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Unread 23-06-2002, 22:55
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#0047 (ChiefDelphi)
 
Join Date: May 2001
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Re: Digital Filters?

Posted by Michael Betts.

Engineer on team #177, Bobcat Robotics, from South Windsor High School and International Fuel Cells.

Posted on 1/31/2000 10:30 AM MST


In Reply to: Digital Filters? posted by Matt Rizzo on 1/31/2000 6:08 AM MST:



: On the Innovation First site they have a white paper on Digital Filters for Joysticks. Could someone
: explain how these filters work?

: Thanks,
: - Matt

Matt,

Kind of hard to explain without a few years of college education behind you. There are two major types of digital filters, an IIR (Infinite Impulse Response) filter and a FIR (Finite Impulse Response) filter. What they have implemented is probably the most simple filter possible... An extremely simplified IIR.

To learn more, ask the electrical engineers on your team. I can give you a couple of references:

A. Embedded Systems Programming, Vol 9 Num 5, May 1996, 'All About Filters' by Jack W. Crenshaw.

B. http://www.analog.com/publications/m.../31-1/DSP.html

The web address for reference A is http://www.embedded.com/ but they do not have that article on-line. You might be able to e-mail Mr. Crenshaw to ask for a reprint (I'm positive he has an electronic file).
BTW, If you do contact Mr. Crenshaw, he lives in Orlando and might be interested in seeing the FIRST nationals.

You might also be able to find a web tutorial with a little searching and the information I have given you here.

One of the best ways to understand the code in the white paper is to 'hand-crank' through the numbers for a few iterations (say, 127 as an input), insert a noise spike on the input (say 230 on the input) and then go back to 127 as an input. What does the output do? Remember that 2-4 is not -2 in unsigned arithmetic, it's 254!

Don't feel bad if this stuff is over your head. It's also over mine a lot of the time. Many engineers understand the math and do not appreciate what it actually means in the real world. Others use it all the time but cannot fathom the theoretical basis. Those who do both are very strange people indeed!

Good luck... - Mike



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