Selecting cables for video surveillance systems.

#Pelco

Selecting cables for video surveillance systems.

Most often, video signals are transmitted between devices via coaxial cable. Coaxial cable is not only the most common, but also the cheapest, most reliable, most convenient and easiest way to transmit electronic images in television surveillance systems (TSS).

Coaxial cable is available in a variety of sizes, shapes, colors, characteristics, and ratings from many manufacturers. The most commonly recommended type of cable is RG59/U; however, this family actually includes cables with a wide variety of electrical characteristics. Similar cables to RG59/U are also widely used in CCTV and other applications involving cameras and video devices, such as RG6/U and RG11/U.

While these cable groups are similar in many ways, each cable has its own physical and electrical characteristics that must be taken into account.
All three of the above groups of cables are part of the same general family of coaxial cables. The letters RG stand for «radio guide» and the numbers represent different types of cable. Although each cable has its own number, characteristics and dimensions, in principle, all of these cables are constructed and operate in the same way.

The Structure of a Coaxial Cable
The most common cables are RG59/U, RG6/U and RG11/U and have a round cross-section. Each cable has a central core covered with a dielectric insulating material, which in turn is covered with a conductive braid or shield to protect against electromagnetic interference (EMI). The outer protective coating over the braid (shield) is called the cable jacket.
The two conductors of a coaxial cable are separated by a non-conductive dielectric material. The outer conductor (braid) shields the central conductor (core) from external electromagnetic interference. The protective coating over the braid protects the conductors from physical damage.

Center Core
The center conductor is the main means of transmitting the video signal. The diameter of the center conductor is usually between 14 and 22 American Wire Gauge (AWG). The center conductor is either solid copper or copper-clad steel (copper-clad steel); in the latter case, the conductor is also called bare copper weld (BCW). The center conductor of the cable for STN systems must be copper. Cables whose center conductor is not solid copper, but only copper-clad, have much higher resistance
circuit at video signal frequencies, so they cannot be used in STN systems. To determine the type of cable, look at the cross-section of its central conductor. If the conductor is steel with a copper coating, its central part will be silver, not copper. The active resistance of the cable, that is, its resistance to direct current, depends on the diameter of the central conductor. The larger the diameter of the central conductor, the lower its resistance. A cable with a large-diameter central conductor (and therefore with lower resistance) can transmit a video signal over a longer distance with less distortion, but is more expensive and less flexible.

If the operating conditions of the cable are such that it can often bend vertically or horizontally, choose a cable with a multi-core central conductor, which is made of a large number of small-diameter wires. Multi-core cable is more flexible than single-core cable and is more resistant to metal fatigue when bending.

Dielectric insulating material
The central core is uniformly surrounded by a dielectric insulating material, usually polyurethane or polyethylene. The thickness of this dielectric insulator is the same along the entire length of the coaxial cable, due to which the operating characteristics of the cable are the same along its entire length. Dielectrics made of porous or foamed polyurethane attenuate the video signal less,
than solid polyethylene dielectrics. When calculating the loss per length for any cable, lower loss per length is desirable. In addition, the foamed dielectric gives the cable greater flexibility, which makes the work easier for installers. But although the electrical characteristics of a cable with foamed dielectric material are higher, such material can absorb moisture, which degrades these characteristics.

Solid polyethylene is stiffer and holds its shape better than foamed polymer, is more resistant to pinching and crushing, but is somewhat more difficult to install. In addition, its signal loss per unit length is greater than that of a cable with a foamed dielectric, and this must be taken into account if the cable length is to be long.

Braid, or shield
On the outside, the dielectric material is covered with a copper braid (shield), which is a second (usually grounded) signal conductor between the camera and the monitor. The braid serves as a shield from unwanted external signals, or interference, which is commonly called electromagnetic interference (EMI) and which can adversely affect the video signal.

The quality of the EMI shielding depends on the copper content of the braid. Commercial quality coaxial cables contain a loose copper braid with a shielding effect of approximately 80%. Such cables are suitable for normal applications where EMI is low. These cables are good when they are installed in a metal conduit or metal pipe, which serves as an additional shield.

