tone in the next code signal is not the proper one, the
pulser circuit 500 reverts to producing pulses. This may contribute
an additional 10 per cent drain. With the communication receiver
220, battery drain is minimized so as to conserve battery life, not
only during stand-by, but also while the alerting tone is
generated.
Reference is made to the graph of FIG. 10 wherein the waveform 620
represents the signal appearing on the conductor 516 which is the
output of the pulser circuit 500, and consists of a series of
pulses 621. For purposes of illustration, the duration of each
pulse is 30 milliseconds, and 1.4 seconds lapses between pulses.
Accordingly, the processing circuit 229 is rendered operative for
the duration of each pulse 621 and inoperative between the pulses
621. If an RF signal impressed on the antenna 230 includes one or
more tones, they will be detected in the processing circuit 229 and
will appear on the conductor 237 during the occurrence of a pulse
621. The waveform 625 represents a code signal consisting of a cue
tone 626 followed by a series of three control tones 627, 628 and
629, it being assumed that these four tones, in this order, will
activate the decoder 240 to produce control signals on the
conductors 465 and 473. The cue tone 626 commences at t.sub.1, and
for purposes of illustration it is assumed that it lasts for 1.5
seconds. Further, it is assumed that, each of the control tones
627, 628 and 629 lasts for 25 milliseconds, there being
substantially no time lag between successive ones of the control
tones. The cue tone 626 terminates at t.sub.4, at which time the
first control tone 627 commences. The second control tone 628
follows the tone 627, and terminates at t.sub.5 which marks the
commencement of the third control tone 629 which terminates at
t.sub.6.
At t.sub.1 when the cue tone commences, the processing circuit 229
is inoperative since no pulse 621 is applied thereto. The cue tone
persists until t.sub.2 when the next pulse 621 is generated, at
which time the processing circuit 229 becomes energized to process
and detect the RF signal, including the cue tone 626, which is then
coupled to the decoder 240. If the cue tone 626 is at the frequency
to which the first filter in the decoder 240 is tuned, a control
signal, represented by the waveform 630 will commence on the
conductor 285 at t.sub.3, which is a few milliseconds after
t.sub.2. As was previously explained, the signal on the conductor
285 will commence at t.sub.3 and persist throughout the first and
third tones as is shown by the waveform 630. The timing circuit 470
will produce an extended control signal represented by the waveform
631 also commencing at time t.sub.3. The control signal 631
terminates at t.sub.7 which is subsequent to the commencement of
the cue tone in the very next code signal. This extended control
signal is applied to the electronic switch 510 in the pulser
circuit 500, causing the same to close and provide a continuous
supply voltage, which is indicated by the numeral 622 on the
waveform 620. The continuous supply voltage is applied to the
processing circuit 229 to permit the rest of the cue tone 626 in
the RF signal to be processed by the processing circuit 229 and to
be applied to the decoder 240. When the cue tone 626 terminates at
t.sub.4, the control signal on the conductor 473 will persist
beyond that time as explained above, thereby to maintain the
processing circuit 229 operative to process and detect the RF
signal containing the control tones 627, 628 and 629.
If the proper tones 627, 628 and 629 are received in the proper
sequence, a second control signal represented by the waveform 632
will appear on the conductor 465 commencing at t.sub.5,
approximately at the commencement of the last control tone 629, and
will terminate with the completion of the last control tone 629 at
t.sub.6. The control signal 632 causes the timer switch 520 to
produce an enabling signal represented by the waveform 633,
commencing at the same time, t.sub.5, as the signal 632. The
enabling signal 633 lasts until t.sub.9 as determined by the RC
time constant of the timer switch 520. The control signal 632 also
causes the latching switch 570 to produce a second enabling signal
represented by the waveform 634 commencing at the same time,
t.sub.5. The enabling signal 634 lasts until t.sub.10 which is when
the user operates the reset switch 576a.
Also, the latching switch 570 produces on the conductor 582 a
further enabling signal represented by the waveform 635, of a
polarity opposite to the enabling signal 634, but with the same
duration. The enabling signal 635 is coupled to the base of the
transistor 474 in the timing circuit 470 to render same
non-conductive, thereby terminating the continuous control voltage
produced by the pulser circuit 500 at t.sub.5. The enabling signal
635 also operates the pulse extender circuit 610, effectively to
couple the capacitor 613 into the pulser circuit 500. This causes
the duration of the pulses developed by the pulser circuit 500 to
increase in duration by an amount dependent on the value of the
capacitor 613. The pulses of increased duration, represented by the
numeral 623 in the waveform 620, make it easier for the user to see
the flashing lamp 600 and/or to hear the "beeps" from the speaker
545. These increased-duration pulses 623 occur only for the
duration of the enabling signal 635. As shown in the waveform 620,
at t.sub.11, after termination of the enabling signal 635, the
pulser circuit 500 again reverts to producing the narrower-duration
pulses 621.
