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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Multifunctional interactive cable television systems. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Телевидение The article is devoted to the consideration of the general principles of constructing multifunctional cable television systems, which are beginning to increasingly interest both users and operators in different regions of the country and in the implementation of which the first steps are being taken. The transition to digital television is the most important scientific and technical problem currently being actively solved in the field of information and telecommunications. The user receives very tangible new opportunities: multi-programming, interactivity, multi-functionality (telephony, data transfer, video conferencing, Internet access, multimedia and a whole range of other services); improving the quality of TV reception. In addition, the radio spectrum, channel capacity, etc. are used more efficiently. Among the methods of delivering TV programs to subscribers, cable television systems (SCTV) have occupied a strong place, especially in large and medium-sized cities. In the West, interactivity began to be introduced into SKTV in the mid-90s. In the USA and Canada, about 1998% of residents had the potential to use interactive cable network services in 11. To create modern information and telecommunication cable networks, it is necessary to solve the problem: modernize previously built systems or build new networks on uniform standard principles. Currently in Russia, most televisions are connected to collective reception systems of various generations: “entry antenna”, a large system of collective television reception (KSKPT) and a cable television system (SKTV). The first option, the “entry antenna,” is a predominantly passive network with directional couplers. If it is necessary to amplify a TV signal, one cascade of channel or range amplifiers or decimeter converters is used. KSKLT and SKTV were built to improve the quality of TV reception; the systems use headends with channel-by-channel signal processing and frequency conversion of channels: broadband trunk and home amplifiers with a bandwidth of 40 - 240 MHz without a return channel and frequency-independent trunk couplers and subscriber splitters. The most active construction of such systems was carried out in the 80s and continues now despite their limited capabilities in terms of increasing the number of organized TV channels, interactivity and use for solving multifunctional network problems. More than 80% of capital investments are spent on the creation of linear cable network structures, so it is very sad that KSKPT and SKTV built according to this principle are completely futile. Reconstruction of the distribution networks of these systems in order to transform them into coaxial nodes (bushes) of any capacity for multifunctional systems is impossible without a complete replacement of all components - coaxial cables, passive elements and amplifiers due to the fact that not only the required frequency range is not provided even when replacing passive and active elements of the network, but it is also impossible on existing networks to achieve the required (subject to the distribution of 40 - 50 analog television channels) minimum level of combination interference, nominal input signal level at the subscriber point, etc. Thus, these distribution networks are doomed and construction they are not historically justified today. To stop this process and give recommendations for expanding or reconstructing the network in each specific case is apparently the task of the Russian Cable Television Association and the Russian Ministry of Communications. Recently, the introduction of broadband SCTV with a return channel has begun (40 - 862 MHz band in the forward direction and 5-30 MHz return channel band). The architecture of these networks is subordinated exclusively to the interests of cable television, and it does not correspond to the tasks solved by broadband multifunctional interactive networks, which are based on the use of fiber-optic systems for transmitting analog and digital signals with high quality over long distances. At the same time, the scope of services should not be limited by the interests of traditional volumes of television and radio broadcasting. Already today it is becoming obvious that there is a need for such services as the provision of paid digital television channels and television on demand (video on demand), multimedia, teleshopping, security and fire alarms, the use of a network for dispatch systems for engineering equipment of residential neighborhoods, the provision of data transmission channels; organization of local and corporate computer networks; connecting subscribers to the Internet; digital telephony, etc. It is likely that simply replacing part of the backbone coaxial network will not ensure full use of the capabilities of fiber-optic communication lines and will sharply limit the size and technical capabilities of SCTV. Consequently, the adaptation of broadband SCTV into telecommunication networks requires a revision of the network architecture of the SCTV itself (see figure).
