ERROR CONCEALMENT TECHNIQUES IN H 264 AVC FOR VIDEO TRANSMISSION OVER WIRELESS NETWORKS Vineeth Shetty Kolkeri University of Texas Arlington Outline Introduction What is Error Concealment Error Concealment Architecture H 264 MPEG 4 AVC Overview Error Concealment algorithm Error Concealment performance analysis test results Conclusions Future Work References Introduction Figure 1 Typical Situation of 3G 4G cellular telephony What is Error Concealment The operation adopted to reconstruct lost information for video transmission over wireless networks Need Compatible with all video streaming devices Applications Recovery of lost information which is comparable with encoded video sequence with minimal complexity Error Concealment Architecture Overview of H 264 AVC Latest Video coding standard Basic design architecture similar to MPEG x or H 26x Better compression efficiency Upto 50 bitrate reduction from the preceding video codec standard Subjective quality is better Advanced functional element Wide variety of applications such as video broadcasting video streaming video conferencing D Cinema HDTV Layered structure consists of two layers Network Abstraction Layer NAL and Video Coding Layer VCL supports 4 2 0 chroma sampling picture format including QCIF and CIF formats Overview of H 264 AVC contd Uses hybrid block based video compression techniques such as Transformation reduction of spatial correlation Quantization bit rate control Motion compensated prediction reduction of temporal correlation Entropy coding reduction in statistical correlation Includes the following features Intra picture prediction 4x4 integer transform Multiple reference pictures Variable block sizes Quarter pel precision for motion compensation In loop de blocking filter Improved entropy coding H 264 AVC Profiles Profiles and Levels for particular applications Profile a subset of entire bit stream of syntax different decoder design based on the Profile Four profiles Baseline Main Extended and High Profile Baseline Main Applications Video Conferencing Videophone Digital Storage Media Television Broadcasting Extended Streaming Video High Studio editing Specific coding parts for the Profiles Specific coding parts for the Profiles contd Common coding parts for the Profiles I slice Intra coded slice the coded slice by using prediction only from decoded samples within the same slice P slice Predictive coded slice the coded slice by using inter prediction from previously decoded reference pictures using more than one motion vector and reference index to predict the sample values of each block CAVLC Context based Adaptive Variable Length Coding for entropy coding Specific coding parts for the Profiles contd Coding parts for Baseline Profile Common parts I slice P slice CAVLC FMO Flexible macroblock order macroblocks may not necessarily be in the raster scan order The map assigns macroblocks to a slice group ASO Arbitrary slice order the macroblock address of the first macroblock of a slice of a picture may be smaller than the macroblock address of the first macroblock of some other preceding slice of the same coded picture RS Redundant slice This slice belongs to the redundant coded data obtained by same or different coding rate in comparison with previous coded data of same slice H 264 Encoder contd H 264 Transform 4x4 multiplier free integer transform Transform coefficients perfectly invertible Hierarchical structure 4 x 4 Integer DCT Hadamard transform Hadamard transform applied when 16x16 intra prediction mode is used with 4x4 integer DCT MB size for chroma depends on 4 2 0 4 2 2 and 4 4 4 formats H 264 Transform contd YUV different formats 4 4 4 is full bandwidth YUV video and each macroblock consists of 4 Y blocks and 4 U V blocks Being full bandwidth this format contains as much as data would if it were in the RGB color space 4 2 2 contains half as much chrominance information as 4 4 4 and 4 2 0 contains one quarter of the chrominance information H 264 Transform contd Codec A video codec is software that can compress a video source encoding as well as play compressed video decompress CIF Common Intermediate Format a set of standard video formats used in videoconferencing defined by their resolution The original CIF is also known as Full CIF FCIF QCIF Quarter CIF resolution 176x144 SQCIF Sub quarter CIF resolution 128x96 4CIF 4 x CIF resolution 704x576 16CIF 16 x CIF resolution 1408x1152 H 264 Scaling and Quantization Multiplication operation for exact transform combined with multiplication of scalar quantization Scale factor for each element in each sub block varies as a function of quantization parameter associated with macroblock that contains sub block position of element within sub block Rate control algorithm in encoder controls value of quantization parameter Encoder performs post scaling and quantization Deblocking filter Adaptive To reduce the blocking artifacts in the block boundary and prevent the propagation of accumulated coded noise Filtering is applied to horizontal or vertical edges of 4 x 4 blocks in a macroblock adaptively on the several levels slice block edge sample H 264 Motion Compensation Predicted rectangular arrays of pixels 4x4 4x8 8x4 8x8 16x8 8x16 and 16x16 Translation from other array positions in reference picture specified with quarter pixel precision In case of 4 2 0 format the chroma MVs have a resolution of 1 8 of a pixel derived from transmitted luma MVs of 1 4 pixel resolution Prediction of variable block A MB can be partitionedsize into smaller block sizes 4 cases for 16 x 16 MB 4 cases for 8 x 8 Sub MB Large partition size homogeneous areas small detailed areas Prediction of variable block size contd Frame divided into multiple macroblocks of 16 x 16 8 x 8 4 x 4 variable size to represent coding profiles No of bits in I and P frames I P I Graph shows the size of the different I and P frames obtained after encoding 19 frames of the Foreman QCIF video sequence Green line shows the average values of the bit lost when it is passed through the lossy algorithm after encoding in a video sequence H 264 Entropy Coding CAVLC Context based Adaptive Variable Length Coding CABAC Context based Adaptive Binary Arithmetic Coding CAVLC makes use of run length encoding CABAC utilizes arithmetic coding codes both MV and residual transform coefficients Typically CABAC provides 10 15 reduction in bit rate compared to CAVLC for the same PSNR All syntax elements are encoded by Exp Golomb codes
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