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From: sjc@mordor.UUCP (Steve Correll)
Newsgroups: net.audio
Subject: Re: dbx and Dolby info wanted
Message-ID: <3529@mordor.UUCP>
Date: Mon, 16-Sep-85 16:41:20 EDT
Article-I.D.: mordor.3529
Posted: Mon Sep 16 16:41:20 1985
Date-Received: Wed, 18-Sep-85 03:09:34 EDT
References: <1153@mhuxt.UUCP>
Organization: S-1 Project, LLNL
Lines: 30

> Can anyone give me information on the relative advantages
> and disadvantages of the dbx and Dolby B&C noise reduction
> schemes?

All three operate by compressing the musical signal before recording
(raising the level of soft passages) and expanding it on playback
(lowering the level of soft passages, simultaneously reducing the noise
added during recording.)

Dolby B reduces noise by at most 10dB; Dolby C by at most 20dB. dbx
compresses and expands the signal by a much greater amount, but in a
less fancy manner. Thus, it provides more noise reduction, but some
people (not all) can occasionally hear it "breathe"--the noise level
modulates because the expander fails to mirror exactly the action of
the compressor. It is also more sensitive to amplitude nonlinearities
if you record on one machine and play back on another. Dolby's patents
cover a scheme for altering the frequency band over which
compression/expansion occurs; this, along with its milder action, seems
to eliminate "breathing", but Dolby circuitry is more sensitive to
frequency response irregularities from machine to machine.

To my ears, Dolby C is good enough to eliminate noise on
first-generation tapes made on reasonably good cassette recorders,
whereas Dolby B is not good enough.  dbx offers a greater margin of
safety which is useful if your ears are finickier than mine or if you
want to play record-studio, recording a song over and over, adding
another musical voice each time.
-- 
                                                           --Steve Correll
sjc@s1-c.ARPA, ...!decvax!decwrl!mordor!sjc, or ...!ucbvax!dual!mordor!sjc


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