If the operating conditions are not very well known and the cable is not installed in a metal pipe, which can serve as additional protection against EMI, then it is better to choose a cable with maximum interference protection or a cable with a dense braid containing more copper compared to commercial quality coaxial cables. Higher copper content provides better shielding due to the higher content of shielding material in a denser braid. Copper conductors are required for STN systems.

Cables in which the screen is aluminum foil or wrapping foil material are not suitable for television surveillance systems (TSN). Such cables are usually used
for the transmission of radio frequency signals in transmission systems and in signal distribution systems from a collective antenna.

Cables with aluminum or foil shields can distort video signals to such an extent that the image quality will drop below the level required for surveillance systems, especially if the cable length is long, so such cables are not recommended for use in surveillance systems.

Outer Jacket
The final component of a coaxial cable is the outer jacket. Various materials are used for its production, but most often polyvinyl chloride (PVC). Cables with jackets of various colors (black, white, tan, gray) are supplied — for both outdoor and indoor installations.

The choice of cable is also determined by the following two factors: the location of the cable (indoors or outdoors) and its maximum length.
Coaxial video cable is designed to transmit a signal with minimal loss from a source with a characteristic impedance of 75 ohms to a load with a characteristic impedance of 75 ohms. If a cable with a different characteristic impedance (not 75 ohms) is used, additional losses and signal reflections occur. Cable characteristics are determined by a number of factors (core material, dielectric material, braid design, etc.), which must be carefully considered when selecting a cable for a specific application. In addition, the signal transmission characteristics of a cable depend on the physical conditions around the cable and on the cable installation method.

Use only high-quality cable; select it carefully considering the environment in which it will be used (indoors or outdoors). Cable with a solid copper core is best for transmitting video signals, unless increased flexibility is required. If the operating conditions are such that the cable is frequently bent (for example, if the cable is connected to a scanning device or a camera that can be rotated horizontally and vertically), a special cable is required. The center conductor in such a cable is stranded (twisted from thin wires). The conductors of the cable must be made of pure copper. Do not use a cable with conductors made of copper-clad steel, because such cable does not transmit signals well at the frequencies used in video surveillance systems.

Foamed polyethylene is best suited as a dielectric between the center conductor and the braid. Foamed polyethylene has better electrical characteristics than solid (rigid) polyethylene, but it is more susceptible to the negative effects of moisture. Therefore, in conditions of high humidity, solid polyethylene is preferable.

A typical CT system uses cables no longer than 750 feet (228 m), preferably RG59/U. If the cable has an outside diameter of about 0.25 in. (6.35 mm), it comes in 500- and 1000-foot reels. If shorter cable is needed, use RG59/U cable with a 22-gauge center conductor, which has a DC resistance of about 16 ohms per 1000 feet (304 m). If longer cable is needed, use 20-gauge cable with a DC resistance of about 10 ohms per 1000 feet (304 m). In either case, cable is readily available with polyurethane or polyethylene as the dielectric material. If cable lengths between 800 feet (244 m) and 1,500 feet (457 m) are needed, RG6/U cable is best. With the same electrical characteristics as RG59/U cable, its outside diameter is also approximately the same as RG59/U cable. RG6/U cable comes in 500 ft. (152 m), 1,000 ft. (304 m), and 2,000 ft. (609 m) reels and is made from a variety of dielectric materials and outer jacket materials. However, RG6/U cable has a larger center conductor diameter (18 gauge), so its DC resistance is lower, at approximately 8 ohms per 1,000 ft. (304 m), meaning that it can travel longer distances than RG59/U cable.

RG11/U cable has higher ratings than RG6/U cable. However, its electrical characteristics are basically the same as the other cables. It can be ordered with a 14- or 18-gauge center conductor with a DC resistance of 3?8 ohms per 1,000 feet (304 meters). Since it has the largest diameter of the three cables (0.405 inches (10.3 mm)), it is more difficult to install. RG11/U cable is typically supplied in 500-foot (152 m), 1,000-foot (304 m), and 2,000-foot (609 m) reels. Manufacturers often make modifications to RG59/U, RG6/U, and RG11/U cables for special applications.

