Audio Signal Processing and Coding

6.10 PRE-ECHO CONTROL STRATEGIES

Several methodologies have been proposed and successfully applied in the effort to mitigate the pre-echoes that tend to plague block-based coding schemes. This section describes several of the most widespread techniques, including the bit reservoir, window switching, gain modification, switched filter banks, and temporal noise shaping. Advantages and drawbacks associated with each method are also discussed.

6.10.1 Bit Reservoir

Some coders [ISOI92] [John96c] utilize this technique to satisfy the greater bit demand associated with transients. Although most algorithms are fixed rate, the instantaneous bit rates required to satisfy masked thresholds on each frame are in fact time-varying. Thus, the idea behind a bit reservoir is to store surplus bits during periods of low demand, and then to allocate bits from the reservoir during localized periods of peak demand, resulting in a time-varying instantaneous bit rate but at the same time a fixed average bit rate. One problem, however, is that very large reservoirs are needed to deal with certain transient signals, e.g., “pitched signals.” Particular bit reservoir implementations are addressed later in conjunction with the MPEG [ISOI92a] and PAC [John96c] standards.

6.10.2 Window Switching

First introduced by Edler [Edle89], this is also a popular method for pre-echo suppression, particularly in the case of MDCT-based algorithms. Window switching works by changing the analysis block length from long duration ...