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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.
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