GB933467A - Synchronous communications systems - Google Patents

Abstract

933,467. Automatic phase control systems; phase detectors; multiplex signalling. NATIONAL RESEARCH DEVELOPMENT CORPORATION. March 1, 1961 [March 9, 1960], No. 8289/60. Class 40 (5). In a multiplex or frequency shift signalling, e.g. teleprinter system, in which characters are represented by tones, a synchronizing tone burst, e.g. 6 cycles, is inserted in the signal at intervals equal to an even number n, e.g. 64 cycles of the synchronizing tone and used for synchronizing a coherent (phase locked) oscillator 1 in the receiver, e.g. a tuning fork oscillator of frequency 384 c/s. The oscillator output (limited to rectangular waveform) is n periodically phase reversed every - (e.g. 32) 2 cycles of the tone and applied to a phase detector 7 together with the synchronizing tone, e.g. 384 c/s., separated from the received input by an attenuator (sync. gate) 2 with gated output only during a sync. burst gating interval of <SP>1</SP>/ 32 sec. controlled from a counter 5 operated from pulses received from the oscillator 1, so that at correct synchronism the phase reversal occurs mid-way of each synchronizing tone burst and the phase detector average output is zero. Incorrect timing of counter circuit 5 however produces an average output from phase detector 7 which is periodically sampled (connected to earth through an output circuit) by a relay 9 to produce pulses of one or other polarity which are respectively amplified by one of amplifiers 10, 11 and applied to operate a gate 4 whereby a timing pulse from the oscillator to the counter 5 is blocked or an additional single pulse added intermediately between the drive pulses from the oscillator. The timing counter circuit 5 is therefore adjusted until the output of phase detector 7 is zero. To allow for slight hunting about the correct timing the attenuation of attenuator 2 is rendered zero during a period centred over twice the synchronizing tone duration.