Re: low-pass and high-pass filter.

Subject: Re: low-pass and high-pass filter.
From: Evan Balster <email@hidden>
Date: Fri, 23 Dec 2016 11:47:22 -0600

It took me a while to get my head around digital filter design algorithms. You'll need to do the following steps, which I recommend you research individually:

Design prototype filters, EG. "8-pole butterworth", made of poles and zeroes.
Transform the filters to the appropriate bandform, EG. high-pass.
Use a bilinear transform to move these filters to the digital domain, still poles and zeroes.
Compute the coefficients of the filter from the poles and zeroes. I recommend using biquad filters, each of which uses two poles and two zeroes.

Step 2 or step 3 will need to involve some kind of "prewarping" to get the filter cutoff in the right location. If you don't prewarp, the filter's cutoff won't end up where you expect it to.

Alternatively, here's a C++ library that does all of this, if it suits your needs: https://github.com/vinniefalco/DSPFilters

– Evan Balster

creator of imitone

On Fri, Dec 23, 2016 at 8:32 AM, Nigel Redmon <email@hidden> wrote:

Maybe this will help? Use four biquads, each set to the same frequency, but with Q set as indicated by the calculator:

http://www.earlevel.com/main/2016/09/29/cascading-filters/

Nigel

On Dec 23, 2016, at 8:59 PM, Waverly Edwards <email@hidden> wrote:

I need to apply an eight-pole Butterworth low-pass and later a high-pass filter. Are there any built-in mechanisms for this?

If there are not, would someone mind providing some algorithmic guidance as to how this could accomplished? Why? I am attempting to reproduce an experiment in which an eight-pole Butterworth filter was used -- a high-pass at 300Hz and a low-pass at 3000Hz. If it matters, this is not for iOS.

Thank you kindly,

W.

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References:

>low-pass and high-pass filter. (From: Waverly Edwards <email@hidden>)

>Re: low-pass and high-pass filter. (From: Nigel Redmon <email@hidden>)

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