Psychoacoustics and audio coding

• Psychoacoustics


  The study of how humans perceive and respond to sound
• Psychoacoustics


  • Knowledge from psychoacoustics is used in lossy compression techniques to reduce the amount of information a sound signal contains without affecting the perceived sound
  • This is achieved by exploiting two particular aspects of how human beings respond to sound
• Human ear sensitivity


  • The sensitivity of the human ear varies according to the frequency of the sound
  • One sound at a given frequency and amplitude may be heard, whereas a sound at a different frequency but with a higher amplitude cannot be heard
  • It is possible to define so-called 'hearing thresholds'
  • By detecting which parts of the signal are below the threshold and removing these components, the amount of information can be reduced
• Masking effect


  • If one of two sound sources is much louder than the other, and if they are of a similar frequency, then the louder sound may totally mask the softer one
  • Lossy audio compression can use this effect to remove the parts of a sound signal that are more than a certain level below a loud sound, and thus would not be heard
  • This again reduces the amount of information that the sound signal contains and hence reduces the number of bits required to store such a signal
• MP3 audio compression


  1. The audio signal is split into 32 frequency bands that match the frequency characteristics of the human ear

  2. The sound content of each band is then analysed and coded using a specialised algorithm which uses the lowest possible amount of data for the given content by exploiting the psychoacoustic aspects

  3. When an MP3-audio is to be played back, decoding requires the data blocks to be separated out so that the frequency data can be reconstructed and used to rebuild the original signal
• MP3 audio compression offers acceptable audio quality with a high compression ratio in the region of 11:1