Digital Television (DTV for short) is a television system that processes signals in a digital way from program acquisition, program production, program transmission to the user end, that is, fromstudioAll links from transmission, transmission and receptiondigital signal, or spread through a digital sequence composed of 0 and 1 digital strings.Digital TV is the third generation TV type after black and white analog TV and color analog TVanalog televisionThe concept of.Compared with analog TV, digital TV has higher picture quality, stronger functions, better sound effect, richer content, and usually has interactivity and communication functions.[1]
On February 28, 2023, the National Bureau of Statistics issued the Statistical Bulletin of the People's Republic of China on National Economic and Social Development in 2022. According to the preliminary accounting, there will be 199 million actual cable TV users in 2022, including 190 million actual cable digital TV users.[18]
Chinese name
digital television
Foreign name
Digital Television
Abbreviation
DTV
Signal propagation
Spread through digital stream composed of 0 and 1 numbers
Digital TV refers to a brand new TV system that uses digital TV signals or processes and modulates digital TV signals in all links from studio to transmission, transmission and reception.All signals of the system are transmitted through digital streams composed of 0 and 1 digital strings,Propagation rateIt is 19.39Mb/s, which ensures the high definition of digital TV and overcomes the inherent shortcomings of analog TV.[3]
The specific transmission process is:TV stationThe sent image and sound signals are digitally compressed anddigital modulation The digital TV signal is formed aftersatellite, Groundair castOr cable transmission, which is received by digital TV, and then through digital demodulation and digital video and audiodecodeProcess and restore the original image and sound.Because the whole process adoptsdigital technique Therefore, the signal loss is small, the reception effect is good, and there is no snowflake and ghosting phenomenon of analog TV.[3]
development history
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Digital technology is one of the fastest developing technologies for many years.In general, the development of the application of digital technology in television can be divided into three main stages:[4]
Phase I: Before the 1880s, research and development of separate local equipment was the main focus. The equipment put into use mainly included digital time base corrector (DTBC), digital frame synchronizer DFS, digital stunt machine (DVE), etc.[4]
The second stage: from the 1980s to the 1990s, this stage was characterized by the successful development of digital TV equipment, such as digital video recorders, digital signal processing cameras, etc.[4]
The third stage: after the 1990s, digital television technology has begun to develop from a single device to the whole system. Some research institutions have proposed fully digital digital television broadcasting standards, such as the digital video broadcasting format in Europe, the digital video broadcasting format in the United StatesATSCFormat, and digital television technology is combined with high-definition television technology. Some developed countries have begun to pilot digital television or digital high-definition television systems.[4]
By 2020, the number of cable TV and direct broadcast satellite users in China will reach 210 million and 130 million respectively. More than 5000 transmitter stations will have 10000 digital TV transmitters covering the vast urban and rural areas, and through multi-channel transmission, they will cover the vast urban and rural areas;Through the multi-channel transmission coverage system, viewers can turn on the TV and watch digital TV programs.[17]July 2020, ChinaTerrestrial digital televisionThe coverage network has been fully completed.[2]By the end of 2020, wireless analog TV will withdraw from the historical stage, and China will enter the era of digital TV in an all-round way.
Advantages and disadvantages
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advantage
1、Image definitionIt can approach the clarity of the source part of the transmitter.One of the reasons is thatSignal-to-noise ratioIt has nothing to do with the number of consecutive processing, becauseClutterMainly superimposed on the transmission signalBinaryDigital signal, through shaping and introducingError detectionTechnology, it is easy to eliminate the influence of these clutters,analog signalDuring transmissionNoiseIt will accumulate gradually, but the digital signal will not produce new noise during transmission, that is, the signal-to-noise ratio can basically remain unchanged, so the analog signal requiresSignal-to-noise ratioS/N≥40dB，The digital signal only needs ≥ 20dB.The second reason is the digital systemnonlinear distortion It is easy to eliminate the influence ofImage distortion。[5]
2. High sound quality and wide range.Because of the information compression technology,channelHigh utilization, allowing high qualitybroadband20Hz~20kHz audio signal and two-way stereo/double sound, or transmissionAC-3Digital stereo, and sound with cinema sound effects.
