[sdiy] Filter cut off DAC

[Tom Wiltshire]

> On 21 May 2018, at 16:52, paula at synth.net wrote:
> 
> Veronica,
> 
>> 12 is a good number in terms of control and cost. If you were to go to
>> 16 you not only have the cost of the DAC but the cost of making the
>> signal path from the DAC match that level. Not too bad at 16 bit but
>> the costs rise.
> 
> totally agree.
> 
>> So what do you want to do that might push the design goal above 13 bit?
> 
> Just thinking, if you add an EG to an LFO and a few other modulation routings I wonder if 12 bits is enough to keep things sounding "smooth".
> 
> Paula

Are we presuming this filter is controlled exponentially? If it’s a linear control, you need more bits, because you basically waste a lot of them on the upper octaves.

So I'll assume exponential control and 12 octaves range; 10Hz to 40KHz - more than enough. Drop an octave if you like and bring the bottom end up a little tiny bit (15Hz-30KHz is 11 octaves).

Assuming 12 octaves, 12-bit is 4096/12 = 341 steps per octave, or 28 steps per semitone. That’s about 3.5 cents, less than what would be perceptible (reckoned as around 6 cents usually), even if the filter was being used as a sine oscillator which is roughly the worse-case scenario.

So I reckon I’m with Veronica. 12 bits will do fine, 13 would be great, and even 14 bit is getting luxurious.

Tom

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