Concepts of Multimedia Processing and Transmission IT 481, Lecture #7 Dennis McCaughey, Ph.D. 19 March, 2007

Concepts of Multimedia Concepts of Multimedia Processing and TransmissionProcessing and Transmission

IT 481, Lecture #7Dennis McCaughey, Ph.D.

19 March, 2007

08/28/2006IT 481, Fall 20062

Direct Video Broadcast (DVB) Systems

Slide: Courtesy, Hung Nguyen

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Processing of The Streams in The Set-Top Box (STB)


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Multimedia CommunicationsMultimedia CommunicationsStandards and ApplicationsStandards and Applications


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Video Coding StandardsVideo Coding Standards

ITU H.261 for Video Teleconference (VTC) ITU H.263 for VTC over POTS ITU H.262 for VTC over ATM/broadband and digital TV

networks ISO MPEG-1 for movies on CDROM (VCD)

– 1.2 Mbps for video coding and 256 Kbps for audio coding ISO MPEG-2 for broadcast quality video on DVD

– 2-15 Mbps allocated for audio and video coding Low-bit rate telephony over POTS

– 10 Kbps for video and 5.3 Kbps for audio Internet and mobile communication: MPEG-4

– Very Low Bit Rate (VLBR) code to be compatible with H.263 Multimedia content description interface MPEG-7

– Description schemes and description definition language for integrated multimedia search engine


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HistoryHistory

H.261:– First video coding standard, targeted for video conferencing over

ISDN. Uses block-based hybrid coding framework with integer-pixel MC

H.263:– Improved quality at lower bit rate, to enable video

conferencing/telephony below 54 kbps (modems, desktop conferencing)

– Half-pixel MC and other improvement MPEG-1 video

– Video on CD and video on the Internet (good quality at 1.5 mbps)– Half-pixel MC and bidirectional MC

MPEG-2 video– SDTV/HDTV/DVD (4-15 mbps)– Extended from MPEG-1, considering interlaced video


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H.261 Video Coding StandardH.261 Video Coding Standard

For video-conferencing/video phone– Video coding standard in H.320 (VTC over switched phone

network) which is an umbrella recommendation– Low delay (real-time, interactive)– Slow motion in general

For transmission over ISDN– Fixed bandwidth: px64 Kbps, p=1,2,…,30

Video Format:– CIF (352x288, above 128 Kbps) - Common Interface Format– QCIF (176x144, 64-128 Kbps) - Quarter CIF – 4:2:0 color format, progressive scan

Published in 1990 Each macroblock can be coded in intra- or inter-mode Periodic insertion of intra-mode to eliminate error propagation

due to network impairments Integer-pixel accuracy motion estimation in inter-mode


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H.261 EncoderH.261 Encoder

F: Loop filter; P: motion estimation and compensation Loop filter: apply low-pass filter to smooth the quantization noise in

previously reconstructed frames before motion estimation and compensation Slide: Courtesy, Hung Nguyen

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Picture Frames - OverviewPicture Frames - Overview

Three frame types: I-Picture (Intra-frame picture), P-Picture (Inter-frame predicted picture) and B-Picture (Bi-directional predicted- interpolated pictures)

I-Picture is being coded by intra-frame coding. When encoding I-Picture, we only reduce the spatial redundancy in the picture without referencing other pictures. The coding process is much similar to JPEG Standard. So encoding I-Picture is less complex than P-frame and B-frame

The basic coding unit is a 8 by 8 matrix block. A macroblock is consists of six block: 4 block of luminance (Y) , one block of Cb chrominance, and one block of Cr chrominance


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Frame TypesFrame Types

Intracoded Frames -> I-Frames– Level of compression is relatively small 10:1 to 20:1– Present at regular intervals to limit extent of errors– Number of frames between I-frames is known as the Group of

pictures (GOP)– 10:1 to 20:1 compression ratio

Intercoded Frames– Predicted Frames-> P-Frames

Significant compression level achieved here Errors are propagated 20:1 to 30:1 compression ratio

– Bidirectional Frames -> B-Frames Highest levels of compression achieved B-frames are not used for prediction, thus errors are not

propagated 30:1 to 50:1 compression ratio

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Macro Blocks & Color Sub-sampling Schemes

A macroblock consists of 4 8x8 pixel blocks


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Sub-sampling of Chrominance Sub-sampling of Chrominance InformationInformation

Transforming (R,G,B)->(Y,Cb,Cr) provides two advantages:

1)The human visual system (HVS) is more sensitive to Y component than the Cb or Cr components.

