Get started: Linux ALSA and JACK

Before we dive into specific music applications, I need to provide a little background information about audio and MIDI support on Linux.

If you’re coming from Mac OS X or Windows, you may not have heard very much about the Linux way of doing audio and MIDI. Seems like the “mainstream media” don’t want to have much to do with Linux. Linux has a very well-developed infrastructure for audio and MIDI. Linux audio is a “stack” (a layer cake) with audio/MIDI applications on top:

  • Audio applications
  • JACK (Jack Audio Connection Kit)
  • ALSA (Advanced Linux Sound Architecture)
  • Linux kernel

You probably haven’t heard about JACK and ALSA before, so a little explaining is in order.

The Advanced Linux Sound Architecture (ALSA) uses the kernel to implement low-level — but extremely powerful — audio and MIDI features. ALSA provides several useful applications, but I like to think of ALSA as a tool to build higher level tools. ALSA is the layer that supports “soundcards,” which is the Linux catch-all term for hardware audio interfaces, MIDI interfaces, and more. Go to the ALSA project homepage to get more information from the developer’s perspective.

You are far more likely to interact with the Jack Audio Connection Kit (JACK) than ALSA. JACK is an audio/MIDI server that provides audio and MIDI services to JACK-based applications (i.e., applications using the JACK API). The list of JACK-enabled applications is impressive. In fact, this list is a rather good summary of the audio and MIDI applications that are available on Linux! Check out the JACK project page to get more information from the developer’s point of view. End-users (us normal people) should read the JACK FAQ which covers some of the finer points about JACK.

ALSA utils

The ALSA utility applications are collectively known as “ALSA utils.” Use the apt-get command to download and install the ALSA utils:

    sudo apt-get install alsa-utils

Here is a list of the ALSA utility applications:

    alsactl    Change and save settings for an audio device
    amixer     Adjust volume and sound controls (ncurses version)
    alsamixer  Adjust volume and sound controls (ncurses version)
    aconnect   Make MIDI connections
    aseqview   Display ALSA sequencer events (e.g., note ON, note OFF)
    aplay      Play back an audio file from the command line
    arecord    Record an audio file from the command line

Let’s take a look at a few of these applications in action.

Test speaker output

Although not strictly part of ALSA utils, speaker-test is a quick way to make sure that the built-in Raspberry Pi audio output is properly connected and configured.

First, connect the RPi2 audio output to your powered monitors using a 3.5mm to whatever patch cable. The Raspberry Pi built-in audio can be routed to either the 3.5mm audio jack (“analog”) or to the the HDMI port. Enter the command:

    amixer cset numid=3 N

to route the built-in audio. Replace “N” with one of the following choices:

    0: auto   1:analog   2:HDMI

In this case, use N=1 to route the audio to the 3.5mm audio jack. Then, run the command:

    speaker-test -t sine -f 440 -c 2

to send a 440Hz tone to the audio output. You should hear a test tone from your speakers.

If you don’t hear a test tone, double check your connections. You may need to add the current user to the audio group: sudo adduser XXX audio, where “XXX” is the user’s name. (I don’t believe this is strictly necessary.)

Play an audio file

Once speaker output is working, why not play an audio file? The aplay program plays an audio file. It supports just a handful of audio formats: voc, wav, raw or au. The default format is WAV.

    aplay -c 2 HoldingBackTheYearsDb.wav

The -c option specifies two channels. (The default is one channel of audio.)

If you listen carefully, you’ll notice that the built-in audio is a little bit noisy. I’ll get into the issue of audio quality in a future blog entry.

The command aplay -l displays a list of all sound cards and digital audio devices.

ALSA mixing

There are two ALSA utility mixer applications: amixer and alsamixer. amixer is a command line tool that controls one or more soundcards. The command (which does not have any command line arguments):

    amixer

displays the current mixer settings for the default soundcard and device as shown below:

    Simple mixer control 'PCM',0
      Capabilities: pvolume pvolume-joined pswitch pswitch-joined
      Playback channels: Mono
      Limits: Playback -10239 - 400
      Mono: Playback -2000 [77%] [-20.00dB] [on]

The output shows a list of the simple mixer controls at your disposal.

The alsamixer application is a bit more visual. alsamixer turns the terminal window into a visual mixer. Try:

    alsamixer

and see. Start alsamixer in one window and play an audio file in different window. Use the UP and DOWN arrows to control the playback gain (volume). Use the escape key (ESC) to exit alsamixer.

