TouchMPE

A browser-based MPE/MIDI controller for touchscreens.

touchmpe_demo.mp4

Getting started

1. Open https://boux.github.io/TouchMPE/ in Chrome on your Android phone or tablet. Optionally tap the menu and "Add to Home Screen" for fullscreen.

2. Connect your phone to your computer via USB. On Android, go to Settings > Connected Devices > USB and select MIDI. If you don't see this option (most likely on Samsung devices), you may need to enable Developer Mode first (Settings > About Phone > tap Build Number 7 times). Once you are in Developer options, there is a setting called Default USB configuration.

   Note: only tested on Android and Samsung devices with Chrome. iOS devices have not been tested — your mileage may vary.

3. In TouchMPE's top bar, tap MPE and select USB Peripheral Port as your MIDI output.

4. In your DAW, your phone should appear as a MIDI device. For example in Bitwig, go to Settings > Controllers > Add Controller > select Roger Linn Design Linnstrument (or any MPE controller), then set your Android device as the MIDI input/output.

5. Play.

CC Panel

• Tap CC in the top bar to open
• Right-click (desktop) or long-press (touchscreen) an empty cell to add a control
• Right-click or long-press a control to configure it (label, CC number)
• Tap a highlighted control then drag to resize
• Use the lock button to prevent accidental edits
• Use the layout button to move/resize the panel

Settings

Tap Settings to adjust pad size, root note, row/column offsets, scale, pressure mode, pitch bend range, timbre distance, and gravity assist.

MPE setup

Default pitch bend range is 48 semitones. Match this in your synth's MPE settings.

Known issues

USB MIDI byte corruption (ghost sustain)

When using Chrome on Android over USB MIDI, high-throughput MIDI streams can cause byte-level corruption in the USB MIDI transport layer. Specifically, the CC number byte of CC 74 (timbre/slide) messages — 0x4A — gets corrupted to 0x40, which is CC 64 (sustain pedal). This activates sustain on the affected MPE member channel, causing notes to ring out forever even after releasing them.

Symptoms

• Notes sound "stuck" — they sustain indefinitely after releasing your finger
• The note visually releases correctly in TouchMPE (noteOff is sent)
• Happens only in MPE mode, not MIDI mode (MIDI mode self-heals since all notes share one channel)
• More likely with many simultaneous touches and fast sliding
• The panic button may not help (CC 123 All Notes Off respects sustain pedal state)

Cause

The corruption occurs outside the application — between Chrome's Web MIDI send() call and the Linux ALSA MIDI driver receiving the bytes. TouchMPE's own debug interceptor confirms clean bytes leaving the app, but a MIDI monitor on the receiving end shows corrupted CC 64 messages arriving. The corruption is triggered by high message throughput overwhelming the USB MIDI transport.

Mitigations applied

1. Message deduplication — pitch bend, timbre (CC 74), and pressure values are only sent when they actually change, dramatically reducing redundant messages during multitouch
2. Single send() per frame — all pending MIDI bytes are flushed in one send() call instead of individual calls per message, reducing USB transfer overhead
3. Pressure disabled — aftertouch/pressure messages are disabled by default since they are unreliable on touch devices and add significant throughput
4. Improved panic — the panic button now sends CC 120 (All Sound Off) + CC 64 value 0 (sustain off) + CC 123 (All Notes Off) on all channels, which properly clears stuck sustain state