[sdiy] 8bits, 10bits or12bits DAC for a midi2cv converter?

[John Mahoney]

At 11:29 AM 10/18/2007, Rainer Buchty wrote:

>Also don't forget filter sweeps. With just 128 or 256 steps they get
>rather, well, steppy.

For filter sweeps and other real-time adjustments, you could use a 
technique like Moog's thing on the Little Phatty, I think it's called 
True Analog Control (TAC). When you call up a patch, all the 
programmed CVs are generated by the DAC (and associated circuitry).

My assumption is that, while making adjustments to a parameter, TAC 
switches from the programmed (DAC-generated) CV to the CV coming 
through a pot -- the pot's "True Analog Control" voltage. If you 
reassign that pot, its TAC voltage is digitized and the synth puts 
the parameter back under control of the programmer. (The CPU then 
assigns the now-unused TAC to the chosen parameter.)

Saving the current patch also digitizes any pots that are "live" under TAC.

There will be CV quantization upon switching from direct to DAC. You 
are unlikely to hear it; it's a potential issue only when you are 
adjusting more than 4 parameters of a preset before changing to a 
different patch. Only then -- or when saving a patch -- will the TAC 
signal be replaced with a digitized voltage. And, the quantization 
will often be inaudible.

TAC is patented, but I think you can use it for DIY. (Correct me, 
please.) It doesn't seem that difficult to implement -- it's "just" 
some electronic switches. Famous last word "just". ;-)
--
john


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