a2jmidi

A one-way static bridge, connecting ALSA-MIDI to JACK-MIDI.

Advanced Linux Sound Architecture (ALSA) and JACK Audio Connection Kit (JACK) are both software frameworks used in LINUX for audio/MIDI applications.

The a2jmidi-utility provides a stable, time-accurate bridge connecting JACK-based applications to MIDI Hardware or to ALSA-MIDI-based software.

Incoming MIDI events will be detected within a fraction of a millisecond and will be aligned into the JACK-buffer with sample precision.

Usage

To create a new bridge, open a terminal-window and do:

$ a2jmidi  [options] [NAME]

Where NAME is a user chosen name for the bridge. The given name will be used to label the client and also the port. If there is already a client with the given name, the bridges name will be made unique by appending a number. A name containing spaces must be enclosed into quotation marks.

Allowed options are:

• -h [ --help ] display help and exit
• -v [ --version ] display version information and exit
• -s [ --startjack ] try to start the JACK server if not already running
• -c [ --connect ] source-identifier identifies a source of ALSA-MIDI events (such as a sequencer-port or a MIDI device) for monitoring. The source will be connected as soon as it becomes available.
• -n [ --name ] (optional) name same as the NAME argument above.

The source-identifier can be specified as the combination of client-number and port-number such as 28:0 or the label of a port such as "USB-MIDI MIDI 1".

To stop the bridge, shutdown the JACK server or do ctrl-c.

Example 1

Start the JACK-server with QjackCtl, then open a terminal and do:

$ a2jmidi "My Midi port"

In the MIDI-window of QjackCtl we will see a new JACK-Midi client called My Midi port. This is the JACK side of the bridge.

The following screenshot shows the connections of ALSA-MIDI sequencer. We see, there is a new ALSA-Midi-client called My Midi port, this is the ALSA side of the bridge.

• Note: the panel labeled "ALSA" appears in the connections-window of QjackCtl, only when we have activated the ALSA Sequencer support in the Misc Tab of the QjackCtl-Setup.

The ALSA- and the JACK- ports remain available as long as the JACK server runs. Even after a short break of system-hibernation the bridge should restart to work normally.

We have now a permanent ALSA to JACK bridge which can be connected by applications that produce ALSA-MIDI events. Like for example Frescobaldi. Frescobaldi is a tool to create professional looking sheet music. The music composition can be proof-read (proof-listened?) by listening to the generated MIDI output. Below, we see how to connect Frescobaldi's midi output to our My Midi port created above.

Finally, the JACK side port can be connected to a virtual MIDI synthesizer as in the example below:

Now, we can hear our music compositions played on the virtual piano.

Example 2 - Connecting USB Hardware

USB Midi hardware can be plugged on and off while the JACK server is running. a2jmidi permits to setup connections to such USB Midi hardware independently whether the hardware is connected or not.

a2jmidi will watch the state of connections and automatically grasp a specific port as soon as it becomes available. To this end we need to know the port name under which the USB Hardware is known to ALSA. To find out, we can use the command aconnect -i as in the example below:

$ aconnect -i
client 0: 'System' [type=kernel]
    0 'Timer           '
    1 'Announce        '
client 14: 'Midi Through' [type=kernel]
    0 'Midi Through Port-0'
client 28: 'USB-MIDI' [type=kernel]
    0 'USB-MIDI MIDI 1 '

In this example we see, there is a device that reports its MIDI output as "USB-MIDI MIDI 1". This device is an USB piano keyboard that we want to include into our JACK Audio setup.

In order to have a2jmidi to automatically connect to this device, we'll start it with the following command:

$ a2jmidi "My Midi port" --connect "USB-MIDI MIDI 1"

In the screenshot below we see that the bridge called "My Midi port" is now connected to the "USB-MIDI MIDI 1" device.

When the USB device is powered off, the bridge will stay alive. As soon as a MIDI device called "USB-MIDI MIDI 1" reappears, the bridge will reconnect to it.

Now we are able to set up complex audio-midi patches including our USB piano keyboard, even before the keyboard is connected or is powered on.

Example 3

In this example we will merge example 1 and example 2 into one startup script.

Create a file with the following content:

#!/usr/bin/env bash

a2jmidi --name "Keyboard" --connect "USB-MIDI MIDI 1" & 
sleep 1
a2jmidi "Sequencer_A" & 
sleep 1 
a2jmidi "Sequencer_B" &  

Save this file as .a2jmidi_setup in your home directory and make it executable.

In QjackCtl go to the Setup/Options panel. Activate Execute Script after Startup and enter $HOME/.a2jmidi_setup as startup script.

When we start the Carla-audio-plugin-host we now see three midi plugs (the red ones) labeled "Keyboard", "Sequencer_A", "Sequencer_B".

The port labeled "Keyboard" will deliver the MIDI events from the keyboard. The ports "Sequencer_A" and "Sequencer_B" can be used to connect ALSA-based software as in example 1.

Build and Install

Instructions on how to build and install can be found in the document INSTALL.md.

Similar Tools

Package a2jmidid

In the official Ubuntu repositories there is the a2jmidid package.

This package contains an executable called a2jmidi_bridge which has very similar functionality to the utility presented here. Unfortunately, the timing accuracy is not perfect. a2jmidi_bridge aligns all MIDI events at the beginning of each JACK-buffer. Especially with larger buffers, this might result in very inaccurate rhythms.

In the same package there is also an executable called a2jmidid which has much better timing accuracy. Like our tool it discovers newly connected ALSA hardware and provides an ALSA to JACK bridge, by mirroring the ALSA-side ports to JACK-side ports. But unlike a2jmidi, you cannot with a2jmidid establish the JACK-side connections unless the ALSA-side Hardware- or Software- ports are alive.

With the a2jmidi-utility, on the other hand, JACK-side and ALSA-side applications can be started independently and in any sequence.