[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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