Joint stereo coding methods try to increase the coding efficiency when encoding stereo signals by exploiting commonalties between the left and right signal. There are two usual joint stereo coding algorithms, namely mid-side (ms) stereo coding and intensity stereo coding. Ms stereo applies a matrix to the left and right channel signals, computing sum and difference of the two original signals. Whenever a signal is concentrated in the middle of the stereo image, ms stereo can achieve a significant saving in bitrate. Even more important is the fact that by applying the inverse matrix in the decoder the quantization noise becomes correlated and falls in the middle of the stereo image where it is masked by the signal.
Intensity stereo coding is a method that achieves a saving in bitrate by replacing the left and the right signal by a single representing signal plus directional information. This replacement is psychoacoustically justified in the higher frequency range since the human auditory system is insensitive to the signal phase at frequencies above approximately 2kHz.
Text © Menno Bakker - Audiocoding