1. 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

2. Outline Brief specification Source coder Video multiplex coder
Transmission coder
Conclusion

3. 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 .

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

5. 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

6. 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

7. - 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

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

9. 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

10. 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 .

11. 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 .

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

13. 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)

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

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

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

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

18. Video multiplex coder(cont.)
Picture layer

19. Video multiplex coder(cont.)
GOB layer

20. Video multiplex coder(cont.)
MB layer

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

22. Video multiplex coder(cont.)
Coefficients reconstruction formula
(REC are in the range 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;

23. Transmission coder
Forward error correction

24. 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

25. Compare with H.263
Source format

26. 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])

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

28. 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

29. 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

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