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

[Gordonjcp]

On Mon, Jun 10, 2019 at 09:11:58PM +0100, Tom Wiltshire wrote:
> I think what he’s looking for is to know why x/(1+x) gives the right warping to get the correct cutoff frequency.
> 
> Since I know very little of how digital filter models are derived from their analog counterparts and the approximations inherent in that process, I’m not the person to ask…
> 
> Tom

That's it exactly.  I'm not great at maths and the kind of algebra that
goes into this sort of thing is a bit much for me, but x/(1+x) gets the
filter tracking absolutely bang on.

-- 
Gordonjcp