How to output digital audio via SPDIF (e.g. when playing DVDs)?
Outputting digital audio via SPDIF in its raw format through a PC sound card can be a tricky process. The key is to allow the original digital audio signal pass to your receiver/amplifier unaltered. There are two major components in the audio path that need to be configured properly to do so. First, your sound card needs to support SPDIF output in raw format. When configuring the sound card, make sure digital output is used (on Windows XP, you can do so from system volume control panel, check the Advanced Control item under Options, and press the Advanced button to the lower left corner, then check Digital Output Only. You may also need to use sound card’s native configuration application to allow Bass to be redirected, and disable sound card based AC3/DTS decoding. Second, your software decoder needs to allow SPDIF to pass through. Most audio decoders have per application settings. So you need to start playback; use the context menu to bring up the property page for the audio decoder
