Several considerations of choosing optical transceiver in security monitoring project

At present, the multi-channel digital video optical transceivers on the market involve a wide variety of technical indicators. Most engineering companies and users also lack the corresponding means to judge the rationality and authenticity of the technical indicators, so they are easily disoriented when choosing products. This article describes two technical indicators that are more critical and easier to verify in the project, and focuses on reliability and practicality, and analyzes how to select multi-channel digital video optical transceivers in security monitoring projects.

1. Technical index considerations

Multi-channel digital video optical transceivers can have multiple functions, and 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 can require the manufacturer to provide third-party test certification. Conditional engineering companies should retest according to the indicators provided by the manufacturer before purchasing equipment to verify the actual technical indicators of the equipment and the integrity of the manufacturer. Here we focus on briefly explaining the following two video indicators from an engineering perspective.

1.1 3dB video bandwidth is sufficient

Video bandwidth is an old issue. If the video bandwidth is insufficient, the details of the monitoring screen will not be clear enough, the horizontal resolution will be low, and even serious color distortion or loss. Because of the importance of video bandwidth to image quality, some unscrupulous manufacturers deliberately exaggerate the product's video bandwidth to deceive customers. A multi-channel digital video optical transceiver produced by a manufacturer claims that its video bandwidth is 8M, but the video sampling frequency announced by it is 12.5M. According to Nyquist sampling law, to truly restore the signal, its sampling frequency should be at least twice the signal frequency. In this way, the theoretical video bandwidth of this manufacturer's optical transceiver cannot exceed 6.25M anyway. Obviously, this is a lie that exaggerates performance and deceives customers, so manufacturers with no commercial reputation should not be considered in the scope of purchase. But not all customers can accurately identify the lies of the manufacturers. For general engineering companies and users, by observing the details of the image, you can roughly judge the video bandwidth of the device.

1.2 APL range is wide enough

APL, the average level of the image, many engineers are not very aware of or familiar with this indicator. However, if it comes to such a problem encountered in some projects: the video signal is transmitted through the optical transceiver, and 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 shake horizontally, then The engineering staff 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 of each line of image signal increases, and the APL will increase, resulting in a decrease in the amplitude of the line synchronization signal. Or lost, the monitor cannot detect the line synchronization and the screen jitter occurs.

2. Reliability considerations

As a security monitoring project, the reliability of equipment should be the first consideration. The reliability of equipment is something that equipment manufacturers must consider during product design. However, some manufacturers may be unwilling to do or do not know how to do this work for some reasons. Here we focus on briefly discussing the following issues from an engineering perspective.

2.1 Convenient power supply, wide power supply range

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. Talking about the reliability of digital video optical transceivers has become a problem. It is more convenient and reliable to use 220VAC-powered video optical transceivers. The 220VAC-powered optical transceivers should be selected as much as possible in the project. There are currently two main power supply methods for this optical transceiver, one is external power supply and the other is internal power supply, 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 current multi-channel digital video optical transceivers use the built-in power supply.

Based on the situation of the domestic power grid and the actual site of the project, a multi-channel digital video optical transceiver with a wide input power range should be selected as much as possible. Generally, you should try to choose the optical transceiver using switching power supply. Compared with the linear power supply, the switching power supply not only has high efficiency, but also has a wide input voltage of the switching power supply, which can basically guarantee the normal operation within the range of 160VAC ~ 265VAC, and its output voltage will not change with the change of the input voltage, thus not affecting the optical transceiver Working performance and technical indicators will not cause the optical transceiver to start repeatedly due to the unstable power grid, and interrupt the transmission of surveillance video and audio signals.

2.2 Strong adaptability to temperature and humidity

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 have a power outage, the optical transceiver will generate heat when it is working, and there will be condensed water droplets when the equipment cools down during the power outage. All of this requires that the multi-channel digital video optical transceiver have more considerations in terms of temperature range and allowable humidity to ensure that it can work stably in harsh environments and meet the needs of security monitoring.

2.3 Grounding considerations

In security monitoring projects, ensuring good grounding of equipment is an important part of ensuring equipment reliability. Good grounding is good for the equipment to resist surge impact, anti-static and lightning strikes, and can significantly improve the reliability of the equipment. Therefore, when choosing an optical transceiver, special attention should be paid to the manufacturer's grounding scheme, which also shows 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 use the national standard 220VAC power cord to connect to the power socket, but can not use a two-core power cord to connect to the power socket for saving trouble. 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 chassis ground and the ground to provide a discharge circuit for the accumulated static charge to prevent the accumulation of static charge and damage the equipment. The good connection between the signal ground of the optical transceiver and the chassis ground and the ground can effectively prevent the damage caused by the induction lightning to the equipment.

