Criteria for selecting equipment for transmitting video images.
In the simplest case, it is enough to connect a video camera to a monitor, and a security television system is ready. However, it is often necessary to transmit a video signal over significant distances or via specific (existing) cable networks.
There are many options. Amplifiers and correctors allow you to transmit further via regular coaxial cables (or transmit at least 200 meters via very bad cables). Passive or active transmitters via twisted pair allow you to use TPP series cables or a structured cable network available in the building (active ones with an amplifier additionally increase the transmission range). Fiber-optic transmitters protect the signal from interference and guarantee transmission over many kilometers. ATM/SDH equipment offers to combine video with general-purpose trunk transmission channels. IP coders and decoders allow (theoretically) to transmit a video signal via the Internet to the other end of the planet.
In what cases and what devices are more appropriate? Each has a very specific purpose. An amplifier-corrector for a coaxial cable is often used not during design, but during commissioning. If the signal quality is low, then an amplifier is added at the last moment to correct the situation. Often, together with the amplifier, it is necessary to install a galvanic isolation device that breaks the «ground loops». Of course, coaxial amplifiers are sometimes included during design, so as not to increase the range of cable products, especially if the project only has a few lines on the edge of the permissible transmission range.Passive twisted pair transmitters (so-called balloons) came into use when a structured cable network, designed with a significant capacity reserve, became the norm for equipping any office building. The existing network is used to transmit analog video signals. In general, this solution does not always work, because interference from co-laid digital lines on an analog video signal can be unexpectedly significant. Crosstalk standards for category 5 digital lines are only 3 dB. Of course, reality is usually better, but there are no guarantees. Let me remind you that 3 dB means that the interference is only 1.5 times less than the signal. Even for category 6, the standards (37 dB) are worse than the minimum permissible signal/noise for a video signal. Of course, the tests are carried out at a frequency of 100 MHz, and in the video signal we are interested in a band of up to 6 MHz, of course, the tests are carried out under the condition of feeding an extraneous signal to all pairs except the one being tested, in life everything is easier, and not all pairs, even occupied ones, constantly carry digital signals, but it is difficult to hope for good quality. In reality, such technology can only be used if you are sure that the channel you are using is laid far enough from digital cables and does not have sections in multi-pair cables together with digital signals.
Active twisted pair transmitters are used as a deliberate design solution for transmission over distances exceeding the 200-300 meters standard for coaxial cable. Unlike coaxial cable amplifiers, such transmitters allow the use of relatively cheap twisted pair of large cross-section (coaxial cable with a central core cross-section of 0.5 mm2 is significantly more expensive and can be bent only with two hands, and sometimes only on the knee). The limits of applicability of analog copper wire transmitters depend on the situation and the manufacturer's marketing activity. In my opinion, 500 meters is quite realistic, a kilometer — only if you have modest requirements for transmission quality. Again, this is a subjective opinion. Many such devices have the means for tuning to a specific line, a specific cable instance, and with careful tuning on site can provide quite decent quality. I remember that the set produced by PHILIPS had a five-band equalizer, and the instructions said that adjustments using a sweep frequency generator and an oscilloscope should be made at least twice a year.
It should be noted that most (but not all!) of both passive and active twisted pair transmitters provide galvanic isolation of the «ground», which is extremely important for long lines connecting equipment installed in different buildings or on a long perimeter.
Fiber optic systems. Certainly, the most noise-resistant devices. Internally, fiber optic systems are divided into digital and analog, as well as multimode and single-mode. Single-mode modifications operate stably at any (within a reasonable object) distance — up to 20-30 km. Multimode (using a cheap LED emitter instead of a laser) operate at 500-5000 meters, often their behavior strongly depends on non-standard cable parameters. All analog systems to some extent depend on the stability of the optical contact on all connections, and on the stability of the fiber itself. Do not forget: ordinary fiber cable is used as a sensitive element of vibration perimeter detection devices. When transmitting a video signal, such sensitivity is an undesirable effect. Of course, with automatic gain control, the effect of changing weather conditions and cable aging may be unnoticeable. Considering that analog devices differ from digital ones by about an order of magnitude in price, it is clear why analog ones are still sometimes used.
Digital systems also come in different forms. The most common ones have an 8-bit signal representation, which even theoretically limits the signal/noise ratio (more precisely, the ratio of the signal to the distortions associated with digitization) to 50 dB. This is not a small amount, but it is also not much.
When are fiber optic transmitters used? Firstly, if the distance exceeds a kilometer. In this case, you should not hope for transmission via a copper twisted pair. Secondly, when there is a developed network of fiber cables at the facility, the installation and operating organization has experience in repairing fiber lines. In this case, fiber should be used for any line longer than 200 meters. Finally, in the presence of powerful electromagnetic interference. I remember a case when sections of coaxial cable of 100-200 meters had to be replaced with fiber optic ones, since they passed by small (by customer standards) ore mills — their drums were only 12 meters high.
