A 32 kbit/s PCM to ADPCM converter

The structure and characteristics of a PCM to ADPCM converter are described. The main feature of the system is a prediction filter implemented with five cascaded recursive and adaptive second order sections. The filter coefficients are adaptively adjusted according to a gradient algorithm; the four bit adaptive quantizer allows a greater and extensible dynamic range in the converter. Stability is a major problem in adaptive filter design and it is solved here in a very simple manner. In spite of its low complexity the algorithm used yields good results with the different kinds of signals usually found in the telephone network, such as speech and voiceband data. It has been implemented in a 60 channel PCM-ADPCM converter, a small volume and flexible equipment which is going to be introduced in various places in digital telecommunication networks.