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From: sjc@angband.UUCP (Steve Correll)
Newsgroups: net.audio
Subject: Re: dubbing Dolby-encoded tapes
Message-ID: <56@angband.UUCP>
Date: Wed, 1-May-85 01:30:08 EDT
Article-I.D.: angband.56
Posted: Wed May  1 01:30:08 1985
Date-Received: Fri, 3-May-85 07:44:49 EDT
References: <530@hlwpc.UUCP>
Distribution: net
Organization: S-1 Project, LLNL
Lines: 31

> How's the best way to copy Dolby-B-encoded music from one tape
> to another?  I assume I should keep the Dolby switch OFF on both
> the playing deck and the recording deck.  I guess it would
> also work to have both switches ON (resulting in decoding the
> signal going out and re-encoding it coming in to the new copy),
> but the first alternative seems simpler.

Unlike dbx, the Dolby system isn't linear with amplitude; thus, if you
choose to copy with the Dolby switch off, you must somehow get the
magnetization level on the target tape to match the magnetization level
on the source tape, or the decoding won't exactly reverse the original
encoding. It isn't adequate to match VU meter readings, since
manufacturers disagree on the relationship between 0dB on the VU meter
and the magnetization (measured in nanowebers per square meter) on the
tape; that relationship is one of the parameters they fuss with in an
effort to optimize S/N, distortion, and frequency response.

Even if the two tape decks are identical, it's not always safe to rely
on the VU meter readings, since manufacturers often economize and
connect them so that they measure slightly different amplitudes on
recording and playback.

Unless you can measure nanowebers per square meter on the tape itself,
you must leave both Dolby switches on, so that the decoders and
encoders, which were tweaked at the factory to "memorize" the
relationship between 0dB on the VU meter and the Dolby standard
magnetization level, will take care of things for you.

-- 
                                                           --Steve Correll
sjc@s1-b.ARPA, ...!decvax!decwrl!mordor!sjc, or ...!ucbvax!dual!mordor!sjc