Multirate sub-band coder with embedded bit stream: Application to digital TASI

Application of sub-band coders to Time Assignement Speech Interpolation systems (TASI) is discussed. After a short review of standard optimum allocation of bits for one single voice port, the extension to multiports is discussed and is shown to present significant drawbacks. A new TASI approach with embedded bit stream is then proposed. The output of the multirate speech compressor has an embedded bit stream, i.e. accepts bit deletion and insertion for dynamic rate conversion /1/. This property is of high importance in a digital communication network, since it allows the bit stream to be flagged at any overloaded node without tandeming or freeze-out. The imbedded operation is obtained by taking advantage both of the sub-band coder architecture and of the dynamic bit allocation. So as to illustrate the method, a multirate version of a sub-band coder has been designed to operate at different rates: 8,16,24,32 kbps, providing a full range of quality from communications quality to toll quality.