GB2210189A - Communication receiver - Google Patents

Abstract

A communication system uses a continuous tone selection system and a decoding unit (18) to identify and pass signals containing a unique continuous tone and to block signals not containing that unique tone, the receiver being individualised to that unique tone by interrupting a selection from a plurality of interruptable links (35), and each link passing (or blocking if interrupted) a control input to the decoding unit and is characterised by a free-running multivibrator (52) injecting a second set of control inputs onto the links (35) so that the decoding unit hunts between looking for one unique tone and at least one other unique tone and in that the multivibrator is stopped when the decoding unit passes a signal so that signal continues to be passed.

Description

COMMUNICATION RECEIVER FIELD OF THE PRESENT INVENTION The present invention concerns a communication receiver for a system consisting of a transmitter and a plurality of receivers, which receiver can receive signals directed to itself independently of the other receivers.

BACKGROUND OF THE INVENTION In the communication system described in British Patent Application 8629861, the desirability of being able to direct messages selectively to one addressee or a plurality of addressees is mentioned. In that Application.

it was thought necessary to have a plurality of decoding units in each receiver so an addressee could receive a message directed to only him by means of one decoding unit and a message shared with others by another decoding unit. Various ways of avoiding the cost and size penalty of using a plurality of decoding units have been tried, such as transmitting multiple signals simultaneously so a plurality of addressees could simultaneously receive a signal. This, whilst initially promising, suffered from call drop-out to an excessive degree.

SUMMARY OF THE PRESENT INVENTION The present invention provides a communication receiver using a continuous tone selection system and a decoding unit to identify and pass signals containing a unique continuous tone and to block signals not containing that unique tone, the receiver being individualised to the unique tone by interrupting a selection from a plurality of interruptable links, each link passing (or blocking if interrupted) control input to the decoding unit, characterised in that a free-running multivibrator injects at least a second set of control inputs onto the links so that the decoding unit hunts between looking for one unique tone and at least another unique tone and in that the multivibrator is stopped in one state when the decoding unit passes a signal.

In decoding units, the control inputs drag points in the unit from an internally set level to another level, say earth level. This is normally achieved by connecting the links to an earth rail and then those links which are not interrupted will drag respective points to earth leaving the other points at the internally set level. In it simplest form, the multivibrator alternates between applying the said another level and the internally set level (or thereabouts) to the links so that the unit sees through the uninterrupted links either the said another level or the internally set level (which set level gives the same effect as an interrupted link). This enables the unique tone used with all links interrupted to be used a group call tone since all receivers will respond to such a tone.

Instead of applying the second set of control inputs to the ends of the links remote from the unit, it is possible to apply them by suitable circuitry to the link ends adjacent to the unit. If a sub-group call facility is desired as well as a group call facility, the multivibrator would be n the form of a ring counter with a plurality of outputs fed to selections of links.

Obviously the number of group and sub-group facilities provided will reduce the number of individual unique tones for other receivers and moreover the time taken for a receiver to cycle through all the facilities will be longer and so the time taken for the receiver to respond to a tone will be longer or the dwell on each facility must be reduced with the danger that the receiver will not respond.

In the preferred embodiment, the multivibrator is also used as part of a timing device to hold the receiver in an operating mode for a predetermined time after termination of a received signal and then to put the receiver in a stand-by mode in which the receiver consumes less power.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a circuit diagram of a receiver as a whole, Figure 2 is a block diagram of a tone decoding arrangement, and Figure 3 is a detail view showing a modification of Figure 2.

DESCRIPTION OF EXEMPLARY EMBODIMENTS The drawings show a receiver which is to be used in a communication system wherein the signal contains an audio frequency component (which is the signal proper) and sub-audio tones (enabling the signal to be addressed to an individual receiver).

In Figure 1, an amplifying chip 11 such as a Plessey - SL6654 receives an incoming signal through a filter 12 on line 14. To improve the input sensitivity of the chip, it is preferred to employ an external oscillator 15. This chip mixes the incoming signal with the oscillator output to give an intermediate frequency signal at about 455 kiloHertz which is amplified and passed through an intier- mediate frequency filter 16 to further amplification and discrimination down to audio frequency.

This audio frequency signal is passed through a filter and D.O. decoupling circuit 17 to a decoding chip 18 of which more will be said in relation to Figure 2. If the chip 18 senses an audio signal and an appropriate sub-audio tone, a signal is passed on line 19 through an audio amplifier 20 to transducers 21 in ear pieces which give the desired sound output to a wearer.

Power for the circuitry is derived from a battery pack 22 which can have associated with it an inductive charging arrangement 23. This power can be taken direct to essential circuitry 24 and through a switching transistor 25 to the rest of the circuitry. This transistor is switched between conducting and nonconducting states by a bistable multivibrator 26 having a momentary action switch 27 to cause the transistor to enter the conducting state and an input line 28 to cause the transistor to enter the nonconducting state.

