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From: bill@thd.tv.TEK.COM (William K. McFadden)
Newsgroups: rec.audio.high-end
Subject: Re: data compression
Message-ID: <11995@uwm.edu>
Date: 9 May 91 12:56:19 GMT
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In article <11954@uwm.edu> 09nilles%cuavax.dnet@netcon.cua.edu (Fiver Toadflax) writes:
>I haven't seen the details(algorithm) of the DCC compression scheme, but
>if it is any decient, nothing will be lost.  Afterall, look at programs
>such as PCZIP, UUencode, compress.  ALL of those programs take programs,
>ASCII text files, etc and shrink them.  Some better then others.  But
>no data is lost unless the file gets corrupted.

This is true.  However, to get the high compression ratio (6:1) needed
for DCC, MUSICAM compression also has to throw away information.

This is done by breaking the signal into a spectrum of critical bands.
Somewhere in the neighborhood of 24 to 32 half- or third-octave bands
are used in most compression schemes.  This correlates well to the
frequency resolution of the human ear.

According to psychoacoustic principles, the presence of a signal at one
frequency masks the presence of another signal near that frequency, but
at a lower amplitude, such that the second signal is inaudible.  There
are other effects, too, such as the presence of a signal masking
another that occurs within a certain time.

MUSICAM and the many similar compression schemes take advantage of
these psychoacoustic effects to throw away the inaudible information
and replace it with noise (which isn't encoded).  Hence, it sounds good
even though the measured SNR is as low as 15 dB.

Since normal audio measurement techniques don't give a very good
measure of the sound quality of these systems, designers must use
listening tests to evaluate them.  In the end, the inaudibility of the
compression is really up to the individual listener.
-- 
Bill McFadden    Tektronix, Inc.  P.O. Box 500  MS 58-639  Beaverton, OR  97077
bill@tv.tv.tek.com,           {hplabs,uw-beaver,decvax}!tektronix!videovax!bill
Phone: (503) 627-6920                 "SCUD: Shoots Crooked, Usually Destroyed"