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U
 UHF signal.  Ultra High Frequency signal.  In television it is defined to belong in the radio
spectrum between 470 MHz and 850 MHz.
Unbalanced signal.  In CCTV, this refers to a type of video signal transmission through a
coaxial cable.  It is called unbalanced because the signal travels through the centre core and
returns via an earthed condutor.  One conductor is a fixed voltage ie earth and only the other
moves.  In a balanced line system neither conductor is referenced to earth.
Underscan.  Decreases raster size in both H and V so that all four edges of the picture are
visible on the monitor.
UPS.  Uninterruptible Power Supply.  These are power supplies used in the majority of high
security systems, whose purpose is to back-up the system when the mains power fails.  The
duration of this depends on the size of the UPS, usually expressed in output VA (or kVA), and
the hold up time expected from fully charged batteries.  There are two common types, "off line"
and "on line".  Off line uses a switch to change from mains to battery operation, thus there can
be a short break in the output.  On Line UPSs are a type where the convertor is always working,
hence there is no switch over break at the ouput..
UTP.  Unshielded Twisted Pair.  A cable medium with one or more pairs of twisted insulated
copper conductors bound in a single sheath, with no overall or individual screen.  Now the most
common method of bringing telephone signals and data to the desktop.