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What is Perceptual Noise Substitution and How it is used in AAC?

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2.2.1 Perceptual Noise Substitution(PNS)

The PNS tool increases the coding efficiency of AAC by representing noise-like signal components with a compact parametric representation instead of coding the exact waveform. Each noise-like scale factor band is represented by a noise substitution flag and the total power of its spectral coefficients, that are not quantized and transmitted. The decoder generates random numbers replacing these coefficients with the received total power.

(quoted from http://www.ibr.cs.tu-bs.de/courses/ss04/skm/mpeg-4-aac.pdf)

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    Note that not many AAC encoders implement PNS tool, as it brings little bitrate reduction (unless the sound is really noisy).
    – Danijel
    May 31 at 18:30
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    Some more info on PNS: ccrma.stanford.edu/~kapilkm/422/…
    – Danijel
    May 31 at 18:34

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