[sdiy] Filter frequency cutoff with linear response?

[Olivier Gillet]

> Is there a 'musically useful' reason (in the widest possible definition...) to have a filter frequency cutoff control with a linear response?

I see one, albeit a very convoluted one:

Let's say you use are using a filter at self-oscillation like a sine
oscillator. You want to do FM with this oscillator, so you need to
apply a linear modulation on pitch (the oscillator frequency must go
between f_0 - f_fm and f_0 + f_fm, and you won't get that symmetry if
the modulation signal gets into the expo converter!).

That's why VCOs have both linear and exponential pitch control -
exponential for "musical" control, and linear for true FM effects. If
you consider a filter as a sine VCO, it might be useful to do it too.