[pulseaudio-discuss] Virtualbox sound management

Fabrice Beauvir fabrice.beauvir at gmail.com
Tue Oct 19 06:15:36 PDT 2010

Hello Colin,
 Thank you for your answer , So I fear that it is not possible to have
several "fake" sinks (one per VM)....

Unfortunatly when I load a other fake one, got 'failed'

see above
administrateur at 23390hd61252:~$ pacmd load-module module-null-sink
Welcome to PulseAudio! Use "help" for usage information.
>>> Module load failed.
>>> administrateur at 23390hd61252:~$

So... I fear that I have to ask for a sound card (it is a server "in box" ,
can be not easy)

Is it right ?

2010/10/19 Colin Guthrie <gmane at colin.guthr.ie>

> 'Twas brillig, and Fabrice Beauvir at 19/10/10 10:41 did gyre and gimble:
> > On a Ubuntu 9.04 server running pulse audio without any sound card I run
> > 2 Windows XP virtual box .
> >
> >  Sound is Ok and on both of them , I am playing on the current virtual
> > sound card (mapped on each vbox  instance) and stream directly using
> > virtual card as input on each .
> >
> > but....
> >
> >  On both stream sound is mixed , I heard the 2 sounds mixed out  on each
> > one !!!
> Ahh right, yes this is simple to explain, but rather more complex to fix.
> Ultimately PulseAudio uses a "module-null-sink" when no real hardware is
> found. This means that a dummy sound card is essentially provided. This
> is created automatically by module-always-sink so that applications
> generally find "somethign" to output to (as without this, alsa
> applications going via alsa->pulse bridge often got, erm, "confused" is
> a nice term :D)
> Anyway, the point is that this module only provide a virtual null
> *output*, and there is no virtual null *input*.
> Well, I say there is no null input but there *is* a "monitor" input for
> the virtual output (a monitor basically acts as a pretend input that
> allows you to record what is being currently played - think of how bands
> perform at live gigs - they use monitors there to make sure they don't
> sound crap!)
> Anyway, when VirtualBox is recording it has to record form some source
> in PA. As a side effect of the virtual output, the *only* input it can
> record from is the montitor of this virtual input. As both VMs output to
> the same virtual null sink in PA, their sound is mixed. Therefore when
> either of them record, they get the product of the mixed audio from the
> monitor.
> So that explains things. How to fix?
> Well there is no module-null-source (there arguably should be on written
> but no one has done this yet).
> So a trick here would be to load a second null sink and set *it's*
> monitor source as the default source, but never actually play anything
> on it.  This way you can happily "record" the silence inside the VMs and
> that should be OK.
> Hope that makes sense!
> You'd have to do something like:
> pacmd load-module module-null-sink sink_name=fake
> pacmd set-default-source fake.monitor
> (you can also put this in your default.pa so it's done automatically)
> You'll likely need to restart your VMs to make sure they then use the
> right devices.
> Hope that helps.
> Col
> --
> Colin Guthrie
> gmane(at)colin.guthr.ie
> http://colin.guthr.ie/
> Day Job:
>  Tribalogic Limited [http://www.tribalogic.net/]
> Open Source:
>  Mageia Contributor [http://www.mageia.org/]
>  PulseAudio Hacker [http://www.pulseaudio.org/]
>  Trac Hacker [http://trac.edgewall.org/]
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