[sdiy] More on Xpander modes was Re: HP from LP ?

rburnett at richieburnett.co.uk rburnett at richieburnett.co.uk
Wed Aug 9 11:22:08 CEST 2017


Yes, would definitely be interesting to see the mixing coefficients for 
some of those more exotic filter responses.

-Richie,


On 2017-08-09 08:02, David Moylan wrote:
> Andy, do you have any coefficients for these shapes? The double
> notches seem interesting.  Was your resonance path always from a 4th
> LP pole? Can you breakdown the abbreviations used for the responses?
> For instance, what's the T suffix?
> 
> Thanks.
> 
> Dave
> 
> On 08/08/2017 09:45 PM, Andrew Simper wrote:
>> In digital you get pretty spot on shapes since your tolerances are 
>> around 0.0001% ;)
>> 
>> I worked on all this stuff over 10 years ago and came up with my 
>> favourite list of responses:
>> https://cytomic.com/files/dsp/cascade-tapped-filter-responses.pdf
>> 
>> I implemented these in a software synth plugin called Strobe over 8 
>> years ago. A while later the guys at Motu liked them so much they 
>> copied the shapes and the names and used them in their software 
>> sampler plugin Mach 5.
>> 
>> Cheers,
>> 
>> Andy
>> 
>> 
>> 
>> On 1 August 2017 at 07:48, David Moylan <dave at westphila.net 
>> <mailto:dave at westphila.net>> wrote:
>> 
>>     I'm just finishing up an add on to a Roland IR3109 to do the pole
>>     mixing, same basic method as the Xpander.  From my early listening
>>     tests I will say that the high pass modes still seem to have some
>>     low pass in them.  Can any Xpander users comment?
>> 
>>     This is due to the fact that achieving HP responses relies on
>>     nulling out terms in the transfer function via mixing the 
>> different
>>     poles.  If you're not exact you don't get a full null and the
>>     attenuation of low frequencies suffers (looks more like a shelf
>>     instead of continuing roll off of low frequencies and usually has 
>> a
>>     little dip in the middle).  I'm using 1% tolerance resistors, but 
>> am
>>     about to sub in some 33.2k for 33k to get even closer to the
>>     theoretically perfect values.
>> 
>>     Even if you had perfect resistors I would imagine any mismatching
>>     between the OTA stages would also cause the attenuation to suffer,
>>     but I haven't modeled that.
>> 
>>     Still, each mode sounds sufficiently different and I particularly
>>     like the modes with notches in them like 1 pole LP + notch.  I
>>     wouldn't expect to like a less "dramatic" filter but I love the
>>     sound of that one.
>> 
>>     FYI, one of the Xpander modes is All Pass / 1P LP or "Phaser" with
>>     first pole switched out.  The shape has a notch off center and its
>>     pole weights are 1 3 6 4.  If you change the weights to 1 3 4 2 
>> the
>>     notch becomes centered and it looks like a 2P band pass with a 
>> notch
>>     at the center and with the first pole switched out it's like a 1P 
>> HP
>>     + notch, mirroring the 1P LP + notch that is already available.    
>>  Something different to try...  The PDF mentioned in the last post
>>     has good drawings of the filter shapes which might help make sense
>>     of this.
>> 
>>     I'm working on this in a Roland GR-300 but the boards could
>>     theoretically work in any synth that uses the IR3109, like the
>>     SH-101 and MC-202.  Not sure how much room is available inside
>>     those, the boards are 1.3" x 2.6" using 3 SOIC chips, a small 
>> number
>>     of 1206 SMT resistors and the rest through hole.  Not very 
>> difficult
>>     to solder.  The filter selection is via a pot into a 3 bit ADC, 
>> and
>>     the first filter pole is switched manually. (Haven't had any 
>> problem
>>     with pops when switching that pole.)  That gives the 16
>>     combinations, of which, there's one duplicate so you get 15 modes.
>> 
>>     If you're interested in a board let me know.  I should have a 
>> video
>>     on YouTube later this week showing it's operation in the GR300.    
>>  Will post here.
>> 
>>     On 07/31/2017 02:28 PM, Tom Wiltshire wrote:
>> 
>>         I’m getting the feeling here that just a simple subtraction of
>>         the lowpass response from the input isn’t enough (ahem). If 
>> you
>>         want better than -6dB/oct, you need to do some more
>>         sophisticated summing of the input and the various outputs, 
>> and
>>         you’d better make sure you’ve got accurate resistor values and
>>         signal levels too, since that provides a limit to how m such
>>         attenuation you can expect in the stop band off you high pass
>>         filter. How good is your signal cancellation? -40db? -60dB?
>>         Unlikely to be better than that.
>> 
>>         That’s not to say that it’s not worth it.. As Paula said, the
>>         Xpander service manual is the original source, and Oliver
>>         Gillet’s documents on the Four Pole Mission board for the
>>         Shruthi expand on that:
>> 
>>         
>> https://mutable-instruments.net/archive/documents/pole_mixing.pdf 
>> <https://mutable-instruments.net/archive/documents/pole_mixing.pdf>
>> 
>>         Basically, generating a multi-pole highpass from a multipole
>>         lowpass response isn't as simple as just taking one off the
>>         other. These are both great filters and worth doing.
>> 
>>         HTH,
>>         Tom
>> 
>> 
>>         ==================
>>                  Electric Druid
>>         Synth & Stompbox DIY
>>         ==================
>> 
>>             On 31 Jul 2017, at 20:50, Julian Schmidt 
>> <elfenjunge at gmx.net
>>             <mailto:elfenjunge at gmx.net>> wrote:
>> 
>>             Have a look at this paper on page 64, Multimode Ladder 
>> Filter
>>             
>> https://www.native-instruments.com/fileadmin/ni_media/downloads/pdf/VAFilterDesign_1.1.1.pdf
>>             
>> <https://www.native-instruments.com/fileadmin/ni_media/downloads/pdf/VAFilterDesign_1.1.1.pdf>
>> 
>>             You can mix the different poles with a simple opamp mixer.
>>             input -4 * Pole1 + 6 * Pole 2 -4 * Pole 3 + Pole 4 will
>>             yield a funky looking HP filter
>> 
>>             julian
>> 
>> 
>>             Am 31.07.2017 um 20:10 schrieb Michael Zacherl:
>> 
>>                 Hi,
>>                 I discussed that briefly in one of the last SDIY 
>> meetings,
>>                 did some experiments but never came to a sound 
>> conclusion:
>> 
>>                 Is there a ‘cheap’ way of turning a low pass filter 
>> into
>>                 a high pass?
>> 
>>                 How’s that possibly approached?
>>                 Just something like subtracting the LP’s output from 
>> its
>>                 input signal?
>>                 If so, what about resonance?
>> 
>>                 My point about doing that cheaply is the (upcoming)
>>                 release of two synths with Moog ladder filter 
>> featuring
>>                 a switchable high pass mode.
>>                 To my knowledge, there was no resonating HP filter 
>> from
>>                 Moog so far in any synth.
>>                 Until now.
>>                 Since both synths (Moog Mother-32 and Behringer Model 
>> D)
>>                 are on the budget side in terms of money I’d rather
>>                 expect a simple solution compared to a, say, DSI Pro-2
>>                 with two fully featured filters in it.
>> 
>>                 Any hints?
>>                 Thanks, Michael.
>> 
>> 
>>                 --
>>                 http://mz.klingt.org
>> 
>> 
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