[sdiy] Ladder filter analysis

[Richie Burnett]

> Colin f wrote:
>
> Another key part of the ladder design is the quirk of any low frequency/DC
> component in the audio input adding to the control current that sets the
> dynamic resistance of the transistors, thereby modulating the cutoff.
> It's quite an audible effect on the minimoog at high resonance, and an OTA
> filter doesn't do it.

The same thing happens with the TB-303 filter too, and it's very audible at 
high resonance.

There's a significant difference in the DC bias level between the sawtooth 
and "squarewave" outputs of the VCO, and you can hear the cutoff frequency 
of the filter wobble at about 8Hz for a short while after you toggle the 
selector switch between the two waveforms!  The step change in DC level 
between the two waveforms kicks the ladder filter into a damped "cutoff 
frequency oscillation".

> Neither do a fair number of 'virtual' ladders, it's the first thing I test
> for.

Poor modelling.  Antti Huovilainen's moog filter paper explains the theory. 
Tim Stinchombe has also written an excellent paper about analogue ladder 
filters.

-Richie,