Backward Adaptive Reencoding: A Technique for Reducing the Bit Rate of µ-Law PCM Transmissions

This paper presents a technique for reducing the bit rate of μ-law PCM-coded speech from 8 bits/sample to a format of 6, 5, or 4 bits/sample. The particular format can be specified dynamically, i.e., it can be made to change from sample to sample. A device based on this technique has been designed using a 3 kbyte ROM and an uncommitted logic array (ULA) circuit to accommodate up to 96 multiplexed channels. The device can be set to operate in either the bit-rate-reducing mode, for removing the redundancy at the transmitter, or in the bit-rate-restoring mode, for recovering the standard PCM speech representation. The device finds useful applications in the context of multichannel telephony. In particular, it can be combined with digital speech interpolation to yield a three to four compression factor [2]. This technique provides approximately equivalent performances to the nearly instantaneous companding (NIC) technique [1] without the need for block processing, delay, or transmission of side information. The principle is based on the use of switched optimum reencoders (quantizers) selected by using a backward adaptation scheme. It further incorporates a redundancy reduction scheme [2], [3] which capitalizes on properties of both companded PCM and speech