[opus] Why does opus scale down frequency from 22k to 20k ?

Timothy B. Terriberry tterribe at xiph.org
Sat Aug 25 07:12:20 PDT 2012

Orange Ubuntero wrote:
> this action will affect the quality of audio.

Any affect will be inaudible to human ears. For a more detailed 
explanation, see http://people.xiph.org/~xiphmont/demo/neil-young.html

> *Can this feature be disable in future ?*

No. It is not possible to encode information for frequencies above 20 
kHz in the bitstream, even if an encoder wanted to.

> *Can opus add no noise and echo in audio at high kbps *(192kbps +) *?*

Opus is a lossy codec, so it will always change the numerical values in 
the actual signal. But if you can find a sample that you can distinguish 
from the original in an ABX _listening_ test (as opposed to by looking 
at a spectrogram) at 192 kbps, then we'd certainly be interested in 
knowing about it.

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