Patent GB-1452433-A (GB)
Title: Communication system incorporating signal delay
IPC: H04W88/18
Filed: 1974-06-21
Abstract: 1452433 Radio communication K H WYCOFF 21 June 1974 [21 June 1973] 27745/74 Heading H4L In a receiver for a selective call communication system in which signals including a code sequence of a reference tone and at least one control tone are transmitted to the receiver from a transmitter co-operating with a plurality of receivers each of which is responsive to a different code sequence, the received sequence is delayed by an amount such that the reference tone coincides in time with at least a portion of the or each control tone of the undelayed sequence and the delayed and undelayed tones are applied to a mixer to produce a frequency representative of the difference between the reference tone and a coincident control tone. In some receivers in which the carrier wave is reinserted at the receiver, the frequencies of the detected tones may differ from those generated at the transmitter and the present arrangement avoids this difficulty since the difference frequency remains constant. Transmitter.-Tone signals from an encoder or speech signals are transmitted as single sideband signals with suppressed carrier. The encoder, Fig. 2, includes an operating key 50 which causes an electronic switch 60 to latch on to energize the power supply of the transmitter and start a clock oscillator 80 coupled to a pulse generator 90 which produces a pulse sequence consisting of a first pulse at 103 followed by a pulse at 108 followed by a pulse at 109. A reset output is coupled back to switch 60 which causes the carrier wave and output of oscillator 80 to be interrupted. The outputs 103, 108, 109 control an electronic switching apparatus 120 receiving as inputs a reference tone from oscillator 140 and a control tone from oscillator 160 or 180 (according to the position of switch 190) the resulting tone sequence, consisting of a reference tone sequentially followed by two control tones of shorter duration, being supplied via amplifier 200 to the modulator. An indicator 110 provides a signal while the tone sequence is being transmitted and when the signal stops the operator can use the microphone. Receiver.-The output of the I.F. amplifier 245, Fig. 6, is supplied to a product demodulator 246 the suppressed carrier being replaced by the output of oscillator 247 and the resulting audio signals being supplied via amplifier 248 to a transducer 252 which is muted in the absence of a signal. The output of amplifier 248 is also supplied via a limiter 260 direct to a mixer 290 and through a delay 270 and bandpass filter 280 passing the reference tone to the mixer. The delay causes the reference tone to coincide with the control tones in the undelayed sequence resulting in a sequence of low-frequency difference signals being applied via filter 300 to a decoder 360 providing an output if the sequence is correct. This enables the switch 420 to remove the muting. A manual switch may be provided to override the decoder 360 and maintain the receiver unmuted. Modifications.-Alternative codes in which the reference tone is in the centre or at the end of a tone sequence and delay arrangements for decoding such codes are described. In some arrangements the bandpass filter 280 may be dispensed with. The system may be used for selective calling or for data transmission.