MPEG-1 and MPEG-2 Digital Video Coding Standards

Author: Thomas SikoraPresenter: 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 QuantizerRun-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 B Y Cb Cr Y Cb Cr

Color Space Transform Sample

Video Filter

Statistical Redundancy

Discrete Cosine Transform

• Closely related to Discrete Fourier Transform

• Performed on 8*8 blockForward DCT

Inverse DCT

DCT Basis Patterns

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

)),(

),((),(

vuQ

vuFndIntegerRouvuFQ

Example Coefficients

Run-Length Encoding

Run-Amplitude Pairs Bit 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 1 Frame 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 1 Frame 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