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Chamberlin Multimode Filter
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Tim Kleinert



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PostPosted: Tue Aug 04, 2015 2:34 pm    Post subject: Chamberlin Multimode Filter
Subject description: Chamberlin filter topology
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Hi chaps,

Skimming through some old DSP papers, I came across this filter design which intrigued me in its simplicity, so I cooked it up on the G2 demo.

It tracks quite well and is amazingly stable -not perfect, but way better then any other G2 DIY filter circuit I've come across so far.

It will self-oscillate at max resonance if you ping it, and has a nice musical sound in my opinion.

Has a little demo step-sequencer thingie attached to it.

Just FWIW.

cheers,
t
UPDATE: I noticed that the resonance has to be negatively biased to completely remove the boost at a setting of zero. New version uploaded. (I threw gain compensation in there as well.)


ChamberlinFiltTK.pch2
 Description:
Chamberlin multimode filter topology (UPDATE: with reso biasing)

Download
 Filename:  ChamberlinFiltTK.pch2
 Filesize:  2.39 KB
 Downloaded:  664 Time(s)


Last edited by Tim Kleinert on Wed Aug 05, 2015 3:50 am; edited 2 times in total
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Tim Kleinert



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PostPosted: Wed Aug 05, 2015 2:23 am    Post subject: Reply with quote  Mark this post and the followings unread

Addendum:

I liked the sound of this filter topology a lot, so I cooked up a 24dB multi-mode version out of two cascaded stages. I added resonance gain compensation to avoid clipping and reduced the resonance range just a smidgen to prevent (very subtle) stability issues in the uppermost cut-off range. So it won't self-oscillate anymore when pinged, but it surely still can howl. Twisted Evil

DSP usage of the filter-circuit:
8.6% cycles ( Shocked cheap! Cool )
17.2% memory

What can I say... me likey. I'm really fascinated and impressed how such a good digital filter (sound, stability, tracking, features) can be built so cheaply. Hal Chamberlin is the man! Hail the Master

UPDATE: I noticed that the resonance has to be negatively biased to completely remove the boost at a setting of zero. New version uploaded.


Chamberlin24dBTK.pch2
 Description:
Two cascaded Chamberlin topology 12dB filter stages for a 24dB multi-mode filter. (UPDATE: With reso biasing)

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 Filename:  Chamberlin24dBTK.pch2
 Filesize:  2.92 KB
 Downloaded:  646 Time(s)

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Tim Kleinert



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PostPosted: Wed Aug 05, 2015 3:52 am    Post subject: Reply with quote  Mark this post and the followings unread

Addendum 2: (sorry... Laughing )

I noticed that the resonance has to be negatively biased to completely remove the boost at a setting of zero. New versions uploaded above.

And last but not least: Just discovered that these designs can take very hot signals and will overdrive quite gracefully. Sounds fantastic. Cool
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Blue Hell
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PostPosted: Wed Aug 05, 2015 4:31 am    Post subject: Reply with quote  Mark this post and the followings unread

Interesting!

Do you have a reference to the paper for me? I googled a bit of course ... but was not clear to me what to use ...

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Tim Kleinert



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PostPosted: Wed Aug 05, 2015 4:49 am    Post subject: Reply with quote  Mark this post and the followings unread

Blue Hell wrote:
Interesting!

Do you have a reference to the paper for me? I googled a bit of course ... but was not clear to me what to use ...


It's treated extensively in the "Effect Design Part 1" by Jon Dattorro. The digital circuit is on page 676 below, Fig.18.

https://ccrma.stanford.edu/~dattorro/EffectDesignPart1.pdf

It's only two Z-1 additions (integrators), two subtractions (negative feedback loops) and three multiplications! Shocked Laughing

(You'll obviously need lin->expo conversion for the cut-off coefficient if you want it to track properly. On the G2 the LevelScaler module does this neatly.)

I think it's absolutely genius! Smile
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Tim Kleinert



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PostPosted: Wed Aug 05, 2015 6:56 am    Post subject: Reply with quote  Mark this post and the followings unread

Another take on this subject:

I compared the 12dB circuit to the stock G2 multi-mode filter, and they can be tweaked to match up quite convincingly. Might well be that the G2 module is based on the same design.

