The quality of television signal transmission has always been the most important thing because it directly affects the viewing performance of TV receivers. In the process of traditional analog TV number transmission, there is always a certain signal-to-noise ratio, which interferes with the propagation and reception of TV signals.

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The noise interference in TV signals mainly comes from:

(1) The original noise in the receiving signal source (refers to the dipole capable signal source).

(2) Spontaneous noise interference from the outside world.

(3) Radio wave interference from other transmitting stations.

(4) Ghost interference due to multipath transmission.

(5) Thermal noise inside the differential and interference caused by nonlinear distortion of the wideband amplifier.

(6) AC interference and combined frequency interference.

For better TV broadcasts, efforts should be made to make the signal-to-noise ratio of the video output of the TV receiver above 45 dB. But it is far from meeting the requirements of digital high-definition television. In recent years, due to the good quality of the satellite channel (relative to terrestrial broadcasting), the coverage is large, no relay is needed, and it is especially suitable for program distribution and broadcasting, so it has been widely used. However, the vast majority of television programs transmitted by satellite are analog. The analog system transmits the occupied bandwidth of the TV program. A 36MHz satellite transponder can only transmit one analog TV signal, so the channel utilization rate is not high.

With the rapid development of digital video rate compression technology and the successful development of VLSI, the digital broadcast control system and the use of satellite to transmit digital broadcast television programs have become reality, so that the image quality is greatly improved to meet the high Broadcast level requirements for definition television. At the same time, due to the use of modern digital video compression technology and channel modulation technology, it can realize 4-6 digital compression TV programs within the occupied bandwidth of analog TV signals, thereby greatly improving channel utilization and reducing the TV program of each channel. Transmission costs, while also reducing noise interference.

Therefore, in digital television, even if digital television signals are generated, it is necessary to improve the reception quality of digital televisions, as well as details of two aspects of the studio and television transmission.

1. Broadcast control system of the broadcast center

In the early TV broadcast control technology, it mainly relied on manual manual control of several video recorders to complete the video and audio switching control. As the channel program increases and the broadcast time is extended, the broadcast control personnel are in a state of high tension throughout the broadcast process, which greatly increases the labor intensity and psychological pressure, which inevitably causes a playback accident, especially during the handover process. The instantaneous jitter noise appears, which seriously affects the broadcast quality.

With the development of microelectronics technology and the popularity of computers, the mode of TV broadcast control has changed with manual switching from semi-automatic and fully automatic broadcast control. Digital tapeless broadcast is about to replace the traditional tape-based broadcast method. The development of computer mass storage technology, digital compression coding technology, optical fiber communication technology, multimedia communication technology, disk technology and the continuous reduction of the cost of digital equipment. It will make a qualitative leap in the broadcast control system, so that the TV broadcast control of the network is completely digitized, thereby greatly improving the broadcast quality of the TV signal.

However, using a disk instead of a tape is not the whole meaning of digitization. The digital broadcast control system should be an advanced automation technology. It consists of multiple servers, workstations, and PCs. On the one hand, it inherits the past tape-based broadcast mode, and uses the program source recorded on a medium carrier such as a magnetic tape or an optical disk. Analog or component video recorders can coexist with new digital devices; on the other hand, broadcast information and program content can be obtained through a functional network.

The design and implementation of an automatic broadcast control system is a very serious engineering and technical issue. First of all, it must be able to reduce the failure factor by improving hardware quality, effective backup and enhanced monitoring means, while providing a variety of emergency means; on the basis of improving the quality of broadcast, as much as possible to improve the automation of the broadcast department, so that the system More specific integrity.

The advanced computer-controlled automatic broadcast control system will be an important means for TV station broadcast equipment upgrade and improve broadcast quality. It can automatically schedule and automatically play each video recorder according to manual operation habits and intelligent management of software. Automatically switch to achieve timed playout, sequential playout, trigger playout and program insertion, which greatly improves the normalization of program videotape.

