The high-speed railway is different from the ordinary railway system. The high-speed railway itself is a large-scale systemic and integrated project. The communication department alone involves more than 10 subsystems, including wired, data, transmission, scheduling, emergency communications, video surveillance, etc. . High-speed rail is very different from ordinary railways or subways. For example, subways usually have a speed of about 60 kilometers per hour and the train interval is about 3 minutes. High-speed rail may reach 300 kilometers per hour, but the time interval may be similar to the subway. This is the communication command system of high-speed rail. It puts forward very high requirements. At the same time, as an important auxiliary facility, the requirements of the video surveillance system are correspondingly very high.
1. Features of high-speed rail video surveillance system
The high-speed rail video surveillance system requires the use of advanced video surveillance technology, based on the IP network of the railway system, to build a digital, intelligent, and distributed network video surveillance system to meet the requirements of public security, safety supervision, passenger transportation, dispatching, vehicle service, and engine maintenance. Business departments, electrical services, vehicles, Power Supply and other business departments and disaster prevention monitoring, rescue and emergency management and other needs, to achieve the sharing of video network resources and information resources. The high-speed rail video surveillance system is generally based on the network architecture and realizes the functions of video collection, encoding, compression, storage, forwarding, and virtual matrix. The video signal collected by the camera is connected to the DVR or encoder through a coaxial Cable to achieve video collection, encoding compression and transmission. The control signal of the PTZ camera is transmitted through RS485; the encoder sends the video stream to the NVR through the network for centralized storage and backup ; Storage server can focus on DVR or NVR video data backup; Streaming media server can perform centralized video forwarding when multiple users access to reduce the pressure on the network and front-end equipment; decoder connected to the TV wall to achieve centralized video The screen display is restored.
2. Video analysis technology
High-speed rail is characterized by a large system span, wide geographical distribution, and a complex video analysis environment. Wind, frost, rain, snow, fog, camera shake, train lights, city lights, insects, cloud shadows and other phenomena are all problems that video analysis may encounter. The VCA system should be able to balance the problems between false negatives and false positives. Railways are different from laboratories. Any adjustment of camera angles, focal lengths, etc. requires a certain amount of manpower and material resources. Video analysis has high requirements for scenes (FOV), and needs to be adjusted continuously in future configurations. Therefore, it is not difficult to understand that the video analysis cameras of most video surveillance systems also use PTZ cameras instead of fixed cameras. After the analysis mode is fixed, the camera FOV is adjusted and you need to enter the analysis settings. Usually, only one mode can be used for one camera video. In railway applications, there are two main VCA modes. One is to set up intrusion detection in important sections and throat areas to identify the physical ways in which people or animals invade high-speed rails (mostly enclosed or fenced along the high-speed rail), but still There may be intrusion and entry); high-altitude falling objects analysis is set up in the public span iron area to prevent the high-altitude falling objects from affecting the operation of the train. At present, these two video analysis application modes have certain applications in railway video surveillance and perform well.
At present, there are two main architectures of video analysis technology, one is based on the back-end server, and the other is the front-end DSP (DVS or IPC). DSP mode, that is, distributed intelligent analysis architecture, the video analysis unit is generally located near the video acquisition device, so that the system can be selectively set, so that the system only transmits video to the control center or storage center when the alarm occurs, relative to the server The method can save network burden and storage space. Video analysis is a complicated process and requires a lot of system computing resources. Therefore, the number of video analysis channels at the same time in the server mode is very limited. For the above reasons, the mainstream video analysis technology on the market mostly adopts the DSP method, based on the camera or the encoder. It should be noted that once the video analysis equipment based on the front-end DSP mode needs to be adjusted, such as adding or canceling the video analysis function, it is usually necessary to replace the DVS or IPC, and the mode based on the back-end analysis is that passenger transportation is directly in the computer room Or the adjustment of the control center is completed without replacing the front-end hardware. Usually the replacement of DVS or IPC is very expensive in railway projects.
3. Redundancy technology (reliability)
The high-speed rail video surveillance system needs to have high reliability to support the various needs of operations, security, and maintenance of different departments. For network video surveillance systems, different redundancy methods can be used to enhance stability. From front-end encoders, transmission networks to forwarding servers, management servers, and storage systems, redundancy technology can be implemented.
·Encoder
The encoder can use dual power supplies and dual network cards to enhance stability; or use "N + 1" redundancy to enhance reliability to ensure that the system can continue to operate when a single machine fails or equipment is replaced.
· NVR
NVR can use "N + 1" redundancy to enhance reliability to ensure continuous operation of the system when a single machine fails or equipment is replaced.
· CMS
CMS can use the "dual-machine redundant hot standby" method to enhance stability to ensure that the system runs without downtime.
·storage
The storage system can use the RAID technology of the disk array to achieve highly reliable data storage.
The fully redundant network video surveillance system architecture shows that from the front-end equipment IPC, DVS, transmission network, NVR, storage device NVR, and core management platform CMS, the redundancy function is realized, which ensures the 24-hour high-reliability operation of the system and reduces System downtime or data loss due to network, power, hardware, storage, software and other failures.
