GB2122000A - Apparatus for use in two way radio systems - Google Patents

Abstract

Apparatus comprising means for detecting receipt by a transceiver of a first predetermined pulse position modulated (PPM) codeword transmitted from another transceiver of a two way radio system, having a band pass filter 14, a pulse shaping circuit 15, a PPM receiver 16 and a binary to decimal converter 17, means responsive to detection of the first codeword to generate a second PPM codeword for transmission by the transceiver to the other transceiver in order to indicate at the other transceiver that the first predetermined PPM codeword has been received comprising a silicon controlled rectifier (SCR), a transistor 5 and a PPM encoder 12, means for generating a third predetermined PPM codeword for transmission in order that the other transceiver may be called which comprises a call button 11 and the PPM encoder 12. The apparatus may also comprise an alarm 18, means for adjusting the relative time periods between pulses which designate a "one" and those which designate a "zero", and means enabling the apparatus to be responsive to a plurality of different PPM codewords. <IMAGE>

Description

SPECIFICATION Apparatus for use in two way radio systems This invention relates to apparatus for use in two way radios.

In recent years there has been considerable demand for the limited space on radio frequency wave bands which have been allocated for two way radio transmissions. On frequencies shared by a number of users, a user is only able to receive transmissions from a particular caller if he listens to ail the transmissions on his frequency regardless of whether or not they may be intended for him. This problem is particularly noticeable for users of the wave bands referred to as Citizen's Band, which have recently become very congested.

In addition, Citizen's Band transceivers are generally of limited range and hitherto the caller has not been able to determine whether or not he is out of range of the transceiver with which he wishes to make contact. In such a system it is necessary for a potential recipient of a call to keep his transceiver turned on whenever he feels that he may be within range, which results in the unnecessary consumption of power. Furthermore, if a caller does transmit a message but does not receive a reply, he is unaware whether or not the person with whom he wishes to make contact is away from his receiver or the receiver is out of range.

According to the present invention there is provided apparatus for use in a transceiver of a two way radio system, the apparatus comprising: means for detecting receipt by the transceiver of a first predetermined pulse position modulated (PPM) codeword transmitted from another transceiver of the two way radio system; means responsive to detection of the first predetermined PPM codeword to generate a second predetermined PPM codeword for transmission by the transceiver to the other transceiver in order to indicate at the other transceiver that the first predetermined PPM codeword has been received; and means for generating a third predetermined PPM codeword for transmission in order that the other transceiver may be called.

Apparatus embodying this invention eliminates or at least reduces the above-mentioned disadvantages since it enables a first transceiver to be selectively called by a caller having a second transceiver, the first transceiver subsequently transmitting an acknowledgement signal (second PPM codeword) in response to the selective call.

The acknowledgement signal is received by the second transceiver where its receipt is indicated, for example by a loudspeaker in the caller's transceiver being caused to buzz. The apparatus preferably comprises means for switching on an alarm in response to the detection of the first predetermined PPM codeword, the alarm preferably operating continuously until being switched off by the dekeying of a microphone.

The alarm enables a person who is absent from the first transceiver at the time that the first transceiver is being called to determine that he has been called during his absence.

The apparatus is preferably constructed so that the relative time periods between successive pulses which designated a "one" in a PPM codeword is substantially two thirds of that for periods between successive pulses designating a "zero", and so that the end of any PPM codeword is designated by a reset which corresponds to a time period of substantially one and a half times that for periods designating a "zero". However.

the apparatus may comprise means for adjusting the time periods between successive pulses in the PPM codewords generated by the means for generating a second PPM codeword, and means for adjusting the time periods between successive pulses which may be detected in the means for detecting the PPM codewords so that the number of different codewords with which the apparatus is compatible may be varied.

Apparatus embodying this invention is preferably employed in two way radio selective call systems which operate on frequency modulation (FM) wave bands. The first, second and third PPM codewords are preferably transmitted by means of FM radio communication, the time taken for the generation of the first predetermined PPM codeword, the detection of the first predetermined PPM codeword and then the automatic generation of the second predetermined PPM codeword occurring within a short time period, such as, between 0.5 and 1.0 seconds.

The means for detecting the first predetermined PPM codeword preferably comprises a binary to decimal converter and a pulse position modulation receiver which are in the form of integrated circuits, and the means for generating the second predetermined PPM codeword preferably comprises a PPM encoder in the form of an integrated circuit. The binary to decimal converter and the PPM receiver may be programmed to be responsive to a plurality of different PPM codewords, and the PPM encoder may also be programmed to generate a plurality of different PPM codewords, thus enabling the apparatus to be set to respond to a plurality of different PPM codewords.