Finally, let's mention digital transmitters «with compression». These are transmitters via telephone lines, via the Internet, via Ethernet. In this case, the problem of transmission range disappears (or rather, is solved by standard means from another area — computer networks). However, all such systems have the following main disadvantages that should be remembered. First: digital compression degrades the signal. To what extent — there are no standard methods for describing such distortions (or rather, they are not used). Second: the computer network must have the ability to fine-tune QoS (or traffic restrictions) so that a considerable flow of data carrying a video signal does not conflict with other possible data in the network (and with itself).
I have listed only the most common systems. There are also systems with multiplexing on radio frequency carriers (similar to cable television networks), microwave and other radio transmitters, but their application is very specific. Modulators in television channels are used if a regular TV is desired on the receiving side, showing several «programs» of security television along with broadcast programs. Radio systems are used for mobile (quickly deployable) complexes or in case of extreme high cost of cable laying (for example, transmission 50 meters to the other side of Tverskaya Street or transmission 5 kilometers in the middle of the Yamal swamps). Radio systems, as a rule, entail problems of certification, licensing, channel allocation. In general, they are used only if there are no other solutions. The only exception is the use of standard IP solutions over standard (unlicensed) Wi-Fi technology.
In addition to the main design features, all devices for transmitting video signals may have several additional useful properties, which, of course, are indicated in capital letters in their advertising descriptions.
The ability to isolate ground loops is mentioned above. When using copper communication cables, a break in the «ground» is absolutely necessary if the source is grounded. In particular, when transmitting («flipping») a signal from one observation post to another, both ends of the cable are grounded, which leads to the formation of «ground loops» and, as a rule, an irreproducible, but unacceptable loss of signal quality. Even if the source is a video camera, seemingly isolated from the «ground», a loop can form with the slightest damage to the outer shell of the coaxial cable, therefore, for all lines outside the premises, it is recommended to use devices with galvanic isolation of the «ground» (common wire).The second useful property is the ability to protect the equipment of the central post. To protect from lightning discharges and other high-power impulse interference. For fiber-optic lines, this problem does not exist (at the receiving end) — impulse interference does not spread along the fiber. And for all systems with copper cable (including relatively short coaxial lines, for which even amplifiers are seemingly not required) protection from impulse interference is a real serious problem. After all, the central post often has very expensive equipment, and it can easily fail if at least one of the incoming lines is induced by a nearby lightning strike exceeds the resistance of the central device itself.
Please note that lightning protection of the transmitting end of the line (from the side of the video camera) is also needed for fiber-optic lines if the camera is located at a significant distance from the fiber-optic transmitter.
Lightning protection is declared as part of many products, lightning protection devices can be purchased separately, but, unfortunately, only very rarely is their actual ability to withstand (and protect the equipment behind them) specific interference amplitudes indicated. But there is an international standard (and its literal Russian translation) describing test methods and recommended equipment classes for various application conditions. Moreover, domestic GOSTs for security equipment clearly indicate the required level of protection. However, even highly qualified manufacturers often produce simplified (cheaper) versions of devices, which, of course, somewhat reduce the likelihood of equipment damage from a thunderstorm, but their data does not allow even a specialist to determine in which cases their use is acceptable and in which cases it is not worth saving.
Have you considered everything? It seems so. But why do some devices, seemingly identical in technology, differ in price by a factor of 5 or even 10? Sometimes the difference in price is due solely to the greed of the seller, but often has a real justification. The elusive concept of «quality» has a strange property. A little bit better costs significant money. For example, the transition from 8-bit to 10-bit digital representation can lead to a twofold increase in the cost of equipment. Are there real quality criteria? Yes, there are. But it's not that simple.
For example, there is GOST R 50725-94 «Connecting lines in image channels». This GOST is directly intended for radio relay, fiber and coaxial transmission lines. However, its origin is broadcast television. The requirements it imposes are excessive for security television systems. And the equipment needed to check the compliance of the transmission line with this GOST costs from 000 (the simplest domestic one). It is unlikely that ordinary consumers (and most manufacturers) have it.A more common method is the so-called subjective quality assessment method, this is not a joke, this is GOST 26320-84. Unfortunately, the use of this GOST often unreasonably overstates the quality assessment. For broadcast television, if the viewer is satisfied, the problem is solved. For security television, the operator must not only be satisfied with a pleasant picture, but also be able to detect the criminal.
Therefore, when applying quality assessment according to GOST 26320, it is necessary to use not the plots recommended in this GOST, typical for TV programs (football, travel, cartoons, close-up of a head), but plots specific to security operational tasks: detection of an intruder in camouflage, classification of actions (usual/unusual). In addition to the subjective point assessment of quality, it is possible to use the objective parameter «operator reaction time» — the time during which he performs a given task (detection of an intruder, confirmation of an alarm, making a decision on admitting a person, searching for a given person among the persons in the frame, etc.). However, such methods ensure quality control not only of the transmission line, but also of the entire end-to-end path, from the lens to the monitor, including even the efficiency of the lighting system. A detailed description of the methods of integrated assessment of the quality of video surveillance systems can be found in the materials of the HOSDB
(http://scienceandresearch.homeoffice.gov.uk/hosdb/cctv-imaging-technology/).
