For effective planning an analog video surveillance system in addition to the consideration of the requirement profile of the employed cameras and recording forms include the choice of connecting cables for the video signal and the operating voltage. In addition to the bridging distance and possibly existing lines and the installation costs play an important role.
For the analog video signal four transmission paths are:
The transmission of the video signal via coaxial
The special form of single-cable, transmission over two-wire line and radio transmission digital signals in the video surveillance can be conducted via network cables or by using special converter via coaxial cable.
Transmission via coaxial
The method most commonly used for connecting video components is a connection over a coaxial cable. The video signal is transmitted directly. As a cable, a special video cable is used with an impedance of 75 ohms. In addition to ready-made BNC cables and the meter can be used, which is equipped only on site with the right plug connectors. The coax cable in this case, the video signal to the recorder, the power of the camera is done separately.
How long the connection cable must be?
This is probably the most frequently asked question. The maximum cable length depends on the attenuation of the cable type used. Simple rule of thumb: The thicker the line, the lower the damping, and the longer the line used must be. The maximum attenuation by the connecting line should be three decibels not exceed, for a reduction of a signal to three decibel means a halving of the level. Especially modern digital recorder show picture disturbance when the level is too low.
For the most commonly used RG-59 cable, the attenuation is at five megahertz 2.4 decibels per 100 meters. If permitted three decibel damping the cable length is a maximum of 125 meters. Now for a cable length of 126 meters, the signal is not equal disappeared, but it show up with increasing cable length fewer fine details and sharp contours, since the high frequencies that cause these signals are attenuated most.
Special Form: The single-cable
Particularly in compact camera systems, single cable technology is used. Here, a cable is only performed to the camera. These camera systems to connect via a coaxial done. So both the video signal as well as the operating voltage to be transmitted simultaneously over the coaxial line. The signals are separated in a respective special supplied power adapter.
By single-cable technology, the number of cables to be laid is reduced and there are material and labor cost savings. But this is not the only advantage. By systemic use of active components in the food adapters, longer line paths are possible. Depending on the cable type used for the interconnector to be up to 500 meters to the power adapter.
The advantages of the single cable can be used with an adapter for cameras that are not prepared for this type of transmission. Such a system always consists of two adapters. Some can not even send audio in addition to video signal and supply voltage. Thus pathways are also possible to 500 meters.
Transmission via two-wire line
The transmission of signals via a two-wire connection offers many advantages:
Spanning long distances, with active systems up to 1,500 meters (color) signal, cheap cable material, CAT 5 UTP cable is sufficient as a rule, multiple use of a cable, to four signals via a CAT-5 cable, less interference by symmetrical signal transmission.
The signal transmission takes place via a respective twisted pair of cable. Here are some of the advantages: There are four pairs in as a CAT 5100UTP line, four signals can be transmitted simultaneously. At four camera signals to be transmitted from a warehouse in an office, no longer need four video cables are laid, but only a CAT-5 cable. At each end of the CAT-5 cable is a passive four-channel transmission unit is required in this case.
A further advantage is the maximum usable length of cable. In passive two-Drahtübertragern the maximum length is 400 meters for color signals and 600 meters for B / W signals. With active individual routes even 1,500 meters or 2,400 meters are possible. In addition to the transmission of video signals, it is also possible with special transformers to transmit audio signals up to 1,000 feet or VGA, Mouse, and keyboard signals of a PC or digital video recorder up to 100 meters.
In some cases, the laying of cables is not possible, for example, when a square or a street is to be bypassed. Here a radio transmission comes with a transmission frequency in the range of 2.4 or 5.8 GHz for use. Here in the 2.4 gigahertz band three of five possible channels and in the 5.8 gigahertz band six of 16 possible channels are available simultaneously. When selecting the frequency band has to be considered that in the 2.4 gigahertz band today also wireless connections are made and the interference can not be excluded. This frequency overlap does not occur in the 5.8 GHz band.
One of the most frequently asked questions about video spark gaps is the question of the achievable range. This can, however, difficult to predict. The frequency of 2.4 or 5.8 GHz does not penetrate metallic obstacles. To metal surfaces or metal gratings, the radiation is reflected, i.e. deflected in a different direction. Other obstacles, such as walls, windows or bushes and trees are indeed penetrated, but it is partly to significant attenuation of the signal.
To achieve an optimum result, should therefore be possible to ensure that between transmitter and receiver is no obstacle (line of sight). Then distances of 250 meters or if external receiving antennas are used, larger ranges are possible with high-quality systems. The attenuation of walls is also the reason why the range in buildings is relatively low, compared to the large distances that can be achieved to outdoors.
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