[sdiy] Choosing a DAC

[Florian Teply]

Am Monday 29 June 2009 02:35:51 schrieb Jerry Gray-Eskue:
> 5 octaves * 12 semitones = 60 steps.
>
> The 1 volt per octave is a liner input at 0.0833... Volts per semitone
> (1Volt/12).
> If you setup your LSB at this voltage an 8 bit DAC can range over 21
> octaves (255/12) however
> that requires over a 21 volt range.
>
> Assuming you are thinking of 0-5v for your output and using 4 steps per
> semitone you can cover 5.3125 octaves using 5.3125 as the Vref. each step
> would be 25cents or 0.02083... Volts.
>
> If you wish to use a 5.00 Volt Vref you will need a higher resolution DAC,
> for Good results you want to always be +- 3 Cents or less, preferably less
> than +- 1 Cent.
>
I'd probably aim at .1 Cents or less, but wouldn't tear all my hair if i don't 
get closer than +/- 1 Cent. So, my best bet would be a 16 Bit (or even 18/20 
Bit) DAC, as they're not that different price-wise. Given the AVR that you 
seem to want to use, 16 Bit seems like a fairly well-matched value to me. 
That leaves enough room for the DACs INL and some possibility to fine-tune 
your scale (just in case you'd change your mind and go for sime different 
scale). Another option would be a 10 octave range with an Voltage doubler at 
the output. Fairly versatile, that's what i'd want form my modular anyways.
Some is to be gained with a fine-tuned reference as well, as David Brown 
already pointed out. And a multiplying DAC ( ref = stepsize wanted, digital 
data = number of steps wanted) can come in handy at non-standard values as 
well.
But: It's hobby after all. Do as you wish ;-)
 
> roughly at 5 Volts Vref it runs
> Bits
> 		  Steps/Octave	S/Semi	C/Step
> 8	255	      51	      4.25	      23.52941176
> 9	511	      102.2	      8.516666667	11.74168297
> 10	1023	      204.6	      17.05	      5.865102639
> 11	2047	      409.4	      34.11666667	2.93111871
> 12	4095	       819	      68.25	      1.465201465
> 13	8191	      1638.2	136.5166667	0.732511293
> 14	16383	      3276.6	273.05	0.366233291
> 15	32767	      6553.4	546.1166667	0.183111057
> 16	65535	      13107	      1092.25	0.091554131
> 17	131071	26214.2	2184.516667	0.045776716
> 18	262143	52428.6	4369.05	0.022888271
> 19	524287	104857.4	8738.116667	0.011444114
> 20	1048575	209715	17476.25	0.005722051
> 21	2097151	419430.2	34952.51667	0.002861024
> 22	4194303	838860.6	69905.05	0.001430512
> 23	8388607	1677721.4	139810.1167	0.000715256
> 24	16777215	3355443	279620.25	0.000357628
>
> As you can see an 12 bit DAC looks very good at 1.46 Cents per step.
> And a 16 Bit DAC is well into the overkill range at 9 Hundredths of a Cent
> per step.
>
Overkill or not depends on the application. But, sure, i don't hear a 
difference of .09 Cents anyways, and my ears aren't even the worst 
imagineable...

Just my pennies,
Florian