Can I use sound card hardware ac3/dts to decode ac3/dts audio stream ?

[adrianmak]

all sound blaster card models after audigy has internal ac3/dts hardware decoder. DVD software players like windvd/powerdvd can use sound card hardware decoder (by switch channel to 5.1 ???).

how about when playback xvid/divx/x264 avi/mkv with ac3/dts audio stream, do software player like MPC, kmplayer able to use sound card hardware decoder ?

Such that it can off load the audio decoder process from cpu.

[Skelsgard]

Not all after Audigy. For example, the Audigy 2 has Dolby but no DTS decoding, while Audigy 2 ZS has both.

Yes, you can use the soundcard´s built-in decoder.
You need to send the audio from the player thru SPDIF. Then select in Soundblaster´s Decoder tab the "Use Built-in decoder" option, and specify the options for Dolby and DTS streams.
Souncards without built-in decoding just output this SPDIF signal thru the SPDIF output, whereas your soundcard will pick-up the signal and decode it as specified (I know this is how Soundblaster cards work, as I own an Audigy 2 ZS myself, but I don´t how other cards with bult-in decoding, if there are others, would configure their output).
If you use Ac3Filter for Ac3 and DTS decoding, the setings must be "Use SPDIF" + PCM 24bit. Neither volume nor any other option in AC3Filter will hace any effect when in SPDIF mode.

Cheers.