First, the composition of the satellite TV reception system
The satellite TELEVISION receiving system is composed of three major parts: the upstream transmission station (upstream transmission), the satellite star (on-board transponder) and the ground receiving (downstream receiving station), as shown in Figures 1 and 2.
Figure 1 Illustration of a satellite television broadcasting system
Figure 2 Illustration of a synchronous satellite
(1) TV antenna: The role of the TV antenna is to receive the electromagnetic wave signal of the transmitter and supply it to the RECEIVing end of the TV.
(2) Amplifier: Amplifiers are divided into antenna amplifiers, channel amplifiers, trunk amplifiers, distribution amplifiers, line extension amplifiers, etc.
1) Antenna amplifier: The antenna amplifier should be used when the magnetic field is weak and far away from the radio. It is mainly used to amplify weak signals, so it is also called a low level amplifier - used to increase the level of the received signal and reduce clutter interference.
2) Trunk amplifier: used to compensate for the energy loss on the trunk, it has the performance of automatic gain control and automatic slope control.
3) Line extension amplifier: It is an amplifier used to compensate for the insertion loss and cable loss of the brancher on the trunk.
4) Channel amplifier: It is mainly used to amplify the amplifier of a channel of full TV signal, it is in the front end of the system, the gain is higher.
5) Distribution amplifier: An amplifier set up to increase the signal level to meet the needs of the splitter and splitter.
(3 Mixer and Splitter: Its role is to combine several different channels of TV signals received by the antenna into a wideband amplifier for amplification, and the role of the mixer is to merge several signals into a way without affecting each other, and can prevent other signals from passing.)
(4) Allocation network. Its function is to transmit the high-frequency TV signal provided by the front end to the brancher through the trunk line, and then to the terminal distributor to provide the user with TV ratings.
Second, the composition of the cable TV system
1. The basic composition of the cable TV system
Cable television (cable television, i.e. CATV) system is an information engineering system that transmits television program signals and applications in the form of cable closed circuit. Cable TV systems are generally composed of signal sources, front-end equipment, transmission trunks and user distribution networks. Fig. 3 is a block diagram of the basic components of a cable television system.
Figure 3 The basic composition of a cable TELEVISION system
Note: The numbers 1, 2, 3、...、6 in the figure represent the building number, and the high-frequency lightning arrester should be installed in front of the building and the building of the overhead line.
The actual system can be a deformation or combination of these parts, depending on the needs. Figure 4 shows the basic components of the adjacent frequency system.
Figure 4 The basic structure of the adjacent frequency system
(1) Signal source. It mainly includes satellite ground stations, microwave stations of the post and telecommunications department, urban cable TV networks, VHF, UHF, FM TV receiving antennas for open-circuit transmission, studio cameras, video recorders, and laser DVD player signals from the production of programs.
(2) Front end. Usually refers to the processing equipment station that provides a high-quality signal for the system, such as band-pass filters, image accompaniment modulators, frequency converters, channel amplifiers, satellite receivers, signal equalizers, power dividers, pilot signal generators and some special service equipment (such as modulation decoders, system monitoring computers, line detection, burglar alarms, etc.). According to the scale and complexity of the system, it can be divided into a local front end, a central front end, and a remote front end that transmits signals to the local center front end through satellite ground reception.
(3) Trunk transmission system. Undertake the task of transmitting the signal processed by the front end to the user distribution network over a long distance, mainly composed of various types of trunk amplifiers and backbone cables, and if two-way transmission programs are required, the two-way transmission trunk amplifier and distributor are used - when the system is a large-scale urban network, the optical cable can also be used as the trunk transmission mode.
(4) User assignment network. It mainly includes distribution amplifiers, line extension amplifiers, distribution lines, splitters, system outlets (user terminal boxes) and cable lines, etc., providing roughly the same level (TV) signal to each user.
2. Division of cable TV system
CatV systems are divided in the following ways:
(1) Classified according to the type of network It belongs to the metropolitan area network, that is, the urban network with 2,000 to 100,000 cable TV subscribers, or a large urban network of more than 100,000 households. Generally operated and managed by the local cable TV authorities, there is a signal source total front end, transmitted to the local sub-front ends through the trunk line, and then enters the user distribution network transmission trunk line multi-optical fiber transmission technology and the user distribution cable network connection, forming a fiber optic cable and a cable hybrid network (ie HFC), and its network topology is shown in Figure 5.
Figure 5 Typical network topology of an HFC
(2) Classification by network type.
1) Lan Area Network. That is, cable TV networks with less than 2,000 subscribers. Generally, the full cable network can be used, and the HFC method can also be used in the case of dispersed users and a large area, and the local area network can also be connected to the metropolitan area network through cables.
2) Two-way transmission of cable digital television network. That is, on the basis of the HFC network, the forward (downstream) channel transmits cable TV analog signals, digital TV signals and various data service signals, and the reverse (upstream) channel transmits various wideband and narrowband digital service signals.
(3) Classified according to the transmission frequency band.
1) Frequency-isolated transmission system. The arrangement of channels on the spectrum is an interval transmission system, that is, VHF (very high frequency) system, UHF (ultra high frequency) system, full channel system (VHE + UHF). Among them, the VHF band has DS1~DS12 channels, and the 15HF band has DS13~DS68 channels.
2) Adjacent frequency transmission system. That is, 300MHz, 450MHz, 550MHz, 750MHz, 862MHz system. Since the 68 standard channels stipulated by the state are discontinuous and jumpy, these discontinuous frequencies can be used inside the system to set up supplementary channels, represented by Z. The channel division and application of the adjacent frequency system are shown in Table 1.
| Table 1 Channel division and application of adjacent frequency system | ||
| The system type | The number of channels transmitted | The channel number that can be transmitted |
| 300MHz adjacent frequency system | 28 channels | DS1~DS12+Z1~Z16 |
| 450MHz adjacent frequency system | 47 channels | DS1~DS12+Z1~Z35 |
| 550MHz adjacent frequency system | 52 channels | DS6~DS22+Z1~Z37 |
| 750MHz adjacent frequency system | 79 channels | DS6~DS42+Z1~Z42 |
| 862MHz adjacent frequency system | 93 channels | DS6~DS56+Z1~Z42 |
Note: 1. Residential quarters and enterprises in small towns can choose 450MHz or 550MHz adjacent frequency system.
2. Residential quarters and enterprises in large and medium-sized cities should choose 750MHz or 862MH force system, and departments with conditions should choose 1GHz system.
3. For the newly built cable TV system, the 550MHz neighbor frequency system is generally selected for one-way transmission, the 750MHz, 862MHz neighbor frequency system is selected for two-way transmission, and the 300MHz and 450MHz neighbor frequency system is only used for the existing old system.
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