[Jack-Devel] synchronization with Jack transport

Christopher Arndt chris at chrisarndt.de
Thu Oct 3 05:23:51 CEST 2019

Am 03.10.19 um 01:04 schrieb Philippe GARCIA-ORTUNO:
> But if I connect seq24 to Jack my Tempest suddenly turns much more 
> slowly, I would say almost half the tempo.

Sounds like seq24 is sending MIDI clock messages to your MIDI output, 
where the Tempest is connected too. You should be able to change this in 
the options window of seq24.

It is also maybe acting as the JACK "timebase master", i.e. setting the 
tempo information for JACK transport clients. I'm not sure, though, 
whether seq24 does this. If not, some other JACK client must be doing 
this, and seq24 is adapting its internal tempo to it, as soon as it is 
set to follow JACK transport. What other JACK programs do you have running?

You can use the jack_transport console tool to act as a timebase master 
(overrides any existing timebase master) and set the tempo:

$ jack_transport
 > tempo 100
 > master
 > play

AFAICS, jack_transport does not support setting the meter (beats-per-bar 
and beat-length) to something other than 4/4, For that you'd need 
another program, e.g. Carla, Ardour, etc. I also wrote a simple Python 
script (based on the jackclient-python library), which supports 
providing the tempo and meter.


The script is not a real-time safe solution, but is nice for testing.


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