Basic knowledge of cable digital TV
1. Overview of Cable TV
1. The transmission form of TV signals As far as the principles of TV technology are concerned, the TV system that transmits active scenes is usually composed of camera, signal processing, transmission, and visualization. The principle of sequential transmission of image information is the basis for the generation of television signals. The basic method is to decompose the image to be transmitted into many pixels, and convert the characteristics of each pixel, such as brightness and color, into the amplitude and time sequence of electrical signals in a certain order and method, and then transmit and process them in sequence, and additionally indicate that each pixel is relatively The characteristic signal of the position-the synchronization signal, in order to facilitate the reproduction and positioning of the pixels when the electro-optical information is restored at the receiving end, this is the principle of TV signal generation in TV technology. For such a full TV signal containing all image information, the basic form of transmission can be divided into the following three methods.
(1) Baseband transmission is the direct transmission of baseband signals through transmission lines or other media. It is generally used where video equipment is concentrated.
(2) Wireless TV transmission is the signal conversion processing method of amplitude modulation of baseband TV signals and sound signals, modulated on the radio frequency carrier, so that the latter can be radiated in the form of high-frequency electromagnetic waves through an appropriate antenna.
For the receiving end of the wireless transmission method, it is necessary to improve the strength and quality of the received signal through various types and sizes of receiving antenna facilities. The level of signal restoration varies from person to place and cannot achieve consistent results and universal stable.
(3) Cable TV transmission After a full amplitude TV signal of a certain amplitude is processed by radio frequency modulation, a radio frequency carrier signal with all audiovisual information characteristics is transmitted through a tangible transmission medium, such as a coaxial cable, optical fiber and other media. To carry out the transmission process. Due to various shortcomings caused by the inherent open-circuit transmission characteristics of wireless broadcasting and television, such as the increase in the number of channels to be expanded by the program source, the result is also limited by the frequency allocation. The cable TV can select, process, demodulate, and modulate various kinds of baseband audio-visual information such as video recorders and microwave relay signals in the front-end studio using video recorders and other equipment, and then transmit them to the closed circuit through the cable distribution system The vast number of users covered in the system network. This form of television signal transmission, which is not limited by frequency usage regulations and is not subject to interference from the natural environment, has developed rapidly, and a basic industry supporting the development of its facilities has gradually formed, promoting the use of coaxial cables as transmission line media and having A cable TV system that handles the characteristics of multiple channels and multiple functions. With the technical application of optical fiber equipment, the coverage and scope of the network have been expanded. The functions and network management of the system have been included in the automatic adjustment control technology, intelligent computer technology and various auxiliary engineering technologies. The application of new technologies has also developed rapidly. Therefore, as an effective means of information transmission, cable TV is constantly developing and changing in the needs of the information society. The transmission form of TV signals will eventually break through the existing forms and develop towards a more market-oriented stage, in order to adapt to the new requirements of society for TV information in the future.
2. Composition of cable TV system At present, China's cable TV system is generally an overall system consisting of five parts: signal source and equipment room equipment, front-end equipment, transmission network, distribution network, and user terminal.
(1) Signal source and equipment room equipment. Sources of cable TV programs include analog and digital TV signals received by satellite ground stations, TV signals transmitted by local microwave stations, TV signals transmitted by local TV stations, uplink TV signals and data, etc. In order to realize the playback of the signal source, the computer room should have satellite receivers, analog and digital players, multi-function consoles, cameras, special effects graphic processing equipment, editing equipment, video servers, user management control equipment, digital signal processing equipment, etc.
(2) Front-end equipment. Front-end equipment is equipment connected between the signal source and the trunk transmission network. After processing the received TV signals, it mixes all the TV signals through the mixer and then sends them to the trunk transmission network to realize the single-channel transmission of multiple signals. The frequency range of the output signal of the front-end equipment can be between 5MHz and 1GHz. The front-end output can be connected to the cable trunk, but also to the optical cable and microwave trunk.
