Battery saver for a tone coded signalling system

I claim:

1. A multiple alert receiver operable in response to receiving a predetermined tone code, said receiver comprising:

receiver means for receiving transmitted signals;

detector means coupled to said receiver means for indicating a detection in response to said predetermined code being received by said receiver means; and

alert means coupled to said receiver means and said detector means for selecting and generating at least one of several alert signals in response to said detector means indicating a code detection and an additional signal being received by said receiver means, the additional received signal determining which of said several alert signals will be generated;

wherein the predetermined code includes at least one tone and said detector means includes circuitry for searching for said tone within a first time window duration and wherein said alert means includes circuitry for searching for said additional signal within a second time window duration, occurring after the code detection, which is greater than said first time window duration, whereby the receiver will be more likely to select a correct alert signal after said predetermined code has been identified.

2. A multiple alert receiver according to claim 1 wherein said additional signal includes at least one tone.

3. A multiple alert receiver operable in response to receiving a predetermined signal code, said receiver comprising:

receiver means for receiving transmitted signals;

detector means coupled to said receiver means for indicating a detection in response to said predetermined signal code being received by said receiver means; and

alert means coupled to said receiver means and said detector means for selecting and generating at least one of several alert signals in response to said detector means indicating a code detection and an additional signal being received by said receiver means, the additional received signal determining which of said several alert signals will be generated;

wherein the predetermined code includes at least a first predetermined signal and said detector means includes circuitry for searching for said first predetermined signal within a first time window duration and wherein said alert means includes circuitry for searching for said additional signal within a second time window duration, occurring after the code detection, which is greater than said first time window duration, whereby the receiver will be more likely to to select a correct alert signal after said predetermined code has been identified.

4. A multiple alert receiver operable in response to receiving a predetermined tone code, said receiver comprising:

receiver means for receiving transmitted signals;

detector means coupled to said receiver means for indicating a detection in response to said predetermined code being received by said receiver means; and

alert means coupled to said receiver means and said detector means for selecting and generating at least one of several alert signals in response to said detector means indicating a code detection and an additional signal being received by said receiver means, the additional received signal determining which of said several alert signals will be generated;

wherein the predetermined code includes at least one tone and said detector means includes circuitry for searching for said tone within a first time window and wherein said alert means includes circuitry for searching for said additional signal within a second time window, occurring after the code detection, said second time window being sufficient to permit detection of said additional signal having a duration greater than said one tone whereby the receiver will be more likely to select a correct alert signal after said predetermined code has been identified.

5. A multiple alert receiver operable in response to receiving a predetermined signal code, said receiver comprising:

receiver means for receiving transmitted signals;

detector means coupled to said receiver means for indicating a detection in response to a predetermined signal code being received by said receiver means; and

alert means coupled to said receiver means and said detector means for selecting and generating at least one of several alert signals in response to said detector means indicating a code detection and an additional signal being received by said receiver means, the additional received signal determining which of said several alert signals will be generated;

wherein the predetermined code includes at least a first predetermined signal and said detector means includes circuitry for searching for said first predetermined signal within a first time window and wherein said alert means includes circuitry for searching for said additional signal within a second time window, occurring after the code detection, said second time window being sufficient to permit detection of said additional signal having a duration greater than said first predetermined signal whereby the receiver will be more likely to select a correct alert signal after said predetermined code has been identified.

6. A multiple alert receiver operable in response to receiving a predetermined signal code, said receiver comprising:

receiver means for receiving transmitted signals;

detector means coupled to said receiver means for indicating a detection in response to a predetermined signal code being received by said receiver means, said predetermined signal code comprising a predetermined sequence of a plurality of individual code signals; and

alert means coupled to said receiver means and said detector means for selecting and generating at least one of several alert signals in response to said detector means indicating a code detection and an additional signal being received by said receiver means after said signal code, the additional received signal determining which of said several alert signals will be generated;

wherein the predetermined code includes at least a first predetermined signal and said detector means includes circuitry for creating a first time window and for searching for said first predetermined signal within said first time window, and wherein said alert means includes circuitry for searching for said additional signal within a second time window, occurring after the predetermined code detection, said second time window being created in response to the detection of said predetermined code, wherein said alert means generates a predetermined one of said alert signals in response to the detection of said additional signal within said second time window, and a different predetermined one of said alert signals in response to the detection of any other signal condition, including the absence of a signal, during said second time window, whereby the identification of a predetermined signal code will insure the generation of an alert signal and the existence of a subsequent additional signal condition determines which alert signal will be generated.

