U.S. patent number 3,798,545 [Application Number 05/224,199] was granted by the patent office on 1974-03-19 for high capacity paging system employing subaudible tones.
This patent grant is currently assigned to Motorola, Inc.. Invention is credited to Norman E. Schultz, Robert J. Schwendeman.
United States Patent |
3,798,545 |
Schultz , et al. |
March 19, 1974 |
HIGH CAPACITY PAGING SYSTEM EMPLOYING SUBAUDIBLE TONES
Abstract
A high capacity paging system includes a transmitter for
transmitting a modulated signal in response to a message including
address signals followed by audio signals. Each transmission is
initiated by actuating the switch and terminated by deactuating the
switch. Control circuitry is operative in response to the switch
actuation to actuate the transmitter. The control circuitry
includes time delay circuitry for holding the control circuitry
operative for a first predetermined time period after deactuation
of the switch. Address circuitry develops the address signals and
is operative in response to actuation of the switch to couple the
address signals to the transmitter for a second predetermined time
period no greater than the first predetermined time period. Audio
delay circuitry is operative in response to operation of the
control circuitry to delay audio signals coupled from a microphone
by the first predetermined time period and couple the delayed audio
signals to the transmitter for transmission thereby. The address
signals of the succeeding message are transmitted simultaneously
with the terminal portion of the audio signals in the preceding
message.
Inventors: |
Schultz; Norman E. (North
Lauderdale, FL), Schwendeman; Robert J. (Pompano Beach,
FL) |
Assignee: |
Motorola, Inc. (Franklin Park,
IL)
|
Family
ID: |
22839667 |
Appl.
No.: |
05/224,199 |
Filed: |
February 7, 1972 |
Current U.S.
Class: |
340/7.3;
340/7.49 |
Current CPC
Class: |
H04W
88/188 (20130101) |
Current International
Class: |
H04Q
7/10 (20060101); H04Q 7/06 (20060101); H04b
005/04 () |
Field of
Search: |
;325/55,64,155,187
;343/177 ;179/1.1Z,15BY,41A ;340/171R,171PF,311 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Safourek; Benedict V.
Attorney, Agent or Firm: Rauner; Vincent J. Dickler;
Marshall N.
Claims
We claim:
1. In a high capacity paging system including a transmitter for
transmitting a modulated signal in response to a plurality of
messages sent in sequence with each message including address
signals and audio signals, and wherein each transmission is
initiated by a switch being actuated, and the message terminated by
deactuating the switch, the combination including; control means
coupled to said switch and transmitter and operative in response to
a first actuation of said switch to actuate said transmitter, said
control means further including time delay means for holding said
control means operative for a first predetermined time period after
a first deactuation of said switch, address means for developing
address signals coupled to said switch and transmitter and
operative in response to said first actuation of said switch to
develop said address signals in a first of said plurality of
messages and couple said address signals to said transmitter for a
second predetermined time period no greater than said first
predetermined time period, audio delay means coupled to said
control means and said transmitter, said audio delay means
operative in response to said control means operation to receive
said audio signals in said first message and delay said audio
signals by a third predetermined time period greater than said
second predetermined time period and no greater than said first
predetermined time period and couple said delayed audio signals to
said transmitter, said control means being further operative in
response to a second actuation of said switch during said first
predetermined time period after said first deactuation of said
switch to maintain said transmitter and audio delay means operative
and to reset said first delay means for holding said control means
operative for another first predetermined time period after a
second deactuation of said switch, said address means being
operative in response to said second actuation of said switch to
develop said address signals in a second of said plurality of
messages and couple same to said transmitter during said first
predetermined time period after said first deactuation of said
switch, said audio delay means being operative in response to said
second actuation of said switch and said maintained operation of
said control means to receive said audio signals in said second of
said plurality of messages and delay said audio signals of said
second message by said third predetermined time period after said
second actuation of said switch and couple said second message
delayed audio signals to said transmitter.
2. The high capacity paging system of claim 1 wherein said address
means includes, means for developing a plurality of address
signals, and selection means for selecting a particular one of said
plurality of address signals to be coupled to said transmitter.
3. The high capacity paging system of claim 2 wherein said address
means includes, circuit means operative in response to said switch
actuation to couple said selected address signals to said
transmitter for said second predetermined time period.