(3) Transmission network. The transmission network is located between the front-end equipment and the user distribution network, and its role is to stably transmit various signals output by the front end to the user distribution section without distortion. The transmission medium can be radio frequency coaxial cable, optical cable, microwave, or a combination thereof, and the most commonly used is optical cable and coaxial cable hybrid (HFC) transmission.
(4) Distribution network. The distribution network of cable TV uses cable transmission, and its role is to distribute the output signal of the amplifier to the building units and users according to a certain level.
(5) User terminal. The user terminal is a user port connected to thousands of households, and the user port is connected to a television. At present, the user port generally adopts a single-port user box or a dual-port user box, or a branch is connected in series. Future user terminals include set-top boxes, cable modems, descramblers, etc.
2. Digital TV cable transmission
1. What is digital TV?
Digital TV refers to a brand-new TV system that includes program production, editing, transmission, transmission, storage, reception and display, all of which use digital processing. It can also be said that digital TV is a TV system that fully realizes digitization and digital processing in three aspects: source, channel, and sink.
2. What is digital TV?
Digital TV (or digital TV) uses a variety of digital processing technologies in the existing analog color TV circuit, such as: digital 100HZ scanning technology, 3D / 5D digital image processing technology, I2C digital bus technology, digital picture-in-picture Technology, NICAM digital sound technology, etc. Color TVs that have adopted these digital processing technologies have achieved better image and sound quality and increased TV functions. Essentially, digital TV is still in the category of analog TV.
3. What are the advantages of digital TV compared to analog TV?
Compared with analog TV, digital TV has the following advantages:
(1) The image transmission quality is high and the distance is long. During the transmission of analog TV image signals, it is affected by the characteristics of the transmission channel (amplitude-frequency characteristics, differential gain, differential phase characteristics) and noise interference. The quality is not high, and after multiple conversions, the channel characteristics and noise interference The accumulation of influences leads to a further decline in image quality, and some of these influences do not exist for digital TV signals. In the transmission process of digital TV signals, interference and noise accumulation will not occur after multiple relays (or duplications). At the same time, error correction coding technology can be used, and the image quality at the receiving end is better, basically consistent with the sending end, and the transmission distance Not restricted.
(2) The channel resource utilization rate is high. Spectrum resources are an important national resource. The spectrum resources of analog TV are limited. A set of analog TV programs takes up 36 MHz of satellite transponders and 8 MHz of terrestrial TV broadcasting and cable TV bands. Digital TV uses compression coding technology to transmit 5 sets of SDTV programs in a 36MHz satellite transponder, with a display resolution of about 400 lines, and more than 4 sets of SDTV programs in an 8MHz channel.
(3) Provide brand-new services and realize high-speed data transmission. In digital TV communication, multiple digital information such as text, data, voice, and still images can be simultaneously transmitted without interference. The digital TV network can be interconnected with computer networks and telecommunications networks, not only to enrich the information sources, but also to increase the interaction between users and various information sources, to achieve users' free on-demand programming, e-commerce, online shopping, online A variety of high-speed data services such as teaching, online medical treatment, and online games (using 64QAM modulation within an 8MHz bandwidth can achieve a data transfer rate of approximately 32 to 38 Mbit / s).
(4) The signal is stable and reliable, and the equipment is easy to maintain and use. After the analog TV signal is digitized, the signal appears in the form of a binary code. It has only two states of "0" and "1". The binary digital signal is not affected by power supply fluctuations and device nonlinearity, so the signal can be stable and reliable. Large-scale integrated circuits can be used to process digital signals, which can reduce the power consumption of the device, reduce the volume, and improve the reliability of the device. At the same time, digital equipment does not need to be adjusted, and maintenance is simple and easy to use.
(5) Save transmission power and cover a wide range. The average power required by digital TV transmission equipment in the same coverage service area is one order of magnitude lower than the peak power of analog TV transmission equipment. For example, the minimum reception level of analog MMDS is 56db, while the reception level of digital MMDS under 64QAM modulation is only 39db Therefore, the coverage of digital TV transmission equipment is several times larger than that of analog TV with the same power.
