[sdiy] Expo converters, lin log etc.

[Andre Majorel]

On 2007-01-31 21:05 +0100, Karl Ekdahl wrote:

> Basically, the response of a "normal" vco (for
> example) is logarithmic, therefore an exponential
> converter is needed to linearize the final response -
> right?

No. As far as I know, VCOs are typically linear, i.e. frequency is
proportional to CV. This is called a "V/Hz" or "Hz/V" response.

An exponential VCO, i.e. one whose frequency is proportional to a
certain power of the CV, is desirable. This is called a "V/oct"
response.

To achieve the desired exponential response with a linear VCO, we
insert an exponential converter between the CV input socket and
the VCO core. That circuit converts voltage x to voltage (or
current ?) y^x where y is some constant chosen so that every time
the you add 1 V to the control voltage, the frequency of the
output of the VCO doubles[1].

The exponential-converter-into-linear-VCO system behaves like an
exponential VCO would and everyone is happy.

[1] Assuming you're aiming for 1 V/oct. For a 1.2 V/oct response,
you would choose y so that the output doubles every time the input
is increased by 1.2 V.

-- 
André Majorel <URL:http://www.teaser.fr/~amajorel/>
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