Traditionally, the architecture of high-capacity SCTV (for five thousand or more thousand subscribers) is a tree structure. For coaxial (especially unidirectional) networks, it provided the best price-quality ratio. However, for an interactive network, such a construction limits the possibilities of creating a reverse channel. In hybrid networks (fiber optic - coaxial), each part, to one degree or another, imposes some specific requirements on each other - fiber optic to the quality of the signal delivered through the coaxial network and, accordingly, vice versa. When designing a SCTV, the calculation of parameters was reduced to determining the optimal signal level at the output of the subscriber connecting device and the output level for series-connected amplifiers, which was limited only by the amount of noise. To include SCTV as a component in a hybrid fiber-coaxial network, it is necessary to recalculate taking into account second-order (CSO - Composite Second Order) and third (STV - Composite Triple Beat) intermodulation distortions. Depending on the parameters of the fiber-optic part of the network preceding the coaxial one, during recalculation the latter may be subject to additional requirements not only for changing the output levels of amplifiers and limiting the number of amplification stages, but also for some rearrangement of the home distribution network in order to increase the minimum signal levels by taps of subscriber splitters. Most likely, such changes will be within the scope of system reconstruction and will not require its global reconstruction, i.e., significant capital expenditures. However, when designing and building broadband interactive SCTV, it is necessary to take into account (and world practice has confirmed this) that at this stage the most cost-effective option for building telecommunication networks that provide subscriber access with multifunctional tasks are hybrid structures using fiber-optic and coaxial cables . In the foreseeable future, telecommunication networks, used in particular for the transmission of TV signals, must have a structure and use transmission systems compatible with traditional analogue and increasingly digital ones. TV signals in broadband networks occupy a huge bandwidth that is incommensurate with other media, and delivering these signals to subscribers in digital form is the most difficult. It must be assumed that in the next decade, hybrid networks will be the dominant structure, with the conversion of digital TV signals into analogue both for a group of subscribers and the use of individual subscriber terminals. The process of creating a subscriber access network (first level) based on fiber-coaxial distribution networks can be accelerated only under the following basic conditions: - development of a regulatory and technical framework that allows us to begin mass design of networks using hybrid technology: - widespread construction of a transport information highway, formed taking into account the transmission of existing and future TV programs; - prompt conversion of broadband interactive SCTV into coaxial nodes, providing the technical capabilities of modern telecommunication networks. Fulfilling these conditions will allow you to avoid investing in obsolete or unpromising technologies. The backbone access level from the transport hub to the coaxial network node (second level) is built, like the previous one, on the basis of a fiber-optic line. but unlike transport, it can be not only digital, but also analog. The third level is the coaxial sub-main and home distribution network, including subscriber coaxial cables to the subscriber terminal. This network covers from several hundred to a thousand or more subscribers. The first two levels are built, as noted, on the basis of fiber-optic communication lines, which have many advantages, which have been repeatedly written about in the magazine. Let us only note that they allow you to organize several dozen television channels. Even when transmitting digital TV at speeds of 2.5 and 10 Gbit/s, the potential capabilities are at least an order of magnitude higher when using modulated radiation of the same wavelength. But today the industry is producing wavelength division multiplexing devices that allow a sharp increase in the number of signal streams. Spectral multiplexing allows you to increase the speed of digital signal streams, and for analog TV signals such multiplexing is a new quality. Today there are two types of transmission of such signals: the first - one fiber is used to organize one channel: the second - transmission of a spectrum containing a certain number of TV channels. Spectral multiplexing makes it possible to optimize fiber-optic links in both cases. The independence of signals propagated in one fiber towards each other at the same wavelength and in the same direction at different wavelengths provides unique opportunities for the implementation of new network architectures, including in relation to interactive networks. Here, not optoelectric and electro-optical converters are used, but optical couplers, which allow branching off a certain share of optical power and inserting an optical signal into the general flow, i.e., a tree-like bidirectional structure is implemented: TV and any other flows are transmitted from the trunk to the branches, and in the opposite direction - narrowband signals of subscriber requests, voice, signals from various sensors, etc. The third advantage is the flexibility to rebuild the network from analog to digital. In a number of Russian cities, work is already underway to create information cable networks with the possibility of integrating the services provided. Let us draw a conclusion based on the above. For any telecommunications system, the most expensive and labor-intensive part of it is the network. Therefore, during the transition to fully digital technologies, distribution networks should be built using traditional structures as much as possible. which is essential, allowing the use of both digital and analogue transmission systems for a certain time. Authors: S.Dmitriev, K.Kukk, B.Exler, Moscow
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