3. High channel utilization.Within the range of one channel of the original 8MHz broadband, one channel of high-definition digital TV can be transmitted, or 2-3 sets of standard definition TV and relevant service data can be transmitted in a bundled manner, or loadedcommunication channel 。[5]
4. Digital TV and analog TV can be channel compatible.Analog TV and digital TV can be transmitted simultaneously on the same channel without interference, which can be added to the original analog TV receiverDigital TV set-top boxIt can also process analog TV signals in digital TV channels, thus expanding the space of receiving channels and making users get more benefits.[5]
5. It is easy to realize encryption/decryption, jamming/descrambling, so as to expand all kinds of paid radio and television services and other professional applications, such as military and commercial services.[5]
6. It is easy to integrate with communication and computer, and is conducive to forming a terminal of national information high-speed transmission network.This is because the function of storing and recording digital signals in computer technology can be usedTV signalUp, for exampleTape recordingWill gradually bedisk、CDReplaced by.Since digital signals only have "1" and "0", which are easy to store, they can be widely used forimage qualityFor example, 100Hz TV stores 50Hz TV pictures one by one, and displays each picture twice, so that the audience can see more stable TV images without feeling flicker.[5]
8、digital circuit Low cost, no needadjustment、tuningIt will not age, so the production cost is reduced and the maintenance is easy.[5]
9. One way propagation.When broadcasting TV data, digital TV does not need to use many attached switching or routing devices of other multicast technologies.In the process of transmission, the stability of the transmitted network signal will not be affected because the signal covers many users and covers a wide range.It will not be affected by the bandwidth pressure in the network during transmission, and will not be attacked by hackers.[6]
shortcoming
Digital TV also has some weaknesses.For example, in the process of samplingquantization errorThe signal damage caused by compression coding is delayed during program production and transmission.Some damages can be repaired without affecting the final quality of the image, while some damages can only be weakened by some compensation measures. These weaknesses will not affect the transformation of the television field to digitalization. Compared with the benefits brought by the digitalization of television signals, these weaknesses are not important.[3]
2. According to image definition, it can be divided into three categories: ①Digital HDTV(HDTV): at least 720 line by line or 1080 line by lineInterlace scanning. The aspect ratio of the screen should be 16:9. Dolby digital audio should be used, which can convert the high-definition format into other formats and can receive and display signals in lower formats,image qualityIt can reach or approach the level of 35mm wide screen film. ②Digital Standard Definition Television (SDTV): must reach 480 linesprogressive scanning, capable of converting 720 line by line, 1080 interlaced and other formats into 480 line by line output, using Dolby digital audio.Corresponding to the resolution of existing TV, its image quality is studio level. ③Digital ordinary definition television (LDTV): the display scanning format is lower than the standard definition television, that is, lower than the 480 line by line scanning standard.Corresponding to existing VCDsresolving power。[3]
3. According to product types, it can be divided into digital TV monitors, digital TV set-top boxes and integrated digital TV receivers.[3]
4. According to the display screen frame ratio, digital TV can be divided into two types: 4 ∶ 3 and 16 ∶ 9 frame ratio.[3]
The analog signal that is continuous in time and amplitude is transformed into the signal that is discrete in time, that is, time discretization.TV signal digitizationsampling frequency The selection ofNyquist sampling theoremThat is, the sampling frequency must be more than twice the signal bandwidth to completely recover the original signal from the sampled signal.For the sampling of digital component coded signals,CCIR601proposalbrightnessThe sampling frequency is 525/59.94 and 625/50Least common multiple6 times of 2.25MHz, i.e. 13.5MHz.For the current TV system, the maximum bandwidth of brightness signal is 6MHz, 13.5MHz>2 × 6MHz=12MHz, so it conforms to Nyquist theorem.andChromatic aberration signalThe bandwidth of the two color difference signals is much narrower than that of the luminance signal, so the sampling frequency of the two color difference signals can be lower during component coding.[7]
It converts the amplitude continuous signal into the amplitude discrete signal, that is, amplitude discretization.By sampling, the analog signal is changed into discrete in timePulse signal。The amplitude of these pulse signals is still analog, so the analog amplitude must be discretized before its amplitude can be expressed digitally.This process of grading the amplitude and rounding off each level is called quantization.The higher the sampling frequency, the quantizationBitsThe larger,Digital rateThe higher, the wider the bandwidth of the required transmission facilities.Among the digital video equipment produced by various manufacturers, the highest quantization bit number is 12.[7]
