Author: Alexander Run-Information Technology Co., Ltd.
If you compare the video conferencing products that use overview and other brands, you will find that under the same network conditions, use the same machine, video clarity and smoothness of viewing products are much higher than other products, viewing How to solve the contradiction between "video quality" and "network bandwidth"?
In video conferencing applications, video quality and network bandwidth are contradictory, usually, the higher the bandwidth of the video stream, the higher the video quality; if high quality video effect, the larger network bandwidth is also large; The key to solving this contradiction is of course video codec technology. Evaluation of the advantages and disadvantages of a video codec technology is compared to which video quality is better under the same bandwidth; which occupies less network bandwidth under the same video quality.
Video codec technology has two sets of standards, ITS (ITU-T) standard H.261, H.263, H.263 , etc., Etc., Etc., MPEG standard MPEG1, MPEG2, MPEG4, etc. H.264 / AVC is the latest standard for the advantages of the two major organizations, the advantages of H.263 and MPEG4, and the most valuable part is undoubtedly a higher data compression ratio. Under the same image quality conditions, the data compression ratio of H.264 is 2 times higher than H.263, 1.5 times higher than MPEG-4.
Here we briefly introduce the concept and development of H.264, and discuss the possibility of practical use of H.264 technology.
H.264 / AVC is? The H.264 / AVC standard is developed by ITU-T and ISO / IEC, which is positioned to cover the entire video application, including: low-rate wireless applications, standard definition, and high-definition TV broadcast applications, on the Internet Video stream application, transmit high-definition DVD video, and high quality video applications applied to digital cameras, etc.
ITU-T is named H.264 (previously called H.26L), and ISO / IEC calls an MPEG-4 advanced video coding (AVC), and it will become the MPEG-4 standard 10 parts. Since AVC is currently expanded by the MPEG-4 standard, it will inevitably benefit from MPEG-4 development well-developed infrastructure (such as system layers and audio, etc.). Obviously, the MPEG-4 AVC of the MPEG-4 Advanced Simple Framework (ASP) will be superior to the current MPEG-4 video compression standard, which will primarily apply high compression ratios and hierarchical quality needs. direction.
Just as seen in the "Video Code Historical" table, ITU-T and ISO / IEC are responsible for customization of all international video compression standards. So far, the most successful video standard is MPEG-2, which has been widely accepted by various market sectors such as DVD, digital TV broadcast (overwriting cables and communication satellites) and digital sets. Since the MPEG-2 technology, the new H.264 / MPEG-4 AVC standard has huge improvement in coding efficiency and quality. Over time, in many existing applications, H.264 / MPEG-4 AVC will replace MPEG-2 and MPEG-4, including some emerging markets (such as ADSL video).
The evolution of digital video codec technology
International standards are usually developed by the International Standardization Organization ISO on the basis of Technical Suggestions for Telecommunication Alliance ITU. The digital video codec standard has also experienced many changes, and its evolutionary process is shown in the figure:
Obviously, the H264 standard has increased moving image compression technology to a higher stage, providing high quality image transfer on lower bandwidth is the high-quality application highlight of H.264. The promotion and application of H.264 is higher in the system of video terminals, gatekeepers, gateways, and MCUs, which will strongly promote the continuous improvement of video conferencing soft equipment in various aspects. H.264 core competitiveness
The most valuable part of H.264 is undoubtedly a higher data compression ratio. The basic principle of compression technology is to filter non-important information in the video file to allow data to be transmitted in the network faster. Under the same image quality conditions, the data compression ratio of H.264 can be 2-3 times higher than the MPEG-2 high of the MPEG-2 high than the MPEG-4 high than the MPEG-4 high. Because of this, after H.264 compressed video data, less bandwidth required during network transmission, and more economical.
When the MPEG-4 requires a 6Mbps transmission rate match, H.264 only requires only 3 Mbps-4Mbps transmission rate. We use transportation to do more metaphors: also use a truck to transport a large box, if MPEG-4 can lose half of the box, then H.264 can lose the box to the original 1/4, When the weight of the truck is not changed, the H.264 is twice as the MPEG-2 is doubled.
H.264 Get superior performance is a substantial increase in computational complexity, such as hierarchical design, multi-frame participation, multi-mode motion estimation, improved intra prediction, etc., significantly increased prediction accuracy, thus obtaining more than other The standard is much more compressed.
Constantly improved hardware processing capabilities and continuous optimized software algorithms are the foundation of H.264. As early as ten years ago, the CPU for several tens of trillion reached top, and now the ordinary desktop, the clue of CPU has been up to several thousand megabytes. According to Moore's law, the capacity of the chip unit is doubled every 18 months, so the computational complexity of H.264 is slightly insignificant relative to performance improvement. What's more, new computing methods are endless, and the H.264's hunger demand for processing speeds is relatively alleviated.
