[sdiy] DACs on-board 12 vs external 16 vs 24 bit

ASSI Stromeko at nexgo.de
Wed Nov 6 20:13:52 CET 2013

On Wednesday 06 November 2013, 02:14:28, Justin Owen wrote:
> For a digital, micro-based oscillator (being used in a modular environment
> if that makes a difference) outputting the standard one waveform at a
> time sine, triangle, saw, pulse waveforms between e.g. 0.1 Hz and 20KHz
> (probably closer to a max of 10KHz) with the regular features of hard
> sync, FM, PWM, Unison etc.

...which probably means that the waveform is always full-scale (no amplitude 
modulation or DCA function).  This already enables a drastic reduction in 
the resolution required, since you don't need extra bits for dynamic 
headroom.  Then if you really want to do sync, FM and PWM and not overtax 
that micro on the computational front you'll really want to use as high a 
sampling frequency as you can muster and doing this with high resolution is 
going to take its toll somewhere.

> ...how much difference is the average human going to hear between 12, 16
> and 24 bit DACs?

Between none and a difference like night and day.  There's a general 
tradeoff between sampling frequency and resolution and which way you go 
depends on secondary requirements.  Your typical 24bit at 44.1kHz audio DAC 
really is 1bit at 2822.4kHz internally for instance because for audio you 
really only care about linearity and a 1bit DAC is superior in that 
department.  If you wanted to have your output DC accurate, then that would 
be a completely different story.

> For extra reference - I'm looking specifically at the 12-bit DACs on the
> STM32F407xx vs. the option of an external DAC of some sort.

In time-honored tradition the data sheet does not tell you the maximum 
update rate for the DAC, but the settling time indicates you could probably 
get it to 200kHz.  That's certainly good enough for what you've outlined 
above (or what I've understood of it, anyway).  All the classic digital 
synths (PPG Wave, DX7, etc.) were in that ballpark, too.

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