T.D.M. transmission of programme channels

Abstract

A programme channel requires greater bandwidth than a normal telephone speech channel. It can be accommodated in a time-division multiplex (t.d.m.) transmission system by using the pulse trains of several speech channels. The paper shows that, when these pulse trains are not equally spaced, distortion components are produced in the output signal.In general, the distortion components are at the channel pulse-repetition frequency and its harmonics, together with upper and lower sideband components. To obtain a sufficiently low distortion level, the permissible departures from equal pulse spacing are only of the order of ±1° or less. Similar distortion is also introduced if the pulses are not identical in height and shape. To obtain sufficiently low distortion level, the differences in height and length of the pulses used for a programme channel may only be of the order of 1%.Although the analysis is made for t.d.m. systems using pulse-amplitude modulation (p.a.m.), its results are shown to be applicable to some systems using other forms of modulation, such as pulse-code modulation (p.c.m.). It is concluded that p.c.m. systems are unsuitable for programme transmission unless the total number of time slots per frame (including synchronising pulses) is an exact multiple of the number of speech channels required to constitute a programme channel.