Stability analysis of APC systems

Adaptive predictive coding (APC) is a useful technique for high-quality digital encoding of speech signals at medium band data rates. A particularly annoying degradation in APC systems, however, is the presence of "glitches" or "beeps" in the output speech. These correspond to frames with signal-to-quantization-noise ratios (S/Q) that are often less than unity, i.e., negative in dB. We show that these degradations are caused by instabilities of the APC system. This paper provides a method for analyzing the stability of APC systems as a function of the encoding bit rate, the quantization algorithm, and the power gain of the APC loop feedback filter. Two methods of improving system performance based on the results of the stability analysis are then developed. Experimental evaluation by listening tests confirm the validity of the techniques.