Bandpass digital filter with band 3Hz...300Hz and Fs 1kHz

Try posting in comp.dsp also.

-- Roberto Waltman

A cycle of 3 Hz is 333 samples at 1 kHz. To be effective, the total length of a FIR filter must be an appreciable part of the cycle time. 63 taps may be even too little, depending on your performance requirements.

When the rate of corner frequency is a small fraction of the sample rate, you should consider an IIR filter instead.

I tried your filter using MATLAB's filterbuilder from the filter design toolbox. Simplified spec: Fs = 1000, fstop1 = 1.5, fpass1 = 3, fpass2 = 300, fstop2 = 500

FIR needs 954 multiplies and 953 additions per sample

IIR needs 34 multiplies and 24 additions per sample (Butterworth)

or 22 multiplies and 15 adds (Chebyshev).

I think the ST DSP library has an IIR function. You should be able to find soemthing free on the web to calculate the coefficients from your own filter spec and off you go.

Michael Kellett

ess

It risk of sounding like I'm asking a dumb question, I've not worked with IIR filters much, but I thought their structure was pretty simple, but with feedback. How can such a structure have more multiplies than adds? Am I missing something or is this just a difference in how it is being described?

Rick

s16 Filter(s16 x) { static s16 z0, z1; s16 y;

x -= z0; z0 += x >> 6; y = x >> 1; x = y + z1; z1 = y;

return x; }

Here we go.

:))))))

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

As mentioned -- use an IIR filter, at least for the low-pass portion. Sometimes you just need a more powerful processor, sometimes you need to get creative about how you're specifying your filter to reduce the computation load.

Thanks to everybody! I used IIR for low-frequency part and FIR - for high.

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