Re: Wave filter question (mainly for Joerg)

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

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