Jack Transport Link

A service that bridges Ableton's Link to and from Jack Transport, allowing applications that use Jack Transport to synchronize their timing with other applications that support Link.

Build Requirements

• a compiler that can build c++17
  • clang++, g++
• cmake 3.16 or higher
• conan
  • pip3 install conan
  • make sure the conan executable is in your path

Building

Make sure you've updated your submodules:

git submodule update --init --recursive

Use cmake to configure, then build:

mkdir build && cd build && cmake .. && make

If everything succeeds, you should have an executable here: ./bin/jack_transport_link.

Linux Systemd Service

There is an optional systemd service file that is enabled by default, at this time it is run as the user pi for the raspi. You can disable that through the cmake -DINSTALL_SERVICE_FILE=Off

If you want to target a different user, you'll have to edit the appropriate file in config/

Installing

You can just run from the bin directory if you want, or copy the executable somewhere, but you can also use the install target:

sudo make install

Or, on debian systems you can use cpack and then install the deb, from the build dir.

cpack && sudo dpkg -i *.deb

Running

There are a few options for running the service, setting the initial tempo and time signature details, indicating if you want the service to start a jack server if there isn't already one to connect to, etc.

Run with the -h switch to discover more details.

Notes

Since jack transport doesn't allow clients to request tempo, we use the metadata API to do tempo requests. You must use jack 1.9.13 or newer for metadata support. You can request the tempo even if the transport isn't running.

The key for bpm is http://www.x37v.info/jack/metadata/bpm and the type is https://www.w3.org/2001/XMLSchema#decimal.

Here is an example of how to set the tempo to 150 beats per minute.

jack_property --client jack-transport-link http://www.x37v.info/jack/metadata/bpm 150.0 https://www.w3.org/2001/XMLSchema#decimal

To get the current bpm property.

jack_property --client jack-transport-link --list http://www.x37v.info/jack/metadata/bpm

TODO

• Latency Compensation computation
• Follower mode (just report transport, don't drive it)
• Option to synchronize the rolling start to a start of a bar.
• Windows support

Acknowledgements

Built using:

• Ableton's Link
• Jack Audio Connection Toolkit
• cpp-optparse

I learned a lot from jack_link by Rui Nuno Capela, which is trying to solve the same problem but has a user interface and I wanted to simply run as a service. jack_link also has some issues with discontinuities that I wasn't happy with.