Figure 1 also shows chips 29 such as a 74HC04, 30 such as a 74H04060, and 31 such as a 74HC 393. The major functions of these chips will be described with reference to Figure 2.

A 4 Megahertz clock 32 in chip 29 provides a clock pulse train to a frequency divider 33 in chip 30 which yields a 1 Megahertz pulse train to a rate multiplier 34 in chip d8. This chip is a continuous tone decoder such as a Consumer Microcircuits Ltd.

FX335L CTCSS decoder which is arranged to respond to one of up to zEirty-eight sub-audible tones by means of interruptable links 35. The output of this rate multiplier is a tone square wave which is fed through a low pass filter 36, a band pass filter 37, a limiter 38, a digital period detector 39 to an output latch 40. The filters 36 and 37 are tuned by programme inputs and the filter 36 compares the tone square wave with the audio signal from the circuit 17, which signal can also be passed on line 41 to the chip 18 to prevent the chip accepting noise as a signal to which it should respond in the absence of an incoming signal (this line 41 is shown' in Figure 1). The audio signal is also passed through a high pass filter 42 to attenuate the tone signal content through an audio switch 43 to line 19. The output of the latch 40 is a signal denoting that the audio signal from circuit 17 contains a predetermined sub-audio tone but this output may be prone to chatter. Thus latch 40 is passed through an integrator 44 (see also Figure 1) to a comparator 45 which yields a signal when the integrated signal exceeds a preset level. The yielded signal is used to drive through logic gating 46 the audio switch 43 to supply a signal to the line 19. The integrated signal is also used to yield a signal denoting that a selection has taken place, all components 34 to 46 are in the chip 18 or associatedwith that chip. The integrated signal is fed through an inverting buffer amplifier 47 in the chip 29 to apply a reset signal to a frequency divider 48 in chip 30 and a hold signal to an oscillator 49 in chip 30.

The oscillator 49 is arranged by means of a resistance capacitance network 50 (Figure 1) to oscillate at about 4 Hertz. The output of this oscillator feeds through an inverting buffer amplifier 51 an edge triggered -bistable multialibrator 52 driving the ends of the links 35.

The oscillator 49, amplifier 51 and multivibrator thus form a free-running multivibrator giving a quarter second duration signals between two voltage levels but which can be frozen in the oscillation by the hold signal. One of these voltage levels would be the level at which the ends of the links in the rate multiplier 34 would be biased to within the chip 18 so the effect would be that of all the links being interrupted whilst the other voltage level would give effect to whether the links were interrupted or not, giving respectively a group call facility and an individual select facility.The output of the oscillator 49 is also fed to the frequency divider 48 which yields a signal after about 17 minutes to operate through line 28 the multivibrator 26 to cause transistor 25 to go non-conductive unless within that 17 minutes the divider has been reset when the 17 minutes will start from the recommencement of operation of the oscillator 495 the divider 48 includes a counter.

In Figure 3, the links, 35 are earthed at their ends remote from the unit 18 and the multivibrator applies pulses earthing the ends of at least some of the links near to the unit through diodes 54 which block positive pulses from the multivibrator, omitting some diodes would block positive and negative pulses from the multivibrator reaching the links. This construction could be extended to provide group call, sub-group call and individual call facilities with a ring counter feeding through a diode network to define the requisite sub-audio tones for the various facilities.

Providing group-call and sub-group call facilities is expensive on the number of sub-audio tones available to a group of receivers since each facility occupies at least one sub-tone and may render other sub-tones unavailable. Some saving on sub-tones may be achieved if the multivibrator applies patterns of both positive and negative (earth) pulses so making sure that each facility occupies a single sub-tone.

Claims (5)

1. A communication receiver using a continuous tone selection system and a decoding unit (18) to identify and pass signals containing a unique continuous tone and to block signals not containing that unique tone, the receiver being individualised to that unique tone by interrupting a selection from a pluralityofinterruptable links (35), each link passing (or blocking if interrupted) a control input to the decoding unit, characterised in that a free-running multivibrator (52) injects a second set of control inputs onto the links (35) so that the decoding unit hunts between looking for one unique tone and at least one other unique tone and in that the multivibrator is stopped in one state when the decoding unit passes a signal.

2. A communication receiver according to claim 1 wherein the multivibrator applies alternately inputs torresponding to internally set level in the decoding unit and inputs corresponding to another level to ends of the links remote from the unit.

3. A communication receiver according to claim 1 wherein the multivibrator applies earthing pulses through diodes or other isolating devices to ends of links closer to the decoding unit.

4. A communication receiver according to any preceding claim wherein the multivibrator also provides timing elsewhere in the receiver.

5. A counication receiver substantially as herein described with refernc to the accompanying drawings.