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Optical and digital coax don't have enough bandwidth capability to carry the new lossless codecs like Dolby TrueHD and DTS-HD. You need HDMI or multi-channel analog connections for those.
Also, if you don't know, you should check how your TV handles audio going from HDMI inputs to the optical output. Some TVs will only output a two-channel stereo signal through the optical output if you are listening to an HDMI source.