[sdiy] Multiplier vs VCA?

[synthplayer88 at spymac.com]

On Wed Aug 18 11:50 , Steve Ridley <spr at spridley.freeserve.co.uk> sent:

>A multiplier multiplies two voltages together.  A ring modulator
>is one type of multiplier - it's proper name is a four quadrant
>multiplier because both inputs can be positive or negative.
>If at least one input is DC coupled (ie it will let a DC CV 
>through), it should work as a VCA. But most "ring modulators"
>have both inputs AC coupled as any DC will unbalance it as
>a ring mod.
>
>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?

A multiplier can take AC signal from both inputs and multiply both input voltage
as the outcome which explains the sound of the ring modulator, creates bursts of
explosions in amplitude?

What a about VCOs? Is there a offshelf monolithic chip that I can use to input
both +ve DC and -ve DC to modulate the output frequency?

Thanks for your help steve!!! :)

Really really appreciated it!!!

PS. Please be patient with me as I am still learning all this stuff, not the
quickest learner........... :(   
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