Sequentially Companded Modulation for Low-Clock-Rate Speech Codec Applications

Abstract

The expansion of the step size of a speech codec may be arranged to change as the number of identical consecutive bits starts to increase. This technique causes the codec to respond partially to a fewer number of identical consecutive bits and more dramatically to larger numbers. In contrast to a typical exponential expansion of the step size, the proposed technique, in addition, expands the exponent. For speech, two distinct advantages have been observed: (i) the improvement of higher frequency audio frequency response at the same clock rate and (ii) the reduction of idle channel noise. In practice we have found that three- and four-bit companding will suffice for a typical 24 kHz, ADM codec. The proposed companding appears to be an acceptable choice between two-, three-, and four-bit companding which leads to better frequency response but worse noise, and four-bit companding which leads to both worse frequency and noise responses.