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
MPEG video Compression 2
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
MPEG video Compression 3
Therefore, Compression required is
Compression = 1.5 Gbps / 18mbps = 83
Hence, Compression ratio required is 83: 1
Contd…
MPEG video Compression 4
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
MPEG video Compression 5
ANSWER is YES, we can use jpeg but we can achievemore compression than that in videos.
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Characteristic of Video
Adjacent frames are similar and changes are due to object motion: Temporal Correlation
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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.
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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…
MPEG video Compression 9
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
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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…
MPEG video Compression 11
Contd…
MPEG video Compression 12
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…
MPEG video Compression 13
B Frame (Contd…)
MPEG video Compression 14
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
MPEG video Compression 15
Motion Compensation
MPEG video Compression 16
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
MPEG video Compression 17
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
MPEG video Compression 18
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
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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
MPEG video Compression 21
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
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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
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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
MPEG video Compression 24
Part 5: Specifies a multi-channel audio coding
algorithm
Part 6 specifies the Real-time Interface (RTI) to
Transport Stream decoders
MPEG video Compression 25
MPEG-2 Video Compression
VIDEO STREAM DATA HIRERARCHY
MPEG video Compression 26
Four Video Compression Techniques:
1. Pre-processing
2. Temporal Prediction
3. Motion Compensation
4. Quantization
Contd…
MPEG video Compression 27
Pre-processing Filters out unnecessary information
Information that is difficult to encode
Not an important component of human visual perception
MPEG video Compression 28
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
MPEG video Compression 29
MPEG video Compression 30
MPEG video Compression 31
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.
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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
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Submergence Handle specific requirements from rapidly developing
multimedia applications
Advantages over MPEG-1 and MPEG-2 Object-oriented coding
MPEG 4
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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
MPEG video Compression 35
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 36