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Transcoding of H.264 bitstream to MPEG-2 bitstream. Sreejana Sharma. What is transcoding?. The process of converting video from one format to another . Transcoding is basically undertaken to meet the constrained transmission bandwidths and terminal capabilities.

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what is transcoding
What is transcoding?
  • The process of converting video from one format to another .
  • Transcoding is basically undertaken to meet the constrained transmission bandwidths and terminal capabilities.
  • Need: To facilitate interoperability between different networks, devices and content representation formats.
applications of transcoding
Applications of transcoding.
  • Adapt the bit rate of a precompressed video stream to the channel bandwidth.
  • To dynamically adapt the multimedia content to the usage environment.
  • To insert new information into a compressed video stream.
  • Also help support VCR trick modes for on-demand video applications.
criteria to be fulfilled for efficient transcoding
Criteria to be fulfilled for efficient transcoding.
  • The quality of the transcoded bitstream should be comparable to the one obtained by direct encoding and decoding 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.
need for h 264 to mpeg 2 transcoding
Need for H.264 to MPEG-2 transcoding
  • H.264 is fast becoming the codec of the future as it can produce MPEG-2 comparable quality at almost half the bandwidth.
  • Widespread adaptation to H.264 is anticipated.
  • However many existing systems such as home receivers and digital TVs use MPEG-2.
  • This motivates the need for an architecture that efficiently leverages the lower cost of H.264 without significant investment in additional video coding hardware.
h 264 avc to mpeg 2 transcoding
H.264/AVC to MPEG 2 Transcoding.
  • Intuitive method is to re-encode the H.264 video with MPEG-2 encoder and then decode with MPEG-2 decoder.
  • However this process is computationally intensive and high in complexity.
  • Solution is to extract various parameters from the encoded H.264 bitstream and decode them using MPEG-2 standards.
h 264 and mpeg 2
Baseline Profile at Level 3

4:2:0 Chroma format

Picture coding type I,P

One reference frame enabled for inter prediction.

Simple Profile Main Level

4:2:0 Chroma format

Picture coding type I,P

One reference frame enabled for inter prediction.

H.264 and MPEG-2.
procedure
Procedure
  • The input sequence is passed through an H.264 encoder and decoder.
  • The output of H.264 is then passed through an MPEG-2 encoder and decoder to get the final output.
quality comparison
Quality Comparison

H.264 input streamMPEG-2 clip obtained by MPEG-2 stream obtained by

30 fpsencoding decoding. Transcoding.

352x288

procedure1
Procedure
  • Motion vector extraction
  • Motion vector resampling.
  • Motion vector refinement.
  • Motion vector reuse.
motion vector extraction
Motion vector extraction
  • Motion vectors are extracted from the H.264 after the variable length decoding stage.
need for motion vector resampling
Need for motion vector resampling.
  • H.264 can have up to 16 motion vectors for a single 16x16 macroblock.
  • MPEG-2 supports only one motion vector per 16x16 macroblock.
  • Need to generate MPEG-2 compliant motion vector with good coding efficiency.
motion vector resampling
Motion vector resampling
  • Only one reference frame used in H.264 encoding stage.
  • Every motion vector from H.264 is scaled according to the area of the block.
  • If more than one motion vector is present for a 16x16 macroblock, the scaled motion vectors are summed to get the final MPEG-2 compliant motion vector.
algorithm
Algorithm
  • For example:
  • If the macroblock is divided into two blocks of 16x8 then a single macroblock is arrived as follows:
  • MPEG-2mv_x= (H.264mv_ax +H.264mv_ bx )/2 MPEG-2mv_y= (H.264mv_ay +H.264mv_by)
algorithm1
Algorithm
  • If the MB is divided into 8x8 blocks.
  • Each block can have sub partitions and up to 16 motion vectors.
algorithm2
Algorithm
  • Derive MV for each 8x8 sub-block.
  • If the first 8x8 block is partitioned into four 4x4 blocks:
  • mv_x0 = ¼(mv_x00+mv_x01+mv_x02+mv_x03)
  • mv_y0 = ¼ (mv_y00+mv_y01+mv_y02+mv_y03)
  • The final MPEG-2 compatible motion vectors is calculated as follows:
  • MPEG-2mv_x=¼(mv_x0+mv_x1+mv_x2+mv_x3)
  • MPEG-2mv_y=¼(mv_y0+mv_y1+mv_y2+mv_y3)
need for motion vector refinement
Need for motion vector refinement
  • The quantization parameters of the incoming bit stream and those selected may differ. When these differences are large it results in quality degradation.
  • H.264 uses variable block size motion compensation modes whereas MPEG-2 uses a full search algorithm for the entire 16x16 block.
  • Achieve compatibility between 1/4 pel MV accuracy in H.264 and 1/2 pel MV accuracy in MPEG-2.
motion vector refinement scheme
Motion vector refinement scheme.
  • Search window of -2 pixels to +2 pixels
  • Horizontal and vertical search method.
  • Instead of checking all points in the search window, only points in the horizontal and vertical direction are checked for the minimum SAD .
motion vectors in p frame
Motion vectors in P frame