2) Cb and Cr are far less correlated with Y than R with G, R with Blue and Blue with G, thus reducing TV transmission bandwidths.

Cb and Cr both require far less bandwidth and can be sampled more coarsely (Shannon).

By doing so we can reduce data without affecting visual quality from a personal view. Slide: Courtesy, Hung Nguyen

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Color Space ConversionColor Space Conversion

In general , each pixel in a picture consists of three components : R (Red), G (Green), B (Blue). (R,G,B) must be converted to (Y,Cb,Cr) in MPEG-1 before processing

We can view the color value of each pixel from RGB color space , or YCbCr color space

Because (Y,Cb,Cr) is less correlated than (R,G,B), coding using (Y,Cb,Cr) components is more efficient.

(Y,U,V) can also be used to denote (Y,Cb,Cr), however it most appropriately represents the analog TV equivalent


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RGB ImageRGB Image

RGB Image

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Compressed Image (QSF=24)Compressed Image (QSF=24)

Compressed Image

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Luminance Plane (Y)Luminance Plane (Y)

Luminance Plane

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Blue Chrominance Plane (Cb)Blue Chrominance Plane (Cb)

Blue Chrominance Plane

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Red Chrominance Plane (Cr)Red Chrominance Plane (Cr)

Red Chrominance Plane

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RedRed

Red RGB Plane

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Green Green

Green RGB Plane

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BlueBlue

Blue RGB Plane

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DCT (discrete cosine transform)DCT (discrete cosine transform)

DCT is used to convert data from the spatial domain to data in frequency domain. The higher frequency coefficients can be more coarsely quantized without a perceived loss of image quality due to the fact that the HVS is less sensitive to the higher frequencies and they contain less energy.

The DCT coefficient at location (0,0) is called DC coefficient and the other values we call them AC coefficients. In general, we use large quantization step in quantizing the higher AC coefficients. Higher precision is required for the DC term in order to avoid blocking in the reconstructed image.

In MPEG-1, we use 8*8 DCT. By using this transform we can convert a 8 by 8 pixel block to another 8 by 8 block. In general most of the energy(value) is concentrated to the top-left corner.

After quantizing the transformed matrix, most data in this matrix may be zero, then using zig-zag order scan and run length coding can achieve a high compression ratio. Slide: Courtesy, Hung Nguyen

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Transform Coding (TC)Transform Coding (TC)

Pack the signal energy into as few transform coefficients as possible

The DCT yields nearly optimal energy concentration A 2-dimensional DCT with block size of 8x8 pixels is

commonly used in today’s image coder Transform is followed by quantization and entropy

coding


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2D DCT and IDCT2D DCT and IDCT

u, v, x, y = 0, 1,2, ….,7


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DCT Scan ModesDCT Scan Modes

The zigzag scan used in MPEG-1 is suitable for progressive images where frequency components have equal importance in each horizontal and vertical direction. (Frame pictures only)

In MPEG-2, an alternate scan is introduced because interlaced images tend to have higher frequency components in the vertical direction. Thus, the scanning order weighs more on the higher vertical frequencies than the same horizontal frequencies. Selection between these two zigzag scan orders can be made on a picture basis. (Frame and field pictures allowed)


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Motion Compensation

Try to match each block in the actual picture to content in the previous picture. Matching is made by shifting each of the 8 x 8 blocks of the two successive pictures pixel by pixel each direction -> Motion vector

Subtract the two blocks -> Difference block Transmit the motion vector and the difference block


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QuantizationQuantization

In MPEG-1, a matrix called the quantizer ( Q[i,j] ) defines the quantization step. If ( X[i,j] ) is the DCT matrix with the same size as Q[i,j], X[i,j] is divided by Q[i,j]*QSF to obtain the quantized value matrix Xq[i,j] . QSF is the Quantization Scale Factor

– Quantization Equation : Xq[i,j] = Round( X[i,j]/(Q[i,j] *QSF))

Inverse Quantization (dequantize) is to reconstruct original value.