However the filter cutoff frequency response (eg. when modulated with fast envelopes) is different, the DIY version somehow appearing to be more snappy and bonky. It also has obvious benefits like resonance modulation (the G2 module doesn't Confused ) and adjustable gain compensation -plus something else: Smile

When futzing around with the patch, I discovered that if you reduce the negative loop-back coming from the second integrator (the LP output), the filter goes into a nice sort of low-end drive, getting progressively more grungy in a "non-digital sounding" way. It however totally wretches the behavior of the entire circuit Twisted Evil : Resonance starts to drown out, the overall cut-off frequency response drops (which however makes it warmer-sounding as it gets grungier), and the different filter modes less pronounced. As unorthodox and probably "wrong" as it is, I think it's totally cool, so I implemented it in this version (which also has two more filter modes I totally forgot in the first version: notch and peak).

Also, the resonance bias and response coefficients have a tad more range (just before the whole thing gets ugly Laughing ).

cheers,
-t

Addendum: Cranking up the aforementioned "grunge" control and feeding the filter with a pure sine oscillator clearly shows a buildup of odd harmonic distortion. Sweeping up the sine oscillator (or any other) however doesn't result in any kind of aliasing within the filter. Seems as if those integrators keep everything in check. Cool

I like this feature so much that I optimized the gain compensation to deal with the interaction between resonance and "grunge" settings.

Therefore, new version uploaded.


ChambFiltGrungTK.pch2
 Description:
Chamberlin 12dB filter topology; all 5 filter modes, plus a totally unorthodox drive/grunge control.

Download
 Filename:  ChambFiltGrungTK.pch2
 Filesize:  2.89 KB
 Downloaded:  734 Time(s)


Last edited by Tim Kleinert on Wed Aug 05, 2015 8:26 am; edited 2 times in total
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Blue Hell
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PostPosted: Wed Aug 05, 2015 7:59 am    Post subject: Reply with quote  Mark this post and the followings unread

Tim Kleinert wrote:
It's treated extensively in the "Effect Design Part 1" by Jon Dattorro. The digital circuit is on page 676 below, Fig.18.


Thanks, got the printer working on that now :-)

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Tim Kleinert



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PostPosted: Wed Aug 05, 2015 8:31 am    Post subject: Reply with quote  Mark this post and the followings unread

Blue Hell wrote:
Tim Kleinert wrote:
It's treated extensively in the "Effect Design Part 1" by Jon Dattorro. The digital circuit is on page 676 below, Fig.18.


Thanks, got the printer working on that now Smile

Those are all great papers, alternating the über-complex math stuff with simple diagrams even a halfwit like me Laughing can understand.
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Blue Hell
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PostPosted: Wed Aug 05, 2015 1:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

There is some more nice stuff in it, yes, and parts II and III seem worthwhile too Smile

The filter seems to be an SVF .. but the implementation I had before uses three multiplications (for the basic filter function that is) and a bit of a different order ... also see http://www.musicdsp.org/showone.php?id=142 and http://www.musicdsp.org/archive.php?classid=3#92 - I currently use the latter. Anyway, that would explain the similarity to the Clavia multi-mode filter too.

There was another remark in the article ... about at high frequency settings the filter no longer working as a filter ... didn't the Clavia filter have this issue too?

Which brings me to : for an SVF to be stable Fc should be limited to about 1/6 * sample_rate .. ah .. ok .. the paper says pi/2 .. which would be 1/4 * sample_rate (or you over-sample, as in that 2nd link above) .. ah and best use not above pi/3 .. ok same thing.

Anyway .. was wondering if it could be useful as a per-grain filter in a granulator thingie .. for now I just made an implementation ... will evaluate it later.

Thanks Tim Smile

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varice



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PostPosted: Wed Aug 05, 2015 4:54 pm    Post subject: Reply with quote  Mark this post and the followings unread

Blue Hell wrote:
...There was another remark in the article ... about at high frequency settings the filter no longer working as a filter ... didn't the Clavia filter have this issue too?...

Yes, the FltLP module has this problem. Tim first mentioned it in this topic:

http://www.electro-music.com/forum/topic-63843.html&postorder=asc

Tim Kleinert wrote:
...An interesting observation: In my original experimentation with IIR filters, I noticed that the stock G2 filters do nothing in the most upper frequency range settings. Shocked I mean, they simply stop working altogether, regardless of slope setting. Oh well.

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Tim Kleinert



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PostPosted: Wed Aug 05, 2015 5:18 pm    Post subject: Reply with quote  Mark this post and the followings unread

varice wrote:

Yes, the FltLP module has this problem. Tim first mentioned it in this topic:

http://www.electro-music.com/forum/topic-63843.html&postorder=asc


Yes, but strangely the FltHP doesn't. I suspect it's just a scaling error.

Blue Hell wrote:
Thanks Tim Smile

You're very welcome. Those Dattorro papers are a goldmine.

