Basics of Digital Cable TV

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Basics of Digital Cable TV

I. Overview of Cable TV
1. Transmission form of TV signals
In terms of the principle of TV technology, a TV system that transmits moving images usually consists of camera, signal processing, transmission, and display. The principle of sequential transmission of image information is the basis for the generation of TV signals. Its basic method is to decompose the image to be transmitted into many pixels, convert the characteristics of each pixel, such as brightness and color, into the amplitude and time series of electrical signals in a certain order and method, transmit and process them in sequence, and add a characteristic signal representing the relative position of each pixel - a synchronization signal, so as to facilitate the reproduction and positioning of pixels when the electrical-optical information is restored at the receiving end. This is the principle of generating TV signals in TV technology. For this full TV signal containing all image information, its basic transmission form can be divided into the following three methods.
(1) Baseband transmission
is to directly transmit the baseband signal through a transmission line or other media. It is generally used in places where video equipment is relatively concentrated.
(2) Wireless TV transmission
is to modulate the baseband TV signal and the accompanying audio signal on the radio frequency carrier through the signal conversion processing method of amplitude modulation, so that the latter can be radiated in the form of high-frequency electromagnetic waves through an appropriate antenna.
For the receiving end of wireless transmission, it is necessary to improve the strength and quality of the received signal through receiving antenna facilities of various forms and sizes. The level of signal restoration varies from person to person and from place to place, and it is impossible to achieve consistent effects and universal stability.
(3) Cable TV transmission
is a method of transmitting and processing the full television signal of a certain amplitude through radio frequency modulation, and then transmitting the radio frequency carrier signal with all the characteristics of audio and video information through a tangible transmission medium, such as a line network composed of coaxial cables, optical fibers and other media. Due to the various disadvantages brought about by the inherent open-circuit transmission characteristics of wireless broadcasting, such as the increase in the number of channels required to increase the number of program sources, the result is also limited by frequency allocation. Cable TV can use video programs from equipment such as video recorders, as well as various baseband audio-visual information such as satellite TV signals and microwave relay signals in the front-end studio to select, process, demodulate, modulate, etc., and then transmit it to the vast number of users covered by the closed-circuit system network through the cable distribution system. This form of television signal transmission, which is not limited by frequency use regulations and is not affected by the natural environment, has developed rapidly. The basic industry supporting the development of its facilities has gradually formed, promoting the cable television system that uses coaxial cable as the transmission line medium and has the characteristics of processing multi-channel and multi-function signals. With the application of optical fiber equipment technology, the coverage and scope of the network have been further expanded. The functions of the system and network management have been included in automatic adjustment and control technology, intelligent computer technology and various auxiliary engineering technologies. The application of new technologies in the source of information in the cable television system has also developed rapidly. Therefore, as an effective means of information transmission, cable television is still constantly developing and changing in the needs of the information society. The transmission form of television signals will eventually break through the existing various forms and develop into a more market-oriented stage to meet the new requirements of future society for television information.
2. Composition of cable television system
At present, China's cable television system is generally an integrated 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 . Cable television program sources include analog and digital television signals received by satellite ground stations, television signals transmitted by local microwave stations, television signals transmitted by local TV stations, uplink television signals and data, etc. In order to play the signal source, the computer room should be equipped with satellite receivers, analog and digital players, multi-function consoles, cameras, special effects graphics processing equipment, editing equipment, video servers, user management and control equipment, digital signal processing equipment, etc.
(2) Front-end equipment . The front-end equipment is the equipment connected between the signal source and the trunk transmission network. It processes the received TV signal, mixes all the TV signals through a mixer, and then sends them to the trunk transmission network to achieve 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 a cable trunk, or to an optical cable and microwave trunk.
(3) Transmission network . The transmission network is between the front-end equipment and the user distribution network. Its function is to transmit various signals output by the front-end to the user distribution part without distortion and in a stable manner. The transmission medium can be RF coaxial cable, optical cable, microwave or a combination of them. The most commonly used one is hybrid optical cable and coaxial cable (HFC) transmission.
(4) Distribution network . Cable TV distribution networks all use cable transmission, and their function is to distribute the amplifier output signal to building units and users at a certain level.
(5) User terminal . The user terminal is connected to the user port of thousands of households, and the user port is connected to the TV. 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. In the future, user terminals will include set-top boxes, cable modems, descramblers, etc. 2.
Digital TV cable transmission
1. What is digital TV ?
Digital TV refers to a new TV system that uses digital processing in all aspects, including program shooting, editing, sending, transmission, storage, reception and display. It can also be said that digital TV is a TV system that fully realizes digitalization and digital processing in three aspects: source, channel and destination.
