Linux VST arpeggiators produce no notes, but Windows ones do.

[DaveyBoy1970]

Bug Summary

Using Carla patchbay I want to add an arpeggiator such as LibreArp or HY-SEQ. I've found that while it works on playback when it comes to export, no arpeggtiation is produced. Where the arpeggiator/sequencer does not take MIDI information from the song editor - such as Stochas - again it works on playback but when it comes to export it's just silence.

I've not been able to get Windows VSTs working in Carla to test, but I asked the question on LinuxMusicians and someone discovered that Windows VSTs (BlueArp in this case) work as expected, producing output on export. They were also able to reproduce the problem with LMMS 1.2.2 using LibreArp and also qmidiarp.

Steps to reproduce

Open Carla Patchbay as instrument. Create notes to arpeggiate in song edior. In Carla add a Linux Arpeggiator (eg LibreArp) link input to MIDI capture, link midi out to a synth (I chose Dexed) and the synth to audio out. When the output is played back the notes will arpeggiate

Expected behavior

When exporting to MP3 or WAV the arpeggiated notes should be in the output.

Actual behavior

Just the bare chords are produced without arpeggiation.

Screenshot

Affected LMMS versions

1.2.2 and 1.3 alpha - both appimage versions downloaded from LMMS.

I have tested this on Ubuntu Studio 21.10 and 22.04

Logs

Click to expand

[DaveyBoy1970]

I've contacted the developers of LibreArp and Stochas and they are able to reproduce the issue, but can't work out why it's not working. Does anyone have an idea what BlueArp is doing right that the others are doing wrong?