Raspberry Pi OSA MIDI Board User Manual

: June 5, 2024
: Raspberry Pi

Table of Contents

Raspberry Pi OSA MIDI Board
Setting Up Raspberry Pi for MIDI
What we need
Assembly
First time setup
Installation
Create the configuration script:
MIDO
PIGPIO
Python Examples
Playback a MIDI File
References
Read User Manual Online (PDF format)
Download This Manual (PDF format)

Raspberry Pi OSA MIDI Board

Setting Up Raspberry Pi for MIDI

This guide will show how to take a freshly installed Raspberry Pi and have it operated as an OS-discoverable MIDI I/O device. It will also provide some examples of using various Python libraries to get MIDI data into and out of the programming environment. UPDATE – Sep 11, 2021.: This guide has been updated to fix some issues with the latest Raspberry Pi OS version, you can also download a full image with pre-installed scripts and fully configured here.

What we need

Raspberry Pi A+/B+/2/3B/3B+/4B
MIDI Board for Raspberry Pi
A MicroSD Card•Set of 4 Nylon M2.5 Screws
Set of 4 Nylon M2.5*11mm Female to Female Standoffs
Set of 4 Nylon M2.5*5mm Male to Female Standoffs

Assembly

Use the nylon screws and standoffs to assembly the Raspberry Pi together with the MIDI Board, as shown on the image below:

Raspberry-Pi-OSA-MIDI-Board-1

First time setup

We tested all the examples in this document on a Pi 4B using Rasperry Pi OS, version May 2020). The first time, it is necessary to use a screen and keyboard to set the Pi up. Thereafter, use your method of choice to access the Pi’s OS. All steps are mandatory unless otherwise stated

Installation

Update/Upgrade
Perform the update and upgrade as described here: https://www.raspberrypi.org/documentation/raspbian/updating.md

Network Configuration (Optional)
If you are SSH’ing from another machine into the Pi, it is worthwhile giving the Pi a fixed IP address: https://www.modmypi.com/blog/how-to-give-your- raspberry-pi-a-static-ip-address-update It is also a good idea to add the network security settings to the Pi so that it will automatically connect to the network: https://www.raspberrypi.org/documentation/configuration/wireless /wireless-cli.md

Set the Pi Up as a USB OTG Gadget
Open a terminal on the Pi and follow this procedure:

Set the USB driver to dwc2
echo “dtoverlay=dwc2” | sudo tee -a /boot/config.txt
Enable the dwc2 driver
echo “dwc2” | sudo tee -a /etc/modules
Enable the lib composite driver
echo “lib composite” | sudo tee -a /etc/modules
Enable the MIDI gadget
echo “g_midi” | sudo tee -a /etc/modules

Create the configuration script:

Create the file
sudo touch /usr/bin/midi_over_usb
Make it executable
sudo chmod +x /usr/bin/midi_over_usb
Edit it with Nano
sudo nano /usr/bin/midi_over_usb

Paste the following into the file, making edits to the product and manufacturer strings as required. cd /sys/kernel/config/usb_gadget/ mkdir -p midi_over_usb cd midi_over_usb echo 0x1d6b > idVendor # Linux Foundation echo 0x0104 > idProduct # Multifunction Composite Gadget echo 0x0100 > bcdDevice # v1.0.0 echo 0x0200 > bcdUSB # USB2 mkdir -p strings/0x409 echo “fedcba9876543210” > strings/0x409/serialnumber echo “OSA Electronics” > strings/0x409/manufacturer echo “MIDI USB Device” > strings/0x409/product ls /sys/class/udc > UDC Exit Nano and save the file (Ctrl+X, Y, return). Add a call to the script to rc.local, so that it executes on every startup. sudo nano /etc/rc.local Add the following line before “exit0” /usr/bin/midi_over_usb Exit Nano and save the file and reboot the Pi. sudo reboot List the available MIDI ports. amidi -l If the MIDI is configured correctly, the last command should output something similar to: Dir Device Name IO hw:0,0 f_midi
IO hw:0,0 f_midi

Install Python Libraries

This section will explain how to install our preferred libraries for Python 2.x.

