[music-dsp] Noise performance of f32 iir filters

[Tom Wiltshire]

I get "DP1" and "DP2" (Direct Form 1 and 2, right?) and "SVF" but what are the other acronyms here? BLT? ("Bacon, Lettuce, and Tomato" is what comes to mind, but it doesn't seem likely...) TPT?

Thanks,
Tom

On 30 Sep 2011, at 10:57, Vadim Zavalishin wrote:

> Just for the reference, I tried to measure the precision of the TPT version of the SVF using DF2 BLT integrators, as described in
> http://www.native-instruments.com/fileadmin/ni_media/downloads/pdf/KeepTopology.pdf
> If I didn't make any mistakes in the measurements, the 32-bit float precision is better than -100dB for cutoffs below 20kHz at 44.1kHz SR. The error seems to be larger at large cutoffs.
> 
> As already mentioned, the trapezoidal integration is pretty much equivalent to TPT with DF1 integrators. There was an observation made by Dominique Wurtz on KVRAudio DSP forum, that DF1 and DF2 integrators should behave fully identically (mathematically, ignoring the precision issues) even in the time-varying TPT case. Which pretty much eliminates (mathematically) the need for DF1 integrators in TPT (except in special cases).
> 
> Regards,
> Vadim
> 
> ----- Original Message ----- From: "Andrew Simper" <andy at cytomic.com>
> To: "A discussion list for music-related DSP" <music-dsp at music.columbia.edu>
> Sent: Tuesday, September 27, 2011 16:19
> Subject: Re: [music-dsp] Noise performance of f32 iir filters
> 
> 
>> Hi Earl,
>> 
>> Since the analog SVF is one of the lowest noise topologies in analog
>> filters I suspect that a fixed point implementation will do well, but
>> I have not tested it yet.
>> 
>> Andy
>> --
>> cytomic - sound music software
>> 
>> 
>> 
>> 
>> On 27 September 2011 21:35, Earl Vickers <sfx at sfxmachine.com> wrote:
>>> Hi Andrew,
>>> 
>>> This looks most impressive. I look forward to seeing your article. Any
>>> thoughts on how suitable this topology would be for fixed-point
>>> implementation?
>>> 
>>> Earl
>>> 
>>> Andrew Simper <andy at cytomic.com> wrote:
>>> 
>>>> I finally got around to following up on my hunch that a slightly
>>>> modified version of the trapezoidal integrated svf (ie a modified
>>>> version of the one I previously posted) should have excellent
>>>> numerical properties. My initial tests confirm this in spectacular
>>>> fashion. I used all sorts of tests, but the one to show up most
>>>> problems was a bell filter with q=2, gain=12 dB, and look at the
>>>> cutoffs 20, 200, 2k, 20k. I compare the modified state variable
>>>> filter, normalised ladder, normalised direct wave form, direct form 1,
>>>> and direct form 2 transposed. The only filter to match the low
>>>> quantization error of the modified svf is the normalized ladder
>>>> filter, but none of the filters can match the coefficient rounding
>>>> error, as is shown in the time domain error of the 20 Hz example:
>>>> 
>>>> http://www.cytomic.com/files/dsp/SVF-vs-DF1.pdf
>>>> 
>>>> The modified SVF works fine down to very low frequencies with all
>>>> single precision computation, which makes it ideal for use even at 192
>>>> kHz sample rates. I'll get around to writing it up some time, but I've
>>>> got a few plugins and other work to get on with for the moment.
>>> 
>>> --
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> 
> -- 
> Vadim Zavalishin
> Senior Software Developer | R&D
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