[sdiy] analog synths with digital controls? how does that work?

[Bob Weigel]

On 1/22/2013 6:43 PM, neil harper wrote:
> hey all,
> the only modern synth I'm really familiar with is my minitaur, but it
> seems that most analog synths these days operate in the same sort of
> way.
>
> so i'm wondering, how are these analog synths able to save/recall
> settings and be modified through a computer?
>
> there's gotta be a digital layer between the knobs and the analog parts
> of the circuit right? so does that mean that it's sampling the position
> of every knob (which i guess it can multiplex) and then using DAC's to
> output the required control voltages for all of the knobs (a dac channel
> for each knob)?
>
> what about things that aren't typically voltage controllable, like the
> resonance control of a filter..
>
>
>    
I did a youtube video on my sounddoctorin channel using the korg polysix 
as an example of a typical way it's done.

Most commonly

1) Knobs are just voltage sweepers between  the range of the adc
2) They occupy a channel of an analog switch chip like a 4051 typically
3) the common / out of the chip goes to either an adc chip OR to a 
comparator leg
4) the other comparator leg in the more typical example is fed by the 
DAC chip.
5) When the DAC is enabled by the computing system (often by a decoder 
chip) then the data from the last read of that knob is placed on the 
data buss
6) The comparator is thus comparing the last held value with the current 
value.  If greater than the cpu bumps the value up and visa versa
7) If cpu only bumps once it assume value not changed.  If more it finds 
the value where the comparator flips and it has the new value to store 
for the next time around.
8) Meanwhile the DAC also goes to the common lines of analog switches 
that are then being selected by the computer to charge up the sample and 
hold capacitor of whatever the currently read parameter is.

That's the essentials I guess

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