7. A multiple alert receiver according to claim 6 wherein said second time window has a duration greater than the duration of said first time window, whereby the receiver will be more likely to select a correct alert signal after said predetermined code has been identified.

8. A multiple alert receiver according to claim 6 wherein said additional signal is a unique signal other than said predetermined plurality of individual code signals from which said sequence of code signals are selected, whereby said receiver is less likely to produce a false code detection by identifying said additional signal as part of said predetermined signal code.

9. A multiple alert receiver according to claim 7 wherein said additional signal is a unique signal other than said predetermined plurality of individual code signals from which said sequence of code signals are selected, whereby said receiver is less likely to produce a false code detection by detecting said additional signal as part of said predetermined code.

10. A multiple alert receiver according to any of claims 6-9 wherein said additional signal comprises a single predetermined tone signal having a predetermined frequency.

11. A multiple alert receiver according to any of claims 6-9 wherein said additional signal comprises a single predetermined tone signal having a predetermined frequency, and the predetermined signal code comprises a tone code comprising a plurality of tone signals having different frequencies.

12. A multiple alert receiver system operable in response to a predetermined signal code, said receiver system comprising:

signal source means for providing said predetermined signal code followed by an additional signal, said predetermined signal code comprising a predetermined sequence of a plurality of individual code signals selected from a predetermined plurality of individual code signals;

receiver means for receiving said signals provided by said signal source means;

detector means coupled to said receiver means for indicating a detection in response to said predetermined signal code being received by said receiver means; and

alert means coupled to said receiver means and said detector means for selecting and generating at least one of several alert signals in response to said detector means indicating said signal code detection and an additional signal being received by said receiver means after predetermined said signal code, the additional received signal determining which of said several alert signals will be generated;

wherein the predetermined signal code includes at least a first individual predetermined signal having a first duration and said detector means includes circuitry for creating a first time window and for searching for said first predetermined signal within said first time window, and wherein said additional signal has a second duration, greater than said first duration, and wherein said alert means includes circuitry for creating a second time window in response to the detection of said predetermined signal code and for searching for said additional signal within said second time window occurring after the code detection, wherein said alert means generates a predetermined one of said alert signals in response to the detection of said additional signal within said second time window, said alert means generating a different predetermined one of said alert signals in response to the absence of the detection of said additional signal during said second time window, whereby the identification of said predetermined signal code will insure the generation of an alert signal and the existence of a subsequent additional signal determines which alert signal will be generated.

13. A multiple alert receiver system according to claim 12 wherein said second time window has a duration greater than the duration of said first time window, whereby the receiver will be more likely to select a correct alert signal after said predetermined signal code has been identified.

14. A multiple alert receiver system according to claim 12 wherein said additional signal is a unique signal other than said predetermined plurality of individual code signals from which said sequence of code signals are selected, whereby said receiver is less likely to produce a false code detection by identifying said additional signal as part of said predetermined signal code.

15. A multiple alert receiver system according to claim 14 wherein said additional signal is a unique signal other than said predetermined plurality of individual code signals from which said sequence of code signals are selected, whereby said receiver to less likely to produce a falst code detection by detecting said additional signal as part of said predetermined signal code.

16. A multiple alert receiver system according to any of claims 12-15 wherein said additional signal comprises a single predetermined tone signal having a predetermined frequency.

17. A multiple alert receiver system according to any of claims 12-15 wherein said additional signal comprises a single predetermined tone signal having a predetermined frequency, and the predetermined signal code comprises a tone code comprising a plurality of tone signals having different frequencies.

