There are
two factors that govern the selection of cable: the location of
cable runs, either indoor or outdoor, and the maximum length of
the individual cable runs.
Video coaxial
cable is designed to transmit maximum signaling energy from a
75 ohm source to a 75 ohm load with minimum signal loss. Excessive
signal loss and reflection occurs if cable rated for other than
75 ohms is used. Cable characteristics are determined by a number
of factors (core material, dielectric material and shield construction,
among others) and must be carefully matched to the specific application.
Moreover, the transmission characteristics of the cable will be
influenced by the physical environment through which the cable
is run and the method of installation.
Use only high
quality cable and be careful to match the cable to the environment
(indoor or outdoor). Solid core, bare-copper conductor is best
suited to video applications, except where flexing occurs. In
locations where the cable must be continuously flexed (i.e., when
used with scanners or pan & tilts), use cable intended for
such movement. This cable will have a stranded wire core. Use
only cable with pure copper stranding. Do not use cable with copper-plated
steel stranding because it does not transmit effectively in the
frequency range used in CCTV.
The preferred
dielectric material is foam polyethylene. Foam polyethylene has
better electrical characteristics and offers the best performance
over solid polyethylene, but it is more vulnerable to moisture.
Use cable with solid polyethylene dielectric in applications subject
to moisture.
In the average
CCTV installation, with cable lengths of less than 750 feet (228
m),RG59/U cable is a good choice. Having an outside dimension
of approximately 0.25 inches, it comes in 500-and 1,000-foot rolls.
For short
cable runs, use RG59/U with a 22-gauge center conductor, which
has a DC resistance of about 16 ohms per 1,000 feet (304 m). For
longer runs, the 20-gauge variety which has a DC resistance of
approximately 10 ohms per 1,000 feet will work well. In either
case, cables with polyurethane or polyethylene as the dielectric
material are readily available.
For installations
requiring cable runs between 800 (244 m) and 1,500 feet (457 m),RG6/U
is best. Having the same electrical characteristics as RG59/U,
its outer dimension also is about equal to that of RG59/U.RG6/U
comes in 500-,1000-and 2000-foot rolls, and it may be obtained
in a variety of dielectric and outer-jacket materials. Due to
its large-diameter center conductor of about 18 gauge,RG6/ U has
a DC resistance of approximately 8 ohms per 1,000 feet (304 m)
and can deliver a signal farther than RG59/U.
Use RG11/U
to exceed the capability of RG6/U. Once again, the electrical
characteristics of this cable are basically the same as the others.
The center conductor can be ordered in 14-or 18-gauge sizes, producing
a DC resistance of approximately 3-8 ohms per 1,000 feet (340
m). Being the largest of the three cables at 0.405 inches, it
is more difficult to handle and install.RG11/U cable usually is
delivered in 500-,1000-and 2000-foot rolls.
Because of
special applications, variations of RG59/U, RG6/U and RG11/U frequently
are introduced by manufacturers.
Due to changes
in fire and safety regulations throughout the country, Teflon
and other fire-retardant materials are becoming more popular as
outer-jacket and dielectric materials. In case of a fire, these
materials do not give off the same poisonous fumes as PVC-type
cables, and therefore, are considered safer.
For underground
applications, direct burial cables, made specifically for that
purpose are recommended. The outer jacket of this type of cable
contains moisture-resisting and other materials that protect the
cable, allowing it to be placed directly into a trench.
With numerous
choices available, finding the right video cable for each camera
application should be easy. After the installation has been properly
assessed, read the equipment specifications and complete the appropriate
calculations.
Cable
Runs
Although coax cable has built-in losses, the longer and smaller
the cable is, the more severe the losses become; and the higher
the signal frequency, the more pronounced the losses. Unfortunately
this is one of the most common and unnecessary problems currently
plaguing CCTV security systems as a whole.
