0 Introduction A typical digital TV system is a digital TV program source (mainly composed of video, audio, etc.) at the transmitting end. The compression-encoded video and audio stream are obtained, followed by source coding, and support for auxiliary data and control data is required. The channel coding implements error detection and error correction functions to improve the anti-interference ability of the digital television transmission signal, so as to adapt to the channel transmission characteristics, and then perform carrier modulation to realize spectrum shifting and finally input to the transmission channel. Currently in the field of digital video, there are two organizations that develop video coding standards, which are ITU-T and ISO/IEC. The standards established by ITU-T include H. 261, H. 263, H. 264, mainly used in the field of real-time video communication, such as conference television. The MPEG series of standards is developed by ISO/IEC and is mainly used for video storage (VCD, DVD), broadcast TV, Internet or streaming media on wireless networks. H. 264 has strong anti-error characteristics, and can adapt to video transmission in a wireless channel with high packet loss rate and serious interference.
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1 coding technology analysis
1.1 Intra Prediction Coding Intra coding is used to reduce the spatial redundancy of an image. In order to improve H. The efficiency of 264 intra-frame coding, taking full advantage of the spatial correlation of adjacent macroblocks in a given frame, adjacent macroblocks usually contain similar attributes. Therefore, when encoding a given macroblock, it can first be predicted according to the surrounding macroblock (typically according to the macroblock in the upper left corner, because this macroblock has been encoded), and then the difference between the predicted value and the actual value. The values ​​are encoded such that the code rate can be greatly reduced relative to directly encoding the frame. As shown in Figure 1.
1.2 Interframe Predictive Coding Interframe predictive coding utilizes temporal redundancy in successive frames for motion estimation and compensation. H. Motion compensation at 264 supports most of the key features of previous video coding standards.
(1) Macroblock partitioning of different sizes and shapes Motion compensation for each 16x16 pixel macroblock can be of different sizes and shapes.
(2) High-precision sub-pixel motion compensation in H. The 263 uses half-pixel precision motion estimation, while in H. Motion estimation at 1/4 or 1/8 pixel accuracy can be used in 264. In the case of requiring the same accuracy, H. 264 The residual after motion estimation using 1/4 or 1/8 pixel precision is better than H. The 263 is smaller by using the residual after half-pixel precision motion estimation. This is the same precision, H. 264 requires a lower code rate in interframe coding.
(3) Multi-frame prediction H. The 264 provides an optional multi-frame prediction function that allows for 5 different reference frames for inter-frame coding, providing better error correction performance, which improves video image quality. This feature is mainly used in the following situations: periodic motion, translational motion, and changing the camera's lens back and forth between two different scenes.
(4) Deblocking filter H. 264 defines a filter that adaptively removes blockiness, which can handle horizontal and vertical block edges in the prediction loop, greatly reducing blockiness.
1.3 Entropy coding The final step in video coding processing is entropy coding, in H. There are three methods for entropy coding in 264. The first is the Exp-Golomb code, which applies to all other coefficients except the transform coefficients. The second is context-based adaptive variable length coding (CAVLC), which is applicable to transform coefficients. The third is context-based adaptive binary arithmetic coding (CABAC). CAV-LC and CABAC perform encoding of the current block according to the condition of neighboring blocks to achieve better coding efficiency. CABAC has a higher compression ratio than CAVLC, but it is more complicated.
(1) Exp-Golomb code: The simplest entropy coding method is to use a single infinitely extensible codeword table for all syntax elements except the quantized coefficients, so that it is not necessary to design a dedicated VLC table for each syntax element. . It applies to all symbols except transform coefficients, such as syntax elements such as the header. It is a variable length code with a regular structure (as in Table 1).
