With the introduction of new standards like DVD and SACD, multi-channel audio systems are growing in popularity. In order to maintain compatibility towards customers that have not migrated to a multi-channel environment, there is a strong demand for the stereo compatibility. Such compatibility was already introduced within the MPEG-2 BC audio standard by means of a matrixed multi-channel extension. The encoded data consists of the stereo downmix stream and the multi-channel extension information. The bitstream syntax allows legacy stereo decoders to decode the downmix, whereas the multi-channel decoders are capable to reconstruct all multi-channel signals. In this paper a matrixed multi-channel extension coding method based on the MPEG-2 AAC standard is introduced. To obtain the highest possible audio quality for both the compatible down-mix and the multi-channel signal in all circumstances, several new solutions were introduced. These include dominant center and dominant surround processing, as well as common window switching. Results of listening tests conducted using both stereo and multi-channel decoded streams are presented.
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