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From: ark@rabbit.UUCP (Andrew Koenig)
Newsgroups: net.audio
Subject: Re: CDs: why no square waves?
Message-ID: <2885@rabbit.UUCP>
Date: Tue, 19-Jun-84 11:15:03 EDT
Article-I.D.: rabbit.2885
Posted: Tue Jun 19 11:15:03 1984
Date-Received: Thu, 21-Jun-84 01:59:51 EDT
References: <28@sunybcs.UUCP>
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 46

Charles E. Pearson says the following:

>  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.
> 
>  The sinusoidal properties that the square wave on CDs display
>  is the perfect example of how the CD theory is either improperly
>  executed or has a basic fault.
>
>  They ring, and you know it.
>  
>  They are not as good as they are supposed to be.  Whether they are
>  better than analogue technology is almost irrelivant except that
>  mediocre examples of alalogue tech. produce better examples of what
>  the digital tech. must produce better.
> 
>  CD get your basics correct first... this 'off by one' attitude will
>  not be tollerated.

Sorry, Charlie, but I've got to disagree with you on this one.
If all the samples in a stream of digital samples up to some point
have the value X and all the samples after that point have the value Y,
you might expect the "correct" output to be perfectly square, with zero
rise time.

However, this is untrue in the presence of bandwidth limiting.
The square wave approximation now so familiar from CD player test reports
is the best possible approximation to a square wave that fits within
the bandwidth limitations of the system.

To put it differently, suppose you had an INPUT signal that looked
like one of these ringy square wave approximations.  The samples taken
from this signal would be indistinguishable from those taken from a true
square wave.  Thus, if you get the square wave "right," you do so
only at the cost of getting some other legitimate (i. e. one that fits
entirely within the prescribed bandwidth) wrong.

In other words, attempts to reproduce square waves result in ringing,
not because the theory is wrong or improperly executed, but because
that is what the theory says should happen.

By the way, 'tolerated' has only one l.


				Andrew Koenig