Increase volume over analogue output in Raspbmc

If you are using Raspbmc with analogue audio out you may notice the volume is too low on certain video files.

This can be fixed for most videos* by setting the speaker configuration to “2.0”, activating “Boost volume level on downmix”, (in XBMC audio settings) and then editing the gain value for this setting using the XBMC advancedsetting.xml file. (This is not possible to do via the GUI)

To get started, run:

cd /home/pi/.xbmc/userdata/
touch advancedsetting.xml
nano advancedsetting.xml

Paste the following into the editor:

<advancedsettings>
  <audio>
    <ac3downmixgain>18.0</ac3downmixgain>
  </audio>
</advancedsettings>

Save the file and reboot. The volume should now be increased on video files with 5.1 audio tracks. Note that high values for ac3downmixgain will cause distortion. Try a loud scene to see if you get any.

Note
The default value for ac3downmixgain is 12.0. I found 18.0 works for my needs.
*Videos with a stereo track will not have their volume increased.

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