In simplest terms, quadraphonic is 4-channel audio, consisting of two front speakers and two rear speakers. The speakers are positioned in four corners of the listening space, and all four channels consist of completely independent audio. Introduced in the 1970s, quadraphonic wasn’t one format, but many competing, and largely incompatible, formats on different media. Quadraphonic was available on records, 8-track, and reel-to-reel. Quadraphonic was a commercial failure. The variety of competing formats coupled with the lack of hardware capable of reproducing quadraphonic sound doomed multi-channel audio to go the way of the dinosaur until the late 1980s, when multi-channel came back to the forefront with the home theater boom in the form of surround sound.
The quadraphonic formats that were available were:
CD-4: One of the less popular vinyl formats due to incompatibility issues and limited frequency response (topping out at 15kHz).
Q4: Said to be the best with fully discrete audio and full bandwidth. A reel-to-reel format.
Quad-8: An 8-track format.
SQ: (Surround Quadraphonic) Matrix quadraphonic for vinyl.
QS: Similar to SQ, but functioned almost exclusively on Sansui receivers.
Matrix H: Developed by the BBC as a means to broadcast quadraphonic audio over the air in a manner compatible with normal stereo receivers.