Video Signals and Control Links

A CCTV transmission system is needed to convey video signals from various cameras at a facility to the security center and to carry commands from the security center to the cameras. Information may be sent by metal cable, radiofrequency transmissions, or optical transmissions.

Metal video cables, such as coaxial cables, are electrical conductors manufactured specifically to transmit frequencies associated with video components. Devices such as video-equalization amplifiers, ground loop correctors, and video-distribution amplifiers may be needed to ensure the highest quality video image.

If the video signal generated by the camera is digital rather than analog, the choice of cables and connectors becomes more important. Inappropriate or degraded cables and connectors can distort the pulses that comprise the video signal, degrading the image quality.

Even when the signal is analog, cable degradation and corrosion on the cable’s connectors can degrade the video signal. When metal video cables and connectors are being installed, require the installer to test the completed installation using a time-domain reflectometer (TDR). A TDR measures some critical properties of a cable-connector run. Better quality TDRs produce a printed graphic display or a stored digital image of the cable’s properties. This data about each cable run should be kept in a maintenance file. As image quality worsens over time, additional TDR tests can be conducted and compared with the initial tests. Comparing the charts from these tests can help determine whether image degradation is caused by cables or connectors, give some indication of the cause, and locate where the degradation occurring.

Radiofrequency transmission may be a good alternative to metal cable and the associated amplifiers in a system with nodes that are separated widely. System information can be transmitted over a point-to-point microwave link for up to 50 miles, if the receiver and the transmitter are in the same line of sight.

A microwave link requires a transmitter and an antenna (usually fairly small and highly directional) at the camera. It also requires a receiver and antenna (also directional) at the receiving end. Microwave link systems must be designed by experienced and competent microwave engineers or technicians to compensate for the effects that weather, climate, foliage, and other factors might have on the signal and ultimately on image quality and reliability.

The same microwave radiofrequency technology that sends the video image from the camera to the receiver site also can be used to send command and control signals back to the camera, controlling features such as pan, tilt, and zoom. Doing so requires additional equipment at both ends. Through a technique called multiplexing, multiple images and command and control information can be sent over one two-way microwave link.

In fiber-optic cable systems, electrical video signals are converted to light signals that are transmitted along the optical fiber. After the signal has been received, it is converted back to electrical energy. An optical driver and a receiver are required for fiber-optic installations. The fiber-optic transmission method provides low-loss, high-resolution transmission 3 to 10 times as far as allowed by traditional metal cable systems. Fiber-optic cable is the transmission media favored by many Federal Government agencies.

A large number of command, control, and communication signals can be multiplexed on fiber-optic cable. Assuming that the fiber has been properly protected from someone cutting it accidentally or intentionally, fiber tends to hold up better than metal cable.

A fiber-optic system requires a small transmitting element, usually located with the camera, and a corresponding receiving element at the point where the optical data is converted to electrical data. Multiplexers are required if the fiber is to carry command and control transmissions as well as image transmissions.

The Internet can be used for video transmission and command and control. An Internet connection offers remarkable flexibility, because any authorized user or multiple users can access the system over the Internet and view images. With proper authorization, users can exert command and control over the system. Video over the Internet can enable a system technician to call up an image and more quickly diagnose problems.

Video over the Internet is not yet the ultimate solution for transmitting security information, partially because the bandwidth needed for real-time transmission of video images is not always available. Video over the Internet is subject to all the vulnerabilities of Internet communication, including unauthorized interception and manipulation.

Wideband radiofrequency transmissions (Wi-Fi, or IEEE 802.11x) technology is being studied as a possible means of transmitting video. Research is underway.