Canon EOS-1DX Instruction Manual page 250

Movie Shooting Cautions
Recording and Image Quality
If you use a card with a slow writing speed, a five-level
indicator may appear on the right of the screen during movie
shooting. It indicates how much data has not yet been
written to the card (remaining capacity of the internal buffer
memory). The slower the card, the faster the indicator will
climb upward. If the indicator becomes full, movie shooting
will stop automatically.
If the card has a fast writing speed, the indicator will either
not appear or the level (if displayed) will hardly go upward.
First, shoot a few test movies to see if the card can write fast enough.
Still Photo Shooting During Movie Shooting
Regarding the image quality of still photos, see "Image Quality" on page
223.
Playback and TV Connection
In autoexposure shooting, shutter-priority AE, or aperture-priority AE
modes, if the brightness changes during movie shooting, the movie may
freeze temporarily. In such cases, shoot movies with manual exposure.
If you connect the camera to a TV set (p.275, 278) 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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