Vorbis dev - Dec 2006 - Short block encode computation time.

Hello!

I've read this list pretty thoroughly before asking. Thanks in advance for
the help.

I'm interested in low-latency streaming of small packets of audio captured
from a sound card.
I want to focus on the compression computational time (not the transport). I
have several questions regarding Vorbis :

(1) Vorbis supports block sizes down to 64 samples according to the spec.
Can I encode, frame, and transport blocks of this size? Will the performance
of a standard vorbis encoder using short sample blocks differ significantly
from performance with the longer block lengths usually used?

(2) Are such short block lengths compatible with Managed Bitrate modes?

(3) What computation time can I expect  in encoding a 64-sample block?
128-sample? (Ordinary, modern P4 platform.)

(4) What are the main sources of computational time in encoding a short
block?

(5) How much of the overall compression is due to perceptual encoding? How
much due to entropy encoding (huffman)?

Thanks in advance.
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On 12/21/06, Doron Veltzer <veltzerdoron@gmail.com> wrote:
> I'm interested in low-latency streaming of small packets of audio
captured
> from a sound card.
> I want to focus on the compression computational time (not the transport).
I
> have several questions regarding Vorbis :
>
> (1) Vorbis supports block sizes down to 64 samples according to the spec.
> Can I encode, frame, and transport blocks of this size?
Yes.  Using Ogg for it, though, will impose a fairly high overhead for
single packets this small.
> Will the performance
> of a standard vorbis encoder using short sample blocks differ significantly
> from performance with the longer block lengths usually used?
Gods, yes.  In a normal encoder setup using block sizes of 256/2048,
the short blocks are typically only 1/4-1/2 the size of the long
blocks.  So, using only short blocks will incur a compression
performance penalty of 2x to 4x and probably cause nasty blocking
artifacts in pure tones.
> (2) Are such short block lengths compatible with Managed Bitrate modes?
The algorithms are compatable.
> (3) What computation time can I expect  in encoding a 64-sample block?
> 128-sample? (Ordinary, modern P4 platform.)
Dunno, never measured it.
> (4) What are the main sources of computational time in encoding a short
> block?
Frame analysis  The coding itself takes practically no time at all..
> (5) How much of the overall compression is due to perceptual encoding?
80-ish %.
> How
> much due to entropy encoding (huffman)?
20-ish %  It depends alot on how you look at it.

Monty