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

David Moylan dave at westphila.net
Wed Aug 9 09:02:32 CEST 2017

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?



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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