[PD] algorithms for drawing filter response curves

[Hans-Christoph Steiner]

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
>
>
> ----------------------------------------------------------------------------
>
>                  ¡El pueblo unido jamás será vencido!
>
>
>
>
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>
> -- 
> Mike Moser-Booth
> mmoserbooth at gmail.com


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