Video signal transmission methods
Caiman Production Group supplying equipment for video surveillance systems suggests different solutions in the aspect of video transmission at a short distance. Depending upon service conditions, the haul and some other conditions different types of conductors and transmitter-receivers are used for data transmission. It does not immediately strike non-specialist’s eyes but video transmission ways are one of the most important links in the chain of video surveillance system. The cost of routing, of conductors and auxiliary transmitter-receivers can compose essential part of general expenditures for video surveillance system, that is why the way of video transmission should be thoughtfully chosen.
Few critical requirements are imposed to the systems of video transmission. Firstly, it is minimal transmitted picture skewing, secondly, the interference immunity and environmental resistance and thirdly, off course, protection against unauthorized interception and transmission of false video data as it runs about security systems. Different ways of video transmission under various highway length and divergent service conditions meet the requirements in a varying degree. In projecting of video transmission system the professionals of Caiman Company take into consideration the haul, peculiarities of itineraries and number of communication lines, electromagnetic environment and climatic conditions on the enclosed object, ways of peripheral equipment’s supply, necessity of few signals transmission by the same lines and certainly the budget of video surveillance system in general. For video transmission depending upon the characteristics of the object few modes can be used.
Transmission of high-frequency analog video by nonsymmetrical line on short distances up to 300 meters is widely-distributed. As a conductor co-axial screened cable is used, such line has high coefficient of signal attenuation which depends a lot upon the cable type. The lower is attenuation the higher is the price of co-axial cable. To decrease losses while transmission by co-axial line on transmitting side the signal amplifier can be used. Co-axial lines have low interference immunity, big video signal losses and expensive price of the cable itself, correct grounding is also required.
One more mode of analog video output is balanced line or twisted-pair cable. In this case low-frequency analog video is transmitted along the line; transmission distance can reach two kilometers. Balanced line is more stable towards interference than single-wire line and the cost of this cable is much lower than co-axial. In dependence upon the types of transceivers the systems of video transmission at the distance of 500, 1000 and 2000 meters may be utilized.
For signal transmission various types of conductors can be used, but in every separate case individual adjustment of transceivers for certain cable variant is needed. In the case of twisted-pair cable utilization for video transmission separate line for every source of video signal should be lined up. In case of single-wire transmission line simultaneous image transmission should be avoided to evade criss-cross interference at the line.
The most qualitative and the most expensive video transmission mode is fiber-optic. The use of fiber-optic cable as a conductor enables to transmit video signal without considerable losses for tens of kilometers, in this case data from various sources of video signal can be transmitted. Optic communication lines cannot be affected by electromagnetic interference, are properly secured against interception and tampering and have huge throughput. High cost of transceivers especially for the ones exceeding 3-4 kilometers distances and excess throughput surpassing requirements towards communication lines for video surveillance systems may be considered as shortcoming. The sum total of these factors makes it possible to recommend fiber-optic video transmission lines for important objects mainly where high expenditures for security systems are quite justified.
Lately more and more attention is paid to digital modes of information transmission. The progress in algorithm elaboration, in digital image compression and transmission sphere makes possible the usage of existing computer channels of communication for analog video transmission, what perceptibly reduces expenditures for video transmission channels. Digital systems of data transmission are mainly based on computer or specialized system of video data logging united with other equipment by digital communication channels. Digital mode of video data transmission has a lot of merits starting from structured data archive with fast access and ending by devices control in video surveillance system. Unfortunately, this mode is restricted by the throughput of digital channels’ basic mass, what not always makes possible thorough use of digital mode of video transmission.