CCTV - Video Surveillance Cameras - Monitors - Security Monitoring Devices
By Al
Colombo
Copyright ©
Basics
Camera Basics
Camera Sensitivity
Video Signal Transmission
Video Monitors
Camera Lenses
Camera Swtichers
Over-The-Phone-Line Video Transmission Technology
The Closed Circuit Television (CCTV) market is one of the fastest segments of the security
industry today. One reason for this is the fact that a picture is worth a thousand words.
This is especially true in a court of law where an eye witness is required who can place
the criminal at the scene of a crime. Otherwise, chances are he won't be convicted.
Because there are so many technical aspects to a CCTV system, that only the basics
pertinent to the production of a usable video signal will be discussed. The depth of the
technical discussion will also be limited so it appeals to the averate, non-technical
audience with limited technical knowledge.
THE SIGNIFICANCE OF CCTV
Security experts in retail long ago learned that video taping a shoplifter in the act
improves the likelihood that the alleged criminal is convicted in a court of law.
"Photographs [video] of a crime scene, or any pertinent segment thereof, are as much
a form of evidence as a gun used in a murder or a knife in an assault case" (Gilbert
B. Stuckey, Evidence For The Law Enforcement Officer, McGraw Hill Book Company, New York,
NY).
Not only is video helpful in establishing the identity of a criminal who has committed a crime, but it also documents what the crime scene looked like at the time that the crime took place. This is ideal for viewing at a later time. Video tape recordings also help establish mood and they help to refresh peoples' memories of a crime incident, long after it has occured.
In the hands of a security manager, a video tape recording of a crime in progress is a god send. It enables security to remove the alledged criminal to the security office or surveillance room where he is made to watch the alleged crime. In most cases, this is enough to convince the culprit to settle the incident out of court.
This is usually a benefit to both the criminal and the store. To the criminal, it saves the embarassment of a public trial in which the culprit is likely to be recognized by acquaintances and friends. In addition, a trial usually means that the alleged name will likely end up in the local newspaper. The retail store also benefits by eliminating the negative press that's sometimes associated with such a trial. This is especially true when the alledged criminal stole "only to support his/her hungry family."
On the other hand, if the matter should go to court, a coviction is usually
assured when there's a video tape clearly showing the alledged committing the crime.
CCTV systems are also helpful in the residential security market. They allow homeowners to
see their callers, thus establishing their identity before they open an outside entrance
door. This is an important feature too, because otherwise, they might open their door to a
criminal.
CCTV EQUIPMENT BASICS
Camera systems often appear to be complicated; but in truth, they can be categorized into
four groups:
Cameras
Signal Transmission Media
Monitors
Peripherals
A camera, for example, collects reflected images from objects in the environment and then
converts them into electronic signals. Cameras require more knowledge and skill to install
than any other part of a CCTV system. For example, installers must consider such things as
light sensitivity, lines of horizontal and vertical resolution, available light at the
target, and the technology behind the imager. All of these things are important because
they help determine how well a camera performs in an environment.
Assuming that the installer installed the right camera for the job, the collected images
are converted from visible light into invisible electronic signals inside a solid-state
imager. These signals then are transported by one of many transmission media to the
monitor, where these signals are converted back to visible light in a CCTV monitor. In
addition, these video signals are commonly routed to other devices designed to manipulate
and/or process these images in a variety of ways. These devices are usually referred to as
peripherals. After installing the right camera and picking the right video signal
transmission technology for a CCTV installation, it's integral that an adequate video
monitor be picked. Although video monitors are not usually as complicated as cameras, they
are important enough for us to take the time to study.
CCTV "peripherals" are also important, as mentioned earlier, although they are
often overlooked. They can also be the most difficult part of a CCTV installation to get
right.
For example, a camera will operate without a lens (the lens is a peripheral) because it will still produce a 1 Vpp video signal-- which fits the classical definition of a video signal. However, the images reproduced by such a monitor will not be usable for identification purposes or visual recognition of others. In fact, without a lens, it's unlikely that there will be an image at all.
Pan-and-tilt mechanisms are another important peripheral that provides camera
scanning and panning capabilities. In fact, camera housings, switchers, splitters,
distribution amplifiers, time- lapse tape recorders, and others are all considered as
peripherals.
A camera is designed to collect the reflected light from objects around them and then to convert them into a usable video signal that measures 1 Vpp (Volt, peak to peak). This video signal then is transmitted to a monitor or some other piece of equipment using any one of several transmission medias.
There are two basic types of cameras on the market, the older CRT (cathod ray
tube) type and the more recent CCD (Charged Coupled Discharge) type.
Tube Cameras
There are several kinds of tube cameras on the market today. The standard Vidicon camera,
Ultricon, Newvicon, and SIT. All of these cameras have one thing in common, they require a
vacuum tube imager to convert reflected light into electrical impulses. This is
accomplished by firing a steady stream of high-velocity electrons at the back side of a
CRT-type imager.
Standard Vidicon -- Vidicon cameras work best in environments where there is full,
consistent light. In office environments, for example, vidicons work well in hallways
where the same lights are used to provide light during business hours. Anywhere, in fact
where lighting is fairly constant, a vidicon camera will perform well.
Standard vidicon cameras are the most inexpensive of both the CRT- and solid-state-based
cameras, and their effective life is generally less: 1 to 2 years.
ULTRICON and NEWVICON -- Ultricon and Newvicon
tube cameras are designed to perform at their best in environments where the light is low
to full. Because this type of camera is equipped with an auto iris lens--one that can
automatically adjust to different levels of light--Ultricon and Newvicon cameras are
effective in environments where the level of light is unpredictable.
