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From: rcd@opus.UUCP (Dick Dunn)
Newsgroups: net.audio
Subject: Re: Square waves (and an oops)
Message-ID: <558@opus.UUCP>
Date: Tue, 19-Jun-84 22:14:21 EDT
Article-I.D.: opus.558
Posted: Tue Jun 19 22:14:21 1984
Date-Received: Fri, 22-Jun-84 07:30:40 EDT
References: <3821@mordor.UUCP> <28@sunybcs.UUCP>
Organization: NBI, Boulder
Lines: 43

First, the oops - I made a stupid statement in a posting yesterday that the
lowest-frequency component present in a 20 KHz square wave but not in a 20
KHz sine wave is at 40 KHz and therefore pretty hard to hear.  That's
wrong; it's at 60 KHz (but still hard to hear!)  Square waves contain only
odd harmonics.  Yes, I know better and I found the error myself - but not
before sending the foolishness to the edges of the known universe.

...but we still don't have Mr. Pearson straightened out...
>  But you missed the point...
>  If the CD logic is sound it must reproduce a perfectly square wave
>  given a properly generated square wave test disk.

Still wrong.  A perfect square wave has substantial components well beyond
audibility.  No audio reproduction system needs this inaudible junk - it
can overload amp stages and fry tweeters.  The best systems will delicately
remove it without removing or phase-shifting the audible spectrum.  It's a
fortunate side-effect of the way CD's work that they don't (more correctly,
shouldn't) produce any components above 22 KHz or so.

It's not necessarily intuitive that you can take the HF components out of a
square wave without damage - but intuition doesn't help here.  I suppose
it's even less intuitive that the CD playback circuitry COULD produce a
very nearly square wave but will not do so if it's working.

>  They ring, and you know it.

And they're supposed to.  The longer-than-infinitesimal risetime and the
overshoot at the top of the rise are both CORRECT in the reproduction of an
audio-frequency signal from the information on a CD.

If you're a reasonably good programmer and you've got some sort of
graphical device, try plotting some "square waves", working up from a sine
wave adding components.  You'll be a little surprised at how many
components you need before it looks decently square.
  
>  CD get your basics correct first... this 'off by one' attitude will
>  not be tollerated.
Hoo, boy - who's doing the "tollerating" (sic) here, anyway?  You can rail
against CD's 'til you turn blue.  You don't set the standards and you
haven't yet stated a problem.
-- 
Dick Dunn	{hao,ucbvax,allegra}!nbires!rcd		(303)444-5710 x3086
	...Cerebus for dictator!