18. A multiple alert function receiver with battery saver circuitry operable in response to receiving a predetermined signal code which includes an initial preamble code signal, an address code signal and an additional signal for determining which of the multiple alert functions will be selected, said receiver comprising:

receiver means for receiving transmitted signals;

power supply means coupled to said receiver means for selectively supplying operating power thereto;

control means, including detector means, coupled to said power supply means for selectively controlling the same,

said control means including,

first circuit means for enabling said power supply means to periodically supply operating power to said receiver means for a first predetermined time duration, and

second circuit means for enabling said power supply means to supply operating power to said receiver means for a second time duration subsequent to and in response to detecting at least said preamble code signal; and

third circuit means coupled to said receiver means for selecting and generating at least one of several alert functions in response to detecting said additional signal subsequent to detecting said preamble code signal and said address code signal.

19. A multiple alert function receiver according to claim 18 which includes means for searching for said address signal more than once after a detection of said preamble code signal and prior to another detection of said preamble code signal.

20. A multiple alert function receiver according to any of claims 18 or 19 wherein said third circuit means includes circuitry for searching for said additional signal within a time window occurring after detection of said address signal, wherein said third circuit means generates a predetermined alert function in response to the detection of said additional signal after detection of said address signal, said third circuit means generating a different predetermined alert function in response to a signal condition other than the detection of said additional signal after said address signal.

21. A multiple alert function receiver according to claim 20 wherein said preamble code signal, said address code signal and said additional signal comprise information signals which modulate the transmitted signals received by said receiver means and wherein said receiver means includes demodulation apparatus for demodulating said transmitted signals.

22. A multiple alert function receiver according to any of claims 18 or 19 wherein said preamble code signal, said address code signal and said additional signal comprise information signals which modulate the transmitted signals received by said receiver means and wherein said receiver means includes demodulation apparatus for demodulating said transmitted signals.

23. A radio receiver for receiving and processing predetermined code signals and having a battery saver circuit, said receiver comprising:

receiver means for receiving code signals; and

control means for detecting said code signals and controlling the receiver means in response thereto,

said control means including,

means for detecting at least a first preamble code signal to selectively control the operation of the battery saver circuit,

means for detecting a second predetermined code signal subsequent to the detection of said first code signal,

means for detecting a third code signal subsequent to the detection of said second code signal, and

means for again detecting said second predetermined code signal at least in the absence of detecting said third predetermined code signal.

24. A radio receiver according to claim 23 wherein said means for again detecting said second predetermined code signal comprises mens for detecting said second predetermined code signal after at leat a previous detection of said second code signal and prior to another detection of said first code signal.

25. A radio receiver according to any claims 23 or 24 which includes means for providing an alert in response to at least the detection of said third code signal after prior detections of said first and second code signals.

26. A radio receiver according to claim 25 wherein said first, second, and third code signals are received by said receiver means as information signals which modulate a carrier signal received by said receiver means, and wherein said receiver means includes demodulator means for receiving said modulated carrier signal and providing said received code signals in response thereto.

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BACKGROUND OF THE INVENTION

The present invention relates generally to battery saver circuits and, in particular, to an improved battery saver circuit especially suited for use in personal paging receivers intended for sequential tone code operation. Such paging receivers produce a single alert signal in response to the reception of a predetermined code.

Battery saving circuits are used to minimize power consumption by periodically supplying power to a receiver in short bursts instead of continuously. Presently known circuits operating in radio receivers periodically supply power, search for the presence of an RF (radio frequency) carrier and then, if a carrier is found, extend the time that power is supplied to permit a further search for a predetermined sequential tone code. Such squelch operated battery savers have a significant disadvantage in that every receiver within the system is activated whenever any transmission of a carrier occurs, regardless of which individual paging receiver is intended to be selectively reached. If paging signals are continuously being broadcasted by a transmitter, this type of battery saving circuit will not save any power since all of the individual receivers will be on all of the time.

A previous partial solution has been to provide battery saving circuits which extend the time that power is applied to the receiver only after a first predetermined tone has been received. In this type of circuit, the first tone (preamble) must be of a sufficiently long duration such that the periodic supplying of power to the receiver will always result in a detection of this first tone, regardless of when this first tone begins. These circuits then proceed to look for the rest of a predetermined sequential code. Whenever the next proper sequential tone is not detected, the preamble tone is again searched for. A disadvantage of such a system, however, is that each individual message must be preceded by the preamble tone which is the first tone in each sequential code. When many messages are to be transmitted, this results in a substantial increase in the total amount of broadcast time. Consequently, the power drain on all receivers having the same first code (preamble) tone is increased and the total number of messages which can be broadcasted in any given time interval is severely limited. It is also possible for such prior battery saver circuits to mistakenly identify a subsequent tone as the first (preamble) tone of the code, and thus incorrectly apply power to a radio receiver.

