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#REDIRECT [[Psychoacoustics]]
== What is Perceptual Audio Coding? ==
Perceptual Audio Coding, also known as PAC is a technique that compresses the digital sound by eliminating frequencies that cannot be perceived by the human ear. PAC decodes the bit stream that sounds just like the original audio, but keeping the compressed file small. PAC used the psychoacoustic algorithms.
Frequencies that are too low or too high are eliminated as well as soft sounds that are drowned out by loud sounds. When several musical instruments are playing simultaneously, depending on frequency and volume at any given moment, the sounds from one instrument can cancel out the sounds of another. The frequencies that cannot be heard are removed.
== Psychoacoustic ==
The term [[psychoacoustics]] describes the characteristics of the human auditory system on which modern audio coding technology is based. The most important psychoacoustics fact is the masking effect of spectral sound elements in an audio signal like tones and noise. For every tone in the audio signal a masking threshold can be calculated. If another tone lies below this masking threshold, it will be masked by the louder tone and remains inaudible too.
== The Steps ==
PAC goes through many steps of coding and algorithms to make the final bitstream.
Perceptual Coding
Perceptual coding is a lossy coding method. This stage uses a model of the destination. It attempts to remove parts of the signal that the human cannot perceive.
Quantization and Rate Control
The purpose of the quantization and rate control parts is to implement the psychoacoustic threshold to the extent possible while maintaining the required bit rate. The goals of this are like many.
1) Enforcing the required rate
2) Implementing the psychoacoustic threshold
3) Adding noise in less offensive places when there are not enough bits
== What uses PAC? ==
Nearly all well known audio codec’s are based on perceptual audio coding, including MP3, Dolby AC-3 and MPEG AAC.
==External links==
*http://www.ece.rochester.edu/~gsharma/SPS_Rochester/presentations/JohnstonPerceptualAudioCoding.pdf
*http://www.eas.asu.edu/~spanias/papers/paper-audio-tedspanias-00.pdf
*http://www.pcmag.com/encyclopedia_term/0,2542,t=perceptual+audio+coding&i=49099,00.asp
*http://www.harmoniccycle.com/hc/music-mp3-info-fraubasic.htm
*http://www.telecomabc.com/p/perceptual.html
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