4. The high capacity paging systey of claim 3 wherein said control
means includes, first switching means operative in response to said
switch actuation to develop a first switching signal for actuating
said transmitter and audio delay means, and time delay circuit
means coupled to said first switching means and said switch and
operative in response to deactuation of said switch to hold said
first switching means operative for said first predetermined time
period.
5. The high capacity paging system of claim 4 wherein said audio
delay means is a tape recorder operative in response to said first
switching signal to record said audio signals coupled thereto, said
tape recorder including reproducing means for reproducing said
audio signals and coupling same to said transmitter said third
predetermined time period after said audio signals are
recorded.
6. The high capacity paging system of claim 5 wherein said circuit
means includes monostable multivibrator means operative in response
to said switch actuation to change state for said second
predetermined time period, and gate means coupled to said
monostable multivibrator means and to said transmitter, said gate
means operative in response to said change of state of said
monostable multivibrator means to couple said selected address
signals to said transmitter.
7. The high capacity paging system of claim 6 wherein said first
switching means is a relay, and said time delay circuit means is a
monostable multivibrator.
8. The high capacity paging system of claim 7 wherein said tape
recorder includes an endless magnetic recording tape for recording
said audio signals, means for rotating said magnetic tape,
recording means for receiving said audio signals, said recording
means operative in response to receipt of said audio signals to
record same on said rotating magnetic recording tape, playback
means for reproducing said audio signals from said rotating
magnetic tape said first predetermined time period after said audio
signals are recorded thereon, and erase means for erasing said
recorded audio signals from said rotating magnetic tape after
reproduction by said playback means.
9. The high capacity paging system of claim 8 wherein said tape
recorder further includes motor means coupled to said endless
magnetic recording tape and operative to rotate said magnetic tape,
and second relay means coupled to said motor means and said first
switching means and operative in response to said first switching
signal to actuate said motor means.
10. The high capacity paging system of claim 9 further including
microphone means for converting speech to said audio signals, said
microphone means being coupled to said audio delay means for
coupling said audio signals thereto.
11. The high capacity paging system of claim 10 wherein said
address signals are subaudible tone signals.
12. The high capacity paging system of claim 11 wherein said means
for developing a plurality of address signals includes, oscillator
means for developing said subaudible tone signals, said selector
means being operative to select particular ones of said subaudible
tone signals.
13. The high capacity paging system of claim 11 wherein said
address signals include, a plurality of subaudible tone signals,
said selector means being operative to select said plurality of
said subaudible tone signals to form said address signals.
14. In a high capacity paging system including a transmitter for
transmitting a modulated signal in response to a plurality of
messages sent in sequence with each message including subaudible
tone signals and audio signals, coupled thereto, and wherein each
transmission is initiated by a switch being actuated, and the
message terminated by deactuating the switch, the combination
including, control means coupled to said switch and transmitter and
operative in response to a first actuation of said switch to
actuate said transmitter, said control means further including time
delay means for holding said control means operative for a first
predetermined time period after a first deactuation of said switch,
address means for developing said subaudible tone signals coupled
to said switch and transmitter and operative in response to said
first actuation of said switch to develop said subaudible tone
signals in a first of said plurality of messages and couple said
subaudible tone signals to said transmitter for a second
predetermined time period no greater than said first predetermined
time period, audio delay means coupled to said control means and
said transmitter, said audio delay means operative in response to
said control means operation to receive said audio signals in said
first message and delay said audio signals by a third predetermined
time period greater than said second predetermined time period and
no greater than said first predetermined time period and couple
said delayed audio signals to said transmitter, said control means
being further operative in response to a second actuation of said
switch during said first predetermined time period after said first
deactuation of said switch to maintain said transmitter and audio
delay means operative and to reset said first delay means for
holding said control means operative for another first
predetermined time period after a second deactuation of said
switch, said address means being operative in response to said
second actuation of said switch to develop said subaudible tone
signals in a second of said plurality of messages and couple same
to said transmitter during said first predetermined time period
after said first deactuation of said switch, said audio delay means
being operative in response to said second actuation of said switch
and said maintained operation of said control means to receive said
audio signals in said second of said plurality of messages and
delay said audio signals of said second message by said third
predetermined time period after said second actuation of said
switch and couple said second message delayed audio signals to said
transmitter.
