PURPOSE: To eliminate a variable resistance attenuator, a hybrid transducer, etc., to make a unit small-size by giving the level difference between program switching control signals and program quality monitor signals digitally and performing digitally the mixture of signals.

CONSTITUTION: Free running multivibrator 41 generates 3.2Hz square wave signals. These signals are frequency-divided by 2 in FF 42, 43 and 44. Time division signals of two frequencies of waveforms (e) and (f) and waveforms (g) and (h) are obtained by decoders 45 and 46 respectively. When applied to gate circuit 49, waveforms (e) and (f) and program quality monitor signals from signal oscillator 54 are coupled digitally and become waveform (m). Waveforms (g) and (h) are waveform-shaped by monostable multivibrators 47 and 48 and become waveforms (i) and (j) respectively, and waveforms (i) and (j) and program switching signals from signal generator 54 are coupled digitally in gate circuit 50 and become waveform (p). Waveforms (p) and (m) are mixed in gate 51.