Poking around on wikipedia (https://en.wikipedia.org/wiki/Compatible_Discrete_4), I learned that true four-channel quadraphonic sound, for LPs, was obtained by further recording two more signal streams in the same groove.
These other channels were recorded with FM (frequency modulation) on top of a 30 kHz carrier. The 30 kHz carrier was just high enough to isolate the quad channels from the 0-15 kHz ordinary left/right stereo channels.
And then, the FM portion, centered around 30 kHz, was added to the regular baseband audio signal (0-15 kHz), and all encoded into the diagonal displacements you mention in your comment.
Brilliant - so that's how the my copy of Mike Oldfield "Boxed" (https://en.wikipedia.org/wiki/Boxed_%28Mike_Oldfield_album%2...) was supposed to work. As I didn't have the cash to buy a quad player I never got to hear it in "Glorious quadrophonic sound", I had to use an "old tin box".
It always baffled me that it also worked on an ordinary stereo - and I sometimes wondered if I had been had.
We now have the tech to encode many channels in one MP4 stream, but don't we don't bother and just stick to 2 for sound. What does that tell us about the future of 3D TV and 360 degree video? Perhaps more of us will stick to the old tin boxes.
Netflix sent me 6 channels for Daredevil, and the sound production was amazing. If you've only heard it via stereo, you missed out on all sorts of excitement.
Good multi-channel sound immerses you in an experience, and is worth a significantly smaller screen as a trade-off, if you have to make it.
That's the perfect movie for demonstrating the potential of multi-channel audio. The only problem is that you're also relying on the viewer to have proper speaker placement, sound levels and speaker calibration. Autocalibration (YPAO) is simple and easy but hardly anyone knows to use it when installing their $300 home-theater-in-a-box.
Master and Commander is another film with great surround sound. A number of years ago now, Dolby Labs used the sound from that movie as a showpiece in their SOMA theatre.
These other channels were recorded with FM (frequency modulation) on top of a 30 kHz carrier. The 30 kHz carrier was just high enough to isolate the quad channels from the 0-15 kHz ordinary left/right stereo channels.
And then, the FM portion, centered around 30 kHz, was added to the regular baseband audio signal (0-15 kHz), and all encoded into the diagonal displacements you mention in your comment.