(6) Flexible and friendly man-machine interface. The flexible and practical human-computer interaction interface is convenient for ordinary people to operate. In addition to the display equipment, it is easy to achieve system integration and mass production, and the price is low to facilitate popularization.
(7) Conditional access is easy to achieve. Digital TV signals are easy to encrypt / scrambling, which is beneficial to information security and facilitates the realization of pay TV, video on demand, and interactive TV.
4. How to digitize analog signals?
The digitization of analog signals goes through three processes of sampling, quantization and encoding to form a binary digital signal. These three processes are collectively called pulse code modulation (PCM).
5. What is high-definition television?
Digital high-definition television (HDTV) is a high-tech television technology currently actively developed and applied in developed countries in the world. It uses digital signal transmission technology, which has stronger anti-interference performance than ordinary analog TV signal transmission, and the image clarity is significantly improved. The aspect ratio of the received image is 16: 9, and with multi-channel digital sound, it can achieve the projection effect of 35mm wide screen movies.
The definition of HDTV in CCIR (International Radio Consultative Committee, now renamed ITU-R) report 801 is: "The design of a high-definition television system requires that viewers can see at a distance approximately three times the screen height Or close to the level of obtaining clear details, to achieve the same feeling as a viewer with normal vision looking at the original scene. "
6. What is pay TV?
Pay TV is different from the existing basic public welfare TV. It serves the fundamental purpose of satisfying the people's personalized, diversified, and multi-level cultural needs, with channel specialization and objectification as the important signs, and charging user service fees as the main profit method. In other words, it uses encryption technology to restrict the viewing of unpaid users. Only paying users can sit at home and order the TV programs they want to see.
3. Digital TV set-top boxes
(1) Concept, function, principle and structure of digital TV set-top box.
The cable TV network is developing in the direction of digitization, networking, and industrialization. One of the key equipment for the cable TV network to provide comprehensive information services is the user terminal equipment digital set-top box. Set-top box STB (Set-Top-Box) originated in Europe and the United States in the early 1990s. The main function of the set-top box is to watch digital TV programs or digital high-definition TV programs with an ordinary analog TV set. Of course, it also has network and conditional access functions. This set-top box is called a digital TV set-top box. According to different transmission media, digital TV set-top boxes are divided into digital satellite set-top boxes (DVB-S), terrestrial digital TV set-top boxes (DVB-T) and cable digital TV set-top boxes (DVB-C). The hardware structure of the three set-top boxes is mainly The difference is in the demodulation part. The set-top box currently used in our county is a cable digital set-top box.
1. The concept of digital TV set-top boxes There is currently no standard definition for set-top boxes. The traditional saying is: "a box placed on top of a TV set." It uses the cable TV network as a transmission platform and the TV set as a user terminal to improve The performance of existing TV sets may increase their functions. Due to their different functions and uses, the concept of "set-top box" is somewhat ambiguous, such as early supplemental channel set-top boxes, teletext set-top boxes, pay-TV set-top boxes, etc.
Digital TV set-top boxes are one of the information appliances. It is a consumer electronic product that enables users to watch digital TV programs on existing analog TV sets and carry out interactive digital entertainment, education, and commercialization activities.
2. The function of digital TV set-top box The basic function of digital TV set-top box is to receive digital TV broadcast programs, and has all broadcast and interactive multimedia application functions. In fact, set-top boxes can support almost all broadcast and interactive multimedia applications, such as digital TV broadcast reception, electronic program guide (EPG), quasi video on demand (NVOD), pay-per-view (PPV), online software upgrades, data broadcasting , Internet access, e-mail, IP phone and video on demand.
(1) Electronic Program Guide (EPG): It provides users with an easy-to-use, friendly interface, and quick access to the programs they want to watch. Users can use this function to see one or more channels or even all channels to be played in the near future TV show.
(2) High-speed data broadcasting: it can provide users with various information such as stock market prices, ticket information, electronic newspapers, popular websites and so on.
