Study and Comparison of MPEG-2 and H Study and Comparison of MPEG-2 and H.264 main profiles and available transcoding methods EE 5359 Priyanka Ankolekar 1000 51 4497

Introduction Development of international video coding standards like MPEG-2 led to a boost in multimedia applications like digital video recording and teleconferencing. On growing demand for better compression performance led to advanced video coding standards like H.264. H.264 is superficially similar to MPEG-2 However, there are significant differences in the details. This project aims to compare the MPEG-2 and H.264 main profiles and to discuss related transcoding methods.

MPEG-2 Second of several standards developed by the moving pictures experts group [16]. Used as the format of digital TV signals and direct broadcast satellite TV systems. MPEG-2 is not optimized for low bit rates like 1Mbps. But it outperforms MPEG-1 at 3Mbps and above. It is used for higher data rates of 4Mbps (DVD) and 19Mbps (HDTV). MPEG-2 devices are back compatible with MPEG-1. MPEG-2/Video is formally known as ISO/IEC 13818-2 and as ITU-T Rec.H.262. [21].

MPEG-2 Profiles A profile is a collection of compression tools that together make up the coding system. A different profile means that a different set of compression tools is available. [22] There are five profiles in MPEG-2, as summarized below. MPEG-2 Profiles[16] Abbr. Name Picture Coding Types Chroma Format Aspect Ratios Scalable modes SP Simple profile I, P 4:2:0 square pixels, 4:3, or 16:9 none MP Main profile I, P, B SNR Scalable SNR (signal-to-noise ratio) scalable Spatial Spatially SNR- or spatial scalable HP High profile 4:2:2 or 4:2:0

MPEG-2 Encoder [10]

MPEG-2 Encoder (contd.) DCT: 2 dimensional 8x8 – for intra frames 8x8 pels – for inter frames 8x8 residual blocks Quantizer: Quantizes DCT coefficients using a default or modified matrix. Motion Estimation and Compensation: In the motion estimation process, motion vectors for predicted and interpolated pictures are coded differentially between macroblocks. For the motion compensation process integer and half pel resolution motion vectors are used to predict from previously decoded pictures. 8 16 19 22 26 27 29 34 16 16 22 24 27 29 34 37 19 22 26 27 29 34 34 38 22 22 26 27 29 34 37 40 22 26 27 29 32 35 40 48 26 27 29 32 35 40 48 58 26 27 29 34 38 46 56 69 27 29 35 38 46 56 69 83

MPEG-2 Decoder [7]

H.264 Developed by the Joint Video Team (JVT). Achieves MPEG-2 quality compression at almost half the bit rate [7]. Significant coding efficiency, simple syntax specifications, and seamless integration of video coding into all current protocols and multiplex architectures. Supports various applications such as video broadcasting, video streaming, and video conferencing over fixed and wireless networks and over different transport protocols. [4]

H.264 Profiles – Comparison [11] Each H.264 profile specifies a subset of entire bitstream of syntax and limits that shall be supported by all decoders conforming to that profile. There are three profiles in the first version: Baseline, Main and Extended. There are four High profiles defined in the fidelity range extensions [19]. Baseline Extended Main High I and P Slices Yes B Slices No SI and SP Slices Multiple Reference Frames In-Loop Deblocking Filter CAVLC Entropy Coding CABAC Entropy Coding Flexible Macroblock Ordering (FMO) Arbitrary Slice Ordering (ASO) Redundant Slices (RS)

H.264 Profiles – Coding parts [1]

H.264 Encoder[9]

H.264 Encoder 4x4 integer DCT: Smaller blocksize leads to a significant reduction in ringing artifacts. Quantization and scan: H.264 standard specifies the math formula for the quantization process. Deblocking filter: To reduce the blocking artifacts in the block boundaries and to stop the propagation of accumulated coded noise. The filtered image is used in motion compensated prediction of future frames and helps achieve more compression. Intra prediction: The encoder derives a predicted block based on its prediction with previously decoded samples (for I frames). Inter prediction: Performed on the basis of temporal correlation and consists of motion estimation and motion compensation. Motion vector resolution is ¼ pel Supports large number of block sizes Multiple reference pictures (upto 32 previously coded frames).

