EE 5359 PROPOSAL H 264 to VC 1 TRANSCODING Vidhya Vijayakumar Student I D 1000 622152 Date September 24 2009 1 H 264 to VC 1 TRANSCODER OBJECTIVE The objective of the thesis is to implement a H 264 bitstream to VC 1 transcoder for progressive compression MOTIVATION The high definition video adoption has been growing rapidly for the last five years The high definition DVD format Blue ray has mandated MPEG 2 3 H 264 2 and VC 1 1 as video compression formats The coexistence of these different video coding standards creates a need for transcoding As more and more end products use the above standards transcoding from one format to another adds value to the product s capability While there has been recent work on MPEG 2 to H 264 transcoding 3 VC 1 to H 264 transcoding 4 the published work on H 264 to VC 1 transcoding is nearly non existent This has created the motivation to develop a transcoder that can efficiently transcode a H 264 bitstream to a VC 1 bitstream DETAILS Video transcoding is the operation of converting video from one format to another 5 A format is defined 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 all kinds of channels like wireless networks Internet dial up networks etc Changes in the characteristics of an encoded stream like bit rate spatial resolution quality etc can also be achieved by scalable video coding 5 However in cases where the available network bandwidth is insufficient or if it fluctuates with time it may be difficult to set the base layer bit rate In addition scalable video coding demands additional complexities at both the encoder and the decoder The basic architecture for converting an H 264 bitstream into a VC 1 elementary stream arises from complete decoding of the H 264 stream and then reencoding into a VC 1 stream However this involves significant computational complexity 6 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 The common transcoding architectures 5 are Open loop transform domain transcoding 2 Fig 1 Open loop transform domain transcoder architecture 5 Open loop transcoders are computationally efficient Fig 1 They operate in the DCT domain However they are subject to drift error Drift error occurs due to rounding quantization loss and clipping functions Cascaded Pixel Domain Architecture CPDT Fig 2 Cascaded pixel domain transcoder architecture 5 This is the most basic transcoding architecture Fig 2 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 As compared to the previous architecture CPDT is drift free Hence even though it is slightly more complex it is suited for heterogeneous transcoding between different standards where the basic parameters like mode decisions motion vectors etc are to be re derived Simplified DCT Domain transcoders SDDT Fig 3 Simplified transform domain transcoder architecture 5 3 This transcoder is based on the assumption that DCT IDCT and motion compensation are linear processes Fig 3 This architecture requires that motion compensation be performed in the DCT domain which is a major computationally intensive operation 3 For instance as shown in the figure 4 the goal is trying to compute the DCT coefficients of the target block B from the four overlapping blocks B1 B2 B3 and B4 Fig 4 Transform domain motion compensation illustration 5 Also clipping functions and rounding operations performed for interpolation in fractional pixel motion compensation lead to a drift in the transcoded video Cascaded DCT Domain transcoders CDDT Fig 5 Cascaded transform domain transcoder architecture 5 This is used for spatial temporal resolution downscaling and other coding parameter changes Fig 5 As compared with SDDT greater flexibility is achieved by introducing another transform domain motion compensation block however it is far more computationally intensive and requires more memory 3 It is often applied to downscaling applications where the encoder end memory will not cost much due to downscaled resolution 4 Choice of basic transcoder architecture DCT domain transcoders have the main drawback that motion compensation in transform domain is very computationally intensive DCT domain transcoders are also less flexible as compared to pixel domain transcoders for instance the SDDT architecture can only be used for bit rate reduction transcoding It assumes that the spatial and temporal resolutions stay the same and that the output video uses the same frame types mode decisions and motion vectors as the input video For H 264 to VC 1 transcoding it is required to implement several changes in order to accommodate the mismatches between the two standards For instance for motion estimation and compensation H 264 supports 16x16 16x8 8x16 8x8 8x4 4x8 4x4 macroblock partitions Fig 6 but VC 1 supports 16x16 and 8x8 only Fig 7 The transform size and type 8x8 and 4x4 in H 264 and 8x8 4x8 8x4 and 4x4 in VC 1 are different and make transform domain transcoding prohibitively complex Hence the use of DCT domain transcoders is not very ideal Fig 6 Segmentations of the macroblock for motion compensation in H 264 Top segmentation of macroblocks bottom segmentation of 8x8 partitions 2 Fig 7 Segmentations of the macroblock for motion compensation in VC 1 2 From Fig 8 it can be inferred that the cascaded pixel domain architecture outperforms the DCT domain transcoders Also for larger GOP sizes the drift in DCT domain transcoders becomes more significant 5 Fig 8 PSNR vs Bit rate graph for the Foreman sequence transcoded with a GOP size 15 using different transcoding architectures as described in Figs 1 2 3 and 5 5 Hence heterogeneous transcoding in the pixel domain is preferred for standards transcoding Standards transcoding When transcoding between two different standards the main factor involved is compatibility between the profile and level of the input stream and that of the output stream for a specific purpose The goal here is to transcode a H 264 bitstream of Baseline profile to VC 1 bit stream of Simple profile The table 1 compares and contrasts the characteristics of both standards Chroma Format Picture
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