MPEG1 and MPEG2 Digital Video Coding Standards Author

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, 25 f/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 -9 Mb/sec Main Profile, Main Level – 720*480 pixels, 30 frame/sec, 15 Mb/sec • Main Profile, High Level (HDTV) – 1920*1280 pixels, 30 frame/sec, 80 Mb/sec

MPEG Video Layers

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

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

Video Filter Color Space Transform RGB Sample Y Cb Cr

Statistical Redundancy

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

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 IPictures • Typical display order

Video Stream vs Display order

Motion Estimation Tree moved down to the right Frame 1 People moved farther to the right than tree 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 Residual Error Picture 2 Predicted Picture

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