jaguarskx is correct. Most TVs do not have a Video Output (especially with a modern HDMI connection). What ever source you use (cable box, DVD player, etc) would have to be connected to the computer, and the video sent from the computer to the TV. This would allow a video capture card to record that video stream.
Keep in mind however, many digital media forms (specifically CD/DVD/Blu-Ray) are encoded to prevent recording due to copyright protections.