LEGO ipMIDI

This post is part 1 of 5 of  LEGO ipMIDI

3 weeks since COVID-19 lockdown.

Bored.

Let’s go back to MIDI and the never completed LEGO Laser Harp idea – instead of usinq MQTT to send codes from the EV3 to my laptop and converting there to MIDI… how can I send pure MIDI?

More than 2 years have past. Python MIDI libraries are better. ev3dev is better. I am bored. Let’s search.

TouchDAW supports two network MIDI protocols: multicast (ipMIDI) and RTP-MIDI. The first one is not exactly a MIDI standard although lots of products seem to support it. But RPT-MIDI is so I’ll try it first.

Found David Moreno’s rtpmidi. It installs a daemon on my Ubuntu laptop. I can use TouchDAW to play music on my laptop through it. Very nice!

But it is not available for ev3dev so I would have to build it… I’m always afraid of that. So maybe there is a python library that can send midi notes through rptmidi? Found none 🙁

OK, ipMIDI then.

‘qmidinet’ on my laptop works. Had to disable jack and sometimes notes get stuck (usually the last note) but it works.

It also works on ev3dev (without GUI, of course):

qmidinet -g -j off

And I can play a MIDI file directly to the laptop:

aplaymidi --port 128:0 happy_birthday.midi

playing heavy MIDI files seemed to stress EV3 (or maybe just Wi-Fi) but small files worked very well – and a single MIDI instrument like my Laser Harp will generate just a few notes per second (at best).

While I was fiddling with different MIDI files I dedided to try ‘multimidicast’, the linux father of ‘qmidinet’. And it also works, just needed to compile from source (not as slow as I expected). Since it is command line only and doesn’t support jack, it uses fewer resources so I’ll use it instead of qmidinet.

I can send a full MIDI file… but what I really need is to send notes, in real time. So I need to generate those notes in python.

‘python-rtmidi’ seems to be the best choice and I remember trying it with the Harp when I was playing MIDI locally on the EV3 (running timidity as a soft synth). At that time, I managed to install ‘python-rtmidi’ and also ‘mido’ that uses it as a backend.

But I couldn’t install it.

There is a ‘python3-rtmidi’ package for armel but not for stretcht. I tryed the buster package but requires python 3.7 (at this moment, ev3dev includes 3.5.3).
So I tried ‘pip’… and after a while I loose network connectivity.
Then i tried downloading the source code and install it… and after a while I also loose network. Even tried installing it without jack support to make it a bit lighter… same thing.

Argh!

So I needed a plan B: playing notes without a python library. And found a post on a Raspberry Pi forum where someone played notes from Ruby using system calls to ‘amidi’:

amidi -p hw:1,0 -S "90 3C 7F"
amidi -p hw:1,0 -S "90 3C 00"

This turns ‘C’ on then off on MIDI channel #0. But it only works with sound cards, not with MIDI connections.

So… there is a kernel module ‘snd-virmidi’ that creates a virtual midi sound card… and it is available in ev3dev !!

sudo modprobe snd-virmidi

This creates 4 MIDI clients:

client 20: 'Virtual Raw MIDI 1-0' [type=kernel,card=1]
    0 'VirMIDI 1-0     '
client 21: 'Virtual Raw MIDI 1-1' [type=kernel,card=1]
    0 'VirMIDI 1-1     '
client 22: 'Virtual Raw MIDI 1-2' [type=kernel,card=1]
    0 'VirMIDI 1-2     '
client 23: 'Virtual Raw MIDI 1-3' [type=kernel,card=1]
    0 'VirMIDI 1-3     '

so I connect one of them to multimidicast:

aconnect 20:0 128:0

and now amidi works!

From python I can now play a ‘C’:

os.system('amidi -p hw:1,0 -S "90 3C 7F"')
time.sleep(0.1)
os.system('amidi -p hw:1,0 -S "90 3C 00"')

Even better: I can do it also from micropython because ‘os.system’ is also available, including the new ‘pybricks-micropython’ library.

Now I need to learn a few hexadecimal MIDI codes and prepare a Raspberry Pi to work as an ipMIDI synth to free my laptop from that role.