Integrated single-channel PCM encoder-filter combination

An encoder for use in PCM trunk transmission is presented. It consists of a 512 kHz Sigma-Delta Modulator followed by a 640th order non-recursive digital filter. The filter output, subsampled at 8 kHz, is compressed according to the A-law and multiplexed. The bandlimiting function of the filter complies with the CCITT Recommendations. Its higher frequency behaviour is such that noise in the band from 4 to 508 kHz contributes negligibly to the PCM output. The system is realized as an LSI circuit plus a few non-critical standard components for the delta modulator and a 2nd order active prefilter.