– Inverse Quantization Equation : X'[i,j]=QSF*Xq[i,j]*Q[i,j]

The difference between actual value and reconstructed value from quantized value is called the quantization error. In general if we carefully design Q[i,j], visual quality will not be affected.


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Quantization (cont’d)Quantization (cont’d)


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Average Distribution of AC CoefficientsAverage Distribution of AC Coefficients

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40Cb Chrominance Plane

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20IQ Cb Chrominance Plane

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10IQ Cr Chrominance Plane

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MPEG (Moving Picture Expert Group)MPEG (Moving Picture Expert Group)

Established in January 1988 Operated in the framework of the Joint ISO/IEC

Technical Committee ISO: International Organization for Standardization IEC: International Electro-technical Commission First meeting was in May 1988 with 25 experts

participated Grown to 350 experts from 200 companies in some

20 countries As a rule, MPEG meets in March, July and

November & could be more often as needed


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MPEG-1 – Coding of Moving Pictures MPEG-1 – Coding of Moving Pictures and Associated Audioand Associated Audio

Request for Proposal (RFP) July 1989 Adopted in 1993 Coding of audiovisual signal at 1.5 Mbps Audio coding is separate from speech at

256 Kbps/per channel PCM Five parts: systems, video, audio,

conformance testing and software simulation


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MPEG-1 OverviewMPEG-1 Overview

In MPEG-1, video is represented as a sequence of pictures, and each picture is treated as a two-dimensional array of pixels (pixels)

The color of each pixel is consists of three components : Y (luminance), Cb and Cr (two chrominance components)– Composite video, aka baseband video or RCA video, is the

analog waveform that conveys the image data in a conventional National Television Standards Committee (NTSC) television signal

– Composite video contains chrominance (hue and saturation) and luminance (brightness) information, along with synchronization and blanking pulses

In order to achieve high compression ratio, MPEG-1 must use hybrid coding techniques to reduce both spatial redundancy and temporal redundancy


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MPEG-1 OverviewMPEG-1 Overview

Audio/video on CD-ROM (1.5 Mbps, CIF: 352x240)– Maximum: 1.856 mbps, 768x576 pixels

Start late 1988, test in 10/89, Committee Draft 9/90 ISO/IEC 11172-1~5 (Systems, video, audio,

compliance, software). Prompted explosion of digital video applications:

MPEG1 video CD and downloadable video over Internet

Software only decoding, made possible by the introduction of Pentium chips, key to the success in the commercial market

MPEG-1 Audio – Offers 3 coding options (3 layers), higher layer have higher

coding efficiency with more computations– MP3 = MPEG1 layer 3 audio Slide: Courtesy, Hung Nguyen

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MPEG-2 vs. MPEG-1MPEG-2 vs. MPEG-1

MPEG-2 is a superset of MPEG-1. Generally, MPEG-1 is used for CD-ROM or Video

CD (VCD) and MPEG-2 is used for broadcast or DVD.

One current difference between MPEG-1 and MPEG-2 is that MPEG-2 has implemented variable bit rate.

MPEG-2 also is what’s known as a closed format, meaning that a license fee must be paid to use the decoding algorithms, where MPEG-1 can be implemented free of charge.


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MPEG2 vs. MPEG1 (cont’d)MPEG2 vs. MPEG1 (cont’d)

MPEG1 only handles progressive sequences specified by Source Input Format (SIF).

MPEG2 is targeted primarily at interlaced, as opposed to progressive for MPEG-1, sequences and at higher resolution.

Different DCT modes and scanning methods are developed for interlaced sequences.

