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@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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