【 Weak current college 】 building weak electricity system---II broadcasting and cable TV system

This section describes the broadcast sound system of classification, composition, and cable TV system works, signal source and signal processing, signal transmission networks, users, Terminal, and more for a more detailed analysis, at the same time on CATV system design based on the three technical parameters and indicators, the design principles described. 1. broadcasting sound system broadcast sound system refers to the building (Group) is self-contained, independent cable systems, is a kind of publicity and communication tools.　　By broadcasting the audio system can broadcast reports, announcements, background music and entertainment programmes, etc.　　1. Radio audio system for the classification of buildings of broadcasting sound system mainly includes public broadcasting radio, conference rooms, guest rooms, all kinds Auditorium acoustics, audio and home audio and simultaneous translation system, etc.　　(1) the public address system the system cable system, now belonging to generally be emergency broadcasting and public broadcasting system integrated with the usual play background music or other program, requires time for emergency broadcast. At design time, it is a fire alarm system design with. Public broadcasting and the fire of zone control. In order to ensure accident auto select partition, normally uses the computer control. Accident broadcasting is prerecorded cassette tape or CD player.　　Public address loudspeaker installation that is in line with the construction decoration. Public broadcasting of each partition are equipped with tuning Control Panel, you can adjust the volume or partition broadcast cut off.　　Multi-function Hall, concert hall and other places of public broadcasting in the control panel is also equipped with automatic switching socket, Visual broadcast radio programs for different cases or connected speech sound amplifying equipment. (2) the auditoria system using professional audio equipment, require high-power speaker system and power amplifier.　　To avoid sound feedback or howl, systems generally use a direct transfer of low resistivity.　　(3) private meeting system the system belongs to the amplification system, according to the nature of the meeting have special requirements such as multiple simultaneous interpretation system, etc. 2. broadcasting of sound system is composed of basic radio audio system including program source device, signal amplification and processing equipment, transmission lines and speaker system in four parts.　　Specific content can refer to the property, equipment and facilities management ".　　Transmission line: generally use low resistance and high current direct feed, transmission line requires a dedicated audio signal line, and the public broadcasting system, as the service area, long distance, in order to reduce the losses caused by the transmission lines, often using high pressure transmission, transmission current small, less demanding on the transmission line.　　Second, cable television (CATV) systems, television is the daily life of important components, cable television and satellite television receiving system has become a residential building basic system must be set. The development of satellite TV to cable television programme source rich provides assurance that cable TV to satellite TV signal coverage provides a reliable guarantee, so they are interdependent, and promote development.　　Sky satellite, terrestrial and cable coverage, has become a 21st century TV main mode. Early antenna television system was developed as a cable television system, application and development of the optical cable into the cable television system. Cable TV system and cable TV systems is often referred to as the CATV system. CATV system is the most common residential buildings and architecture of the system must be set. CATV systems generally use a coaxial cable and fiber-optic cable to transmit signals. Coaxial cable with good shielding, cable transmission, optical wave signal is stronger with strong anti-electromagnetic interference capability, therefore, CATV systems, transmission of TV signals are of high quality, clear imaging, transmission capacity, can provide a rich program.　　Two-way CATV system allows for data transfer, interactive TV and video functions, making it the entire society component of the integrated information network. At present, the city generally achieved through optical fiber CATV networking, forming a large system, units or large building CATV system within the small can be seen as the major system of distribution system, or you can, in turn, the city large system sent signals as these independent small system program source.　　People in work in regular contact with these small systems, a small system and large system similar to similarly divided into front end and distribution branch, three sections, just small system no less complex than large systems.　　CATV development very quickly in recent years, today's CATV system brings together contemporary electronic technology in the field of high technology and new achievements, including TV, radio, digital communication, automatic control and computer, and other new technologies and new results.　　Cable TV (CATV) systems, the front end, signal transmission distribution network and user terminal consists of three parts.　　1. front end system front-end system with signal source and signal processing in two parts.　　(1) signal source signal source main equipment including satellite TV signal reception antenna, terrestrial television signal reception antenna, MW antenna, recording and playing like machines, cameras and pilot signal generator, etc, there are some front-end TV editing equipment, computers, management control equipment, etc. Antenna to receive TV signals in the symbol space. Only receive a single channel is called a channel dedicated to receive antenna.　　Receive 1 ~ 5 channel called VHF low band receiving antenna; can receive 6 ~ 12 channel called VHF high band receiving antenna; 1 ~ 12 channels can be received as VHF-channel receiver antenna; can receive 13 ~ 30 channels or 31 ~ 44 channel UHF low frequency band known as the receiving antenna; can receive 45 ~ 68 channels known as UHF receiver antenna for high frequency band; 13 ~ 68 channels can be received as UHF antennas throughout the channel. (2) front-end signal processing of the signal source of road signals the necessary processing and control, and output high-quality signal to the main part of the transmission part is called the front-end signal processing part. The main role in front of the following. ① the antenna to receive TV signals of each channel to a certain level, respectively, and then the mixture into the mixer. ② necessary TV signal into a signal of the channel, and then press the channel signal processing. ③ to trunk amplifier provides for automatic gain controlAnd automatic control of the pilot signal slope. ④ own webcasts through the modulator to a channel after becoming a TV signal into the mixer. 　　⑤ satellite TV receiving equipment output video signal through a certain channel modulator become TV signal into the mixer.　　Front-end signal processing part of the main equipment including antenna amplifiers, channel amplifier, frequency converters, custom webcasts equipment, satellite TV receiving equipment, pilot signal generator, modulator, mixer and connection cables, parts. 2. signal transmission distribution network distribution network is divided into passive and active. Passive distribution network only distributor, Branch Manager and transmission cables, passive components, which can be connected users. Active distribution networks increased line amplifier, so the number of users that can connect.　　Line amplifier uses full-channel amplifier, to compensate for the increased number of users, line growth of signal loss. The dispenser is put all the input signal energy equal to two or more output devices, and they have three second Distributor, Distributor, Quad splitter.　　Divider output cannot open circuit or short circuit, causing a serious mismatch input port, also affect other outputs.　　The branch is on the route, from the main line of coupling part signal energy, and one or multiple output devices. Allocation between the various components in the system are connected with feeder, it is to provide signal transmission pathway separation as the main trunk, the trunk and branch lines. Trunk line on top and transmission distribution network.　　Trunk used to assign the network signal transmission, the branch line is used to allocate network and user terminal connection. 3. user terminal antenna television system user terminal is supply TV TV signal connector, also known as user terminal box.　　Divided into cassette and out box.　　4.CATV system design (1) should follow the basic principles for CATV system to carry out to meet the requirements of high-quality television signal, the CATV system design must comply with China was promulgated on cable television broadcasting system specification requirements should also be consistent with national and other national standards promulgated by the Department and its specifications. (2) design based on the three technical parameters and indicators in the cable television broadcast system technical specifications of CATV system technical parameters 27 indicators, but the most important of which is on TV image quality most of the three, which is the system output level, CNR and carrier combinations than the third beat. The importance of other technical parameters than the three small, the system is also in acceptance to the three main technical parameters of indicators, as long as the third parameter to the index value, the transfer of TV signal quality generally meets the requirements. So, usually the third parameter of the index as the main system design. ① system output level. Also often called user-level refers to the system output to output the value of the signal level. System output signal level is too low, the signal will be the noise generated by the system itself, as well as user TV noise drowns; output signal level is too high, it causes the TV to work within the circuit, the nonlinear area of TV image appears nonlinear distortion noise, the technical specification of system output level. Currently in the works, to avoid interference of nonlinear distortion occurs, generally according to the design of the level. ② carrier to noise ratio. Refers to the transmission of television signals through the system, the carrier power and noise power ratio. This parameter is mainly reflect the system internally generated noise on the image quality size. If the system internally generated noise small, the TV signal transmission after the carrier to noise ratio is large, the image quality is good; otherwise, the TV signal system transfer of carrier to noise ratio on the small, poor image quality. System of a major carrier to noise ratio depends on the system using various amplifier output signal of the carrier to noise ratio. Amplifier output signal of the carrier to noise ratio in addition and Amplifier noise coefficient for itself, and the input signal level for the input signal level higher output signal carrier to noise ratio.