If, for example,
your monitor is located 1,000 feet (304 m) from the camera, approximately
37-percent of the high frequency information will be lost in transmission.
The unfortunate aspect of this condition is that it is not obvious.
You cannot see information that is not there and may not even
realize that information has been deleted. Because many CCTV security
systems have cable runs that exceed several thousand feet, unless
you are aware of this characteristic of cable, your system may
be providing a seriously degraded image.
So, if your
cameras and monitors are separated by lengths greater than 750
feet (228 m), you should check to make certain that some provision
has been made to guarantee the video signal's transmission strength.
Cable
Termination
In video security systems, camera signals must travel from the
camera to the monitor. The method of transmission is usually "coax"
cable. Proper termination of cables is essential to a system's
reliable performance.
Because the
characteristic impedance of coax cable ranges from 72 to 75 ohms,
it is necessary that the signal travels on a uniform path along
any point in the system to prevent any picture distortion and
to help ensure proper transfer of the signal from the camera to
the monitor. The impedance of the cable must remain constant with
a value of 75 ohms. To properly transfer power between two video
devices with acceptable losses, the signal output from the camera
must match the input impedance of the cable, which in turn must
match the input impedance of the monitor. The end point of any
video cable run must be terminated in 75 ohms. Usually, the cable
run will end at the monitor, which will ensure that this requirement
is met.
Usually the
video input impedance of the monitor is controlled by a switch
located near the looping video (input/output) connectors. This
switch allows for either 75 ohm termination if the monitor is
the "end point",or Hi-Z for looping to a second monitor.
Check equipment specifications and instructions to determine the
proper termination requirements. Failure to terminate signals
properly usually results in a high contrast, slightly grainy picture.
Ghosting and other signal imperfections also may be evident.
Selecting
Video Cable
There
are two factors that govern the selection of cable: the location
of cable runs, either indoor or outdoor, and the maximum length
of the individual cable runs.
Video coaxial
cable is designed to transmit maximum signaling energy from a
75 ohm source to a 75 ohm load with minimum signal loss. Excessive
signal loss and reflection occurs if cable rated for other than
75 ohms is used. Cable characteristics are determined by a number
of factors (core material, dielectric material and shield construction,
among others) and must be carefully matched to the specific application.
Moreover, the transmission characteristics of the cable will be
influenced by the physical environment through which the cable
is run and the method of installation.
Use only high
quality cable and be careful to match the cable to the environment
(indoor or outdoor). Solid core, bare-copper conductor is best
suited to video applications, except where flexing occurs. In
locations where the cable must be continuously flexed (i.e., when
used with scanners or pan & tilts), use cable intended for
such movement. This cable will have a stranded wire core. Use
only cable with pure copper stranding. Do not use cable with copper-plated
steel stranding because it does not transmit effectively in the
frequency range used in CCTV.
The preferred
dielectric material is foam polyethylene. Foam polyethylene has
better electrical characteristics and offers the best performance
over solid polyethylene, but it is more vulnerable to moisture.
Use cable with solid polyethylene dielectric in applications subject
to moisture.
In the average
CCTV installation, with cable lengths of less than 750 feet (228
m),RG59/U cable is a good choice. Having an outside dimension
of approximately 0.25 inches, it comes in 500-and 1,000-foot rolls.
For short
cable runs, use RG59/U with a 22-gauge center conductor, which
has a DC resistance of about 16 ohms per 1,000 feet (304 m). For
longer runs, the 20-gauge variety which has a DC resistance of
approximately 10 ohms per 1,000 feet will work well. In either
case, cables with polyurethane or polyethylene as the dielectric
material are readily available.
For installations
requiring cable runs between 800 (244 m) and 1,500 feet (457 m),RG6/U
is best. Having the same electrical characteristics as RG59/U,
its outer dimension also is about equal to that of RG59/U.RG6/U
comes in 500-,1000-and 2000-foot rolls, and it may be obtained
in a variety of dielectric and outer-jacket materials. Due to
its large-diameter center conductor of about 18 gauge,RG6/ U has
a DC resistance of approximately 8 ohms per 1,000 feet (304 m)
and can deliver a signal farther than RG59/U.