3. Coding
According to certain rules, discrete signals in time and amplitude are represented by corresponding binary or multi base codes.Digital TV channelCode rateToo high. If no compression is applied, one channel will occupy more than 100 MHz bandwidth.Therefore, in order to simplify the transmission and storage of signals, it is necessary to compress the code rate.Common methods are as follows:[7]
(1) Reduce sampling frequency
Reducing the sampling frequency can compress the code rate, but Nyquist sampling theorem points out that when the sampling frequency (fs）Less than baseband frequency (fm）2 times of,spectrumIn case of overlap, uselow pass filter The original signal cannot be recovered without distortion.Since the energy of TV image signal is mainly concentrated inRow frequencyThere is a large gap in the narrow range with each harmonic frequency as the center. Therefore, if the sampling frequency is selected as odd times of half line frequencys<2fmThe overlapping part of the frequency spectrum that occurs when is half line interleaved.Therefore, we can adoptComb filterIt is selected to restore the original signal with less distortion.[7]
(2) Differential pulse code modulation
Standard digital TV adoptsMP@MLMain class and main level.In summary,MPEG2The principle of image compression is to use two characteristics of the image: spatial correlation and temporal correlation.Any scene in a frame image is composed of several pixels, so apixelIt is usually associated with some pixels around it in brightness andchromaThere is a certain relationship on the, which is called spatial correlation;A plot in a program is often composed of image sequences composed of several consecutive images. There is also a certain relationship between the front and rear images in an image sequence, which is called temporal correlation.These two kinds of correlation make the image have a lot of redundant information.If we can remove these redundant information and only transmit a small amount of irrelevant information, we can greatly save the transmission band.andReceiverUsing these non relevant informationdecodeThe algorithm can restore the original image on the premise of ensuring a certain image quality.A good compression coding scheme is to remove the redundant information in the image to the maximum extent.[7]
(3) Linear transform coding
Linear transform coding does not directly encode image data, but first performs some linear transformation on image data to obtain a set of transform coefficients, and then quantizes, codes, and transmits these coefficients.At the receiving end, the corresponding inverse transform is performed on the received transform coefficient, and then the image signal is restored.Theoretically, the reason why linear transform coding can compress the code rate is that the image signal has a strong correlationFrequency domainThe reflection of is that the power domain filtering method can obtain largeCompression ratio。[7]
network structure
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For a digital TV cable network, according to the transmission direction of its digital TV signal stream, it can be divided into front-end broadcastcomputer room, middle end transmission network media and end set-top box users.[8]
(1) Front end broadcasting room
The front broadcasting room is like a human brain, which is at the core of the whole system.Therefore, ensuring the safe broadcasting of the front-end broadcasting room system is undoubtedly the primary condition for the safe transmission of cable television network.The front-end broadcasting room of the cable TV network is a very complex and diverse system, covering many different subsystems, mainly including encryption subsystem, modulation/demodulation subsystem, on-demand subsystem and emergency processing subsystem.[8]
(2) Medium end transmission network media
The medium end transmission network media is like humancentral nervous system, transmitting digital TV signals sent from the front-end broadcasting room.In this system, digital TV signals are transmitted in three stages, namely the main amplifier stage, the branch amplifier stage and the building amplifier stage.The medium layer of the midrange transmission network is widely distributed in every corner of the city.Unlike the front end broadcasting room layer, which is relatively concentrated in a relatively closed area of a television station, the middle end transmission network media layer is a system that is completely "naked" in the development space of a city. In terms of distribution characteristics, it is most vulnerable to attacks.Facts have proved that the overwhelming majority of attacks against digital TV signals come from this link.[8]
(3) End set-top box users
Set top box equipment consists of two parts. One part is the basic hardware of the set top box, which is the part to complete the basic functions;The other part is the card on the set-top box, which is the memory for writing encryption programs.In the user's home, the set-top box is responsible for decryption and on-demand functions.[8]
key technology
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The good operation of digital TV requires the following key technologies.
