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Path: utzoo!linus!decvax!harpo!floyd!vax135!cornell!uw-beaver!tektronix!tekcad!shauns
From: shauns@tekcad.UUCP
Newsgroups: net.audio
Subject: Re: warning about dbx
Message-ID: <8@tekcad.UUCP>
Date: Wed, 31-Aug-83 17:32:23 EDT
Article-I.D.: tekcad.8
Posted: Wed Aug 31 17:32:23 1983
Date-Received: Thu, 1-Sep-83 21:43:57 EDT
Organization: Tektronix, Beaverton OR
Lines: 52

Ah, yes.  Mr. Francois has found out the problem with extreme single-band
compansion.  I have used a dbx 228 with my Nakamichi for 5 years, and it
works well except on source material with radical rapid changes in amplitude
and a high noise level, such as piano music on LPs.  Getting into the Pink
Floyd discussion, dbx also breathes on the heartbeat at the beginning and end
of Dark Side of the Moon.

Outboard dbx units or better tape decks do not solve the breathing problem; the
cause lies in the fact that dbx uses a 2:1 compression ratio independent of
frequency (which translates to >40dB of noise reduction).
If the recorded signal does not mask the noise by being in the same
frequency range, the result is perceptible noise floor modulation.

Dolby addresses this problem by limiting the amount of compansion to <20dB and
by using a sliding cutoff filter to constrain compansion to the frequency
range of the source.  Gain is thus modulated only in the band that the signal
occupies, which reduces breathing.  Because of this, on the source material
mentioned above Dolby C will have less breathing than dbx with comparable
noise reduction.

Dolby B and C, however, are absolute level sensing variable compansion ratio
systems and therefore are extremely sensitive to tape deck frequency response
aberrations and
Dolby reference level mismatch.  Dolby C, with its greater noise reduction, is
correspondingly more sensitive than Dolby B.  This is one reason why it's
difficult to transport tapes between decks and obtain the same frequency
response.

Since dbx's compansion ratio is constant over amplitude,
one can trade a few dB of ultimate S/N for high frequency headroom
by moving the compressed input signal around within the recorder's
dynamic range.
Dolby can't do this; its fixed threshold makes it strictly a noise reducer.
To pull off this trick, processing must occur before record levels are set,
easy with an outboard dbx.
I suspect that most onboard dbx systems process the signal after record levels
are set, thereby eliminating what is for me dbx's most useful feature.

dbx does not breathe if source noise levels are low, such as in the recording
of live music.  For LP transcription, Dolby C is completely sufficient.
What I would like to see from dbx is a 2-band compander along the lines of their2bx dynamic range expander.  This would eliminate breathing and negate Dolby
C's claimed advantages.  Unfortunately, for most source material and most
listeners dbx works fine; dbx isn;t about to fix something that ain't broke.

Oh, well - nothing's perfect...

-- 
				Shaun Simpkins

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