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Path: utzoo!watmath!clyde!floyd!cmcl2!seismo!ut-sally!mordor!sjc
From: sjc@mordor.UUCP
Newsgroups: net.audio
Subject: Re CDs and square waves
Message-ID: <3979@mordor.UUCP>
Date: Mon, 25-Jun-84 02:51:32 EDT
Article-I.D.: mordor.3979
Posted: Mon Jun 25 02:51:32 1984
Date-Received: Thu, 21-Jun-84 00:35:33 EDT
Organization: S-1 Project, LLNL
Lines: 49

Quotes from two different postings:

>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.

>As for the validity of testing auudio equipment with 20 khzsquare waves,
>Charles is RIGHT ON.  *I* can hear the difference between a 20 Khz
>square wave, and a 20 Khz sine wave.  The only difference is due to sine
>wave components at frequencies > 20 Khz.  Therefore, extended frequency
>range is VITAL to accurate reproduction of very high frequency sounds,
>esp. percussion.  This includes having speakers that are capable of
>reproducing the high frequencies, too.  (My Infinity Quantum-2's are
>flat to 30 Khz, and gradually roll off beyond that.  My Kenwood High Speed
>amp is flat to > 100 Khz.  Even my Ortofon MC-20FL cartridge is fairly flat
>to about 25 Khz.)

A test disk may provide a series of samples which (when viewed as
unsigned integers) alternate between 0 and 65535, but a correctly
functioning CD player must not produce from that a perfect square wave,
because the Nyquist theorem says that a series of samples at 2*N Hz
will contain only invalid information at frequencies at or above N Hz.
The CD player must filter out such frequencies, and as a result will
produce from that test disk a sine wave.

This has nothing to do with whether a CD player uses purely analog or
partly digital filtering.  No matter how ideal the filter--whether made
by Kyocera or Sears Roebuck--its purpose is to attenuate frequencies
above (44)/2 kHz as completely as it can.

I will not only admit that a CD player cannot reproduce a perfect 20kHz
square wave, I will claim that the more nearly ideal the antialiasing
filter is, the more nearly it will turn a 20kHz square wave into a
20kHz sine wave. The CD system will give you >90dB dynamic range,
remarkably low added-harmonic- and IM-distortion, complete freedom from
wow and flutter, and disks that will last indefinitely--but it will not
give you 20kHz square waves.  If you require 20kHz square waves, CDs
are not for you.  (Neither, of course, are analog tape recorders, FM
broadcasts, or vinyl records that have been played numerous times.)

Clearly people vary in ability to hear high frequencies (particularly
with age and exposure to Mick Jagger at 120dB).  Note that there is no
difference between a 20kHz square wave and a 20kHz sine wave until you
get to 40kHz.  When comparing sinusoids with square waves by ear, one
must be careful to use a signal generator which produces the same RMS
magnitude for each.

                                                           --Steve Correll
sjc@s1-c.ARPA, ...!decvax!decwrl!mordor!sjc, or ...!ucbvax!dual!mordor!sjc