MIDI patch-bay

ALSA provides a virtual MIDI patch-bay that lets you interconnect MIDI senders and receivers. MIDI data is communicated from sender ports to receiver ports. A port may belong to either a MIDI hardware interface or a software application. The virtual patch-bay allows for very flexible, powerful MIDI data routing.

The aconnect utility application both displays the status of the virtual patch-bay and makes connections. First off, we need to know the available sender and receiver ports. The command:

    aconnect -i

displays a list of the sender ports including external MIDI input ports. External MIDI input ports (-i) are ALSA sender ports because they send MIDI data to ALSA receiver ports. I connected a Roland UM-2ex MIDI interface to one of the RPi’s USB ports and got the following output with aconnect -i:

client 0: 'System' [type=kernel]
    0 'Timer           '
    1 'Announce        '
client 14: 'Midi Through' [type=kernel]
    0 'Midi Through Port-0'
client 20: 'UM-2' [type=kernel]
    0 'UM-2 MIDI 1     '

The UM-2ex has one 5-pin MIDI IN (client 20, port 0).

Likewise, the command:

    aconnect -o

displays a list of the receiver ports including external MIDI output ports. External MIDI output ports (-o) are ALSA receiver ports because they receive MIDI data from ALSA sender ports. Here is the output when the UM-2ex is connected:

client 14: 'Midi Through' [type=kernel]
    0 'Midi Through Port-0'
client 20: 'UM-2' [type=kernel]
    0 'UM-2 MIDI 1     '
    1 'UM-2 MIDI 2     '

The UM-2ex has two 5-pin MIDI OUTs (client 20, port 0 and port 1).

The notions of sender and receiver may seem a little confusing especially in the context of external MIDI INs and OUTs. Please keep in mind that “send” and “receive” are defined with respect to ALSA itself (and ALSA objects).

Now, I want to really blow your mind. Let’s connect both the Roland UM-2ex and an M-Audio Keystation Mini 32 to the RPi2. Here is the output generated by aconnect -i:

client 0: 'System' [type=kernel]
    0 'Timer           '
    1 'Announce        '
client 14: 'Midi Through' [type=kernel]
    0 'Midi Through Port-0'
client 20: 'UM-2' [type=kernel]
    0 'UM-2 MIDI 1     '
client 24: 'Keystation Mini 32' [type=kernel]
    0 'Keystation Mini 32 MIDI 1'

We can see the MIDI IN for the UM-2 and the Keystation.

Here is the output generated by aconnect -o:

client 14: 'Midi Through' [type=kernel]
    0 'Midi Through Port-0'
client 20: 'UM-2' [type=kernel]
    0 'UM-2 MIDI 1     '
    1 'UM-2 MIDI 2     '
client 24: 'Keystation Mini 32' [type=kernel]
    0 'Keystation Mini 32 MIDI 1'

We see the MIDI OUTs for the UM-2 and the Keystation.

Let’s patch the Keystation (client 24, port 0) to the MIDI OUT of the UM-2ex (client 20, port 0):

    aconnect 24:0 20:0

The sender port is (24:0) and the receiver port is (20:0). MIDI messages are sent from the Keystation to the MIDI OUT of the UM-2ex. If you physically connect the MIDI IN of a tone module or synthesizer to the UM-2’s MIDI OUT, you can now play the tone module or synth using the Keystation. Guess what we just built? A USB MIDI to 5-pin MIDI bridge. Ever need to control an old school 5-pin MIDI synth using a new school USB-only MIDI controller? Now you can with Raspberry Pi and ALSA!

Run aconnect -l to display the connection status. Here is the output for the virtual patch bay:

client 0: 'System' [type=kernel]
    0 'Timer           '
    1 'Announce        '
client 14: 'Midi Through' [type=kernel]
    0 'Midi Through Port-0'
client 20: 'UM-2' [type=kernel]
    0 'UM-2 MIDI 1     '
	Connected From: 24:0
    1 'UM-2 MIDI 2     '
client 24: 'Keystation Mini 32' [type=kernel]
    0 'Keystation Mini 32 MIDI 1'
	Connecting To: 20:0

The output shows the connection from the Keystation to the UM-2ex.

To break the connection, run the command:

    aconnect -d 24:0 20:0

Run aconnect -l, again, and you’ll see that the connection has been removed.

More resources

If you’re a long-time reader of my site, you know that I blogged about the USB to 5-pin MIDI bridge technique before. If you have a Raspberry Pi and know how to run aconnect, you have a bridge!

The Ardour folks have two good articles about JACK on Linux (here and here).

New to Linux (Raspbian Jessie) on Rapsberry Pi? Then be sure to check out my article about getting started with Raspbian Jessie and Raspberry Pi.