Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.1 6/24/83; site pur-phy.UUCP
Path: utzoo!watmath!clyde!burl!mgnetp!ihnp4!inuxc!pur-ee!CS-Mordred!Pucc-H:Physics:hal
From: hal@pur-phy.UUCP (Hal Chambers)
Newsgroups: net.audio
Subject: 20kHz Square Waves
Message-ID: <1361@pur-phy.UUCP>
Date: Mon, 25-Jun-84 18:10:32 EDT
Article-I.D.: pur-phy.1361
Posted: Mon Jun 25 18:10:32 1984
Date-Received: Thu, 28-Jun-84 02:33:48 EDT
Organization: Purdue University Physics Dept.
Lines: 25


    All this talk of digital audio being unable to reproduce frequencies
above 22kHz is all very nice but has nothing to do with 20kHz square wave
reproduction.  Similarly arguments about summing sine waves to produce a
square wave and the presence of "Gibb's ears" are also very true and totally
irrelevant.  The digital reproduction process doesn't do a Fourier trans-
formation or Fourier synthesis of the audio signal.  The signal is digitized
(like reading a voltmeter) and that number later converted back to a voltage
by the D/A converter.  Sampling at 44kHz, a 22kHz square wave can be reproduced
exactly (alternate samples are a high voltage then a low voltage).

    This afternoon, I set up the D/A converter on a microcomputer here in
the lab to sample at 40kHz; fed it data of alternating +N and 0; and
observed the output on an oscilloscope.  Result: A 20kHz square wave.
The only limitation being the finite rise time (2 usec.) of the D/A converter.
(Note: at 20kHz the period is 50usec.).

    In commercial players, any ringing on the square wave output is due to
the use of digital filtering; an analog filter with a high frequency
resonance; or something else after the D/A conversion.

Hal Chambers
Physics Dept.
Purdue Univ.
(...pur-ee!pur-phy!hal)