[sdiy] MIDI CC with Pots

Tom Wiltshire tom at electricdruid.net
Sat Oct 4 14:14:53 CEST 2014

On 4 Oct 2014, at 06:35, Jacob Watters <jacob at joviansynth.com> wrote:
> -= PROBLEM 1: MIDI CC and Pots =-
> The synth has several pots for adjusting various controls of the sound
> (knob per function). The controls will also respond to MIDI CC. There
> can be a conflict in how the synth responds to both messages.
> OPTION 1: I recommended having the synth use whatever message it last
> received, whether a pot change or a CC message. This provides
> flexibility in usage and allows the use of both MIDI CC and the pots
> at the same time. This is how most of my synths seem to work.

I think this is clearly the best option, partly for the reasons you state, but also because it's also the way that people have come to *expect* things to work. Therefore, if you're going to do something that confounds people's expectations, you'd better have a damn good reason to do so, because they're certainly not going to like it at first. The Model T mentioned earlier is a case in point!

The "jump to" or "catch up" or "pass through" or whatever is a matter of taste. I suppose my ideal would be to have a option for this deep in some menu somewhere, since it's a preference that each person will set once to the way they like it, and then leave it alone ever after.

> -= PROBLEM 2: Mod Wheel =-
> OPTION 1: I pointed out that the mod wheel is usually used for
> modulating an existing control that has its own value already. It adds
> more LFO to the oscillators or opens the filter more than the current
> cutoff setting. It is not usually a dedicated control on its own. This
> means that it only adds to the current value of a control that is set
> with a pot on the synth. Since he agreed to add an extra pot for the
> thing that the mod wheel will control, this is very easy to employ and
> won't slow the synth down much (just add the mod CC to the current pot
> value and limit it to the max value of the control). This also means
> the pot for the control will have its own dedicated CC for setting the
> base level, and the mod CC will only add to that.

Again, there's a clear precedent set here, and I think you'd be wise to follow it. A dedicated control sets the base level of some parameter, and the mod wheel adds "extra" on top of that. That's how most kit works, and that's what people expect.
One important difference between a joystick and a mod wheel to watch out for is that a joystick might well have a centre-zero, and therefore be a bipolar +/- modulation, whereas a typical mod wheel has zero at the bottom and goes up to 100% at the top.

Finally, if you're receiving CC's or Mod Wheel data from MIDI, you might want to think about how you smooth it. The Mod Wheel on my Axiom MIDI controller sends data about every 100-150mS, which is audibly "steppy" on synths that don't smooth the incoming data. Some sort of filtering or interpolation between the incoming data points goes a long way to making things feel smooth and "analog".


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