2. What is digital TV?
Digital TV (or digital TV) uses a variety of digital processing technologies in the circuit of existing analog color TVs, such as: digital 100HZ scanning technology, 3D/5D digital image processing technology, I2C digital bus technology, digital picture-in-picture technology, NICAM digital audio technology, etc. Color TVs that use these digital processing technologies have achieved better image and sound quality and increased the functionality of TVs. In essence, digital TV still belongs to the category of analog TV.
3. What are the advantages of digital TV
over analog TV? Compared with analog TV, digital TV has the following advantages:
(1) The image transmission quality is higher and the distance is longer. During the transmission process, analog TV image signals are affected by the transmission channel characteristics (amplitude-frequency characteristics, differential gain, differential phase characteristics) and noise interference, and the quality is not high. Moreover, after multiple conversion transmissions, the channel characteristics and noise interference accumulate, resulting in further deterioration of image quality. However, some of these influences do not exist for digital TV signals. During the transmission process, digital TV signals will not accumulate interference and noise after multiple relays (or copies). At the same time, error correction coding technology can be used. The image quality at the receiving end is better and basically remains consistent with that at the transmitting end, and the transmission distance is not limited.
(2) High utilization of channel resources . Spectrum resources are important national resources. The spectrum resources of analog television are limited. One set of analog television programs needs to occupy a satellite transponder with a bandwidth of 36MHz and occupies 8MHz of terrestrial television broadcasting and cable television bands. Digital television uses compression coding technology to transmit 5 sets of SDTV programs in a 36MHz satellite transponder, with a display clarity of about 400 lines, and transmit more than 4 sets of SDTV programs in an 8MHz channel.
(3) Provide new services and realize high-speed data transmission. In digital television communication, multiple digital information such as text, data, voice, and still images can be transmitted simultaneously without interfering with each other. Digital television networks can be interconnected with computer networks and telecommunications networks, which not only enriches information sources, but also increases the interactivity between users and various information sources, and realizes multiple high-speed data services such as users' free on-demand programs, e-commerce, online shopping, online teaching, online medical care, and online games (using 64QAM modulation in an 8MHz bandwidth can achieve a data transmission rate of about 32~38Mbit/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 binary code, which has only two states: "0" and "1". The binary digital signal is not affected by power supply fluctuations and device nonlinearity, so the signal can remain stable and reliable. Large-scale integrated circuits can be used to process digital signals, which can reduce the power consumption of the equipment, reduce the volume, and improve the reliability of the equipment. At the same time, digital equipment does not need to be adjusted, and is easy to maintain and use.
(5) Save transmission power and have a wide coverage range . The average power required by digital TV transmitters in the same coverage service area is an order of magnitude lower than the peak power of analog TV transmitters. 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 range of digital TV transmitters is several times larger than that of analog TV transmitters with the same power.
(6) Flexible and friendly human-machine interface . The flexible and practical human-machine interaction interface is easy for ordinary people to operate. In addition to display equipment, it is easy to achieve system integration and large-scale production. The low price makes it easy to promote and popularize.
(7) It is easy to achieve conditional reception . Digital TV signals are easy to encrypt/scramble, which is beneficial to information security and convenient for realizing pay TV, video on demand and interactive TV.
4. How to digitize analog signals ?
The digitization of analog signals requires three processes: sampling, quantization and encoding to form binary digital signals. These three processes are collectively called pulse code modulation (PCM).
5. What is high-definition TV?
Digital high-definition TV (HDTV) is a high-tech TV technology that is actively developed and applied by developed countries in the world. It uses digital signal transmission technology and has stronger anti-interference performance than ordinary analog TV signal transmission. The clarity of the image is significantly improved. The aspect ratio of the received image is 16:9. With multi-channel digital audio, it can achieve the projection effect of 35mm widescreen movies.
The definition of HDTV in the 801 report of CCIR (International Radio Consultative Committee, now renamed ITU-R) is: "The design of high-definition television systems requires that viewers can see or nearly get clear details at a distance of about three times the height of the screen, achieving the same feeling as viewers with normal vision watching the original scene."
6. What is pay TV?
Pay TV is different from the existing basic public welfare TV. Its fundamental purpose is to meet the personalized, diversified and multi-level cultural needs of the people, with channel specialization and object-oriented as important signs, and charging user service fees as the main profit method. In other words, it uses encryption technology to restrict non-paying users from watching, and only paying users can sit at home and order the TV programs they want to watch.
3. Digital TV set-top box
(I) The concept, function, principle and structure of digital TV set-top box.
