MPEG 2 Compression_Methods

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Compression Methods

MPEG-2 and JPEG



Compression, why and how ?

We want to remove as much data as possible withoutaffecting the picture quality (bandwidth = money)

Our eyes are less sensitive to high frequency componentsin a video frame

The decoding must be done in realtime (but encoding cantake some time)

MPEG-2 (Moving Picture Expert Group) defines a set toolsfor compression.

Compression rates can vary over time

The video stream indicates the compression tools that areactually used together with parameters needed fordecoding.



Bitrates of uncompressed video

8 bits/sample

720 samples/line

576 lines/frame

25 frames/s

gives a bitrate of 83 Mbit/s for each of the three components Y, Cr and Cb

Symbolrate of a typical transponder is 27 MSymbols/s which correspondsto a bitrate of 36Mbit/s for video

Each transponder contains in average 6 different video streams

The compression factor has to be (2 * 83) / ( 36 / 6) = 28 !!!



Compression of Fixed Pictures

8

8

Divi de the pic ture i nto Mac roblocks to faci l i tate DS P.



Two Digital FormatsMacroblock in 4:2:2 Macroblock in 4:2:0

= luminance sample

= crominance sample

Cr Cb Y YCr Cb

16

16

4 4

4

16

1616



DCT (Discrete Cosine Transform)Each macroblock (8x8 pixels) can be transformed fromthe time domain to the frequency domain.

A matrix indicates the amplitude of eachfrequency component.

2 components

3 components 4 components

´more components gives more details...

...but our eyes are less sensitive to highfrequency components´



Contribution of the DCT Coefficients

Vertical Frequency

Horizontal Frequency

DC Coefficient



Picture Quality

Only one DCT coefficient 3 Coefficients is enough for anice picture.

Seems like we don¶t have to transmit the high frequqency components.....



Quantization and Treshholding



Read the Coefficients the Zig Zag way...



Apply RLC (Run Length Coding)....

No Coding :G G G G G G G G G G G G G G G G B B B B B B B B B B R R R R R

Run Length Coding : 16*G 10*B 5*R

G = Greean

R = Red

B = Blue

Equal element s i n a row do not hav e to be c oded i nd ivi dually, just transmi t the element-c ode and the number of succ essiv e occ uranc es !



..and VLC (Variable Length Coding)Example : alphabet coding

(most frequent letters arecoded with fewer bits)

A 0 40

C 10

G 111E 1100

H 11011

B 110100

F 110101

V i deo sig nal s al so c ontai ns more or less frequent element s ( DCT c oeff ici ent s)

?

0 1

10

11

110

1101

11010

110100

111

1100

11011

101100

110101



Some Remarks..

Picture data is now compressed by a factor 8 (at least)...

The process is the same as for JPEG

Compression of Y, Cr and Cb is done separately

The decompression must be done in realtime

The process is referred to as intra-frame-coding(removes redundancy within a frame)

There are still a lot of redundancy to remove if we consider

several frames (inter-frame-coding)



Compression of Moving Pictures

MPEG-1 : maximum bitrate = 1.5Mb/s (CD_ROM storage)

MPEG-2 : allows higher bitrates, supports interlaced pictures

MPEG-2 is back compatible with MPEG 1 MPEG-2 can be divided into several profiles and levels

± The level describes the resolution

± The profile describse the set of tools used for the compression

± Digital (DVB) TV uses MP@ML (Main Profile@ Main Level)

MPEG1/2 contains compressing tools for audio too...



I, P and B Pictures I (Intra) pictures

± Coded without reference to other pictures

± Contains all necessary ino for their reconstruction

± Comparable with a JPEG picture

± Relatively low compression rate (about 1:8)

P (Predicted) pictures

± Coded from preceeding I or P pictures using motion compensation ± Higher compression rate (about 1:25)

B (bidirectional) pictures

± Coded by bidirectional interpolation between preceeding and following I and P pictures

± High compression rate (about 1:70)

I, P and B pictures are arranged in a Group of Pictures (GOP)

± MP@ML has the parameters (M,N) = (3,12)



MPEG Video Layers

I B B P B B P B B P B B I

Group of Pictures Layer (GOP)

Slice Layer

Macroblock Layer

Block Layer

Y1 Y2

Y4Y3Cr Cb++

.....

Sequence Layer

4

4

16

16



The Elementary Stream

Sequence header Sequence header Video sequence Video sequence

GOP GOP GOPSequence header

GOP header Picture Picture Picture

Picture Header Slice Slice Slice

Slice Header Macroblock Macroblock Macroblock

Macroblock header Block1(Y) Block2(Y) Block3(Y) Block4(Y) Block5(C) Block6(C)



Reordering of pictures

B BI P B B P B B P B B I

2 341 5 67 8 910

11

1213

I B B P B B B BP IB B

1 2 3 4 5 6 7 8 9 10

11

12 13

P

Display Order

Video Stream Order

Both I and P pictures are needed in advance to code/decode the B picture



Motion Estimation

Frame1 Frame2

Search Window(Block Matching)

Only the motion (displacement) vectors of the macroblocks are transmitted



Coding P pictures I and P pictures can differ a lot so a difference picture isencoded (pixel by pixel difference)

Also called predicted error

For the same reason as for I pictures, DCT, VLC and RLC codingis applied to the predicted pictures Both motion vectors andpredicted errors defines the P picture (motion compensation)



Coding B picture

Linear Interpolation (of prediction error and motion vectors) between previous and following I and(or) P pictures

I and P pictures needed in advance to create B pictures

I P B BI B B P

MPEG 2 Compression_Methods

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