SIT -- The SIT (Semiconductor Intensified Tube) camera is
designed to operate at its best in extremely low-light conditions. Like the Vidicon,
Ultricon and Newvicon cameras, the SIT uses a CRT. However, a solid-state intensifier is
added, providing additional light sensitivity. Because this camera is designed to offer
usable pictures in super low-light conditions, images are typically grainy during the day.
CCD Cameras
CCD cameras are made using a semiconductor target instead of a vacuum tube. CCDs are
typically designed to satisfactorily collect images in environments where the light level
is low-to-full and somewhat variable. The advantage that a CCD model realizes over that of
a tube-type camera lies in how camera scenes are formed.
Because there's a slab of semi-conductor instead of a tube in these cameras, they are not
susceptible to many of the same maladies, such as image bloom. In addition, because CCDs
generate less heat within them, the electronic components found inside of them last
longer. The life expectancy of a typical CCD camera is 5 to 25 years.
CMOS-based CCDs -- This camera is excessively sensitive to
infrared light, so it is not usually used outside in bright sunlight. It is also less
sensitive in low-light situations, making this chip technology less than desirable for
outdoor night use. It does, however, perform well in environments where the light is
consistent, like an inner office hallway where the same interior lights are on. CMOS CCD
technology is also less desirable than some of the others because of the way that the
light-sensitive pixels report the intensity of the light that strikes them. For example,
in a CMOS-based CCD chip, each row of pixels provide light-sensitivity information by
moving the information downward across other pixels below them.
Other chip technologies perform the same task by adding vertical shift registers between
each pixel column, making the transfer of light-sensitivity data faster and more
efficient. In addition, CMOS chips sometimes develop a white, fish bone pattern on a
monitor screen when the level of light that strikes the CMOS chip drops below a certain
level.
INTERLINE TRANSFER CCDs -- This type of CCD is effective in
applications where the light level is low- to full and varies from one moment to the next.
Although interline transfer CCDs are not technically low-light cameras, with certain types
of lighting enhancements they will perform well in select low-light conditions.
This chip technology is more effective than the CMOS technology because
vertical shift registers are placed between each column of pixels. This reduces the
effective resistance to the flow of information flowing from the pixels.
The chip used in this type of CCD camera is much faster than its CMOS counterpart. It's
also especially sensitive to IR light, which means it supports the use of IR light
enhancements.
Although the Interline Transfer CCD camera performs well in a diversity of
applications experiencing a variety of light levels, this camera requires a lens with a
wider aperture. This is because the pixels are generally smaller than in CMOS chips.
Smaller pixels are necessary to accommodate the added vertical shift registers.
This camera's sensitivity to IR light in the 1200 nm range also makes it a good candidate
for use in certain types of low- light applications. This requires the removal of the IR
cut filter in the lens.
The IR cut filter essentially filters out IR light when a camera is used in
normal daylight conditions. A cut filter is required because this camera can see heat.
This characteristic can be and usually is counterproductive when this camera is used in
outside applications where the light level is high, as on bright, sunny days. This can
also be a problem in applications where certain objects in the camera scene are physically
hot. This same characteristic can also cause smearing, which is the overload and shutdown
of one or more rows of pixels. In either case, the net result is blinding to the camera as
well as an observer at the monitor. To eliminate the problem, all that's necessary is to
put the IR cut filter back into the camera.
Frame Transfer CCDs -- In an effort to further improve CCD
technology, designers have designed a multiple-layer chip that eliminates almost all the
IR problems found in the Interline Transfer CCD models. The removal of the vertical shift
registers resulted in larger pixels, which eliminates smearing. In addition, new and
improved semiconductor materials are used in this Frame Transfer CCD model.
Because this semiconductor material is less prone to IR light, an IR cut filter is not
necessary. Frame-transfer chip cameras, however, are still IR sensitive enough that some
IR enhancements are still successful at producing usable images in low-light conditions.
There's still down side to Frame Transfer CCDs. First, these chips cost more than Interline Transfer chips, making cameras more costly. This, in turn, has resulted in fewer chip manufacturers that care to produce Frame Transfer CCDs.
Before a camera will produce an image in any setting, there must be enough
light to stimulate the camera. The human eye and CCTV camera shares a common
characteristic--both do not directly see an object using direct light from the source.
Instead, light is first reflected off of the object before someone's eye or a camera can
"see" it.
The amount of reflected light that's available in any setting depends to a large extent on
the amount of illumination available and the object's reflective characteristics. The
latter includes, among other things, color and a corresponding wavelength. This
characteristic in turn helps determine the amount of light that reaches the camera. In
general, in simple terms, the darker the object appears to the eye, the less light that
the object will reflect.
For example, the amount of light that bounces off a black ashphalt parking lot on a hot
summer day is by far less than that of the same parking lot on a cold, snowy winter day.
The reason for this lies in the different wavelengths of light that black ashphalt
absorbs. In this case, almost all the illumination energy is absorbed, resulting in very
little of the original light at the camera.
On the other end of the scale, a white object reflects all seven basic colors of the
rainbow. When the color red, orange, yellow, green, blue, indigo, and violet are combined,
the result is white light. An object painted with an individual color (wavelength), such
as red, results in the reflectance of that color and the absorption of all the others.
One way to remember the seven basic colors is to group the first letter of each one
(R,O,Y,G,B,I,V) into an easy-to-remember name: ROY G. BIV.
Determining Camera Sensitivity
To determine what camera to use in any given application, it's necessary to first
determine the minimum light level required to cause the camera to "see," which
is commonly called "camera sensitivity."
