Fiber optic closed-circuit TV surveillance systems

The first part of this article examined the benefits of using fiber optics and discussed the principles of signal transmission in such systems. In this part, we will look at how to practically design and install a fiber optic system for use in a closed-circuit TV or similar system.

The development process usually includes two components: How do fiber optics work

Selection of suitable active components of the transmission path based on the required function (s), the type and number of fibers available or offered, and the maximum transmission distance.

Passive fiber infrastructure designs, including trunk cable types and specifications, junction boxes, fiber patch panels.

Components of the transmission path

First of all, what components are actually required to meet the system specifications?

Fixed camera systems - These systems are extremely simple and usually consist of a miniature fiber optic transmitter and either a modular or rack-mountable receiver. The transmitter is often small enough to be mounted directly in the camera body and is equipped with a coaxial bayonet connector, a 'ST' optical connector, and terminals for connecting a low-voltage power supply (typically 12V DC or AC). Many surveillance systems consist of several of these cameras, signals from which are transmitted to a central control room, in which case the receivers are rack mounted on a standard 19 ”3U card with a common power supply.

Systems on controlled cameras with PTZ devices- such systems are more complex, since an additional channel is required to transmit camera control signals. Generally speaking, there are two types of remote control systems for such cameras - requiring unidirectional transmission of remote control signals (from the central station to the cameras) and requiring bi-directional transmission. Bidirectional transmission systems are becoming more and more popular, as they allow each camera to receive confirmation of the receipt of each control signal, and therefore provide greater accuracy and reliability of control. Within each of these groups, there is a wide range of interface requirements, including TTL, RS232, RS422, and RS485. Other systems do not use a digital interface, but transmit data as a sequence of audio signals over an analog channel, similar to two-frequency tone dialing signals in telephony. There are also systems that do not require a separate remote control channel at all, but transmit control signals via a coaxial video cable - either during a blanking interval or by modulating a high-frequency carrier.

All of these systems can work with fiber optic cables using the appropriate equipment from Fiber Optic. Fortunately, the customer is not required to delve into the intricacies of PTZ remote control in order to determine their requirements; it is simply required to indicate the preferred camera control system and estimate the maximum communication length so that the appropriate equipment, fiber and operating wavelength can be recommended.

There is, however, one question that the customer usually has to answer himself - should the remote control signals for cameras mounted on a PTZ device be transmitted over the same optical fiber as the video signal, or over a second fiber.

Under normal circumstances, simultaneous transmission of optical signals along the same fiber in opposite directions is undesirable, since mutual interference occurs due to diffuse reflections in the fiber. In closed-circuit TV systems, this effect creates noise in the image whenever the camera controls are activated.

To achieve bi-directional, interference-free transmission over a single fiber, it is necessary that the transmitters at different ends of the fiber operate at different wavelengths, for example, 850 nm and 1300 nm, respectively (Figure 2.2). A wavelength division multiplexer (WDM) coupler is connected to each end of the fiber to ensure that each receiver receives only the correct wavelength (e.g. 850 nm) light from the transmitter at the opposite end of the fiber. Unwanted reflections from the near-end transmitter are in the “wrong” range (ie 1300 nm) and are rejected accordingly.

Bi-directional single-fiber transmission incurs additional costs comparable to the cost of a dual-fiber system. This, however, can be offset by savings in the reduced fiber count.

Additional features - although the choice of a fixed camera or a PTZ camera satisfies the requirements of most closed-circuit TV surveillance systems, there are a number of systems that require additional features, for example, audio transmission for general announcements, auxiliary messages to the consumer, or intercom communication with a remote post ... On the other hand, contacts of sensors that are triggered in the event of a fire or the appearance of strangers can be part of an integrated security system. All of these signals can be transmitted over optical fiber - either over the same one used by the network, or over another.

Video Multiplexing - Up to 64 video and 128 audio or digital data signals can be multiplexed on a single single-mode fiber, or slightly less on multi-mode. In this context, multiplexing refers to the simultaneous transmission of full-screen video signals in real time, rather than the low-frame or split-screen display, which is often referred to as the term.

The ability to carry many signals and additional information over multiple optical fibers is very valuable, especially for long distance CCTV surveillance systems, such as highways or railways, where minimizing the number of fiber optic cables is often vital. For other applications, with shorter distances and highly scattered cameras, the benefits are not so obvious, and here the first thing to consider is using a separate fiber line for each video signal. The choice of whether to multiplex or not is quite complex and should only be made after considering all the considerations, including system topology, overall costs, and last but not least, network fault tolerance.