Alternative to MIDI-CV
jocke at netcontrol.fi
Wed Feb 14 10:08:06 CET 1996
On Tue, 13 Feb 1996, J.D. McEachin wrote:
> Thinking about the best DAC to use for MIDI-CV...
> Why not use a 16bit R-2R ladder? Since linearity and clock rate aren't
> as crucial w/ CVs as for audio, couldn't you get away w/ an R-2R built w/
> 10k resistor networks? Is there some reason R-2Rs don't work w/ 16 bits?
R-2R ladders are good for CV generation, but for 16-bit accuracy,
you'd have to have resisitors with a tolerance of 15 ppm (0.0015
percent), and you'd have problems with temperature tracking.
Someone suggested using an 8-bit DAC for CVs, but I doubt whether
this is enough. Say, for instance, that you feed the MIDI note
number (0...127) to the seven most significant bits of the DAC.
As 8-bit DACs typically have an accuracy of +/- 0.5 LSBs, your
CV will have an accuracy of +/- 0.25 half-note steps, or +/-
25 cents, which is hardly acceptable.
What you really need is a 7-bit DAC with, say, 12-bit accuracy.
This can be achieved by using a 12-bit DAC and feeding the note
number to the 7 most significant bits of the DAC, and tying the
unused bits to gnd. With 12-bit accuracy (+/- 0.5 LSBs) your CVs
will be accurate to 3 cents.
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