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

David Moylan dave at westphila.net
Fri Aug 11 05:35:35 CEST 2017

As promised (but slightly tardy), I posted a video up to YouTube:


As stated in the description, synth audio starts at 5:30.  Sync with the 
synth output is off a bit, seems to lead the commentary, but I don't 
think it's too detrimental to getting the explanation across.

I mention in the video that my DG408 is running very hot, but I'll start 
a new thread on that.



>> 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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