15. The high capacity paging system of claim 14 wherein said
address means includes means for developing a plurality of
subaudible tone signals, and selection means for selecting
particular ones of said plurality of subaudible tone signals to be
developed.
16. The high capacity paging system of claim 15 wherein said means
for developing a plurality of subaudible tone signals includes a
plurality of oscillator means, said selector means being operative
to select particular ones of said subaudible tone signals.
17. The high capacity paging system of claim 16 wherein said
address means includes circuit means operative in response to said
switch actuation to sequentially couple said selected subaudible
tone signals therefrom for said second predetermined time
period.
18. The high capacity paging system of claim 17 wherein said
control means includes, first switching means operative in response
to said switch actuation to develop a first switching signal for
actuating said transmitter and said audio delay means, and time
delay circuit means coupled to said first switching means and said
switch and operative in response to deactuation of said switch to
hold said switching means operative for said first predetermined
time period.
19. The high capacity paging system of claim 18 wherein said audio
delay means is a tape recorder operative in response to said first
switching signal to record said audio signals coupled thereto, and
including reproducing means for reproducing said audio signals and
coupling same to said transmitter said third predetermined time
period after said audio signals are recorded.
20. In a high capacity paging system including a transmitter for
transmitting a modulated signal in response to a message coupled
thereto including address signals having a second predetermined
time period followed by audio signals, and wherein said
transmission is actuated by operation of a switch and said message
terminated by deactuation of said switch, the method of
transmitting said message comprising the steps of; actuating said
switch to initiate a first transmission, initiating said
transmission of a first message address signals for said second
predetermined time period upon actuation of said switch, developing
said audio signals of said first message simultaneously with
transmission of said first message address signals, delaying said
transmission of said audio signals by a third predetermined time
period not less than said second predetermined time period and no
greater than said first predetermined time period, terminating said
first message by deactuating said switch, maintaining transmission
for said first predetermined time period after termination of said
first message, actuating said switch to initiate a second
transmission and initiating said transmission of a second message
address signals for said second predetermined time period during
said first predetermined time period after termination of said
first message, developing said audio signals of said second message
simultaneously with transmission of said second message address
signals, delaying said audio signals of said second message by said
third predetermined time period after said second actuation of said
switch.
21. The method as set forth in claim 20 wherein said address
signals are selectable from a plurality of address signals and
including the step of selecting the desired address signals for
transmission.
22. The method as set forth in claim 21 wherein said address
signals are subaudible tone signals and including the step of
selecting the desired subaudible tone signals for transmission.
23. A method for rapidly transmitting a plurality of messages in
sequence in a paging system wherein each message includes address
signals having a first predetermined time period and audio signals,
and wherein a transmitter transmits said signals said transmitter
being initiated by actuating a switch, and the message terminated
by deactuating the switch, including the steps of:
a. transmitting a first message including said address signals
followed by said audio signals;
b. transmitting the address signals of a second message
simultaneously with a terminal portion of the audio signals of said
first message;
c. transmitting the audio signals of said second message after the
audio signals of said first message and the address signals of said
second message.
24. A method for rapidly transmitting a plurality of messages in
sequence in a paging system wherein each message includes address
signals having a first predetermined time period and audio signals,
and wherein a transmitter transmits said signals, said transmitter
being initiated by actuating a switch, and the message terminated
by deactuating the switch, including the steps of:
a. actuating said switch for actuating said transmitter and
coupling said address signals and audio signals to said
transmitter;
b. delaying transmission of said audio signals by a second
predetermined time period at least as long as said first
predetermined time period;
c. deactuating said switch;
d. maintaining said transmitter actuated for third predetermined
time period at least as long as said second predetermined time
period;
e. actuating said switch a second time during said third
predetermined time period;
f. coupling address signals and audio signals of a second message
to said transmitter during said third predetermined time
period;
g. delaying transmission of said audio signals of said second
message by a second predetermined time period.
