[sdiy] Article "Analyzing the Moog Filter"

Brian Willoughby brianw at audiobanshee.com
Fri Aug 23 08:30:09 CEST 2019


First of all, where is the specification for the Moog Ladder Filter that shows it having accurate voltage control over a defined range? Is it really that accurate? Doesn’t it require calibration periodically?

How bad is the CV accuracy for other filters? Don’t the others calibrate to the same end results?

I’m familiar with the various pass-band shape tradeoffs when resonance is applied to various filter topologies, so I’m sort of leaving resonance out of the question. I’m primarily interested in whether Moog has significantly more CV accuracy without resonance when compared to any other synth filter.


Second, if you were to build a 4-pole SVF, would it be considered two 2-pole low-pass sections in series, or would the two dual integrators make some folks call it “four” single-pole low-pass sections in series?

Since I haven’t tried this, I’m not familiar with how global feedback might cause problems with 2 SVF in series. I do recall some nice discussions around here, though.

Brian


On Aug 22, 2019, at 11:05 PM, Donald Tillman <don at till.com> wrote:
> On Aug 22, 2019, at 6:07 PM, Tom Wiltshire <tom at electricdruid.net> wrote:
>> 
>> I don’t like to disagree with you, Don, but I’m not sure what you’re thinking. Moog or SVF are definitely *not* the only two filter options.
>> OTA+cap-to-ground+buffer? VCA+Integrators?
> 
> Consider a description of a filter as a sort of "taxonomy" with three layers:
> 
>     Top Layer: the filter spec, number of poles, response
> 
>     Second Layer: the topology that implements that filter function
> 
>     Bottom Layer: implementation details, including the control element
> 
> So a Moog Ladder would be:
> 
>    Top Layer: 4 pole, low-pass, with resonance
> 
>    Second Layer: 4 single-pole low-pass sections in series, with feedback
>   
>    Bottom Layer: the ladder circuit
> 
> And a State Variable filter would be:
> 
>    Top Layer: 2 pole, multi-mode
> 
>    Second Layer: 2 integrators and an inverter, in a loop
> 
>    Bottom Layer: the circuit, perhaps OTAs 
> 
> And so forth.  
> 
> This analysis also works really well with oscillators and other functions.
> 
> Here's a Moog style VCO:
> 
>     Top Layer: VCO with sine, square, triangle, sawtooth waves
> 
>     Middle Layer: block diagram with exponential current source, sawtooth core, waveshapers
> 
>     Bottom Layer: the circuit details
> 
> So if I dismiss the implementation details, as defined this way, it limits the number of unique filter designs.
> 
> You know I'm a big fan of implementation details.  And you'd want to make sure that the implementation details didn't have a significant functional effect as you draw these lines.  That's part of the craft.
> 
> But if I'm characterizing filter types, I think it's reasonable to pay attention to the implementation topology and ignore the implementation details.
> 
>   -- Don
> --
> Donald Tillman, Palo Alto, California
> http://www.till.com
> 



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