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Re: Determining the usefulness of compression



On Wed, Dec 04, 2002 at 08:44:28PM -0800, Vineet Kumar wrote:
> * Charlie Reiman (creiman@kefta.com) [021204 18:26]:
> > filename suffixes (never bother with gz, tgz, bz2, zip, jpg, jpeg, gif, z,
> > Z, mpg, mpeg, avi, wav, mov....)
> 
> Huh? some AVIs and all WAVs are uncompressed, and will benefit
> enormously from compression.  The theory here is correct, though:
> don't try to compress already-compressed data; it won't work.

I think the point re WAVs is that gzip and its kin don't compress them
very well so you waste a lot of cpu and get something that if you're
lucky is 90% of the original file size. I think there's a program
called 'flac' that will compress them to around 60-70% of original
size, so you could make the script intelligent about this.

Compressing WAVs works better for a better quality original recording.
More or less all music has been produced with some analogue signal
processing somewhere in the chain. This introduces noise. (Is it
possible to make a digital guitar? Does the idea even make sense?) DSP
with insufficient precision also introduces noise. Frequently some
digital stage will add dither, which is sort of like 'nice' noise.
Noise, being random, screws up the predictor algorithm used by the
compressor, and makes it less effective.

Cleanup software may help, but I am uneasy about this kind of thing,
try it on "She moves through the fair" by All About Eve and see what
it does to the percussion with various amounts of added background
white noise...

Pigeon



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