This invention will now be described by way of illustrative and non-limiting example with reference to the accompanying drawings in which: Figure 1 shows a block diagram of two FM transceivers of a two way radio selective call system, each transceiver incorporating an automatic call confirmation means embodying the invention; Figure 2 shows a circuit of the call conformation means of Figure 1; Figure 3 is an example of a waveform from a PPM codeword may adopt; and Figure 4 shows a table of connections to a PPM encoder of the call confirmation means of Figure 2 which correspond to particular PPM codewords.

The operation of a two way radio selective call unit incorporating an automatic call confirmation means will now be described with reference to Figure 1, in which a block diagram representing two FM transceivers A and B of a two way radio system is shown. This arrangement enables a caller stationed at the transceiver A to call a person stationed at the transceiver B and to enable the caller to determine whether or not his call has been received at the transceiver B. The transceiver B receives a first predetermined pulse position modulation (PPM) codeword transmitted by the caller's transceiver A, and in response, automatically sends out a second predetermined PPM codeword (comprising for example, a series of binary zeros) back to the transceiver A, thus acknowledging receipt of the first predetermined PPM codeword.

The caller at the transceiver A transmits the first codeword by pressing a switch 1 a which is connected to a call confirmation means 2a of the FM transceiver A. The first codeword is generated by the call confirmation means 2a and is then transmitted by means of a known type of FM transmitter 3a and an aerial 4a. The first codeword is picked up by an aerial 4b of an FM receiver 5b of known type. If a call confirmation means 2b of the transceiver B is set to receive the first codeword then the call confirmation means 2b generates the second codeword comprising binary zeros and transmits it by means of a known type of FM transmitter 3b and the aerial 4b. The call confirmation means 2b also causes an alarm (not shown in Figure 1) to operate, thus signalling to the person stationed at the transceiver B that he is being called.The second codeword of binary zeros serves to acknowledge receipt of the first codeword and is subsequently picked up by the caller's transceiver A. The call confirmation means 2a is not responsive to the second codeword which comprises, for example of binary zeros, but instead a receiver 5a of the transceiver A responds to the second codeword by causing a loudspeaker 6a to make a buzzing sound for the duration of receipt of the second codeword. It is noted that the two transceivers A and B are similar and so the transceiver B may be operated to call the transceiver A in a similar way.

Referring now to Figure 2, the operation of a circuit of the call confirmation means 2a or 2b will now be described in detail, in the case where a caller stationed at the transceiver A wishes the call the person stationed at the transceiver B.

A call button 11 is connected between terminals A5 and Al 5 of a pulse position modulation encoder 12. The encoder 12 has eighteen terminals each of which is designated by one of the reference numerals Al to Al 8. When the call button 11 is depressed for a period of, for example, one second, the encoder 12 generates a plurality of identical first predetermined PPM codewords which are coded according to the particular manufacture of PPM encoder used (in this case an SL 490 type integrated circuit). The time period between successive pulses i.e. the time interval plus the pulse time duration, in a PPM codeword may be referred to as one frame, the frame rate for the transmission of the codewords being controlled by a capacitor C1 and a variable resistorVR1 connected as shown in Figure 2.The time period between successive pulses in a PPM codeword determines the presence of a "zero" or a "one". In this embodiment, the time period between pulses designating a "one" is, for example 3.8 milliseconds which is two thirds that for time periods designating a "zero" which are, therefore, 5.8 milliseconds. The end of a PPM codeword is designated by a reset which is identified by a time period between successive pulses of about two times that for time periods which designate a "zero", that is, 11.6 milliseconds as indicated in Figure 3. The first predetermined codeword is generated at the terminal A3 of the encoder 12 and is then delivered through a deviation adjustment resistor VR2, a serially connected capacitor C2 and a resistor R1 to an output terminal T1.The output terminal T1 is connected to an FM transmitter of a known type, i.e. that corresponding to the transmitter 3a of Figure 1.

In this embodiment, an SL 490 integrated circuit is used for the encoder 12, and is used in combination with a binary to decimal converter and a PPM to binary converter (which will be introduced later), and as a result is capable of supplying a total of eighteen different predetermined PPM codewords at the terminal A3. The particular PPM codeword which may be supplied to the terminal T1 depends on which terminals Al to Al 8 are connected together by the call button 11. Hence, by connecting up to eighteen different pairs of the terminals Al to Al 8, for example according to the code table shown in Figure 4, up to eighteen different codewords (labelled A to I and P to X in Figure 4) may be generated at the terminal A3, and so the transceiver A would be capable of calling eighteen different transceivers B.Furthermore, when the call button 11 is depressed, an internal voltage regulator in the PPM encoder 12 switches on a transistor TR1 by generating a 5v voltage at the terminal A17 which switches on the transceiver A into a transmit mode. In addition, the first PPM codeword is delivered at the output terminal T1. Since the terminal Tl is connected to an FM transmitter the first codeword is transmitted.

