As part of the VST I am producing, I have designed an SK filter analog where the loading of the first stage by the second is removed to ease implementation. This only affects the filter Q which then has an easy translation of the poles to compensate. Implementing it as CR filter simulation reduces the basic calculation. This is then expanded on by a Zero Delay design, to better its performance.
ZDF filters rely on making a better integral estimate of the voltage over the sample interval to better calculate the linear current charge delta voltage. More of a trapezoid integration than a sum of rectangles. There is still some non-linear charge effects as the voltage affects the current. The current sample out now not known, just then needs a collection of terms to solve for it. Given a high enough sample rate, the error of linearity is small. Smaller than without it, and the phase response is flat due to the error being symmetric on the simulated capacitor voltage, and drive, and not just the capacitor voltage.
The frequency to the correct resistive constant is a good match, and any further error is equivalent to a high frequency gain reduction. There is a maximum frequency of stability introduced in some filters, but this is not one of those. Stability increases with ZDF. The double pole iteration is best done by considering x+dx terms and shifting the dx calculation till later. Almost the output of pole 1 is used to calculate most of the output of pole 2 multiplied by a factor, added on to pole 1 result, and pole 2 result then finally divided. These dx are then added to make the final outputs to memorize.
