Matrixing has been used for a long time in M-S stereo to provide backwards compatibility with monophonic reproduction. The MPEG phase 2 multichannel coding system uses matrixing to provide backwards compatibility to existing two-channel stereo MPEG phase 1 decoders. It is also used for transmission of more than two audio signals in two channels, e.g. Dolby Surround. The combination of matrixed audio signals and low bitrate coding based on perceptual principles may lead to artefacts due to an unmasking effect of the quantisation noise if no countermeasures are taken. The paper discussses means to code matrixed signals with low bitrates and without artefacts.
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