Re: Encoding multiple tiny audio files

On Mon, Mar 30, 2009 at 4:52 AM, Michał Czuczman
<[email protected]> wrote:
> Hi all,
>
> I want to make a static database containing hundreds of thousands of very
> short audio files, each having not more than 100 milliseconds. These are made
> by splitting larger audio files into tiny pieces.
>
> I encode all the little files separately, but do not store 3 vorbis header
> packets, which are the same for all the files. I do not use ogg stream, only
> store plain vorbis packets.
>
> When I encode the longer files, which altogether have the same audio length,
> I get 20% higher compression ratio. However, in order to have fast lookup
> times, I cannot deal with decoding such large files, only to get a little
> chunk in them.
>
> Do you know of a way to compress the little chunks, but achieve compression
> ratio of those primary audio files? Where does the difference in compression
> rate come from?
>
> Is it possible to get even higher compression rates without quality loss,
> by tweaking any parameters or blocksizes hardcoded in libvorbis source?

What you are doing is fairly far outside of the design of Vorbis.
You're talking
about using chunks comparable to the Vorbis block length and Vorbis uses a 50%
block overlap. So you're losing compression from the wasted overlap on
each side,
you're also leaving the psychoacoustic model unprimed.

You could reduce the block lengths used by Vorbis, but you would not
be satisfied
with the performance.

For these granule lengths you would have better luck by using CELT:
 http://celt-codec.org/

CELT also has overlapped blocks but the typical block size is much smaller, and
the overlap is reduced.

Although, since you are clearly processing the audio, you should be aware that
lossy compression may just be generally unsuitable for your application as a
lossy compressor makes assumptions about how the audio will be heard that your
processing may violate.
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