(3) Online software upgrade: it can be regarded as one of the applications of data broadcasting. The data broadcasting server broadcasts the upgraded software according to the DVB data broadcasting standard. The set-top box can recognize the version number of the software, receive the software when the versions are different, and update the software stored in the memory.
(4) Internet access and e-mail: The digital set-top box can easily realize the Internet access function through the built-in cable modem. Users can surf the Internet and send e-mail through the built-in browser of the set-top box. At the same time, the set-top box can also provide various interfaces to connect to the PC and use the PC to access the Internet (at present, we do not have this service in Yancheng).
(5) Support interactive applications such as video on demand, interactive games, etc.
(6) Conditional access: The core of conditional access is scrambling and encryption, and the digital set-top box should have descrambling and decryption functions.
3. The principle and structure of the digital TV set-top box The digital TV set-top box receives digital TV and various data information from various transmission media, through demodulation, demultiplexing, decoding and audio and video encoding (or through the corresponding data analysis module), in Watch digital TV programs and various data information on analog TV. Take the Skyworth series of cable digital TV set-top boxes we are using as an example. Its working principle is as follows: the cable digital TV set-top box receives digital TV programs, processes data services, and completes the analysis of various applications. The source completes two levels of coding before entering the cable TV network, one is channel coding for transmission, and the other is the source coding of audio and video signals and all sources are encapsulated into a transmission stream. Corresponding to the front end, the receiver set-top box first extracts the channel coded signal from the transmission layer to complete the channel demodulation; secondly, it restores the compressed source coded signal, restores the original audio and video streams, and simultaneously completes the reception and analysis of data services and multiple applications .
According to the requirements of receiving digital TV broadcasting and Internet information, the hardware structure of a digital TV set-top box is composed of signal processing (channel decoding and source decoding), control and interface.
(2) Signal requirements of digital TV set-top boxes The set-top box selected by our county is a series of products of Skyworth, mainly including the early Zhongshilian 5800, Irdeto 5800 and the currently used Genie 5800 models. Their functions and performance are basically the same.
1. Signal requirements. For the signal requirements of the set-top box, the general household level of analog TV is 69db ± 5db according to the standard we are currently implementing, and the level of digital TV is greater than 37db according to practice. There is no problem with analog and digital signals. The level difference between them is 15db ± 5db, and we are now running a difference of 20db.
2. Connection of set-top box signals. The signal from the user box is connected to the signal input terminal of the set-top box through the connecting line. The set-top box has two signal outputs, one is the analog signal to the RF input terminal of the TV, and the other is the audio and video (A / V) signal to the TV audio and video Input. Note: Regardless of whether the power of the set-top box is turned off, it will not affect the TV's viewing of analog signals.
3. In the connection port of the RF input signal of the set-top box, the signal input of Shilian 5800 model is British F5 and ï¼5 conversion head, the signal input of Irdeto 5800 model is ï¼5 direct input, and the signal input of Genie 5800 model is British F5 input. .
4. Distribution of existing digital TV programs and channels (see Table 1)
(3) Basic operation (see the instruction manual of the cable digital TV set-top box for details)
(4). Common faults and troubleshooting Digital TV is the development of technology and the innovation of technology. It is different from the transmission and transmission of analog TV, but it has the advantages that analog TV cannot compare with. I will talk about the maintenance method of digital TV from a practical point of view, specifically starting from three aspects:
First, detect the user's entry level. Generally, the level of the analog TV is greater than 58db, and it can be watched normally. However, according to the practice, the level of the digital TV is greater than 37db, but it is better to be greater than 40db. Then, when the home level of the digital TV is measured to be greater than 37db, then the home level of the previous analog TV is measured, and it can be inferred that when the digital set-top box is normally watched, the home level of the network in this area is required. Secondly, the search problem of set-top boxes. The search can be divided into automatic search, manual search and full channel search. Before these three methods are carried out, the set-top box must be restored to the original settings. Can solve some general faults, such as the inaccurate time on the set-top box, and the number of search channels has not changed. Finally, the upgrade problem of the set-top box. It can also be performed in two ways, namely automatic upgrade and manual upgrade. The automatic upgrade is that the user puts the set-top box to the standby or power-on state, and the front end of the digital television company issues an automatic upgrade instruction to complete. For manual upgrade, select online upgrade directly in the set-top box and wait for 5-10 minutes to complete. The prerequisite for these two upgrade methods is that the front end of the digital TV company must have upgrade software supporting different set-top boxes.
