The MPEG Surround idea extends traditional approaches for coding of two or more channels in a way that provides significant advantages, both in terms of compression efficiency and user features.
A compact set of parameters representing the spatial image of the original surround signal is transmitted along with an automatically generated mono or stereo downmix. Alternatively, an externally created downmix signal (»artistic downmix«) may be used.
On the receiving side, a legacy decoder will disregard the MPEG Surround spatial parameters and will reconstruct the downmix. In this way backward compatibility is achieved. An MPEG Surround enabled decoder will expand the transmitted downmix signal into a high quality multi-channel output based on the spatial parameters.
In addition to this regular method of multi-channel reproduction, the MPEG Surround technology offers a true-to-life virtual surround rendering for headphones. In this mode of operation, a "binaural" stereo output signal is rendered based on the same bit stream as for the multi-channel case. When this "binaural" stereo signal is played back over standard headphones it yields a virtual surround sound impression. As a consequence, MPEG Surround enables an introduction of high quality surround sound to mobile devices. Traditional techniques for surround virtualization require the calculation of all virtual output channels as an intermediate step of processing. This typically leads to a high complexity. Within MPEG Surround this complexity burden has been mitigated as it directly computes the stereo output signal from the transmitted downmix signal and the spatial parameters.
MPEG Surround offers an additional set of features that allow an even wider scope of applications.
In order to streamline the introduction of MPEG Surround into home A/V reproduction equipment it offers a matrix compatible downmix mode. In this mode the MPEG Surround encoder produces a downmix that gives an optimal reproduction quality for existing matrix surround systems. Hence, existing A/V reproduction equipment can directly convert the transmitted matrix compatible downmix to multi-channel making use of the present matrix surround systems. Alternatively, A/V equipment enhanced with MPEG Surround can reproduce the original multi-channel signal without repercussions as a result of the downmix modifications.
Finally, MPEG Surround allows rendering of a multi-channel audio signal from stereo content without making use of the transmitted spatial parameters. In this mode, referred to as the "Enhanced matrix mode", the spatial parameters are estimated based on properties of the received stereo signal. Comparison of the Enhanced matrix mode with a state-of-the-art matrix surround system showed a superior performance of the MPEG Surround Enhanced matrix mode.
For maximum operability, apart from the downmixes generated by the MPEG Surround encoder also legacy stereo material can be rendered to multi-channel audio at a high quality.
A Report on MPEG Surround Verification Test is available from the MPEG Home Page.