[sdiy] digitally controlled potentiometers

[Mattias Rickardsson]

On 06/05/07, Andre Majorel <aym-htnys at teaser.fr> wrote:
> On 2007-05-06 12:39 -0500, Paul Schreiber wrote:
> > Just something to think about:
> >
> > The average person, turning a standard 300 degree rotation pot,
> > can set that pot to ~50 *repeatable* positions. Or within +-3
> > degrees. Digital pots having 128 discrete steps actually have
> > *better resolution* than you yourself turning a pot and then
> > having it "remembered" (like say a P5 or OB-8).
> >
> > No one will even notice 128 steps as a 'loss of accuracy',
> > zipper or no zipper. Certainly 256 steps (8-bit binary weighted)
> > is overkill.
>
> On an MS-20, the fine tune pot for VCO2 has a range of a little
> above +/-1 octave. At 100 Hz, you can reliably set VCO2 to within
> about 0.2 Hz of VCO1. That's less than +/-0.003 octave.  With a
> digital pot you'd need about 350 steps to have the same
> resolution.

Another comparison;
On the Minimoog, the Cutoff Frequency pot spans 7 octaves in its mid
180 degree turning range. 7 octaves means 84 semitones, so there is a
new semitone each 2 degrees when turning the pot. And although the pot
is 30 years old, you can still tune it within tinier frequency
differences than a semitone.

> But it's a pain to do. As a user I would certainly prefer two
> 7-bit pots in coarse/fine arrangement to a single 9-bit (or 14-bit
> !) pot.

With a new & good & smooth pot, and a decent knob, it's not that much
of a pain, actually. The precision when controlling it is higher than
you might expect.

On the other hand, few parameters in a typical hard-wired synth is
this sensitive. VCO & cutoff frequencies are probably the only ones.
But if you make strange modulations or patch a modular synth, even the
smallest rotations can make a big change in the sound.

On the third hand, we're talking about analog circuits, and their own
little tricks and temperature deviations can cause larger changes than
this. It's mostly a matter of not getting annoyed by low resolution
when turning the pot - because what you get in the end can vary from
time to time even with infinite digital precision. Perfect
repeatability and automation in complex analog systems will never
happen. Wohooo! :-)


/mr