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Thispaper discusses an implementation of an experimental variable bit-rate codec (VRC) that processes speech signals at 16 kbps to 48 kbps.In previous papers, we proposed a multiple quantizer scheme that improves the transmission characteristics of low bit-rate ADPCM[I]. We investigated this scheme to achieve variable-rate coding of speech.We propose the combined evaluation method of constant SNR and constant noise to optimum bit-rate control of VRC. This algorithm was implemented with six digital signal processor (DSP) chips. The average bitrate of the prototype codec for conversation through the telephone channel was about 23 kbps, including supplementary information, and a quality better than that specified in G.721 (32kbps ADPCM) was observed.