Joint stereo is a stereo technique that has higher compression than true stereophony because certain channel information is combined. This technique, which is used in MP3, is also known as M/S stereophony or middle/side stereo coding.
Since in normal stereo transmissions there is some redundancy between the signals from the left and right channels, joint stereo relies on redundancy reduction. In joint stereo, therefore, a sum signal and a difference signal are transmitted, which are formed from the signals of the two channels. The difference signal is made up of the differences between the frequency bands and the volume. Since the lower frequency bands are not direction-sensitive in sound reproduction, they do not have to be transmitted and radiated separately.
Due to the redundancy reduction, less data is generated and the transmission can be done with a lower bit rate. Joint stereo only works if the audio signals from the left and right channels have small differences.