GB1501874A - Telecommunications apparatus - Google Patents

Abstract

1501874 Speech transmission DEFENCE SECRETARY OF STATE FOR 3 June 1975 [7 June 1974] 25475/74 Heading H4R In a pulse code modulation system, the logarithmically compressed amplitudes of, and time intervals between, successive stationary points (maxima and minima) of an input analogue waveform are encoded, and the decoder includes a filter whose response time is controlled in accordance with the received time interval signals. Coder.-Stationary points are detected by a differentiator 2 and zero cross detector 4 which controls the sampling instants for the amplitude coder 6 and the time interval coder 7, either or both of which may have a compressive characteristic. Decoder.-Timer 15 controls read out of time interval and amplitude codes from buffer 13 in accordance with the time interval codes. Amplitude decoder 17 produces a stepped wave analogue output in which each level has a duration corresponding to a respective time interval code and this stepped wave is smoothed in a second order controllable filter 19, details Fig. 3 (not shown) comprising impedances which are switched in and out of circuit with a variable duty-cycle determined by the time interval decoder 16 to match the rise time of the filter to the duration of each step in the output of the amplitude decoder 17. Transmission link.-In order that the P.C.M. signals may be transmitted at a predetermined regular bit rate, despite irregular sampling, buffer stores 8, 13 are provided and the transmission link has a slight excess capacity. When multiplexer 9 finds an empty store location, a zero time interval code is transmitted together with a repetition of the previous amplitude code.