Canon 1D X - Manual (Page 219)

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Shooting Movies
The live image displayed on the camera’s LCD monitor
can be recorded as a movie to the card. The movie
recording format will be MOV.
Cards that Can Record Movies
When shooting movies, use a large-capacity card with a
fast writing/reading speed as shown in the table.
If you use a slow-writing card when shooting movies, the movie
may not be recorded properly. Also, if you play back a movie on
a card with a slow reading speed, the movie may not play back
properly.
If you want to shoot still photos while shooting a movie, you will
need an even faster card.
To check the card’s reading/writing speed, refer to the card
manufacturer’s Web site.
Compression Method
(p.233)
IPB 10 MB/sec. or faster
ALL-I (I-only) 30 MB/sec. or faster
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