On November 2, 2010, 2010 security was everywhere. In order to crack down on various crimes, video surveillance showed its powerful functions. We also saw it on the highway.

1. Large capacity: There are usually dozens or even hundreds of video signals in a monitoring sub-center.

2, image quality requirements: image requirements up to 480 lines, weighted signal to noise ratio S / N requirements of not less than 57dB; PAL image requirements to reach 25 frames (50 fields / s), NTSC system requirements to reach 30 frames (60 games / s).

3. Multi-level control is required: At present, the monitoring and management of video signals in the expressway project can be divided into the following station level, road segment center, district center, and provincial center four-level monitoring system.

(1) Station level monitoring system: each toll station needs to monitor the local video signal

(2) Central monitoring of road sections: A highway usually has one road section sub-center to monitor and manage a certain road section.

(3) Area center monitoring system: The unified management of multiple expressways is usually divided into several areas according to the area.

(4) Provincial Center Monitoring System: The provincial highway administrations need to uniformly monitor, manage and dispatch all highways in the province. Current mainstream video transmission product introduction

The point-to-point video optical transceiver is a commonly used video transmission device (see FIG. 1 for the schematic diagram), and can be mainly divided into analog optical transceiver and digital optical transceiver (non-compressed). Advantages of point-to-point video optical transceivers:

1. Simple and flexible to use, high video quality and good real-time performance.

2. In addition to transmitting multiple video signals, it can also transmit audio signals, asynchronous data, Ethernet data and other signals.

The main disadvantages of point-to-point video optical transceivers:

1. It is impossible to set up the network, does not have the protection function, and the system can be poorly maintained.

2, can not achieve long-distance transmission. If the distance is too far, only the optical transceiver cascade can be used. The cascade of multi-level optical transceivers causes a sharp attenuation of the signal.

As the name implies, the optical signal emitted by the optical head on the analog optical transceiver is an analog optical modulation signal, which is called amplitude modulation, frequency modulation, phase modulation optical transceiver, which varies with the amplitude, frequency and phase of the input analog carrier signal. .

The optical signal emitted by the optical head on the digital optical transceiver is a digital signal, that is, 0 or 1 corresponds to two states of strong and weak optical signals, and different combinations of 0 and 1 represent video, audio and data signals of different amplitudes.

1. The analog signal transmission input and output processing methods are different, and the video, audio and data signal signals caused by distortion, distortion and interference are different.

Since the analog optical transceiver is to perform amplitude modulation, frequency modulation, and phase modulation, the amplitude of the analog signal changes and the amplitude, frequency, and phase changes caused by the modulation of the carrier signal are in a one-to-one linear relationship, which becomes the quality of the optical transceiver. The key, so far, it is difficult to achieve true linear modulation, non-linear shape will inevitably cause signal distortion; at the same time, the modulated carrier signal signal also modulates the optical signal, the nonlinearity of the optical signal is also a very important factor, as is well known, optical devices The nonlinearity has a great influence on the environmental temperature change, the stability of the working voltage, and the light emission power. Therefore, the optical device needs to undergo 7-10 days of thermal cycle aging during production, such as process screening, aging, and testing. In order to control the transformation in a certain range; the optical signal is transmitted over a long distance in the optical fiber, which causes power attenuation of the optical signal, transmission frequency drift, phase distortion, and optical signal dispersion effect also causes optical signal distortion; the optical signal arrives At the receiving end, the receiving optical device still has to cause nonlinear distortion, and the analog signal from the photoelectric conversion enters the solution. Tuning, demodulation, and modulation produce nonlinear distortion.

Therefore, the integrated analog optical transceiver, from the input signal modulation - electro-optical conversion - optical transmission - photoelectric conversion - demodulation five processes, will cause non-linear distortion, and these signal distortion distortion is inherent, so it can not be eliminated, so the analog optical transceiver transmission Video images, audio quality, and data effects are difficult to achieve with satisfactory results.

The digital optical transceiver only has the quantization error of analog-to-digital conversion (such as only 2.5mv when the 1V video signal is 12bits), which is not enough to cause signal distortion. Therefore, there is no such defect.

2. Intermodulation distortion caused by simultaneous signal transmission of multiple signals.

In field monitoring applications, users may have many kinds of signals, such as video images, audio, data, Ethernet, telephone or other user-defined signals. In order to improve the efficiency of fiber utilization and reduce costs, various signals must be The optical transceivers are multiplexed for transmission on a pair or one of the optical fibers. For FM, AM, and phase-modulated optical transceivers, mixing multiple channels of video, audio, or data signals with frequency modulation, amplitude modulation, and phase modulation on a certain carrier will inevitably cause various mirroring and intermodulation interference. Therefore, there are many well-known foreign brands in the market, such as FM, AM, and phase-modulated optical multi-channel video, audio, and data inter-interference. These unstable phenomena are inherent shortcomings of analog modulation technology. .

Digital optical transceivers transmit digital signals, making it easy to perform large-capacity multiplexing without mutual interference.

3, the stability is different

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