Description
BACKGROUND
Paging systems currently employed which transmit audio messages to
the desired paging receiver, contact the desired paging receiver by
first transmitting an address signal for selecting the desired
paging receiver. The audio message is transmitted immediately
following the address signal. The address signals generally consist
of one or more audio frequency tone signals, which may be
simultaneously or sequentially transmitted for a predetermined
period of time. The address signals in most systems are transmitted
for one to four seconds, whereas the audio signals generally extend
from four to eight seconds. At the desired receiver, the audio
frequency tones are received and detected, causing actuation of the
pager receiver audio amplifier. With the audio amplifier actuated,
the following demodulated audio signals can be heard by the
user.
In systems such as that described above, the address portion of the
message accounts for one-third to one-half of the total time
necessary to transmit a message. As the number of users in a paging
system increases, a point will be reached when the system reaches
full capacity. A maximum number of messages can then be transmitted
during a one hour period, with messages being transmitted
continuously. During the one hour period, one-third to one-half of
the time is spent transmitting the address portion. Transmitting an
address portion of a message prior to the audio portion therefore,
seriously limits the number of message transmissions per hour time
period, and the system message capacity.
SUMMARY
It is, therefore, an object of this invention to provide a high
capacity paging system.
Another object of this invention is to provide a high capacity
paging system which requires substantially less time for address
portion transmission, and allows substantially more time for audio
portion transmissions.
Yet another object of this invention is to provide a high capacity
paging system employing subaudible tone signals for the address
portion.
Still another object of this invention is to provide a high
capacity paging system wherein the address portion is transmitted
during the audio portion of a preceding message.
In practicing this invention, a high capacity paging system is
provided which includes a transmitter for transmitting a modulated
signal in response to a message, including address signals and
audio signals. The address signals are two sequentially transmitted
subaudible tone signals. Each transmission is initiated by
actuating a push-to-talk switch, and the message terminated by
deactuating the push-to-talk switch. Control circuitry is coupled
to the push-to-talk switch and the transmitter and is operative in
response to actuation of the push-to-talk switch to actuate the
transmitter. A monostable multivibrator in the control circuitry
maintains the control circuitry in an operative condition for a
predetermined time period after deactuation of the push-to-talk
switch.
Address circuitry is provided for developing the sub-audible tone
signals in the address portion. Selectors are provided for manually
selecting the tones necessary to form a desired address. The
address circuitry is coupled to the push-to-talk switch and the
transmitter, and is operative in response to the push-to-talk
switch actuation to couple the subaudible tone signals to the
transmitter for a second predetermined time period, no greater than
the first predetermined time period. monostable multivibrators in
the address circuitry provide the necessary timing for the second
predetermined time period.
An endless loop tape recorder is also provided and is coupled to
the microphone, transmitter and control circuitry. The tape
recorder is operative in response to operation of the control
circuitry to record the audio signals coupled from the microphone.
The recorded audio signals are reproduced at the end of the first
predetermined time period and these delayed audio signals are
coupled to the transmitter for transmission thereby.
THE DRAWINGS
FIG. 1 is a simplified block diagram of a high capacity paging
system in accordance with this invention;
FIG. 2 is a timing diagram which shows the timing for the various
portions of the message;
FIG. 3 is a combined schematic and block diagram of a high capacity
paging system in accordance with this invention.
DETAILED DESCRIPTION
Referring to FIG. 1, a simplified block diagram for a high capacity
paging system is shown, which transmits messages including
subaudible tone signals followed by audio signals. Tone selector
and tone oscillator 10 contains a plurality of tone oscillators for
developing the desired subaudible tone signals. These subaudible
tone signals extend in a range from 65 Hz to 250 Hz. A plurality of
tone selectors are also provided in tone selector and tone
oscillator 10, which allow manual selection of the desired tone
oscillators and tones for the address portion of the message.
Actuation of particular tone selectors will allow the desired tones
to be transmitted when a message is initiated.
Push-to-talk switch 11 is connected to control circuit 13. Upon
action of push-to-talk switch 11, control circuit 13 operates, and
causes the actuation of recorder 14 and transmitter 15. Transmitter
15, when actuated, develops a radio frequency (RF) signal, which
may be modulated by subaudible tone signals and audio signals. The
RF signal is transmitted via antenna 16 to a portable paging
receiver such as that shown at 17.
