MPEG Surround represents the state of the art in multi-channel audio compression. Standardized in ISO/IEC 23003-1, MPEG Surround is the forceful continuation of using parametric descriptions of key signal characteristics in high quality audio coding. Similar to SBR for the high frequency range or Parametric Stereo for the stereo image, MPEG Surround relies on a parametric description of a multi-channel audio signal instead of coding all channels of an audio signal separately. The result: Where discrete multi-channel audio codecs would require 2-3 times the bit rate compared to stereo coding, MPEG Surround adds only a very small amount of additional data.

Building blocks of an MPEG Surround system

MPEG Surround audio coding systems comprise a core audio codec such as aacPlus, and the MPEG Surround technology on top of it. The encoding process can be described in three steps: the parameters describing the surround information in the respective audio channels are extracted, a mono or stereo downmix of a multi-channel signal is being created, and the downmix is encoded with the core audio codec for transmission along with the surround parameters embedded.

On decoder side, the stereo downmix is being decoded and the full multi-channel signal is being recreated based on the parameterized surround information. The MPEG Surround data is embedded in the ancillary data part of the bit stream and is thus only recognized by MPEG Surround decoding systems. A legacy stereo decoder version corresponding to the core encoder deployed in the MPEG Surround encoding system, will simply ignore the MPEG Surround data and decode the downmix signal generated by the encoder.

One major advantage of the MPEG Surround technology besides its unparalleled efficiency is based on this architecture: Full backwards compatibility to existing legacy decoding equipment. As the transmitted downmix, in most cases in stereo, is available for decoding as the output signal, existing stereo decoder equipment can stay in use and will not suffer any noticeable quality degradation. This approach provides an easy upgrade path to multi-channel audio for existing digital broadcast systems without affecting legacy stereo-only decoders. Combining e.g. MPEG Layer-2 audio with MPEG Surround enables broadcasters to offer multi-channel content to consumers with MPEG Surround enabled equipment, whereas any stereo decoder would still play back the stereo signal. Most importantly, the added value of multi-channel content can be introduced without the need for simultaneous transmission of stereo and multi-channel content, and even at very moderate transmission bandwidth costs of only a few kilobits per second.

Efficiency

Combining the new MPEG Surround technology with aacPlus provides high audio quality at bit rates in the range of 64 - 96 kbps for 5.1 multi-channel audio signals. This further gain of coding efficiency by a factor of 2-3 extends the existing aacPlus family of codecs towards an even more powerful standard suitable for all kinds of application scenarios from mobile music to home theatre in a forward and backward compatible way.

For more information on MPEG Surround, please visit www.mpegsurround.com or our whitepapers section.

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