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Path: utzoo!mnetor!seismo!rochester!pt!speech1.cs.cmu.edu!phd
From: phd@speech1.cs.cmu.edu (Paul Dietz)
Newsgroups: sci.electronics
Subject: Re: 3800 Hz notch filter [will] be used on CD masters
Message-ID: <1002@speech1.cs.cmu.edu>
Date: Mon, 29-Jun-87 04:26:48 EDT
Article-I.D.: speech1.1002
Posted: Mon Jun 29 04:26:48 1987
Date-Received: Sun, 5-Jul-87 20:17:20 EDT
References: <598@neoucom.UUCP> <67@sdeggo.UUCP> <1597@ihuxi.ATT.COM> <921@argus.UUCP>
Organization: Carnegie-Mellon University, CS/RI
Lines: 38
Keywords: with possible icky results!
Summary: Fill up the hole!

In article <921@argus.UUCP>, ken@argus.UUCP (Kenneth Ng) writes:
> In article <1597@ihuxi.ATT.COM>, store2@ihuxi.ATT.COM (Kit Kimes) writes:
> 
> It will degrade the quality of the copy, but mixing in a low volume
> sound that is notched to match the CBS copycode scheme should bypass
> the scheme...
> ...  Also, CBS claims that no normal music is present in the notched
> area.  But if there isn't anything there, what does the notch do?  ...

I hate to disagree with CBS, but "normal" music certainly has interesting
things happening around 3800 Hz! Anyone who is unsure about this should
take a quick look at the spectrum of most orchestral instruments. Maybe
CBS is implying that only instruments with a big hole near 3800 are
normal! :^)

Ken brings up an interesting point. In order to trick the spoiler, he
suggests filling the hole. What if instead of just filling the hole, we
fill it proportional to the energy in the two adjacent bands? This only
requires a single, fairly broad filter that spans the notch and the
two adjacent bands. If you wanted to get tricky, you might even be able
to use one of the spoiler chips to do the detection. Then one could use
a cheap digital noise source for the filler.

How good would this sound? Probably not so great. If that notch is
200 Hz wide, it's on the order of a musical half step. Partials can
easily be entirely eaten by a notch that wide. Thus, the two adjacent
bands don't really contain enough information for reconstruction. So,
who's taking bets on how long before "Reconstruction" Filters appear
on the market. (Next to the Monster Coax Speaker cable 8^)! )

--- Paul Dietz

phd@speech1.cs.cmu.edu on ARPA, or ...!ucbvax!phd@speech1.cs.cmu.edu
... or something like that.

"I take no responsibility for this post. My aides never informed me
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