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What are the classifications of radio and television optical fiber transmission systems

2022-09-22
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Radio and television have become an indispensable part of life, and now optical fiber is the main medium for network information dissemination. How much do you know about the composition of digital TV optical fiber transmission system? A comprehensive understanding can be made below. The front-end part includes two parts: source coding and channel coding and modulation. The source coding includes video and audio coding; the channel coding and modulation part includes channel coding, QAM modulation, and optical transmitter.

 

The client side is the exact opposite of the front-end process. The transmission line is fiber optic cable. From the perspective of the entire system of the cable digital TV network, the optical transmitter mainly performs digital processing and modulation processing on the signal source before, which is not the focus of this book. This book focuses on the part of optical fiber transmission. Generally speaking, it refers to the part that outputs the optical signal from the optical transmitter and is amplified by the optical amplifier, and then reaches the user's home through the optical distribution network.

 

Optical fiber transmission system

Radio and TV Optical Fiber Transmission System

Classification of Optical Fiber Transmission Systems

The radio and television optical fiber transmission system is an important part of optical communication, which is mainly composed of optical fibers, light sources, optical amplifiers and optical receivers. The characteristics and light receiving methods are different, and different types of optical fiber transmission systems are divided.

FTTH Cable

(1) According to the number of wavelengths, it can be divided into single wavelength and multi-wavelength.

(2) According to the signal, it can be divided into digital and analog optical fiber transmission.

(3) According to the modulation format, it can be divided into direct modulation and external modulation.

(4) According to the receiving method, it can be divided into direct receiving and coherent receiving.

(5) According to the wavelength, it can be divided into short wavelength, long wavelength and ultra-long wavelength.

(6) According to the fiber characteristics, it can be divided into multimode fiber and single mode fiber.

(7) According to the working mode, it can be divided into coherent optical fiber transmission, wavelength division/time division/code division multiplexing and all-optical transmission.

 

There are many types of optical fiber transmission, but the optical fiber transmission used in cable TV systems basically adopts the intensity modulation/direct receiving system. In wired FTTH, considering the large number of applications of optical fiber and optical power, most of them use AM operating at 1550nm wavelength. -VSB external modulation optical fiber transmission system, which will be introduced in the following text.

 

Subcarrier Multiplexing Optical Fiber Transmission System

Using sub-carrier multiplexing (SCM) technology can realize the transmission of multiple channels of video and audio signals on one optical fiber. The so-called sub-carrier refers to radio frequency electromagnetic waves (sine waves) to distinguish the The transmitted optical carrier, that is, at the transmitting end, the signals to be transmitted are modulated (amplitude modulation or frequency modulation) on different subcarriers, and then each subcarrier with the signal (or double sideband quadrature balanced amplitude modulation of the suppressed carrier) Signal) is combined with a power combiner (mixer) to form a broadband composite radio frequency (RF) signal, and then this composite radio frequency (RF) signal is used to modulate the light intensity of the transmitter light source, and the modulated optical carrier is transmitted to the optical fiber through the optical fiber. Each optical receiver restores the sub-carriers through the photoelectric detection module, and then uses the demodulator to demodulate the multi-channel modulated sub-carriers to obtain the multi-channel video and audio signal sources respectively.

 

The basic principle of sub-carrier multiplexing optical fiber transmission system, the multi-channel AV signal is modulated and mixed into a radio frequency signal containing multi-channel TV programs. After long-distance transmission, it reaches the optical receiver. The optical receiver first restores the multiplexed radio frequency (RF) signal, and then uses the modulator (the digital TV system uses a set-top box, and the analog TV system uses a TV set). The high-frequency tuner is completed) from the demodulation of various AV original signals. Each demodulator corresponds to a band-pass filter of the corresponding frequency band, which is matched with a specific sub-carrier to prevent cross-talk between channels.

 

The modulation of the light source by the SCM optical fiber transmission system is generally IM (light intensity modulation).

 

FM (Frequency Modulation) Optical Fiber Transmission System

Its advantage is that it takes full advantage of the high sensitivity of frequency modulation to light receiving, and the transmission distance is relatively long, and the general light receiving sensitivity can reach below -15dBm. The disadvantage is that each FM wave occupies a wide frequency band, and the same bandwidth carries relatively few program contents.

 

AM-VSB (Amplitude Modulation Vestigial Sideband) Optical Fiber Transmission System

The vestigial sideband AM is a modulation method of analog TV signals commonly used by radio and television in our country. This method has unique advantages in transmitting cable radio and television. It is mainly reflected in two aspects. First, its TV signal modulation/demodulation system (AM-VSB) is completely in line with the reception system of home TVs, and does not need to be converted. Therefore, the signal is directly input to the TV after photoelectric conversion for viewing. . The second is that it effectively compresses the bandwidth of each channel to only 8MHz (PAL-D system), so as to accommodate more TV channels in the VHF/UHF band designated by the country.

 

Of course, the disadvantage caused by this is: the linearity requirements of the laser are very high, and the nonlinear index requirements are very strict, so that a relatively high optical modulation system cannot be achieved, and the sensitivity of the optical receiver is very low. Therefore, you will find that the optical cable transmission indicators are very strictly controlled in the transmission of radio and television optical networks, and the average link loss and joint loss have very clear requirements. This does not mean that the technical requirements of the radio and television system itself are higher than other applications, but As a last resort.

 

Otherwise, the indicators are not well controlled in the initial stage of construction. After a period of time, the indicators will drop slightly, and there will be problems in signal transmission. Of course, with the rapid development of technology, combined with the actual needs of the market, new products such as optical transmitters and optical amplifiers with higher power output, lower noise and high linearity are continuously launched, which also greatly promotes AM-VSB (Amplitude Modulation of Vestigial Sideband) Fiber optic transmission systems move forward.

 

DM (Digital Modulation) Optical Fiber Transmission System

This method is the current mainstream method of cable digital TV transmission, which reflects the huge advantages of economic feasibility and mature technology. Today's digital modulation/demodulation technologies mainly include MPSK (multi-ary phase shift keying) and M2QAM (multi-ary amplitude). Modulation), of course, there are modulation technologies that multiplex multiple carriers, such as OFDM (Orthogonal Frequency Division Multiplexing), etc. These technologies are effectively used in cable digital broadcasting and television systems according to their different characteristics. Among them, M2QAM (Orthogonal Amplitude) Modulation) is used in the downlink broadcast transmission of digital TV signals due to its advantages of high spectrum utilization, strong anti-interference, and simple product circuit structure.

 

And technologies such as MPSK/OFDM are applied to the digital TV upstream channel with complex space environment because of its strong anti-interference ability and multiplexing efficiency. At present, the subcarrier multiplexing optical fiber transmission technology is still the supporting technology in the TV transmission system. In the wired FTTH network, the DM mode and the VSB-AM mode are mostly used, but the DM mode has become the mainstream mode.

 

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