A low-rate vocoder based on an adaptive subband formant analysis

A formant-based analysis technique has been developed that avoids some of the problems inherent in formant tracking by fitting a second order spectrum to separate subbands of the measured spectrum. Synthesis is performed using a high quality channel vocoder for which the modulator gains are set by sampling the envelope obtained from the formant-based analysis. By using a fixed bandwidth versus frequency rule, the number of parameters to be transmitted has been limited to the resonant frequency and gain of each of three subbands. By using 4-bit logarithmic coding and frame-fill interpolation methods, successful vocoding at 800 bps has been achieved.