1. Digital signal transmission frequency.A stable transmission frequency can make up for the poor program quality caused by the lack of traditional TV signals. Therefore, in order to realize the combination of high-frequency transmission and low-frequency transmission of digital TV transmission frequency, several aspects should be considered.First, consider the situation that the wave frequency of electromagnetic wave is interfered by UHF frequency during transmission, which affects the overall shaping effect of TV image when receiving the signal.Secondly, because the chips used as a whole are generally used to adjust TV lines, the overall scope of application is relatively limited.Thirdly, digital TV should work in the form of LD-MOS for comfort.Finally, because its transmission line signal is vulnerable to interference from external factors, especially in areas close to water sources or humid areas, attention should be paid to different problems caused by different changes in wave frequency.[9]
2. Transmission antenna.The transmitting antenna can ensure the accuracy of TV broadcasting and the authenticity of image effect transmission.Therefore, electromagnetic waves are required to radiate signals in space continuously, so as to effectively change the electromagnetic field.[9]
3. Positioning system.GPS global positioning system is required in the process of signal reception and transmission to ensure the coverage area of digital signal transmission and the stability of digital signal transmission, thus reducing the dependence on public transmitters.[9]
4. Transmission of electromagnetic waves.Comprehensive wireless digital signal coverage has high requirements for the location and construction site of the signal tower, and the establishment of the transmission antenna must meet the specific needs of the current stage.The most commonly used method in China is to build the signal tower with vertical transmission antenna components to enhance the overall performance and ensure the uniformity of signal distribution.[9]
5. Site selection technology.The location of antenna has a great impact on the transmission of electromagnetic waves.In general, the relevant construction personnel need to consider the actual application when selecting the site of the antenna.First, the radiation range required by the electromagnetic wave in the radiation process should be considered to ensure that the overall coverage is wide enough after installation.Secondly, in the transmission process, try to reduce the changes caused by environmental factors to ensure the overall communication quality.[9]
Main business
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1. Radio and TV programs transmitting digital signals
This is the basic business of digital TV. Unlike analog TV transmission in the past, digital TV can transmit 6-8 times or more TV channels due to the use of digital compression technology. In addition to digital broadcasting of the original analog radio and TV channels, digital TV has also added a large number of professional pay channels, such as fishing channelsPregnant women's guide, online exam, beauty channel, etc.TV programs transmitted by digital TV can support both standard definition format and high-definition format.[10]
2. Information service
Including government affairs, finance and economics, life, health, education, entertainment and other classified information.
3. Electronic Program Guide (EPG)
Program preview guide with classified query function, so that users can easily and quickly find their favorite channels or programs.[10]
4. Video on Demand (VOD)
Digital TV users can watch movies and TV programs they want to watch on demand at any time.[10]
5. Enhanced TV
Relevant data can be attached to TV programs through hyperlinks, and interactive operations can be carried out at any time.For example, if information about people, background or a product is attached to a video program, users can click to open it with a remote control.[10]
Record TV programs on the hard disk, which is in the set-top box and can hold 20-60 hours of programs.PVR can store programs according to standards such as time, title, actor or program type, automatically skip advertisements, and allow users to pause, playback or fast forward live TV programs at will.[10]
7. Interactive TV programs
Interactive TV programs allow users to enter different viewing situations by selecting different shooting angles, personas, etc. when watching programs, and even let users guide the ending of the story.[10]
8. TV Business
Through the platform of digital TV, you can handle shopping, payment, stock trading and various banking businesses[10]。
9. Distance education and telemedicine
Receive instant distance education or medical services through digital TV.[10]
10. Game lottery
Online interactive games are played through digital TV to buy and sell various lottery tickets.[10]
11. SMS/MMS
Send and receive SMS or MMS via digital TV.[10]
12. Video call
Remote video conversation via digital TV.[10]
Interactive television system
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The interactive TV system built on the digital TV platform can provide many interactive applications, such as:
1. Data broadcasting.The digital audio, video, image, animation and other information will be transmitted to the user's digital TV receiver, computer and other intelligent devices through the digital TV broadcast channel, including stock trading information, electronic program guide, public information query, (aircraft flights, weather forecast, housekeeping services), web browsing, etc.[11]
2. TV Bank.Enter the ID number and login password with the remote control to enter the TV banking interface, and press
All buttons will appear on the interface, and click to enter the corresponding function.[11]
3. Program browsing.[11]
4. Internet access.There are two access methods, one is digital TV receiver, the other is personal computer.Digital TV network access enables viewers to use TV to access the Internet.The network access through personal computer is to connect the computer with the set-top box, which can provide users with higher network access speed.[11]
5. TV shopping.[11]
6. Interactive advertising.When the audience is interested in a certain advertising information content, they can select it through the remote control button to enter, so that the audience and the advertiser can connect and communicate through the server.[11]
Comparison between digital TV and analog TV
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1. Sound image:
The analog transmission TV signal is vulnerable to interference during transmission, and the noise will continue to overlap, and the image quality is only superVCDThere are only two kinds of accompaniment: mono and stereo.