SUMMARY OF THE INVENTION

Accordingly, one object of the present invention is to provide an improved battery saver circuit which overcomes the aforementioned deficiencies.

A more particular object of the invention is to provide an improved battery saver circuit for a tone coded receiver in which subsequent tones cannot be mistaken for the preamble or first code tone.

Another object of the invention is to provide an improved battery saver circuit for a receiver which receives a series of several individual tone coded messages collectively preceded by a single tone coded preamble.

Still another object of the invention is to provide a multiple alert receiver which selects and generates one of several alert signals in response to the reception of a predetermined code and an additional signal.

The present invention contemplates an improved battery saver circuit for use with a personalized radio pager operable on receipt of a predetermined sequential tone code comprising a plurality of separate tones. The receiver produces corresponding demodulated tone signals at an output reference terminal. A power supply apparatus is coupled to the receiver for selectively supplying power thereto for predetermined durations of time in response to signals from a control circuit. A switchable frequency detector is coupled to the receiver output terminal for detecting when the demodulated tone signals have a predetermined frequency which is selected by "select" signals that are likewise produced by the same control circuit. The control circuit periodically enables the power supply apparatus to supply the receiver with power for a first predetermined time duration and also causes the detector to select a first frequency which corresponds to the first tone of the sequential code. When the first tone is detected during one of these first time durations, the control circuit enables the power apparatus to supply power for a second predetermined time duration and also causes a second frequency to be selected which corresponds to the second tone of the sequential code. When the second tone is properly detected, the control circuit causes power to be supplied for a third time duration and selects the third tone of the code. When this third tone is not detected within a predetermined time window after the second tone is detected, the control circuit then reselects the second code tone.

Accordingly, the battery saver sequentially searches for a first, second, and third tone of a predetermined code. If the first two tones are properly detected but the third tone is not properly detected, the battery saver will then search again for the second tone. By searching for the second tone when a third proper detection is not made, rather than searching for the first tone, a string of individual messages proceeded by a single preamble tone can be used to signal a number of different paging receivers each having the same first tone (preamble) as the first tone of its code.

Another aspect of the invention is that the first code tone must be detected several times before the second code tone is searched for. This insures that the first code tone must exist for at least a minimum period of time. When this minimum period of time is greater than the time period of any subsequent code tone, a subsequent tone cannot be identified as the initial preamble tone. Also by searching for a first tone rather than a carrier signal, only receivers having this first tone as part of their sequential code will have battery power applied to their receivers and switchable filters for an extended duration.

Still another aspect of the invention is that one of several alert signals is selected and generated in response to an additional signal being received after the entire predetermined code has been detected.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the invention reference should be made to the drawings, in which:

FIG. 1A is a block diagram of a battery saving circuit for a personal radio pager;

FIG. 1B is a chart labeling some of the interconnecting lines illustrated in FIG. 1A;

FIG. 2A is a graph representing a number of tone coded messages proceeded by a tone coded preamble;

FIG. 2B is a graph representing power which is periodically supplied to a receiver;

FIG. 2C is a graph representing power which is supplied to a receiver in response to the sequential detection of predetermined tones;

FIG. 2D is a chart of the typical time values for the wave forms shown in FIGS. 2A-2C;

FIG. 3 is a schematic diagram of one of the component blocks illustrated in FIG. 1A;

FIG. 4 is a schematic diagram of another one of the component blocks shown in FIG. 1A;

FIG. 5 is a schematic diagram of still another one of the component blocks illustrated in FIG. 1A;

FIG. 6 is another schematic diagram of another one of the component blocks illustrated in FIG. 1A; and

