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From: newton2@ucbtopaz.CC.Berkeley.ARPA
Newsgroups: net.audio
Subject: Re: copying Dolby'ed cassettes
Message-ID: <510@ucbtopaz.CC.Berkeley.ARPA>
Date: Thu, 19-Jul-84 03:58:12 EDT
Article-I.D.: ucbtopaz.510
Posted: Thu Jul 19 03:58:12 1984
Date-Received: Tue, 24-Jul-84 03:45:48 EDT
References: <601@opus.UUCP>
Organization: Univ. of Calif., Berkeley CA USA
Lines: 40

There are a few things to keep in mind with consumer Dolby applications:

It's desirable to optimize the flatness of the channel between a pair of
encode-decode processors; the sliding-band compandor is sensitive to frequency-response errors in the spectral regions of dominant program energy.
In practice this means that ragged band-edge response can cause problems
when there is substantial hf energy in the program. 

A deck with a signle pair of processors (not a three head or "double-Dolby"
job) is more likely to have truly complementary record-play response than
one using two separate processors, one pair for rew3cord and another}i for
replay. The components are likely to be poorly (i.e., cost-effectively)
matched in the latter case, while in the former case, identically the
same components (in conjugate configuration, as it were) are used for
record and replay.

It's very important to make a "staight dub", i.e., a direct copy
of the Dolbized master to another cassettte without intermediate
coding-decoding, so that the copy is made with the same absolute
fluxivity as the original- that is to say "Dolby level" mustr
be accurately observed so that particular signals are recorded at
the same flux level on the copy as on the master. This is necessary
to afford a fighting chance at accurate decoding using Dolby
circuits adjusted for standard conditions. I think Dolby level
on cassette is 200 nWb/M.

It's essential that no superaudible tones or beats be accidentally
recorded together with the intended signals, as these will cause
serious decoder mistracking if they are reproduced. All acceptible
Dolby-equipped products have effective multiplex pilot-tone traps and
bias filters *which are incorporated in the noise reduction circuitry*.
If they are omitted from the signal path when NR is switched off,
a straight dub my include 19 kHz leakage from a receiver or other sources
of high-frequency garbage. The presence of unintended high-frequency
energy applied to the decoder will slide the side-chain filter up in
frequency and markedly reduce the noise-reduction effect (more hiss,
abnormally augnmented high-frequencies [seldom a complaint about
Dolby!]).

And so on. Why not do this: Make a noise-reduced and re-encoded dub 
and see if it sounds acceptible; if YES, you are DONE.