This option is only available for the 90xx-ST, 96xxNI-ST/RT and 7716NI-SP/16 series with firmware v3.0.2+. Log into the DVR/NVR locally,
Go to MENU, select SYSTEM CONFIGURATION and then select GENERAL (on left side).
Under MORE SETTINGS tab, make sure the option "Enable VGA/HDMI Synchronous Output" is unchecked.