Video Compression

H.261

Standards Presentation Qaiser-Ahmed Patel ECE 8873 – Data Compression and Modeling Spring 2004

Contents       H.261 Overview Motivation Features of H.261

H.261 Coder Summary References

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

   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

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 29.97 frames per second with 4:2:0 chrominance sub sampling (Y:C B :C R )

Features of H.261 – Video multiplex arrangement  Group of blocks structure     Picture – coded as luminance and two color difference components (Y, C B and C R ) Group of blocks (GOB) MacroBlock (MB) Block

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

H.261 Picture Syntax (1/4)

H.261 Picture Syntax (2/4)

H.261 Picture Syntax (3/4)

H.261 Picture Syntax (4/4)

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

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

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

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

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

Discrete Cosine Transformation (DCT)  Overview: 64 2-D DCT basis functions:  FFT vs. DCT: Like FFT, but approximates lines with fewer coefficients

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

Demo  Miss America (QCIF)

Summary   H.261

 Used in video/teleconferencing Techniques used:        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)

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 http://www-mobile.ecs.soton.ac.uk/peter/h261/h261.html