The first step in this process is to determine the illumination available at the scene. To
do this a light meter calibrated in foot candles (fc) or lux (lx) is used. If a light
meter is not available, use Table 1 and Table 2 that follows.
--------Outdoor Illumination-------
Direct sunlight = 10,000 to 13,000 fc
Full daylight = 1,000 to 2,000 fc
Overcast day = 100 fc
Dusk = 10 fc
Twilight = 1 fc
Deep twilight = 0.1 fc
Full moon = .01 fc
Quarter moon = .001 fc
Moonless night = .0001 fc
Overcase night = .00001 fc
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Table 1
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Indoor Illumination
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Manufacturing assembly line
Rough assembly work = 20 fc
Fine assembly work = 100 fc
Extra-fine assembly work = 300 fc
Retail Stores = 50 fc
Banks Lobby = 20 fc
Offices and Teller
stations = 50 fc Hospitals
Operating Table = 1800 fc
Examination Table = 50 fc Offices
General work area = 30 fc
Accounting & Bookkeeping = 50 fc
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Table 2
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The second step is to determine the actual amount of reflected light available at the
camera. To do this you must determine the reflectance of the surface being observed by the
camera. One way to do this is to use Table 3.
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Surface Reflectance
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Black asphalt = 5%
Bare earth = 7%
Gravel surface = 13%
Human face, white = 18% to 25%
Trees on grass = 20%
Red (brick) = 25%
Old concrete = 25%
Unpainted concrete
building = 40%
New concrete = 40%
Green (grass) = 40%
Old white paint = 55%
New white paint = 75%
Smooth aluminum
building = 65%
Old snow = 65%
New snow = 85%
Glass windows = 70%
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Table 3
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To determine the minimum light sensitivity of the camera in a particular application,
multiply the minimum amount of light available at the camera and the percentage of
reflectance of the observed area. Again, be sure to use worse-case numbers.
The formula you use should look like this:
Illumination x Reflectance = Usable light (at the camera)
Look at the following example. The minimum amount of available light (illumination) at a
scene to be observed is 1 fc. Let's say that the surface area to be observed by the camera
is 10-year-old concrete. Therefore, to determine the amount of light that's available at
the camera, multiply 1 fc of illumination by the reflectance of "old concrete,"
which, in Table 3, is 25%.
Before we can do the math, however, this reflectance value (expressed in a percentage)
must be converted into a decimal equivalent. This is done using the formula:
Percentage/100 = Numeric Value
In this case, the 25% reflectance value of old concrete is converted into its decimal
equivalent by dividing 25 by 100 (25/100), which equals 0.25.
Now armed with the numeric equivalent of reflectance, the necessary math can now be
performed to find theamount of useable light that's available at the camera:
1 fc x .25 (reflectance) = .75 fc
Foot Candles and Lux
Illumination and camera sensitivity are often listed in manufacturers' specification
sheets in foot candles (fc) or lux (lx). For this reason, it's sometimes necessary to
convert foot candles to lux and visa versa. The relationship between these two terms is as
follows:
.0929 fc = 1 lx
To make the job of conversion easier, experts often say that it's alright to express this
relationship as:
.1 fc = 1 lx
If a camera specification sheet rates light sensitivity in lux, you can convert it into
foot candles using the formula:
10 x fc = lux
or, in the previous example:
10 x 0.25 fc = 2.5 lx
There will also be times when it's necessary to convert lux into fc. Simply reverse the
original formula and perform the math using the following formula:
0.1 x lux = fc. In the previous example, 0.1 x 2.5 lx = 0.25 fc
Thus, for a camera to satisfactorily see a scene with 1 fc of illumination against
"old concrete," the camera must have a light sensitivity rating of at least 0.25
fc or 2.5 lx. A camera with an even lower light sensitivity rating will be even better,
and it will in all probability cost more money to buy it too.
A camera alone is of no use to anyone if the video signal it produces is not transported elsewhere else for observation and/or recording. The most common transmission method used is that of a coaxial cable.
Coaxial Cable
A coaxial cable consists of a center conductor with an outer shield. In most cases, the
shield consists of a metallic web of conductors with or without an additional metal-foil
wrapping surrounding the center conductor. The entire assembly is then wrapped with a
plastic covering, called a sheath.
In most cases, the shield is connected to an equipment ground, which is accomplished by
connecting one end of the shield to the chassis of the equipment that it's connected to.
The chassis, in turn, is connected to earth ground by the neutral connection of the power
cord and receptical.
Ground Loops
In most cases, the shield on a coaxial cable is connected to only one ground to eliminate
ground loops. Ground loops occur when there is more than one ground along the path of a
video signal. It occurs because there's a sizable potential difference (voltage) between
the two grounding points.
This potential difference causes electrons to flow between the two ground
points--a current that is not associated with the video signal. As the ground current
combines with the current on the common side of the video signal, it causes the flow of
electrons (current) to vary according to the electrical interference present in the ground
current.
Because the most common electrical current flowing through ground is 60 cycle power from
the power line bus, a 60-Hz. component is added (modulated) onto the video signal. This,
in turn, effectively corrupts the otherwise near-perfect video signal, causing any one of
several effects. By eliminating one of the grounds, the ground loop current is essentially
stopped.
Coaxial Cable Quality
The single most cause of problems in CCTV systems is low-quality coaxial cables. That's
why it's important that installers use a quality coaxial cable. Although coaxial cable is
probably the least expensive part of a CCTV system, it is by no means the least important.
