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KwangWoon University Multimedia Lab. Seoung-Jun Oh sjoh@media.kwangwoon.ac.kr

INTERNATIONAL STANDARDS FOR VIDEO COMPRESSION: H.261. KwangWoon University Multimedia Lab. Seoung-Jun Oh sjoh@media.kwangwoon.ac.kr. 목 차. 개 요 The ITU-T Rec. H.261 Summary. 개 요. Why Standard for Video Compression? Reduce the high cost of video compression codecs.

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KwangWoon University Multimedia Lab. Seoung-Jun Oh sjoh@media.kwangwoon.ac.kr

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  1. INTERNATIONAL STANDARDSFOR VIDEO COMPRESSION:H.261 KwangWoon University Multimedia Lab. Seoung-Jun Oh sjoh@media.kwangwoon.ac.kr

  2. 목 차 • 개 요 • The ITU-T Rec. H.261 • Summary

  3. 개 요 • Why Standard for Video Compression? • Reduce the high cost of video compression codecs. • Resolve the critical problem of inter-operability of equipments from different manufacturers. • What standards for Video Compression? • ITU[T Rec. H.261 or p x 64 Video Codec for Audiovisual Services at p x 64 Kbits/s CCITT Study Group XV, August 1990 • JPEG (Joint Photographic Experts Group) Continuous-Tone Still Picture Compression Standard ISO/IEC JTC1/SC2/WG8 CCITT SGVIII, August 1990 • MPEG (Motion Picture Experts Group) Coding of Motion Pictures and Associated Audio for Digital Storage Media at up to about 1.5Mbits/s ISO/IEC JTC1/SC2/WG11, December 1990

  4. Major Organizations • CCIR [ International Radio Consultative Committee]: ITU-R • Defining the format of television system and the recoding requested for studio applications. • CCIR 601 : recommend for digital video (e.g., 4:2:2 720 pels/line) • CCITT [ International Telegraph and Telephone Consultative Committee ]: ITU-T • Defining the coding required for the provision (visual telephony and videoconference) • CMTT [ Committee for Mixes Telephone and Television, Joint CCIR/CCITT ] • Defining the coding required for transmitting television over the telecommunication networks for distribution purpose. • IEC [ International Electrotechnical Commission ] • Defining the interface and the recording.

  5. The CCITT Rec. H.261 • The CCITT Study Group XV • Need for providing ubiquitous video services for ISDN • Group on Coding for Visual Telephony in 1984 Recommend a video coding standard for transmission at m x 384 kbps, m = 1, 2, …, 5. • Finally, CCITT Rec. H.261, at p x 64 Kbit/s, p = 1, …, 30. • Scope of Rec. H.261 • Describes the video coding and decoding methods for the moving picture component of AV services at p x 64 kbps, p = 1, …, 30.

  6. 352 pels/line Y 288 lines /frame 176 176 Cb Cr 144 144 176 88 88 Y 144 Cr 72 72 Cb H.261 : Video Format • Non-Interlaced Pictures occurring 29.97 fps • CIF ( Common Intermediate Format ) : Optional • CIF was chosen because it can be easily converted to and from both NTSC ans PAL formats to make use of readily available cameras and monitors. • QCIF ( Quarter CIF ) : Mandatory • Every 1, 2, or 3 frames may be dropped at the encoder.

  7. x x x x x x x x o o o o x x x x x x x x x x x x x x x x o o o o x x x x x x x x x x x x x x x x o o o o x x x x x x x x H.261 : Block Structure • Block : 8 x 8 pixels : Unit of DCT/IDCT operation • Macro Block (MB) : 4 Y blocks + 2 chro. ( Cb and Cr ) blocks • The chrominance blocks have reduced resolution because the human visual system is less sensitive to color detail than luminance detail. • Group of Blocks (GOB) : 3 x 11 MB’s • Picture : 3 GOBs / QCIF , 12 GOBs / CIF x : Luminance sample o : Chrominance Samples : Block Boundary

  8. H.261 : Block Arrangement • Block ( 8 x 8 ) • MB ( 16 x 16 ) • GOB ( 48 x 176 ) • QCIF ( 144 x 176 ) • CIF ( 288 x 356 ) 4Y + Cb + Cr 3 x 11 MB’s 3 GOB’s 12 GOB’s

