Setting up surround sound
After spending your hard earned cash on a surround sound system, it would be
nice to link this into your MythBox so that all sound is sent to your
amplifier as a data stream. For music and recordings, the benefit of this is that
the sound quality is maintained between your amp and box. Also, most amps
can feed sound through a Pro Logic matrix, meaning that your amp will analyse
the sound stream and predict what sounds should be fed through the rear
surround channels. This is better than mirrored stereo channels at the front
and rear because sound can be directed through any one channel.
In the case of DVD playback, the surround sound information is stored in the Dolby
Digital or DTS soundtracks on the disc so no prediction by your amp is
necessary. However, to allow your amp to utilise these soundtracks properly we
need to output a bytestream rather than a downmixed stereo version. To do all
of this, carry out the following steps:
- We need to make sure all sound is output at 48k including music, so that
ALSA sound doesn't break. Edit
/etc/modules.d/alsa, and add:
# Make all output 48k
options snd-via82xx dxs_support=5
Next we need to create /home/mythtv/.asoundrc and add the following:
pcm.!default {
type plug
slave.pcm "spdif"
slave.rate 48000
}
- Run
modules-update so that the changes take effect.
- Now run
alsamixer and adjust the IEC958 fader to be 0. This will enable the
SPDIF output.
- To enable AC3 pass-through in Xine, we need to edit the config file in
/home/mythtv/.xine/config. Edit the line beginning with
audio.output.speaker_arrangement to read as follows:
audio.output.speaker_arrangement:Pass Through
- We shouldn't need to make any changes to Myth TV for it to start
outputing sound through SPDIF. However, to avoid problems check out the
General menu from the main Setup page. Check that the sound device used is
ALSA:default. Also ensure that the internal volume controls and
aggressive sound card buffering are not being used.
Troubleshooting:
- On some channels, the audio and video seems to be really choppy!
There are some issues with using earlier versions of ALSA causing video and audio to become choppy.
Since we are using the version of ALSA that comes with the kernel, you need to make sure you are
using at least kernel 2.4.16 (this is the version I have it working with).
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