Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!rutgers!psuvax1!psuvm!psuecl!peg
From: peg@psuecl.bitnet
Newsgroups: sci.electronics
Subject: Re: Audio Spectrum Analyzer
Message-ID: <53928@psuecl.bitnet>
Date: 7 Sep 89 23:33:53 GMT
References: <3906@mentor.cc.purdue.edu>
Organization: Engineering Computer Lab, Pennsylvania State University
Lines: 37

In article <3906@mentor.cc.purdue.edu>, aic@mentor.cc.purdue.edu (George A. Basar) writes:
>   I'd like to build one of these beasties(the SA, not the pink noise source)
> to interface with a PC (C64 to be precise).  I've had a few ideas on how
> to go about this but I would appreciate any pointers to information on this
> subject.


Hey!

The sweepable notch filter sounds like a good idea--the fellow at
Motorola should be thanked for that one--but I have another.

It is my understanding that a lot of the commercial spectrum analyzers
(especially the lower cost ones) are using switched-capacitor filter
IC's.  These are really nifty IC's that give excellent filter
performance and are easy to use.  Examples are the MF 4 and MF 10.

These IC's are tuned by inputting a digital clock at a multiple of
the desired center frequency.  Typically, the clock is at 50 or 100
times fc.  For audio, this would mean a clock of 2kHz to 200kHz.
You would want to watch your signal routing, but you could probably
keep the clock out of your audio signals.

The big advantage I see of this approach is:  by using a
programmable divider IC, you could keep your CPU fairly free.
You could load a new divisor, say, 60 times a second.  That seems
like you could be doing other things (especially if you had an
Amiga computer!!!! :-).  Of course, if you want really fast
display update, this might not matter....  But most SA's I've
seen have a peak hold type display.

Well, I hope that idea might help you.  I've been wanting to
do this project for a while, but never find time....  Good
luck!

Paul

P.S.  The MF4 + 10 I have are by Texas Instr.