PURPOSE: To easily and accurately detect function failure by setting the period of a check signal which is supplied to a decoder to a time equal to the total processing time of a signal processing system from the decoder to an encoder.
CONSTITUTION: In a check signal generation circuit 18, digital signal which is generated in the period equal to the total of respective signal processing time in which signals are inputted from the encoder 15, are fed back through a selector 5 and are fetched to the output of a cross fader 9 are formed, and the signals are supplied to a selector 14 and a digital comparator 19. If the digital signal of a period (s) equal to the total processing time (t) is supplied from the circuit 18 to the selector 14, for example, the signal is fetched from the fader 9 after (t) hours through the encoder 15, the decoder 7, and the like. At that time, the totally equal signal which is delayed by one period is fetched from the circuit 18, and it is supplied to the circuit 19, whereby the detection of disagreement is executed. Namely, if the signal of the circuit 18 agrees with the output of the fader 9, it can be decided that there is not any failure in the functions of respective circuits.