Use RG11/U
to exceed the capability of RG6/U. Once again, the electrical
characteristics of this cable are basically the same as the others.
The center conductor can be ordered in 14-or 18-gauge sizes, producing
a DC resistance of approximately 3-8 ohms per 1,000 feet (340
m). Being the largest of the three cables at 0.405 inches, it
is more difficult to handle and install.RG11/U cable usually is
delivered in 500-,1000-and 2000-foot rolls.
Because of
special applications, variations of RG59/U, RG6/U and RG11/U frequently
are introduced by manufacturers.
Due to changes
in fire and safety regulations throughout the country, Teflon
and other fire-retardant materials are becoming more popular as
outer-jacket and dielectric materials. In case of a fire, these
materials do not give off the same poisonous fumes as PVC-type
cables, and therefore, are considered safer.
For underground
applications, direct burial cables, made specifically for that
purpose are recommended. The outer jacket of this type of cable
contains moisture-resisting and other materials that protect the
cable, allowing it to be placed directly into a trench.
With numerous
choices available, finding the right video cable for each camera
application should be easy. After the installation has been properly
assessed, read the equipment specifications and complete the appropriate
calculations.
Cable
Runs
Although coax cable has built-in losses, the longer and smaller
the cable is, the more severe the losses become; and the higher
the signal frequency, the more pronounced the losses. Unfortunately
this is one of the most common and unnecessary problems currently
plaguing CCTV security systems as a whole.
If, for example,
your monitor is located 1,000 feet (304 m) from the camera, approximately
37-percent of the high frequency information will be lost in transmission.
The unfortunate aspect of this condition is that it is not obvious.
You cannot see information that is not there and may not even
realize that information has been deleted. Because many CCTV security
systems have cable runs that exceed several thousand feet, unless
you are aware of this characteristic of cable, your system may
be providing a seriously degraded image.
So, if your
cameras and monitors are separated by lengths greater than 750
feet (228 m), you should check to make certain that some provision
has been made to guarantee the video signal's transmission strength.
Cable
Termination
In video security systems, camera signals must travel from the
camera to the monitor. The method of transmission is usually "coax"
cable. Proper termination of cables is essential to a system's
reliable performance.
Because the
characteristic impedance of coax cable ranges from 72 to 75 ohms,
it is necessary that the signal travels on a uniform path along
any point in the system to prevent any picture distortion and
to help ensure proper transfer of the signal from the camera to
the monitor. The impedance of the cable must remain constant with
a value of 75 ohms. To properly transfer power between two video
devices with acceptable losses, the signal output from the camera
must match the input impedance of the cable, which in turn must
match the input impedance of the monitor. The end point of any
video cable run must be terminated in 75 ohms. Usually, the cable
run will end at the monitor, which will ensure that this requirement
is met.
Usually the
video input impedance of the monitor is controlled by a switch
located near the looping video (input/output) connectors. This
switch allows for either 75 ohm termination if the monitor is
the "end point",or Hi-Z for looping to a second monitor.
Check equipment specifications and instructions to determine the
proper termination requirements. Failure to terminate signals
properly usually results in a high contrast, slightly grainy picture.
Ghosting and other signal imperfections also may be evident.
Cable
Termination
In video security
systems, camera signals must travel from the camera to the monitor.
The method of transmission is usually "coax" cable.
Proper termination of cables is essential to a system's reliable
performance.
Because the
characteristic impedance of coax cable ranges from 72 to 75 ohms,
it is necessary that the signal travels on a uniform path along
any point in the system to prevent any picture distortion and
to help ensure proper transfer of the signal from the camera to
the monitor. The impedance of the cable must remain constant with
a value of 75 ohms. To properly transfer power between two video
devices with acceptable losses, the signal output from the camera
must match the input impedance of the cable, which in turn must
match the input impedance of the monitor. The end point of any
video cable run must be terminated in 75 ohms. Usually, the cable
run will end at the monitor, which will ensure that this requirement
is met.
