[sdiy] Linear vs. Exponential control of VCA's

[jhaible at t-online.de]

>   The Yamaha CS line uses a VCA that has both linear and 
> exponential CV 
> inputs.  In nearly all cases the expo input is used for 
> an offset 
> trimmer/CV reject trimmer.  The linear inputs are 
> attached to the EG 
> outputs, panel controls (for things like aftertouch VCO 
> mod depth, etc.), 
> touch response detectors, and so on.  I would love to 
> make a SMT micro-PCB 
> equivalent of the IG00151 at some point.  Yamaha's precis 
> on the chip is 
> not very informative, but from what I can tell it is just 
> an OTA with a 
> simple linear V/I and simple expo V/I summed together to 
> the Iabc of the 
> OTA.

I would have thought it has the usual expo converter with
a (linear) reference current input. Then the linear input
would be a summing node for several linear CVs, and the
expo input would be used for (unity or whatever) *gain* 
adjustment rather than offset or CV reject.

JH.