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

Andrew Simper andy at cytomic.com
Wed Aug 9 06:45:30 CEST 2017

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:

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.



On 1 August 2017 at 07:48, David Moylan <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
>> 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> 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
>>> 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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