You make some good points DrFrankencopter.
I'm not using a full spice model (well, mathematically it's identical, but practically, much simpler to evaluate), since obviously, for real-time, you need as few operations as possible. What I AM doing, however, is using the same Modified Nodal Analysis that SPICE uses, to get my basic equations.
For instance, to model the two-diode bipolar rectifier used in this side-chain, you need to model two diodes charging a capacitor with a drain resistor for release. This can be modelled in simple ways, or using Newton-Raphson iteration (which is what I am doing) to get a result that matches the SPICE simulator (and my measurements of the real thing) to a precision I doubt I could get better than in software. When you do this you see that there are a bunch of subtle effects that a simpler modelling approach doesn't display. And to me, that's important. It might not be to other people, but really this is, to some degree, an experiment in "is it really worth it?" and "can we really do better". To which, so far, I think the answer is yes.
As for the RC circuits for release and attack, for the attack, yes, you can simply use a trapezoidal-integrated low pass filter model (standard stuff), but the release is the combination of a low pass filter with the rectifier feeding it, which has a far more complex response. In some ways, you could say it's a voltage controlled low pass filter, since the higher the input voltage, the lower the diode resistance. But in order to solve it as the real circuit does, it's a balancing equation (current in = current out), so you need to either iterate, or you can directly solve it using the Lambert W function (which I also do sometimes).
The Opto model I'm using I'm still working on. So far I've modelled the LED directly by measurement, but the Photoresistor is something I'm still working on. I have a static model, but I am additionally working on a dynamic model (to include the attack and release properties of the optical cell itself).
I'll post some audio once I'm done and you can all decide for yourself if I've been wasting my time ;-) The fact is, I've learned LOADS about circuit analysis by doing this, and it's going to massively help me with my analog designs in the future.
cheers for the comments.