Good coaxial cable, for example, has a low DC resistance because its center conductor is
large enough to effectively carry the signal.
A good quality coaxial cable also has a shield that's rated higher in its shielding capability than less expensive coaxial cables. This rating is commonly expressed as a percentage, reflecting the degree of shielding that's provided by a particular cable. It's not uncommon, for example, to see coaxial cables with shielding efficiencies of 80% to 95%.
A good coaxial shield is crucial to the proper, uninterupted and uncorrupted
operation of a CCTV system. The shield around the center conductor of a good oaxial cable,
for example, will stop stray electromagnetic radiation (EMR) that occurs in the
environment from entering the cable and interfering with the video signal. Sources of
stray EMR, for example, are common in homes and commercial buildings, as well as along the
street. Examples of EMR sources are electric power lines, motors, power transformers, and
radio broadcast transmitters (i.e. television and commercial radio transmitters as well as
citizen band and amateur ham radio transmitters in the neighborhood).
The type of center conductor inside the coaxial cable an installer uses is also important.
Experts say that nothing less than a 100% copper center conductor will eventually result
in the replacement of the cable, especially in outdoor applications where the cable is
constantly exposed to EMR sources of all types. The issue of whether to use a solid or
stranded center conductor must also be considered. Cable with a solid center conductor is
less expensive than stranded, but this type of cable should never be used on a camera
that's mounted to a pan & tilt mechanism. Instead, stranded cable should be used in
pan and tilt applicatons, as well as any other situation where the camera must move from
time to time. Using a cable with a solid core usually results in a broken center conductor
at some point in the future.
The type of cable sheath used is also important to the longevity of a video installation. Installers should not use indoor-type coaxial cable in outside applications, for example. When they do, ultraviolet radiation from sunlight can cause cracks to develop in the cable sheath. This enables moisture and contaminants to penetrate the sheath, changing the impedance of the cable. The net effect will be disruption of the video signal that flows along the center conductor.
Choosing The Right Coaxial Cable
Installers must select the right coaxial cable for each application that they encounter.
The first consideration is cable impedance. Coaxial cable is available in
several impedances. This is because the coaxial cable must match the output impedance of a
video device to the input of another. An impedance match makes it possible for the optimum
exchange of energy between the two devices.
It's the dielectric insulator between the center conductor and braided shield (ground) of
a coaxial cable that ultimately determines impedance. There are a number of dielectric
materials available for this purpose. The type of dielectric material and its thickness
determine the impedance of a particular coaxial cable.
Like the CCTMA industry, the video industry uses an impedance of 75 ohms. Thus, installers
must be certain to use a 75-ohm coaxial cable when installing CCTV systems. Otherwise, the
quality of the video signals carried by the coaxial cable that they do use will likely be
less than desirable. This is because the attentuation of the video signal will vary with
the instantaneous frequency of the signal. In other words, the quality of the output video
signal will vary from one moment to the next because the coaxial cable will transport some
portions of the signal with more ease than others. This can cause ghosting, snow, and a
generally poor video picture.
In addition, the type of coaxial cable selected will also determine the distance that
video signals will travel. Coaxials, for example, are categorized according to size and
their distance- carrying capabilities. For example, the most common coaxial cable used
today is RG59/U. This coaxial cable will carry signals for distances of up to 1,000 feet.
The next most popular coaxial is RG6/U, which is capable of carrying video signals up to
1,500 feet. RG11/U cable will likewise carry video signals for up to 3,000 feet without
any appreciable degradation of signal.
Fiber-Optic Cable
Although coaxial cables are probably the widest used and most accepted form of video
transmission today, it is slowly loosing ground to its fiber-optic counterpart. One very
good reason for this is the wider bandwidth realized by fiber-optic cable.
Although fiber-optic cable is not new, it's use in the CCTV market is relatively new.
Fiber-optic cable is now being used to transport both video and audio signals for short
and long distances. This is made possible by modulating a video/audio signal(s) onto a
beam of coherent light, which is generated by a solid-state laser. The modulated light is
then passed through a single, minutely-small strand of nearly-pure glass fiber. Because
this method uses light to carry the intelligence, data can be carried up to 3 miles or
even more without utilizing a repeater of any kind.
In many ways, a fiber-optic cable looks like a smaller version of a coaxial cable, until you closely examine the connectors and what's inside. For example, inside the center of a fiber-optic cable is a nearly-pure glass fiber. This center glass fiber (core) is protected by several layers of material.
The first layer nearest the core is called the cladding. Cladding is comprised of a less-than-pure film of glass. Although the core carries the major portion of the modulated light, and so the intelligence, the cladding aids in the return of light that's commonly lost through refraction.
The final layer is usually referred to as the jacket, or buffer. The buffer is designed to absorb some of the physical shock encountered by the fiber-optic cable in its environment. This layer has no optical properties, but it's sole purpose is actually to aid in the protection of the innter glass fiber layers.
Benefits
Installers are using more fiber-optic cable than ever before because of the following
reasons:
1.There are more channels of communication over which to transmit video images, audio and
other data. This means more images on a single cable than is possible with metallic
coaxial cable.
2.Longer signal transmission distances with less signal attenuation than has ever been possible with coaxial cables without some type of repeater.
3.Fiber-optic cable is not susceptible to electromagnetic interference (EMI), like their metallic coaxial counterparts.
4.Fiber-optic cables are generally smaller than their metallic coaxial counterparts when it comes to the number of communication channels that's available.
