Patent Document ID: 4520491
Application ID: 06439146
Patent Flag: 1

Claim One:
1. A digital information transmission system using differential pulse code modulation and comprising an encoder for converting an incoming linear PCM signal into a differential PCM signal with a reduced number of bits, and a remote decoder for carrying out the inverse conversion, the encoder comprising a difference circuit forming a difference signal between the incoming linear PCM signal and a prediction signal, a first quantizer operating with a quantizing step and quantizing the difference signal to form the differential PCM signal and a local decoder receiving the differential PCM signal and forming the prediction signal, the local decoder and the remote decoder being constituted in the same manner and comprising each a second quantizer operating inversely to said first quantizer of the encoder and a filter constituted by a cascade arrangement of filtering sections, each section being provided with a round-off (or truncation) circuit operating with a step equal to the quantizing step of the first quantizer, characterized in that the filters of the local decoder and the remote decoder comprise several filtering sections of the second order having two coefficients, at least one of which is controllable, the controllable coefficients of corresponding sections of the filter of the local decoder and the filter of the remote decoder being controlled by identical control circuits receiving the same signal derived from the differential PCM signal and comprising means for calculating the controllable coefficients by successive iterations so as to minimize a predetermined function of the difference signal produced in the encoder, as well as means for testing, for each section and at each iteration, whether each controllable coefficient affected by a calculated modification term is larger than an upper limit value or smaller than a lower limit value, said means for testing determining that these limit values are such that the poles of the transfer function in Z of said section lie just inside a circle of unit radius, the modified value retained for said controllable coefficient being the nearest limit value when the test is positive and being the value affected by the calculated modification term when the test is negative.