Access audio hardware directly (ALSA, JACK)

Sean Fern edited this page Jan 21, 2017 · 14 revisions

For most use cases, audio works just fine with CRAS; ALSA to CRAS is supported out of the box by crouton, and JACK to CRAS can be set up via these steps.

However, direct access to hardware may be desired for more latency-sensitive audio applications. The following is a guide on setting up ALSA and JACK access to hardware on a crouton chroot.


ALSA to hardware

Permissions

On your chroot, add yourself to the hwaudio group.

sudo usermod -a -G hwaudio "$USER"

Log out and log back in.

Test:

$ groups
slee2 hwaudio video sudo plugdev audio input

Stop CRAS

Make sure you've stopped CRAS, or else it will hold on to your audio device. In either ChromeOS or your chroot, run sudo stop cras

Configure ALSA

On your chroot, run cat /proc/asound/cards to see the names of your audio devices.

You should see something like this:

 0 [HDMI           ]: HDA-Intel - HDA Intel HDMI
                      HDA Intel HDMI at 0xe1218000 irq 65
 1 [bdwrt5677      ]: bdw-rt5677 - bdw-rt5677
                      bdw-rt5677

For the purposes of this tutorial, we will be wanting to talk directly to bdwrt5677, the internal audio device on the Pixel 2015. Your setup may be different, so replace bdwrt5677 with your device in the following steps as appropriate.

Override your chroot's audio settings (which by default routes to CRAS) by creating the file ~/.asoundrc with the following contents:

# ~/.asoundrc
# overrides default alsa settings
# To revert back to CRAS, change "type hw" with "type cras". You do not have to change "card".

pcm.!default { 
    type hw
    card bdwrt5677
}

ctl.!default { 
    type hw
    card bdwrt5677
}

Test

Test ALSA controls with alsamixer. If you see the controls for your desired hardware device and not CRAS, congrats!

To go back to running audio on ChromeOS, change type hw to type cras in your ~/.asoundrc and run 'sudo start cras'.


JACK to hardware

Install JACK

JACK1 and JACK2 are almost completely compatible and both are actively maintained. That being said, you can only choose one...

jackd2

sudo apt-get install jackd2

I found jack2 to be quite finicky with DBUS issues, so make sure you uncheck Enable DBUS Interface in Qjackctrl -> Setup -> Misc

jackd1

sudo apt-get install jackd1

You may need to remove jackd2.

A prompt may pop up to enable realtime process priorities. Enter yes.

Set up ALSA

Make sure ALSA is talking directly to the hardware via the earlier steps, as you will be running ALSA as your JACK driver backend.

Stop CRAS

sudo stop cras

Set up realtime process priorities

Edit security policy configuration

On Ubuntu, the JACK installation should bring up a prompt if realtime process priorities should be enabled. If you missed it, you can re-run it with sudo dpkg-reconfigure -p high jackd. This creates a a file at /etc/security/limits.d/audio.conf with PAM rules granting your user to unlimited memory access and higher CPU priorities.

You may also do this manually, appending the following lines to the bottom of your /etc/security/limits.conf:

@audio -  rtprio     95
@audio -  memlock    unlimited

Enable PAM

Source

Crouton uses su for login, which by default in Ubuntu has PAM user limits disabled. We need to enable it.

Uncomment the following line in /etc/pam.d/su

# Sets up user limits, please uncomment and read /etc/security/limits.conf
# to enable this functionality.
# (Replaces the use of /etc/limits in old login)
session    required   pam_limits.so

Test

Log out and log back in. Run ulimit -r -l to see if your user now has a more permissive security policy.

Your output should be as follows:

$ ulimit -r -l
-l: locked-in-memory size (kbytes)  unlimited
-r: max rt priority                 95

Fiddle with qjackctrl/jackd

Either with qjackctl(gui) or jackd (cli), point your audio device to hw:bdwrt5677 or whatever your audio device is called. You may have to fiddle with the settings/parameters, as certain combinations of options will not run with your device. It can also help to prefix these with the pasuspender keyword, to temporarily suspend pulseaudio while running the gui or cli.

Example ~/.jackdrc: (automatically generated via qjackctl)

/usr/bin/jackd -dalsa -dhw:bdwrt5677 -r44100 -p1024 -n2

Another example ~/.jackdrc:

/usr/bin/jackd -dalsa -dhw:Audio -r48000 -p1024 -n2 -P

Note the -P, which is the flag for playback only. For this particular USB audio device, JACK threw errors and refused to run duplex mode. Your audio device may behave similarly and require fiddling with.

Test

Once you have JACK running error free, test it with mplayer -ao jack song.mp3. Change your volume settings via alsamixer. If it all works, congrats! Now you have realtime audio superpowers on your Chromebook!

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