[sdiy] digital control of CV

[Robert Shanks]

Hi all  -

> Using Octal DACs rather than a single DAC and sample & hold buffers  
> _might_ be more expensive, but not hugely.

Won't you still need sample and holds or lag circuits/filters for  
smoothingthe DAC output?

Robert

On Sep 2, 2007, at 7:37 PM, Tom Wiltshire wrote:

> 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
>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy