How to get HDMI sound out of a Radeon Device in Ubuntu Karmic

After much searching of the Interweb I've managed to get sound out of my TV when connected to my Acer Aspire 5536 via HDMI.

It turns out that Ubuntu Karmic has full support on the Audio side for the sound part of the RS780 chipset. Pulseaudio picks it up and the new volume control lets you select the output effortlessly.

Unfortunately the Radeon Video driver that Ubuntu uses by default doesn't support 'HDMI Sound pass through', so the bits never get to the TV.

Fortunately there another Radeon driver - 'radeonhd' and that does have HDMI sound support, but it doesn't detect it automatically so you have to configure it in the good old 'xorg.conf'.

To get the driver, first make sure you have the 'universe' repository activated and then install the driver via apt.

apt-get install xserver-xorg-video-radeonhd

and then edit /etc/X11/xorg.conf to add in the following:

Section "Device"
      Identifier      "ATI Radeon HD 3200/RS780"
      Driver          "radeonhd"
      Option          "DRI" "On"
      Option          "Audio" "On"
      Option          "HDMI"  "all"
EndSection
Section "DRI"
      Mode 0666
EndSection

(Change the Identifier as required so that it ties in with the rest of the configuration).

The DRI Option activated direct rendering, which again doesn't active by default for certain chipsets. The DRI section allows non-privileged commands to access direct rendering.

Details of the options and default are on the radionhd manual page that is installed alongside the driver.