[sdiy] Newbie question: Diode ladders

[Antti Huovilainen]

On Fri, 16 Sep 2005, Johannes Öberg wrote:

> But am I understanding things correctly; you do get a very non-linear and
> non-ideal CV-to-resistance response, right? And that's why you rarely see
> "diode VCRs" for other things than VCF's and other non-critical
> applications?

At least in transistor ladder the resistance is inversely proportional to 
Ictrl. Since cutoff is proportional to conductance and inversely 
proportional to resistance, the cutoff is linearly releated to Ictrl.

>> One thing to note about the behaviour of the ladder filter is that the 
>> audio signal also has an effect on R and therefore Fc.
> Does this have anything to do with the special sound of ladder filters? I

Another way of putting it is to say that the distortion in ladder filter 
stages acts as a level dependant gain and therefore affects the cutoff 
frequency.

The differential equation (missing various scaling which is more or less 
irrelevant) for transistor ladder stage is

dV/dt = Ictrl/C * (tanh(Vin) - tanh(V))

The tanhs can be seen to modify the gain (Ictrl/C) depending on signal 
level. If you remove the tanh's and replace Ictrl with 1/R, you end up 
with normal RC filter.

Antti

"No boom today. Boom tomorrow. There's always a boom tomorrow"
   -- Lt. Cmdr. Ivanova