[opus] opusenc constant quality setting

[Gregory Maxwell]

On Thu, Nov 6, 2014 at 8:55 PM, Larry Fenske <opus at towanda.com> wrote:
> I'm a bit confused about VBR with Opus.  I see that the default is VBR,
> but I don't see any way to specify a quality setting.  I can set a
> target bitrate, but that's definitely not what I want; I want a constant
> quality level, like "-q" in oggenc, and for the encoder to select
> bitrates based on the desired quality.
>
> For example, for playback through earbuds or laptop speakers, I wouldn't
> need anything better than low quality; through good headphones or decent
> stereo speakers, medium quality; and for playback through $5000
> speakers, high quality.  Bitrate is not relevant in these cases, except
> I don't want the file to be larger than necessary for the desired quality.

The VBR settings are 'constant quality'.  Bitrate is just used as a
real-world unit indexed against a large corpus of diverse content.
E.g. bitrate 128 is the quality that got the average rate for the
whole corpus to 128.

This avoids problems with an opaque quality setting having no external
meaning (and drifting over time and/or having to add crazy values like
-2 later ) and only making sense relative to itself.

If you'd like, you can just imagine that the bitrate numbers in VBR
mode are "quality" scaled by a somewhat funny 512kbit/sec/channel.