The first codeword, which is transmitted by its frequency modulating a carrier signal, is detected by an FM receiver 5b (which may also receive PPM codewords from other sources) in the transceiver B. The FM signal is converted into an audio frequency signal which reproduces the first codeword, the audio frequency signal then being delivered to an output of the receiver 5b which is situated before an audio amplifier (not shown) in the receiver 5b. The level of the audio signal may be sufficiently low that a loudspeaker 6b in the receiver 5b does not produce any sound which is audible to a person stationed at the transceiver B.

The audio frequency signal which carries the PPM codewords is delivered to an input terminal T2.

The audio signal is passed from the input terminal T2 to a resistor R2 and a capacitor C3 and then to a transistor TR2 and a band pass filter 14. The band pass filter 14 is set so that only the frequency of the audio signal carrying the PPM codewords is transmitted. The output of the band pass filter 14 is passed to a pulse shaping circuit 15 which comprises a transistor TR3, a resistor R3, capacitors C4 and C5, and a diode Dl. The pulse shaping circuit,15 serves to improve the profile characteristics of the PPM codewords received from the band pass filter 14. This is so that the codeword may be detected by a PPM receiver 16, which in this embodiment is a ML 926 or an ML 927 type chip or integrated circuit.

If an ML 926 chip is used in conjunction with the SL 490 chip, then nine of the eighteen codewords which may be generated by the SL 490 chip will be selected. If the ML 927 chip is used, then the remaining nine codewords will be selected. Therefore, by choosing the appropriate PPM receiver 1 16 chip, the transceiver B may be set to be responsive to any one of eighteen codewords.

A PPM receiver 1 6 is connected to the output of the pulse shaping circuit 1 5 and serves to receive the PPM codewords generated by the PPM encoder 12 which have the frame rate characteristics corresponding to those set by the variable resistorVR1 and the capacitor C1 in the transmitting stage of the transceiver A as mentioned earlier. It is a characteristic of the PPM receiver 1 6 that a codeword having the correct frame rate characteristics must be detected twice in succession in order for an output to be produced. Frame rates accepted by the PPM receiver 1 6 may be adjusted by means of a variable resistor VR2 (for fine tuning) and a capacitor C6 (for coarse tuning).Therefore, once a PPM codeword has been chosen for the respective transceivers A and B, it is necessary to adjust the frame rate transmission characteristics of the variable resistor VR 1 and the capacitor C1 to equal the frame rate acceptance characteristics of the PPM receiver 16 by means of the variable resistor VR2 and the capacitor C6. It is noted that even though the apparatus described hereinbelow is restricted to a possibleighteen PPM codewords by virtue of the fact that the PPM encoder SL490, a BCD decoder 4028 and a PPM receiver ML 926/7 are employed, it is clear that a pair of transceivers A and B tuned to a particular frame rate would not interfere with a pair of transceivers A and B operating on a different frame rate even though the "zero" and "one" sequence of the eighteen PPM codewords may be the same.Hence, the number of variations is not limited to eighteen codewords. Alternative types of PPM encoder, BCD decoder or PPM receivers may be employed in this system, which will also have the effect of varying the number of PPM codewords available in a system.

The output of the PPM receiver 16 is in the form of a binary coded decimal (BCD signal) corresponding to the codewords having the correct frame rate characteristics. The BCD signal is then passed to a binary to decimal converter BCD chip 1 7 (in this embodiment a CMOS 4028), which can select, from the codewords arriving from the PPM receiver 16, the nine predetermined PPM codewords to provide an output at each one of nine output terminals B1 to B9. One end of a resistor R4 is shown in Figure 2 to be connected, via a diode D2, to the output terminal B1 to which the PPM codeword corresponding to the first predetermined PPM codeword is delivered. The arrival of an output at the terminal B1 causes the voltage level to "pulse", i.e. to rise for the time duration of the first predetermined PPM codeword detection.The other end of the resistor R4 is shown connected to the base of a transistor TR4 and to one end of a capacitor C7 which itself is connected in parallel to a diode D3, the other end of the capacitor C7 being connected to the emitter of the transistor TR4. The emitter is also connected to a capacitor C8 which charges up through the resistor R4 and the diodes D2 and D3 when the voltage level rises or "pulses" at the terminal B 1. When the first PPM codeword ceases to be detected and the level at the terminal B1 returns to its steady state or "zero" value, the capacitor C8 discharges through the base-emitter junction of the transistor TR4 (which turns the transistorTR4 on), and in turn switches on a silicon controlled rectifier (SCR) call-latch via a resistor R6, and also switching on a transistor TR5 via a resistor R7.

