Stability study of the SB-ADPCM coder Subband ADPCM coder uses ADPCM to quantize frequency components. A practical ADPCM uses always the backward adaptive predictor in order to satisfy that the encoder and the decoder work synchronously. Because of this backward loop the destabilization is introduced into the subband coder system. In this paper a system-theoretic framework is used to study such unstability. The conditions for the stability of SB-ADPCM are given and a closed form expression of the optimum bit allocation taking into account of the stability is presented.