[vorbis] Ogg Vorbis v1.1 suggestion

[Carsten Haese]

On Tue, 2003-11-25 at 16:37, Robert Jessop wrote:
> On Tuesday 25 November 2003 20:36, Carsten Haese wrote:
> > On the few files that I have in .wav here, I get quite respectable
> > results with oggenc --advanced-encode-option lowpass_frequency=12
> > --resample 32000 -b 40 -M 32. I have no idea how this stacks up against
> > Real Audio since I don't have that available. Of course, the CPU hit is
> > still there.
> >
> 
> ABR mode works by making several versions of each packet and then choosing the 
> one that gives the best quality/bitrate compromise over a number of packets.
> The number is versions of each packet it encodes is defined in 
> codec_internal.h
> #define PACKET_BLOBS 15
> 
> If you reduce it and recompile ABR mode should be quicker, though if you go 
> too low quality will be really bad. (I haven't tested this)
> 
> Robert Jessop
This idea must be taken with a very big CAUTION sign on it. If you
reduce PACKETBLOBS...

1) ...all sets of tuning parameters that come in PACKETBLOBS different
versions will be overinitialized (and produce a ton of compiler warnings
at best).

2) ...the "center" case (at PACKETBLOBS/2) that true VBR encoding uses
will turn out to be a mode that ordinarily would only ever be considered
for lowering the bitrate in managed situations. Hence, true VBR encodes
will use too few bits.

3) ...for reasons analogous to 2, managed encoding will use too few bits
as well, so you'd have to arbitrarily fudge your bitrate settings to get
to where you want to be.

I'm sure Monty would have many more reasons to add to this list, but I
think the above reasons are enough to illustrate that this suggestion,
while well-meaning, is an ugly hack at best. A much cleaner solution
would be to leave PACKETBLOBS alone, find the loops that go from 0 to
PACKETBLOBS, and restricting their range to a smaller interval that
contains PACKETBLOBS/2 and whose size would ideally be configurable
through an --advanced-encode-option. (Maybe something really awkward
like bitrate_management_search_radius=something to discourage its use :)

Best regards,

-Carsten

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