MPEG-1 and MPEG-2 Digital Video Coding Standards Author: Thomas Sikora Presenter: Chaojun Liang

Overview Introduction Intra-picture Coding Techniques Inter-picture Coding Techniques

What is MPEG? Stands for Moving Picture Expert Group Nickname to a family of International standards used for coding audio-visual information in a digital compressed format. Includes MPEG-1, MPEG-2, and upcoming MPEG-4.

MPEG-1 Finalized in 1991 Optimized to work at 352*240, 30 f/s or 352*288, 25f/s Bit-rate is optimized for application of around 1.5 Mb/sec Defined for progressive frames only

MPEG-2 Finalized in 1994 Digital television broadcasting Target bit-rate raised to 4-9Mb/sec Main Profile, Main Level –720*480 pixels, 30 frame/sec, 15Mb/sec Main Profile, High Level (HDTV) –1920*1280 pixels, 30 frame/sec, 80Mb/sec

MPEG Video Layers

Intra Frame Coding Techniques Video Filter DCT Quantizer Run-length Encoding Bitstream Buffer Bit-Rate Control

Video Filter Subjective Redundancy –Eyes more sensitive to changes in Luminance, less to variations in Chrominance R G B -> Y Cb Cr Sample Chrominance 2:1 in both directions

R G BY Cb Cr Color Space Transform Sample Video Filter

Statistical Redundancy

Discrete Cosine Transform Closely related to Discrete Fourier Transform Performed on 8*8 block Forward DCT Inverse DCT

DCT Basis Patterns

Increasing Vertical Frequency (Down) Increase Horizontal Frequency (Right)

Variance Distribution of DCT Coefficients

DCT Coefficient Quantization Higher frequency -> Larger step size -> more coarsely quantized Force as many of the DCT-Coefficients as possible to zero or near zero

Example Coefficients

Run-Length Encoding Run-Amplitude PairsBit Stream Variable Length Code Table

Inter Frame Coding Techniques Exploit temporal redundancy Use Motion Estimation to eliminate temporal redundancy

Picture Types Intra Pictures (I-Pictures) Predicted Pictures (P-Pictures) Bi-directional Pictures (B-Pictures)

Intra Pictures Coded with only info in the picture itself Random access points into the compressed data Moderate compress: about 2 bits per coded pixel

Predicted Pictures Coded with respect to the nearest previous I-Picture or P-Picture

Bi-directional Pictures Use both a past and future picture as a reference.

Video Stream Composition Encoder chose frequency and location of I- Pictures Typical display order

Video Stream vs Display order

Motion Estimation Tree moved down to the right People moved farther to the right than tree Frame 1Frame 2

Motion Estimation To adequately represent the change, or the difference, between 2 video frames Frame-wise direct minus not effective Macroblock-wise minus 2-D spatial search for each luminance macroblock

Macroblock Matching Frame 1Frame 2

Picture 1 Predicted Picture Picture 2 Residual Error Picture

Coding of Residual Errors Similar to Intra frame coding Different quantization matrix Motion vectors are also coded