Spatial Audio Processing: MPEG Surround and Other Applications

5.2 Interaction between core coder and spatial audio coding

So far, the core coder and the parametric stereo coder have mostly been described as separate processes. It is well known that core coders result in audible artifacts if the bitrate is relatively low. Such artifacts include audible pre-echos resulting from quantization noise just before transients, ‘warbling’ or ‘underwater’ effects resulting from spectral holes, and alike. On the other hand, a parametric stereo coder may also result in audible artifacts if employed at low bitrates. Examples of such artifacts are a loss of ‘width’, audible crosstalk or instable sound source positioning. When these coding methods are combined in a single audio codec, it is likely that audible artifacts resulting from the core coder as well as parametric stereo will determine the overall perceived quality, and that the artifacts may ‘add up’ in some way when the perceived quality of the complete system is determined.

As discussed in Chapter 1, bitrate reduction in conventional lossy audio coders is obtained predominantly by exploiting the phenomenon of masking. Therefore, lossy audio coders rely on accurate and reliable masking models, which are often applied on individual channel signals in the case of a stereo or multi-channel signal. For a parametric stereo extended (mono) audio coder, however, the masking model is applied only once on a certain combination of the two input signals. This scheme has two implications with respect to masking ...