From the TV image quality to meet the subjective evaluation standards of the 4 levels of the above considerations, the technical specifications of the system of the carrier to noise ratio should be not less than 43dB. ③ carrier beat combo three times. Measuring system signal by nonlinear distortion noise levels of several parameters, including carrier combined third beat ratio, crosstalk, carrier modulated than combination second beat ratio, etc. Because the current system for the transmission of television channels and more generally in more than 35, so the combination of three beat interference and more prominent, so the parameter is a combination of three carrier beat ratio, and as long as the parameters to achieve targets, several other general indicators will also be able to meet the requirements. The technical specifications of the system of carrier beat combo three times than not less than the 54dB.

In an optical cable transmission system, because the combined second beat interference also more prominent, so in addition to the requirements of carrier combined third beat indicators than meet the requirements, generally also requires a separate calculation carrier combinations than the second beat, so that it meets the requirements. 　　In the technical specification of carrier combination is not listed in the second beat than the parameters and requirements of it from reaching the targets, but works in practice generally require this parameter is less than 53dB. Systems of nonlinear distortion parameters are mainly depends on system amplifier nonlinear distortion in the size of the parameter.　　Amplifier nonlinear distortion parameters in addition to the size and performance of the amplifier itself, and its output signal level of the input signal level of the degree of difference, the output signal level is low, the input signal level difference of each channel, the greater the nonlinear distortion parameters. This shows that the carrier to noise ratio and nonlinear distortion parameters both on the signal level is contradictory, seek to a signal level of best value, so that both can meet the requirements, the system design of one of the main tasks.