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MP3 Audio Compression Layer: Features, Performance, and Encoding

Understand the advanced audio quality, bandwidth modes, and bitrate reduction ratios of MP3 compression technology. Discover how the psycho-acoustic perceptual model, filter banks, quantization, and joint stereo coding contribute to efficient encoding. Learn about the encoding process, including the rate control loop, distortion/noise control loop, and the intricate steps involved in decoding audio signals. Dive into the principles of audio perception and the technical aspects behind achieving optimal sound quality through MP3 compression.

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MP3 Audio Compression Layer: Features, Performance, and Encoding

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  1. MPEG Audio Compression Layer 3 (MP3) Hai Tao

  2. 1:4 by Layer 1 (corresponds to 384 kbps for a stereo signal), 1:6...1:8 by Layer 2 (corresponds to 256..192 kbps for a stereo signal), 1:10...1:12 by Layer 3 (corresponds to 128..112 kbps for a stereo signal), History • MP3 originally developed by Fraunhofer IIS partly under EUREKA project EU147, Digital Audio Broadcasting (DAB). Then adopted as ISO-MPEG Audio Layer-3 (1991, There are Layer 1 and 2 also) • For 44.1 KHz sampling, stereo 16 bit audio, the bit rate is 44100x2x16=1.41Mbit/s, Using MP3, good quality sound can be achieved even with >1:24 compression ration • Layer 3 is the most advanced in all three methods

  3. sound quality bandwidth mode bitrate reduction ratio telephone sound 2.5 kHz mono 8 kbps * 96:1 better than short wave 4.5 kHz mono 16 kbps 48:1 better than AM radio 7.5 kHz mono 32 kbps 24:1 similar to FM radio 11 kHz stereo 56...64 kbps 26...24:1 near-CD 15 kHz stereo 96 kbps 16:1 CD >15 kHz stereo 112..128kbps 14..12:1 Performance • Typical performance for various sources

  4. Basic Diagram

  5. Psycho-Acoustic Perceptual Model • The basic perceptual model used in MP3 is that louder frequencies mask out adjacent quieter ones. People can not hear a quiet sound at one frequency if there is a loud sound at another • This can be explained better by the following figures presented by Rapha Depke

  6. Audio perception • Human perception of audio signal

  7. Perceptual model • People can not hear a quiet sound at one frequency if there is a loud sound at another

  8. Perceptual model • The louder the sound, the larger the mask area • The bottom area is called absolute threshold of hearing (ATH), defining the weakest sound that can be heard

  9. Filter Banks • The audio signal passes through 32 filters with different frequency

  10. Dominant band and the mask • Dominant band is found and the corresponding mask is applied

  11. Quantization of Audible Sound • The components exceed the mask are quantized and encoded using the Huffman coding method

  12. Joint Stereo • Joint stereo coding takes advantage of the fact that both channels of a stereo channel pair contain similar information • These stereophonic irrelevancies and redundancies are exploited to reduce the total bitrate • Joint stereo is used in cases where only low bitrates are available but stereo signals are desired.

  13. Encoder • A typical solution has two nested iteration loops • Distortion/Noise control loop (outer loop) • Rate control loop (inner loop)

  14. Rate control loop • For a given bit rate allocation, adjust the quantization steps to achieve the bit rate. • This loop checks if the number of bits resulting from the coding operation exceeds the number of bits available to code a given block of data. • If it is true, then the quantization step is increased to reduce the total bits. This can be achieved by adjusting the global gain

  15. Distortion control loop • This loop shape the quantization steps according to the perceptual mask threshold • Start with a default factor 1.0 for every band • If the quantization error in a band exceeds the mask threshold, the scale factor is adjusted to reduce this quantization error • This will cause more bits and the rate control loop has to be invoked every time the scale factors are changed • The distortion control is executed until the noise level is below the perceptual mask for every band

  16. Decoder • Decoder side relatively easier. The gain, scale factor, quantization steps recovered and used for reconstruct the filter bank responses • Filter bank responses are combined to reconstruct the decoded audio signal

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