- Polyphase quadrature filter
A polyphase quadrature filter, or PQF, is a
filter bankwhich splits an input signal into a given number N (mostly a power of 2) of equidistant sub-bands. These sub-bands are subsampled by a factor of N, so they are critically sampled.
This critical sampling introduces
aliasing. Similar to the MDCT time domain alias cancellationthe aliasing of polyphase quadrature filters is canceled by neighbouring sub-bands, i.e. signals are typically stored in two sub-bands.
Note that signal in odd subbands is stored
PQF filters are used in
MPEG-1 Audio Layer Iand II, Musepack(which was based on MPEG-1 layer II), in MPEG-1 Layer III with anadditional MDCT, in MPEG-4 AAC-SSR for the 4 band PQF bank, in MPEG-4 V3 SBRfor the analysis of the upper spectral replicated band, and in DTS.
PQF has an advantage over the very similar stacked
quadrature mirror filter(QMF). Delayis much lower, computational effort is much lower. PQF filter parameter
There are different formulas possible, most of them are based on the MDCT but are slightly modified.
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