The automatic broadcast control system is composed of many links. Therefore, it must have a hierarchical monitoring and interactive monitoring method to perform subjective evaluation on the broadcast signal in real time; it can use the oscilloscope and the integrated tester to perform quantitative technical monitoring. Make the audio and video technology quality level requirements meet industry standards.

With the continuous improvement of people's living standards, the development of TV stations is required to increase the number of channels, increase the broadcast time, increase the number of programs, increase the number of live programs, and increase the number of program output points. At the same time, it must ensure the high reliability of the broadcast, the high quality of the signal, the ease of operation and the good capacity of the system. This requires that the development of the broadcast control system of the studio will be the complete automation of the application of robot technology, otherwise it will be difficult to adapt to the network of digital audio and video signal transmission technology in the future.

2. Digital TV transmission method

Due to the rapid development of digital video rate compression technology, digital compression TV programs continue to increase, TV population coverage requirements continue to increase, a perfect video and audio transmission method, whether TV programs can be distributed and broadcasted to the home, has become more and more Attract people's attention.

At present, some privately-developed countries have adopted satellite, microwave and cable transmission networks. China has also begun to use satellite to transmit multi-channel digital compression TV programs, as well as network-to-home formed by microwave relay systems and CATV systems.

In November 1995, China's CCTV successfully broadcast five sets of digitally compressed TV programs, including sports and movies, to the nation's G-band. In August 1996, the United States SA company's MPEG-2/DVB-compliant equipment was used to broadcast three digital compressed TV programs in the central and central units.

At the beginning of 1997, China's MPEG-2/DVB standard used the 3B transponder of Asia 2 satellite to transmit digital compressed TV programs in five provinces including Henan, Inner Mongolia, Qinghai, Guangxi and Hunan in SCPC mode; The 6B transponder transmits digitally compressed TV programs from five provinces including Fujian, Guangdong, Hubei, Jiangxi, and Liaoning (also using SCPC). The satellite broadcasting of digital compression technology has injected new vitality into the development of digital TV in China.

There are usually two ways to send multiple sets of digital compressed TV programs to satellites in China: one is to modulate one set of each program and send it to the satellite, and the other is to combine the data streams of several sets of programs into one data. Then, modulate a carrier and send it to the satellite. The former is called SCPC (single-channel single carrier) mode, and the latter is called MCPC (multiple single carrier) mode. The SCPC method adopted by various provinces in China shares a transponder, and the central station adopts the MCPC method because it transmits multiple sets of programs.

At the receiving end, there are two ways of receiving according to different purposes: one is to convert the received satellite digital TV signal into an analog signal through the IRD service provided by the cable television station, and send it to the cable television network in a conventional manner; the other is Directly to the household, but the family should be equipped with a home IRD, satellite digital TV signals can be sent directly to the TV, making it very convenient for users to receive TV programs.

At present, CATV systems have been established in towns and cities across China, and are also being developed at a higher level, enabling large industrial and mining enterprises to network with CATV systems in towns or cities, thereby expanding the transmission distance from several kilometers to A range of dozens to a few hundred miles. Due to the increasing number of business contents, in addition to the high performance of various devices, the transmission distance must be solved by means of long-distance transmission. The high-quality coaxial cable currently in common use, even if a reasonable amplifier is designed, can transmit up to 14km for VHF, and the whole channel system can only transmit 2.75km. Therefore, it can solve long-distance transmission, except for the use of tens of kilometers. In addition to fiber optic technology, the main method is to use microwave technology, that is, microwave as the delay of the CATV system. The microwave is an electromagnetic wave having a frequency in the range of 30 MHz to 300 GHz and has a wavelength of 1 m to 1 mm. Due to the characteristics of frequency bandwidth, good stability, linear propagation, directional transmission and reception, microwave transmission systems are widely used in long-distance transmission in TV transmission networks in various countries. The same is true in China, which has entered the stage of using TV microwave technology in large CATV systems.

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