Speech coding at a very slow rate of delivery (8 Kbit/s) with delta modulators

The great design simplicity of “delta modulators” has indicated a considerable development potential for this technique. However, at a very slow rate of delivery, the quality achieved deteriorates considerably to the extent of limiting the use of delta modulation to a few very specialized functions. Based on extremely simple delta modulators, some speech signal processing methods have nevertheless shown satisfactory results. These methods are described in this article and call for separate processings (alternate or joint) of the signal sign and envelope and, in a more performing version, a variable frequency sampling of the rectified envelope. For channel by channel processing, the transmission is achieved at the fixed rate of 8 Kbit/s. Moreover, in order to reduce digital computations, the delta coding of a speech signal digitized after a logarithmic compression of voice modulated amplitude is considered on the basis of previous methods. The purpose of this article has been limited to presenting original speech coding methods which may prove interesting for specific applications, in the light of preliminary results.