Joint (audio engineering)
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In audio engineering, joint refers to a joining of several channels of similar information in some way, in order to obtain, for example, higher quality or smaller file size.
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[edit] Joint frequency encoding
Joint frequency encoding is an encoding technique used in audio data compression to reduce the data rate.
The idea is to merge a given frequency range of multiple sound channels together so that the resulting encoding will preserve the sound information of that range not as a bundle of separate channels but as one homogenous data stream. This will naturally destroy the original channel separation for good, as the information cannot be accurately reconstructed, but this process will greatly lessen the amount of required storage space.
[edit] Joint stereo
The term joint stereo has become prominent as the Internet has allowed for the transfer of relatively low bit rate, acceptable-quality audio with modest Internet access speeds. It should be noted that there are many types of joint stereo encoding. Not all of them are forms of joint frequency encoding. Two types are described here, both of which are implemented in various ways with different codecs, such as MP3, AAC and Ogg Vorbis.
[edit] Intensity stereo coding
The intensity stereo coding form of joint stereo encoding functions on the principle of sound localization. The dominance of inter-aural time differences (ITD) for localization (by humans) is only given for lower frequencies. That leaves inter-aural amplitude differences (IAD) as the dominant location indicator for higher frequencies. The idea of intensity stereo coding is to merge the upper spectrum part into just one channel (thus eliminating phase differences) and to transmit a little side information about how to pan certain frequency regions to recover the IAD cues. Intensity stereo coding does not perfectly reconstruct the original audio because of the loss of data resulting in the simplification of the stereo image, and can produce unwanted artifacts under certain conditions (switch-point too low or inaccurate merging by the encoder for example). However, for very low bitrates this tool usually provides a gain of perceived quality. It is supported by most of the audio compression formats (including MP3, AAC and Vorbis) but not by every encoder.
[edit] M/S stereo coding
M/S stereo coding transforms the left and right channels into a mid channel and a side channel, hence the name. The mid channel is the sum of the left and right channels, or L + R. The side channel is the difference of the left and right channels, i.e., L − R. Unlike intensity stereo coding, M/S coding retains the audio perfectly and does not introduce artifacts by itself. It is a special case of transform coding. However, when used in conjunction with a lossy form of compression, artifacts can become apparent.
This kind of coding is also sometimes known as matrix stereo, and is utilized in many different forms of audio processing and recording equipment including broadcasting. It is therefore not limited to digital systems, and can even be created with passive audio transformers or analog amplifiers.
One example of the use of M/S stereo is phonograph records, where the mid channel (L + R) is cut into the groove horizontally, and the side channel (L − R) is cut vertically. Another use is in FM stereo broadcasting, where L + R modulates the carrier wave and L − R modulates a subcarrier.