5.Signal transmissions are more secure because the signals traveling on a
fiber-optic cable do not emit electromagnetic radiation. This makes it more difficult to
tap into a fiber-optic cable with the intent of eavesdropping. To do so in an unauthorized
manner will also introduce extreme signal loss or even the total disruption of the signal.
More Communication Channels
Fiber-optic cable has a wider signal frequency bandwidth than it's metallic coaxial
counterpart. This means more available channels of communication.
For example, metallic coaxial cable has an effective bandwith of 10 MHz. By
comparison, fiber-optic cable has an effective bandwidth of 44.6 MHz.km. This means an
effective potential of more than 670 simultaneous telephone conversations over one glass
fiber.
Longer Transmission Distances
Fiber-optic cable can carry light-modulated signals for longer distances than metallic
coaxials because there's less signal attenuation. Metallic coaxial cable experiences a
higher degree of signal attenuation because of the inductive and capacitive properties of
the wire that carries the video signal. The very nature of a metallic coaxial causes a
higher degree of attenuation than fiber-optic cable.
Fiber-optic cable, on the other hand, experiences far less attenuation because glass fibers offer little resistance to the passage of light. In fiber-optic cable, it's more a matter of glass-fiber purity that determines the degree of attenution.
Not Susceptible To EMI
Fiber-optic cable is not susceptible to EMI, which includes nearby sources of radio-
frequency (RF) energy, such as high-power radio and television broadcasting transmitters,
CBs, ham radios, diathermy equipment, or induction heat-treatment furnaces and other
equipment. This makes fiber-optic cable an excellent choice for environments likely to
experience high levels of RF, such as airports, microwave installations, and radio and
television stations.
Smaller In Size
Fiber-optic cable is smaller than coaxial cable because the attenuation of a single glass
fiber is much less. This is because light does not require a large surface area through
which to travel. This makes it possible for fiber- optic cable to transmit more
communication channels than metallic coaxial per unit size.
This property also makes fiber-optic cable lighter in weight than coaxial cable.
More Secure Communications
The communication carried by a coaxial cable, no matter how good the shield may be, can be
compromised. There are a variety of ways in which to do this from simple induction to
actually tapping into the cable itself.
Although losses may occur when a coaxial is tapped, the losses are still far less than
that of an optic fiber under the same circumstances. In the case of fiber-optic cable, the
signal at the other end of the cable would in all likelihood be unusable. This, of course,
would result in the immediate inference that the fiber- optic cable has been tampered
with.
How Fiber-Optic Cable Works
The transmission of video, audio, and other data over fiber-optic cable involves the
movement of light through a nearly-pure glass fiber made of glass silica. This beam of
light is first modulated by video, audio or some other type of data by impressing the
electronic information onto the light beam and then directing the light beam into one end
of a fiber-optic cable. The modulated information then travels with the light beam to the
other end where it is then retrieved. Here it then is demodulated and converted back into
it's original electronic form.
One of the properties responsible for this operation is coherent light.
Coherent light, for example, exhibits only one color by transmitting light at one
wavelength. White light, on the other hand, is the conglomeration of all the colors
(wavelengths) of the rainbow. The coherent property is what empowers light to travel for
long distances inside a glass fiber cable that's no more than 100 microns, or 0.004 in. in
diameter.
Another property that makes glass fiber inherently better for long-distance video
transmission is its relatively low resistance to the flow of light particles/waves.
Metallic wire, on the other hand, is made up of atoms and molecules that naturally resist
the flow of electrons. To force this current through a coaxial cable, a potential
difference (voltage) must be introduced across the two conductors of the coaxial cable.
Internal reflectance, which is the optical property that enables light to bend at an angle
as it travels through a glass silica fiber, is also to a large extent responsible for the
successful transmission of video over long distances. Coaxial cables, on the other hand,
continue to struggle against the flow of electrons, making the use of line amplifiers
necessary.
Fiber-Optic Cable Quality
To gauge the quality of a fiber-optic cable, engineers use a mathematical property called
"refractive index." The refractive index of a fiber-optic cable is expressed as
a ratio. It is determined by measuring the difference in the speed of light in a vacuum to
the speed of light through a particular medium, such as a fiber-optic cable.
To prove the validity of this principle, one has only to pass white light through a prism.
The result is the refraction of all the colors. The light that escapes through the other
end is then separated into the basic colors of the rainbow: Red, Orange, Yellow, Green,
Blue and Violet. It's because the wavelength of each color is different that these colors
are viewed seperately as they exit the prism. The wavelength of Red is shorter than
Orange, for example, so the angle of refraction is also less. "Internal
reflection" is another factor that helps determine the quality of a fiber-optic
cable. This property greatly minimizes the loss of light when the angle of refraction is
equal to or greater than the critical angle. Thus, in better fiber-optic cables, nearly
all the light transmitted is reflected back to the center of the fiber-optic cable. The
glass cladding around the center glass core also helps to reflect some of the refracted
light back toward the center of the fiber-optic cable.
Fiber-Optic Cable Modes
Some types of fiber-optic cable has the ability to transport more than one beam of light,
or "mode." A mode is simply the path that a beam of light takes as it travels
inside a fiber-optic cable. There are several types of fiber-optic cables on the market
today that can transport 1 to more than a thousand beams of light over multiple paths, or
modes.
The number of modes that a fiber-optic cable can transport is determined by the size of the glass fiber and other factors that determine it's capacity and quality.
The last category is that of CCTV Peripherals, which consists of camera monitors, lenses, switchers and splitters, as well as event recorders, time-lapse tape recorders and pan & tilt mechanisms. In this discussion we will discuss lenses and camera control devices, which include camera switchers of various types, microprocessor-based matrix control systems, computer-driven camera control systems, and over-the-phone camera control systems.