Cable TV networks are developing towards digitalization, networking and industrialization. One of the key equipment for cable TV networks to provide comprehensive information services is the user terminal device 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 ordinary analog TV sets. Of course, it also has network and conditional reception functions. This set-top box is called a digital TV set-top box. According to the different transmission media, digital TV set-top boxes are divided into three types: 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 structures of the three set-top boxes mainly differ in the demodulation part. At present, the set-top box used in our county is the cable TV digital set-top box.
1. The concept of digital TV set-top box
There is no standard definition of set-top box at present. The traditional term is: "a box placed on top of the 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 the existing TV set or increase its functions. Due to different functions and uses, the concept of "set-top box" is somewhat vague, such as the early supplementary channel set-top box, teletext set-top box, pay TV set-top box, etc.
Digital TV set-top box is one of the information appliances. It is a consumer electronic product that allows users to watch digital TV programs on existing analog TV sets and conduct interactive digital entertainment, education and commercial activities.
2. Functions of digital TV set-top box
The basic function of digital TV set-top box is to receive digital TV broadcast programs, and it has all broadcast and interactive multimedia application functions. In fact, the set-top box can support almost all broadcast and interactive multimedia applications, such as digital TV broadcast reception, electronic program guide (EPG), near video on demand (NVOD), pay-per-view (PPV), online software upgrade, data broadcasting, Internet access, email, IP phone and video on demand.
(1) Electronic Program Guide (EPG): It provides users with an easy-to-use, user-friendly interface that allows them to quickly access the programs they want to watch. Users can use this function to see the TV programs that will be broadcast on one or more channels or even all channels in the near future.  
(2) High-speed data broadcasting: It can provide users with various information such as stock market trends, ticket information, electronic newspapers, popular websites, etc.
(3) Online software upgrade: It can be regarded as one of the applications of data broadcasting. The data broadcasting server broadcasts the upgrade software according to the DVB data broadcasting standard. The set-top box can identify the version number of the software, receive the software when the version is different, and update the software stored in the memory.
(4) Internet access and e-mail: The digital set-top box can conveniently realize the Internet access function through the built-in cable modem. Users can surf the Internet and send e-mails 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 (currently we have not opened this service in Yancheng).
(5) Support interactive applications such as video on demand, interactive games, etc.
(6) Conditional reception: The core of conditional reception is scrambling and encryption. The digital set-top box should have the functions of descrambling and decryption.
3. Principle and structure of digital TV set-top box
Digital TV set-top box receives digital TV and various data information from various transmission media, and watches digital TV programs and various data information on analog TV through demodulation, demultiplexing, decoding and audio and video encoding (or through corresponding data parsing modules). Taking the Skyworth series of cable digital TV set-top boxes we are using now as an example, its working principle is as follows: Cable digital TV set-top box receives digital TV programs, processes data services and completes the parsing of various applications. The source completes two levels of coding before entering the cable TV network. One is the channel coding for transmission, and the other is the source coding of audio and video signals and the encapsulation of all sources into a transmission stream. Corresponding to the front end, the receiving end set-top box first extracts the channel coding signal from the transmission layer and completes the channel demodulation; the second is to restore the compressed source coding signal, restore the original audio and video streams, and complete the reception and parsing of data services and various applications.
According to the requirements of receiving digital TV broadcasts and Internet information, the hardware structure of a digital TV set-top box consists of signal processing (channel decoding and source decoding), control and interface.
(II) The requirements of digital TV set-top boxes for signals
The set-top boxes selected by our county are the series products of Skyworth Company, mainly including the early Zhongshilian 5800, Irdeto 5800 and the current Elf 5800 models, and their functions and performances are basically the same.
1. Signal requirements. The requirements of the set-top box for signals are that the household level of general analog TV is 69db±5db according to the standard we are currently implementing, and the level of digital TV is no problem if it is greater than 37db according to practice. The level difference between analog signal and digital signal is 15db±5db, and we are currently running a difference of 20db.
2. Connection of set-top box signals. The signal from the user box is connected to the signal input end of the set-top box through a connecting line. The set-top box has two signal outputs, one is the analog signal to the RF input end of the TV, and the other is the video and audio (A/V) signal to the video and audio input end of the TV. Note: Whether the power of the set-top box is turned off or not, it will not affect the TV's viewing of analog signals.
3. The signal input of the RF input signal of the set-top box is the British F5, -5 conversion head for the signal input of the Vision 5800 model, the signal input of the Irdeto 5800 model is the -5 port direct input, and the signal input of the Elf 5800 model is the British F5 input.