H.264 input stream MPEG-2 bitstream obtained by MPEG-2 bitstream obtained by

30fps proposed method. Encoding and decoding in

352x288 1 Mbps MPEG-2 at 1 Mbps.

quality comparison1
Quality comparison
  • MPEG-2 encoding.
  • Proposed method.
conclusions
Conclusions
  • Information of the incoming stream is used as much as possible.
  • Low in complexity.
  • Fairly acceptable loss in PSNR.
  • All the expectations for a good transcoder are met.
future research
Future Research
  • This transcoder can be applied on PVR or multimedia server to play the delivered H.264 content in the existing MPEG-2 equipment.
  • This thesis is based on transcoding of H.264 from baseline profile to MPEG-2 simple profile with no B frames.
  • The same process could be extended to transcoding from the main profile of H.264 to main profile of MPEG-2 with the presence of B frames.
references
[1] Jun Xin, Chia-Wen Lin, Ming-Ting Sun , “Digital Video Transcoding” , Proceedings of the IEEE, Vol. 93, Issue 1,pp 84-97, January 2005.

[2] Jing Wang et. al.,“An AVS to MPEG-2 Transcoding System”, Proceeding of 2004 International Symposium on Intelligent Multimedia, Video and Speech Processing, October 20-22, 2004 Hong Kong.

[3] 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.

[4] Soon-kak Kwon, A. Tamhankar and K.R. Rao, “Overview of H.264 / MPEG-4 Part 10 (pp.186-216)”, Special issue on “ Emerging H.264/AVC video coding standard”, J. Visual Communication and Image Representation, vol. 17, pp.183-552, April 2006.

[5] J. Youn and Ming-Ting Sun , “Motion Vector Refinement for high-performance transcoding”, in IEEE Int. Conf. Consumer Electronics, Los Angeles, C.A., Vol. 1, Issue 1, pp 30-40,March 1999.

[6] Hari Kalva, “Issues in H.264/MPEG-2 Video Transcoding”, Computer Science and Engineering, Florida Atlantic University, Boca Raton, FL.

[7] B. Haskell, A. Puri and A. Netravali, “Digital Video: an introduction to MPEG-2”, Chapman and Hall, 1997.

[8] MPEG-2 software (version 12) from MPEG software simulation group http://www.mpeg.org/MPEG/MSSG/#source.

[9] H.264 software JM (10.2) from: http://iphome.hhi.de/suehring/tml/download/

[10] T. Wiegand et. al., “Overview of the H.264/AVC Video Coding Standard”, IEEE Transactions on Circuits and Systems for Video Technology, Vol. 13, Issue 7, July 2003.

[11] P.N.Tudor, “ Tutorial on MPEG-2 Video Compression”, IEE J Langham Thomson Prize, Electronics and Communication Engineering Journal, December 1995.

References
references1
References

[12] Iain E.G.Richardson, “H.264 and MPEG-4 Video Coding for Next Generation Multimedia”, Wiley 2003

[13] P Kunzelman and H.Kalva, “Reduced complexity H.264 to MPEG-2 transcoder”, ICEE paper 10/02/2006.

[14] J Chu, W Lu, Y Liu, Y Song, X Song, S Yu, “H.264/MPEG-2 Transcoding based on Personal Video Recorder Platform”, IEEE.

[15] Test streams obtained from: http://www.cipr.rpi.edu/resource/sequences/sif.html

[16] Commercially available transcoders, PSP Video 9, http://www.pspvideo9.com

[17] J McVeigh et. al., “A software based real time MPRG-2 video encoder”, IEEE Trans. CSVT, Vol 10, pp 1178-1184, Oct.2000.

[18] Information Technology – Generic coding of moving pictures and associated audio information: Video, ITU-T Rec H.262 (2000 E).

[19] T Shanableh and M Ghanbari, “Transcoding architectures for DCT domain heterogenous video transcoding”, Proc.IEEE ICIP, Vol.1 pp 433-436, Thessaloniki, Greece, Sept 2001.

[20] R Periera, “Efficient transcoding of MPEG-2 to H.264”, Master’s thesis DEC 2005, EE Department, U.T.A.

[21] ITU-T, “Advanced video coding for generic audio visual services”, H.264, March 2005

[22] K.R.Rao and J.J.Hwang, “Techniques and standards for Image, Video and Audio Coding”, Prentice Hall

[23] I.E.G Richardson, “Video Codec Design: developing image and video compression systems”, Chichester: Wiley, 2002.