As is explained with reference to the transmitter 20 forming part
of the communication system described herein, the transmitted
signal consists of a carrier wave modulated by a sequence of groups
of code signals, each of which code signals includes a sequence of
four tones (a cue tone followed by three control tones). All of the
code signals in the first group contain a common cue tone, and all
of the code signals in the second group also have a common, but
different, cue tone, etc. The cue tone of the first code signal in
each group is longer than the gap between adjacent pulses from the
pulser circuit 500. If, for example, the pulses are separated from
each other by 1.4 seconds, the cue tone of the first code signal in
each group should be, say, 1.5 seconds. Then, the cue tone will
necessarily be present during occurrence of at least one of the
pulses 621. Such cue tone will cause the processing circuit 229 to
be continuously energized, not only for the duration of the three
remaining tones in the first code signal, but will also be
energized through the gap between that code signal and the
following code signal, even though as few as none of the ensuing
three tones corresponds to this decoder circuit.
Further details concerning the manner of operation of the receiver
220 may be had by reference to FIG. 11. FIG. 11A illustrates the
output of the pulser circuit 500, FIG. 11B illustrates the output
of the discriminator 236, FIG. 11C represents the output of the
decoder 240, FIG. 11D represents the control signal at the output
of the timing circuit 470 and FIG. 11E represents the control
signal on the conductor 465. It is assumed that the cue tone 650 is
that to which the decoder 240 is tuned. In the presence of a pulse
from the pulser circuit 500 and the cue tone 650, a control signal
660 is developed on the conductor 285 commencing at t.sub.1, at
which time the extended control signal 670 on the conductor 473
also commences. The termination of the cue tone at t.sub.2 causes
termination of the control signal 660. Due, however, to the timing
circuit 470, the control signal 670 is extended beyond t.sub.2. If
the first, second and third control tones are each 25 milliseconds
in duration and the gap between successive code signals is 25
milliseconds, then the timing circuit 570 is to be constructed to
cause the extended control signal 670 to persist for more than 100
milliseconds (the sum of the durations of the control tones and the
gap between successive code signals) beyond t.sub.2. For example,
the extended control signal 670 will persist for at least 150
milliseconds beyond time t.sub.2. If by that time no signal is
present on the conductor 285, the control signal 670 will
terminate.
In the example of FIG. 11, it is assumed that none of the proper
control tones of the first code signal is received. A sequence of
tones 650a then follows, none of which corresponds to the second
tone to which the decoder is to respond. However, the second code
signal includes the same cue tone as indicated by the waveform 651,
but for shorter duration of, for example, 25 milliseconds. This
causes a control signal 661 to appear on the conductor 285 having a
duration equal to the duration of the tone 651. The control signal
661 begins anew the timing by the timing circuit 470, thereby
causing the control signal 670 to be extended at least another 150
milliseconds beyond termination of the control tone 651 at
t.sub.3.
Again, however, it is assumed in the example in FIG. 11 that none
of the proper control tones of the second code signal is received.
A sequence of tones 651a then follows, none of which corresponds to
the second tone to which the decoder is to respond. No output is
present on the conductor 285 as indicated by the gap 662, but,
because of the timing circuit 470, the control signal 670 persists,
at least until commencement of the third code signal.
A third code signal 652 is shown as commencing at t.sub.4 and again
includes the same cue tone 653 persisting until t.sub.5. This
causes a control signal 663 to appear on the conductor 285 having a
duration equal to the duration of the tone 653. The control signal
663 begins anew the timing by the timing circuit 470, thereby
causing the control signal 670 to be extended at least another 150
milliseconds beyond termination of the control tone 653 at
t.sub.5.
The code signal 652 includes a second tone (first control tone) 654
to which the decoder 240 is to respond. The code signal 652
includes a third tone (second control tone) 655 to which the
decoder 240 responds, which causes the control signal 664 to be
present on the conductor 285. The signal 664 again extends the
control signal 670, so that it will persist for at least 150
milliseconds beyond termination of the signal 664 at t.sub.6. It is
finally assumed that the code signal 652 includes the fourth tone
656 to which the decoder 240 is tuned, during which signal a second
control signal 680 (FIG. 11E) is produced on the conductor 465, as
previously described. The extended control signal 670 maintains
energizing power for the processing circuit 229 on the conductor
516 for substantially its entire duration, as depicted by the
signal 640 (FIG. 11A). The control signal 670 terminates at
t.sub.7, about 150 milliseconds (in this example) after
t.sub.6.
The signal appearing on the conductor 582 which commences with the
commencement of the last control tone 656 causes the continuous
supply voltage 640 to terminate at t.sub.6. The pulser circuit 500
again begins to produce pulses.
