Simple MIDI Fader

By Kevin Cuzner

Project Status

• PCB complete and working (with an additional load added to reduce regulator noise)
• Firmware feature-complete and mostly working
  • Emulates as HID and USB-MIDI devices
  • Not tested on Windows yet
• Case OpenSCAD in progress
• Configuration host software in progress

Featureset

• Two 60mm analog fader inputs
• USB Audio Class interface, aiming for driverless on Windows 7 and 10
• Driverless HID interface for configuration
  • It might be trivially simple to echo the fader status in the HID reports...

Main Component Options

• Faders (note that they have different footprints and different levers):
  • Expensive faders ($4.74, very light movement force): PTB6053-2010APB103-ND
  • Cheap faders ($1.73): PTA6043-2015CPB103-ND
• Microcontrollers:
  • 497-17354-1-ND: STM32F070F6P6, $1.48, 32K flash, 6K RAM, 20-TSSOP.
    • No CRS, external XTAL required.
  • 497-15099-ND: STM32F070CBT6, $2.52, 128K flash, 16K RAM, 48-LQFP
    • No CRS, exteranl XTAL required.
  • 497-17344-ND: STM32042F6P6, $2.25, 32K flash, 6K RAM, 20-TSSOP.
    • Has a CRS
    • 24.6mA with all peripherals and HSI48 enabled
• 609-4052-1-ND: Micro-USB, $0.76
• 455-1790-1-ND: JST-SH header for programming, $0.63
• Power supply solution:
  • MCP1603T-330I/OSCT-ND: Cheap buck regulator, but not too cheap. $1.01, TSOT-23
    • Noise issues at low currents (leaves PWM mode and skips pulses).
  • 576-4764-1-ND: MIC5504 buck regulator, $0.11, SOT23-5.
    • At a 100mA load, the case will heat 42.6C. Not over the top, but really quite hot.
    • We could do this AND the buck regulator, but just have this for the analog.

Notes

Notes about the logical MIDI protocol

• MIDI has two types of bytes STATUS and DATA.
• A STATUS byte has its MSB set
• A DATA byte has its MSB cleared (hence why there are only ever 0 to 127 data values)
• Usually, the lower nybble of the STATUS byte is a channel number (there are 16 channels).
• The number of data bytes to follow varies by which STATUS byte preceded them.
• Interesting STATUS commands (upper nybble shown) for this project:
  • 0xB: Control Change. Followed by a controller number (1 data byte) and a controller value (1 data byte). Although there are possibly 127 controller numbers, numbers 120-127 are reserved for "Channel Mode Messages". Several of these are commonly used.
  • 0xE: Pitch Bend. Followed by two DATA bytes encoding a 14-bit value, LSB first. 0x2000 is center.
• https://www.cs.cmu.edu/~music/cmsip/readings/MIDI%20tutorial%20for%20programmers.html
• https://www.midi.org/specifications-old/item/table-1-summary-of-midi-message

Notes about USB and MIDI

• Windows XP and above seem to support USB MIDI except Elements.
  • "Elements" are things that process MIDI
  • "Jacks" are supported, and they are exactly what they sound like. This will implement a Jack.
  • Elements and Jacks are "Entities"
• http://www.usb.org/developers/docs/devclass_docs/midi10.pdf