  9. H.261 : Data Rate • Raw Bit Rate • CIF : 352 x 288 x 8 x 29.97 x 1.5 ( 4:2:0 ) = 36.46 Mbps • QCIF : 176 x 144 x 8 x 29.97 x 1.5 ( 4:2:0 ) = 9.115 Mbps • Video Transmission Rate • Between approximately 40 kbps and 2 Mbps • p = 1 ( 64 kbps ) : QCIF, 10 frames/s ( 47.52 : 1 compression ) • p > 6: CIF may be used • Symmetry of Transmission • Operating in real time with minimum delay for bi-directional and unidirectional visual communication. • Error Handling : BCH ( 511,493 ) FEC code ( optional )

  10. H.261 : Video Encoder • Hybrid Coder : Transform Coding + Temporal Prediction Coding Step Size Video In DCT Q VLC 0 1 Buffer - Error Q-1 Intra/Inter Mode Decision IDCT 0 Motion Vector 0 1 + Motion Comp Frame Memory Loop Filter Prediction Motion Estm

  11. H.261 : Motion Compensation • H.261 doesn’t specify when & how to perform MC in the encoder. • The decoder will accept 1 vector set / MB. • Integer value MV’s ( not exceeding ± 15) • MV for color difference blocks is derived by halving the component values of the MB motion vector and truncation the magnitude parts toward zero to yield integer components. • When MC is performed, the loop filter is normally activated to remove the blocking effects associated with MC • The filter is separable and non-recursive with coefficients of 1/4, 1/2, 1/4 except at block boundaries. • The 3 x 3 loop filter processes the data in each 8 x 8 block without overlap between blocks.

  12. Inter VAROR Intra ths MSE H.261 : Inter-Intra Decision • The inter-intra mode decision is made for each MB on the basis of the energies of the Y prediction error and the original Y signal. • H.261 does not specify how this decision is made. • In general, inter-mode operation is preferred since the DC value of intra-coded blocks are not transmitted when all quantized DCT coefficient are zero. • Typical example :

  13. u i v j Transform Domain (8 x 8) Spatial Domain (8 x 8) H.261 : DCT Algorithm • Separable 2-D DCT of size 8 x 8

  14. H.261 : Quantization and Coding • Q step size • the same for all coefficients within a MB, but can be changed for each MB. • Controlled by the amount of space left in the buffer. • The quantized coefficients are zig-zag scanned. • Run-Length Coding (RLC) • characterize the quantized DCT coeff’s by the non-zero values • # of 0’s ( RUN ) between these non-zero values ( LEVEL ). • [RUN, LEVEL] is variable length coded & combined with the header information ( Q step size, mode, MVs ). • The synchronization & check bits are added • BCH ( 511, 493 ) is used for error correction.

  15. Step Size Inter-Intra Mode Select VLC Buffer Q-1 IDCT 0 Decoder Data 0 1 + Motion Comp Frame Memory Loop Filter Prediction Motion Vector H.261 : Video Decoder • Inverse Operation of the Encoder

  16. H.261 : 요약 • CCITT Rec. H.261 - Standard for Video Teleconferencing • p x 64 Kbit/s ( p = 1, …, 30 ) : 64 Kbit/s ~ 2 Mbit/s • Non-Interlaced Video Signals at 29.97 Pictures/s • CIF optional, QCIF mandatory, ITU-R 4:2:0 format • Motion Compensated DCT-Based Entropy Coding • Separable and Non-Recursive Loop Filter • Shaped the MPEG Standard

  17. References 1. CCITT Recommendation H.261 - Video Codec for Audiovisual Services at p x 64 Kbit/s, August 1990. 2. JPEG Technical Specification, Revision 8, August 1990. 3. Digital Compression and Coding of Continuous-tone Still Images, ISO/IEC JTC1 Committee Draft 10918, 1991. 4. Coding of Moving Pictures and Associated Audio, MPEG Draft, May 1991. 5. Communications of the ACM, vol. 34, No. 4, April 1991. 6. Liou, M.L., Overview of the px64 Kbps Video Coding Standard, Commun. ACM, April 1991. 7. Wallace, G.L., The JPEG Still Picture Compression Standard, Commun. ACM, April 1991. 8. Le Gall, D.J., MPEG : A Video Compression Standard for Multimedia Applications, Commun. ACM, April 1991.

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