Wang Daguang Yulin Teachers College Yulin, Guangxi 537000
Abstract: The video conference system can transmit images and sounds in real time, and it is a timely and vivid information exchange tool, which is favored and applied by various industries. Colleges and universities use video conference system as a modern teaching method, which is widely used in distance education and cross-campus teaching. However, the video conference system exposes the problems of low definition and poor effect in practical applications. This article discusses the realization of high-definition of the video conference system.
Keywords: video conferencing system; high definition
The video conference system can transmit video images and sound in real time. It is a communication tool for timely, accurate, vivid, comprehensive transmission and exchange of information. Over the years, it has been used by governments, communities, education, transportation, security, medical, commercial, financial and other industries. Favor and application. Especially in colleges and universities, the video conference system is used as a modern teaching method, which is widely used in distance education and cross-campus teaching. However, the video conference system has also exposed problems such as low resolution, time extension, poor stability, and unsatisfactory results in practical applications, which seriously affect the development and use of the video conference system.
As a means of information transmission, the video conferencing system's fidelity and clarity are extremely important, and it directly affects the use effect. For example, the use of video conferencing systems for distance education teaching and academic exchange research often requires experimental procedures, reaction processes, X-rays, MRI MRI images, ECG electrocardiograms, ultrasound examination results, data, design drawings, flowcharts, maps and Photographs and other information content, detailed analysis, interpretation, discussion and diagnosis. This requires the video conference system to deliver high-definition information content with high fidelity. The research and development of high-resolution, high-definition video conference system (HDVShow) has positive practical significance.
This article mainly analyzes the reasons that affect the definition, time extension and picture-synchronization of the video conference system, and discusses the related technologies, system architecture, and technical strategies to achieve high-definition video conference systems.
1. Analysis of the reasons that affect the resolution of the video conference system 1. The low resolution of the video conference system stems from the early analog TV system. The analog TV we used for decades is NTSC system and PAL system. 30 frames per second, PAL line is 625 lines, 24 frames per second. Scan the odd lines of the still picture first, and then scan the even lines. Two pictures are displayed on the display at the same time, called 50/60 fields per second. This odd / even scanning mode is called interlaced scanning, which saves transmission bandwidth. This resolution is called 480i and provides a 240-line picture. TVs under 25 inches at home look relatively clear and smooth. However, if you use a TV set larger than 25 inches, or use a large screen projection, the image quality will be significantly reduced, such as visual noise, jagged lines, fading, and blurred outlines. Therefore, the clarity of the video conference system based on early analog TV is necessarily low.
2. The low resolution of the video conference system results from the low data rate exchange format (CIF)
In the past, video conferencing system data was encoded based on the Common Interchange Format (CIF). The International Telecommunication Union-Telecommunication Standards Department (ITU-T) has developed video standards called H.261 and H.263. The H.261 standard only defines QCIF and CIF formats. 1/4 of the CIF (QCIF) format is only used for meetings with the lowest data rate (64 kilobits / second and lower) .The video conferencing system encodes based on the low data rate exchange format (CIF), and the video conference image information is inevitable Suffered greater losses, resulting in a reduction in picture clarity.
3. The low picture resolution of the video conferencing system stems from the poor transmission network conditions. Early video conferencing systems were transmitted through coaxial cables. The inherent frequency transmission characteristics of the coaxial cable make the TV image signal have a large loss after being transmitted over a long distance through the coaxial cable. 3. Noise increases and picture clarity decreases. The video conference system uses a narrow-band network transmission, and the picture information needs to be transmitted after being processed with a high compression ratio. The video conference picture is inevitably degraded because of the large amount of data reduced during network transmission.
4. The low resolution of the video conference system is due to the low hardware level. (1) The video camera system on-site camera equipment does not use professional-grade cameras, most of them use amateur-level and domestic camera equipment, and the grade is low, resulting in the video conference system on-site video recording. The received image signal source has poor quality and low definition.
