[sdiy] tanh() and approximations and warping, why is my maths wrong?

[Neil Johnson]

Hi,

Andrew Simper <andy at cytomic.com> wrote:
> The pre-warping of the cutoff for a digital filter (ie mapping cutoff to gain factors needed to compute the filter) depend on the digital filter structure being used, not the non-linearities in the filter structure. Typically the non-linearities will cause a small change in the cutoff, either upwards on downwards depending on the shape of the non-linearities, but just on self oscillation they won't make much difference.

I suspect that Gordon is not talking about a digital filter at all,
just that for linear-in-CV control of cutoff you need to tweak the
control voltage a bit at the high end, hence the original post saying
"you need to warp Fc a bit for the frequency response to "track"
properly at higher cutoff settings."

Achim wrote up some notes on transistor control and the appearance of
tanh a while back: http://synth.stromeko.net/diy/OTA.pdf

Neil


Neil