Myth BOX
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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:
  1. 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
    }
    
  2. Run modules-update so that the changes take effect.
  3. Now run alsamixer and adjust the IEC958 fader to be 0. This will enable the SPDIF output.
  4. 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
    
  5. 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).