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by b1c837696ba28b 1168 days ago
FM radio: https://en.wikipedia.org/wiki/FM_broadcasting

> left and right, but with opposite phase

The classic mid-side mic technique uses two capsules. The first (middle) is a cardioid or omnidirectionsl facing the subject, the latter (side) is a figure-8 turned 90 degrees. The capsules should be as close to each other as possible to avoid low-frequency phase errors. The signals are converted to L+R via a simple Mid/Side network:

L = 0.707 * (M+S), R = 0.707 * (M-S)

which is where "opposite phase" comes into play.

https://www.uaudio.com/blog/mid-side-mic-recording/