More sophisticated motion estimation methods (frame/field prediction mode) are developed to improve estimation accuracy for interlaced sequences.

MPEG2 has various scalability modes. MPEG2 has various profiles and levels, each

combination targeted for a different applicationSlide: Courtesy, Hung Nguyen

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MPEG EncodingMPEG Encoding

Frame TypesI Intra Encode complete image, similar to

JPEG

P Forward Predicted Motion relative to previous I and P’s

B Backward Predicted Motion relative to previous & future I’s & P’s

• • • • • •

I1 B1 B2 B3 P1 B4 B5 B6 P2 B7 B8 B9 I2


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– I frame complete

image– P frames provide

series of updates to most recent I frame

• • •• • •

I1

P1 P2

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I1+P1 I1+P1+P2

Frame Reconstruction (I & P Frames Only)Frame Reconstruction (I & P Frames Only)


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Using Forward-Backward PredictionUsing Forward-Backward Prediction

If only forward prediction is used, there are uncovered areas (such as block behind car in Frame N) for which we may not be able to find a good match from the previous reference picture (Frame N-1).

On the other hand, backward prediction can properly predict these uncovered areas since they are available in the future reference picture, i.e. frame N+1 in this example.

New objects such as an airplane moving into the picture, cannot be predicted from the previous picture, but can be predicted from the future picture.

Backward PredictionForward Prediction


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– B frames interpolate between frames represented by I’s & P’s

• • •

I1 I2I1+P1 I1+P1+P2

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B1 B2 B3 B4 B5 B6 B7 B8 B9

Interpolations

Frame Reconstruction (cont’d)Frame Reconstruction (cont’d)


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Transmission Order of the Frames


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Intra-frame Encoding ProcessIntra-frame Encoding Process

Decomposing image to three components in RGB space Converting RGB to YCbCr Dividing image into several macroblocks (each macroblock has 6

blocks , 4 for Y, 1 for Cb, 1 for Cr) DCT transformation for each block After DCT transform , Quantizing each coefficient Then use zig-zag scan to gather AC value Use DPCM to encode the

DC value, then use VLC to encode it Use RLE to encode the AC value, then use VLC to encode it

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I-Picture Encoding Flow ChartI-Picture Encoding Flow Chart


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Inter-frame CodingInter-frame Coding

The kind of pictures that are using the intra-frame coding technique are P pictures and B pictures

Coding of the P pictures is more complex than for I pictures, since motion-compensated macroblocks may be constructed– The difference between the motion compensated

macroblock and the current macroblock is transformed with a 2-dimensional DCT giving an array of 8 by 8 transform coefficients.

– The coefficients are quantized to produce a set of quantized coefficients. The quantized coefficients are then encoded using a run-length value technique.


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Inter-frame Encoding ProcessInter-frame Encoding Process

Decomposing image to three components in RGB space Converting RGB to YCbCr Perform motion estimation to record the difference between the

encoding frame and the reference frame stored within the frame buffer

Dividing image into several macroblocks (each macroblock has 6 blocks , 4 for Y, 1 for Cb, 1 for Cr)

DCT transformation for each block Quantizing each coefficient Use zig-zag scan to gather AC value Reconstruct the frame and store it to the frame buffer if necessary DPCM is applied to encode the DC value, then use VLC to encode it Use RLE to encode the AC value, then use VLC to encode it


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Predictive CodingPredictive Coding

Predictive coding is a technique to reduce statistical redundancy. That is based on the current value to predict next value and code their difference (called prediction error). If we predict next value more precisely, then the prediction error will be small.

So we can use less bits to encode prediction error than actual value. In MPEG-1, we use DPCM (Difference Pulse Coded Modulation) techniques which is a kind of predictive coding. And it is only used in DC coefficient


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Motion Compensation (MC) And Motion Compensation (MC) And Motion Estimation (ME)Motion Estimation (ME)

Motion Estimation is to predict a block of pixels' value in next picture using a block in current picture. The location difference between these blocks is called Motion Vector. And the difference between two blocks is called prediction error.

