At present, multi-channel digital video optical transceivers on the market involve a wide variety of technical indicators. Most engineering companies and users lack the corresponding means to judge the rationality and authenticity of technical indicators, so it is easy to lose direction when selecting products. So, what is a video optical transceiver? How to judge the performance indicators of video optical transceivers?
What is a video optical transceiver?
The video optical transceiver is a device that converts one or more analog video signals into optical signals through optical fibers through various codes. Since the video signals are converted into optical signals, the two technologies of analog conversion and digital conversion are adopted. Therefore, video optical transceivers are divided into analog optical transceivers and digital optical transceivers. The principle of the optical transceiver is to modulate the signal onto the light and transmit the video through the optical fiber.
How to judge the performance index of video optical transceiver
1 Technical indicators considered
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.
1.1 3dB video bandwidth is sufficient
Video bandwidth is a common problem. 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 frequency of the signal.
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. But not all customers can accurately identify the lie of the manufacturer. For the average engineer and user to observe 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 image level, is a measure of the average video brightness level and expressed as a percentage of the maximum white level. When APL is low, the image is dark, and when APL is high, the image is lit. Many engineers are not very concerned or not familiar with this indicator.
However, if there is 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, causing the amplitude of the line sync signal to decrease. 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 the 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
The power supply is the primary consideration for ensuring the reliability of digital video optical transceivers. The power supply cannot be guaranteed to be reliable and reasonable. The reliability of digital video optical transceivers has become fishing in the water. The video optical transceiver with 220VAC power supply is convenient to take power and has high reliability. In the project, 220VAC power supply optical transceiver should be selected as much as possible. At present, the optical transceiver mainly has two power supply modes, one is a powerful external type, and the other is a powerful 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 the built-in power supply.
Based on the domestic power grid situation, combined with the actual project site, you should choose a multi-channel digital video optical transceiver with a wide input power range. 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 working 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 High 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, the air humidity is large, and some field monitoring points occasionally lose power. 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 the surge, anti-static and lightning strikes, which can significantly improve the reliability of the equipment. Therefore, when choosing 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, the multi-channel digital video optical transceiver should be connected to the three-eye power socket by the national standard 220VAC three-core power cord, and can not be connected to the power socket 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 during 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 to prevent static charge accumulation and damage 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 be provided with 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 is extremely vulnerable parts. Static electricity, surge, and induced lightning strikes may cause damage to the port of the optical transceiver. Except that the project should be carefully operated in strict accordance with the 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 cannot do practicality 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. In general, the indoor multi-channel digital video optical transceiver is 19-inch wide to fit in a standard 19-inch cabinet. It should also take into account the need to place the optical transceiver on the desktop or wall in the project. In addition, multi-channel digital video optical transceivers must also have good thermal performance and electrical contact performance.
3.2 Rich interface and 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.
There are many types of multi-channel digital video optical transceiver interfaces, and there are a number of mode setting dial switches. The user interface of all these elements should be laid out reasonably, and there is considerable operation space between the interfaces to facilitate engineering. Installation and repair. For example, the video interface is too dense. In actual applications, there will be an embarrassing situation in which one video connector must be removed and other normal video connectors must be unplugged.
3.3 The meaning of the indicator light is clear, which facilitates the opening and maintenance of the project.
In order to facilitate project opening and engineering maintenance, multi-channel digital video optical transceivers should be marked with clear-cut indicators. In addition to the power indicator, the transceiver of the multi-channel digital video optical transceiver must have a video indicator light to indicate whether the video channel has video signal input or output. The engineers and users are in the project opening phase and engineering maintenance phase. According to the indication of the video indicator, you can judge the situation during opening and maintenance, locate the fault point, and solve the problem that may occur as soon as possible.
In addition, for the audio and data sections, the corresponding indicator light should also be provided. The simple way is to refer to the practice of telecommunication equipment, provide synchronization indicator and loop indicator light, and engineers and equipment manufacturers can judge whether the equipment system is faulty or the interface part is faulty, so as to make a correct response.
For the Ethernet interface, the connection/activity indicator, full-duplex/half-duplex indicator, and the 10M/100M indicator should be provided in accordance with the general requirements of the data communication equipment.
The multi-channel digital video optical transceiver should also provide system indicators to indicate whether the sending device is normal and whether the receiving device is normal so that the engineering personnel can distinguish whether the device is faulty or the fiber link is faulty.
3.4 Video can be up and down, convenient networking
Some security monitoring projects, multiple optical fibers along the way to be transmitted to the central monitoring room; and some security monitoring projects, and the former is just the opposite, one or more video signals are transmitted to the receiving points along the fiber; There are also security monitoring projects, multi-channel video signals need to be transmitted a long distance, and several levels of relays may be used in the middle.
This requires multi-channel digital video optical transceivers to have video signal insertion and extraction functions and lossless relay functions (relaying signals directly at the transmission layer rather than relying on signals at the user interface layer, which will degrade the technical specifications of the video signal).
More powerful vendors have not only provided such products, but also provided products that cross-connect any video channel signals, any audio channel signals, and any data path signals, and have been put into practical use. If there is such a demand in the project, priority should be given to the use of such equipment, which not only reduces the point of failure, but also improves the reliability of the system, ensures technical specifications, reduces engineering costs, and saves user investment.
The above is what is the video optical transceiver and how to judge the performance of the video optical transceiver. In summary, the user can consider the technical indicators, reliability, and practicability of the device when selecting the device, according to the above method. Do more comparisons, according to the actual situation of the project to do the program, and finally will be able to find the most suitable multi-channel digital video optical transceiver. Shenzhen Electric
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