Matrixing enables multichannel audio to be made two-channel compatible. If bit-rate reduction based on psychoacoustic models is applied to the two-channel downmix, unmasking of the quantization noise may result after demixing. The general principle to solve this problem is presented: the application of so-called prequantization and masked threshold adaptation. It is shown how minimum bit rates are obtained by dynamic switching between the possible matrix equations. The necessary refinements to enable such dynamic switching are discussed. Further attention is needed in the two situations where the center signal dominates and where the compatible masked threshold drops below the multichannel one. Methods to handle these situations are given. Compatibility matrixing is part of the ISO IEC International Standard 13818-3, also known as MPEG-2.
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