The switching on of the SCR call-latch causes a current to flow from the terminal T3 to which a voltage is applied, to the earth rail via a transistor TR6 and a resistor R8. An alarm 1 8 is connected in parallel between the terminals of the resistor R8 which comes into operation as a result of current flowing from the terminal TR3 to the earth rail, so providing a response acknowledging the receipt of the first PPM codeword by the receiver 5b and the call confirmation means 2b. The alarm 18 may be in the form of a buzzer or a light buib or other suitable alarm means. Since the SCR calllatch remains on even after the first PPM codeword ceases to be detected, the alarm 1 8 remains on until the transceiver is keyed by means of a switch of a microphone 19b. When the keying of the transceiver B takes place, i.e. the person stationed at the transceiver B has realised that he is being called, a voltage supply rail 20 is earthed, thus switching off the SCR call-latch and turning off the alarm 1 8.

The switching on of the transistor TR5 causes the terminal A5 of the PPM encoder 12 to be earthed, which has the effect of short-circuiting the terminals A5 and Al together. When the terminals A5 and Al are short-circuited, a predetermined codeword of, for example binary zeros is generated automatically by the PPM encoder 1 2 and transmitted as described above with reference to the transmission of the PPM codewords generated by the depression of the call button 11. The binary zero codewords are received by the transceiver A but the PPM receiver 16 and the BCD chip 17 are not set to be responsive to a binary zero codeword, the detection of this codeword is carried out by the receiver 5A.The binary zero codeword causes the loudspeaker 6a which is connected to the receiver 5a to sound or "buzz" for the time during which the codeword is received. Therefore, receipt of the binary zero codeword by the transceiver A acknowledges the receipt of the first predetermined PPM codeword by the transceiver B.

Although this embodiment has been described in terms of a pair of transceivers using the same first predetermined PPM codeword, i.e. only one call button 11 and only one terminal at the BCD decoder 17. If several call buttons 11 are wired up to the PPM encoder as described earlier then a plurality of different PPM codewords may be transmitted to a variety of transceivers, all of whom will acknowledge by transmitting a binary zero codeword. However, if a person wishes his transceiver to acknowledge receipt of a plurality of different PPM codewords each originating from a separate transceiver, additional diodes (not shown) may be connected between a plural number of the terminals B1 to B9 and the resistor R4.

An optional "bleep" circuit may be incorporated into the circuit shown in Figure 2, which "bleep" circuit comprises a link (which may be cut if this circuit is not required), a capacitor C9 and a resistor R9. The "bleep" circuit serves to cause the circuit of Figure 2 to generate an acknowledgement signal when the microphone 19a or 19b is keyed.

Claims (9)

Claims

1. Apparatus for use in a transceiver of a two way radio system, the apparatus comprising: means for detecting receipt by the transceiver of a first predetermined pulse position modulated (PPM) codeword transmitted from another transceiver of the two way radio system; means responsive to detection of the first predetermined PPM codeword to generate a second predetermined PPM codeword for transmission by the transceiver to the other transceiver in order to indicate at the other transceiver that the first predetermined PPM codeword has been received; and means for generating a third predetermined PPM codeword for transmission in order that the other transceiver may be called.

2. Apparatus according to claim 1, wherein an alarm is provided in response to the detection of the first predetermined PPM codeword.

3. Apparatus according to claim 2, comprising means responsive to dekeying the associated transceiver to discontinue the alarm.

4. Apparatus according to claim 1, claim 2 or claim 3, wherein the spacing of successive pulses of the PPM codewords designating a first binary level is substantially equal to two thirds of that of successive pulses of the PPM codewords designating the other binary level.

5. Apparatus according to any one of the preceding claims, comprising means for adjusting the spacing of successive pulses of the PPM codewords designating a first binary level and the spacing of successive pulses of the PPM codewords designating the other binary level.

6. Apparatus according to any one of the preceding claims, wherein the means for detecting receipt of the first predetermined PPM codeword and the means for generating the third predetermined PPM codeword are capable of detecting and generating a plurality of different PPM codewords respectively.

7. Apparatus for use in a transceiver of a two way radio system, the apparatus being substantially as herein described with reference to Figures 2,3 and 4 of the accompanying drawings.

8. A transceiver fitted with apparatus according to any one of claims 1 to 7.

9. A transceiver according to claim 8, in which the transceiver is a frequency modulation (FM) transceiver.

1 0. A transceiver according to claim 9, substantially as herein described with reference to Figures 1 to 4 of the accompanying drawings.