[sdiy] Multiplier vs VCA?

[john mahoney]

----- Original Message ----- 
> >
> >A typical VCA will be a two quadrant multiplier because the
> >audio input can be positive or negative while the CV input
> >can usually only be positive.
>
> Right, so if I understand this correctly.
> A VCA will only accept 1 input with AC signal and the other must be DC
voltage
> either +ve or -ve, which makes an audio expander or compressor if the DC
voltages
> are generated from a logrithmic function?
> Isn't there a VCA chip that can take both +ve DC and -ve DC to be
ultilised both
> as a compressor and expander know as compunder? or would you need 2
seperate VCAs
> one takes +ve input and the other -ve to achieve both compressing and
expansion
> seperately?

Not quite. Even when acting as a compressor, a VCA accepts only positive
CVs. (As a 2-quad multiplier must do, by definition.)

An amplifier has what's called a "transfer curve" that determines the
relationship between the input signal and the output signal.

A normal amplifier [ideally] produces gain in a linear fashion. The ratio of
input to output signal strength is linear.

Oops! Then again, there are VCAs with exponential response. So the real
answer is more complex than I said above. Something about the number of
asymptotes in the transfer curve, perhaps.

When acting as a compressor or expander, however, the input:output ratio is
non-linear. A non-linear CV is what determines the nature of compression or
expansion. Typically there are, say, 2 levels of gain; one amount of gain
when the input signal is below a set threshold, and another when the input
signal exceeds that threshold.


Some modules can function as VCAs or balanced modulators (like ring
modulators) depending on the nature of the CV. Positive-only CV gives you a
VCA, a bipolar CV yields a balanced modulator. Examples include PAiA and
Blacet, if memory serves.
--
john (getting in over my head!)