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From: newton2@ucbtopaz.CC.Berkeley.ARPA
Newsgroups: net.audio
Subject: Re: Defeating Dolby
Message-ID: <702@ucbtopaz.CC.Berkeley.ARPA>
Date: Sat, 2-Feb-85 19:04:19 EST
Article-I.D.: ucbtopaz.702
Posted: Sat Feb  2 19:04:19 1985
Date-Received: Wed, 6-Feb-85 04:05:53 EST
References: <1063@opus.UUCP>
Organization: Univ. of Calif., Berkeley CA USA
Lines: 24

I guess you're saying the Dolby (tm) noise reduction effect goes away when
the Walkman is near the terminal. Most likely the horizontal scanning waveform
is penetrating the audio path and paralyzing the sliding-band filter that
is the core of the Dolby B (and C) NR system. The NR thinks there's
substantial high-frequency energy coming off the tape, and responds by 
opening up its bandpass fully, which is the "correct" behavior- if the
signal really *had* been recorded on the tape, the recording NR processor
would likewise have slid upward to make way for the high-level 15 kHz "signal".

Well designed Dolby circuits must have an effective low-pass filter at the
*encoder* input to prevent signals too high in frequency for the tape to
capture from triggering the encoder, and must *also* have an effective low-pass
filter at the input to the playback processor to keep out spurious high
frequencies which are *not* on tape. Most common source of these problem signals
is tape recorder bias (on three-head so-called "double-Dolby" machines that
try to reproduce decoded Dolby recordings while they're being made) and
19 kHz FM stereo pilot tone leakage. Dolby C is *very* much more sensitive
to such leakage than is Dolby B. Incidentally, bias leakage can be a problem
in the *record* processor if it couples into the Dolby circuit by
somehow circumventing the otherwise adequate low-pass described above-- say
capacitively. That's my guess for what's occuring in your situation,
albeit in playback mode. Symptom would be: tape noise rises as much as 8 dB
(Dolby B; 15 dB or more for Dolby C), while whatever low level highs are
present are emphasized (Dolby C effect would extend down to mid-freqs).