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From: gregr@tekig1.UUCP
Newsgroups: net.audio
Subject: Re: Warning about dbx
Message-ID: <1295@tekig1.UUCP>
Date: Wed, 7-Sep-83 00:25:51 EDT
Article-I.D.: tekig1.1295
Posted: Wed Sep  7 00:25:51 1983
Date-Received: Sat, 3-Sep-83 04:50:13 EDT
References: <1797@allegra.UUCP>
Organization: Tektronix, Beaverton OR
Lines: 28

Your problem with DBX is characteristic of this type of noise reduction unit.
It is caused by the fact the DBX uses a much higher expansion/compression ratio
than Dolby, and is not frequency or signal level selective.  This method used bydBx produces a much better signal-to-noise ratio improvement than Dolby, but 
at the cost of producing "breathing".  At low signal levels (or between record 
cuts) the dBx system compansion (expansion/compression) reduces tape hiss, or in
the case of encoded records, background noise almost entirely.  However, should 
loud solo passages occur which have narrow frequency content the system opens 
up and reveals the background noise as well.  If sufficient frequency content 
in the midrange-high end is included in the passage it masks the noise level, 
otherwise you hear the noise which appears to be modulated be the varying signal
level.  This is the "breathing" you hear.  In many types of music, particularly
popular music, the masking effect caused by a constantly wide frequency 
spectrum can give very acceptable results.  In music that features a lot of solo
instruments the breathing can often be unacceptable.  

Unfortunately Dolby B doesn't offer a complete solution either.  This system
depends on accurate tracking of absolute signal levels (the reference level
has to be set) and precise matching of circuit parameters between the
encoding and decoding circuits.  And IF everything works perfectly the amount
of noise reduction is still insufficient to reduce tape hiss completely as
you have seen.  My subjective opinion is that the system you will be happiest
with depends on both the type of music you listen to, and on your sensitivity
to the side effects of both systems.

There is no free lunch!  CD disk players cost about $800-$1000 and PCM digital
tape encoders cost about $1500-$2500.  These are the only acceptable solutions
for me.
                                         Greg  (still pushing CD's)