MIDO

Mido is an easy-to-use library for handling MIDI data. It relies on the rt- midi backend, the asound library, and Jack. Input the following commands in sequence: The output should show one ‘Midi Through’ port and one additional port. If this is the case, all is well. *Note: in Mido, the port name is the entire string enclosed in single quotes, but it is possible to truncate the name to the string before the colon. On this machine, the string is: ‘f_midi:f_midi 16:0’. For example, these two commands are equivalent

PIGPIO

We use the pigpio library to interface with the GPIO pins. We have found this library to be more stable and flexible than the standard method of interfacing with the Pi’s hardware (RPi.GPIO). If you want to use another library, edit the code accordingly. To install the pigpio library, follow the instructions here: http://abyz.me.uk/rpi/pigpio/download.html Prior running all of the examples below, you should start the pigpio service if not done:

Python Examples

The examples also use the numpy library’s interp function as an easy way to map between two ranges. We used Reaper to send and receive data. The Pi is configured as a Hardware MIDI output in Reaper’s preferences menu.

Control GPIO with Note Data (example_1_key_press.py) This example shows how to:

Listen for 3 specific note-on and note-off events using a simple condition
Catch the exceptions that arise when non-note data is sent to the Pi (e.g. transport data from a sequencer)
Map the note velocity to the PWM of the output pin

Import the relevant libraries, create the pi object from the pigpio library, and open the output port: The try/catch block is to catch the errors that arise from other types of MIDI data being sent (e.g. transport controls etc.). while True: try: #This filters out all non-note data for msg in port.iter_pending(): # if there is a message pending if(msg.type == ‘note_on’): # if it is Note On message out = interp(msg.velocity, [0,127],[0,255]) # scale velocity from 0-127 to 0-255 #filter the data by note number if(msg.note == 53): pi1.set_PWM_dutycycle(2, out) elif(msg.note == 55): pi1.set_PWM_dutycycle(3, out) elif(msg.note == 57): pi1.set_PWM_dutycycle(4, out) else: # if the message is not Note On (e.g. Note Off) if(msg.note == 53): pi1.set_PWM_dutycycle(2, 0) elif(msg.note == 55): pi1.set_PWM_dutycycle(3, 0) elif(msg.note == 57): pi1.set_PWM_dutycycle(4, 0) except AttributeError as error: print(“Error excepted”) pass

Control GPIO with Mod and Pitch Wheels (example_2_wheels.py)
This example shows how to:

Listen for Pitch and Mod Data and filter them by type
Map the data to the PWM of the output pin

This example is similar to the above, with these message types:

The Pitch wheel is type pitchwheel with a value of msg.pitch
The Mod Wheel is a Continuous Controller type control_change with a control parameter of msg.control = 1 (the CC number) and a value of msg.value

Output MIDI Data from a GPIO Event (gpio_event.py)

This example shows how to:

Use an interrupt to detect a button press
Send MIDI data from the Pi to another device

Open the output port, create two messages and setup the GPIO pin as an input. This example assumes there is a button tied to pin 21, so that pin goes HIGH when the button is pressed: The following are the callback functions called when the button is pressed or released. The output ports send() function simply sends the message out of the port: The callback listeners run in the background and do not need any more attention:

Playback a MIDI File

This example shows how to:

Load a MIDI file in the programming environment
Playback the file .

This examples assumes you have a MIDI file called midi_file.mid in the same directory as your python script: import mido from mido import MidiFile from mido import MetaMessage port = mido.open_output(‘f_midi’) mid = MidiFile(‘midi_file.mid’) while True: for msg in MidiFile(‘midi_file.mid’).play(): port.send(msg)