IPC is a single device, and the way to improve stability is to store locally when the network is temporarily interrupted; for a large number of DVS, you can use the "N + 1" method for redundancy to prevent single device hardware and software failure; for NVR, You can use the "N + 1" method for backup to prevent single device hardware and software failure; for storage devices, you can use mature RAID technology to achieve redundancy protection; for the network, you can use "dual network" to achieve high reliability data transmission ; For the CMS, the dual-machine hot backup mode is adopted, and the dual-machine shares the RAID disk array to achieve redundancy.
Key technologies of high-speed rail video surveillance system
1. Encoding compression technology
Video encoding and compression is the premise and foundation of network video surveillance. Uncompressed video data is massive. Current typical video coding compression standards are MPEG-4 and H.264. The MPEG-4 standard still uses a basic coding framework similar to the previous standards (H.261 / 3 and MPEG-1 / 2), that is, the typical three steps: predictive coding, transform quantization, and entropy coding. The new compression coding standards are all designed based on the idea of ​​optimization, and some technologies in the previous standards are improved. For example, the motion compensation technology with 1/4 and 1/8 pixel accuracy is proposed on the original basis, which greatly improves the performance of predictive coding. The MPEG-4 standard not only gives specific compression algorithms, it is formulated for the needs of integration and compression technologies such as digital TV, interactive multimedia applications, and video surveillance. MPEG-4 integrates a variety of multimedia applications in a complete framework, providing corresponding categories (Profile) and levels (Level) for different applications. H.264, which is also the tenth part of the MPEG-4 standard, was proposed by the Joint Video Team (JVT, JointVideoTeam) composed of the ITU-T Video Coding Experts Group (VCEG) and the ISO / IEC Motion Picture Experts Group (MPEG) The high compression rate video coding standard. Like the previous standard, H.264 is also a mixed coding mode that uses predictive coding plus transform coding. It combines the advantages of previous coding standards, and absorbs the experience accumulated in the process of standard setting, and obtains better coding methods than before. Much compression performance. The biggest advantage of the H.264 standard is that it has a high data compression ratio. Under the premise of the same image quality, the compression ratio of H.264 encoding is 1.5 to 2 times that of MPEG-4. H.264 adopts a "network friendly" structure and syntax, which is beneficial to the processing of error codes and packet loss, to meet the needs of different rates, different resolutions, and different network transmission and storage occasions.
In the network video surveillance system, the ability of inter-coding and inter-decoding between products is very important, which can reduce the difficulty of system integration, facilitate expansion and protect costs. At present, it is not ideal to compile and solve different equipment from different manufacturers, that is to say, the compatibility is not good. The main reason is that the standard profiles and levels of coding adopted by different manufacturers are different. In addition, the data encapsulation format is incompatible, followed by the added private information (in the basic stream / packet).
4. Video storage application technology
The storage deployment should be flexible, and you can choose alarm triggered storage, preset schedule storage, manual start and stop storage, etc .; the storage architecture should be mainstream architectures such as DAS, NAS, SAN, etc., generally using RAID5 redundancy. The storage requirement is generally to separate normal recording and alarm recording and set different periods, for example, 7 days for normal recording and 30 days for alarm recording. The planning and design of the storage system should be deployed based on project requirements and network conditions.
It should support manual backup storage of video image information; video information recording storage, event triggered storage and planned storage functions; the system should support users to retrieve and playback based on various conditions such as time, place, event, etc .; the system should support multiple users simultaneously Call to retrieve video images; DVR and NVR can work in multiple storage and archiving modes; the system should support downloading to local playback video and remote direct playback video.
The video archive storage service (secondary storage, alarm storage) has the following characteristics:
· The storage architecture is a completely independent secondary architecture;
· Archive Server (Archive Server) is usually divided into "important video archive" and "alarm video archive";
· Archive Server (Archive Server) can be deployed anywhere on the network;
An archive server (Archive Server) can archive multiple DVRs or NVRs;
· Users do not need to specify to play back recordings from DVR or NVR or Archive Server, the system will automatically index;
· You can choose to archive a part of a DVR or NVR channel.
5. Integrated video platform
The system software platform includes the core data part and the client workstation. The core data server contains the system database and core software. The workstation system should include user management, authority management, configuration management, fault management, log management and other functions. The platform should support the monitoring and maintenance of various video equipment resources and the operating status of the equipment, the configuration of system equipment parameters through software, and the management, registration, authentication, deletion, and rights allocation of system users. The system should have a log management function, which can record the time when operators enter and exit the system and the main operating conditions through the operation log, and support log information query and report making functions.
Data management: centrally "register" all types, versions, quantities, and numbers of all equipment in the database; monitoring function: real-time image calls can be made through the network for all monitoring outlets of the entire line, providing reference for leading emergency command and decision-making display ; Network management function: monitor and manage the operation of network element equipment of all monitoring systems across the line, real-time display of the operating data and alarm information of various equipment; alarm management: record and display alarms of equipment, systems and communication parts, record alarm handling The situation can be exported; control function: PTZ control and relay output node control of all monitoring outlets of the whole line; voice intercom: real-time voice intercom to the monitoring point of the front-end installation of voice equipment, one-way "shout" function; data storage : Support local storage and central backup of video, with multiple automatic and manual storage methods.
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