How to Wire a Digital CCTV System
The objective of this article, the second installment of our special, six-part series on digital CCTV, is designed to educate dealers in the proper selection, installation and integration of the many different forms of transmission media found in the security industry today.
Transmission media is the true lifeline of any security installation. Understanding how each one works and knowing why and when to select them will increase system performance and bring about desirable results.
The three major areas for video, data and audio transmission are coaxial cable, network cable (UTP or unshielded twisted pair, e.g. Cat 5) and fiber optics. (See cabling comparison chart above.) Presently, coaxial cable is the most popular choice. However, as costs are reduced and dealers become more comfortable with UTP and fiber optics, coax will eventually be limited to the interconnecting of racked equipment.
Coax’s Center Conductor Size Determines Distance
Let’s begin with the video portion of our transmission section. As stated in the first part of this series (see February’s issue), most security cameras, even if they are listed as Digital Signal Processed (DSP), still have an analog baseband output. By “analog baseband video,” we mean a single channel, point-to-point system with an analog bandwidth of DC to 8MHz.
The first and most common method is that of using coaxial cable. The purpose of coax is to accurately convey electronic signals from the source to their destinations. Since the impedance of video source and load devices is 75 ohms, the cable has to be 75 ohms in order to properly transmit the signal.
Because we are transmitting baseband video, the cable selection must meet the following parameters. First, the cable must have a 75-ohm impedance to match the equipment characteristics. Second, the center conductor should have a low resistance and, therefore, the selected center conductor should be either solid or stranded copper.
The use of a copper weld or copper-coated steel offers high resistive losses to the video signal and will produce a poor image quality. The most common types of coaxial cable in CCTV are selected for their allowable distances between the camera and monitor site. As the size of the center conductor of the cable increases, the allowable distance between the equipment also increases.
In specifying cable, it is necessary to determine how much loss is acceptable.The last requirement for coaxial cable used in CCTV is that of the outer shield. Most cable manufacturers offer several different shielding materials. However, only one material is suitable for CCTV applications, and this material is 95-percent copper braid.
As an added note – to newcomers to the CCTV industry – some manufacturers offer digital coaxial cables. However, this does not relate to CCTV. They are referring to digital satellite or digital downlink cables, which are designed for RF signals and will not match the parameters for CCTV equipment.
UTP Impervious to Noise, Ground-Fault Problems
This method of transmitting signal over network wire is not new, but it is greatly improved. There are three key situations for using UTP in analog video applications. First, the majority of video equipment uses coaxial connectors, usually BNCs. Secondly, the output impedance of the coaxial systems is 75 ohms, while UTP has an impedance of 100 ohms. Thirdly, UTP is a “balanced-line” system, while coax, which is one shielded conductor, is “unbalanced.”
As for cabling, UTP wire, gauge 24 or thicker, stranded or solid, Category 2, 3, 4, 5 or greater, can be used. Multipair wire with an overall shield can be installed, but untwisted wire, or “quad-wire,” is not recommended and will cause a very unstable image.
UTP systems offer several advantages over coaxial cable. Unlike coax, UTP can reside in high-noise environments, such as elevator traveling cables, or near fluorescent lights, radio transmitters, motors or generators. However, the single greatest improvement compared to coax is that UTP devices in the active mode (those requiring AC or DC voltages to operate) prevent ground-fault contamination.
In most cases, the maximum loop resistance for any system is found to be between 26 ohms and 52 ohms of DC resistance. (Wire resistance may be measured with an ohmmeter by shorting the two conductors together at the far end, and measuring the resistance at the other end.) Also note that the network wire or UTP must be installed with no bridge-taps, loading coils, or MOV-type (metal oxide varisters) surge protectors.
Fiber Delivers More Information, Fidelity Than UTP or Coax
The final transmission media is fiber optics, which uses a modulated light to transmit signals. Fiber optics is a more expensive form of transmission. However, fiber extends operating distances and improves performance, which makes it ideal for long-haul communications.
Today’s low-loss glass fiber-optic cable offers almost unlimited bandwidth and many advantages vs. all previously developed transmission media. The basic fiber-optic transmission system consists of three elements: the optical transmitter, the fiber-optic cable and the optical receiver.
The fiber-optic cable consists of glass fibers that act as wave-guides for the optical signal. Fiber-optic cable is similar to electrical cable in its construction, but provides special protection for the optical fiber within. Just as in coaxial cable, there are many different types of fiber-optic cables. For ease of explanation, only the most common types will be discussed.
It is the combination of decibel losses – as well as losses generated by connectors, patch panels and splices – that determines equipment selection. The optical receiver simply converts the received optical signal back into a replica of the original electronic signal.
Presently, fiber optics can transmit more than 64 channels of video as well as data and audio via a single fiber. Fiber-optic cable can also support much higher data rates, and at greater distances, than coaxial, making it ideal for the transmission of serial digital data.
On the installation side, fiber-optic cables, even ones containing many fibers, are usually much smaller and lighter in weight than a wire or coaxial cable with similar information-carrying capacity, making it much easier to handle and install.
Last but not least, fiber-optic cable is ideal for secure communications systems because it is very difficult to tap into, and fiber emits absolutely no electrical radiation.
With all of these advantages, why do many companies still rely on coaxial cable for transmission of signals? Two main reasons come to mind: the cost of interface equipment, and the training requirements of installation personnel.
It is between the transmitter and receiver that most of the cost is found in a fiber-optic system. The interfaces that convert the signals at each end can range from $500 for a single video channel to more than $22,000 for a 20-channel multiplexed unit.
Amount of Light Is Crucial in Fiber System Design
When designing a fiber-optic system, there are many factors that should be considered. Without the correct amount of light reaching the receiving end, the entire system will not operate properly.
First, determine the correct optical transmitter and receiver combination based upon the signal to be transmitted (analog video, digital video, audio, RS-232, RS-422, RS-485). Once the receiver and transmitter are selected, determine the operating voltage for each. The type of fiber, size and connector is usually specified by the equipment manufacturer and should be carefully followed.
Now comes the hard part – calculating the total optical loss (in dB) in the system by adding the cable loss, splice loss, and connector loss. These parameters should be available from the equipment an
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