MPEG Layer III Filter Bank

In MPEG 1& 2, Layer III (the popular ``MP3 format''),

The original MPEG 1&2, Layers I and II, based on the MUSICAM coder, contained only 32 subbands (each band approximately 650 Hz wide, implemented using a length 512 lowpass-prototype window, lapped (``time aliased'') by factor of 512/32 = 8, thus yielding 32 real bands with 96 dB of stop-band rejection, and having a hop size of 32 samples) [132, §4.1.1]. It was found, however, that a higher coding gain was obtained using a finer frequency resolution. As a result, the MPEG 1&2 Layer III coder (based on the ASPEC coder from AT&T), appended a Princen-Bradley filter bank [198] having 6 to 18 subbands to the output of each subband of the 32-channel PQMF cosine-modulated analysis filter bank [132, §4.1.2]. The number of sub-bands and window shape were chosen to be signal-dependent as follows:

• Transients use subbands, corresponding to relatively high time resolution and low frequency resolution.
• Steady-state tones use subbands, corresponding to higher frequency resolution and lower time resolution relative to transients.^12.1
• The encoder generates a function called the perceptual entropy (PE) which tells the coder when to switch resolutions.

The MPEG AAC coder is generally regarded as providing nearly twice the compression ratio of ``MP3'' (MPEG 1-2 Layer III) coding at the same quality level.<sup>12.2</sup> MPEG AAC introduced a new MDCT filter bank that adaptively switched between 128 and 1024 bands (length 256 and 2048 FFT windows, using 50% overlap) [132, §4.1.6]. The nearly doubled number of frequency bands available for coding steady-state signal intervals contributed much to the increased coding gain of AAC over MP3. The 128-1024 MDCT filter bank in AAC is also considerably simpler than the hierarchical MP3 filter bank, without requiring the ``cross-talk aliasing reduction'' needed by the PQMF/MDCT hierarchical filter bank of MP3 [132, §4.1.6].

The MPEG-4 audio compression standard (there was no MPEG-3), included a new transform coder based on the AAC filter bank [132, §4.1.7].

See, e.g., [15] for much more on MPEG coders and related topics. Chapter 4 of the dissertation of Scott Levine [132] contains an excellent summary of MPEG, Sony ATRAC, and Dolby AC-n coders up to 1998.

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written by Julius Orion Smith III
Julius Smith's background is in electrical engineering (BS Rice 1975, PhD Stanford 1983). He is presently Professor of Music and Associate Professor (by courtesy) of Electrical Engineering at Stanford's Center for Computer Research in Music and Acoustics (CCRMA), teaching courses and pursuing research related to signal processing applied to music and audio systems. See http://ccrma.stanford.edu/~jos/ for details.