Push-to-talk switch 11 is also coupled to tone selector and tone
oscillator 10. Upon each actuation of push-to-talk switch 11, the
selected subaudible tone signals for the address portion of a
message are coupled from tone selector and tone oscillator 10 to
transmitter 15. The subaudible tone signals modulate the
transmitter RF carrier, and are radiated via antenna 16 to the
desired portable paging receiver 17. FIG. 2 waveform A shows the
timing for the subaudible tone signals. In the embodiment shown,
two tones are sequentially transmitted. The two tones have a total
time period of approximately 4 seconds.
Speech information to be transmitted is coupled to microphone 12
where it is converted into audio signals. These audio signals are
coupled from microphone 12 to recorder 14. FIG. 2, waveform B,
shows the timing for the audio signals coupled to recorder 14.
Recorder 14 is an endless loop type tape recorder which is capable
of recording the audio signals coupled from microphone 12 and
reproducing the same audio signals a predetermined time period
after they are recorded. The delay must be as long as, or longer
than, the time period necessary to transmit the tones from tone
selector and tone oscillator 10.
Control circuit 13 includes a time delay circuit 18. Upon
termination of the message, push-to-talk switch 11 is deactuated.
Each deactuation of push-to-talk switch 11 causes delay circuit 18
to maintain control circuit 13 in an operative condition for a
predetermined time period. The predetermined time period for which
control circuit 13 is maintained operative is the same as the
predetermined time period for which the audio signals reproduced by
recorder 14 are delayed. Maintaining control circuit 13 operative
for this predetermined time period after deactuation of
push-to-talk switch 11 allows recorder 14, and transmitter 15, to
remain actuated during this time period, and thereby allows the
delayed audio signals to be coupled from recorder 14 to transmitter
15 where they modulate the RF carrier signal and are transmitted to
portable paging receiver 17. FIG. 2 waveform C shows the timing for
the delayed audio signals coupled to transmitter 15.
In operation, a calling sequence is initiated by an operator
actuating push-to-talk switch 11 after selection of the desired
subaudible tone signals in the address. This causes the subaudible
tones to be coupled from tone selector and tone oscillator 10 to
transmitter 15. Two subaudible tones are sequentially transmitted
during the four second time interval immediately following
actuation of push-to-talk switch 11 as shown in FIG. 2, waveform A.
Shortly after actuation of push-to-talk switch 11, the operator
begins speaking into microphone 12. Microphone 12 develops audio
signals in response to the operator's speech, and couples the audio
signals to recorder 14 where they are recorded, then reproduced a
predetermined period of time after recordation. These delayed audio
signals are shown in FIG. 2 waveform C. The audio signals are
delayed by a period of time greater than that necessary to transmit
the subaudible tone signals as shown in FIG. 2, waveforms A and C.
These delayed audio signals are coupled from recorder 14 to
transmitter 15 where they modulate the RF carrier signal. While the
operator is speaking itno microphone 12 she selects the desired
address for the next portable paging receiver to be contacted by
actuating the proper tone selectors. Upon termination of the first
message, push-to-talk switch 11 is deactuated, then again actuated
to start a second message. When push-to-talk switch 11 is again
actuated the delayed audio signals in the first message are still
being coupled to transmitter 15 for modulation of the RF carrier
signal, due to the operation of delay circuit 18. Upon reactuation
of push-to-talk switch 11 the subaudible tone signals for the
second message are coupled to transmitter 15 where they modulate
the transmitter RF carrier signal simultaneously with the delayed
audio signals from recorder 14. FIG. 2, waveforms A and C shows the
simultaneous presence of the address portion of the second message,
and the delayed audio portion of the first message at transmitter
15. Simultaneously with the transmission of the tones for the
second message, the operator again begins speaking into microphone
12, reciting the information in the audio signal portion of the
second message. Microphone 12 converts the speech to audio signals
and couples them to recorder 14.
The subaudible tone signals for the second message terminate
shortly after the delayed audio signals from the preceding message
as shown in FIG. 2. Shortly after termination of the subaudible
tone signals from the second message, the delayed audio signals for
this second message modulate the RF carrier signal developed by
transmitter 15 and are radiated by antenna 16 to the desired
portable paging receiver 17. The subaudible tone signals for each
succeeding message are, therefore, transmitted simultaneously with
a terminal portion of the audio signal portion of a preceding
message. By transmitting the subaudible tone signals simultaneously
with the audio signals in a preceding message, the time period
necessary for transmitting an entire message is substantially
reduced, thereby allowing a greater system call capacity.
