There are two steps with BT midi. First, connect the BT to the device using the standard procedure like:
https://www.raspberrypi.org/forums/viewtopic.php?t=214373
Make sure you trust it. Then you will have created a MIDI device that will re-connect. From there you need to connect the stream to zynth, but you need to do it via the OS MIDI thru port like this:
root@zynthian:~# aconnect -l
client 0: 'System' [type=kernel]
0 'Timer '
1 'Announce '
Connecting To: 128:0
client 14: 'Midi Through' [type=kernel]
0 'Midi Through Port-0'
Connecting To: 128:0[real:0]
Connected From: 128:0
client 16: 'f_midi' [type=kernel,card=0]
0 'f_midi '
client 129: 'UD-BT01' [type=user,pid=695]
0 'UD-BT01 Bluetooth'
Then you need to connect the device (the UDBT01 in this example) to the MIDI through port:
root@zynthian:~# aconnect 129 14
and you get this:
root@zynthian:~# aconnect -l
client 0: 'System' [type=kernel]
0 'Timer '
1 'Announce '
Connecting To: 128:0
client 14: 'Midi Through' [type=kernel]
0 'Midi Through Port-0'
Connecting To: 128:0[real:0]
Connected From: 128:0, 129:0
client 16: 'f_midi' [type=kernel,card=0]
0 'f_midi '
client 129: 'UD-BT01' [type=user,pid=695]
0 'UD-BT01 Bluetooth'
Connecting To: 14:0
That should work. Unfortunately, it doesn’t re-connect the MIDI port on re-boot.
Of course, you could probably just connect the USB cable…
Gonzo