[sdiy] More on Xpander modes was Re: HP from LP ?
paula at synth.net
paula at synth.net
Fri Aug 11 08:26:01 CEST 2017
Dave,
Thanks for sharing, sounds great!
Paula
On 2017-08-11 04:35, David Moylan wrote:
> 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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