Usually the
video input impedance of the monitor is controlled by a switch
located near the looping video (input/output) connectors. This
switch allows for either 75 ohm termination if the monitor is
the "end point", or Hi-Z for looping to a second monitor.
Check equipment specifications and instructions to determine the
proper termination requirements. Failure to terminate signals
properly usually results in a high contrast, slightly grainy picture.
Ghosting and other signal imperfections also may be evident.
Vertical
Interval Switching
With the increased
use of time lapse video tape recorders, it has become important
to eliminate frame loss due to video sequential switching (switching
on a random basis). The significance of the loss of even a single
frame may possibly have an effect on information required in utilization
of the video tape recorder. Vertical interval switching in sequential
switchers eliminates information loss, which greatly enhances
the video tape recorders value in CCTV security applications.
Vertical interval
switching is employed in all the Pelco sequential switchers in
order to eliminate visual flashes, jitter, or roll on the monitor
or VTR.
With vertical
interval switching, the person watching the monitor may do so
with ease. (Viewing a rolling picture over a period of time tends
to be an annoying or fatiguing situation.)
What
is Vertical Interval Switching?
Vertical interval switching is a method of switching video during
that portion of the scan between video displays. The electronics
inside the Pelco sequential switchers look for the vertical sync
pulse of each of the input cameras and then switches the video
during this vertical pulse – eliminating the vertical roll
or jitter on the monitor.
For
Proper Vertical Interval Switching
For roll-free switching conditions from a vertical interval sequential
switcher, certain requirements must first be met. First, all the
cameras should have their vertical sync pulse occurring during
the same time period. To do this you must select cameras that
have a line-locked circuitry. This circuitry causes the vertical
sync pulse to occur at the same time in multiple camera installations,
that is, when the cameras are powered by the same AC power phase.
This is one of the reasons why 24 VAC power cameras are used a
great deal in larger CCTV systems because power to the camera
can be easily wired to the same AC power source.
But, what
if cameras in a larger system get their AC power from different
power phases and 24 VAC cameras cannot be used? Do you now have
to live with the vertical roll on the monitor? NO. There are a
few more options in the marketplace.
1. With cameras
that are equipped with external sync or genlock capability you
can supply external sync to all of the cameras to produce a vertical
lock. However, you must use more cable and a master sync generator
to do this, which could get quite costly.
2. The next
option available in some cameras is adjustable phase control.
This control lets the installer or dealer adjust for the differences
in the vertical sync timing which occurs between different AC
power phases. This feature again would give you a nice, roll-free
switching action
Splitting/Amplifying
the Video Signal
Keep in mind
the video signal used in CCTV equipment is nominally a one volt
peak-to-peak signal and is impedance sensitive to 75 ohms for
ideal video reproduction at the monitor. If these parameters are
not kept, then the video will degrade.
Distribution
Amplification
If the installation of a system requires viewing the video at
multiple locations from a single camera, there are a few different
ways of accomplishing this. One way is through using a distribution
amplifier. This device basically takes the single video signal
and reproduces the exact signal into multiple outputs; and in
the case of the Pelco DA104DT you would get four identical outputs.
So, if the
input signal is a one volt peak-to-peak signal you will get four
output signals of the same amplitude. Providing the run distance
for the type of coax used is kept within the specified length,
no other equipment will be needed to reproduce a nice clear video
display on each monitor. Another timesaving feature of the Pelco
DA104DT is that there are not adjustments required. Just connect
the unit, turn it on, and the installation is complete. If the
need arises where more than four signals are required, multiple
units can be linked together by simply using one of the output
signals as an input signal to the next unit, and so on.
