PURPOSE: To record the interchangeable 4-channel sound information, by applying the FM processing to a subcarrier by means of a subchannel signal and then giving the FM processing to a main carrier by means of the output of the FM processing of the subcarrier and a main channel.

CONSTITUTION: In a pair of stereo modes, a main signal, i.e., an addition output of an input signal is delivered to output lines 3 and 4 of matrix circuits 1 and 2 on the basis of a control signal and then turned into each input of mixers 7 and 8 respectively. While a subchannel signal, i.e., a subtraction signal is delivered to output lines 5 and 6 of circuits 1 and 2 respectively. The subchannel signal is supplied to modulators 9 and 10 and gives FM modulation to the subcarrier. These outputs are turned into the other input of mixers 7 and 8 respectively to be synthesized with signals 3 and 4 and supplied to the inputs of modulators 11 and 12 respectively. The outputs of these modulators are mixed by a mixer 14 together with the video signal undergone the FM processing through a modulator 13 and turned into the video disk recording signals via a limiter 15.