[vorbis-dev] Impulses

[Mark Taylor]

> Date: Fri, 19 Nov 1999 08:50:53 -0500 (EST)
> From: Gregory Maxwell <greg at linuxpower.cx>
> X-Sender: greg at link.z.aubbs.cx
> Content-Type: TEXT/PLAIN; charset=US-ASCII
> Sender: owner-vorbis-dev at xiph.org
> Precedence: bulk
> Reply-To: vorbis-dev at xiph.org
> X-UIDL: hBkd9e6]!!IV@!!/j[d9
> X-UID: 632
> 
> 
> After playing with the vorbis code for a while and doing tons of hacks and
> analysis on it, I've found it to perform very poorly with impulse signals.
> 
> The MDCT seems to cause lots of spreading, and it seems to result in much
> worse impulse performance then mp3. 
> 
> 
> What do you think of this:
> 
> input wav -> DWT -> sum non-impuse factors -> iDWT -> MDCT ... (like above)
>               \
>                -> -> sum impulse factors -> iDWT -> LPC -> LSP -> quant 
> 

Do you guys really think window switching is so bad?  It clearly
works very well and is not just 'mp3' quailty, since it is used
in AAC which is pretty much the best encoder out there.
The only problem I can see is that the encoding
is not as efficient - you always need to allocated extra bits for
short MDCT windows.  But except for extreme cases like castanets.wav,
the amount of attacks/pulses is usually less than 5%.  Assuming 50%
more bits for the lossless encoding, a more sophisticated technique
would save at most 2.5%.

Also, I believe Vorbis is using a 2048 sample MDCT window?  (like AAC,
but almost twice that of  mp3).   Such a large window results in
more spreading, making short windows even more important?

Monty: you've mentioned comparisions between Vorbis and AAC in
the past - which AAC encoder/decoder were you using?  If you get
a chance, could you the output from a decoded AAC encoding of 
castanets.wav?

Mark

--- >8 ----
List archives:  http://www.xiph.org/archives/
Ogg project homepage: http://www.xiph.org/ogg/