Forums

Re: Wave filter question (mainly for Joerg)

Started by Tim Wescott May 11, 2006
Joel Kolstad wrote:
> Simple question on wave filters, that I've heard about but never used: Rumor > has it that wave filters are just the digital simulation of a classical L-C > (ladder-type) filter structure. True? > > I skimmed the TI application note on DTMF detection using wave filters, and > there's an awful lot of unfamiliar (to me) material there. Somewhere I have a > print out of some IEEE tutorial on the subject, but I can't place my hands on > it at the moment. > > What I'm really after is trying to find efficient (in terms of, e.g., > multiply/accumulate cycles) tone detectors... > > ---Joel Kolstad > >
I'm taking the liberty of cross-posting this to the DSP group because you'll get a lot of good answers there, too. I would look at either plain old bandpass filters of the form (z+1)(z-1) H(z) = ------------------------- z^2 - 2 d cos(th) z + d^2 or I would consider modulating the incoming signal with inphase and quadrature versions of the tone you're looking for, and low-pass filtering the result. Note that I do _not_ have the famous Goertzel filter on this list. Odd, that. In the bandpass filter case you'll have to think carefully about the form of the filter; the states can easily overflow, and you sometimes need an astonishing precision to your variables for a high-Q filter. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html