Joint stereo
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Joint stereo, in audio engineering, is the technique of encoding stereo audio into a "mid channel" which is a full channel average of the combined left and right channels, with a side channel which has separation information on how to re-create two distinct stereo signals. This can aid compression for a large majority of music, and may be used in MP3, AAC and Ogg Vorbis audio encoding techniques.
This is the same kind of encoding used in broadcasting FM Stereo. The main channel contains the "mono" composite signal, and the left-right separation is modulated on a 19 kHz subcarrier.
There is no significant difference in file size or subjective sound quality between the two methods of stereo MP3 encoding.