CSE679: MPEG

MPEG-1 MPEG-2

MPEG

MPEG: Motion Pictures Experts Group Standard for encoding videos/movies/motion

pictures Evolving set of standards

MPEG-1, MPEG-2, MPEG-4

Exploit motion prediction Apply the similar techniques as JPEG to

individual image

MPEG Compression Steps

Data preparation Similar to JPEG

Data processing Motion prediction DCT

Quantization Entropy encoding

Data Preparation

Subsample Chrominance signals 4:1:1 in MPEG-1 NTSC: Y=352*240, Cb=Cr=176*120

PAL: Y=352*288, Cb=Cr=176*144

Macroblocks 16*16 samples (pixels) for the luminance

component. 8*8 samples (pixels) for the chrominance

component.

Data Processing

Apply motion prediction at the Macroblock level In most cases, predictive encoding only makes sense

for parts of images and not for the whole image.

Motion prediction used to figure out where this macroblock may be in the next frame

Forward prediction Prediction based on previous frames

Backward prediction Prediction based on future frames

Bidirectional prediction Prediction based on both previous and future frames

IPB Frames

I frames Independently coded and no temporal prediction Can start decoding once we have an I-frame

P Frames Forward predicted frames Based on previous I or P frames

B frames Bi-directionally predicted frames, based on previous and

following I and P frames Group of Pictures (GOP): Set of frames between two I

frames IBBPBBPBBI or IPPPI, or II

MPEG decoders may have to buffer extra frames of data besides being displayed

DCT

Apply DCT on the 8*8 data blocks DC coefficients AC coefficients

Quantization

Apply Quantization on the frequency domain coefficients

Favor low frequency coefficients

Entropy Encoding

Convert 2 dimensional symbols into 1 dimensional series Use zig-zag or vertical scans

Apply run-length coding on the 1-D series Apply Huffman coding

Summary of MPEG-1

Successive frames may have significantly same data

MPEG exploits motion prediction Apply motion prediction at the Macroblock level I, P, and B frames

The standard allows the use of I-frame only, I and P frames only or I-, P- and B-frames.

More about MPEG……

MPEG-2

MPEG-2 strives for a higher resolution. MPEG-1 is near the maximum data rate of about

1.5Mbits/s. MPEG-2 targets at 40Mbits/s => high resolution.

MPEG-2 supports four levels Low, main, high 1440 and high

There are five profiles associated with each level

The low level of MPEG-2 is compatible with MPEG-1.

Conclusion

MPEG MPEG-2

Targets at high resolution Profiles and levels Compatible to MPEG-1