4. Distribution of existing digital TV programs and channels (see Table 1)
(III) Basic operation (see the instruction manual of cable digital TV set-top box for details)
(IV) Common faults and troubleshooting

Digital TV is a technological development and innovation. It is different from the transmission and transmission of analog TV, but it has advantages that analog TV cannot compare with. I will talk about the maintenance method of digital TV from a practical perspective, specifically from three aspects:
First, detect the user's home level. Generally, the home level of analog TV is greater than 58db and can be watched normally, while the level of digital TV is greater than 37db according to practice, which is no problem, but it is better to be greater than 40db. Then when the home level of digital TV is greater than 37db, the home level of the previous analog TV is measured again, which can be used to deduce the home level of the network in the area when the digital set-top box is watched normally. Secondly, the search problem of the set-top box. 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 factory settings. It can solve some general faults, such as inaccurate time on the set-top box, no change in the number of search channels, etc. Finally, the upgrade of the set-top box. It can also be done in two ways, namely automatic upgrade and manual upgrade. Automatic upgrade is that the user puts the set-top box in standby or power-on state, and the front end of the digital TV company issues an automatic upgrade command to complete it. For manual upgrade, directly select online upgrade in the set-top box and wait for 5-10 minutes to complete it. The premise of these two upgrade methods is that the front end of the digital TV company must have upgrade software for different set-top boxes.
1. Mosaic or freeze phenomenon?
① Is the attenuation of the signal transmission cable too large
? ② Is 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 ends of the analog signal level should not be less than 60dB, and the error of the high and low end levels should be less than 3dB. Then check whether the analog TV's five (sports channels) are clear. If they cannot be achieved, 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 requirement of 37dB is not met, the TV screen will have mosaics or freezes in a certain program package or several program packages, and users cannot watch normally. During maintenance, the level of the digital TV signal should be adjusted to above 40dB.
2. Can the user watch normally?
If the TV interface shows that the current signal has been encrypted, please contact the supplier or ask for authorization. There are several possibilities for this situation:
a) The fee has not been paid, ask the user the payment deadline;
b) CPU has an operating failure. Solution:
cut off the power of the set-top box, wait for 10 seconds, and then restart the set-top box. If it does not work, proceed to the next step. Press the menu key → point to system settings → restore factory settings → automatic search.
c) There is no automatic upgrade. Solution: cut off the power of the set-top box, press and hold the "Confirm" key on the panel and turn on the power. Release "Confirm" after the "Automatic Upgrade" screen appears. At this time, "Frequency 0371.000, Symbol Rate 06.875, Modulation Mode QAM64, Service Number 7000" will appear on the TV. Press "Confirm". Use the up and down keys to adjust to the "Confirm and Cancel" dialog box below. Press the "Confirm" key on the "Confirm" dialog box. At this time, the set-top box upgrade signal dialog box will appear. If the current "Version is 1.1, upgraded to 2.0, do you need to update?" dialog box appears at this time, please press the Confirm key again. The set-top box will automatically upgrade. After the upgrade is complete, it will appear that the current program is empty and whether you need to automatically search for channels. Press the Confirm key again, and the upgrade and search are complete. (The frequency of the blue card is 0307.000, and the frequency of the red card is 0371.000)
③ If the TV displays a dialog box that asks for authorization because the E30 program is encrypted, you need to turn off the power of the set-top box and turn it on after 30 seconds. If the dialog box that asks for authorization because the program is encrypted appears, you need to open the set-top box menu, switch the main menu to program management → switch the submenu to edit the program list, press F3 to delete the encrypted channel → exit → confirm.
3. If duplicate channels appear or some channels cannot be received
, restore the factory settings → search all channels.
4. If 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
If the above operations cannot be viewed normally, a factory upgrade is required. First restore the factory settings → turn off the power → press the confirmation key and the down key on the panel at the same time → turn on the power → the TV will display the factory upgrade dialog box → wait for the upgrade to be completed → search all channels. If the above operations cannot be performed normally, the set-top box may be damaged.
All the above manual mandatory operations are required to be completed by maintenance personnel. User operations are strictly prohibited. Be careful during the operation and ensure that the power is not cut off and the ongoing operation is not forcibly interrupted during the upgrade process.
A special fault: A user in Changle Community reported that the digital TV programs can be watched normally, but Phoenix Satellite TV often has mosaic phenomenon. The above operations can be normal for a period of time, but it still does not work. The user level is adjusted to 68db, but it does not work. After repeated treatment, the effect is still not achieved. Finally, the replacement method is used to replace the splitter one level at a time. When the first-level amplifier under the light spot is replaced, the fault is eliminated.

Keywords:Cable Reference address:Basics of Digital Cable TV

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