Channel coupling

Channel coupling is a method used to reduce bitrate demand by coding the stereo channel information more efficiently. there are several channel coupling methods. In mp3 the general term is joint stereo.

MP3 can use 2 different channel coupling methods: mid/side-coding and intensity stereo. Mid/Side-coding calculates a "mid"-channel by addition of left and right channel (l+r)/2 and a "side"-channel (l-r)/2. With more mono-like signals one can use less bitrate to encode the side-channel, so that the overall bitrate will be less than encoding the left and right channel. intensity stereo destroys phase information, so it's used only at low bitrates.

Correctly implemented MP3 mid/side stereo (like in LAME) does very little or no damage to the stereo-image and increases compression efficiency either by reducing size or increasing overall quality.

Ogg Vorbis and several lossless audio compressors can also use lossless channel coupling. In lossless channel coupling the channels will remain exactly the same as with stereo coding after the decoding.

Ogg Vorbis uses lossless channel coupling for high bit rates only.