Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site mordor.UUCP 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