Path: utzoo!attcan!uunet!lll-winken!lll-lcc!ames!oliveb!sun!pepper!cmcmanis
From: cmcmanis%pepper@Sun.COM (Chuck McManis)
Newsgroups: comp.sys.amiga
Subject: Re: Sampling at 29KHz (long)
Message-ID: <53788@sun.uucp>
Date: 19 May 88 01:47:27 GMT
References: <2845@polya.STANFORD.EDU> <734@eos.UUCP>
Sender: news@sun.uucp
Reply-To: cmcmanis@sun.UUCP (Chuck McManis)
Organization: Sun Microsystems, Mountain View
Lines: 25

In article <734@eos.UUCP> phil@eos.UUCP (Phil Stone) writes:
>Compute a 256-point sine wave and put it in memory (32-byte sine waves
>don't cut it in my book - even a tin ear can hear the interpolation
>noise in fixed, jagged steps that big).

Phil you still don't get it do you? If take a 32 sample sine wave
and run it through a spectrum analyzer, you will find that it has
a big fundamental at the frequency of interest, and a bunch of 
higher order frequencies that are caused by the 'interpolation' noise.
The key feature is that *all* of these frequencies are above the
'nyquist' frequency. The Amigas low pass filters start cutting of 
frequencies above 7Khz and pretty much eliminate everything above
14Khz. That means that even your golden ear may have difficulty
in hearing the differences. (You will always get .4% distortion 
because that is the monotonic difference between to 'points' in 
space.) Where's my DFT when I need it. So to determine when a 32
sample waveform is 'good' enough, run it through a DFT, and look
at the first frequency component above the fundamental. Is it below
your filter point? If so you need to lengthen your sample, if not
then your sample *cannot* be made any better so why waste the bytes?
The constants in the equation are the low pass filter, the sample
frequency, and the DAC resolution. 

--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.