H.264 Decoder [7]

Comparison between H.264 and MPEG-2 Algorithm Characteristic MPEG-2 H.264 General Motion compensated predictive, residual transformed, entropy coded Same basic structure as MPEG Block size 8x8 16x16, 8x16, 16x8, 8x8, 4x8, 8x4, 4x4 Macroblock size 16x16 (frame mode) 16x8 (field mode) 16x16 Intra Prediction None Multi-direction, Multi-pattern Quantization Scalar quantization with step size of constant increment Scalar quantization with step size of increase at the rate of 12.5% Entropy coding VLC CAVLC, CABAC Weighted prediction No Yes

Comparison between H.264 and MPEG-2 (contd.) Algorithm Characteristic MPEG-2 H.264 Reference picture One picture Multiple pictures Motion Estimation Blocks 16x16 16x16, 8x8, 8x4, 4x4 Entropy Coding Multiple VLC Tables Arithmetic Coding and adaptive VLC Tables Frame Distance for Prediction +/- 1 Unlimited forward/backward Fractional Motion Estimation 1/2 Pixel (MPEG2) 1/4 Pixel Deblocking Filter None Dynamic edge filters Scalable coding support [2] Yes, layered picture spatial, SNR, temporal scalability With some support on temporal and SNR scalability Bit rates with same quality HD video with resolution (1920 x 1080) 12 -20 Mbps 7 – 8 Mbps Transmission rate 2 – 15 Mbps 64 kbps – 150 Mbps

Performance comparison between H.264 and MPEG-2 – Simulations Test streams used: Foreman (CIF), News (CIF), Carphone (QCIF) [26]. Codecs used: MPEG-2 [25] and H.264 [24] Profiles for which the simulations were run: Main and Simple/Baseline. Compression ratio = Original file size/Compressed file size Formula to compute PSNR for MPEG-2 (MAXI = 255)

Performance comparison between H.264 and MPEG-2 –Simulation (Foreman) H.264 Main Profile H.264 Main Profile H.264 Main Profile MPEG-2 Main Profile

Performance comparison between H Performance comparison between H.264 and MPEG-2 –Simulation results (Foreman) Parameter MPEG-2 H.264 Input video resolution 352 x 288 (CIF) fps 30 # frames encoded 90 GOP I-P-B-B-P-B-B I-B-B-P-B-B-P PSNR (Y) (dB) 30.42 37.03 PSNR (U) (dB) 39.1 41.08 PSNR (V) (dB) 39.6 43.81 Bit rate (kbits/second) 481.00 481.06 Original file size (.yuv) (MB) 13.3 Compressed file size (KB) 179 172 Compression ratio 74:1 78:1

Performance comparison between H.264 and MPEG-2 –Simulation (News) MPEG-2 Main Profile H.264 Main Profile

Performance comparison between H Performance comparison between H.264 and MPEG-2 –Simulation results (News) Parameter MPEG-2 H.264 Input video resolution 352 x 288 (CIF) fps 30 # frames encoded 90 GOP I-P-B-B-P-B-B I-B-B-P-B-B-P PSNR (Y) (dB) 37.02 39.1 PSNR (U) (dB) 41.0 PSNR (V) (dB) 39.02 42.0 Bit rate (kbits/second) 376.00 Original file size (.yuv) (MB) 13.3 Compressed file size (KB) 141 134 Compression ratio 94:1 99.7:1

Performance comparison between H.264 and MPEG-2 –Simulation (Carphone) MPEG-2 Main Profile H.264 Main Profile

Performance comparison between H Performance comparison between H.264 and MPEG-2 –Simulation results (Carphone) Parameter MPEG-2 H.264 Input video resolution 176 x 144 (QCIF) fps 30 # frames encoded 90 GOP I-P-B-B-P-B-B I-B-B-P-B-B-P PSNR (Y) (dB) 30.46 37.6 PSNR (U) (dB) 36.36 40.9 PSNR (V) (dB) 36.5 41.5 Bit rate (kbits/second) 128 127.6 Original file size (.yuv) (MB) 3.3 Compressed file size (KB) 47.2 45.6 Compression ratio 69.6:1 72.6:1

Conclusions from simulations For the same bit rate and video resolution, the PSNR (dB) values are greater for H.264 encoded videos than for the MPEG-2 encoded videos indicating better video quality. This can be verified from the screen shots. The compression ratio for H.264 encoded video is also better than that for MPEG-2 encoded video inspite of better quality video in H.264.

Transcoding: Introduction Transcoding is the coding and recoding of digital content from one compressed format to another to enable transmission over different media and playback over various devices [29]. High demand for efficient usage of available bandwidth. Interoperability between different networks and devices is gaining importance. Transcoding is a step in this direction. Many legacy systems like digital TVs use MPEG-2. Therefore the need for a transcoding architecture that employs the lower cost of H.264 video and does not require a significant investment in additional video coding architecture.

Transcoding: The criteria To achieve optimum results, the following criteria should be satisfied: The quality of the transcoded bitstream should be comparable to the one obtained by direct decoding and re-encoding of the output stream. The information contained in the input stream should be used as much as possible to avoid multigenerational deterioration. The process should be cost efficient, low in complexity and achieve the highest quality possible.

