> 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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