[sdiy] phaser cv law

[JH.]

>One thing about incandescent vactrols though, is the
>thermal inertia of the filament, which has a considerable
>effect on the overall current/resistance transfer, particularly
>at high frequencies.

Yes, absolutely.
Some non-optoelectronic phasers approximate this with an RC low pass, but 
it's only an approximation.
My Compact A clone (I don't know if the original has this or not) shows a 
crazy extra effect: The front panel lamp (which indicates the inverse of the 
lamp that drives the LDRs) sometimes "jumps" a little before it gets to its 
maximum brightnes / sweep turning point. This only happens _sometimes_, 
every few cycles. I suspect it's a self-heating thermal effect - Vbe of the 
driving transistor decreasing with temperature, look at these crazy 
shematics! - and gives a nice randomizing effect.

>One popular phaser LFO waveform is derived by taking
>a sine wave, full wave rectifying it, and inverting the
>result (giving a slowly swept botom & fast swept peak).

I bet it's quite close to the Tau / Aries 1/x function. (or law?)
This helps to achieve a thru-zero modulation effect. And it (1/x at least) 
would be the best approximation for a moving sound source, i.e. rotary 
speaker effect.
The Compact A and Mutron, as far as I can tell, are doing just the opposite 
of this, though. Where -abs(sinwt) modulation or a triangle modulation with 
1/x law accelerate the frequency sweep towards the high range, an 
incandescent lamp would decelerate.

JH.