PURPOSE: To obtain a spectral distribution at the same point of time by sampling channel signals in the same frame of a time-divided input signal at a low frequency respectively and extracting all channel signals in the same frame.
CONSTITUTION: The input signal IN supplied to a terminal 17 is sampled when control signals for switches 11-1∼11-4 turn off, and accumulated in capacitors 13-1∼13-4 respectively. The voltage corresponding to charges accumulated in a capacitor 16 is outputted to the output terminal 18 of an operational amplifier 14 from the time when control signals for switches 12-1∼12-4 turn on to the time when a control signal for a switch 15 turns on. Thus, when time-divided channel signals in the same frame of the input signal are thinned out, all channel signals in the same frame are extracted by plural sample holding circuits provided in parallel and variation with time in spectrum at the same point is detected.