Finally, the RS485 data terminal of the multi-channel digital video optical terminal should also provide a ground wire, so as to connect with the ground terminal of the peer 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 to damage. Static electricity, surges, and induced lightning strikes may cause damage to the optical transceiver. In addition to careful operation in accordance with the construction requirements in the project, the equipment itself is also Necessary port protection should be provided. When selecting an optical transceiver, special attention should be paid to the port protection measures of the manufacturer.

3. Practical considerations

Every manufacturer should provide mature products to users and engineering companies, rather than scientific research prototypes, so it must have practical considerations. When choosing an optical transceiver, users and engineering companies can consider more from the perspective of the practicality of the equipment. First, it is easy for users and engineers to judge the practicality issues; second, it is difficult to believe that manufacturers who cannot do good practicality can do a good job in the reliability and technical indicators of optical transceivers. Here we focus on the practicality of the optical transceiver from the following angles.

3.1 Elegant 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 addition to the general indoor multi-channel digital video optical transceivers, in addition to the use of the 19th cabinet, it should also take into account the need to place the optical transceiver on the desktop or on the wall in the project. In addition, the multi-channel digital video optical transceiver must also have good heat dissipation performance and electrical contact performance.

3.2 Rich interfaces and reasonable layout

In addition to requiring sufficient video interfaces, multi-channel digital video optical transceivers may also require or may require other feature-rich interfaces in the future, 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 ability to upgrade the system, so as not to completely replace the device due to network upgrades or system function changes, thereby protecting the user's previous investment.

There are many types and numbers of multi-channel digital video optical transceiver interfaces. There may be some mode setting dial switches to match these interfaces. The user operation interface composed of all these elements should be laid out reasonably, and considerable operation space should be left between the interfaces to facilitate engineering. Installation and maintenance. For example, if the video interface is too dense, in practice, there will be an embarrassing situation that you must unplug other normal video connectors in order to repair a certain video connector.

3.3 The meaning of the indicator is clear, which is convenient for project opening and maintenance

In order to facilitate project opening and project maintenance, multi-channel digital video optical transceivers should be marked with clear indicators. In addition to the power indicator, the transceivers of multi-channel digital video optical transceivers must have a video indicator, indicating whether the corresponding video channel has video signal input or output. Engineering personnel and users can According to the indication of the video indicator, you can judge the status of the activation and maintenance, locate the fault point, and solve the possible problems as soon as possible.

In addition, for the audio and data parts, corresponding indicators should also be provided. The simple way is to refer to the practice of telecommunications equipment, provide synchronization indicator and loop indicator, based on which engineers and equipment manufacturers can determine whether the equipment system is faulty, or the interface part is faulty, so as to make the correct response.

For the Ethernet interface, the connection / activity indicator, full-duplex / half-duplex indicator, and 10M / 100M indicator should be provided in accordance with the general requirements of data communication equipment.

Multi-channel digital video optical transceivers should also provide system indicators to indicate whether the sending equipment is normal and the receiving equipment is normal, so as to facilitate engineering personnel to distinguish between equipment failure and fiber link failure.

3.4 Video can be up and down, easy to network

In some security monitoring projects, there are multiple channels of video along a fiber to be transmitted to the central monitoring room; while in the case of security monitoring projects, exactly the opposite of the former, one or more video signals are transmitted to various receiving points along the fiber; In some security monitoring projects, multi-channel video signals need to be transmitted over a long distance, and several levels of relays may be used in the middle. In this way, multiple digital video optical transceivers are required to have video signal insertion and extraction functions and lossless relay functions (relay signals directly at the transmission layer instead of at the user interface layer, which will degrade the technical indicators of the video signal). Stronger manufacturers have not only provided such products, but also provided products that are alternately connected between any video channel signal, any audio channel signal and any data channel signal, and have already put it into practical use. If there is such demand in the project, priority should be given to the use of such equipment, which not only reduces the point of failure, improves the reliability of the system, guarantees the technical indicators, but also reduces the engineering cost and saves the user's investment.

In summary, users can consider the technical indicators, reliability, and practicability of the device when selecting the device. According to the aforementioned methods, make more comparisons, make suitable solutions according to the actual situation of the project, and finally find the most suitable. Own multi-channel digital video optical transceiver.