Present by 楊信弘 Advisor: 鄭芳炫 Video Codec for Audiovisual Services at p x 64 kbit/s ITU-T Recommendation H.261 Present by 楊信弘 Advisor: 鄭芳炫

Outline Brief specification Source coder Video multiplex coder Transmission coder Conclusion

Brief specification Video input and output: Source coding algorithm: based on a common intermediate format(CIF). Source coding algorithm: A hybird of inter-picture prediction and transform coding is adopted. Bit rate: between approximately 40 kbit/s and 2 Mbit/s .

Brief specification (cont.) Error handling: The transmitted bit-stream contains a BCH code(511,493)forward error correction code .

Source coder Motin estimation and compensation Motin compensation Encoder Decoder Motin estimation and compensation Motin compensation Discrete cosine transform Inverse Discrete cosine transform Quantizer Inverse Quantizer Coding Model Decoding Model Entropy coder Entropy decoder Transmission

Source coder (cont.) External control Coding control Source coder Video multiplex coder Transmission buffer Transmission coder Video signal Coded bit stream (a) Video coder Source decoder Video multiplex decoder Receiving buffer Receiving decoder (b) Video decoder

- Source coder (cont.) + CC T Q Q-1 T-1 F P p t qz Video in q To video multiplex coder Q-1 T-1 + F P v f

Source coder (cont.) Source format: pictures are coded as luminance and two colour components (Y, Cb and Cr ) . input values of 1 through 254 .

Source coder (cont.) Two picture scanning format CIF: quarter-CIF: luminance sampling structure: 352x288 chrominance sampling structure: 176x144 quarter-CIF: luminance sampling structure: 176x144 chrominance sampling structure: 88x72

Source coder (cont.) Video source coding algorithm The main elements are prediction, block transformation and quantization . The prediction error or input picture is subdivided into 8x8 blocks . Further, four luminance blocks and two colour difference blocks are combined to form a macroblock .

Source coder (cont.) Prediction Motion compensation The prediction is inter-picture and may be augmented by motion compensation and a spatial filter . Motion compensation Both horizontal and vertical components of these motion vectors have integer values not exceeding +15~ -15 .

Source coder (cont.) Transformer (DCT, IDCT)

Source coder (cont.) Quantization INTRA dc coefficient -> number of quantizers=1 (linearly quantized , stepsize=8) All other cofficient -> number of quantizers=31 (linearly quantized , stepsize=an even value in the range 2 to 62)

Video multiplex coder A hierarchical structure with four layers picture; Group of block (GOB); Macroblock (MB); Block .

Video multiplex coder(cont.) H.261 image structure GOB organization within a encoded bit-stream

Video multiplex coder(cont.) Arrangement of GOBs (176x48)in a picture Arrangement of macroblocks (16x16) in a GOB

Video multiplex coder(cont.) Arrangement of blocks in a macroblock

Video multiplex coder(cont.) Picture layer

Video multiplex coder(cont.) GOB layer

Video multiplex coder(cont.) MB layer

Video multiplex coder(cont.) Block layer Transmission order for transform coefficients

Video multiplex coder(cont.) Coefficients reconstruction formula (REC are in the range -2048 to 2047) (level are in the range -127 to 127) REC=QUANT * ( 2*level+1 ); level>0 REC=QUANT * ( 2*level -1 ); level<0 QUANT=“odd” REC=QUANT * ( 2*level+1 ) - 1; level>0 REC=QUANT * ( 2*level -1 ) +1; level<0 QUANT=“even” REC=0; level=0;

Transmission coder Forward error correction

Telecomms-related Multimedia Standards H.320 N-ISDN Videoconference and Videophone system H.321 B-ISDN(ATM) Videoconference and Videophone system H.322 LAN Videoconference and Videophone system H.324 PSTN/Mobile Video(tele)phone system

Compare with H.263 Source format

Compare with H.263 (cont.) Motion compensation Motion vectors have only integer values in H.261 But have integer or half integer values in H.263 Advanced Prediction mode There are one or four vectors per macroblock . Unrestriction Motion Vector mode The maximum range of vector components is [-31.5, 31.5](with restriction are [-16,15.5])

Compare with H.263 (cont.) PB-frames mode

Compare with H.263 (cont.) Loop filter VLC table Multiplex coding Loop filter(Low-pass filter) is no more adopted in H.263 VLC table Multiplex coding

Conclusion H.261 over ISDN BRI (2B+D) one B channel，16K for Audio，48k for Video two B channel，64K for Audio，64k for Video two B channel，16K for Audio，112k for Video H.261 is suitable for visual services over ISDN H.263 is suitable for visual services over PSTN or Mobile

Conclusion (cont.) QCIF, sub-QCIF CIF 4CIF, 16CIF Videophone , very low bit rate videoconference CIF Videoconference 4CIF, 16CIF HDTV, Video on Demand, etc.