Portable paging receivers such as shown by paging receiver 17 are
designed to have an audio response characteristic between 300 and
3000 cycles. That is, audio signals in the 300 to 3000 cycle range
will be reproduced by the speaker and heard by the portable paging
receiver user. Signals below 300 cycles and above 3000 cycles are
greatly attenuated by the paging receiver and will not, therefore,
be reproduced by the speaker nor heard by the user. This audio
response characteristic is common to all portable paging receivers
presently in use and is required by Federal Communications
Regulations. All frequencies below 300 cycles are therefore
considered in radio pager applications to be subaudible signals.
The subaudible tone signals developed by tone selector and tone
oscillator 10 extend in a range from 65 to 250 cycles, as
previously mentioned. If the subaudible tone signals in the address
portion of a succeeding message are transmitted simultaneously with
the terminal portion of the audio signals of a preceding message,
the user of the portable paging receiver hearing the preceding
audio message will not hear the subaudible tones. He will not,
therefore, be disturbed by the addressing of the succeeding pager
to be contacted while he is receiving his audio message.
Referring to FIG. 3, there is shown a more detailed combined
schematic and block diagram for the high capacity paging system.
Oscillators 23 are the oscillators in tone selector and tone
oscillator 10 which develop the subaudible tone signals. In the
preferred embodiment 10 oscillators are provided, however, the
system may be increased to 30 oscillators, or as many as are
desired.
Two columns of switches are also shown, the switches in one column
being labelled 24 and those in the other column being labelled 25.
Switches 24 and 25 are the selectors in tone selector and tone
oscillator 10, for selecting the two subaudible tone signals which
form the address portion of the message to be transmitted. In the
preferred embodiment, ten switches labelled as 24 are provided, and
ten switches labelled as 25 are provided. One of the switches
labelled 24 is manually actuated to select any one of the 10
subaudible tone signals for the first transmitted tone; and one of
the switches 25, is manually actuated to select any one of the
remaining nine subaudible tone signals for the second tone. If the
system capacity is increased to thirty tones, additional switches
are provided. Actuation of one switch 24 and one switch 25, will
couple the subaudible tone signals from oscillators 23 through
switches 24 and 25 to tone gates 26 and 27, respectively. The
operation of tone gates 26 and 27 will be further explained in a
subsequent portion of this application.
Push-to-talk switch 11 is shown including two form A contacts,
labelled 29 and 30. Upon actuation of push-to-talk switch 11, a
ground potential is coupled through form A contact 29 to control
circuit 13.
In control circuit 13 the ground potential is coupled through diode
31 to one terminal of control relay 32, causing the relay to
energize and close form A contacts 33 and 34. The closure of
contact 33 provides a ground potential through contact 33 to
transmitter 15, causing the transmitter to actuate and develop a
radio frequency carrier signal which is coupled to antenna 16. The
radio frequency carrier signal is radiated by antenna 16 to the
portable paging reciever 17. The closure of contact 34 provides a
ground potential through contact 34 to relay 36 in recorder 14,
causing relay 36 to energize and close form A contact 38. The
closure of form A contact 38 couples an AC signal from AC plug 37
through contact 38 to tape drive motor 39, causing the motor to
rotate. Actuation of tape drive motor 39 causes the endless loop
magnetic tape 40 in recorder 14 to begin to rotate and record,
playback and erase any information coupled thereto.
Actuation of push-to-talk switch 11 also couples an A+ signal
through contact 30 to differentiator circuit 43 in tone selector
and tone oscillator 10. This differentiator circuit, consisting of
resistors 44, 45 and 46, capacitor 47 and diode 48, produces a
positive going pulse in response to the A+ signal coupled thereto.
The positive going pulse is coupled from diode 48 in differentiator
circuit 43 to monostable multivibrator 49 in tone selector and tone
oscillator 10. Multivibrator 49 is a standard type multivibrator,
commonly used in the art, consisting of a first transistor 50 and a
second transistor 51. Transistor 50 is normally nonconductive or
"off," and transistor 51 is normally conductive or "on." The
positive going pulse from differentiator circuit 43 is coupled to
base 52 of transistor 50 causing transistor 50 to switch to a
conductive or "on" state. Transistor 51, in response to the change
of state of transistor 50, switches to a non-conductive or "off"
state, causing collector electrode 53 of transistor 51 to increase
positively in potential towards supply voltage. Monostable
multivibrator 49 will remain in this unstable, switched state for a
predetermined period of time, determined by the period of time
required to charge capacitor 54. In the embodiment shown,
monostable multivibrator 49 will remain in this switched state for
approximately 1.3 seconds. While in this switched state the
positive voltage developed at collector 53 of transistor 51 in
monostable multivibrator 49 is coupled to tone gate 26.
