As I have mentioned, I am building a MIDI controller using the PIC18F4550. The controller has a MIDI in port, and acts as a MIDI to USB converter as well.

Features:

  • 10 potentiometers (knobs)
  • MIDI in
  • USB MIDI interface

Missing hardware:

  • Drill and mount the front panel
  • Create and mount the PCB
  • Mount the guts in the case

Missing firmware:

  • Active sense handling
  • MIDI channel selection

As always more details, source code, schematic, and PCB will follow.

I am in the process of building a MIDI USB device, using a PIC18F4550. I have trimmed the Microchip USB framework MIDI example, and is using this code as my base. It is a blessing to not have to code the entire USB stack with descriptors, from scratch, but it is a pain using someone elses code, and learning to use this framework as a sort of black box.

The goal is a combined MIDI to USB converter and knob box.

MIDI

MIDI is an interface for sending musical notations and messages through a wire. I will only brush up on the protocol of MIDI in this text.

A MIDI message consists of a status byte and up to 2 data bytes, the protocol is serial, and runs at 31250 baud. A table with all messages can be found here. On thing that was not obvious to me was that only the bytes needed are send, no zeroes. As an example an "active sense" message only sends the status byte. This may seem obvious, but starting from the USB side of this stuff made it seem otherwise.

Another thing that I wasted a fair amount of time discovering, is "running status". When you sent the same voice or mode command multiply times you can omit the status byte from all but the first message.  An example, you want to sent "note on" on channel 1, key number 60, and a velocity of 128, when a key is pressed. Then "note on" on channel 1, key number 60, and a velocity of 0, when a key is depressed. For the second package you only send the data, e.g. key number 60, velocity 0, the receiver assumes the status and channel are the same as the first message.

USB MIDI

MIDI packages on the USB cable are 4-byte fixed length packages, unused bytes are zeroed. The first byte is the cable number (16 virtual cables) and the Code Index Number, which acts sort of the same as the first 4 bits of a MIDI status. The following 3 bytes are an ordinary midi message. The standard is here.

MIDI to USB MIDI

For a long time, a note on page 17 of the USB MIDI standard eluded me. I thought that since the  Code Index Number almost mirrors the ordinary status of a MIDI message, I would have to parse and classify every message coming from the MIDI port. It turns out that you can send each byte received along without out classifying it, and set the Code Index Number to 0xF. Using this Code Index Number every byte received on the MIDI port is simply packaged in a 4 byte package, first 4 bits, the cable number, then 0xF, then the byte from the MIDI port, and in the end padded with 2 zero bytes.

Generated on 2018-05-03 01:14:21.920372