[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:
https://www.youtube.com/watch?v=hJ4JQYdH1fo
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.
Enjoy.
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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