(2) The display terminal equipment of the video conference system is of low grade. The use of ordinary analog TV sets or the use of projectors with low lumens and low resolutions results in poor quality and low resolution of the live video images displayed on the video conference system.
5. The low resolution of the video conference system results from the poor lighting of the conference room. Generally, the existing academic lecture halls, conference rooms or halls are used to install the video conference system. Enough attention was paid to lighting without referring to the requirements of the TV studio. The conventional conventional lighting in academic lecture halls, conference rooms and auditoriums is far from the requirements for brightness and color temperature of professional camera environment lighting. What's more, in order to allow participants to view the projection screen with low brightness at the video conference site, most of the lights at the conference site are always turned off. The camera of the video conference system can only be used in an environment with extremely low illumination The shooting caused the video scene on the conference site to be gray and low in definition.
2. Analysis of the reasons for the extension of the video conference system and the unsynchronized picture and sound. When the video conference system selects the encoding / decoding technology format with low processing capacity, the video conference transmission picture signal encoding and receiving end picture signal decoding process requires more time. The delay in the processing process causes the video conferencing system to prolong the viewing signal, and the picture and sound are out of sync.
The video conference system transmits a large amount of data information of the picture signal through the network, which takes time. When the transmission network bandwidth is too narrow, it consumes more time, resulting in a delay in the processing process. The video conferencing system prolongs when the picture signal appears, the picture and the sound The out-of-sync phenomenon is even more serious.
Third, the realization of high-definition video conference system 1. Choose high-definition video format In order to improve the clarity of the TV screen, people have developed progressive scanning technology, 480 lines of image information lines are merged into one frame, and then automatically correct the picture quality, purely from digital From the above, the progressive scan technology doubles the 240-line picture provided by the interlaced scan. Progressive scanning reduces the jagged picture on the large screen, makes the picture smoother, and improves the picture quality. This resolution is called 480p, commonly known as standard definition television. For true high-definition video, the image resolution is 1080i or 720p. The amount of image information provided by the HD format is more than 4 times higher than the interlaced 480i format, and a higher number of pixels greatly improves the picture quality. Obviously, to realize the high-definition of the video conference system, it is impossible to choose the interlaced scanning 480i format and the progressive scanning 480p format, but should use the high-definition video 720p format.
2. Adopting high-definition encoding / decoding technology adopts the recommendations of the International Telecommunication Union-Telecommunications Standards Department, and high-definition video conference uses the new video compression standard H.264. Compared with H.261 and H.263, H.264 has higher processing performance, provides high-quality video transmission and low-latency codec, H.264 encoding standard provides a common platform for interconnection and interoperability, and has a stronger flexibility. The most prominent advantage is that it reduces the entire video file, and the video file can be transmitted through a smaller capacity network connection (lower data rate / broadband) to provide high-quality pictures at a relatively low data transmission rate. The efficiency of H.264 is twice that of H.263, and the video quality at a specific line rate is also twice as high, making the video stream smoother and more natural. The enhanced H.264 technology also has an error concealment algorithm for interactive video, which can automatically adjust the video processing. Even in the case of excessive network load, instability or high error rate, it also guarantees free operation and provides high-quality video. flow. Now, H.264 has become a mandatory standard for HD-DVD (High Definition DVD) and broadcasting, cable, video conferencing and consumer electronics products.
3. All hardware devices in the high-definition hardware video conference system must support high-definition, which is the key to realizing the high-definition of the video conference system. Video conferencing system front-end equipment, especially video, graphic image source shooting, production, and collection equipment, such as camera units: main camera, auxiliary camera, graphic camera, video recorder, electronic whiteboard, etc., choose to support 1080i image resolution Full HD professional level Equipment to ensure the high-definition quality of the image signal source on the scene.
The multipoint control unit MCU of the video conferencing system, the selection of conference terminal equipment, must be able to support 1080i HD. The terminal display device of the video conference system uses a full HD digital projector, LCD TV and plasma TV that support 1080i image resolution to ensure that the video conference picture display is clear and smooth, and the colors are realistic.
