[sdiy] Article "Analyzing the Moog Filter"

Donald Tillman don at till.com
Fri Aug 23 16:57:12 CEST 2019


On Aug 22, 2019, at 11:30 PM, Brian Willoughby <brianw at audiobanshee.com> wrote:
> 
> 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?

If a filter is tuned with a transistor's exponential base-to-collector function, then it will probably be accurate enough to throw into oscillator mode and be used to play a melody.

That's not the case with filters tuned with diodes, FETs or vactrols.

That said, if I'm buying something called a "filter", then it's not reasonable to assume that I can play a melody on it.


> 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.

There are multiple ways to build a 4-pole SVF. 

  -- Don
--
Donald Tillman, Palo Alto, California
http://www.till.com


> 
> 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



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