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.
The book · Volume 4 of 7
Audio for Video: A Complete Guide to Sound in Video Products: From PCM and Codecs to Loudness, Accessibility, and Real-Time Voice Processing
Joint stereo is a large part of why AAC and Opus sound good at modest bitrates, and also why some stereo images collapse when bits run short. This volume explains joint stereo in depth with mid/side and intensity coding, channel layouts and the codecs that use them, so you can set stereo bitrates with a clear view of the trade-offs. Written by Nikolay Sapunov, CEO at Fora Soft.
Hearing a narrow or smeared stereo image at low bitrates because of joint stereo coding?
Send us test clips and target bitrates and we will recommend codec and joint stereo settings that keep your stereo image intact. 250+ video products shipped since 2005.