Monitors
After a camera converts an image into an electrical signal, the video signal is
transported to a remote point for viewing and possibly recording. This is common in
applications where one or more cameras are monitored at a security kiosk or an
administrative office. Here the video signal is then converted from its electronic form
back to light, via a CCTV monitor, so security guards, office personnel, or others can
perform surveillance duties.
CCTV monitors are designed to convert the electrical video signals transmitted over
coaxials, fiber-optic cables, or a radio- based media into light energy for the eye to
see. Although these monitors share some similarities to conventional television sets, in
reality there are many differences between the two that set them a world apart.
Televisions, for example, are designed to receive commercial video and audio signals that
are broadcasted over the UHF and VHF frequency bands. CCTV monitors, on the other hand,
are designed to receive composite video signals direct over a coaxial cable, whether they
arrived over the same coaxial cable, a fiber-optic link or a microwave/radiated
radio-frequency link.
There are other differences between conventional TV sets and CCTV monitors that also
should be noted. The most significant is durability and life expectancy.
Where a typical conventional television set is designed to operate up to 5
years at eight-hour-a-day intervals of operation, for example, a commercial-grade CCTV
monitor will operate up to 5 years when operated on a 24-hour-a- day basis. This is not to
say that a conventional television set cannot be used as a CCTV monitor. With the use of a
video-to-RF modulator, composite video signals can be converted to operate over the VHF
television frequency band. However, conventional television sets will not operate for
extended periods of time as CCTV monitors will.
Viewing Distance and Resolution
Commercial CCTV monitors come in a number of sizes. As a general rule, the farther away
security personnel are from the monitor, the larger the monitor should be.
One accepted method of determining monitor distance and size is based on the mathematical
formula:
Monitor (inches) - 4 = Viewing Distance (+/- 25%)
For example, to find out how far away a 12-in. monitor should be used, plug the values
into the equation and do the math:
12 - 4 = 8 ft.
To establish the upper and lower viewing limits, multiply the value of 8 ft. by 1.25 and
.75, or:
8 x 1.25 = 10 ft.
8 x .75 = 6 ft.
Thus, a 12-in. monitor should be viewed effectively at a distance of 6 to 10 ft.
The same formula can be used to calculate the approximate viewing distance of a common
9-in. monitor:
9 - 4 = 5 (+/- 25%)
The viewing range is then calculated in the same manner:
5 x 1.25 = 6.25 ft.
5 x .75 = 3.75 ft.
In this case, a 9-in. monitor can be viewed effectively from a distance of 3.75 to 6.25
in.
Another concern when purchasing a monitor is its resolution. The number lines of
resolution is usually determined by the resolution of the camera(s) used.
For example, if a monitor with a 300 line resolution is used with an 800- to 1000-line
camera, the result will not be as good as if a 700-line monitor were used. On the other
hand, when a 700- line monitor is used with a 300-line camera, the result will usually be
just as good. This is because the 700-line monitor will reproduce the 300 lines of
available video by dividing them over the 700-line screen. This will produce video images
that may not as good as the originals produced by the 300-line camera.
The last category is that of CCTV Peripherals, which consists of camera lenses,
monitors, switchers and splitters, as well as event recorders, time-lapse tape recorders
and pan & tilt mechanisms. In this discussion we will discuss lenses and camera
control devices, which include camera switchers of various types, microprocessor-based
matrix control systems, computer-driven camera control systems, and over-the-phone camera
control systems.
Lenses
A basic definition of a lens is "A device that collects light from an environment,
narrowly focusing it onto either a cathod ray tube or solid-state target."
In the scheme of things, lenses are considered to be a camera peripheral rather than part of the camera itself. This is because it does not require a lens to produce a 1-Volt-peak-to-peak (Vpp) video signal at a camera's output. In fact, all that is necessary to create a video signal is to have light strike the target inside the camera. However, without the proper lens, video images cannot be focused onto the light-sensative target in a video camera to produce a "usable" video signal.
Among the many issues that pertain to a CCTV syste, the most important that
should be considered before buying a lens are:
camera format,
whether the situation calls for a fixed-iris lens or one with an auto-iris,
whether the operator needs to zoom in on particular parts of the scene,
and the size of the viewing area involved.
Format
Lenses and cameras now come in four formats: 1/3-, 1/2-, 2/3-, and 1-in. Lens format is
actually determined by the size of the opening in a camera where the lens attaches. When
the diameter of this opening measures 1/3 in., the camera is said to be a 1/3-in. formated
camera. On the other hand, when the diameter is 2/3 in., the camera is said to have a
2/3-in. format.
For proper viewing, experts say to use only a lens that's formatted the same or
higher than the camera used. For example, if the opening in a camera (camera format) is
1/2 in., a 1/2- to 1- in. formatted lens should be used for optimum viewing. Or, if a
camera with an opening of 1 in. is used, use a 1-in. formatted lens. To do otherwise will
result in the projection of only part of the viewing area onto the target in the camera.
Fixed-Iris, Manual-Iris and Auto-Iris Lenses
Whether to use a fixed-iris lens or one with an automatically-operated iris depends
largely on the light conditions where a camera and lens will be used. For example, a lens
with an auto-iris feature should be used in environments where the light intensity can
vary from minute by minute, hour by hour, or day by day. This is almost a necessity in
outdoor applications and where lights are switched on during business hours and off after
closing.
Fixed-iris and manual-iris lenses, on the other hand, can be used in situations where the
light essentially stays the same all of the time. This type of application is often found
inside of buildings where the lights never vary, no matter what time of day it may be.
