[sdiy] Digital VCA

[Matthew Smith]

Hi Folks

My digital ADSR/LFO is no longer "Vaporware" - PCBs have come back from 
the fab, all parts soldered on, just waiting for some connectors to hook 
up the LCD. Then it's a case of getting the pots, LCD, rotary encoder 
mounted on something and the not-so-trivial task of writing the software.

Whilst I will be testing this out using a "Dixon VCA," I have been 
mulling over what would be involved in producing a digital VCA. My idea 
at the moment is NOT to have this integrated with the ADSR/VCO, but to 
have these as totally distinct modules, that can be mix'n'matched with 
pure analogue gear.

 From a digital perspective, is a VCA really as simple as multiplying 
the digital value of the signal input with the digital value of the 
control input, then dropping the least significant bit? (That's assuming 
I'm multiplying two 12-bit numbers, then dropping the last bit of the 
13-bit result to get 12 bits again.)

Thinking of zero-cost value-adds that I could put in this, 
sample-and-hold of the control signal springs to mind.

The main issue I have identified so far is how to handle signals that 
are too large. The CV input of my ADSR/LFO has Zener protection, to snip 
the incoming voltage off at 5.1V (running the CPU at 5V, and am 
standardising on this for my control voltage level, if not signal 
level.)  As far as the VCA is concerned, I was thinking of doing the 
same, but having "clip" LEDs that light whenever the the digitised value 
of the signal hits 4095. (Or maybe a certain number of samples hit that 
value in a specific time. I'll probably be running the ADCs/output DAC 
at 50ksps - a bit fast for the eye to perceive!) I guess I could have 
other LEDs that come on at lower values, too.

Cheers

M

-- 
Matthew Smith

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