[sdiy] Re: ADuC702x (Re: programmable synth module)

[Eric Brombaugh]

It all depends on what you want to do with the converter. There are a 
lot of other parameters besides data width that come together to make a 
converter ideal for your application:

* Bitwidth
* Sample rates (min/max)
* Interface (serial/parallel, registered/uC compatible/complex protocol)
* Analog I/O (differential/single-ended, voltage/current)
* Range
* etc

16-bits gives you 1 part in 64k resolution - that's ~96 dB SNR for 
audio applications and it gives you less than 0.2 mV resolution on a 
0-10V CV. On a 1V/Octave system, that's about 0.18 cents!

Now, if you're willing to put up with complex interfaces and fixed 
sample rates, you can get stereo ADC/DACs in the form of PC-style AC97 
codecs that have reasonable specs and don't cost a lot. The problem is 
that the AC97 protocol is a beast to use unless you're assembling a 
synth with FPGAs. On top of that, you'll have to deal with teeny tiny 
surface mount parts. And these parts often have poor electrical 
isolation, so digital noise leaks into your analog signals.

For DIY style converters, Tom is right - the cheap ones are pretty much 
useless and the useful ones aren't cheap.

Precision wise - if you've got 16 bits you can always round/truncate to 
lower resolution in software. But if you don't need that precision, why 
pay for it?

So, it's best to go for the minimum that makes sense for the precision 
your app needs. For CVs, 12 bits will give you about 3 cents resolution 
on a 1V/Oct system and about 72 dB SNR on audio. 10 bits is 4x coarser 
- about 12 cents frequency resolution and 60 dB SNR. I'm not sure what 
the tracking accuracy on most VCOs is, but it doesn't make sense to 
have your CVs much more accurate than the VCO. For VCAs, you only need 
enough resolution to prevent 'zipper' effects, unless that's what you 
want and then you can get it by quantizing in software.

Eric


On Feb 16, 2006, at 9:44 PM, john mahoney wrote:

> ----- Original Message -----
> From: "Tom Arnold"
>>
>> Actually, we cant agree on 16bit converters because most of them suck 
>> and
>> the ones that dont suck are incredably expensive. :-)
>
> What's the best we can get for a reasonable cost? 10 bits? 12? 14?
>
> Will 16-bit precision support additional functions that 12- or 14-bit
> precision won't?
>
> Finally, is it reasonably easy to allow for a few options? Perhaps a 
> jumper
> setting on the board for DAC bit depth?
> --
> john <--(apparently got a surplus lot of question marks)
>