To transmit a voice at a low data rate and to encode it to a bit stream being stiff against a background noise and a channel error by including a step, etc., for synthesizing a redundant error control bit with a spectral bit encoded from two continuous blocks to one by frame with a specified velocity.

A digitized voice signal is passed through a voice encoder 80 and the signal is transmitted through a communication link by a transmitter 90. The encoder 80 includes an adaptive voice activity detector(VAD) identifying the respective sub-frames of 22.5 msec as any of a voice, a background noise or a tone according to adaptive quantizing processing. VAD algorithm separates a voice subframe from a background noise by using local information. The burst time-division multiple acess(TDMA) having a 90 msec frame is utilized for this communication link.