1. Mosaic or freezing phenomenon?
①Whether the attenuation of the signal transmission cable is too large ②Is the connection between the cable TV signal line and the set-top box reliable? First measure the entrance of the user ’s set-top box. The high and low end of the analog signal level should be no less than 60dB, and the error of the high and low end level should be less than 3dB To see whether the fifth (sports channel) in analog TV is clear, if none can be reached, first check the signal. Because the digital TV signal is now transmitted between 18-31, and the level attenuation is 20dB. If the bottom limit of 37dB is not met, the TV screen will appear a certain program package or several program packages mosaic or freeze phenomenon. The user cannot watch normally. It is better to adjust the level of digital TV signal to over 40dB during maintenance.
2. The user cannot watch normally?
If the current signal is encrypted on the TV interface, please contact the supplier or ask for authorization. There are several possibilities for this situation:
a) If the fee is not paid, ask the user the deadline for payment;
b) The CPU has a malfunction and the solution:
Turn off the power of the set-top box, after 10 seconds, restart the set-top box, if not, then proceed to the next step, press the menu key → point to system settings → restore factory settings → automatic search
c) There is no automatic upgrade, the solution: cut off the power of the set-top box, press and hold the "confirm" button on the panel, and then turn on the power. When the "automatic upgrade" screen appears, release the "confirm", then the "frequency point 0371.000" will appear on the TV , Symbol rate 06.875, Modulation mode QAM64, service number 7000 "Press" Confirm ", use the up and down keys to adjust to the" Confirm and Cancel "dialog box below, press the" Confirm "button on the" Confirmation "dialog box, and the set-top box appears Upgrade signal dialog box, if the current "version is 1.1, upgrade to 2.0, do you need to update?" Dialog box, please press again to confirm the key, the set-top box is automatically upgraded, after the upgrade, the current program will appear empty, If it is necessary to automatically search for the channel, press again to confirm the health, then the upgrade and the channel search are all completed. (The frequency of the blue card is 0307.000, and the frequency of the red card is 0371.000)
③If the TV shows the dialog box of E30 program is encrypted, please authorize, you need to turn off the power of the electric set-top box, and it will be turned on after 30 seconds. If the program is encrypted, please authorize the dialog box, you need to open the set-top box menu, adjust the main menu to program management → submenu to edit the program table, delete the encrypted channel, press F3 → exit → confirm
3. If there are duplicate stations or some of them cannot be seen, perform factory reset → full channel search
4. Two different sounds appear on the TV screen. Use the remote control to adjust the audio channel to the left channel (L). Because the right channel (R) is an FM radio program.
5. Factory upgrade When the above operations cannot be viewed normally, a factory upgrade is required. First restore the factory settings → turn off the power → simultaneously press the confirmation key on the panel, down key → turn on the power → the TV will appear the factory upgrade dialog box → wait for the upgrade is complete → full channel search. If none of the above operations can be watched normally, the set-top box may be damaged.
All the above manual manual operations are required to be completed by maintenance personnel. User operations are strictly prohibited. Care must be taken during the operation process and ensure that the continuous operation is not interrupted during the upgrade process without power.
A special failure: A user in Changle District reported that the digital TV program is basically normal, but the Phoenix TV often has a mosaic phenomenon. The above operation can be normal for a while, but it still does not work. The user level can't be adjusted to 68db. After repeated processing, it still can't achieve the effect. Finally, the replacement method is used, and the branching device is replaced level by level. When switching to the first level amplifier under the light spot, the fault is eliminated.
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