Tone gate 26 is a gate for coupling the first subaudible tone from
oscillators 23 and switches 24 to transmitter 15. Tone gate 26 may
consist of a single transistor, having base, emitter and collector
electrodes which is biased to operate as a switch. A typical switch
configuration which has the emitter electrode connected to switches
24, the collector electrode to transmitter 15, and the base
electrode to collector electrode 53 of transistor 51.
Upon application of the positive voltage from collector 53 of
transistor 51 to tone gate 26, the tone gate would be rendered
conductive, allowing the first subaudible tone signal to be coupled
to transmitter 15. Tone gate 26 will remain conductive for the same
period of time as monostable multivibrator 49 remains in its
switched or unstable state, allowing the subaudible tone signal to
modulate the RF carrier signal at transmitter 15 for that period of
time.
When monostable multivibrator 49 reverts to its stable state,
transistor 51 will revert to a conductive condition, and transistor
50 will revert to a nonconductive condition, causing the voltage at
collector 57 of transistor 50 to increase towards supply potential.
Capacitor 58, resistors 59 and 60, and diode 61 form a
differentiation circuit which develops a positive going pulse in
response to the increased positive voltage developed at collector
57 of transistor 50. This positive going pulse is coupled to base
electrode 62 of transistor 63 in a second monostable multivibrator
65, in tone selector and tone oscillator 10.
The operation of monostable multivibrator 65 is identical to
monostable multivibrator 49. It will change to an unstable state in
response to the positive going pulse at base 62 of transistor 63,
and will remain in this unstable state for a predetermined period
of time determined by the value of capacitor 66. In the preferred
embodiment, monostable multivibrator 65 will remain in an unstable
state for approximately 2.5 seconds. When in an unstable state
transistor 63 is rendered conductive and transistor 67 is rendered
nonconductive, causing the voltage at collector 68 of transistor 67
to approach supply potential. This voltage is coupled to tone gate
27, which is identical to tone gate 26, allowing the second
subaudible tone signal in the address portion of the message to be
coupled from oscillators 23 and switches 25 to transmitter 15 where
it modulates the RF carrier signal. Tone gate 27 just as tone gate
26 will allow the subaudible tone signal to be coupled to
transmitter 15 for the period of time that monostable multivibrator
65 remains in its unstable state. Monostable multivibrators 49 and
65 and tone gates 26 and 27 in tone selector and tone oscillator 10
therefore allow the selected two subaudible tone signals in the
address portion of the message to be coupled to transmitter 15 for
a period of time of approximately four seconds, upon actuation of
push-to-talk switch 11.
After actuation of push-to-talk switch 11, and during the interval
that the subaudible tone signals in the address portion are being
transmitted by transmitter 15 the operator will speak the
information into microphone 12 where it is converted into audio
signals. Again, the timing of the audio signal portion of the
message is shown by waveform B, FIG. 2. The audio signals are
coupled to input amplifier 73 in recorder 14, where they are
amplified and coupled to a recording head 74. Recording head 74
causes the audio signals to be recorded on magnetic tape 40. A
reproduction or playback head 75, is physically positioned in
recorder 14 so that a point passing record head 74 will pass
playback head 75 four seconds later. This time period is selected
to be the same as the time period for which transmitter 15 and
recorder 14 remain operative after push-to-talk switch 11 has been
released, and is as long as, or longer than the time period
necessary to transmit the subaudible tone signals in the address
portion of the message. The audio signals recorded on magnetic tape
40 by recording head 74 are therefore reproduced by playback head
75 4 seconds after they are recorded, producing a delayed audio
signal. The delayed audio signals are amplified in output amplifier
76, and coupled to transmitter 15 where they modulate the RF
carrier signal which is radiated at antenna 16. The delayed audio
signals reproduced by playback head 75 are erased by erasing head
77, positioned after playback head 75, so that audio signals in a
later portion of the message, or the audio signals for a subsequent
message may be recorded on magnetic tape 40 by recording head
74.