Digital TV adopts MPEG-2 coding, which has strong anti-interference and error correction ability in transmission, clear picture, image quality can reach the level of DVD, and can provide a variety of audio modes such as mono, stereo, multi language, multi channel and surround sound.[3]
2. Number and type of programs:
Analog TV can only transmit TV programs and FM radio, with only 79 sets at most.
Digital TV can transmit more than 500 sets of TV programs and radio programs.It can also provide multiple services at the same time, such as audio broadcasting, data broadcasting, interactive TV, TV shopping, etc. The service content is no longer limited to traditional video programs.[3]
3. Personalized service:
Traditional analog TV does not provide personalized services. All users receive the same program, and there is no special data service channel. To learn about the program content, you can only check radio and television newspapers or watch TV commercials.
Digital TV users can cooperate with conditional access system to provide various specific personalized services according to user preferences and economic conditions, such asvideo on demand, audio on demand, etc;Digital TV also has a wealth of data services, which can be broadcast 24 hours a day without interruption. There is a special Chinese electronic program menu to check the name of each channel and program related content at any time, such as program name, broadcast start and end time, content introduction, etc.[3]
From the difference between digital TV and analog TV, digital TV has the following three characteristics:[12]
1. The information source code of video information source and audio information source should be digital compression code, that is, analog audio and video signalA/D conversionCompression code after;[12]
The role and significance of cable digital TV development has gone beyond its own scope, and will have a huge impact on the whole social economy, promote the development of a series of industries, and then have a profound impact on the whole society.Specifically, the significance of cable digital TV is mainly reflected in the following aspects:[13]
1. Digital TV will promote China's information construction
As one of the infrastructures of urban national economy and informatization construction, the cable TV network of digital TV will vigorously implement digital transformation, promote and popularize the application of digital TV, which will greatly accelerate the informatization process in China, create an international information and cultural environment for the city, improve the informatization degree of the city, and improve the international influence and competitiveness.Radio and television is the most popular information tool and the best information carrier. Digital television enables every family to have a multimedia information terminal that integrates public communication, information services, culture and entertainment, and communication and interaction. Through the cable digital television network platform, a large amount of information can be transmitted to users, making digital television a platform for social informatization,So as to promote the informatization process of our country.[13]
2. It can accelerate the development of radio and television
Generally, cable TV distribution network can only transmit 40~50 sets of analog TV programs, and channel resources are quite tight.The adoption of digital technology will enable theInformation capacityIt can provide hundreds of digital channels and channel resources are extremely rich, which fundamentally solves the contradiction of channel resource tension.The development of digital TV has also greatly improved its service mode and expanded its service field. TV is not only a tool for watching TV, but also an information tool, a life tool and a service tool.It enables radio and television to provide professional, diversified and personalized market services, increase new profit models, cultivate new cultural and entertainment consumption markets, and transform radio and television from relying solely on advertising to a variety of profit models.[13]
3. It can drive the formation and development of domestic digital TV industry chain
Digital TV still brings us a huge industry.The modern cultural industry is an industry that integrates with modern technology. The full implementation of digital television will form a diversified pattern of television industry. The development of digital television can drive the formation of new cultural and entertainment consumption hotspots and promote the transfer of traditional cultural industries to emerging cultural industries.It will also drive the formation and development of China's digital TV components, set-top boxes, digital TV, digital TV application software, digital TV content supply and other industries, which is conducive to building a new digital TV value chain, cultivating new economic growth points, and promotingRadio and television industryThe development of information industry and cultural industry will promote the development of the national economy.[13]
