[sdiy] digital control of CV

[Tom Wiltshire]

Jure,

> tom - wouldn't that be much more expensive?

Using Octal DACs rather than a single DAC and sample & hold buffers  
_might_ be more expensive, but not hugely. Farnell Online stock the  
AD5346 for £3.70 in the UK. This deals with eight channels.

> how many CV channels do you plan on having?

I'd been working with 64 channels as an upper limit, Eg. 8 chips.  
This costs (roughly) 8x£4 = £32 or about $60US. Whether I require all  
of those depends on the final number of parameters in the synth  
voice, which I haven't decided for sure.

> also, you'd probably need more accuracy than 8-bit
> for VCO control, right?

The Prophet 5 used 7-bit parameter control for most of its  
parameters, I'm told, with 14-bit resolution for the oscillator CV.  
I'd thought to use 8-bit control of most parameter CVs, but was  
intending to use a digital oscillator which would be mostly  
controlled digitally. For a monosynth, this would probably be a  
direct MIDI link to the oscillator(s). 8-bit parameter CVs to the  
oscillator would only control things like Envelope or LFO Depths,  
fine tuning (+/-100 cents maybe in one cent intervals - smallest  
perceptible difference is around 6 cents.) or semitone offset.

> well, there's also 12-bit version

There is, which means you could upgrade the whole thing to 12-bits  
without having to alter anything much, except make sure you had  
enough pins on the processor that stores patches. I thought 8-bits  
would be enough. Certainly for the first try!

>  if i understand correctly... this "octal" DACs are
> essentialy really nothing more than a single DAC followed by... a s&h?
> do i read it correctly?

Well, I don't know exactly what's on the silicon, but the datasheet  
block diagram shows eight separate 8-bit resistor string DACs, so  
there's no S&H's of any kind, and hence no droop. That was one of the  
advantages, together with the low chip count and consequently simple  
circuit.

Regards,
Tom