peeling as I understand it (was Re: [vorbis] When will quality increase be unnoticable?)

[Graham Mitchell]

>> Is bit-peeling going to be real (or just a rumor forever)?

> Apparently the RC3 streams are capable of being bit peeled, however the
> tool to do so was looking likely to be quite complex. I believe the plan
> was to have RC4 produce streams that left better hints for the peeling
> tool, so as to make the tool simpler and faster, but I doubt we'll see it
> until post-1.0. 

NOTE: this is my understanding of the situation.  I am not a Vorbis developer.

RC3 streams are peelable.  No tools exist to do so. (Except a 
proof-of-concept hack that (I think) Segher made at some point.  Or maybe 
Monty.)

RC4 is focused on tuning the lower sample rates (below 44.1 kHz) and on 
getting a proper bitsteam management engine in place.  This is great news for 
people doing streaming with Vorbis, since rc3 had noticeable quality 
improvements over rc2, but broke -M maximum-bitrate functionality.  RC4 
doesn't address peeling at all, AFAIK.

I believe that RC4, coupled with proper documentation (file formats, 
bitstream formats, API docs, etc), will become 1.0 relatively soon after 
RC4's release.

After 1.0 is out the door, then some attention will be paid to "hinting" to 
make peeling easier, and peeling tools will finally be written.

It is suspected that RC3 or RC4 or 1.0 files may not peel perfectly.  That 
is, if a file is encoded at (for example) q9, and subsequently "peeled" to 
q6, it may not sound quite as good as the same file originally encoded at q6. 
 So they'll peel, and the bitrate (and filesize) will be right where you want 
it, but the quality may be sub-optimal.  But this depends probably on how 
"smart" the peeler is.

It is also suspected that post-1.0 files, with their "hints" or whatever the 
heck is going on, will peel "perfectly".  That is, a file encoded at q9 and 
then later peeled to q6 will be identical* to the same file originally 
encoded at q6.

(* By "identical", I mean that the two Vorbis files will decode to the same 
WAV and not necessarily that they are byte-for-byte identical, though maybe 
they will be.  I just don't know enough about it to say for sure.)

So that's the situation as far as I know.  But remember, I'm not a Vorbis 
developer so I could be completely wrong.


-- 
Graham Mitchell - computer science teacher, Leander High School
"Compassion should never determine our beliefs about sin; it should 
only determine our response to those who struggle with it."
	-- Harry Schaumberg, "False Intimacy"

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