[PD] algorithms for drawing filter response curves

Mike Moser-Booth mmoserbooth at gmail.com
Thu Feb 10 05:07:53 CET 2011


Hey Hans,

I had some time to go over this today, and I think I've got the response
working. I didn't document my changes as I went along (sorry), but I think I
remembered them all and wrote them at the top of the attached script. Also,
I should take back what I said earlier about the bandwidth. Given this gui,
it makes sense for it to be BW.

.mmb

On Tue, Feb 8, 2011 at 12:08 AM, Hans-Christoph Steiner <hans at at.or.at>wrote:

>
> I'll check out the settings.  As for running this, its a script, you can
> just run it on the Terminal.  On GNU/Linux, it should just work.  On Mac OS
> X, you'll probably need to have run it like this:
>
> /usr/bin/wish filterview.tcl
>
> .hc
>
> On Feb 7, 2011, at 11:20 PM, Mike Moser-Booth wrote:
>
> I don't really know how to test this and I don't really know tcl, but I
> tried looking at the code to see if I could make some sense of it anyway,
> because why the hell not. ;-) Anyway, the only part I really understood was
> the calculation of the coefficients, and I noticed something wrong. You are
> calculating alpha for bandwidth instead of Q. I'm guessing you're just
> borrowing what is in the ggee externals (given the code's comments), because
> it's done the same way there, but it's not really conventional for a lowpass
> filter. That's really more for bandpass, notch, and peaking filters. Using Q
> is also a simpler calculation: alpha = sin(omega)/(2*Q). With Q set to .7071
> (or -3 dBfs), you have a filter with no resonance, which makes sense because
> the cutoff frequency for a lowpass filter is defined as the frequency where
> the response falls be 3 dB. This is also how Max's [filtergraph~] works.
>
> It's all defined here in Robert Bristow-Johnson's Cookbook:
>
> http://www.musicdsp.org/files/Audio-EQ-Cookbook.txt
>
> The ggee externals, as far as I can tell, are all based on this cookbook,
> though with mistakes like using BW instead of Q.
>
> Hope that helps,
> .mmb
>
> On Mon, Feb 7, 2011 at 10:03 PM, Hans-Christoph Steiner <hans at at.or.at>wrote:
>
>>
>> On Feb 7, 2011, at 8:35 PM, Spencer Russell wrote:
>>
>>  On Sun, Feb 6, 2011 at 11:15 PM, Mathieu Bouchard <matju at artengine.ca>
>>> wrote:
>>>
>>>> On Sun, 6 Feb 2011, Spencer Russell wrote:
>>>>
>>>>  [1] gives the magnitude frequency response given the biquad
>>>>> coefficents, and it linked to from several places around the net. Is
>>>>> this the sort of thing you're looking for?
>>>>> [1] http://bit.ly/eFck4j
>>>>>
>>>>
>>>> This link got me directly to :
>>>>
>>>>  USB complete: everything you need to develop custom USB peripherals
>>>>  Par Jan Axelson
>>>>  Chapitre 12
>>>>  Page 364
>>>>
>>>> and nothing else. (??)
>>>>
>>>>
>>> Whoops, I meant to send this: http://bit.ly/gUCBu4. Sorry.
>>>
>>> Here he's basically just taken the transfer function of a biquad and
>>> made the z = exp(j*w/Fs) as Mike suggested, followed by taking the
>>> magnitude and log-scaling for dB, but he's done the algebraic legwork.
>>> Just note that w (lower-case omega) is 2*pi*f and you should be good
>>> to go. He's also done some trig-substitution for more numerical
>>> precision.
>>>
>>
>> I'm making good progress, but now am getting lost in the filter math
>> meeting the pixel math.  Can someone mathy check my math?  Or even try it
>> out?
>>
>>
>> https://github.com/pd-projects/filterview
>>
>> .hc
>>
>>
>>
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>>
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>>
>>
>>
>>
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>
>
>
> --
> Mike Moser-Booth
> mmoserbooth at gmail.com
>
>
>
>
> ----------------------------------------------------------------------------
>
> "A cellphone to me is just an opportunity to be irritated wherever you
> are." - Linus Torvalds
>
>


-- 
Mike Moser-Booth
mmoserbooth at gmail.com
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