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Multi-channel digital video optical transceivers can have a variety of functions, each type of function has corresponding technical indicators, such as video indicators, audio indicators, asynchronous data indicators, Ethernet indicators and so on. Specific technical indicators may require manufacturers to provide third-party test certificates. Conditional engineering companies should re-test according to the indicators provided by the manufacturers before purchasing equipment to verify the actual technical specifications of the equipment and the integrity of the manufacturers. Here we focus on the following two video indicators from an engineering perspective.
1.13dB video bandwidth is sufficient
Video bandwidth is an old growing issue. The video bandwidth is insufficient, the details of the monitor picture are not clear enough, the horizontal resolution is low, and even the color distortion or loss occurs. Because of the importance of video bandwidth to image quality, some unscrupulous vendors deliberately exaggerate the video bandwidth of their products to deceive customers. A multi-channel digital video optical transceiver produced by a manufacturer claims that its video bandwidth is 8M, but its externally announced video sampling frequency is 12.5M. According to Nyquist's law of sampling, to truly restore the signal, the sampling frequency should be at least twice the signal frequency. In this way, the theoretical video bandwidth of this manufacturer's optical transceiver may not exceed 6.25M in any case. Obviously, this is a lie that exaggerates performance to deceive customers, so that vendors without commercial reputation should not be considered for purchase. However, not all customers can accurately identify the manufacturer's lies. For general engineers and users, by observing the details of the image, the video bandwidth of the device can be roughly judged.
1.2 APL range is wide enough
APL, the average level of the image, is not very noticed or familiar to many engineers. However, if you have a problem encountered in some projects: the video signal is transmitted through the optical transceiver, when there is a large area of ​​white in the picture (such as when the camera is illuminated by strong light), the picture on the monitor will horizontally jitter, then Engineers must be familiar. This is because the APL range of video optical transceivers produced by some manufacturers is too narrow. When there is a large area of ​​white in the image signal, the DC component in each line of image signals increases, and the APL will increase, resulting in a reduction in the amplitude of the line sync signal. Or lost, the monitor cannot detect the line sync and the picture is shaken.
2, reliability considerations
As a security monitoring project, the reliability of equipment should be the first consideration. The reliability of the equipment must be considered by the equipment manufacturer when designing the product. However, some manufacturers may not be willing or do not know how to do this work for some reasons. The focus here is on the following issues from the engineering perspective.
2.1 Power supply is convenient, the power supply range is wide
Power supply is the primary consideration for ensuring the reliability of digital video optical transceivers. Power supply cannot be guaranteed to be reliable and reasonable. The reliability of digital video optical transceivers has become a fishing in the water. The video optical transceiver with 220VAC power supply is convenient to take power and has high reliability. The 220VAC power supply optical transceiver should be selected as much as possible in the project. At present, the optical transceiver mainly has two power supply modes, one is a power external type, and the other is a power built-in type, that is, the power supply and the optical transceiver are integrated. From the perspective of engineering applications, the built-in power supply is not only easy to install, but also has high reliability. Therefore, most of the current multi-channel digital video optical transceivers are built-in power supply.
Based on the domestic power grid situation, combined with the actual project site, multi-channel digital video optical transceivers with a wide input power range should be selected. Generally, you should choose the optical transceiver that uses the switching power supply. Compared with linear power supply, switching power supply is not only efficient, but also has a wide input voltage of switching power supply. It can basically guarantee normal operation in the range of 160VAC~265VAC, and its output voltage will not change with the input voltage, so it will not affect the optical transceiver. The performance and technical indicators will not cause the optical transceiver to restart repeatedly due to the instability of the power grid, interrupting the transmission of monitoring video and audio signals. 2.2 Temperature and humidity adaptability
In some security monitoring projects, the working conditions of the optical transceiver are relatively harsh, the temperature variation range is wide, and the air humidity is large. Some field monitoring points will occasionally lose power, and the optical transceiver will be hot when working. When the equipment is powered off, there will be condensation water droplets during cooling. All of these require multi-channel digital video optical transceivers to have more considerations in terms of temperature range and allowable humidity to ensure stable operation in harsh environments and to meet security monitoring needs.
2.3 Grounding considerations
In the security monitoring project, ensuring the good grounding of the equipment is an important part of ensuring the reliability of the equipment. Good grounding is good for the equipment against surge, anti-static and lightning strikes, which can significantly improve the reliability of the equipment. Therefore, when selecting an optical transceiver, pay special attention to the manufacturer's grounding scheme, and thus the strength of the manufacturer. A good grounding should have the following considerations:
First, the power supply of the optical transceiver must be grounded. As an industrial grade product, for the safety of equipment and personnel, multi-channel digital video optical transceivers should be connected to the power sockets using the national standard 220VAC power cord, and can not be connected to the power sockets for the convenience of using a two-core power cord. Moreover, the ground wire of the power socket should be reliably connected to the earth in the project.
Secondly, the signal ground of the optical transceiver should be connected to the ground of the casing and the earth to provide a bleed circuit for the accumulated static charge, preventing static charge accumulation and damaging the equipment. The signal ground of the optical transceiver is well connected to the ground of the casing and the earth, which can effectively prevent the damage caused by the induced lightning.
Finally, the RS485 data terminal of the multi-channel digital video optical transceiver should also provide a grounding wire to be connected to the ground terminal of the opposite device in the project to avoid damage to the data port due to the ground potential difference between the devices at both ends of the RS485 channel.
2.4 port protection
The video port, audio port and data port of the optical transceiver are extremely vulnerable parts. Static electricity, surge, and induced lightning strikes may cause damage to the optical transceiver port. In addition to careful operation in accordance with construction requirements, the device itself is also The necessary port protection should be provided. When selecting an optical transceiver, special attention should be paid to the manufacturer's port protection measures.
3. Practical considerations
Every manufacturer should provide users and engineers with mature products, not scientific prototypes, so it must have practical considerations. When selecting an optical transceiver, users and engineers can consider more from the practical point of view of the device. First, the practical problems of users and engineering companies are easy to judge; Second, it is hard to believe that manufacturers that can not be practical can do the reliability and technical indicators of optical transceivers. Here we focus on the practicality of optical transceivers from the following perspectives.
3.1 Appearance and reasonable structure
The multi-channel digital video optical transceiver has high technical content, and its appearance should be small and exquisite, beautiful and elegant, and the overall structure must meet the engineering installation requirements as much as possible. Generally, the indoor multi-channel digital video optical transceiver should take into account the need to place the optical transceiver on the desktop or the wall in addition to the 19-inch cabinet. In addition, multi-channel digital video optical transceivers must also have good thermal performance and electrical contact performance.
3.2 rich interface, reasonable layout
In addition to requiring sufficient video interfaces, multi-channel digital video optical transceivers may require or may require other feature-rich interfaces, such as high-fidelity audio interfaces, ordinary telephone interfaces, asynchronous data interfaces, Ethernet interfaces, and digital interfaces. This requires that the selected multi-channel digital video optical transceiver must have the system upgrade capability, and will not completely replace the equipment due to network upgrade or system function change, thus protecting the user's upfront investment.
Buying engineering optical transceivers must see three tips
[ Huaqiang Security Network News ] In the current security market, the development of optical transceivers, most engineering companies and users also lack the corresponding means to judge the rationality and authenticity of technical indicators, so it is easy to get lost when choosing products. This paper describes two technical indicators that are relatively critical and easy to verify in engineering, and focuses on how to select multi-channel digital video optical transceivers in security monitoring engineering from the aspects of reliability and practicability. 1. Technical indicators are considered