FIG. 7, shown on separate pages as FIG. 7A and FIG. 7B, is a combined schematic diagram of two of the component blocks illustrated in FIG. 1A.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, a block diagram of a battery saver circuit 10 is shown generally in FIG. 1A. This circuit has been constructed in accordance with the present invention and is particularly adapted for use in a personal radio pager. The circuit 10 includes an antenna 11 coupled to a conventional radio receiver apparatus 12 which includes a modulation output reference terminal, indicated at 13. This terminal is coupled to a switchable frequency tone filter and detector 14. A power supply apparatus 15 (shown dashed) is coupled to receiver apparatus 12 and to filter 14 and supplies operating power thereto. A control circuit 16 (shown dashed) is coupled to the filter 14 and to the power supply 15 for the selective control thereof as will be more fully described subsequently.

The supply circuit 15 includes a source of power, such as battery 15a. The battery is connected to a semiconductor switch 15b which receives control signals from control circuit 16 on an interconnecting line PC. The power supply 15 supplies operating power for rendering receiver apparatus 12 and filter 14 operative in response to the signals received on the referenced line PC. Semiconductor switch 15b functions as a solid state relay and couples the battery 15a to the receiver 12 and filter 14 in response to the control signals on line PC. The receiver apparatus 12, when rendered operative, receives tone modulated input signals from antenna 11 and produces output tone signals, corresponding to the transmitted modulation tones, at output terminal 13.

The filter and detector apparatus 14 receives the demodulated tone signals present at terminal 13 and produces a detect signal in response thereto when these signals have a selected predetermined frequency. This predetermined frequency is determined by "select" signals which are received from control circuit 16 on interconnecting lines T-P through T-6, and the detect signal is coupled to the control circuit on line K. As may be appreciated, the control circuit 16 generates the power control signals for the power circuit 15 and the referenced frequency "select" signals for filter 14 in response to the detect signals received on line K.

The control circuit 16 includes a clock 17 for producing appropriate timing pulses, preferably at a fixed frequency of 4 kHz, which are in turn coupled to a multi-timer stage 18. The multi-timer comprises a digital down counter (not specifically shown) which counts the generated clock pulses and produces timing pulses on lines A through J which occur at fixed times after the counter has been reset by a signal received on line O. The control circuit 16 also includes a detection processor circuit 19, a detect counter and tone detector control circuit 20, a timer control logic circuit 21, a message memory circuit 22, a battery saver control circuit 23, an alerting logic circuit 24, and an amplifier 25 driving a loud speaker 26, all interconnected substantially as shown in FIG. 1A.

Various parts of the stages 17 through 23 of control circuit 16 periodically generate a control signal on line PC for supplying the receiver apparatus 12 with power for a first predetermined duration of time. Further, a select signal is also generated on lines T-P through T-6, so that filter 14 will produce a detect signal on line K when the output tone signals at terminal 13 have a first predetermined frequency.

After a first detect signal is produced, the control signal on line PC then extends the time that power is supplied to receiver 12 and filter 14. Additionally, the detector portion of filter 14 is reset by a signal received from control circuit 16 on line L and the output of terminal 13 is again sampled by filter 14. If a second detect signal is then produced, the foregoing process is repeated. Once a third detect signal is produced, the control circuit 16 recognizes that a preamble tone which exists for at least a predetermined minimum time duration has been detected and, in that event, produces a power control signal which will keep the receiver on to search for the rest of a predetermined sequential code. Circuit 16 also sets the switchable filter 14 to detect the second tone in the predetermined sequential code. These general system functions can be more easily understood when viewed in conjunction with the wave forms shown in FIGS. 2A-C.

Referring to FIG. 2A, a sequence of individual tones is represented by a series of pulses having the same amplitude but existing for different durations of time. A long preamble tone 30 is followed by a gap 31, three individual messages 32, 33, and 34, and a final nth message 35. Each individual message consists of five sequential tones of identical durations followed by either a gap or a sixth unique tone having a slightly longer duration. Thus FIG. 2A represents a graphical plot of the modulation tones that would be used to signal a group of individually coded receivers. For a receiver to be alerted, a complete sequential code must be identified as exactly corresponding to the sequential code of the individual receiver. A typical sequential code would consist of a first long, or preamble, tone 30 followed by five sequential shorter tones, such as, for example, tones 33-1 through 33-5. Accordingly, the wave form depicted in FIG. 2A would alert four different individual radio receivers. Even more radio receivers would be alerted if additional messages are included between messages 34 and 35.

