H261
H261
H261
H.261
Standards Presentation
Qaiser-Ahmed Patel ECE 8873 Data Compression and Modeling Spring 2004
Contents
H.261 Overview
ITU-T (International Telecom Union) Recommendation in 1990 Precursor to coding schemes found in H.263 and MPEG-1
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
Uncompressed video and audio data are huge Compression ratio of lossless methods is not high enough Target networks are p * 64Kbps, 1p30
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 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
Bit rate
Picture formats
The target bit rate is ~ 64Kbps to 1920Kbps CIF (Common Intermediate Format) NTSC & PAL QCIF (Quarter Common Intermediate Format)
Picture coded as luminance and two color difference components (Y, CB and CR) Group of blocks (GOB) MacroBlock (MB) Block
Cb Y
Cr
Assumption
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
a) Video coder
Video multiplex decoder Receiving buffer
b) Video decoder
Encoder
Intra/inter
Coding control
Mode Qp
DCT
Q Q-1
VLC
Video out
Decoder
Step size Error Correction Buffer Intra/inter Input data Motion Vector + Frame Memory Decoded data VLC Decoder Q-1
IDCT
MC
Overview:
FFT vs. DCT: Like FFT, but approximates lines with fewer coefficients
Demo
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)
References
Video codec for audiovisual services at p x 64 kbit/s ITUT (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