Introduction to Cable TV
CATV (Cable Television) for short Or fiber optic copper cable hybrid system. The earliest cable TV service appeared in 1948, when the cable TV network was used to realize the simple retransmission of broadcast and TV programs. In the early 1980s, cable television networks achieved large-scale coverage all over the world. Cable TV network companies are operating companies that provide cable TV services to users.
Advantages
Because the TV signal is transmitted through cables and is not blocked by high buildings and mountains, the viewing quality is good.
Cable TV can use adjacent frequency transmission. Unlike radio, which is regarded as preventing interference, it must use frequency-separated transmission in a region. Therefore, spectrum resources can be fully utilized and more channels can be provided.
Cable TV can also realize the two-way transmission of signals through cables, can provide interactive two-way services, and can also easily realize charge management and carry out a variety of paid services.
The cable TV station does not need expensive transmitters and huge iron towers, so the cost of building the station is low, which is conducive to rapid development.
The cable television system not only receives and rebroadcasts programs from major television networks, but also provides self-recorded programs with different thematic content, providing subscribers with more program choices.
History
The TV system first appeared in the United States in the 1940s. The original system was used in the shared antenna TV of apartment buildings; it is impossible for residents in apartment buildings to have every household on their roof Install the antenna, so the shared antenna TV system came into being. The shared antenna TV is very simple. It consists of a shared antenna TV amplifier installed near the main antenna and wired cables to each household. Later, due to the development of high-power broadband amplifier technology, a cable TV system with a transmission range including a city appeared. Since the shared antenna TV system has a broadband amplifier and a cable and signal distributor for signal transmission, it can be regarded as a city-wide cable TV system. The predecessor.
The development of China’s cable television began in 1974, which can be summarized in three stages:
1) The first stage: the shared antenna stage from 1974 to 1983 . Its technical feature is that it uses frequency-separated transmission in all channels, and a shared antenna system can transmit five to six sets of TV programs. This stage can be called the primary development stage of cable TV.
2) The second stage: the cable TV stage from 1983 to 1990. Its technical feature is that it adopts adjacent frequency transmission for enterprises or metropolitan area networks that are mainly constructed by means of cables, and can transmit about 10 sets of TV programs. Starting from the cable TV network established in Shashi in 1985, the cable TV network evolved from the shared antenna stage to the cable TV stage. However, at this stage, some local and municipal cable television networks have begun to use optical cables for long-distance transmission.
(3) The third stage: from 1990 to present. This stage was marked by the "Interim Regulations on the Management of Cable TV" promulgated by the government on November 2, 1990. Since then, China's cable TV network has entered a high-speed, standardized and legalized development track.
Cable TV system
Currently, a typical cable TV system is mainly composed of the following four parts: signal source part, front-end equipment part, trunk transmission part and user distribution part, as shown in the figure 1 shown.
The signal source part
The signal source of the cable TV system is divided into two categories: one is the various TV signals that are transmitted from the air. Including satellite TV signals, V-segment, U-segment TV signals, and other TV signals transmitted by cable stations via microwave (or optical cable); the other type is TV programs run by the cable TV system. Its main equipment includes satellite ground station, microwave station, V-segment and U-segment receiving antennas, cameras, video recorders, TV OB trucks, broadcast control equipment and system management computers.
Front-end system part
The front-end system is used in the cable TV system to process and make the TV signal received through the antenna and self-organized A series of equipment suitable for channel transmission. It is located between the signal source and the main line transmission system, and its function is to perform necessary processing on multiple sets of TV signals sent by the signal source, and then mix them into one signal and send it to the main line transmission system. Its main equipment includes receivers, modulators, channel amplifiers and converters, pilot signal generators, mixers, demodulators, transponders and time base correctors. Since the signal source and the front-end equipment are usually together, at present, they are collectively referred to as the front-end system in the system.
