[music-dsp] Tweakable filters of higher orders?

Nigel Redmon earlevel at earlevel.com
Wed Sep 29 10:36:21 EDT 2004


Yes, of course you can do it that way, but you're still talking about 
factoring it into identical filters, right? I mean, such filters are 
typically factored into all biquads (assuming even order)--or a 
24dB/oct filter might be two SVF's. Particularly, for a "tweakable" 
filter, you'd choose a filter topology that you already know how to 
make tweakable (something that you can change the coefficients on in 
"real" time easily and without artifacts).


On Sep 28, 2004, at 11:44 PM, Joshua Scholar wrote:

> I'm no expert in this, but it's my impression that you can do much 
> better
> than cascading identical lower order filters.  Instead make an optimum 
> high
> order filter and then factor it into lower order filters...  I believe 
> the
> math is that this is always possible.
>
> I've done this before (turned a high order filter into a cascade of 
> lower
> order ones) - I just looked up the math for it online and use Maxima 
> to do
> the calculations.
> The result was a filter with identical response, but with less 
> sensitivity
> to round off error.
>
> It's not entirely free.  The operations count of the lower order filter
> cascade is higher than the single higher order filter it replaced.
>
> ----- Original Message -----
> From: Nigel Redmon
> To: a list for musical digital signal processing
> Sent: Tuesday, September 28, 2004 11:28 PM
> Subject: Re: [music-dsp] Tweakable filters of higher orders?
>
>
> Yes, that is the usual thing to do--higher order filters get too
> sensitive to coefficient precision. Most DSP texts point this out, and
> suggest using series or parallel combinations of lower order filters. I
> wanted to add that--it may or may not be obvious--when you cascade
> several lower order filters, remember that the frequency cutoff moves
> (because it's defined as the -3dB points, and since the cascade changes
> the slope...).
>
>
> On Sep 28, 2004, at 3:17 PM, Koen Tanghe wrote:
>
>> On Sunday, September 26, 2004 1:46 AM [GMT+1=CET],
>> Koen Tanghe <xxxkoen at smartelectronix.comxxx> wrote:
>>
>>> Can I just use a cascade
>>> of multiple identical instances of a biquad to get "sharper" filters?
>>> Or
>>> are there any special things to do?
>>
>> Well, that's what I ended up with for now. Appears to work fine...
>> Koen
>
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