A four-channel stereo receiver capable of making the four-channel indication, and automatically changing over the four-channel mode operation and the two-channel mode operation or muting the monaural and the two-channel signal, based on the presence or absence of at least one of the second and the third...http://www.google.de/patents/US4013841?utm_source=gb-gplus-sharePatent US4013841 - Four-channel stereo receiver
