[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?
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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