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Then encode a surround sound signal as a Dolby Pro Logic II stereo signal and you’ve got a surround sound record!

https://en.m.wikipedia.org/wiki/Dolby_Pro_Logic

DPLII was based on the fact that if you invert one channel of a stereo signal, it sounds effectively the same. So, they had some way of encoding front and back signals as some sort of symmetry/anti-symmetry between the left and right signal.

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DPLII was originally worked out as a blind upmixer using analog circuitry. See this interview with Jim Fosgate, who designed it:

https://www.stereophile.com/interviews/1204fosgate/index.htm...

Later Fosgate worked with Roger Dressler at Dolby and Dolby's engineers converted the analog circuit to digital. See these two posts from Dressler for additional detail:

https://www.avsforum.com/threads/dolby-pro-logic-ii-vs-dolby...

The encoder came later, after the DPLII blind upmixer was already designed.

They actually did that: https://en.wikipedia.org/wiki/Compatible_Discrete_4

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