0 Preface
The i.Mx27 multimedia application processor is the latest addition to Freescale Semiconductor's DragonBall family. It integrates H.264/MPEG4/H.263 full-duplex hardware codec video processing units for video surveillance and digital video recorders. Network advertising machine, V2IP video phone, IPTV set-top box, smart phone, portable multimedia player, mobile TV, car multimedia products.
A mobile video surveillance terminal based on i.MX27 is designed and applied to the bus video surveillance system. The video data and GPS location information collected by the terminal can be transmitted to the bus dispatch monitoring center in real time. In the case of poor network, the video data and GPS location information can also be saved locally, and the high-speed transmission can be concentrated through the WIFI wireless network after the bus arrives at the station. Go to the monitoring center.
1 Bus video monitoring system composition and function description
1.1 System components
The whole system is divided into front-end vehicle monitoring system, communication line, monitoring platform front-end vehicle monitoring system, including vehicle DVR, surveillance camera, monitor head, alarm button, alarm flash, LCD screen, intercom headset, GPS and other communication lines. Front-end mobile transmission (EDGE/CDMA 1X) and background command center Ethernet network monitoring platform include monitoring software and a series of service groups such as GIS, streaming media, database, application server, etc., which can be used in public transportation bureaus, municipal public security bureaus, police stations, etc. The unit builds a multi-pole monitoring platform, and the logical relationship of the level can be flexibly configured according to requirements, and the system has good scalability.
The structural diagram of the system composition is shown in Figure 1.
1.2 System Function Description
(1) Video surveillance function. The vehicle equipment supports up to 4 cameras, which can monitor the front and rear doors and compartments before and after;
(2) GPS positioning function. The video surveillance device should obtain the current time, latitude and longitude information from the GPS device, and embed the recorded video file or video stream in the form of a time tag (ie, subtitle mode); the GPS information (1 per second) is simultaneously written to the log file, Value-added processing in the later stage: the video file saved inside the video device has a one-to-one correspondence with the GPS log file;
(3) Batch communication function. When the vehicle is located in a parking lot, station, etc. with a high-speed wireless network such as WIFI/WinMax, it can enter the background data transmission working mode at any time. The transmitted content includes video files and GPS logs;
(4) Real-time communication function. When the terminal is equipped with the GPRS/CDMA network communication module function, real-time video data and GPS location information can be transmitted.
2 Mobile video monitoring terminal hardware design
The mobile video surveillance transmission terminal adopts i.MX27 as the core device, and the GPS module and the WIFI module are expanded to realize the functions of collecting, storing and wirelessly transmitting mobile video. At the same time, the real-time location of the mobile device can be accurately located through the GPS information.
The system hardware structure is shown in Figure 2.
3 mobile video monitoring terminal software design
The terminal system uses an embedded Linux operating system to improve service reliability and system efficiency. The main flow chart of the software is shown in Figure 3.
3.1 terminal application software
3.1.1 Video capture, encoding, storage
Video coding uses the H.264 standard to increase higher video resolution with less data. Video local storage uses a large-capacity SD card or USB flash drive. Do not use a mobile hard disk to avoid damage to the mobile hard disk caused by vibration during bus operation.
3.1.2 Wireless network transmission
The terminal system is designed with WIFI high-speed wireless network and GPRS/CDMA network to adapt to different applications. WIFI wireless network is used for high-speed data transmission in places where WIFI hotspots are everywhere. In other cases, data can be transmitted through GPRS/CDMA wireless network.
Wireless network technology covers a wide range of global voice and data networks that allow users to establish long-range wireless connectivity, as well as infrared and RF technologies optimized for short-range wireless connectivity. Devices commonly used in wireless networks include portable computers, desktop computers, handheld computers, personal digital assistants (PDAs), mobile phones, pen computers, and pagers. Wireless technology is used for a variety of practical purposes. For example, a mobile phone user can use a mobile phone to view an email. Passengers using laptops can connect to the Internet through base stations installed at airports, train stations, and other public places. At home, users can connect to desktop devices to sync data and send files. Wireless network transmission mechanism Information transmission mechanism of wireless network technology.
3.1.3 Acquisition and processing of GPS information and other information
The terminal system saves real-time GPS information every second and synchronizes time with the video data. At the same time, other peripheral signals such as alarm detection are collected regularly for system processing.
3.2 Key Technology Implementation Principles
3.2.1 Video real-time transmission processing mechanism
In the case of unstable or poor network bandwidth, in order to ensure the integrity and real-time performance of the video data transmitted by the network video server to the server as much as possible, the system adopts a dual processing mechanism:
(1) Compare the number of data packets in the video history buffer per unit time. If the number of data packets is found to be greater than 500 packets, perform a clear history buffer operation, and then close the open encoder to encode a new IDR frame for use. transmission;
(2) Timing comparison between the time of the current coded frame and the time of the data packet to be sent. If the time is greater than 5 s, the operation of clearing the history buffer is performed, and the encoder is turned off again to encode a new IDR frame. For transmission;
(3) In addition, in order to ensure the full use of the bandwidth, the code stream transmitted to the server can be played normally. Now, when the history buffer is cleared, the integrity judgment of the I frame is added, and it is necessary to ensure that the currently transmitted video data is not an I frame. Perform the operation of clearing the history buffer;
3.2.2 Specific implementation of the video capture and encoding part
4 Conclusion
This paper expounds the realization principle of mobile video surveillance terminal based on i.MX27 and its application in bus dispatch monitoring, and introduces the design scheme and implementation method of mobile video surveillance terminal, and details the implementation of some key technologies. description of. The system has been tested in bus monitoring and has achieved good results.
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