In FIG. 2B, a pulse 36 having a time period 37 is illustrated and represents the time duration in which power is periodically supplied to the receiver 12 in FIG. 1A. During this time period, the receiver 12 is rendered operative and is permitted to search for the initial tone of its predetermined sequential code. The time period 37 is chosen to be less than the duration of the preamble tone 30 so that the repetitive pulse 36 must occur within the duration of tone 30.

In FIG. 2C, a graph is set forth to illustrate the power applied to receiver 12 as a function of the sequential detection of the predetermined code tones. The power supplied is shown as a series of pulses. The pulses are illustrated as having different amplitudes merely in the interest of clarity and not as an intended limitation. At a time t.sub.1, during the time duration of pulse 36, a valid preamble tone may be assumed to be detected by the battery saving circuit 10. The control circuit 16 then terminates the pulse 36, creates a power supply pulse 38, and maintains filter 14 in a condition to again detect the preamble tone 30. When a second valid preamble detection occurs at a time t.sub.2 within the duration of pulse 38, a pulse 39 (shown dashed) is created by control circuit 16 and the filter 14 is kept set to detect the preamble tone. When a third valid preamble detection occurs within pulse 39 at a time t.sub.3, the control circuit 16 supplies power to the receiver 12 for a long duration of time (an initial time period) represented by a pulse 40, and the tone filter 14 is then set to detect the second tone in the receiver code which corresponds to the first tone in one of the messages following the preamble tone, such as 32-1. The pulse 40 has a duration sufficiently long enough to keep the receiver operative throughout the reception of at least the first tone of two messages following the preamble tone. Upon the identification of a valid second code tone (first message tone) at a time t.sub.4, power is supplied to the receiver 12 for a duration of time (a maintenance time period) represented by a pulse 41 (shown dashed). The pulse 41 is shown having a duration sufficient to keep the receiver power on for at least the first tone of two subsequent messages.

A chart of the typical time durations for the referenced pulses depicted in FIGS. 2A-C is tabulated in FIG. 2D. From this chart it can be seen that the preamble tone 30 exists for a substantially longer time than any of the subsequent messages, such as 32, 33, or 34. The wave forms as shown in FIGS. 2B and 2C are representative of the output or operating power as supplied by power supply apparatus 15, the duration of which are effectively controlled by the control circuit 16. The multi-timer 18 produces timing pulses which are used to generate these wave forms.

Considering FIGS. 1 and 2 together, the general system operation of the battery saver circuit 10 can now be explained in detail. The typical time duration values indicated in FIGS. 1B and 2D are used for the following explanation illustrating the reception of a typical code which includes a preamble tone and five subsequent message tones.

The control circuit 16 uses the 52 ms (millisecond) pulse and the 512 ms pulse created by multi-timer 18 to generate the wave form in FIG. 2B. When the multi-timer 18 receives a reset pulse on line O, the battery saver control 23 is turned on by a pulse on line M and it remains on until a reset pulse on line N is produced by the timer control circuit 21 in response to the multi-timer 18 producing a pulse on line F after 52 ms. If no detection has occurred within this 52 ms period, the battery saver 23 will be turned off and the multi-timer 18 will generate another pulse after 512 ms on line G which will cause the timer control logic 21 to generate a reset pulse on line O. Thus power is supplied to the receiver 12 and tone filter 14 for 52 ms out of every 512 ms.

The 12 ms pulse (FIG. 1B) on line B (FIG. 1A) is used to create a corresponding 12 ms De-Q signal on line L, which shorts the detector part of filter 14 for 12 ms after a reset signal occurs on line O. This De-Q pulse is used to minimize the possibility of power-up transients causing a false detect signal on line K. The De-Q signal on line L shorts the stored up charge on a capacitor to insure that the detector portion of filter 14 starts from a zero initial condition. This detector portion has a normal response time of 15 ms before a detect signal is generated on line K. Therefore a modulating tone must be present at terminal 13 for 15 ms after the De-Q signal has terminated before a detect signal will be produced. The frequency which will produce a first detect signal is determined by the control circuit 16 through select signals received by the filter 14 on lines T-P through T-6. This first frequency is periodically searched for in a 40 ms period (52 ms minus 12 ms) every 512 ms, and this searching continues until a detection occurs within one of these 40 ms periods.

