Transmission: Possible

Your mission, should you decide to accept it, is to read and comprehend the final installment of our special six-part series on digital CCTV, which will help explain some of the advances in transmission methods incorporated into today’s closed-circuit security systems.

In the days of old, a single coaxial cable, along with a multiconductor cable, were required for each remote camera location in order to provide video information as well as limited control of remote location functions, such as pan/tilt and zoom.

Today’s systems demand a greater number of remote camera locations, increased operating distance and multiple controlling locations. Therefore, they can no longer afford the limitations created by the older transmission methods. Fiber optics, unshielded twisted-pair wire, networking, and radio-frequency and laser communications are helping facilitate traffic flow on the digital video superhighway.

Fiber Optics Offer a Bounty of Benefits

Fiber optics is a method of carrying video, audio or control information over a thin strand of glass or plastic fiber using light as the transmission media.

Fiber-optic cable offers many benefits over conventional coaxial cabling. When comparing the data transmission performance of fiber optics with conventional cabling, the true benefits of fiber optics become apparent in a hurry. Other advantages include:

Because fiber is not susceptible to electromagnetic interference (EMI), it offers a much cleaner signal than copper.

Signals transmitted by fiber optics do not degrade as quickly. Hence, cable lengths of up to 6,500 feet using multimode fiber are possible.

Fiber is virtually unaffected by outdoor atmospheric conditions, allowing it to be lashed directly to telephone poles or existing electrical cables without concern for extraneous signal pickup.

Fiber eliminates ground loops that can cause problems in more than 60 percent of all security systems. The reason for ground loops found in coaxial cable systems is simple; coaxial cable uses current rather than light to transmit information and this can cause differences of electrical potentials in larger systems or systems incorporating many different power sources.

Since fiber-optic cables do not carry current, they are ideal for volatile environments where a spark from a conventional broken copper line could result in explosive consequences. If a fiber-optic cable is broken, there is no risk of electrical shock.

Glass fibers will not corrode, as do the copper wires traditionally used for CCTV installations. This reduces the loss generated by the transmission system over a length of time and operation.

Basic Fiber Theory Class Is Now in Session

A basic fiber link consists of an optical transmitter, which converts an electrical signal into a light signal. The drive circuit changes the electrical signal (composite video, pan/tilt control, audio, etc.) into an optical signal, which is launched into the fiber by a light-emitting diode or laser. Fiber-optic cable is the medium for carrying the light.

The cable consists of a thin strand (core) of glass or plastic surrounded by another layer of glass or plastic (cladding). An additional coating (jacket) consisting of different materials protects the core and cladding from shock, weather and other corrosive elements.

The receiver accepts the light and converts it back into an electronic signal. The receiver consists of two basic parts: the photo detector that captures the light pulses and the output circuit that converts, amplifies and reshapes the signal. The transmitter and receiver specifications, as well as the size and type of fiber, and the number of splices and connectors in the system determine the maximum operating distance.

Shedding Light on Fiber System Design

When designing a fiber-optic system, many factors that must be considered all contribute to ensuring that enough light reaches the receiver. With insufficient amounts of transmitted light, the entire system will fail to operate properly. There are no in-betweens.

In a standard coaxial cable application, as the signal propagates down the cable, the signal strength weakens due to the DC resistance of the cable. Whereas in a fiber optic-system, as long as the dB budget loss of the system is not exceeded, the video signal remains at full strength.

The following procedures are helpful when designing any fiber-optic system:

1) Determine the correct optical transmitter and receiver combination based on the signal to be transmitted (analog, digital, audio, video, RS-232, RS-422, RS-485, etc.).

2) Determine any necessary special modifications, such as system impedance, bandwidth requirements, connectors or fiber size.

3) Calculate the total optical budget loss (in dB) of the system. This loss is determined by adding the loss generated by the cable, and splice and connector losses. These parameters should be available from the manufacturer of the electronics and fiber.

4) Ensure that the fiber system’s bandwidth is adequate to pass the desired signals.

Typical bandwidth for multimode fiber cable is approximately 660Mbps. This broad bandwidth allows for multiplexing of many signals to be transmitted over a single fiber.

Fiber Installation Is an Entirely Different Animal

The requirements for fiber connections are very different from those required by coaxial cable. Normally, a simple crimp or multipin connector can be used to make video, audio and control connections. A fiber-optic connection, however, is a precise alignment of two mating fiber cores.

Ensuring proper contact between the two fibers can be challenging. Connectors are used to couple two fibers or to connect fibers to transmitters or receivers, and connectors are designed to be demountable.

The two most common connectors used in the fiber industry are the ST and SC configurations. The most frequently used connector is the ST type because the construction of this connector allows the installer to mechanically couple the connector to the receiving or transmitting equipment with a push-and-turn motion.

As for splicing, there are two types—fusion and mechanical.

Fusion splicing is accomplished by welding the two fibers together, usually with an electrical arc. It has the advantages of low loss, high strength and long-term reliability.

Mechanical splices use an alignment fixture to mate the fibers and an optical matching gel or epoxy to minimize back reflection. Some mechanical splices use bare fibers in an alignment bushing, while others closely resemble connector ferrules without all the mounting hardware.

Unshielded Twisted-Pair (UTP) Transmission 101

The use of an unshielded twisted-pair (UTP) cable for transmission of video, data or audio is not new. However, the introduction of reliable, high-performance, temperature-stable, noise-immured and surge-protected systems has increased the popularity of this method in recent years.

UTP allows the transmission of video, audio and data over 24-gauge telephone wire (Cat 2-Cat 6). The signal is converted to a 100-ohm balanced signal, allowing it to travel over the twisted telephone pair without distortion. The twisted pair should have a maximum DC resistance of 26 ohms for a distance of 500 feet.

As with any other video transmission method, there are some basic considerations when specifying and installing twisted-pair equipment. The use of shielded twisted pair, or a pair of wires with multiple splices, is not recommended and it is also very important that no resistance, capacitance or inductive devices be connected to the same pairs used for video or control transmission. This includes telephones and certain surge and lightning protection devices.

4 Convincing Arguments in Favor of UTP Usage

Transmission via UTP offers several

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