The high-speed railway is different from the general railway system. The high-speed railway itself is a systematic and integrated large-scale project. Only the communication department involves more than 10 subsystems, including cable, data, transmission, dispatch, emergency communication, 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, trains are about 3 minutes apart, and high-speed trains may reach 300 kilometers per hour. However, the time interval may be similar to that of subways. This is the communication command system for high-speed rail. High requirements were put forward, and at the same time, as an important auxiliary facility, the requirements of the video surveillance system were correspondingly very high.
1. High-speed rail video surveillance system features
The high-speed rail video surveillance system requires 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 public security, security, passenger transport, dispatch, traffic, and locomotive. Workspace, electricity, vehicles, power supply and other business departments and disaster prevention monitoring, rescue and emergency management and emergency management, etc., to achieve video network resources and information resources sharing. The high-speed rail video surveillance system is generally based on the network architecture, and realizes the functions of video acquisition, 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 realize video acquisition, encoding compression and transmission. The control signal of the PTZ camera is transmitted through RS485; the encoder sends the video stream to the NVR for centralized storage backup through the network. The storage server can perform key backup of the video data of the DVR or the NVR; the streaming media server can perform centralized video forwarding when multiple users access, and reduce the pressure of the network and the front-end equipment; the decoder is connected with the video wall to realize the centralized video. The screen displays Restore.
2. Video analysis technology
High-speed rail is characterized by large system span, wide geographical distribution, complex video analysis environment, wind, rain, snow, fog, camera shake, train lighting, urban lighting, insects, cloud shadows, etc. are all problems that video analysis may encounter, good The VCA system should be able to balance the gap between false negatives and false positives. The railway is different from the laboratory. It requires a certain amount of manpower and material resources to adjust any angle and focal length of the camera. The video analysis has high requirements for the scene (FOV), and it needs to be constantly adjusted in the future configuration. Therefore, it is not difficult to understand that video analysis cameras of most video surveillance systems use PTZ cameras instead of fixed cameras. After the analysis mode is fixed, the camera FOV is adjusted and needs to enter the analysis settings. Generally, one camera video can only perform one mode. In railway applications, there are mainly two VCA modes. One is to set up intrusion detection in important sections and throat areas to identify human or animal intrusion into high-rail rails (physical methods such as mostly closed or fence protection along high-speed rail, but still There may be intrusions into the area; high-altitude analysis of the high-altitude iron area to prevent high-altitude objects from affecting train operation. At present, these two video analysis application modes have certain applications and good performance in railway video surveillance.
At present, video analysis technology mainly has two architecture modes, one is based on the back-end server mode, and the other is the front-end DSP mode (DVS or IPC). In the DSP mode, that is, the distributed intelligent analysis architecture, the video analysis unit is generally located near the video capture device, so that the system can be selectively set, so that the system transmits the video to the control center or the storage center only when the alarm occurs, relative to the server. Ways can save network load and storage space. Video analysis is a complex process that requires a large amount of system computing resources, so the number of ways in which the server can simultaneously perform video analysis is very limited. Based on the above reasons, the mainstream video analysis technology on the market currently adopts DSP mode, based on camera or encoder. It should be noted that, based on the front-end DSP-based video analysis device, once the video analysis point needs to be adjusted, such as adding or canceling the video analysis function, the DVS or IPC is usually required to be replaced, and the model based on the back-end analysis is directly carried in the equipment room. Or the control center adjustment is completed without replacing the front-end hardware. The DVS or IPC method is usually replaced and the cost in the railway project is very high.
3. Redundancy technology (reliability)
High-speed rail video surveillance systems need to be highly reliable to support the operation, security, and maintenance of different departments. For network video surveillance systems, different redundancy methods can be used to enhance stability. Redundant technologies can be implemented from front-end encoders, transmission networks to forwarding servers, management servers, and storage systems.
·Encoder
The encoder can be stabilized by dual power supply or dual network card mode, or the "N+1" redundancy mode can be used to enhance the reliability to ensure continuous operation when the single machine fails or the device is replaced.
·NVR
The NVR can use "N+1" redundancy to enhance reliability to ensure continuous operation of the system when a single machine fails or when equipment is replaced.
·CMS
CMS can use "two-machine redundant hot standby" to enhance stability to ensure that the system runs without downtime.
·storage
Storage systems can use disk array RAID technology to achieve highly reliable data storage.
