Video Monitoring optical terminal principle encoding decryption and Mode Analysis

With the rapid development of broadband digital optical transmission device technology and digital video technology, digital video optical transceiver has begun to enter the market and gradually replaced the analog optical transceiver market, ushered in the era of rapid development of digital video optical transceiver, it also presents a mature technology, superior performance, wide application, and vigorous demand.

Optical Transceiver principle and coding

The optical transmission system consists of three parts: light source (Optical Transmitter), transmission medium, detector (optical receiver), and optical source and detector. The optical terminal is to transmit multiple E1 (a data transmission standard for a relay line with a speed of 2.048 MbPS, which is adopted by China and Europe) a device that converts a signal into an optical signal and transmits the signal (it is mainly used to realize electrical-optical and optical-electrical conversion ). There are two types of Optical Transceiver: analog optical transceiver and digital optical transceiver:

1. analog optical terminal

The simulated optical transceiver uses the PFM modulation technology to transmit image signals in real time. The transmitting end first performs PFM modulation on the analog video signal (generally, there are several methods of modulation, phase adjustment, and amplitude modulation, so that the analog optical terminal is divided into several types of Optical terminals, such as FM, phase adjustment, and Amplitude Modulation ), after the optical signal is transmitted to the receiving end, the optical-optical conversion is performed, and then the PFM demodulation is performed to restore the video signal. Thanks to the PFM modulation technology, the transmission distance can easily reach about 30Km, and the transmission distance of some products can reach 60Km or even hundreds of kilometers. In addition, after the image signal is transmitted, the distortion is very small, and the signal-to-noise ratio and non-linear distortion are very high. By using wavelength division multiplexing technology, two-way transmission of images and data signals can also be realized on an optical fiber.

2. Digital Optical Transceiver

Since digital technology has obvious advantages over traditional simulation technology in many aspects, just as digital technology has replaced simulation technology in many fields, the digitization of Optical terminals is also an inevitable trend. Currently, there are two main digital image optical transceiver Methods: MPEGII Image Compression digital optical transceiver and non-Compressed digital image optical transceiver.

Image compression digital optical terminal generally uses MPEGII image compression technology, which can compress active images into N × 2 Mbps data streams through standard telecom communication interfaces or directly through optical fiber transmission.

Because of the image compression technology, it can greatly reduce the signal transmission bandwidth, so that the image signal can be transmitted with a small amount of resources. At the same time, the standard interfaces of N × 2 Mbps can be used to transmit monitoring images through rich channels of existing telecom transmission equipment, which brings convenience to engineering applications. However, image compression digital optical transceiver also has its inherent disadvantages. Its critical weakness is its inability to ensure real-time image transmission. Because it takes a certain amount of time to compress and decompress the image, the latency of the transmitted image is generally 1-2 seconds. Therefore, this kind of device is only suitable for places with low real-time requirements. In addition, after compression, the image will produce a certain degree of distortion, and the price of this optical terminal is also high.

The principle of A non-Compressed digital image optical terminal is to re-connect the analog video signal to the voice, audio, data, and other signals after A/D conversion, and then transmit it through optical fiber. It uses a high data rate to ensure the quality and timeliness of video signal transmission. Because the bandwidth of the optical fiber is very large, this high data rate does not impose a high requirement on the transmission channel. Non-Compressed digital image optical transceiver can provide excellent image transmission quality (signal-to-noise ratio is greater than 60dB, differential phase distortion is less than 2, and differential gain distortion is less than 2%), achieving broadcast-level transmission quality, and the image transmission is real-time. Because of the use of digital technology, mature communication technologies such as reconnect technology and optical transceiver technology can be used in devices, which improves the reliability of devices and reduces costs.

Optical terminal Mode Analysis

The difference between a single-mode optical transceiver and a multi-mode optical transceiver is that the refraction angle of light waves in the core is different from the fiber diameter of the core. A single-mode optical transceiver is used for long-distance data transmission, multimode is mostly used for data transmission within 2 km. In the past, the device price difference between single-mode optical terminal and multi-mode optical terminal is gradually decreasing. The following will analyze the differences between single-mode and multi-mode optical terminal and choose between them.

Currently, the commonly used optical fiber can be divided into two categories by mode: Multimode Optical Fiber and single-mode optical fiber. Multi-mode optical fiber cables are the first optical fiber cables to be commercially used. Due to problems such as serious dispersion, high attenuation, narrow available bandwidth, and high cost, it has withdrawn from the optical fiber network platform of telecom operators as the main battlefield of optical fiber cables, but it reserves a certain market share in the optical fiber cabling of smart buildings.

When multi-mode optical fiber cables are used for video image transmission, the maximum value is 3 ~ The transmission distance is about 5km, and the bandwidth (for Analog modulation) and transmission rate (for digital) of the video terminal are limited. It is generally applicable to scenarios with short distance, small capacity, and simple application. In particular, the influence of dispersion greatly limits the application of non-Compressed digital optical transceiver with high-speed transmission in terms of transmission distance and capacity.

Single-mode optical fiber cables made of single-mode optical fiber (mainly G.652 optical fiber) have become the mainstream of optical fiber communication because of their excellent features and low prices. Currently, new technologies are emerging in the field of optical fiber communication. In terms of long distance, large capacity, and multi-service, none of them are developed for single-mode optical fiber. The optical cable has a service life of more than 20 years, which is longer than the actual service life of the optical terminal. From the perspective of strategic resources, we need to consider the laying of optical cables. In the future, we must consider the expansion, equipment updates, the number of Optical Fiber cores is sufficient, and we can continue to carry new services.

China Telecom's optical fiber transmission equipment has replaced several generations, but the cables laid over more than a decade or two years ago are still in normal use, which is the best example. It is worth noting that the device price difference between single-mode optical terminal and multi-mode optical terminal is gradually decreasing, and the price difference between multiple video devices is even zero. Moreover, single-mode optical fiber cables are cheaper than multi-mode optical fiber cables, therefore, the cost of using multimode devices must be updated. From this point of view, single-mode optical fiber cables are currently recommended. Of course, single-mode optical fiber cables must also be used for Optical terminals. Generally, the module/Multimode Optical Fiber/optical fiber is selected based on the number of kilometers and cost, and then the corresponding optical terminal type is selected.