Part of the trunk transmission system
The role of the trunk transmission system is to transmit system signals. It is mainly composed of various types of trunk amplifiers, trunk cables, and trunk optical cables. , Optical transmitter, optical receiver, multi-channel microwave distribution system and FM microwave relay and other equipment and equipment. Its task is to transmit the high-frequency television signal output by the front end to the user distribution network with high quality. The transmission mode of the trunk system mainly includes coaxial cable transmission, optical fiber transmission and microwave transmission and their mixed transmission.
User distribution system part
The user distribution system is the last part of the cable TV system, and its function is to distribute the signals from the transmission trunk to thousands of households , It includes user distribution amplifiers, distributors, splitters, user terminal boxes and other equipment and devices. The function of the distribution amplifier is to compensate for the signal loss of the branch line book and amplify the signal power to support more users. Distributors and splitters are passive devices for distributing the signal to each branch and each user, and require good isolation and proper output level. User distribution networks generally use thinner coaxial cables to reduce costs and facilitate construction.
In addition, in the composition of the entire cable television system, there are two problems to be noted: one is the problem of system power supply, and the other is the problem of system lightning protection. The power supply of the front end of the system is generally not a big problem, but the power supply of the main line part and the distribution part should be properly considered according to the local power supply environment. Generally, there are two modes of centralized power supply and decentralized power supply. In a cable TV system, in order to improve the condition of receiving signals and enable the receiving antenna to provide high-quality TV signals to the front end, the antenna is usually erected at a high place, so the antenna is the most vulnerable part of the system to lightning strikes. In order to prevent lightning strikes, lightning rods should be installed in the area of the receiving antenna, and a security device should be installed at the output end of each antenna. In addition, the erected cables are also vulnerable to lightning strikes, so when the cable TV system transmission trunk line is long, the outer conductor of the cable can be grounded at an appropriate distance (200-300m).
Key technology
Broadband access network technology
With the maturity of Internet technology, high-definition transmission through the network Multimedia data such as audio, video, image, etc. are constantly swallowing limited channel bandwidth. Therefore, cable TV broadband access network technology has undoubtedly become a hot spot today. This two-way access mode of broadband network based on cable TV has been accepted by more and more users due to its flexible application, fast speed, and low cost. It has become the current commercial households and Broadband access technology commonly used in private housing.
Currently, the existing cable TV networks are generally unidirectional fiber-coaxial hybrid networks (HFC), generally using AM-VSB modulation and star-tree topology, and the front-end system converts CATV signals into optical Then, the optical receiver is used to convert the optical signal into an electrical signal, and then the electrical signal is transmitted to the end user through the coaxial cable distribution network.
In the cable TV broadband access network structure, analog TV and digital TV, telephone and data services are integrated in the central office. Analog and digital TV, telephone and data services share a downlink optical transmitter, which uses an optical fiber to quickly transmit the downlink service to the corresponding optical node, and then converts these downlink optical signals into radio frequency signals. The optical nodes use a star-tree network topology, each with multiple coaxial two-way electrical thin lines to cover about 500 users. Uplink signals are converted into uplink radio frequency signals at the integrated service subscriber unit, and then quickly converted into optical signals by optical nodes, and then transmitted back to the central office through optical fibers, uplink transmitters, etc., and converted into radio signals. , And finally realize the interconnection of the data signal with the data network at the router, and smoothly send the VOD upstream control signal to the VOD server.
Digital television technology
Digital television technology can be understood as a television device with digital signals. This technology compresses and modulates the TV signal from the cable station into a digital TV signal. The signal is transmitted through cable TV optical cable and TV satellite. After receiving the signal, the TV is equipped with digital TV. The set-top box demodulates and decodes the digital signal to restore the original TV image and sound. The entire signal transmission process is completed through digital technology processing. Therefore, this technology is called digital television technology.
Digital TV technology has the following advantages:
1) The transmitted signal is more stable. The application of digital TV technology in cable television networks can be said to be a historic advancement. Compared with previous TV transmission technologies, digital TV technology has higher signal stability during signal transmission, which plays an important role in the transmission of TV signals. Role.