4. The use of optical fiber broadband transmission small local video conference system, such as multi-campus teaching video conference system in colleges and universities, because the transmission line distance is not long, the construction scale is small, the investment is not large, you can consider the use of dedicated optical fiber physical end-to-end mode transmission. Conference video and audio signals are uploaded and downloaded, and control signals are uploaded and downloaded using separate fiber-optic core channels, each doing its own thing. This transmission mode has two major advantages: First, it avoids the loss of video and audio signals through the mixing / demodulation processing link, so that the information of the screen information of the conference site can be almost lost. The second is that there is no transmission bandwidth limitation, so as to ensure that the video conference system has a fixed signal quality and achieve high fidelity of sound and picture.
For wide area video conferencing systems, it is best to use dedicated physical fiber end-to-end mode to transmit signals when conditions permit. If it can only be transmitted through the network, there must be sufficient network bandwidth. Under normal circumstances, the data transmission rate of the video conference can reach the high-definition effect of DVD or more under the bandwidth of 2Mbit / s. In order to avoid the problem that the transmission bandwidth is narrow and the information data of the video conference screen is excessively compressed to reduce the resolution sharply, the time is prolonged, and the picture and sound are out of sync, etc., make full use of the high bandwidth to ensure the stability and high security of the video and audio of the video conference true. Considering various influencing factors comprehensively, the actual data transmission rate is preferably set to be above 10 Mbit / s.
5. Reasonable lighting of the video conference system venues. With reference to the requirements of the TV studio, lighting the video conference system venues (report hall, meeting room, hall), mainly to ensure that the ambient lighting brightness and color temperature in the shooting area of ​​the venue basically meet the standards. Ensure that the live video source captured by the video conference system is clear and the picture quality is good.
Fourth, high-definition video conferencing system architecture and examples 1. High-definition video conferencing system based on SIP High-definition video conferencing system based on SIP, can transmit 1080iHD full-scene high-definition video, remotely reproduce vivid and realistic conference live. The video conference system uses Reliable Hybrid Multicast Protocol (RCMP) as the multimedia transmission protocol. RCMP (as shown in Figure 1) combines the advantages of IP Multicast Application Layer Multicast (ESM) and solves the problems caused by the traditional C / S structure. The server is overloaded. Using SIP as the session control protocol has good scalability and compatibility. The RTP protocol is used on top of the SARLM-based reliable hybrid multicast protocol (RCMP), which enables high-efficiency video distribution and packet loss processing to realize the transmission of HD on the Internet. The video conference system has an adaptive bandwidth function, which can make full use of idle bandwidth while preventing network congestion, and adjust the quality of multimedia transmission in real time according to the fluctuation of bandwidth. The system uses a combined retransmission and FEC encoding mechanism to recover video frames lost during network transmission, thereby effectively improving the video quality when the network condition is poor. Compared with traditional video conferencing, SIP-based high-definition video conferencing systems can make full use of available bandwidth under high bandwidth, resulting in higher quality video and audio, and more stable video and audio quality under low bandwidth. It also has good interactivity and reliability.
2. Small local area HD video conference system architecture (as shown in Figure 2)
3. Architecture of wide area high-definition video conference system (as shown in Figure 3)
The unit where the author is located is a multi-campus school. The video conferencing system of the whole school adopts a small local area high-definition video conferencing system architecture, and the dedicated optical fiber is transmitted in end-to-end physical mode. The experience of many years of application and practice is that the signal transmission of the video conference system is very stable, and the sound and picture quality has achieved high fidelity, which achieves and exceeds the DVD effect. The school's external video conference system adopts a wide-area high-definition video conference system architecture. After the trial stage, it has been tested. The video conference signal has good interactivity, stability and smoothness, and the video conference image quality has high definition.
References [1] Xu Qi. An SIP-based interactive high-definition video conferencing system [J]. Computer Simulation, 2007, 6
[2] Yu Ke. On the real high-definition video conference system and its advantages [J]. Video communication, 2008, 7
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