Elsevier Editorial System tm for Journal of Visual Communication and Image Representation Manuscript Draft Manuscript Number Title Efficient transcoding of an MPEG 2 bit stream to an H 264 bit stream Article Type Regular Article Keywords MPEG 2 H 264 video transcoding Corresponding Author Mr Att Kruafak Sc M EE Corresponding Author s Institution The University of Texas at Arlington First Author Rochelle Pereira MS EE Order of Authors Rochelle Pereira MS EE K R Rao PhD Att Kruafak Sc M EE Abstract The objective of this paper is to develop a technique for transcoding from MPEG 2 main profile to H 264 main profile and compare its performance with other transcoding architectures The proposed transcoder reuses information from the MPEG 2 bit stream talking into account the improved techniques such as multiple block size motion estimation in loop deblocking filter intra directional prediction etc adopted in H 264 The proposed method achieves low complexity comparable quality and reduced bit rate in the transcoding process Cover Letter Enclosed is the manuscript Efficient transcoding of an MPEG 2 bit stream to an H 264 bit stream for possible publication in Journal of Visual Communication and Image Representation Please address all your correspondence to me K R Rao Electrical Engineering Department The University of Texas at Arlington 416 Yates Street Box 19016 Arlington Texas 76019 USA Phone 1 817 272 3478 Fax 1 817 272 2253 Email rao uta edu and Att Kruafak Electrical Engineering Department The University of Texas at Arlington 416 Yates Street Box 19016 Arlington Texas 76019 USA Phone 1 682 553 5358 Fax 1 817 272 2253 Email att kruafak uta edu Manuscript E cient transcoding of an MPEG 2 bit stream to an H 264 bit stream Rochelle Pereira a 1 K R Rao b 2 and Att Kruafak b 3 a NVIDIA Corp Santa Clara CA 95050 USA b Electrical Engineering Department The University of Texas at Arlington 416 Yates Street Box 19016 Arlington Texas 76019 USA Abstract The objective of this paper is to develop a technique for transcoding from MPEG 2 main pro le to H 264 main pro le and compare its performance with other transcoding architectures The proposed transcoder reuses information from the MPEG 2 bit stream talking into account the improved techniques such as multiple block size motion estimation in loop deblocking lter intra directional prediction etc adopted in H 264 The proposed method achieves low complexity comparable quality and reduced bit rate in the transcoding process Key words MPEG 2 H 264 video transcoding PACS 1 Introduction MPEG 2 1 2 has been a widely accepted video coding standard for various applications ranging from DVD to digital TV broadcast A large variety of products based on the MPEG 2 standard are available in the market The most important goal of MPEG 2 is to make the storage and transmission of digital AV material more e cient The latest standard H 264 AVC 3 5 has an even broader perspective to support high and low bit rate multimedia applications on existing and future networks The advantage in terms of better quality at a lower bit rate is why H 264 is fast replacing MPEG 2 6 However 1 Email address rpereira81 yahoo com Email address rao uta edu Phone 1 817 272 3478 Fax 1 817 272 2253 URL http www ee uta edu dip 3 Email address att kruafak uta edu 2 Preprint submitted to Elsevier Science 23 February 2007 the user end hardware such as set top boxes DVD players had previously been adapted for MPEG 2 coded bit streams This gives rise to a need for portability between MPEG 2 and H 264 Video transcoding is the operation of converting video from one format to another 7 10 A format is de ned by characteristics such as bit rate spatial resolution etc One of the earliest applications of transcoding is to adapt the bit rate of a compressed stream to the channel bandwidth for universal multimedia access in various channels like wireless networks Internet dialup networks etc Changes in the characteristics of an encoded bit stream like bit rate spatial resolution quality etc can also be achieved by scalable video coding 8 However in cases where the available network bandwidth is insu cient or if it uctuates with time it may be di cult to set the base layer bit rate In addition scalable video coding demands additional complexities at both the encoder and the decoder 2 Transcoding Architectures The basic architecture for converting an MPEG 2 elementary stream into an H 264 elementary stream arises from complete decoding of the MPEG stream and then re encoding into an H 264 stream However this involves signi cant computational complexity 7 Hence there also is a need to transcode at low complexity Transcoding can in general be implemented in the spatial domain or in the transform domain or in a combination of the two domains Several transcoding architectures have been proposed earlier i e 2 1 Open Loop Transform Domain Transcoding Fig 1 8 Open loop transcoders are computationally e cient They operate in the DCT domain However they are subject to drift error Drift error occurs due to rounding quantization loss and clipping functions 2 2 Cascaded Pixel Domain Architecture CPDT Fig 2 8 This is the most basic transcoding architecture The motion vectors from the incoming bit stream are extracted and reused Thus the complexity of the motion estimation block is eliminated which accounts for 60 of the encoder computation 11 As compared to the previous architecture CPDT is drift 2 Fig 1 Open loop transform domain transcoder architecture Fig 2 Cascaded pixel domain transcoder architecture free Hence even though it is slightly more complex it is suitable for heterogeneous transcoding among di erent standards where the basic parameters like mode decisions motion vectors etc are to be re derived 3 Fig 3 Simpli ed transform domain transcoder architecture Fig 4 Transform domain motion compensation illustration 2 3 Simplified DCT Domain Transcoders SDDT Fig 3 8 This transcoder is based on the assumption that DCT IDCT and motion compensation are linear processes This architecture requires that motion compensation be performed in the DCT domain which is a major computationally intensive operation 12 For instance as shown in Fig 4 the goal is trying to compute the DCT coe cients of the target block B from the four overlapping 4 Fig 5 Cascaded transform domain transcoder architecture blocks B1 B2 B3 and B4 Also clipping functions and rounding operations performed for interpolation in fractional pixel motion
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