[sdiy] Polymoog Resonator - no-pots equivalent

sleepy_dog at gmx.de sleepy_dog at gmx.de
Sun Aug 27 12:35:54 CEST 2017


Ok, now I'm wondering about one thing, if I were to do a resonator with 
VCFs:

Which ranges to use for the bands?
Initially, I was going to go by the polymoog's 60..300, 300..1500, 
1500..7500 Hz bands.
And thought I'd make the control ranges such that they have some small 
gaps in between, as I thought if you can make them overlap and resonance 
is high up, mayhem will ensue.

Now, I've looked at e.g. formant tables such as this one:
https://www.cs.sfu.ca/~tamaras/project468_rdone/Formanttable.png

If I was to use the first 3 formants to produce vowels, look at "A", 
that's already ouf of what the polymoog ranges can do.
Well in the demo it did manage to do that, so I guess they were just all 
shifted down, and the ratios roughly kept or something like that?
But then look at other vowels in the table.
I don't really or primarily want to do speech synthesis with this analog 
thing, but stumbling upon the table made me question my idea that I 
should make the ranges non-overlapping.

Now if I was to make this more versatile, what would be a good idea to do?
Allow all bands to have wider, overlapping, ranges and add some sort of 
circuitry preventing anyone's head (or circuitry) from exploding?
Or perhaps... keep it non-overlapping, but add  range switches which 
shift *all* band's ranges somewhat up or down?
But that's less versatile again? Look at how close together are even the 
4th and 5th formant to the rest, but then there's probably a reason why 
the Polymoog's 3 bands are space more apart with their centers.

Now, of course, I could dynamically prevent overlap in the digital 
control voltage generating part.
But should the adjustment anywhere ever be off for whatever reason, 
that's not going to ensure that...

- Steve


Am 21.08.2017 um 19:46 schrieb Tom Wiltshire:
> Hi Steve,
>
> Yes, three multimode filters would be the same as the PolyMoog resonator. That’s what it is:
>
> http://www.synthfool.com/docs/Moog/polymoogresonator.jpg
>
> Note that the schematic shows us that the filters are state-variable filters (which we’d guessed anyway because of the HP, BP, and LP outputs) and that they’re *not* VCFs. They’re just based on op-amps. You also mention the slopes. A 12dB SVF has 6dB slopes for the BP output. That’s normal, since that’s a 2-pole bandpass filter, and it’s a 2-pole filter. The way I think about it is that you need to filter off both the low and the high frequencies, so you have to share the poles out to do the job!
>
> The Moog schematic is about as simple as such a thing could get, since it is pretty much just the three SVFs joined together. Personally I don’t like the way you have to change the response of all the filters together instead of individually, but that would be easy to change. Compare with this schematic:
>
> http://sound.whsites.net/articles/st-var-f1.gif
>
>
> Obviously you could go wild with this. Some years ago I built SVF+VCA modules around the 2164. This would be perfect for this, since you get a 2-pole VCF with VC resonance and a VCA all on the one chip.
>
> http://electricdruid.net/images/svf+vca.jpg
>
> I don’t seem to have the schematic online, but ask me if you’re interested.
>
> HTH,
> Tom
>
> ==================
>         Electric Druid
> Synth & Stompbox DIY
> ==================
>
>> On 21 Aug 2017, at 17:16, Steve <sleepy_dog at gmx.de> wrote:
>>
>> Hey experienced SDIYers,
>>
>> say, would slapping together 3 {VCA, multi-mode VCF 12 dB/oct with Q control} combos be the functional equivalent of the Moog Resonator? (with modified VCF control ranges, limited to 1 of those 3 bands each)
>> Although I faintly remember some of those multi mode VCFs having only 6 dB/oct slope on BP. Maybe not those. Are there designs with the same slope for all, or is this a general feature of such VCFs?
>>   
>> My goal is to make something that can shape sounds similarly to the Moog thing, as demo'ed on YT by some guy enhancing vocal and brass sounds.
>> https://www.youtube.com/watch?v=_XUiJi5153Y
>>
>> But in a much smaller box, without the battalion of pots.
>>
>> I thought about just taking J.Haible's schematic and replacing all the pots with MCP41HV digipots, but that'd be 14 (some are stereo), a bit expensive *and* wiper noise, that doesn't taste well together ;)
>>
>>
>> - Steve
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>




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