The concept of spatial audio coding as employed in the MPEG Surround standard  is outlined in Figure 6.1. A multi-channel input signal is converted to a down-mix by an MPEG Surround encoder. Typically, the down-mix is a mono or a stereo signal, but more down-mix channels are also supported (for example a 5.1 down-mix from a 7.1 input channel configuration). The perceptually relevant spatial properties of the original input signals that are lost by the down-mix process are captured in a spatial parameter bitstream. The down-mix can subsequently be encoded with an existing compression technology. In the last encoder step, the spatial parameters are combined with the down-mix bitstream by a multiplexer to form the output bitstream. Preferably, the parameters are stored in an ancillary data portion of the down-mix bitstream to ensure backward compatibility.
The right panel of Figure 6.1 outlines the MPEG Surround decoding process. In a first stage, the transmitted bitstream is split into a down-mix bitstream and a spatial parameter stream. The down-mix bitstream is decoded using a legacy decoder. Finally, the multi-channel output is constructed by an MPEG Surround decoder based on the transmitted spatial parameters.
The use of an MPEG Surround encoder as a pre-processor for a conventional (legacy) codec (and a corresponding post-processor in the decoder) has important advantages over existing multi-channel compression methods.