MIDI: Audible artifacts for knobs with center position caused by absoluteLin()

[uklotzde]

Bug Description

The following code causes audible artifacts:

filterKnob = function(_channel, _control, value, _status, group) {
    let filterParam = script.absoluteLin(value, 0.0, 1.0);
    engine.setParameter(group, "super1", filterParam);
};

Similar code can be found in various controller scripts for symmetric parameters with the value range 0..1 and the center value 0.5.

When the knob is centered it sends the MIDI value 0x40 (= 64). This value should be mapped to 0.5 to disable the effect/filter. Unfortunately, the simplified linear interpolation in absoluteLin() results in a slightly different value 0.503937008 and the filter is active all the time. Depending on the selected effect this noticeably degrades the sound quality!

The following code works as expected:

filterKnob = function(_channel, _control, value, _status, group) {
    var filterParam = script.absoluteLin(value, 0.0, 1.0);
    if (value === 0x40) {
        // Fix value for center position.
        value = 0.5;
    }
    engine.setParameter(group, "super1", filterParam);
};

Version

No response

OS

No response

[uklotzde]

Using absoluteNonLin() instead might work, but Math.round() looks brittle. For the typical MIDI range the exact calculated center value is (127 - 0) / 2 = 63.5. Rounding errors could flip it into either direction, e.g. either 63 or 64.

Even if it works for MIDI it might not work correctly for other ranges.

For a solid implementation of absoluteNonLin() the input center value also needs to be provided as an argument instead of calculating it from min and max.

[Swiftb0y]

Can confirm. The problem is that there is no middle element in the $[0; 127]$ range. A "fix" could be to set the max to $128$, but then the maximum return value will be off. A fix would be to split the values into two ranges, eg $[min,63]$ maps to $[0, 0.5]$ and then $[64, max]$ to $[0.5, 1.0]$. While this would result in a small step (the function becomes discontinuous), that may even be desirable to implement to mimic some slight snapping. This hack is applied here for instance:

mixxx/res/controllers/midi-components-0.0.js

Lines 60 to 67 in 4c1b881

	inValueScale: function(value) {
	// Hack to get exact center of pots to return 0.5
	if (value > (this.max / 2)) {
	return (value - 1) / (this.max - 1);
	} else {
	return value / (this.max + 1);
	}
	},

Edit, obviously substituting in low, high, min, max, etc instead of the above hardcoded values.

[uklotzde]

script.absoluteNonLin = function(value, low, mid, high, min, max, center) {
    if (!min) {
        min = 0;
    }
    if (!max) {
        max = 127;
    }
    if (!center) {
        center = (max - min) / 2;
    }
    if (value === center || value === Math.round(center)) {
        return mid;
    } else if (value < center) {
        return low + (value / (center / (mid - low)));
    } else {
        return mid + ((value - center) / (center / (high - mid)));
    }
};

[uklotzde]

If someone complains about sound quality with a controller connected consider this bug as a possible cause.