Transcoding: Architectures [7,15] Open Loop Transcoding Operate in transform domain. Computationally efficient – since they operate directly on DCT coefficients. Suffer from drift problem.

Transcoding: Architectures [7,15] Cascaded Pixel Domain Transcoding Drift-free architecture Concatenation of simple encoder and decoder Reduced complexity since the encoder reuses the motion vectors along with other information extracted from the input bit stream.

Transcoding: Architectures [7,15] Simplified DCT Domain Transcoding Based on the assumption that DCT, IDCT and motion compensation are all linear operations. Less memory required as compared to CPDT. Computationally intensive, since motion compensation is performed in the DCT domain. Due to linearity assumptions, problem of drift may occur.

Transcoding: Architectures [7,15] Cascaded DCT Domain Transcoding Used for spatial and temporal resolution downscaling. Greater flexibility as compared to SDDT. Greater cost and complexity – since additional DCT motion compensation and frame memory is used. Adopted for downscaling operations where the encoder side DCT-MC and memory will not cost much.

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References (contd.) [12] Kunzelmann, P.; Kalva, H., “Reduced Complexity H.264 to MPEG-2 Transcoder”, ICCE 2007, pp. 1-2, January 2007. [13] Kamaci, N., Altunbasak, Y., “Performance Comparison of the Emerging H.264 Video Coding Standard with the existing standards”, ICME, Vol.1, pp. 345-8, July 2003. [14] Jun Xin, Chia-Wen Lin, Ming-Ting Sun , “Digital Video Transcoding”, Proceedings of the IEEE, Vol. 93, Issue 1,pp 84-97, January 2005. [15] A. Vetros, C. Christopoulos and H. Sun, “Video transcoding architectures and techniques: an overview”, IEEE Signal Processing magazine, Vol. 20, Issue 2, pp 18-29,March 2003. [16] “MPEG-2”, Wikipedia, 14 February 2008. Available at <http://en.wikipedia.org/wiki/Mpeg_2> [17] “Introduction to MPEG 2 Video Compression” Available at <http://www.bretl.com/mpeghtml/codecdia1.HTM> [18] “H.264/MPEG-4 AVC”, Wikipedia, 18 February 2008. Available at < http://en.wikipedia.org/wiki/H.264> [19] “H.264 A new Technology for Video Compression” – Available at < http://www.nuntius.com/technology3.html> [20] R Periera, “Efficient transcoding of MPEG-2 to H.264”, Master’s thesis Dec 2005, EE Department, University of Texas at Arlington. [21] “H.262 : Information technology - Generic coding of moving pictures and associated audio information: Video”, International Telecommunication Union, 2000-02. Available at < http://www.itu.int/rec/T-REC-H.262> [22] “MPEG-2 White paper”, Pinnacle Technical Documentation, Version 0.5, Pinnacle Systems, Feb. 29, 2000.

References (contd.) [23] M. Ghanbari, “Standard Codecs : Image Compression to Advanced Video Coding,” Hertz, UK: IEE, 2003. [24] H.264 software (version 13.2) obtained from: <http://iphome.hhi.de/suehring/tml/> [25] MPEG-2 software (version 12) obtained from: <http://www.mpeg.org/MPEG/video/mssg-free-mpeg-software.html> [26] Test streams (Foreman, News, Carphone) obtained from: <http://www-ee.uta.edu/dip/Courses/EE5356/ee_5356.htm> [27] Implementation Studies Group, “Main Results of the AVC Complexity analysis”, MPEG document N4964, ISO/IEC JTC11/SC29/WG11, July 2002. [28] A. Joch et al., “Performance comparison of video coding standards using Lagarangian coder control”, Int. Conf. of Image Processing, Vol. 2, pp. II-501 to II-504, Sept. 2002. [29] I. Sylvester, “Transcoding: The future of the video market depends on it”, IDC Executive Brief, Nov. 2006. Available at < http://www.ed-china.com/ARTICLES/2006NOV/2/2006NOV10_HA_AVC_HN_12.PDF> [30] R. Hoffner, “MPEG-4 Advanced Video Coding emerges”, Available at < http://www.tvtechnology.com/features/Tech-Corner/F_Hoffner-03.09.05.shtml> [31] S. Wagston and A. Susin, “IP core for an H.264 Decoder SoC”, 2007, Available at< www.us.design-reuse.com/news/?id=15746&print=yes> [32] S. Krishnamachari and K. Yang, “MPEG-2 to H.264 Transcoding: Why and How?”, Dec. 1, 2006, Available at < http://broadcastengineering.com/infrastructure/broadcasting_mpeg_transcoding_why/index1.html>