Using ALSA’s dmix plug-in with VirtualBox

 Audio, Gentoo, virtualization  Add comments
Jun 172009
 

Introduction

I recently switched from using Linux’s KVM to using Sun’s VirtualBox for virtualizing a Windows XP guest on a Gentoo Linux host, and I have been quite pleased. The only feature that I don’t have working is clipboard-sharing, but that’s a problem for another day. Previously, I had major problems with sound using my EchoAudio Mia card. Ultimately, I used ALSA’s dmix plug-in to mix audio streams in software, before feeding it to the card. That worked great! However, VirtualBox was complaining about the new setup, and start-up was defaulting the audio component to “null”. In other words, my VM guest had no audio support!

Two problems, Two Solutions

The primary problem was that I had enabled the “esd” USE flag for my Gentoo system, which enables Enlightenment’s Enlightened Sound Daemon (ESD, or ESounD). Apparently, the latest version of SDL (libsdl) does not function properly in the ALSA environment, if the esd USE flag is set. Well, VirtualBox depends on SDL for audio support; consequently, my VM guest could not produce audio. The solution was to add the “-esd” USE flag to /etc/make.conf and rebuild the dependent packages (emerge -uDN world).

The second problem stems from my use of ALSA’s dmix plug-in. Although this works fine for playback, the dmix plug-in cannot be used for capture. Therefore, an environment variable must be set to specify an alternative capture source, like so:

# Unset any output customizations, use default
$ unset VBOX_ALSA_DAC_DEV
# Specify capture (analog-to-digital converter) device - can also be added to ~/.bashrc
$ export VBOX_ALSA_ADC_DEV="hw:0,0"
# Launch my Windows guest VM
$ VBoxManage startvm "Windows XP SP2"
# wait for it...
$ sleep 3
# check log for results:
$ grep -Pi '(alsa|audio)' VBox.log
VirtualBox Command Line Management Interface Version 2.2.4
(C) 2005-2009 Sun Microsystems, Inc.
All rights reserved.
 
Waiting for the remote session to open...
Remote session has been successfully opened.
00:00:00.808 [/Devices/AudioSniffer/] (level 2)
00:00:00.808 [/Devices/AudioSniffer/0/] (level 3)
00:00:00.808 [/Devices/AudioSniffer/0/Config/] (level 4)
00:00:00.808 [/Devices/AudioSniffer/0/LUN#0/] (level 4)
00:00:00.808   Driver <string>  = "MainAudioSniffer" (cch=17)
00:00:00.808 [/Devices/AudioSniffer/0/LUN#0/Config/] (level 5)
00:00:00.808   Driver </string><string>  = "AUDIO" (cch=6)
00:00:00.808   AudioDriver </string><string>  = "alsa" (cch=5)
00:00:01.090 Audio: Trying driver 'alsa'.
00:00:01.093 Audio: set_record_source ars=0 als=0 (not implemented)
00:00:01.095 ALSA: ADC frequency 44100Hz, period size 1024, buffer size 4096
00:00:01.099 ALSA: DAC frequency 44100Hz, period size 940, buffer size 3763</string>

Now I could hear audio output from my VM guest, simultaneously while listening to audio from host applications, and I could also capture audio!

Incidentally, I extended my default ~/.asoundrc file a little, although it did not seem to make a difference:

pcm.!default {
    type plug
    slave.pcm "dmix"
}
pcm.dsp0 {
    type plug
    slave.pcm "dmix"
}
ctl.mixer0 {
    type hw
    card 0
}

References

  1. http://forums.virtualbox.org/viewtopic.php?f=7&t=18835
  2. http://forums.gentoo.org/viewtopic-p-5714588.html#5714588
  3. http://www.virtualbox.org/ticket/1908
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