One of the main directions of the development of digital TV technology is high-definition TV, which has a very high definition. At the same time, the technical requirements are more stringent and demanding. It has high requirements for the quality of digital signals, reception and transmission technologies.The so-called high-definition digital television, understood literally, emphasizes the clarity of picture and sound, and improves the resolution from 720 × 576 to 1920 × 1080.People have higher and higher requirements for the definition of TV programs. High definition words can be seen in TV stores and various video websites.The development prospect of HD digital TV is very broad, and it is a key part of the development trend of digital TV technology.[14]
The network has become an indispensable part of modern people's life, work and entertainment.People search and get more information on the Internet.Network TV is to take the Internet as a carrier to bring information to the audience.Compared with ordinary TV, IPTV has more abundant program content. At the same time, the terminal equipment of IPTV is set-top box or PC, through which network interaction can be realized. Compared with one-way TV programs, people are more willing to order their favorite programs at will.This super interaction makes IPTV have a huge impact on traditional TV. In addition, due to the rapid development of network technology, the quality of IPTV programs is also increasing, and its definition can also meet the requirements of users, which also makes more users prefer to choose IPTV.Therefore, with the development of network technology and digital television technology.The development trend of its network TV will become more and more obvious.[14]
3. Satellite live TV technology
Different from the former two, satellite live TV technology is a new type of digital TV program that transmits signals through satellite.The development of science and technology has promoted satellite technology, and the function of the transponder of communication satellites has become very powerful.Satellite transponderIt can completely realize the transmission of digital TV signals from the sending end to the receiving end.Compared with the above two digital TV technologies, the outstanding feature of satellite live TV technology is that it covers a wide range, and the transmission of digital signals worldwide can be achieved through satellite TV technology.Moreover, its equipment is also very simple. Users can see high-quality digital TV programs through the antenna.[14]
Common transmission modes of terrestrial wireless digital include: digitalMMDSTransmission modeMUDSTransmission mode and "Mobile TV" mode.The specific mode needs to be analyzed by local TV stations according to their own conditions.The three transmission modes are applicable to different situations.For example, in areas with a large area, poor residential concentration and underdeveloped economy, digital MMDS transmission mode and MUDS transmission mode should be selected. Although the "mobile TV" mode can achieve good results, its cost is high and one-time investment is large;In areas with high residential density and good economic conditions, it is more advantageous to use "mobile TV".The signal of terrestrial wireless digital TV is transmitted throughelectromagnetic waveWhen the transmission is completed, many links are saved and the equipment complexity is reduced, which is different from the cable digital TV and satellite TV mentioned earlier.In addition, the cost of wireless digital TV is low, because its service range is small, which makes the ground wireless digital equipment simple and low construction cost, which forms its unique advantages.At the same time, the management of terrestrial wireless TV is simple, which can realize unmanned monitoring and save labor costs.The most important thing is that due to the small service scope of terrestrial wireless television technology, its local characteristics are distinctive. Different regions can broadcast TV programs that conform to local culture according to local conditions, and the production of programs is more flexible.Therefore, terrestrial wireless digital TV also has good prospects for development.[14]
Related events
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On March 31, 2020, the National Leading Group for Terrestrial Digital Television 700 MHz Frequency Migration notified the Work Plan for Terrestrial Digital Television 700 MHz Frequency Migration, which explained and deployed the overall idea, work objectives, implementation principles and strategies, overall arrangement, work plan and operation guarantee of the migration work.[15]
Heilongjiang Radio and Television BureauIt was announced that all central terrestrial analog TV programs would be closed by August 31, 2020, and all provincial, municipal and county terrestrial analog TV programs would be closed by December 31, 2020.Heilongjiang Province has entered the era of digital TV.[16]
