QNAP Security VioStor NVR (Version: 3.3.2) - Manual (Page 179)

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 Pre-/Post-alarm Recordings
 Start recording video…second(s) before the event occurs: Enter the
number of seconds to start the recording before an event occurs.
 Stop video recording…second(s) after the event ends: Enter the number of
seconds to stop the recording after an event ends.
The maximum number of seconds for the above settings is 300, i.e. 5 minutes.
Note: All the settings will not be effective until you click ‘Apply’. When applying the
changes, the recording will stop for a while (maximum 1 minute) and then restart.