2) Strong operability. When processing the transmitted signal, digital TV technology is more convenient and effective for signal processing, and has stronger operability. Digital TV technology uses asynchronous processing methods in signal processing, which can increase the processing speed of the transmitted signal and increase the signal Processing efficiency.
3) The definition of the TV screen is high. The traditional TV signal transmission process focuses on the usability of the signal. With the rapid development of science and technology, people have higher requirements for the quality of TV pictures, and the application of digital TV technology has largely met people's requirements.
4) You can expand the range of channels you can watch. Digital TV technology can transmit more TV signals through its technical advantages, and with the continuous improvement of digital TV technology, the range of channels that users can watch will expand, providing viewers with more excellent programs.
5) Realize the integration with Internet technology. Digital TV technology can realize technical services that cannot be realized by traditional TV transmission technologies, such as channel opening on-demand and TV information encryption processing. Combining digital TV technology with the Internet is also the development trend of the future TV media industry.
Decoding information resources
Decoding technology in digital cable TV mainly refers to video decoding technology. Digital cable technology has gradually spread. Compared with my country’s previous high-tech technology, the advantage of such a TV system is that it effectively improves the overall reduction technology of the signal and video that is difficult to control. The relevant staff can use the 1920 display mode to achieve compression and processing of digital signals, The code rate drops to 30Mbit/s, which is 1/33 of the original. After the digital signal is processed by the digital compression of the system, effective information resources can be provided to the computer processing system.
Audio coding
In the digital TV technology, the main function of audio coding is to effectively compress the sound information. In general, after The audio quality of the digitally compressed sound signal is better than before. Therefore, when broadcasting digital cable TV, the sound transmission inside is not as people imagine, so it is in the process of processing digital TV sound. In the process, effective measures should be taken to increase the compression coding process. At present, the audio coding and decoding in my country is mainly based on the MPEG-2 audio compression standard frequently used in Japan, Europe and other countries. With the advancement of social science and technology, the development of digital TV technology now requires a higher level of audio coding and compression standards. With the development of cable TV now, some areas no longer use the previous audio coding standard, but use the more advanced MPEG4AVC standard.
Network business
Usually we divide the business of the cable TV network into three categories, basic business, extended business, and value-added business.
Value-added services
It is a multi-function service developed on the wired network, including Internet access, IP telephone, video conferencing, bandwidth rental, TV business, etc. These new services will expand the service content of the cable network from television to voice and data communications, finance, education and other fields, greatly expanding the business development space of the cable network. At present, the value-added services of the cable network are still in the stage of development and experimentation in various regions, and the income is still very small, but its future potential is incalculable.
IP phone
After the cable TV network is upgraded and transformed, using VOIP technology can provide very low-priced IP phone services, reducing the phone charges to the current ordinary phone charges. One-tenth. Traditionally, people are worried about the quality of voice services provided by Cable. However, the development of DOCSIS technology has greatly improved the support of cable networks for delay-sensitive services and improved the Qos (Quality of Service) of services. At present, many new cable networks in the United States Television operators can already provide telecom-level voice services. In the future, this service will have a greater impact on telecommunications' circuit-switched telephone service, but it is expected to become an important source of income for wired networks.
TV business
As one of the most important information media today, once TV is connected to financial institutions and enterprises, it will have the preliminary conditions for business development. Compared with e-commerce, TV commerce is more realistic and feasible, and has more room for development. Many consulting agencies predict that within ten years, TV commerce will become a trillion-level industry. China has the world's largest cable TV network and the largest number of cable TV users. It is uniquely equipped to develop TV business. The revenue from this business is expected to reach more than 100 billion within ten years.
Video conferencing
Using the cable TV network, through special channels and recording and playback equipment, it can provide video conferencing with very smooth sound and images, and realize effective real-time communication between multiple points. Achieve communication effects that cannot be achieved under traditional technology.
In addition to the above business, the value-added services of the cable TV network also include distance education, telemedicine, remote securities trading, electronic automatic meter reading and home security monitoring, TV games, etc. These businesses also have huge future Development space.