Finalized in 1992, the MP3 compression format has become a synonym for personalized music enjoyment for millions of users. The paper presents a novel extension of this popular format which adds support for the coding of multi-channel signals, including the widely used 5.1 surround sound. As a prominent feature of the extended format, complete backward compatibility with existing stereo MP3 decoders is retained, i.e. standard decoders reproduce a full stereo downmix of the multi-channel sound image. The paper discusses the underlying advanced technology enabling the representation of multi-channel sound at bitrates that are comparable to what is currently used to encode stereo material. Results for subjective sound quality are presented and related activities of the MPEG standardization group are reported.
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