Midi

MIDI Extension Proposal 2008-11-23 (Draft)

To enable support for newer state based MIDI controllers (i.e. controllers which dynamically remap buttons by hitting certain mode buttons) the Mixxx MIDI mappings specs will need to be extended.

The extensions will enable controls to be mapped to QtScript (aka Javascript/EMCAScript) functions stored in a functions library file.

Scope of required changes:

1. Changes to the current and new MIDI mapping file formats
2. Additional script option MIDI event type handlers (configobject.cpp midiobject.cpp)
3. A script file loaded at start-up that contains a library of functions, add all those functions will need to be parsed and made available to midi learning.
4. Interface definition for arguments passed to mapped script methods (to include the raw MIDI event details, which channel, and any options associated with the mapping)

Example of Hercules Mk2 FX/Cue/Loop mode switch button

For the purposes of this document we will use the Hercules Mk2 controller to illustrate how these changes would be configured. The Mk2 is a simple controller that features a FX/Cue/Loop Mode selector button (see area 3, selector is triangle shaped), and 3 trigger buttons numbered 1, 2, and 3 respectively (also part of area 3).

Old -> New Mapping

XML

Currently, Mixxx does not support modes, so the function of the mode selector button is mapped to reverse. The new mapping would change that to call a javascript function, the function would be designated by a "<script-binding/>" option tag. Similarly the mappings for the buttons would be changed from their current mappings to call the second script method, though that is not shown in the snippets here. (mapping for Tom's branch is at the bottom)

Old mapping to reverse:

        <control>
            <group>[Channel1]</group>
            <key>reverse</key>
            <miditype>Ctrl</miditype>
            <midino>0x07</midino>
            <options>
                <switch/>
            </options>
        </control>

New mapping to mode selection:

        <control>
            <group>[Channel1]</group>
            <key>HerculesMk2.fx_cue_loop_mode</key> <!-- changed -->
            <miditype>Ctrl</miditype>
            <midino>0x07</midino>
            <options>
                <script-binding/> <!-- changed -->
            </options>
        </control>

QtScript Function

When a midi event arrives in controlobject/midiobject the <script/> tag triggers an evaluation of the functions stored in the library. Here is an example implementation of the library which shows the basics of managing the controller's state. The implementation isn't complete as the callbacks to control objects to trigger the various button actions or set the LED values are only comments or stubs to alert().

(Note: you'll need to compile Mixxx with option script=1 to enable QtScript parsing.)

function HerculesMk2() {}
HerculesMk2.mode_store = { "[Channel1]":0, "[Channel2]":0 };
HerculesMk2.fx_button_map = { 13:3, 14:2, 15:1, 16:1, 17:2, 18:3 };
HerculesMk2.mode_def = { "[Channel1]": { "min":15, "inc":-1, "max":13 } , "[Channel2]": { "min":16, "inc":1, "max":18 } };
HerculesMk2.modes = { 13:"loop", 14:"cue", 15:"fx", 16:"fx", 17:"cue", 18:"loop" };

HerculesMk2.fx_cue_loop_mode = function (msg) {
        if (msg.midino.value == 0) return; // ignore button up
        var ch = msg.channel;
        var B0 = 176; // Hex MIDI code for Hercules Mk2 LED output
        var mode = HerculesMk2.mode_store[msg.channel];
        if (mode != 0) {
            midi.send(B0, mode, 0) // clear previous LED status
        }
        if (mode == HerculesMk2.mode_def[msg.channel]["min"] || mode == HerculesMk2.mode_def[msg.channel]["min"] + HerculesMk2.mode_def[msg.channel]["inc"]) { // In one of the first two modes
            mode = mode + HerculesMk2.mode_def[msg.channel]["inc"];
        } else { // either uninitialized (mode == 0) or in the final mode and need to roll back to first mode.
            mode = HerculesMk2.mode_def[msg.channel]["min"];
        }
        HerculesMk2.mode_store[msg.channel] = mode;
        midi.send(B0, mode, 127) // set new LED status
    }
    
HerculesMk2.fx_cue_loop_button = function (msg) {
        if (msg.midino.value == 0) return; // ignore button up
        var mode = HerculesMk2.mode_store[msg.channel];
        if (mode == 0) { HerculesMk2.fx_cue_loop_mode(msg); mode = HerculesMk2.mode_store[msg.channel]; }
        var trigger_no = HerculesMk2.fx_button_map[msg.midino];
        switch (HerculesMk2.modes[mode]) {
            case "fx": /* trigger trigger_no on/off toggle event for fx on msg.channel */ ; break;
            case "cue": /* seek trigger_no cue point on msg.channel */; break;
            case "loop": /* trigger_no 1 to loop in/out, 2/3 to lengthen/shorten loop */ ; break;
        }
        alert(msg.channel + " " + HerculesMk2.modes[mode] + " button #" + trigger_no + " hit.");
    }

QtScript Test

Test code, which cycles through the buttons modes and triggers a few button events.

// --- Test stuff below
function midi(){
}
midi.send = function (status, midino, value) {
       alert("midi.send - status: " + status + " midino: "+ midino + " value: " + value);
}

function msg(){
}
msg.channel = "[Channel1]";
msg.midino = 13;
msg.value = 127; // button down

function msg2(){
}
msg2.channel = "[Channel2]";
msg2.midino = 17;
msg2.value = 127; // button down

HerculesMk2.fx_cue_loop_mode(msg);
HerculesMk2.fx_cue_loop_button(msg);
HerculesMk2.fx_cue_loop_mode(msg2);
HerculesMk2.fx_cue_loop_button(msg2);
HerculesMk2.fx_cue_loop_mode(msg);
HerculesMk2.fx_cue_loop_button(msg);
HerculesMk2.fx_cue_loop_mode(msg);
HerculesMk2.fx_cue_loop_button(msg);
HerculesMk2.fx_cue_loop_mode(msg);
HerculesMk2.fx_cue_loop_button(msg);
HerculesMk2.fx_cue_loop_mode(msg2);
HerculesMk2.fx_cue_loop_button(msg2);

Tom's branch midi mapping

Old mapping to reverse:

                <control>
                    <group>[Channel1]</group>
                    <key>reverse</key>
                    <miditype>Ctrl</miditype>
                    <midino>7</midino>
                    <midichan>1</midichan>
                    <controltype>button</controltype>
                    <options>
                        <button/>
                    </options>
                </control>

New mapping to mode selection ??:

                <control>
                    <group>[Channel1]</group>
                    <key>HerculesMk2.fx_cue_loop_mode</key> <!-- changed -->
                    <miditype>Ctrl</miditype>
                    <midino>7</midino>
                    <midichan>1</midichan>
                    <controltype>script</controltype> <!-- changed -->
                    <options>
<!--
                        <button/>
-->
                    </options>
                </control>