Path: utzoo!utgpu!water!watmath!clyde!bellcore!faline!thumper!ulysses!andante!mit-eddie!ll-xn!ames!pasteur!ucbvax!decwrl!sun!pepper!cmcmanis
From: cmcmanis@pepper.UUCP
Newsgroups: comp.sys.amiga.tech
Subject: Re: Is 29 Khz really the fastest audio playback rate?
Message-ID: <53701@sun.uucp>
Date: 18 May 88 17:57:33 GMT
References: <8805171810.AA07554@cory.Berkeley.EDU>
Sender: news@sun.uucp
Reply-To: cmcmanis@sun.UUCP (Chuck McManis)
Organization: Sun Microsystems, Mountain View
Lines: 23
Posted: Wed May 18 13:57:33 1988

In article <8805171810.AA07554@cory.Berkeley.EDU> (Matt Dillon) writes:
>	I think everybody is missing a major point here...  If the limit
>to the audio system is 29KHz/channel, it means that the highest frequency
>you can get out of the thing is about 14KHz... and that would HAVE to be a 
>square wave!
>						-Matt

Matt, this is what the low-pass filter is for. You are correct that the
maximum frequency that this setup can produce is 14Khz. However on the
output is a low pass filter that filters out any frequencies above 7Khz
or so. Ideally this would have a vertical cutoff at 14Khz so that when
you send a wave of 0xff 0x00 to the output the filter removes all of
the 'artifacts' above 14Khz and you get a pure 14Khz sinewave out. 
Sudden transitions in the output are heard as 'quantization' noise and
are generally much higher frequencies than the waveform you are trying
to produce. That's why a filter is essential in any digital sound synthesis
system. The end result is that a two byte sample to play a sine wave at
10Khz has just as much fidelity as a 256 byte sample at 78Hz would. 


--Chuck McManis
uucp: {anywhere}!sun!cmcmanis   BIX: cmcmanis  ARPAnet: cmcmanis@sun.com
These opinions are my own and no one elses, but you knew that didn't you.