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MPEG Audio Codec Family ("Layer 1, 2, 3")
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• Q: Talking about MPEG audio coding, I heard a lot about "Layer 1, 2 and 3". What does it mean, exactly?
• A: MPEG describes the compression of audio signals using high performance perceptual coding schemes.
  It specifies a family of three audio coding schemes, simply called Layer-1,-2,-3, with increasing encoder complexity and performance (sound quality per bitrate) from 1 to 3.
  The three codecs are compatible in a hierarchical way, i.e. a Layer-N decoder is able to decode bitstream data encoded in Layer-N and all Layers below N (e.g., a Layer-3 decoder may accept Layer-1,-2 and -3, whereas a Layer-2 decoder may accept only Layer-1 and -2.)

• Q: So we have a family of three audio coding schemes. What does the MPEG standard define, exactly?
• A: For each Layer, the standard specifies the bitstream format and the decoder.
  
  
•  Q: What have the three audio Layers in common?

• A: 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 have a similar sensitivity to biterrors. They 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 support the insertion of program-associated information ("ancillary data") into their audio data bitstream.
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