Quadraphonic CD-4 records from the 1970's encode 4 discrete channels into a single record groove! https://en.wikipedia.org/wiki/Compatible_Discrete_4 https://e

Quadraphonic CD-4 records from the 1970's encode 4 discrete channels into a single record groove! https://en.wikipedia.org/wiki/Compatible_Discrete_4 https://en.wikipedia.org/wiki/Compatible_Discrete_4 The trick is to encode two additional channels on a 30kHz carrier signal above human hearing range, then demodulate those channels in the receiver.