Ghid sisteme de supraveghere - Amplificatoare semnal video

The range of analog video signal transmission is limited by attenuation of transmission line. Here are example distances, over which signal can be transmitted without amplification:
  • coaxial cable, maximal distance for e.g. TRISET-113 - 500 m,
  • telephone cable, 50-70 m depending on the type,
  • computer twisted-pair CAT5 cable, up to 1800m (with special devices used),
  • fiber optic cable, a few tens of kilometers.
Longer distances can be achieved by the use of wireless technology, however it performs processing of the signal and its amplification:
  • radio transmission - a few tens of kilometers,
  • infrared laser - a few kilometers.
For comparison, with telephone line supported by xDSL technology the total achieved distance of transmission of digitalized and compressed video data may reach thousands of kilometers.
Signal amplification is used to compensate attenuation of cables. Attenuation figure depends on cable type and its length. Higher frequencies are more attenuated than lower, therefore we can observe distortion in resolution and color quality. If we want to transmit signal on long distances, we have to amplify it. However, for the reason of not uniform attenuation of lower and higher frequencies, we should use amplifiers with correction of frequency characteristic. Such an amplifier has reverse attenuation characteristic than the cable.
Illustration of correction of cable attenuation
In surveillance installations special amplifiers are used to increase distance over which video signal can be conveyed through twisted-pair or coaxial cable. There are commonly used amplifiers for single lines, e.g. the amplifier AMP-1/2R M1840, which enables adjustment of amplification level. Additionally, it has two outputs, and can work as active video splitter. There are also amplifiers dedicated for 3, 4, 5, 6, and even more lines. Amplification is from a few to a dozen dB and frequency correction range is equal a few dB.
Typical application of video amplifier
Amplifiers are powered either directly from 230V AC power network, or from 12V DC source (usually AC/DC adapter).
Alike other CCTV devices, amplifiers use BNC connectors, sometimes we can find them with F as well as with RCA connector. Input and output impedance is 75Ohm at signal's amplitude 1Vp-p
Video Signal Amplifier AMP 1/2R
Amplifier and video splitter AMP-1/2R M1840
Video splitters.
It is kind of amplifiers equipped with many outputs, so called video splitters or multiplexers. When we want to deliver signal from one camera to several receivers, we should use such a splitter. Obviously, it can't be the same kind as used in antenna installations. Splitters in CCTV, as well as any other video processing devices, must ensure matching input and output impedances and proper levels of video signal. So they must have input and output impedance equal 75 ohm and unit amplification (or 0 dB). Only active splitter can have such parameters, as it has an amplifying element. For that reason such a splitter is also called video distribution amplifier.
Distribution amplifiers are powered either from 230V AC power network or from 12V DC source. BNC connectors (inputs/outputs with 75 ohm impedance) are used. The standard amplification is 0 dB, however some manufacturers produce them with 1 dB gain, which somewhat compensates loss in cables.
Example of using video splitter
Some models of video splitters are shown below:
Active Video Signal Splitter RV-4/8 (4-in, 8-out)
M1704 - Video signal multiplexer RV-4/8 (4-in/4x2-out)
Video Signal Multiplexer RV-1/2P
M1707 - Video signal multiplexer RV-1/2P (1-in/2-out)
Video Signal Multiplexer RV-1/4
M1708 - Video signal multiplexer RV-1/4 (1-in/4-out)
Video Signal Multiplexer RV-2/4
M1709 - Video signal multiplexer RV-2/4 (2-in/4-out)