Joint Vs. Normal Stereo

Different techniques reduce the amount of data contained in an audio stream while preserving as much of the fidelity as possible. One such method, joint stereo, reduces the two distinct channels in a stereo stream into a much smaller and simpler stream of data. When properly applied, joint stereo processing can enhance the apparent quality of a file at low bit rates, allowing you to shrink audio files substantially.

Intensity Stereo

Intensity stereo takes advantage of the human ear’s inability to perceive the location of sound at certain frequencies. Using this technique, the encoder searches for areas in the audio stream where the content of the left and right channels is very similar, and blends the higher frequencies into a mono stream. It then includes directional cues to play that stream louder in one channel or the other to simulate differences in the stereo effect. This technique can substantially reduce file size, especially at low bit rates, but the results may include detectible audio artifacts.

Mid/Side Stereo

The mid/side technique is similar to the intensity technique, although it uses a different calculation. For each frame of audio, the encoder calculates a “mid” value, consisting of the data in the stream equal on both sides, and a “side” value, which represents the difference between what remains on the left and right. Depending on the bit rate, this technique can be very good at preserving audio data while reducing file size, as the calculations needed to reconstruct the audio stream from the encoded data are simple. Many lossless codecs use mid/side encoding as a way of preserving audio quality.

Advantages

Joint stereo techniques have the advantage of significantly reducing the amount of data required to store an audio stream in a digital format. In true stereo, the encoder must preserve two completely separate streams, often containing nearly identical information. This redundancy leads to large file sizes, and the extra overhead can add up when storing large quantities of audio.

Disadvantages

The chief disadvantage of joint stereo processing is that it usually involves a lossy encoding scheme, reducing the fidelity of the audio stream in exchange for a smaller file size. The actual alterations may be too subtle for the average listener to detect, but if you decompress and recompress files encoded with these techniques, the audio artifacts can become noticeable and detract from the quality of the sound. Maintaining two distinct stereo channels may take up more space, but it ensures a much higher level of fidelity.