Alternative to MIDI-CV

[Joachim Verghese]

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.

-joachim