1. Standards Presentation ECE 8873 – Data Compression and Modeling
Video Compression H.261
Standards Presentation
Qaiser-Ahmed Patel
ECE 8873 – Data Compression and Modeling
Spring 2004

2. Contents H.261 Overview Motivation Features of H.261 H.261 Coder
Summary
References

3. H.261 Overview
An ITU-T H-series standard applicable to videophone or video conferencing
Video coding algorithm is designed for transport using the Real-time Transport Protocol (RTP)
Operates in real-time with limited delay
Transmission bit rate is at multiples of 64Kbit/s
ITU-T (International Telecom Union) Recommendation in 1990
Precursor to coding schemes found in H.263 and MPEG-1

4. H.261 (p x 64) Motivation Uncompressed video and audio data are huge
Compression ratio of lossless methods is not high enough
Target networks are p * 64Kbps, 1p30
64Kbps (p=1) < data rate < 1920Kbps (p=30)
Covers transmission from ISDN base rate (64 Kbps) up past
the T-1 data rate (1.54 Mbps)
Maximum delay of 150 ms
Coding algorithm is a hybrid of:
Inter-picture prediction - removes temporal redundancy
Transform coding - removes the spatial redundancy
Motion compensation – uses motion vectors to help the
codec compensate for motion
Data rate can be set between 40 Kbit/s and 2 Mbit/s
Input signal format
CIF (Common Intermediate Format) and QCIF (Quarter CIF)
available

5. Features of H.261 – Source format
Bit rate
The target bit rate is ~ 64Kbps to 1920Kbps
Picture formats
CIF (Common Intermediate Format) – NTSC & PAL
QCIF (Quarter Common Intermediate Format)
At frames per second with 4:2:0 chrominance sub-sampling (Y:CB:CR)

6. Features of H.261 – Video multiplex arrangement
Group of blocks structure
Picture – coded as luminance and two color difference components (Y, CB and CR)
Group of blocks (GOB)
MacroBlock (MB)
Block

7. H.261 - Video multiplex arrangement
4 layers in the compressed stream
Picture, GOB (Group Of Block), MB (MacroBlock), Block 11 3
GOB
QCIF
CIF MB Cb Cr Y

8. H.261 Picture Syntax (1/4)

9. H.261 Picture Syntax (2/4)

10. H.261 Picture Syntax (3/4)

11. H.261 Picture Syntax (4/4)

12. H.261 Coding Frame Types Intra-Encoded Frames (I-Frames)
Similar to JPEG compression
Spatial filtering – still objects (transform coding)
Ex.: Black and White background pattern
Predicted Frames (P-Frame)
Predicted based on earlier frame
Temporal filtering – Inter-picture prediction
Ex.: Application User

13. H.261 - Motion Compensation
Assumption
As parts of an image moves, its colors stays mostly constant
Idea
Find similar parts in other images
Encode where it was found (i.e. motion vector)
Previous decoded image – Reference image
Image to code – Target Image
Encode the residual only

14. H.261 Coder Outline block diagram for H.261 codec External control
Coding control
Source coder
Video multiplex
coder
Transmission
buffer
Video
Signal
Coded
bitstream
a) Video coder
Source decoder
Video multiplex
decoder
Receiving
buffer
b) Video decoder

15. H.261 coder structure (1/2) Encoder Coding control Mode Intra/inter Qp
DCT Q
VLC
Video out -
Q-1
Video in
DCT-1 + MC
Frame
buffer MV ME

16. H.261 coder structure (2/2) Decoder Step size Error Correction Buffer
VLC
Decoder
Q-1
Intra/inter
IDCT
Input data
Motion Vector +
Decoded data MC
Frame
Memory

17. Discrete Cosine Transformation (DCT)
Overview: D DCT basis functions:
FFT vs. DCT:
Like FFT, but
approximates
lines with fewer
coefficients

18. Discrete Cosine Transformation (DCT) Computation
Factoring reduces transformation to a series of 1-Dimensional DCTs

19. Demo
Miss America (QCIF)

20. Summary H.261 Techniques used: Used in video/teleconferencing
Two dimensional (2-D) 8 X 8 DCT to remove intra-frame correlation
Zig-zag order to scan the transform coefficients
Run Length coding for zero-valued coefficients after quantization
Motion estimation is applied to video sequence to improve the prediction between successive frames
Transmission rates control in the range of p X 64 Kbps
Error resilience including synchronization and concealment technique required in transmission code, to cover up channel errors
Common Intermediate Format (CIF) and Quarter CIF (QCIF) for a single solution to different video formats (NTSC / PAL)

21. References
Video codec for audiovisual services at p x 64 kbit/s ITU-T (International Telecommunication Union – Telecommunication Standardization Sector) Recommendation H.261, 1993
Stephen J. Solari, Digital Video and Audio Compression, McGraw-Hill Professional; (March 1, 1997)
K. Rijkse, “H.261 : Video Coding for Low-Bit-Rate Communication”, IEEE Communications Magazine, pp. 42 – 45
H.261 Video Coding