Joint stereo is a family of techniques for coding a stereo pair together rather than as two independent channels, exploiting the fact that left and right usually share most of their content. The main forms are mid/side coding, which stores a sum and difference channel, and intensity stereo, which shares high-frequency content between channels and codes only its panning. By not spending full bits on redundant information, joint stereo improves quality at a given bitrate, which is why MP3, AAC, and Opus all use it. Encoders switch it on and off per frame as the stereo content varies, since a wide, decorrelated mix can actually be hurt by aggressive joint coding.