It should be noted that the cue tone frequency can persist for an
extended period of time (for example, 10 seconds or more). If the
proper set of tones 653, 654, 655 and 656 are transmitted at the
beginning of such period of time, the cue tone will be present long
after termination of the tone 656. Without the connection along the
conductor 582, the continuous supply voltage 640 would be present
for the entire period of time, and no pulses would be available
intermittently to energize the circuits 540 and 590. In that case,
the tone from the loud speaker 545 would be continuous as would be
the light from the lamp 600.
Whereas FIGS. 10 and 11 show instantaneous switching, it should be
understood that, as a practical matter, some delay occurs between
the commencement of a tone and the actual switching.
The communication system described herein is capable of
accommodating a much greater number of code signals per unit time
than might be expected in a system incorporating battery-saving
circuitry. Increasing the gap between adjacent pulses to improve
battery life would seem to require that the duration of each code
signal be increased to exceed the duration of the gap between
pulses. Of course, longer code signals increases the time it takes
for transmission thereof and decreases the time available for
transmission and thus the number of code signals transmitted per
unit time.
However, this apparent inconsistency is overcome by grouping the
code signals such that all those having one common cue tone are
transmitted first, followed by transmitting all those having a
second common cue tone, followed by transmitting all those having a
third common cue tone, etc. The first cue tone in the first code
signal of a signal group, of course, will have to have a duration
exceeding the gap between pulses. Thus, if the gap is 1.4 seconds,
the duration of the cue tone may be 1.5 seconds. However, the
timing circuit 470 enables the processing circuit 229 to be
activated, not only for the duration of the cue tone and for the
succeeding tones in that code signal, but also beyond the ensuing
gap between code signals. Thus, the cue tone in the next code
signal may be short in duration, such as having a duration the same
as the duration of the succeeding control tones in that code
signal. The first cue tone in the first group has already insured
that the processing circuit 229 will be activated, and the cue
tones of all the rest of the code signals in that group will merely
replenish the supply voltage for the processing circuit 229.
After all of the code signals having the common first cue tone have
been transmitted, the code signals in the next group with a second
common cue frequency are then sequentially transmitted. Again, the
first cue tone in the first code signal of the second group insures
activation of the processing circuit 229 in those receivers
selected, and the cue tones in the rest of the code signals in that
group merely maintain the processing circuit 229 activated.
An example of such a system is as follows. With four tones, the
first being referred to as a cue tone, the next three being
referred to as control tones, there are 10,000 possible code
signals, if each tone is selected from a group of 10 frequencies.
For example, the first cue tone in a group has a duration of 1.5
seconds, and the duration of each of the rest of the cue tones,
each of the control tones, and each of the gaps between code
signals is 25 milliseconds. In that case, 67 code signals could be
transmitted in a 10-second period, or 6.7 code signals per
second.
The above system is capable of 10,000 code capacity. That capacity
may, of course, be changed by changing the number of tones. For
example, the code signal may include seven digits, the first digit
of which is either 1 or 2, whereby a dialed number commencing with
either such digit would automatically be routed to a paging
transmitter of the type previously described herein. In such case,
the system would have a 2,000,000 code capacity. The transmitter
for such a system, instead of having 10 silo stores, would have
twenty arranged in two banks of 10 each. The number 1 would
activate one bank while the number 2 would activate the other bank.
Whereas the number of codes per second in such a system would be
reduced since it would take longer for each code to be transmitted
using the same tone durations the overall code capacity of the
system would be increased.
Facilities may be provided for each silo store to apprise a caller
if it is full. For example, if a number is dialed which would be
stored in the fourth silo store and it is full, a busy signal will
be returned. In such case, the caller would have to call back at a
future time. Also, in order to encourage the caller to hang up upon
entry of his number into the associated silo store 48, a different
kind of alerting signal may be returned to the caller, or
alternatively, a message such as, "your number has been completed
and you need not hold the line" may be returned to the caller.
The sequential switch 100 will skip any of the silo stores 48 that
has no information stored therein. Also, the transmitter 20 may be
operated continuously or coincident with the scanner 110. During
light load periods, it may be desirable to operate the transmitter
only periodically, such as every 5 minutes.
The random access memory unit 45 may provide additional functions
to those previously attributed to it. For example, it may validate
numbers applied thereto. In other words, if an incorrect number is
called, the memory unit 45 will so advise the caller. Also, in
seven digit call sequence system it may automatically allow a four
digit number to complete the call. Such a scheme would be useful
where there may be 2,000,000 different codes available in the
system but that no one area has more than, for example, 10,000
users. The advantage in having 2,000,000 code capacity, in such
case, would be that a user could go from one area to another and
still have the use of his paging receiver. However, since only
10,000 of the 2,000,000 codes are to be called, the memory unit 45
may be suitably programmed to send the necessary seven digit number
in response to a four digit call.
While there has been described what is at present considered to be
the preferred embodiment of the invention, it is to be understood
that various changes and modifications can be made therein without
departing from the spirit and the 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.
* * * * *