I would like to provide the MIDI out functionality to the backend so that I can use the drum sounds of my Roland JV-1080 sound generator. In this log entry, I explain how I setup the MIDI out and what difficulties I faced.

MIDI follows the communication protocol of a serial port with two special aspects:

• Some dedicated wiring is necessary, since the MIDI ports rely on opto-isolators.
• MIDI uses a baudrate of 31250Hz, which is not a standard serial port baudrate.

On a RaspberryPi, it is straightforward to create a MIDI output from one of the serial ports as described in this article. Since operating a serial port from python does not require a special driver, I wanted to use the same approach for my x86 based backend. So when I was selecting the backend mainboard, I carefully looked for one that provides an onboard serial port and I ended up with a MSI Z97SLI.

However, I had to learn that an RS232 output (which my mainboard provides) uses different voltage levels than the UART that a RaspberryPi has. I now use a USB-powered converter (€4,60).

But still, the communication did not work and it took me some time to figure out that even though I could set the baudrate to 31250Hz without error message, the serial port was actually operating at the next standard baudrate which is 38400Hz.

It turns out that the 16550A chipset that my mainboard uses for the serial port just does not support custom baudrates different from the default baudrates (at least I did not manage to configure it accordingly). On the RaspberryPi the MIDI baudrate of 31250Hz could be configured without difficulties from the python script opening the port.

Now I use a dedicated PCI card that provides serial ports using an AX99100 chipset (€17,95). There is a dedicated linux driver on the manufacturer's homepage that I could compile without problems ('make'). In order to permanently add the driver, I had to

• place the binary module ax99100.ko in /lib/modules/5.15.0-91-generic/kernel/drivers ('5.15.0-91-generic' being the kernel I use as returned by 'uname -r')
• add the name 'ax99100' to /etc/modules
• run 'sudo depmod'

With the driver comes a command line tool 'advanced_BR' that can be used to configure custom baudrates. I could successfully run MIDI out communication with the following configuration:

advanced_BR -d /dev/ttyF0 -b 1 -m 0 -l 250 -s 16

It configures a base clock of  125MHz (-b 1), a divisor of 250 (-m 0 -l 250) and a sampling of 16 (-s 16), yielding a nominal baudrate of 125MHz / (250 * 16) = 31250Hz.

So now, after a lot of trial-and-error, I have a serial-port-based MIDI out for ~20€, that performs very well. I still wonder, how a USB-MIDI adapter would compare to my solution in terms of linux-driver-hassle and latency / jitter.