Quadraphonic has 2 more channel 'encoded' in a high frequency carrier (50 kHz maybe) which can't be heard on regular mono or stereo equipment. So each of the 2

Quadraphonic has 2 more channel 'encoded' in a high frequency carrier (50 kHz maybe) which can't be heard on regular mono or stereo equipment. So each of the 2 stereo channels will have this carrier signal encoding one additional sound channel.