The iris itself is actually a small aperture through which light must pass before it can
strike the light-sensitive target inside a camera. In low-light, for example, the aperture
is fully open and in bright-light, it will nearly be closed. In medium-lit conditions, the
same aperture will physically be open somewhere between its fully-open and closed
position.
To measure the size of an aperture opening, the lens industry adopted the F-stop
measurement. The larger the F-stop number, for example, the smaller the opening. Thus,
when a camera is set to an F-stop of f-1.4, the size of the aperture is larger than when
the same lens on the same camera is adjusted to f-8 or f-22.
In a fixed-iris lens, the aperture is set and cannot be changed. In manual- and auto-iris
lenses, however, the aperture can be changed to suit the amount of reflective lighting in
the environment. The manual-iris model, for example, requires someone to change the
aperture setting to suit the situation, where the auto-iris model can make these aperture
adjustments automatically without intervention of any kind.
Manual-iris lenses are commonly adjusted using a thin ring at the base of the lens where
it screws onto the camera. On most models the f-stop setting is clearly shown on the ring.
To readjust the aperture of the lens, all the dealer has to do is turn the ring. A pointer
on the stationary part of the lens points to the f-stop settings as the ring is turned.
Common F-stops included on manual-iris lenses are 1.4, 2.0, 2.8, 4.0, 5.6, 8.0, 11, 16,
and 22.
Generally, the higher the f/stop number, the less light that a camera lens can
pass. In addition, the lower the f/stop number, the more light that is passed.
Auto-iris lenses, on the other hand, automatically adjust the amount of light that passes
through them. This is done with a motorized aperture that adjusts itself according to the
amount of light that's available in the environment. To perform this task, an auto-iris
lens is equipped with either a tiny motor and gears or a magneto-array ring. An electronic
controller board or video sampling board are used to control the setting of the lens
aperture, using the amount of video signal generated inside the camera as the adjustment
criteria. In turn, the amount of video signal generated by a camera depends on the amount
of available reflective light in the environment.
This type of lens is ideal for use in environments where the ambient light changes
regularly. Automatic iris lenses are able to adjust the amount of light that passes
through them using a built- in optical sensor that tells the lens what the light level is
in the environment. In low-light conditions, for example, the optical sensor prompts the
lens to open up, which allow more light to fall on the target in the camera.
Field of View
An important aspect that must be considered before choosing a camera lens is the field of
view (FOV), or the actual area that the camera is expected to "see." When all
else is the same, the expected FOV of a particular lens will change when the lens size is
either increased or decreased.
Using a FOV calculator, for example, a 2/3-in. 25 mm lens will allow a camera to see a FOV
of 5 ft.(h) x 3.8 ft.(w) at a distance of 15 ft. Using a 1/2-in., 25 mm lens will change
the field of view to 3.8 ft.(h) x 2.8 ft.(w).
The more accurate way to determine FOV is to do it mathematically using the following
formulas and chart:
Scene Width = Horizontal Format x Distance)/Focal Length
Scene Height = Vertical Format x Distance/Focal Length
+--------------------------------------+
| Horizontal & Vertical Camera Formats |
+---------+--------------+-------------+
| | Horizontal | Vertical |
| Lens | Camera | Camera |
| Format | Format | Format |
+---------+--------------+-------------+
| 1/3 | | |
+---------+--------------+-------------+
| 1/2 | 6.2 mm | 4.65 mm |
+---------+--------------+-------------+
| 2/3 | 8.8 mm | 6.60 mm |
+---------+--------------+-------------+
| 1 | 12.7 mm | 9.50 mm |
+---------+--------------+-------------+
In the above example, using the formulas and chart, the exact height and width of the
2/3-in., 25 mm lens under the same conditions is 3.96 ft.(h) x 5.28 ft.(w). Although the
dial-up Lens Calculator was a slight bit off, the degree of inaccuracy is hardly worth the
mention.
Focal Length
Another very important aspect to be considered when choosing a lens for a particular
application. The Focal Length (FL) of a lens is actually the distance from the center of
the lens to the surface of the tube or solid-state target. A lens with a FL that's shorter
than the FL of a standard lens--for a given format, is considered to be a wide-angle lens.
A lens with a longer FL than a standard lens is considered as a telephoto lens.
Optical Speed
The optical speed of a camera lens actually refers to it's light-gathering ability. The
more light that a lens is able to collect and convey to the target in a camera, the better
the resulting picture will be. The larger the lens, the more light that the lens can
collect and use to generate a picture. The optical speed of a lens is measured in f/#.
A slow lens, for example, has a higher f/number assigned to it than a fast lens. For
example, an f/2 lens is slower than a model rated at f/1.6. To determine the f/number of a
lens, use the formula: f/# = FL/d
(FL=Focal Length, d=Diameter)
Fixed Focal Length Lens
One of the most common lenses in use today, the fixed focal length lens is relatively
economical. However, it offers only one focal length, usually between f/1.3 to f/1.8,
which cannot be changed. Because the optics in this type of camera is simple and
to-the-point in nature, these lenses are usually considered faster than variable-
focal-length models. Fixed focal length lenses come in standard, wide-angle, telephoto and
variable zoom.
Standard lenses essentially mimic what the eye sees. In terms of magnification, a standard
lens has a magnification factor of 1. In terms of a numeric measurement, stated in
millimeters (mm), to be a standard lens, a 1-in. formatted lens must have a focal length
(FL) of 25mm, a 2/3-in. must have a FL of 16mm, a 1/2-in. lens must have a FL of 12mm, and
a 1/3-in. lens must have a FL of 8mm.
