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From: sethb@fid.Morgan.COM (Seth Breidbart)
Newsgroups: rec.audio.high-end
Subject: Re: data compression
Message-ID: <12182@uwm.edu>
Date: 16 May 91 13:00:25 GMT
Sender: news@uwm.edu
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Approved: tjk@csd4.csd.uwm.edu
Originator: tjk@csd4.csd.uwm.edu


In article <12149@uwm.edu> 09nilles%cuavax.dnet@netcon.cua.edu (Fiver
Toadflax) writes:

>> And sometimes they just can't compress a file at all.  But at least no
>> data is lost, so who cares?  But with music, I want one second of
>> music per second.  If this current second happens to be hard to
>> compress, I don't want to wait 3 seconds to hear it.
>
>This is a matter of 'horse power' and only a real problem when actually
>encoding the music.  Decodeing can be done faster on the same machine or
>just as quickly on a slower machine.

Not so.  If a file is random (in the Kolmogorov/Chaitin sense), then
it _cannot_ be compressed without loss of information.  In general,
better compression algorithms do take longer, but time is no
guarantee.  Besides, there's only a fixed amount of 'horse power' in
any particular tape deck.

Proof that most files cannot be compressed:

There are 2**8000000 different files 1 megabyte long.  There are
2**7999990-1 different files with lengths less than 1 megabyte-11 bytes.
The ratio of these is approximately 0.001.  Therefore, under 0.1% of
all 1 megabyte files can be compressed by at least 11 bytes (which is
0.000011% compression).


>What I mean by that is that not all the data/music is going to appear as
>a worst case situation where it takes at least as much space to encode as
>it takes up unencoded.  So it will loose data when those worst case
>situations are encountered.  But on average the music is not a worst case
>and then no data would be lost.

You don't mean "on average".  You mean "usually".  It's not clear
whether data need be lost for real music.  Stuff like 1 kHz test tones
wins.  I don't care.  Complex orchestral music, recorded live, with
hall and audience dynamics, will lose bits.  I don't know if I'll be
able to hear the difference, but the bit loss will be there.

Seth		sethb@fid.morgan.com