Patent GB-1011031-A (GB)
Title: Improvements in or relating to radio communication systems
IPC: H04B1/50
Filed: 1962-04-17
Abstract: 1,011,031. Electric selective signalling. MARTIN MARIETTA CORPORATION. April 17, 1962 [May 2, 1961], No. 14813/62. Heading G4H. [Also in Division H4] A multiple station radio telephone system using pulse position modulation comprises means at each transmitter for deriving a position modulated pulse train from the information to be transmitted, delay means for obtaining from the pulse train a number of derived pulse trains coded in time spaced manner in accordance with the address of the receiver, means for transmitting each derived time spaced pulse train on a particular frequency, receiver means for separating the received signal into its component frequencies and time delay and coincidence circuit means for recombining into the position modulation pulse train only those time spaced pulse trains addressed to the receiver. Transmitter.-Microphone 52 is connected via a compressor amplifier and bandpass filter 53 to an adder circuit 56 which also receives an input from a sawtooth generator 55 triggered at 8 K.P.P.S. from a clock circuit 58. The output of the adder circuit is applied to a Schmitt trigger circuit 57 which provides output pulses having a time duration which are a function of the amplitude of the audio signal. These pulses are applied to a differentiation circuit 59 to produce position modulated pulses. These pulses are applied via a voice operated switch 61. to an address coding arrangement. This comprises a delay line 63 with a number of output taps 64 each connected to a stationary contact on each of three separately adjustable switches S1A, S2A and S2B which converts each modulation pulse train to three output pulse trains having a delay coding corresponding to the address of the co-operating receiver. Each output pulse train is applied to a separate channel and via a fixed time delay 67, 68, 69 triggers an R.F. oscillator 77, 78, 79. The oscillators operate at frequencies spaced by one megacycle and their output signals are combined in unit 81 before transmission. The time delays 67-69 enable a duplexer 75 under control of a solid state unit 74 to switch the aerial 76 from the receiver input to transmitter author. Receiver.-The aerial is connected via a diode clipper to an R.F. amplifier circuit 86 which is followed by a connectional mixer 87 and broad band I.F. amplifier 89. The output of amplifier 89 is applied to three filters 91, 92, 93 which separate the three frequency component signals and these are detected in units 94, 95, 96 and applied to three delay lines 97, 98, 99 which are identical to the transmitter delay line 64, and are each provided with a permanently connected tap. Correctly addressed signals undergo such delay that three coincident signals occur at the input of a coincidence gate 101<SP>1</SP> which provides position modulated pulses in its output circuit. These pulses are applied via a unit 161, which controls a gate circuit 164 connected in the audio frequency path to a speaker 166 1  to disconnect the circuit when no modulation is present and to a demodulating circuit. This comprises a voltage controlled oscillator 100 operating at 8 kc./s. which controls the generation of a sawtooth waveform in unit 104 and a sampling gate circuit to which the sawtooth signal and the position modulated pulses are applied. The sawtooth signal is sampled when the gate is opened by the pulses and the sampled signal charges a capacitor in an integrating circuit. The signal across the capacitor is applied via a D.C. amplifier 120 a bandpass filter 125 and audio amplifier 126 to the speaker 166 and the D.C. component from the amplifier 126 is fed back to the oscillator 100 to syncronize its frequency of operation. Three further fixed taps are provided on the delay lines of each receiver in the system and connected to a second coincidence gate 102<SP>1</SP>. The further tap positions on each receiver are identical so that from any transmitter, command signals coded for this further tap address can be transmitted to all receivers, in the system. Additionally three adjustable taps, controlled by switches S1B, S2B, S3B, ganged to the local transmitter switches S1A, S2A, S3A, and connected to coincidence gate 100<SP>1</SP> may be switched to any address to allow conferences between a number of stations. Ringing between stations is accomplished by amplitude modulated pulses. Specifications 947,679 and 985,828 are referred to.