While BCC aims at compressing multi-channel audio by using a single down-mix channel, a different trade-off between compression ratio and audio quality can be achieved by using more than one down-mix channel. A more general scheme, encoding a multi-channel audio signal with an arbitrary number of down-mix channels and spatial cues, has been often denoted spatial audio coding.
In the next chapters, some commercially available audio coders that employ spatial audio coding techniques will be discussed. ‘parametric stereo’, described in the next chapter, which is an integral part of the aacPlus v2 codec, employs a mono down-mix accompanied by parameters to encode two-channel audio. MPEG Surround, described in Chapter 6, is a very flexible coder in terms of number of input and down-mix channels. An important use case for MPEG Surround is an operation mode based on a stereo down-mix. The use of two down-mix channels results in an improvement in the overall quality and provides backward compatibility with stereo systems, which is an important property for many applications.