[mythtv-users] How to set digital audio passthrough in 0.24.1-fixes?

Jean-Yves Avenard jyavenard at gmail.com
Sat Jul 2 23:05:42 UTC 2011


On Sunday, 3 July 2011, Victor <vperez at rwserver.com> wrote:

&gt;
&gt; That is how my settings are, yet it doesn't work, see screenshot
at http://twitpic.com/5k3yle
&gt;
&gt; I get audio but it is somehow processed by mythtv, no
passthrough. Most of my HDHomerun recordings have Dolby 5.1 audio, my
Onkyo receiver used to light up the little red &quot;Dolby&quot; logo
when I played them before the upgrade. It also lighted up a little
&quot;DTS&quot; logo in the mkv videos that had it. It doesn't
anymore, I just get plain old stereo :-(
&gt;
&gt; Little audio debugging, playing back a mkv with DTS audio:
&gt;
&gt; 2011-07-02 10:47:38.304 AO: enc(0), passthru(0), canAC3(1),
canDTS(1), canLPCM(1), configured_channels(6), 6 channels supported(1)
&gt; 2011-07-02 10:47:38.304 AO: Opening audio device
'hdmi:CARD=NVidia,DEV=1' ch 6(6) sr 48000 sf signed 16 bit reenc 1
&gt; 2011-07-02 10:47:38.305 ALSA: SetParameters(format=2, channels=6,
rate=48000, buffer_time=500000, period_time=16)
&gt;
&gt; Btw: hdmi:CARD=NVidia,DEV=1 is the only one that outputs audio
&gt;

It is outputting 6 channels as your output is multi channels capable.

For AC3 audio to be decoded by myth and output as 6 channels PCM means
some audio processing is being performed.. This can happen when you
enable time stretch, volume control, up mixing etc...

Not seeing the configuration, I'm guessing you've enabled software
volume control..

But everything is behaving properly here as far as myth is concerned:
it is outputting 5.1 LPCM in your log.

Now if all you are hearing is stereo, it means your amplifier, while
hdmi capable does support multi channels PCM which is rather crap
being the whole point of hdmi.
If that's the case, in the advanced audio option, there is a StereoPCM
option that you will have to check. This will force multi channels
audio to be re-encoded as AC3.

Keep in mind that AC3 is a lossy codec, and it is better to keep LPCM
than re-encoding to AC3 and this is why the decision to no re-encode
to AC3 when processing the audio was taken (that and it uses less CPU)


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