The fully redundant network video surveillance system architecture shows that the front-end equipment IPC, DVS, transmission network, NVR, storage device NVR, and core management platform CMS all implement redundancy functions, ensuring high-reliability operation of the system for 24 hours. System downtime or data loss due to network, power, hardware, storage, software, etc. failures.
IPC is a single device. The way to improve stability is to store locally when the network is temporarily interrupted. For large-channel DVS, you can use the “N+1†mode for redundancy to prevent the failure of the hardware and software of the single device. For the NVR, You can use the "N+1" mode for backup to prevent the failure of the hardware and software of the single device. For the storage device, you can use the mature RAID technology to achieve redundancy protection. For the network, you can use the "dual network" to achieve high reliability data transmission. For CMS, dual-system hot standby mode is adopted, and dual-system shared RAID disk arrays are used for redundancy.
Key technologies of high-speed rail video surveillance system
Code compression technology
Video coding compression is the premise and basis of network video surveillance. Video data without compression is massive. The 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), which is a typical three-step process: predictive coding, transform quantization, and entropy coding. The new compression coding standards are designed based on the idea of ​​optimization, and some of the techniques in the previous standards have been improved. For example, on the basis of the original, motion compensation techniques of 1/4 and 1/8 pixel precision are proposed, which greatly improves the performance of predictive coding. The MPEG-4 standard not only provides specific compression algorithms, but is also tailored to the needs of integration and compression technologies such as digital television, interactive multimedia applications, and video surveillance. MPEG-4 integrates multiple multimedia applications into one complete framework, providing profiles and levels for different applications. H.264, also the tenth part of the MPEG-4 standard, is 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). High compression rate video coding standard. Like the previous standards, H.264 is also a hybrid coding mode that uses predictive coding plus transform coding. It concentrates on the advantages of previous coding standards and absorbs the experience accumulated in the standard-setting process. It is better than other coding methods. Much more 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 facilitates the processing of bit errors 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 inter-compilation capability between products is very important, which can reduce the integration difficulty of the system and facilitate the expansion and protection of costs. At present, the inter-decoding of devices from different vendors is not ideal, which means that the compatibility is not good. The main reason is that the standard profiles and levels of coding used by different manufacturers are different, and the data encapsulation format is not compatible, followed by the added private information (in the basic stream/package).
4. Video storage application technology
The storage deployment should be flexible. You can choose alarm-triggered storage, preset schedule storage, manual start-stop storage, etc. The storage architecture should be mainstream architectures such as DAS, NAS, SAN, etc., and RAID5 redundancy is generally adopted. The storage requirement is generally that the normal recording and the alarm recording are separated and set to different periods, for example, normal recording for 7 days and alarm recording for 30 days. The planning and design of the storage system should be deployed according to the project requirements and network conditions.
Should support the manual backup storage of video image information; video information storage, event trigger storage and plan storage function; the system should support users to retrieve and play back according to various conditions such as time, place, event; the system should support multiple users at the same time Call to retrieve video images; DVR, NVR can work in a variety of storage archive mode; the system should support the way to download 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 is usually divided into "important video archive" and "alarm video archive";
· Archive Server can be deployed anywhere on the network;
· An Archive Server can archive multiple DVRs or NVRs;
· The user does not need to specify to play back the video from the DVR or NVR or Archive Server, the system will automatically index;
• You can choose to archive some part of a DVR or NVR channel.
5. Integrated video platform
The system software platform includes a core data part and a client workstation. The core data server includes a system database and core software, and the workstation system should include functions such as user management, rights management, configuration management, fault management, and log management. The platform should support the monitoring and maintenance of various video equipment resources and equipment running status, configure the system equipment parameters through software, and manage the system user registration, authentication, deletion and authority allocation. The system should have a log management function, which can record the time and main operation of the operator entering and exiting the system through the operation log, and support the functions of log information query and report creation.
Data management: The type, version, quantity and number of all equipments in the whole line are “registered†in the database; monitoring function: all monitoring points of the whole line can be called through the network for real-time image, which provides reference for leading emergency command and decision scheduling display. Network management function: monitors and manages the operation of network element devices in all monitoring systems of the entire line, and displays the running data and alarm information of various devices in real time. Alarm management: records and displays alarms of devices, systems, and communication parts, and records alarm processing. The situation can be derived; control function: PTZ control and relay output node control for all monitoring points of the whole line; voice intercom: monitoring points for the voice equipment installed at the front end, real-time voice intercom, one-way “calling†function; data storage : Supports local storage and central backup of video, with multiple automatic and manual storage methods.
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