Begriff »MP3«

Short for MPEG Layer 3, a type of audio data compression that can reduce digital sound files by a 12:1 ratio with virtually no loss in quality.

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About ISO/MPEG Audio Coding

Basic scheme

The ISO/MPEG Audio Coding Standard 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, Layer 2 and Layer 3.

Compression gain (Sound quality per bit) and encoder complexity increase from Layer 1 to Layer 3.

All Layers use the same basic structure. The coding scheme can be described as perceptual noise shaping or perceptual subband/transform coding.

The encoder analyses 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 task is to synthesize an audio signal out of the encoded spectral components.

Compression rates

You can achieve a compression rate of

• 1:4 with Layer 1 (or 192 kbps per audio channel),
• 1:6..8 with Layer 2 (or 128..96 kbps per audio channel), and
• 1:10..12 with Layer 3 (or 64..56 kbps per audio channel),

and the reconstructed audio signal will maintain a CD-like sound quality.

Quelle: Fraunhofer-IIS, http://www.iis.fhg.de/departs/amm/layer3/sw/