# Joint stereo

It has been suggested that Mid-side stereo be merged into this article or section. (Discuss)

Joint stereo coding methods try to increase the coding efficiency when encoding stereo signals by exploiting commonalities between the left and right channel signal. In the context of MP3 encoding, there are actually 2 totally different method of Joint Stereo: Mid-side Stereo and Intensity Stereo.

## Mid-side Stereo

Mid-side stereo coding calculates a "mid"-channel by addition of left and right channel, and a "side"-channel, i.e.:

$Left = L \qquad Right = R\,$

$Middle=\frac{L+R}{2} \qquad Side=\frac{L-R}{2}$

$Left=Middle+Side \qquad Right=Middle-Side$

Whenever a signal is concentrated in the middle of the stereo image (i.e. more mono-like), mid-side stereo can achieve a significant saving in bitrate, since one can use less bitrate to encode the side-channel. 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.

Unlike intensity stereo which destroys phase information, mid-side coding keeps the phase information pretty much intact. Correctly implemented mid-side stereo does very little or no damage to the stereo-image and increases compression efficiency either by reducing size or increasing overall quality.

## Intensity Stereo

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.

Intensity stereo is by definition a lossy coding method thus it is primarily useful at low bitrates. For coding at higher bitrates only mid-side stereo should be used.

Some more details about joint stereo & mid-side coding:

• Bugs and/or not-optimized encoders may implement mid-side coding incorrectly, making mid-side coding sounds worse than simple stereo, while in reality (see the formulas above) there should be no difference in quality between mid-side stereo and simple stereo.
• Modern/optimized encoders will use mid-side coding or simple stereo coding as necessary, depending on the correlation between the Left and Right channels.