PURPOSE: To restore an audio signal to a substantial dynamic range at a receiver side, by using a data corresponding to an amount of compression, when the audio signal is compressed in a broadcast station as a multiplex data signal.

CONSTITUTION: After L, R signals are compressed at a limiting amplifier consisting of variable attenuators 1, 1', and level detectors 2, 2', the signals are inputted to a stereo modulator 4 to be a stereo composite signal. A compression control voltage being an output signal of the detectors 2, 2' is coded at A/D converters 3, 3' and synthesized into a signal code train at a multiplexer 5. The output of a multiplex subcarrier wave oscillator 6 is modulated with this code train, and the modulated output and a stereo composite signal from the modulator 4 are synthesized at an adder 8. The output signal of the adder 8 is used, the carrier wave is modulated at an FM modulator 9 to obtain an FM signal where the stereo signal and data signal are multiplexed.