Film surround matrix decoders are primarily 4-channel devices with, in some cases, a decorrelation circuit to make the single rear channel sound less monaural. Conventional decoders with multiple rear output channels suffer from low separation during music and environmental effects, which is most of the time in a film. Current decoders also suffer from various defects, such as pumping of the music during dialog. This paper discusses a mathematical design method for producing a matrix decoder with optimal properties-maximum rear channel separation during music, accurate placement of directionally encoded material, and constant music level in the presence of any directionally encoded effect. The design of a 5-channel encoder is also presented. This encoder in combination with a 5- or 7-channel decoder reproduces 5-channel discrete films with very little subjective loss. In fact, playback through the 7-channel decoder (with 4 rear channels) is sometimes subjectively preferable to 5-channel discrete reproduction.
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