When a first preamble detection occurs, the detection processor 19 produces a signal on line Q that is coupled to the detect counter 20 and timer control logic 21. This detect signal causes multi-timer 18 to be reset and counter 20 to be indexed to a count of one. The multi-timer 18 is always reset after every detection unless an alert signal is being generated. The count of the number of detections, which is indicated on lines S, T, and U, is monitored by the detection processor 19 and the timer control logic 21. Since the multi-timer 18 was reset before the 52 ms pulse on line F occurred, the power supplied to the receiver 12 and filter 14 will continue until a reset pulse is received by the battery saver control 23 on line N. Since the detect counter 20 now has a count of one, a pulse on line N will not be created when the 52 ms pulse of line F occurs, but when the 42.5 ms pulse of line D occurs. This pulse selection is accomplished by the timer control logic circuit 21 through the use of OR gates as will be more fully explained subsequently. When the multi-timer 18 is reset, a De-Q signal is created on line L which terminates 2 ms after the reset pulse on line O. The selection of the 2 ms pulse instead of the 12 ms pulse for the De-Q period is similarly accomplished by OR gate logic in the detection processor 19 which also monitors the count of the detect counter 20. Thus power is extended to receiver 12 and filter 14 for 42.5 ms, and a search window of 40.5 ms is provided for the filter 14 to again search for the preamble tone.

When the preamble tone is detected for a second time by filter 14, an identical sequence of events occurs and the filter remains set to detect the preamble tone for a third time. When the preamble has been detected for a third time, a preamble detect latch in the timer control logic circuit 21 creates a positive logic signal on preamble detect line Z, which in turn trips a latch in the detect counter 20 that creates a high logic signal on line V which indicates the detection of a valid preamble.

After this third preamble detection, the timer control logic 21 selects the timing pulse on line I as the pulse which will reset the battery saver control 23 by causing a pulse on line N. Thus once a valid preamble has been detected, all radio pager receivers which have this preamble tone as the first tone in their sequential code will be turned on for a period of time corresponding to pulse 40 in FIG. 2C. Therefore, a receiver having a preamble tone 30 as its first tone will be turned on if this tone was selected by the filter 14 in response to the select signals on lines T-P through T-6, and this tone was detected three consecutive times.

One advantage of the present invention is that by requiring several detections of the preamble tone, a valid preamble is only detected when the preamble tone exists for a duration of time substantially longer than any of the subsequent code tones. Thus in the present example the preamble 30 must exist for a minimum of 49 ms before a valid preamble is detected. The figure of 49 ms is obtained by multiplying the 15 ms response time of the filter 14 by three and adding two of the 2 ms delays caused by the De-Q signals on line L. Therefore in the present system, it is impossible to mistake an individual message tone, such as 32-2, for a preamble tone. The multiple sampling of the preamble also improves the noise immunity of the disclosed battery saver.

Once a valid preamble has been detected, the detect counter and tone detector control 20 supplies different select signals to filter 14 along lines T-P through T-6 for selecting the second predetermined frequency of the code. This second code tone corresponds to the first tone of one of the messages that follow the preamble, such as 32-1. The time duration of pulse 40 is sufficient to keep the receiver and filter on throughout the remainder of the preamble 30 and for an additional time which is long enough to interrogate the first two messages that follow the preamble. If no tone detection occurs during this initial time period (ITP), the multi-timer 18 will receive a reset signal on line O when the timing pulse on line I occurs and the preamble latch in the detect counter 20 will be reset. This results in the renewal of the periodic searching for the preamble tone.

The occurrence of a detection during the initial time period will effectively terminate the power supply pulse 40 and initiate a maintenance time period (MTP), as illustrated by pulse 41 in FIG. 2C. This maintenance time period keeps the power on long enough (681 ms) for the first tone of two additional messages to be interrogated. The detect counter 20 is indexed by the detection of this second tone and select signals are generated to search for the third code tone. The detection processo