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From: rfg@hound.UUCP (R.GRANTGES)
Newsgroups: net.audio
Subject: Re: Square Wave Response (Now Hold On!)
Message-ID: <524@hound.UUCP>
Date: Tue, 12-Jun-84 12:35:27 EDT
Article-I.D.: hound.524
Posted: Tue Jun 12 12:35:27 1984
Date-Received: Wed, 13-Jun-84 01:10:50 EDT
References: <270@whuxj.UUCP>
Organization: AT&T Bell Labs, Holmdel NJ
Lines: 27

Now hold on just one darned minute, folks. I'm sure I have seen
square waves from a CD test record that looked better than anything
I've ever seen from a disc (except maybe with the old Weathers'
pickup). Mitchell, here you are going on about 20 khz square
waves. Almost any amplifier is going to mess that one up today,
not to mention the small problem of how do you radiate it acoustically.
We  be talking about 1000 hz square waves. The CD medium is
inherently capable of producing -off the disc- a "perfect" square
wave, where I define perfect as flat topped (in this case response
to DC so even a 100 hz or 10 hz square wave would look flattopped) with
"vertical" transitions (in this case, a rise time of less than one
sample period -24 microseconds).  True, some players will show ringing.
Most of the photos I have seen show less ringing than what we have
become accustomed to with phono cartridges - of which moving coil
designs seem to be the worst (?).
THe initial article led off with a wild claim, contrary to fact, that
everyone seems to have accepted - that CD square wave response is bad.
It's not. It's superb! For that matter, so is the impulse response.
Somebody donate big bucks to the cause and we'll buy some test discs
to take pictures at home. Meanwhile, I'll go by what I've seen printed.
A well known rule of thumb is that for a square wave to look good,
you need flat response and linear phase between 0.1 and 10 times
the repetition rate of the square wave. With rep rates to 1000 hz,
the CD medium, even with the poorest "filtering", provides almost
perfect response - certainly far better than any other medium in
common use. I hate this overkill, why didn't yoiu guys kill this one
earlier (Quick, Henry, the Flit!)    Dick Grantges hound!rfg