[pulseaudio-discuss] Help with CMI8768 Please

Lennart Poettering lennart at poettering.net
Sun Sep 13 10:49:33 PDT 2009

On Sat, 12.09.09 10:54, Jim Duda (jim at duda.tzo.com) wrote:

> load-module module-alsa-sink device=a52encode rate=48000 channels=6
> sink_name=alsa_surround

PA should be able to detect a properly set up a52 alsa module
automatically. Debian does it right afaik. 

> Then I get the proper upmixing provide by pulse.  The alsa a52 plugin is doing
> the analog to digital encoding to dolby digital for my surround
> system.

Hmpf. No. a52 has nothing to do with "analog" or "digital". if you use
the "a52" or "spdif" alsa output always outputs digital audio on the spdif
port, however when a52 is used it is encoded in a AC-3 compressed
stream while when using "spdif" it is encoded in uncompressed PCM.

> Unfortunately, although this works great for music, attempting to play mpeg
> videos is a problem.  When I use mplayer to play mpeg videos with 5 channel
> sound, mplayer uses liba52 to decode the channels.  When playing through
> pulse and subsequently a52 (to go back to digital), the sound is all choppy.
> I suspect this has to do with the a52 plugin as I've read in various posts
> that it may not be thread safe.

Please note that a52 is little tested. Fedora does not ship this ALSA
pugin for patent reasons and hence I never actually tested it. Also,
first decoding AC3 to PCM in mplayer and then encoding it again to AC3
in PA certainly won't help audio quality. I fear as long as PA can
only deal with PCM properly it's best to bypass it if you want to do
AC3 digital output.


Lennart Poettering                        Red Hat, Inc.
lennart [at] poettering [dot] net
http://0pointer.net/lennart/           GnuPG 0x1A015CC4

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