[sdiy] Multiplier vs VCA?

[synthplayer88 at spymac.com]

On Thu Aug 19 10:49 , 'john mahoney' <jmahoney at gate.net> sent:

>----- 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.)


I think the THAT CORP VCA uses -ve control voltage?

http://thatcorp.com/vcas.html


>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.
>
>



Right, as I understand it. In the feedback configuration the DC is from
rectifying the output and then fed into a logrithmic amplifier to control the VCA
and in feedforward configurations, the dc comes from a RMS detector parallel to
the VCA at the input. The threshold is the point where you determine to rectify
the signals?
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