[alsa-devel] Can module snd-aloop (virtual looback device) be made clock slave of a real audio device ?

Jaroslav Kysela perex at perex.cz
Mon Aug 20 12:26:57 CEST 2012

Date 20.8.2012 11:51, James Warden wrote:
> Hello alsa-devel's,
> A while back, I had written a rather hackish method to hook up the
> ALSA virtual loopback device to jack (jackaudio.org) via Jack clients
> alsa_in / alsa_out so that non jack-aware applications that can
> however use ALSA devices "directly" (via alsa-lib) can have their
> audio stream redirected to jack (see my rather lengthy prose at
> http://alsa.opensrc.org/Jack_and_Loopback_device_as_Alsa-to-Jack_bridge).
>  There are a few advantages in this method (depends on your use-cases
> of course). But it could be improved (or so I think) by removing the
> following drawbacks:
> The alsa_in/out jack clients are resampling the audio stream to
> synchronize with the device that jack is using. The resampling, done
> by libsamplerate, takes some CPU power. Since many users are using my
> recipe on laptops, this is not always welcome.
> Because snd-aloop is a virtual device, couldn't it be made a clock
> slave of the h/w device that jack is operating on ? This would in
> principle remove the need for resampling. I could imagine that the
> module could be passed an option at loading time (e.g. modprobe
> snd-aloop clock-master=<real h/w device index> or something like
> that)

Use 'PCM Rate Shift 100000' mixer control to fine tune the rate (timing)
for the snd-aloop PCM devices. An example using this solution is the
alsaloop application from the alsa-utils package.

Also note that other controls can expose the audio format on the
playback side, so you can avoid resampling in the alsa-lib completely
for ALSA audio apps. The alsaloop application can give you more hints.


Jaroslav Kysela <perex at perex.cz>
Linux Kernel Sound Maintainer
ALSA Project; Red Hat, Inc.

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