In MPEG-1, encoder must calculate the motion vector and prediction error. When decoder obtain these information , it can use this information and current picture to reconstruct the next picture.

We usually call this process as Motion Compensation. In general, motion compensation is the inverse process of motion Estimation Slide: Courtesy, Hung Nguyen

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Motion Estimation (ME)Motion Estimation (ME)


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Motion Compensation (MC)Motion Compensation (MC)


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P-Frame Encoding: Macroblock P-Frame Encoding: Macroblock StructureStructure

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P-Frame Encoding: Encoding ProcedureP-Frame Encoding: Encoding Procedure

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Example Frame Sequences Example Frame Sequences

I and P Frames Only

I,P and B Frames

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Coding of P PicturesCoding of P Pictures

As in I pictures, the encoder needs to store the decoded P pictures since this may be used as the starting point for motion compensation. Therefore, the encoder will reconstruct the image from the quantized coefficients.

In coding P pictures, the encoder has more decisions to make than in the case of I pictures

– Selection of Macroblock Type: There are 8 types of macroblock in P pictures.

– Motion Compensation Decision: The encoder has an option on whether to transmit motion vectors or not for predictive-coded macroblocks.

– Intra/Non-intra Coding Decision Coded/Not Coded Decision:After quantization, if all the coefficients in a block is zero then the block is not coded.

– Quantizer/No Quantizer Decision: Quantizer scale can be altered which will affect the picture quality. Slide: Courtesy, Hung Nguyen

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The Inter-frame Encoding Flow The Inter-frame Encoding Flow ChartChart


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Coding of P Pictures (cont’d)Coding of P Pictures (cont’d)


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Coding of B PicturesCoding of B Pictures

B pictures are divided into slices in the same way as I and P pictures. Since B pictures are not used as a reference for motion compensation, errors in B pictures are slightly less important than in I or P pictures. Consequently, it might be appropriate to use fewer slices for B pictures

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Decisions to be made when Decisions to be made when coding the B picturescoding the B pictures

Selection of Macroblock Type: There are 12 types of macroblock in B pictures. Compare with P pictures, there are extra types due to the introduction of the backward motion vector. If both the backward and backward motion vectors are present, then motion-compensated macroblocks are constructed from both previous and future pictures, and the result is averaged to form the "interpolated" motion-compensated macroblock.

Selecting Motion Compensation Mode Intra/Non-Intra Coding Coded/Not Coded Decision Slide: Courtesy, Hung Nguyen

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Coding of B-PicturesCoding of B-Pictures

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Variable Length Coding (VLC)Variable Length Coding (VLC)

In MPEG-1, the last of all encoding processes is to use a Huffman Code to reduce data redundancy and the first step in decoding process is to decode VLC to reconstruct image data

Encoding and decoding processes with a Huffman Code must refer to a code table having two entries– The original data and the corresponding codeword. – In MPEG-1 standard , multiple code tables are defined in

MPEG-1 Standard 2-ANNEX C. The use of multiple code tables improves the compression ratio.


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MPEG-2 vs. MPEG-1MPEG-2 vs. MPEG-1

MPEG-2 is a superset of MPEG-1. Generally, MPEG-1 is used for CD-ROM or Video

CD (VCD) and MPEG-2 is used for broadcast or DVD.

One current difference between MPEG-1 and MPEG-2 is that MPEG-2 has implemented variable bit rate.

MPEG-2 also is what’s known as a closed format, meaning that a license fee must be paid to use the decoding algorithms, where MPEG-1 can be implemented free of charge.


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MPEG2 vs. MPEG1 (cont’d)MPEG2 vs. MPEG1 (cont’d)

MPEG1 only handles progressive sequences specified by Source Input Format (SIF).

MPEG2 is targeted primarily at interlaced, as opposed to progressive for MPEG-1, sequences and at higher resolution.

Different DCT modes and scanning methods are developed for interlaced sequences.

More sophisticated motion estimation methods (frame/field prediction mode) are developed to improve estimation accuracy for interlaced sequences.

