Well, are you watching HDTV or regular analog? Big difference. Is it an All-in-Wonder card? The AIW version has a NTSC tuner for analog, no ATSC for HDTV.
Most cards have an S-Video OUTPUT, meaning that you watch your computer on your TV, not vice-versa. You need an All-in-Wonder card for an input.
Or you could just buy a TV Tuner, which has coaxial inputs.
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