Standard Analog Video Cable
Description:
Closed circuit simply means that the signal is not
being broadcast any where, but rather is traveling
by wire from the source to the receiver.
Usually a closed-circuit television (CCTV) system
uses a standmounted or hand-held video camera
to project a magnified image onto a video moniter
or a television (TV) screen. Traditionally video
transmission in CCTV is done using 75 ohm coaxial
cables. The video signal transmitted is generally
normal composite video signal, although some
systems use RF signals. A CCTV video signal is
comprised of both low frequency components
(horizontal and vertical sync pulse information)
and high frequency components (video information).
In order to transmit this full spectrum of frequencies
with little distortion or attenuation, it is important
to select the correct coaxial cable that meets the
specifications for CCTV transmission.
Center Conductor material made of bare copper
is recommended for optimum performance in CCTV
signal transmission. Because a CCTV video signal
is a base-band composite video with fairly low
frequency components compared to a CATV RF
signal, the low D.C. resistance that copper provides
will greatly improve the video signal transmission.
Dielectric Material of a coax cable is also another
key area that should be addressed. The dielectric
material and its composition is critical as it sets up
the electrical characteristics such as capacitance,
velocity of propagation, impedance, and attenuation
of the cable. These parameters will determine signal
strength and transmission distance. It is recommended
to choose a dielectric with excellent electrical properties
such as polyethylene or Foam PE. Such material will
give you lower capacitance and a higher velocity of
propagation. This results in a cable with low-loss
characteristics and reduced attenuation of the signal.
To improve the electrical properties even further, a gas
injected foamed composition of these materials
is recommended.
A Braided Shield is the proper type of shield for
CCTV and has two key purposes. One is to provide
a low D.C. resistance ground path and the second
is to provide shielding of outside interference from
distorting the video signal. The shielding should be
constructed of bare copper to provide a low D.C.R.
return path. It should have a 95% or better braid
coverage in order to provide adequate shielding from
outside electrical interferences. Anything less is usually
not acceotabke for CCTV. To provide increased shielding
in the RFI range, a construction of an aluminum foil tape
is acceptable as long as a high percentage copper braid
is still used to provide the low D.C.R. return path.