Re: Wave filter question (mainly for Joerg)

Tim Wescott <tim@seemywebsite.com> wrote:

>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.

Goertzel doesn't work for detecting tones reliably especially when a
few percent of error is allowed.

-- 
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