MPEG video compression standard

A seminar on MPEG 1 and 2

Presented ByAnurag Jagetiya

Shiv Prasad SharmaME (CSE) Modular-12

Under the guidance of

Ms. Shano Solanki

Astt. Prof (CSE), NITTTR, CHD

MPEG video Compression 1

Need of Video Compression

Basics of video compression

MPEG-1

MPEG-2

References

Outline


Example: HDTV broadcast has resolution of 1920*1080 at 30 fps using 8 bits to represent primarycolors.

This leads to a total of 1.5 Gbps data rate

But, Channel B.W. is only 6MHz that supports around 19.2Mbps data rate only.

But, channel also has to support audio, and other data.

Effective data rate reduced to only 18 mbps

Need of Video Compression


Therefore, Compression required is

Compression = 1.5 Gbps / 18mbps = 83

Hence, Compression ratio required is 83: 1

Contd…


Video is made of multiple images called frames.

Images are having highly redundant data i.e. adjacent pixels are highly correlated.

JPEG compresses images.

Then can’t it be used to compress framesindividually???

Basics of Video Compression


ANSWER is YES, we can use jpeg but we can achievemore compression than that in videos.


Characteristic of Video

Adjacent frames are similar and changes are due to object motion: Temporal Correlation


Key idea

Predict a new frame from previous frame and specify theprediction error.

Prediction error can be coded using image coding methods. (e.g.JPEG)

Predictions from past frame are known as forward prediction.

Prediction error can be coded with fewer bits

Regions those can’t be predicted well, are coded directly.


Previous of next frame is known as reference image.

Video compression makes use of MOTIONCOMPENSATION to predict a frame from the previousand/ or next frame.

Motion vectors that describes the transformation fromone image to another that are applied to the target andsynthesized to produce next image.

Contd…


Macro Blocks: Compression methods works on ablock of 16*16 pixels called macro blocks.

I (intra) frames

independent frames

Coded without reference to other frames

Key terms


P (predictive) Frame

Not Independent

These frames are predicted from a past frame (I or P)

Coded by a forward predictive coding Current macro block is predicted from similar macro block in

previous I or P frame. And the difference between macro blocks is coded

Contd…


Contd…


MPEG introduced a new B frame

B frame (bi-directional predictive-coded)

Due to unexpected movement in real scenes, thetarget macro block may not have a good matchingin previous frame.

Therefore, B frame is coded with reference to bothprevious and future reference frames (either I or P)

Contd…


B Frame (Contd…)


MPEG-1

Finalized in 1991

Initial audio/video compression standard

Used by VCD’s

MP3 = MPEG-1 audio layer 3

Target of 1.5 Mb/s bitrate at 352x240 resolution

Only supports progressive pictures

MPEG 1: Evolution


Motion Compensation


Chroma (C): Color Information

Luma (Y): Brightness [Black and white]

FACT: Human eyes are more sensitive to brightnessinformation then color.

Optimization can be achieved by storing more lumainformation than chroma.

Therefore MPEG uses YCbCr model instead of RGB

Chroma Subsampling


Y represents Luma

Cb and Cr jointly represents Chroma

Cb: Blue color difference

Cr: Red color difference

A macro block can be represented in several different YCbCr color model

E.g. 4:4:4 means Y=4, Cb=4 and Cr=4 [Full Bandwidth]

E.g. 4:2:2 means Y=4, Cb=2, Cr=2 [half BW then first one]

YCbCr


Frame Sequence

Actual frame sequence is determined by encoder and is specified in videoframe header.

Display order: I B B P B B P B B IDecoding Order: I P B B P B B I B B

NOTE: Decoder can’t work with B frame without its succeeding P or I frameMPEG video Compression 19

Example


Different Source format CIF: 352x288, SIF: 352x240

Using bi directional temporal prediction More precise motion compensation based predictions.

Difference in quantization tables

MPEG allows random access

MPEG 1 Vs H.261


In real time transmission of mpeg need of buffering hence the inevitable delay is an important issue.

MPEG-1 Can be used to store and play video on CD on a single computer

Low bit rate

Issue with MPEG 1


Project started in 1990 and finalized in 1994.

Provided high quality video with bit rate around 4 Mbps

It was developed as a standard for digital tv broadcast.

It gained wide popularity in terrestrial, satellite, or cable network.

Adopted in DVDs.

MPEG -2


Part 1: Combine video and audio data into single/multiple

streams

Part 2: Offers more advanced video compression tools

Part 3: Is a multi-channel extension of the MPEG-1 Audio

standard

Part 4: Specifies protocols of managing MPEG-1 & MPEG-2

bit streams

Parts of MPEG-2


Part 5: Specifies a multi-channel audio coding

algorithm

Part 6 specifies the Real-time Interface (RTI) to

Transport Stream decoders


MPEG-2 Video Compression

VIDEO STREAM DATA HIRERARCHY


Four Video Compression Techniques:

1. Pre-processing

2. Temporal Prediction

3. Motion Compensation

4. Quantization

Contd…


Pre-processing Filters out unnecessary information

Information that is difficult to encode

Not an important component of human visual perception


Temporal Prediction: Uses the mathematical algorithm Discrete Cosine

Transform (DCT) to:

Divide each frame into 8X8 blocks of pixels

Reorganize residual differences between frames

Encode each block separately


Quantization: Lossy Compression

Compressed range of value by a single quantum value

Quantization Matrix is designed to provide moreresolution to perceivable frequency

Set much of the unnecessary elements to Zero

Uses zig zag ordering and run length encoding.


MPEG1 only handles progressive sequences (SIF) MPEG2 is targetedprimarily at interlaced sequences and at higher resolution

More sophisticated motion estimation methods frame/field predictionMode are developed to improve estimation accuracy for interlacedsequences.

Different DCT modes and scanning methods are developed for interlacedsequences.

MPEG2 has various scalability modes.

MPEG2 has various profiles and levels, each combination targeted for adifferent application

MPEG-2 Vs MPEG-1


Submergence Handle specific requirements from rapidly developing

multimedia applications

Advantages over MPEG-1 and MPEG-2 Object-oriented coding

MPEG 4


Another ISO/IEC standard being developed by MPEG

Content representation standard for information search

Makes searching the Web for multimedia content as easy assearching for text-only files

Operates in both real-time and non real-timeenvironments

MPEG 7


Y. Wang, J. Ostermann, Y. Q. Zhang, Video Processing and Communications, Prentice Hall, 2002. Chapters 9,11,13

Fundamental of Multimedia, Li-Drew, Prentice Hall

John Wiseman, An Introduction to MPEG videocompression

References


MPEG video compression standard

Engineering

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