[vorbis] FAQ submission: quality vs. bitrate

[L. D. Steiner]

I would like to see the following information added to the FAQ at 
vorbis.com, appended to the "Audio Quality" section. "Uiver," "boa," 
and "jonI" from OPN's #Vorbis channel contributed and reviewed it.

"What does the 'quality' setting mean?"

Beginning with vorbislib 1.0-rc3, audio quality is no longer measured 
in kilobits per second, but on an arbitrary scale of 0 to 10, called 
"quality." This change in terminology was brought about by a tuning of 
the variable-bitrate algorithm that produces better sound quality for a 
given average bitrate, but which does not adhere as strictly to that 
average as a target.

This new scale of measurement is not tied to a quantifiable 
characteristic of the stream, like bitrate, so it's a fairly subjective 
metric, but provides a more stable basis of comparison to other codecs 
and is relatively future-proof. As Segher Boessenkool <segher at xiph.org> 
explained, "if you upgrade to a new vorbis encoder, and you keep the 
same quality setting, you will get smaller files which sound the same. 
If you keep the same nominal bitrate, you get about the same size 
files, which sound somewhat better." The former behavior is the aim of 
the quality metric, so encoding to a target bitrate is now officially 
deprecated for all uses except streaming over bandwidth-critical 
connections.

For now, quality 0 is roughly equivalent to 64kbps average, 5 is 
roughly 160kbps, and 10 gives about 400kbps. A jump in bitrate takes 
place at 5, when channel coupling data is compressed losslessly, so 
most people seeking very-near-CD-quality audio encode at a quality of 5 
or, for a decrease in file size, 4.99. The default setting is quality 
3, which at approximately 110kbps gives a smaller filesize and 
significantly better fidelity than .mp3 compression at 128kbps.

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