MPEG2 has various scalability modes. MPEG2 has various profiles and levels, each

combination targeted for a different applicationSlide: Courtesy, Hung Nguyen

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MPEG2 OverviewMPEG2 Overview

A/V broadcast (TV, HDTV, Terrestrial, Cable, Satellite, High Speed Inter/Intranet) as well as DVD video

4~8 Mbps for TV quality, 10-15 Mbps for better quality at SDTV resolutions (BT.601)

18-45 Mbps for HDTV applications– MPEG-2 video high profile at high level is the video coding

standard used in HDTV Test in 11/91, Committee Draft 11/93 ISO/IEC 13818-1~6 (Systems, video, audio, compliance,

software, DSM-CC) Consist of various profiles and levels Backward compatible with MPEG1 MPEG-2 Audio

– Support 5.1 channel– MPEG2 AAC (Advanced Audio Coding): requires 30% fewer bits

than, and not backward compatible with, MPEG1 layer 3 or MP3


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Features Supported by the MPEG-2 Algorithm

Different chrominance sampling formats (i.e., 4:2:0, 4:2:2, and 4:4:4) can be represented

Video in both the progressive and interlaced scan formats can be encoded

The decoder can use 3:2 pull down to represent a ~24 fps film as ~30 fps video

The displayed video can be selected by a movable pan-scan window within a larger raster

A wide range for picture qualities can be used Both constant an variable bit rate channels are supported ISO/IEC 11172-2 bit streams are decodable Bit streams for high and low (hardware) complexity decoders

can be generated Editing of encoded video is supported The encoded bit stream is resilient to errors


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MPEG-2 Slice and Macro-block Structure


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MPEG-2 Bit Stream Syntax

GOF: Group of FramesSlide: Courtesy, Hung Nguyen

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Progressive vs. Interlaced ScanningProgressive vs. Interlaced Scanning

In the Interlaced video, each displayed frame consists of two interlaced fields, with the scanning lines in Field 1 located between the lines of Field 2.

On the contrary, the Progressive video has all the lines of a picture displayed in one frame. Thus, progressive video requires a higher picture rate than the frame-rate of an Interlaced video, to avoid a flickering display.

(a) Progressive Scan (b) Interlaced ScanSlide: Courtesy, Hung Nguyen

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Disadvantage of Interlaced ScanningDisadvantage of Interlaced Scanning

A moving object may appear distorted when two fields are merged into a frame.

Since a moving ball will be at different locations in the two fields in the Interlaced Format, the ball will look distorted when two fields are put into a frame

Interlaced video also tends to cause horizontal picture details to dither thus introduces more high frequency noises

(a) Progressive Scan (b) Interlaced ScanSlide: Courtesy, Hung Nguyen

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Field vs. Frame DCTField vs. Frame DCT

Frame-based DCT: Suitable for the blocks in the background or in a still image having little motion because these blocks have high correlation between pixel values from adjacent scan lines.

Field-based DCT: Suitable for blocks having motion because motion causes distortion and may introduce high-frequency noises into the interlaced frame.


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HDTV StandardsHDTV Standards

Standard Samples/Line Number of Lines Aspect Ratio

Advanced television (ATV) 1280 720 16/9

Digital Video Broadcast (DVB) 1440 1152 4/3

Multiple Sub-Nyquist Sampling Encoding (MUSE) 1920 1035 16/9

ITU-R HDTV 1920 1152 16/9

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SummarySummary

H.261:– First video coding standard, targeted for video conferencing over

ISDN. Uses block-based hybrid coding framework with integer-pixel MC

H.263:– Improved quality at lower bit rate, to enable video

conferencing/telephony below 54 bkps (modems, desktop conferencing)

– Half-pixel MC and other improvement MPEG-1 video

– Video on CD and video on the Internet (good quality at 1.5 mbps)– Half-pixel MC and bidirectional MC

MPEG-2 video– SDTV/HDTV/DVD (4-15 mbps)– Extended from MPEG-1, considering interlaced video


Documents

Concepts of Multimedia Processing and Transmission IT 481, Lecture #7 Dennis McCaughey, Ph.D. 19 March, 2007