Joint stereo is a technique used in audio coding to improve the efficiency of the coding of the left and right channel of a stereophonic audio signal. Joint stereo coding takes advantage of the fact that both channels of a stereophonic audio signal contain to a certain extent the same information.
There are two techniques used in joint stereo coding:
Mid/Side Stereo (MS Stereo) uses one channel to encode information that is identical on the left and right channel and the other channel to encode the difference between the two channels. The result is a channel with the monophonic information (Left + Right) which is called the Middle channel and another channel with the difference (Left - Right) which is called the Side channel.
MS Stereo itself is lossless. It doesn't filter information out. It only encodes the stereo channels in another way. MS Stereo is widely used in both lossy and lossless audio coding techniques.
Intensity Stereo makes use of the fact that the human ear is not able to locate the spacial location of sounds with full accuracy for rather high frequencies above about 2 kHz. Intensity Stereo encodes these frequencies in a monophonic signal and adds some information about the direction.
Because Intensity Stereo is based on imperfections of the human ear it throws away some information. The stereophonic image is not perfect anymore. Therefore, Intensity Stereo is not lossless but lossy. Intensity Stereo is primarily used for stereo coding at low bit rates.