Patent GB-1117292-A (GB)
Title: Supervisory apparatus for telecommunication systems
Inventor: LORD MICHAEL ROBERT
IPC: H04J3/14
Filed: 1966-02-02
Abstract: 1,117,292. Monitoring time division multiplex systems. STANDARD TELEPHONES & CABLES Ltd. 2 Feb., 1966, No. 4572/66. Heading H4L. In a time division multiplex telecommunication system providing an equal number of two-way signalling and information channels, a test tone is transmitted over a selected channel only during idle periods of that channel, an alarm being operated when the amplitude of the received and transmitted test tones differ by more than a predetermined amount. The arrangement is described as applied to a p.c.m. system, Fig. 1, which provides N two-way channels each channel comprising an audio and a signalling circuit. At each transmitter the audio and signalling circuits are connected to electronic selector switches 1 and 2 respectively and the audio signals are supplied via a common coder 3 to a combiner 4 where a further digit for synchronizing and to indicate the signalling condition on a channel is added. At the receiver the synchronizing signalling and audio information are separated at 8, the signalling digits being supplied to selector 11 and the audio signal being supplied via a common decoder 12 to a selector 10. At each terminal station A and B a supervisory logic circuit, Fig. 2, is provided which has terminals C, D, E connected to corresponding terminals in Fig. 1. It is assumed that channel J is to be used for supervisory purposes and when this channel is idle there will be no D.C. signal on terminal D and a signal will therefore be provided by inverter 16 which is applied via a circuit 15, which passes signals only if their duration exceeds a specified value, to open a gate 13 so that an output from audio oscillator 14 appears at terminal C and is injected into audio channel J. This signal is converted into p.c.m. at 3 and at the receiver (station B) is converted back into the original audio signal and appears at terminal E and therefore there will be no output from inverter 19. At station B there will be no signal at D (channel J idle) so that a test tone from the oscillator 14 at station B will appear at terminal C and will be transmitted back to station A via channel J and the common coding and decoding equipment to provide a signal at terminal E. Therefore there will be no output from inverter 19 controlling gate 17. If a fault occurs in the common equipment in the A to B direction, the signal at terminal E station B will vanish so that a signal will be supplied via gate 17 and terminal F to operate an alarm. If the fault is in the B to A direction, i.e. no signal at E, the alarm at station A will be operated similarly. The sensitivity of gate 17 or the amplitude of the signal applied to it is adjusted so that the alarm will operate when the transmission loss has reached a value considered to indicate a fault condition. In a modification, there is no oscillator 14 or alarm at station B and the test tone received at B is re-transmitted back to station A. Additional circuitry may be provided, Fig. 4 (not shown), when the " go " and " return " circuits are not synchronous with each other. Two digits of the code representing the audio test signal on channel J are extracted from the outgoing line, stored in bi-stable units, gated by the synchronizing signals at the receiver and injected into the receive line during the appropriate time slots of channel J.