PURPOSE: To minimize deterioration in quality of voice and to transmit voice and data simultaneously by allowing a coding section and a decoding section to switch the code rule synchronously when a forecast coefficient detection circuit of transmission side and a reception side detects respectively that the forecast coefficient is at the outside of a prescribed range.
CONSTITUTION: A tone signal generator 1 uses a data transmission request signal RE to output a tone signal of a predetermined specific frequency and a switch 3 switches a voice signal into a tone signal for a prescribed time to input it to a low-frequency coding section 2. When the tone signal is cut off after a prescribed time, a forecast coefficient detected by forecast coefficient detection circuits 5, 9 exceeds a prescribed range. Thus, the other coding rule reducing bit number is switched to the low-frequency coding section 2 and the low-frequency decoding section 10, which are operated in the data simultaneous transmission mode. Further, a multiplex circuit 8 at the transmission side inserts a data signal D to a low-order 1 bit of e.g., a low-frequency coding section 2, outputs the result multiplexedly and a separating circuit 14 at the reception side outputs separately the corresponding bit as the data signal D.
KO MASAHIRO
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