The delay provided by recorder 14 allows the address portion of the
first message in a sequence of messages to be transmitted prior to
the audio signal portion. The audio signal portion of the message
is, however, coupled to tape recorder 14 at the same time that the
address portion is being transmitted, eliminating the time period
required in previous systems during which no audio signals were
developed. During the time that the audio signals are developed,
and after completion of the address signal, the operator may select
the address for subsequent message and operate the corresponding
switches 23 and 24.
Upon termination of a message the operator will deactuate
push-to-talk switch 11. When push-to-talk switch 11 is deactuated,
the ground potential supplied through form A, contact 29 for
operation of control circuit 13 is removed. Removal of this ground
potential causes a positive pulse to be developed by capacitor 80
in the delay circuit 18 of control circuit 13, which is coupled
through resistor 79 to base electrode 81 of transistor 82 in
monostable multivibrator 83, also in delay circuit 18. Monostable
multivibrator 83 includes transistors 82 and 84, and will switch to
an unstable state for a predetermined amount of time in response to
a positive pulse applied to base electrode 81 of transistor 82. In
the stable state transistor 82 is nonconductive and transistor 84
is conductive. In the unstable state transistor 82 is rendered
conductive and transistor 84 is rendered nonconductive. The time
period for which the monostable multivibrator 83 remains in this
unstable state is determined by the value of capacitor 85. In the
preferred embodiment, this time period is equal to the time period
for which the audio signals are delayed by recorder 14, and must be
greater than the time period necessary to transmit the subaudible
tone signals in the address portion of the message. As the recorder
14 delays the audio signals coupled thereto by 4 seconds,
monostable multivibrator 83 must remain in its unstable state for
four seconds.
When monostable multivibrator 83 is in its unstable state the
voltage at collector 86 of transistor 82 will approach ground
potential. This ground potential is coupled to base 87 of
transistor 88, rendering transistor 88 nonconductive. With
transistor 88 nonconductive, the voltage at collector 89 of
transistor 88 increases and is coupled to base 90 of transistor 91,
rendering transistor 91 conductive. With transistor 91 conductive,
the ground potential at emitter 92 of transistor 91 is coupled to
collector 93 of transistor 91 and through diode 94 to control relay
32, causing relay 32 to remain in an actuated condition during the
time period that monostable multivibrator 83 remains in its unstab
e state. With relay 32 held in an actuated condition, recorder 14
and transmitter 15 will also be held in an actuated condition for
the time period that monostable multivibrator 83 remains in its
unstable state. This allows the last four seconds of the audio
signals coupled to recorder 14 to be reproduced and coupled to
transmitter 15 for modulating the RF carrier signal. When
monostable multivibrator 83 reverts to its stable state, the ground
potential for actuating control relay 32 will be removed allowing
recorder 14 and transmitter 15 to deactuate.
During the interval, after deactuation of push-to-talk switch 11,
and while transmitter 15 and recorder 14 remain actuated, the
operator can again actuate push-to-talk switch 11. As each
actuation of push-to-talk switch 11 will cause the subaudible tone
signals in the address portion of a message to be transmitted, the
address portion of a subsequent message will be coupled to
transmitter 15 and transmitted simultaneously with a portion of the
delayed audio signals. The reactuation of push-to-talk switch 11
will cause monostable multivibrator 83 to revert to its stable
state; however, it will also provide a ground potential which will
maintain control relay 32 in an operative condition, thereby
maintaining transmitter 15 and recorder 14 in an operative
condition so as to allow the audio signals in the subsequent
message to be immediately coupled to the recorder.
As can be seen, a high capacity paging system has been provided
which requires substantially less time for the address portion
transmissions and allows substantially more time for audio portion
transmission so as to allow an increased system message
transmission capability. This is accomplished by transmitting
subaudible tone signals for the address portion simultaneously with
the transmission of the audio portion of a preceding message, and
by providing a delay in the audio message so as to facilitate
transmission of the subaudible tone signals for a succeeding
message during the terminal portions of the audio signal portion of
a preceding message.
* * * * *