Description
This article is from the MPEG FAQ, by
Frank Gadegast phade@cs.tu-berlin.de with numerous contributions
by others.
15 So we have a family of three audio coding schemes. What does the MPEG standard define, exactly? (MPEG-Audio)
For each Layer, the standard specifies the bitstream format
and the decoder. It does *not* specify the encoder to
allow for future improvements, but an informative chapter
gives an example for an encoder for each Layer.
What have the three audio Layers in common? (MPEG-Audio)
All Layers use the same basic structure. The coding scheme can
be described as "perceptual noise shaping" or "perceptual
subband / transform coding".
The encoder analyzes the spectral components of the audio
signal by calculating a filterbank or transform and applies
a psychoacoustic model to estimate the just noticeable
noise-level. In its quantization and coding stage, the
encoder tries to allocate the available number of data
bits in a way to meet both the bitrate and masking
requirements.
The decoder is much less complex. Its only task is to
synthesize an audio signal out of the coded spectral
components.
All Layers use the same analysis filterbank (polyphase with
32 subbands). Layer-3 adds a MDCT transform to increase
the frequency resolution.
All Layers use the same "header information" in their
bitstream, to support the hierarchical structure of the
standard.
All Layers use a bitstream structure that contains parts that
are more sensitive to biterrors ("header", "bit
allocation", "scalefactors", "side information") and parts
that are less sensitive ("data of spectral components").
All Layers may use 32, 44.1 or 48 kHz sampling frequency.
All Layers are allowed to work with similar bitrates:
Layer-1: from 32 kbps to 448 kbps
Layer-2: from 32 kbps to 384 kbps
Layer-3: from 32 kbps to 320 kbps
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