I have to sheepishly admit that I didn't read all to deep into the mathematical descriptions as my math skills are quite limited. I just saw that simple circuit diagram and thought "Wait a minute, I can patch this in a breeze". And it worked right off the bat, which amazed me because very often these kind of things, uh, don't. Laughing
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redmar



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PostPosted: Tue Aug 25, 2015 3:43 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi Tim,
this filter design sounds just great...also, the grunge control has a nice feel, I bet it sound even nicer in a mix. Just one thing, what's the purpose of the high logic signal from the clock active out to the cutoff mixer? Should I add the same amount of bias in a classic architecture synth patch?
As always, thanks for sharing your g2 skills with us..
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Tim Kleinert



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PostPosted: Tue Aug 25, 2015 4:08 am    Post subject: Reply with quote  Mark this post and the followings unread

redmar wrote:
Just one thing, what's the purpose of the high logic signal from the clock active out to the cutoff mixer? Should I add the same amount of bias in a classic architecture synth patch?

I just used the clock active output as a freebie +64 constant in that little demo thingy in order to get things to sound as I wanted there -just quick'n'dirty patching. Smile It's of no relevance.

I built exponential tracking into the filter. It will respond in the usual fashion to modulations. Even the tuning isn't all that bad.
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redmar



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PostPosted: Tue Aug 25, 2015 6:35 am    Post subject: Reply with quote  Mark this post and the followings unread

Great...thanks!
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Electromagnetic Wave



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PostPosted: Tue Jan 30, 2018 11:54 pm    Post subject: Reply with quote  Mark this post and the followings unread

After reading the following I ask myself few questions. "A cascade of two Chamberlin filters can be considered as the digital counterpart to the Moog VCF".

Question :
How to deal with the saturating amplifier and the 4 identical saturating lowpass stages between each of the 4 poles (6dB/oct) if the ladder filter employed fourth-order all-pole and the Chamberlin filter use 2(12 dB/Oct) poles?

I appreciate the facts in your patch that 'these designs can take very hot signals and will overdrive quite gracefully' and 'the DIY version somehow appearing to be more snappy'. Smile

I'm not trying to get the exact sound of Moog. But just to find this sound so pleasant when I drive the resonance very high and I sweep the frequency range with the filter. None of the patches I tested (Moog clone, TB303, etc.) with the G2 have this sound so pleasant (filter sweep) with a strong resonance gain. Despite the DSP / resolution constraints, I'm sure it's possible to get a good result for our ears with the G2.

Effect Design Part 1" by Jon Dattorro, page 674 :
Quote:
The classic Moog analog synthesizers, for example, employed
fourth-order all-pole voltage-controlled filters (VCFs).
His constant-Q design was also known as the Moog ladder,
after the appearance of the schematic [20]. A cascade of two
Chamberlin filters can be considered as the digital counterpart
to the Moog VCF because some of the same characteristics
are shared. They are both all-pole constant-Q designs tuned by
a single sweepable parameter. Rossum [21] of Emu considers
essential nonlinear



I was looking at this paper before re-reading this post and be more interested in this Chamberlin filter patch and "Effect Design Part 1" :

https://aaltodoc.aalto.fi/bitstream/handle/123456789/14420/article5.pdf
and
https://aaltodoc.aalto.fi/bitstream/handle/123456789/14420/article6.pdf
after seeing this article from Arturia (Mini V filter) :
Quote:
The heart of the filter circuit is functionally equivalent to a series of a saturating amplifier and 4 identical saturating lowpass stages. On one hand the lowpass filtering part indiscriminately attenuates the signal spectrum past the cutoff, and on the other the saturation part adds only frequency content that is harmonically related to the input, thus enhancing the “overall musicality” of the signal, if used properly.

The system also includes a global feedback path, whose gain is controlled by the “emphasis” knob. When this is set to the minimum, the “cutoff frequency” knob is solely responsible for the actual cutoff of the filter. Otherwise, as the “emphasis” increases, resonant behavior is introduced and becomes progressively more apparent. Meanwhile, the actual cutoff first decreases sensibly, then it slowly increases up to the point in which the filter goes into self-oscillation. In other words, the global feedback allows for complex behavior and richer tone, thus enhancing the musical qualities of the filter even more.

https://www.arturia.com/mini-v/behindthemini-v

I will tweak this Chamberlin Multimode Filter again because I love the sound too. Very Happy

If you are here Tim another small thing I don't understand in what you said before :
Quote:
Yes, the FltLP module has this problem. Tim first mentioned it in this topic:
http://www.electro-music.com/forum/topic-63843.html&postorder=asc

Yes, but strangely the FltHP doesn't. I suspect it's just a scaling error.


