Canon EOS Rebel T3 18-55mm IS II Kit Instruction Manual page 151

Movie Shooting Cautions
Playback and TV connection
If the brightness changes during movie shooting, that part might look
momentarily still when you playback the movie.
If you connect the camera to a TV set with an HDMI cable (p.191) and
shoot a movie, the movie being shot will be displayed at a small size on
the TV. But the movie will still be properly recorded in High-Definition
(HD) quality.
If you connect the camera to a TV set and shoot a movie, the TV will not
output any sound during the shooting. However, the sound will be
properly recorded.
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