PURPOSE: To mix and multiplex a low speed data signal and a sound signal by arranging the signaling bit for a sound signal at the frame bit position of a prescribed frame, mixing, multiplexing and transmitting the sound signal and the low speed data signal and preserving the (6+2) envelope form with a low speed data signal.

CONSTITUTION: In 20-multi-frame constitution with the 8 bits of bits B1-B8 as a unit, a low speed data signal and a sound signal are transmitted. The low speed data signal is multiplexed to multi-frames 1-10 by the (6+2) envelope form arranged in the sequence of a frame bit F of one bit, a data bit D of 6 bits and a stator bit S of one bit. An 8k system sound signal is multiplexed to the multi-frames 11-20. A bit B1 of the multiframes 1-8 becomes a frame bit F for synchronizing a main multi-frame. A bit B1 of the multi-frames 9 and 10 arranges the signaling bit SIG for a sound signal and can correspond to the two lines of a voice. As the result, the low speed data signal and the sound signal can be mixed and multiplexed in a 64kb/s.