[vorbis] Bit Rate Peeling Quality

J C Fitzgerald v7022 at wave.co.nz
Mon Jun 23 02:26:48 PDT 2003

Your comprehensive reply is appreciated, Graham.

> Assuming the files were encoded with current encoders, c.ogg would probably
> be noticeably less good than d.ogg, ...
I'm using oggenc on the assumption that it's state of the art (am
I wrong?).

So following on from your answer (and also that given by Daniel):  Is
there a quality higher than 2 with which c.ogg could be encoded to yeild
it as good as d.wav, or is it just that quality doesn't (currently)
scale in this way?

> > Also a related question (in terms of my proposed use):  Are there likely
> > to be any changes to the Vorbis codec which will cause problems doing
> > bit rate peeling in, say, four years time on a file encoded today with v1?
> Erm, you're using terminology a little loosely here.
I know, but I haven't been able to glean a good definition of 'codec'
yet (and my specific request for a definition went unanswered).

> In which case, I'm pretty sure the answer is, "Don't worry, in this 
> hypothetical future you'll be able to peel your then-old vorbis files just as 
> well as files you'd encode then.  Or pretty darn close, anyway."
That's good to hear.

> However, Segher claims (and I believe him) that it's possible to losslessly 
> re-encode a v1 vorbis stream in such a way to make a peeler's job much 
> easier.  That is (to quote my man Bill Nye), "consider the following":
> 	a.wav --> b.ogg (at q6) --> [MAGIC] --> c.ogg (also q6) --> d.wav
> 	a.wav --> b.ogg (at q6) -------------------------------------> e.wav
> Anyway, it's assumed that eventually the (not-yet-written) code from the 
> [MAGIC] phase would be incorporated into the stock encoder.
And, presumably, the into peeler for use when processing older files.

In my case I have two input and two output cases which I'm trying
to balance.  On the input side I have sources which will always be
available to me (unless catastrophe strikes), and others which will
never again be available.  On the output side I want to hold for home
use the files at better than required quality, and in the future I also
want smaller copies at average quality for portable use.

So it seems to me that my best path is to encode now at q6, then in
the future reencode available sources at q2 while peeling my q6s from
no longer available sources at q2 (or maybe q3 if this would compare
better with native q2 encoding).

Sound reasonable?

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