As a result of changes in fire safety and safety regulations in various countries, fluoroplastic (Teflon®) and other fire-resistant materials are becoming increasingly popular as a material for the dielectric and sheath. Unlike PVC, these materials do not emit toxic substances during a fire and are therefore considered safer.

For underground installations, a special cable that is laid directly into the ground is recommended. The outer shell of such a cable contains moisture-resistant and other protective materials, so it can be laid directly into a ditch. With a wide variety of video cables for cameras, you can easily choose the most suitable for specific conditions. After you decide what your system should be, familiarize yourself with the technical characteristics of the equipment and make the appropriate calculations.

Cable length
The signal is weakened in each coaxial cable, and this weakening is greater the longer and thinner the cable. In addition, the signal weakening increases with the frequency of the transmitted signal. This is one of the typical problems of security television surveillance systems (STN) in general.

For example, if the monitor is 1000 feet (304 m) away from the camera, the signal is attenuated by about 37%. The worst thing about this is that the loss may not be obvious. Since you cannot see the information that is lost, you may not even know that such information was there. Many STN video security systems have cables that are several thousand feet or more long (on the order of hundreds and thousands of meters), and if the signal loss in these cables is high, the images on the monitors will be seriously distorted. If the distance between the camera and the monitor exceeds 750 feet (228 m), special measures must be taken to ensure good transmission of the video signal.

Cable Termination
In CCTV systems, the signal is transmitted from the camera to the monitor. Usually, the transmission is via a coaxial cable. The correct termination of the cable significantly affects the image quality.

The characteristic impedance of a coaxial cable is in the range of 72 to 75 ohms; it is necessary that the signal is transmitted along a uniform line at any point in the system to prevent image distortion and to ensure proper signal transmission from the camera to the monitor. The impedance of the cable must be constant and equal to 75 ohms along its entire length. In order for the video signal to be transmitted from one device to another correctly and with low loss, the output impedance of the camera must be equal to the impedance of the cable, which, in turn, must be equal to the input impedance of the monitor. The termination of any video cable must be equal to 75 ohms. Usually the cable is connected to the monitor, and this alone ensures that the above requirement is met.

Typically, the impedance of the monitor's video input is adjusted by a switch located near the loop-through (input/output) jacks used to connect an additional cable to another device. This switch allows you to enable a 75-ohm termination if the monitor is the end point of the signal transmission, or enable a high-impedance (Hi-Z) termination and pass the signal to a second monitor. Check the equipment specifications and instructions to determine the required termination. If the termination is incorrect, the image will usually be too contrasty and slightly grainy. Sometimes the image will appear double, or there may be other distortions.

Typical Requirements for Coaxial Video Cable

* Minimum cable requirements:
•75 Ohm impedance
•Copper center conductor
•Braided copper shield with 95%
insulator coverage by braid

Recommendations for transmitting signals according to the RS-485 standard
The maximum cable length (with 24 gauge wire) for RS-485 communications is 4,000 feet (1,219 m). Pelco recommends using shielded twisted pair cables such as Belden 9843 or equivalent that, at a minimum, meet the basic requirements of the Electronic Manufacturers Association (EIA) RS-485 standard.

Voltage Conversion Formulas
On our product data sheets, we often list the power consumption and supply voltage.
To perform the necessary power supply calculations, use the following formulas.

How to calculate Current (Amps) (from Power (Watts)):
watts/volts = amps (example: 85.5 watts/24 volts = 3.56 amps)

How to calculate Current (Amps) (from Power (Volt-Amps)):
volt-amps/volts = amps (example: 75 VA/24 volts = 3.12 amps 75 VA/115 volts = 0.64 amps)

How to calculate Power (Watts) (from Current (Amps)):
volts x amps = watts (example: 24 volts x 3.56 amps = 85.44 watts)

How to calculate Power (Volt-Amps) (from current (amperes)):
amperes x volts = volt-amperes (example: 3.12 A x 24 V = 74.88 VA 0.64 A x 115 V = 73.6 VA)

* Watts and VA are the same in case of DC, but they are different in case of AC. Volt-amperes, which are greater than watts in case of AC, are used to calculate the power consumption in an AC circuit. But the calculations in both cases are performed using the same formulas.