Philips PCA646VC Installation Manual page 18

Framerate
The framerate setting determines the number of pictures per second
in the video stream.
The set of framerates from which the user may choose depends on
the currently selected video image format and the available bandwidth
on the USB bus (the number of devices connected to your USB port
and the amount of data they transport).The buttons for invalid
framerates will be automatically greyed out and cannot be selected.
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