[pulseaudio-discuss] AC3 passthrough on SPDIF

[Jon Smirl]

On 5/27/08, Rick W. Chen <stuffcorpse at gmail.com> wrote:
> Jon Smirl said the following on 26/05/08 16:13:
>
> > Mplayer is failing inside pulse when it opens the audio stream. Other
>  > apps seem to work.
>  >
>  > ---------------------------------------
>  >
>  > (totem:10043): GStreamer-CRITICAL **: gst_object_unref: assertion
>  > `object != NULL' failed
>  >
>  > (totem:10043): GStreamer-CRITICAL **: gst_object_unref: assertion
>  > `object != NULL' failed
>  > ** Message: Error: Failed to connect stream: OK
>  > pulsesink.c(411): gst_pulsesink_prepare ():
>  > /play/abin/audiosinkbin/audio-sink/bin6/pulsesink1
>  >
>  >
>  > (totem:10090): GStreamer-CRITICAL **: gst_object_unref: assertion
>  > `object != NULL' failed
>  >
>  > (totem:10090): GStreamer-CRITICAL **: gst_object_unref: assertion
>  > `object != NULL' failed
>  >
>  >
>
>
> for mplayer I just tell it to use the default alsa device, which is the
>  pulseaudio pcm from the guide. it's working nicely.

mplayer is failing when I set it to use "AC3 passthrough". Setting it
to "5.1-channel" works.

Pulse is probably rejecting the direct AC3 feed.

Sending pulse 5.1 audio causes an unneccessary AC3 decode/encode load
to occur. This is continuously consuming about 30% of one of my cores.
An optimization to pulse would be to take the AC3 stream directly from
mplayer and then only decode/reencode if mixing is required. It would
probably be more efficient if pulse decoded/reencoded the ac3
internally too. That would let you control latencies better.

I'm experiencing a few problems
   VLC sound is garbled, other app are ok
   I get occasional drop outs, probably due to realtime decoding latencies.
  my stereo music is getting copied to all 6 channels, that defeats
the DSP processing in my amp (which sounds way better than a simple
copy).

On the plus side it works on 64b with flash.

This is a good feature to add to pulse. I suspect that most people
using SPDIF don't know that they are only getting stereo sound.  The
feature is mostly working, but it is way too hard to set up.

Fully implementing this is probably tied to pulse supporting
processing on compressed streams. For example pulse could allow mp3,
ac3 and pcm inputs. Then depending on which streams were active and
what the output device was it could decide if decode/reencode was
necessary and to what format. Adding this to pulse would allow network
support of compressed streams, something I'm interested in.

AC3 has patents and royalties, but the licensing site says the royalty
is zero for under 1M copies per product. Are any of the distributions
exceeding 1M installs? If the 1M copy limit is a problem we could
always ask for an exception. Of course there is also the mp3 strategy
where you install it on your own.


>
>
>  --
>
>   Rick
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-- 
Jon Smirl
jonsmirl at gmail.com