(2) CAVLC coding: Context-based adaptive variable length coding is a method for encoding a zig-zag-scanned 4×4 (and 2×2) block transform coefficient of a difference. CAVLC utilizes several features of quantized 4x4 blocks: 1 The blocks after prediction, transformation, and denaturation are generally sparse (including many zeros). CAVLC uses run length coding to compactly represent a long string of zeros. The highest non-zero coefficient after 2zig-zag scanning is often a sequence of +1/-1. The number of non-zero coefficients of 3 neighboring blocks is related. The number of coefficients is encoded using a look-up table, and the choice of the look-up table depends on the number of non-zero coefficients in the neighboring block. The magnitude of the 4 non-zero coefficients is relatively high at the beginning of the rearranged array (close to the DC coefficient) and low at the high frequencies. CAVLC uses this to appropriately select the VLC look-up table of the amplitude parameters based on the magnitude of the most recent encoding.
(3) CABAC method: Arithmetic coding enables the use of probability models of all syntactic elements (transform coefficients, motion vectors) on both sides of both encoding and decoding. In order to improve the efficiency of arithmetic coding, the basic probability model can be adapted to the statistical characteristics that change with video frames through the process of content modeling. Content modeling provides conditional probability estimates of coded symbols. With appropriate content models, the correlations that exist between symbols can be removed by selecting the corresponding probability model of the coded symbols that are currently adjacent to the coded symbols. Different syntax elements are usually maintained. Different models. It can be seen that the basic steps of the context-based adaptive binary arithmetic coding algorithm are: 1 For each syntax element, the probability model is selected according to its context. 2 Based on local statistical adaptive adjustment probability statistics 3 using arithmetic coding. The implementation block diagram is shown in Figure 2.
Context probability selection: Select "contextual profile" based on past observations. The model is a probability model of one or several binary sign bits, which can be selected from valid models, which are derived from the statistics of the data symbols of the nearest probabilistic code. The context model stores the probability that each binary symbol is 1 or 0. Binarization: Converts non-binary symbols (transform coefficients or motion vectors) into binary codes. ABAC only processes 1 and 0.
Arithmetic Coding Engine: A probability model selected by an arithmetic coder encodes each bit. There are only two subintervals (equivalent to 0 and 1) for each bit. The selected context model is updated according to the actual encoded value (if the bit value is "1", the frequency of the "1" is added) .
2 H. 264 application in video conferencing Currently, most video conferencing systems use H. 261 or H. 263 video coding standard, and H. The emergence of 264, at the same rate, H. 264 can be better than H. 263 reduces the code rate by 50%. In other words, users can enjoy H even if they only use 384kbit/s bandwidth. High quality video service up to 768kbit/s under 263. H. 264 not only helps to save huge expenses, but also improves the efficiency of resource utilization, while at the same time making video conferencing services with commercial quality have more potential customers.
For IP and wireless environments H. The 264 draft contains tools for error cancellation to facilitate the transmission of compressed video in error-prone, packet-loss multi-environment environments, such as the robustness of transmissions over mobile channels or IP channels.
In order to resist transmission errors, H. Time synchronization in the 264 video stream can be accomplished by employing intra-frame image refresh, which is supported by slice structured coding. At the same time, in order to facilitate resynchronization after error, a certain resynchronization point is also provided in the video data of one image. In addition, the intra macroblock refresh and the multi-reference macroblock allow the encoder to consider not only the coding efficiency but also the characteristics of the transmission channel when determining the macroblock mode. In addition to using the quantization step size to adapt to the channel code rate, in H. In 264, the method of data segmentation is often used to cope with changes in the channel code rate. In general, the concept of data partitioning is to generate video data with different priorities in the encoder to support quality of service QoS in the network. For example, using a syntax-based data partitioning method, each frame of data is divided into several parts according to its importance, which allows discarding less important information when the buffer overflows. A similar temporal data partitioning method can also be employed, by using multiple reference frames in P and B frames.
In wireless communication applications, we can support bit rate changes in wireless channels by changing the quantization accuracy or spatial/temporal resolution of each frame. However, in the case of multicast, it is impossible to require the encoder to respond to varying bit rates. Therefore, unlike the method of Fine Granular Scalability (FGS) adopted in MPEG-4 (lower efficiency), H. 264 uses stream switched SP frames instead of hierarchical encoding.
3 Conclusion
As a result, video conferencing products pay for the H.264 protocol, and researchers are committed to popularizing H.264, a new industry standard. We will be able to experience the H.264 video service to bring us high quality enjoyment.
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