What is “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.

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