Wide-angle lenses enable more of a scene to be witnessed than when looking at it with the
naked eye. To be considered a wide- angle lens, a 2/3-in. camera must have a FL of 4.8mm
and have a 96-deg. horizontal and 72-deg. vertical angle of view.
Camera switchers are devices that allow CCTV system operators to switch between
cameras--either manually or automatically. Manual switchers, for example, enable operators
to periodically switch from one camera to another, as the need arises. This is the most
basic switcher available and certainly the most inexpensive type tobuy and maintain.
Sequential switchers, on the other hand, automatically switch between camera scenes,
allowing system operators to perform other tasks while they keep watch on certain areas of
a facility. This type of switcher, for example, allows operators to write reports, answer
the telephone and perform other duties at the same time that they observe camera scenes.
There are several types of sequential switchers that installers use.
Bridging Switchers - A Bridging switcher provides two
seperate monitor outputs. The first monitor typically is set to display only one of the
entire group of camera signals. The other monitor displays the other camera scenes in
ascending order.
Homing Switchers - Homing switchers are similar to Bridging switchers in that each of the two monitors can be operated seperately. However, where the Bridging switcher channels one particular camera to a second monitor--without including it in the sequentially-observed camera images, the Homing switcher allows users to pick one of the normally sequenced scenes to view on a second monitor. This arrangement is especially useful when a particular entrance or area of a building requires special treatment.
Looping Switchers - Looping switchers also provide a seperate 75-ohm output for each camera input. This allows installers to route particular cameras to another switcher, monitor, or time-lapse tape recorder.
Alarming Switchers - Automated switching using an alarm switcher makes it possible to focus on a particular area in a mechanized fashion without user intervention. This method usually begins with either a set of screw terminals or some other contact method to which any normally-open, dry-contact switch or relay output is connected. Alarming switchers, in short, enable users to automatically call up a particular camera scene when a detector in the vicinity of the camera is violated.
Combination Switchers - CCTV manufacturers have also engineered a line of switchers that have many of the characteristics of two or more of the above switcher types. This approach enables installers and their clients the luxury of mixing and matching features without getting stuck with features that no one wants.
Microprocessor-based Matrix Switchers -
Microprocessors that utilize LSI (Large-Scale Integration) and VLSI (Very-Large-Scale
Integration) technology have made it possible to automate camera functions in a way never
before possible. Where in the past the typical function of a camera switcher was to
sequentially switch one camera scene at a time on a given monitor, matrix switchers make
it possible to assign particular cameras to any one of a number of monitors.
In addition, focus, pan-and-tilt, and zoom functions can be placed under the control of a
matrix switcher so when someone violates a door or interior motion detector, not only does
the associated camera scene automatically appear on any one of several monitors in any one
of several remote locations, but the appropriate camera can be made to automatically
return to its preset position. At the same time the lens automatically is focused, and--if
included--zoomed to a given degree. Essentially, by the time the camera returns to its
preset position over looking the door or area involved, the matrix controller has in all
likelihood already positioned the lens to the proper focus and zoom.
The transmission of video over the phone line is not new. Commonly known as
slow-scan video, this method enables users to conduct surveillance activities from a
remote location without installing a dedicated coaxial cable or establishing some kind of
microwave or radiated- RF link between the monitored location where the camera is and the
operator.
Recently, however, digital technology has made it possible to improve the typically slow
refresh times associated with yesterday's slow-scan technology. Dubbed by some as
"fast-scan," the result has rendered a remarkably efficient method of
controlling cameras over the phone line in a close-to-real-time fashion. For example,
where it once took 8 to 16 seconds to transmit one frame of video using the older analog,
slow-scan method, pictures can now be transmitted at the rate of 8 to 10 frames every
second for low resolution images, or 1 frame in 4 seconds for higher resolution images.
These over-the-phone-line camera control systems also provide near-real-time camera
control and selection. This usually includes control over pan-and-tilt mechanisms, lens
focus and zoom. Not only does this refer to prepositioning, but also to the preview of
solicited changes in camera position.
When performing a pan or tilt maneuver, for example, the operator is able to see in near
real time the effect of each command enacted at the remote sight through a small window.
Although the resolution of the image shown in the window is too low for identification
purposes, it gives the operator a quick idea of where the camera is pointed. As images are
then refreshed at the selected rate, the operator will then see the delayed effect of his
or her commands on screen later.
Security with this type of system is usually performed at both sending and receiving ends.
For example, before an operator can dial up a remote site, he or she must enter a unique
passcode. A unique identification number is also encrypted in the software on both ends so
when an operator dials up the "transmitter" at the remote site, the
identification number at the "receiver" is checked. In some cases, connection is
then severed by the transmitter and the transmitter then dials up the receiver. This
feature assures that the proper receiver gains control of the system.
Computerized Camera Control - On-site computer-driven camera control systems also provide
a myriad of features and benefits unequaled by yesterday's standards. Using a soft screen-
generated or hard keyboard, users can run sequential camera tours based on any number of
criteria. Use of a soft keyboard is usually accompanied by the implementation of a mouse
and a Windows-based softwareprogram.
Like their microprocessor counterparts that use embedded firmware, computerized camera
control systems provide presets so when a door or motion detector are violated, cameras
are automatically returned to "home" and lens focus and zoom settings are
performed.
Computerized systems also provide other benefits, such as the retention of camera images
for personnel and visitor identification, the retention of individual video frames during
alarm, and the integration of alarm devices and access control functions.