Which one has a scaling error ? The FltLP because the filter no longer working as a filter at high frequencies or the FltHp because the filter is working as a filter at high frequencies ?
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Tim Kleinert



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PostPosted: Wed Jan 31, 2018 2:07 am    Post subject: Reply with quote  Mark this post and the followings unread

Electromagnetic Wave wrote:
After reading the following I ask myself few questions. "A cascade of two Chamberlin filters can be considered as the digital counterpart to the Moog VCF".

No. But there are many classic synths that use two state variable filters in series for steeper slopes, eg. the Roland Jupiter 8 (and 6, AFAIK). Since both filter stages are resonant, these designs can exhibit two separate resonance peaks, slightly apart because of filter cutoff scaling imperfections. (Btw. imitating that alone already goes quite a way towards that Roland filter sound.)

Quote:
Question :
How to deal with the saturating amplifier and the 4 identical saturating lowpass stages between each of the 4 poles (6dB/oct) if the ladder filter employed fourth-order all-pole and the Chamberlin filter use 2(12 dB/Oct) poles?

I never experimented with putting a nonlinearity somewhere within the feedback loop of a single Chamberlin structure and cannot comment on that. When using two in series, I'd probably just place it in between them.

OTOH I tried the Stilson-Smith approach to the Moog ladder on the G2 and wasn't exactly wowed by the results. Maybe the G2 saturation module hasn't got the right curve, who knows. But anyway, the thing gets nasty very quickly even at 96kHz, and band-limiting the feedback path warps the cutoff frequency response beyond recognition. Clearly a case for copious oversampling.

Quote:
I'm not trying to get the exact sound of Moog. But just to find this sound so pleasant when I drive the resonance very high and I sweep the frequency range with the filter. None of the patches I tested (Moog clone, TB303, etc.) with the G2 have this sound so pleasant (filter sweep) with a strong resonance gain. Despite the DSP / resolution constraints, I'm sure it's possible to get a good result for our ears with the G2.

In terms of filter design, insurmountable barriers for the G2 are the impossibility of oversampling (a necessary requirement if you want to implement nonlinearities) and lack of math modules to implement implicit methods (aka "zero delay feedback"). So I'd rather use a "black box" or perceptual approach, and beef up the filters with whatever pre-/post-processing sounds good to my ears.


Quote:
If you are here Tim another small thing I don't understand in what you said before :
Quote:
Yes, the FltLP module has this problem. Tim first mentioned it in this topic:
http://www.electro-music.com/forum/topic-63843.html&postorder=asc

Yes, but strangely the FltHP doesn't. I suspect it's just a scaling error.


Which one has a scaling error ? The FltLP because the filter no longer working as a filter at high frequencies or the FltHp because the filter is working as a filter at high frequencies ?

It's the 1-pole lowpass which has that bug. Somewhere above 16kHz (don't remember exactly) it just bows out completely.

cheers,
t
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Electromagnetic Wave



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PostPosted: Wed Jan 31, 2018 9:36 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
imitating that alone already goes quite a way towards that Roland filter sound

as many other here I used this patch and added some saturation (and other shapers) stage, adjusted the input/output gain between each circuit, removed/added frequencies, delay and/or change a bit the last frequency from the last ladder filter, without succes.... for now.

Quote:
When using two in series, I'd probably just place it in between them

I already tested it!

Quote:
Clearly a case for copious oversampling.

You are right.

Quote:
It's the 1-pole lowpass which has that bug.

Ok. I was not sure because of what Blue Hell said before in this topic about the remark in the article 'at high frequency settings the filter no longer working as a filter'. I thought if the two worked like this (Chamberlin and Clavia filter) this is not a bug (well yes surely for an application for a musician).

It's good to see you here again sometimes!

Thank you and have a nice day!
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varice



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PostPosted: Wed Jan 31, 2018 7:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

Tim Kleinert wrote:
...
Quote:
If you are here Tim another small thing I don't understand in what you said before :
Quote:
Yes, the FltLP module has this problem. Tim first mentioned it in this topic:
http://www.electro-music.com/forum/topic-63843.html&postorder=asc

Yes, but strangely the FltHP doesn't. I suspect it's just a scaling error.


Which one has a scaling error ? The FltLP because the filter no longer working as a filter at high frequencies or the FltHp because the filter is working as a filter at high frequencies ?

It's the 1-pole lowpass which has that bug. Somewhere above 16kHz (don't remember exactly) it just bows out completely.

cheers,
t...

Yes, the FltLP module has this problem (at all pole settings). It appears from my testing (G2 Demo with software spectrum analysis) that none of the other filter modules with low pass settings have this problem. So, it appears to me that this is a bug in the function of the FltLP module.

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