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From: rcd@opus.UUCP (Dick Dunn)
Newsgroups: net.audio
Subject: Re: Copy protected records?
Message-ID: <1080@opus.UUCP>
Date: Tue, 12-Feb-85 02:09:28 EST
Article-I.D.: opus.1080
Posted: Tue Feb 12 02:09:28 1985
Date-Received: Wed, 13-Feb-85 19:04:07 EST
References: <305@boulder.UUCP> <8221@brl-tgr.ARPA>
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Organization: NBI,Inc, Boulder CO
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> > 	I have heard rumors from a couple of sources now that some record
> > companies are adding a high frequency signal to their records that 
> > interfere with the bias on tape decks, thus making it difficult to make
> > tapes of said records.  Does anybody know if this is really true, and
> > if so, just what the technique is?

It's not clear whether this might refer to the actual bias for the
recording or (rumors being what they are) to introducing a bias which would
screw up Dolby tracking.

> 'Twould seem to me that any signal high enough to be inaudible and also
> interfere with my tape deck bias would also be severely diminished by
> the fact that my stereo rolls off the high end very quickly above what
> I find to be inaudible...

Good point.  Consider another related one--a record company might want to
be damned careful about recording ultrasonics which can fry tweeters (when
passed thru a superb cartridge and one of those idiotic DC-to-light preamp/
amp combinations).  And yet another--do you want music or a new way to call
your dog?  I can think of lots of reasons that make the rumor unlikely.

> ...all you'd have to do is zap
> your equalizer to not try to record above the top level that cassettes
> are capable of recording.

Another good way would be to kick in the MPX filter that's present on most
decks--this is a filter designed to get rid of any residual of the 19 KHz
"pilot" signal in FM stereo signals.  In fact, the reason you have the MPX
filter is just to prevent Dolby mistracking.  I'm pretty sure that these
filters are lowpass (rather than notch--can anyone confirm or correct?), so
this should handily get rid of any HF junk.

> The only thing I could think of is that they are beating the bias
> frequency down to something audible to ruin the recording.

This doesn't seem likely either--bias is commonly some 105 KHz, so to beat
it on down the line (oops) to audio, you've got to come in above 85 KHz.
That's a tall order for most cartridges, and there are lots of other
places in the system to lose a frequency that high.

It wouldn't surprise me to know that record companies are working on
something to make it difficult to tape LP's--i.e., finding one more way to
screw up the waning fidelity of the garden-variety LP.  I just wish they'd
attack the problem by making decent tapes available instead.
-- 
Dick Dunn	{hao,ucbvax,allegra}!nbires!rcd		(303)444-5710 x3086
   ...Cerebus for dictator!