Comparison of H.264 and MPEG-4
1. In the case of a very low rate (32-128 kbps), H.264 has a performance multiplication effect compared to MPEG-4, ie, the same rate of H.26L media streams and MPEG-4 media streams. H.26L has a gain of approximately 3 decis (the quality level is multiplied). 32kbps H.26L media stream, its hiker is similar to 128K MPEG-4 media flow. That is, in the same picture quality, the syrup rate of H.264 is only a quarter of MPEG-4.
2, H.26L compares to MPEG-4 at medium and low yards: H.26L is multiplied with MPEG-4 in the case of medium and low yard ratio (32-128kbps).
The H.264 standard launched only one year, most publicity support H.264 terminal vendors mainly support the basic grade of H.264. Because H.264 complexity increases, challenges the video processing capabilities of terminal vendors. Existing platforms, or you can't do the H.264 codec, or you cannot support codec under high yield rate. And the viewing video conference product supports 640 * 480, the video standard uses the latest high-rate codec technology, and the image is clear and smooth. On the basis of bandwidth savings, the signal-to-noise ratio of video quality is 40% higher than the similar products, and is the best coding technology in current quality.
H.264 / AVC Core Technology Overview is just like it is seen in the figure, this new standard is composed of several processing steps: inter-frame and intra prediction transformation (and reverse transform) quantization (and inverse quantization) Loop filter entropy encoding singular image flow forms video, which can be divided into "macroblock" of 16x16 pixels, which simplifies the processing of each step in the video compression algorithm. For example, a picture taken from the standard definition standard video stream solution (720x480) is divided into 1350 (45x30) macroblock, and then further processing in the macroblock level. Motion estimation of inter predictive improvement. Motion estimation is used to determine and eliminate time redundancy between different pictures existing in the video stream. When the motion estimation search is based on the picture in the past direction, the encoded picture is called "P frame picture", when the search is based on the two directions in the past and future, then the encoded picture is called "B frame picture" . In order to improve the coding efficiency, the macroblock is split into smaller blocks in order to include and separated the moving macro block in the "H.264 Motion Estimation-Improved Motion Estimation). Then, the motion vector of the previous or future picture is used to predict a given block. H.264 / MPEG-4 AVC invented a smaller block, which has better flexibility, and can have higher prediction accuracy in the motion vector. H.264 Motion Estimation - Improved Motion Estimation Inframe Prediction Included from the motion estimation, using intra estimation to eliminate spatial redundancy. Internal estimates predict the current block by pumping adjacent pixels in different directions in different directions in a predefined set. Then the prediction blocks and the real blocks are encoded. This method is especially useful for H.264 / MPEG-4 AVC, especially for a flat background that often has space redundancy. An example is the "H.264 internal estimation" displayed below. H.264 Internal estimation transformation motion estimation and internal estimation results After converting from spatial domains to the frequency domain. H.264 / MPEG-4 AVC uses an integer DCT4x4 transform. MPEG-2 and MPEG-4 use floating point DCT8X8 transform. The smaller block H.264 / MPEG-4 AVC reduces block effects and obvious artificial traces. The integer coefficient eliminates the accuracy loss caused by the floating point coefficient operation in MPEG-2 and MPEG-4. The coefficients after the quantization transformation of the transformation is quantified, and the predictive amount of integer coefficients and the elimination of not perceived high frequency coefficients. This step is also used to control the bit rate of the output remaining in a substantially constant constant. H.264 Quantization / Code Rate Control loop filter H.264 / MPEG-4 AVC standard defines a decimal block filtering process for the 16x16 macroblock and 4x4 block boundaries. In this case, the purpose of filtration is to eliminate artificial traces caused by different motion estimates (such as motion estimates and internal estimates) due to adjacent macroblocks. In this case in this case, the purpose of filtration is to eliminate artificial traces that may cause differences due to transformation / quantization and from phase neighboring block motion vectors. The loop filter is modified by a content adaptive nonlinear algorithm two pixels in the same side of the macroblock / block boundary. Entropy encoding prior to entropy coding, the quantization coefficient of 4x4 must be reordered. According to these coefficients, the prediction algorithms used for motion estimates or the different scan types are selected to create a different scan type to create a reordering serialization. The scan type is sorted in the order from low frequency to high frequency. Since most of the high frequency coefficient tends to zero, then the number of zero can be reduced by using the run coding, thereby efficiently reaching the purpose of entropy coding. The serialization of the entropy coding-coefficient of entropy encoding the motion vector, quantization coefficient and macroblock header by the byte stream of the entropy encoding step by the mapping symbol. Entropy encoding indicates frequently used symbols by designing a few bit bits, and more bitits are used to represent symbols that are not currently used.
