MPEG video compression standard

A seminar on MPEG 1 and 2
Presented By
Anurag Jagetiya
Shiv Prasad Sharma
ME (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 primary
colors.
 This leads to a total of 1.5 Gbps data rate
 But, Channel B.W. is only 6MHz that supports around 19.2
Mbps 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 frames
individually???
Basics of Video Compression

 ANSWER is YES, we can use jpeg but we can achieve
more 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 the
prediction 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 MOTION
COMPENSATION to predict a frame from the previous
and/ or next frame.
 Motion vectors that describes the transformation from
one image to another that are applied to the target and
synthesized to produce next image.
Contd…

 Macro Blocks: Compression methods works on a
block 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, the
target macro block may not have a good matching
in previous frame.
 Therefore, B frame is coded with reference to both
previous 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 brightness
information then color.
 Optimization can be achieved by storing more luma
information 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 video
frame header.
Display order: I B B P B B P B B I
Decoding 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 more
resolution 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 targeted
primarily at interlaced sequences and at higher resolution
 More sophisticated motion estimation methods frame/field prediction
Mode are developed to improve estimation accuracy for interlaced
sequences.
 Different DCT modes and scanning methods are developed for interlaced
sequences.
 MPEG2 has various scalability modes.
 MPEG2 has various profiles and levels, each combination targeted for a
different 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 as
searching for text-only files
 Operates in both real-time and non real-time
environments
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 video
compression
References

MPEG video compression standard

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