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1.my sound card is THX certified but my speakers aren't, have I a better sound quality or not ? Because my speakers don't have THX certified.
THX certified speakers would
probably sound better. But, I'm sure that there are plenty of really good non-certified speakers. A certified amp/speakers
probably has more power and bigger woofers. As I understand it, the amp/speakers are supposed to reproduce theater-quality sound at theater-volumes in your home.
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2.The audigy 2 zs has dts and dolby EX decoder, does anyone knows if it is hardware based or software emulation ?
I don't know, but you can check your CPU utilization (task manager) to see if enabling either of these increases your CPU usage.
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3.If I want to enjoy dts and dolby digital ex sound , I must have a dts-dolby ex sound card, my speakers must support dts-dolby ex... ?
You need a decoder
somewhere. If you are hooked-up to the soundcard's analog outputs, the decoding has to be done by the soundcard before the digital-to-analog conversion. If you have a S/PDIF connection (coax or optical), the decoding can be done externally.
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...and the movie must have dts-dolby ex sound ?
Maybe... With Dolby EX, the additional EX channel(s) is not discrete-digital. It's derived-matrix (like Pro logic). So, with a 5.1 channel mix, you'll probably get
something out of the "extra" speaker even if the movie isn't EX encoded. As I understand it, DTS-EX
does have an additional discrete-digital channel. But again, I assume
something will be derived for this speaker/channel with a 5.1 channel DTS track.
P.S.I don't think your G51 is an "EX" speaker system... For EX, you a need 6.1 or 7.1 system.
Most "current" DVDs have a Dolby 5.1 track
and a Dolby "stereo track". The stereo track (LPCM or Dolby AC3) is usually matrix-encoded with "Dolby Surround" (like most VHS tapes), and it can be decoded with a Pro Logic system set to the "movie mode".
DTS is optional, and the DTS decoder works
only if you have a DTS track.