multimedia communication systems 1 - صفحه...

Multimedia Communication Systems 1

“MULTIMEDIA SIGNAL CODING AND TRANSMISSION”

DR. AFSHIN EBRAHIMI

1

Basics: Video and Animation 2

Video and Animation• Basic concepts• Television standards• MPEG• Digital Video Broadcasting• Computer-based animation

Video Signal Representation3

Video signal representation includes:• Visual representation• Transmission• Digitization

Several important measures for the visual representation:

1. Vertical Detail and Viewing DistanceSmallest detail that can be reproduced is a pixel– Ideally: One pixel for every detail of a scene– In practice: Some details fall between scanning linesKell factor: only 70% of the vertical details are represented, due to the fact that some of the details of the scene fall between the scanning lines (determined byexperience, measurements)

Visual Representation4

The Kell factor of ≈ 0.7 is independent of the way of scanning, i.e. whether• the scanning is progressive (sequential scanning of lines) or• the scanning is interlaced (alternate scanning, line 1, line 3, ... line n-1, line 2, line 4, ...)

Scan lines

A “detail” where only 2 out of 3components can be represented


2. Horizontal Detail and Picture WidthPicture width for conventional television service is 4:3 · picture height

Geometry of the television image is based on theaspect ratio, which is the ratio of the picture width W to the height H (W:H).

• The conventional aspect ratio for television is 4:3 =1.33

• Modern systems use 16:9 = 1.77

The viewing distance D determines the angle αsubtended by the picture height H.

This angle is measured by the ratio of the picture height to viewing distance: tan(α) = H/D.


3. Total Detail Content of the ImageTotal number of picture elements= number of vertical elements · number of horizontal elements

= vertical resolution� · aspect ratio= 525� · 4/3 (for NTSC)

4. Perception of Depth (3D impression)• In natural vision: angular separation of the images received by the two

eyes• Television image: perspective appearance of objects, choice of focal

length of camera lens, changes in depth of camera focus


5. Luminance and ChrominanceUsually not RGB, but YUV (or a variant) is used

6. Temporal Aspects of IlluminationMotion is represented by a rapid succession of slightly different still pictures(frames). A discrete sequence of pictures is perceived as a continuous sequence ofpicture (due to “Lucky weakness” of the human brain).Between frames, the light is cut off briefly.For a realistic presentation, two conditions are required:– Repetition rate must be high enough to guarantee smooth motion– The persistence of vision must extend over the interval between flashes

7. Continuity of MotionTo perceive continuous motion, the frame rate must be higher than 15 frames/sec.For smooth motion the frame rate should be 24 - 30 frames/sec.


8. FlickeringThrough slow motion a periodic fluctuation of brightness, a flicker effect, arises.How to avoid this disturbing effect?

• A first trick: display each picture several timesE.g.: 16 pictures per second: very inconvenient flicker effect→ display every picture 3 times, i.e. with a refresh rate of 3 · 16 = 48 Hz

• To avoid flicker, a refresh rate of at least 50 Hz is needed

• Computer displays achieve 70 Hz of refresh rate by the use of a refresh buffer

• TV picture is divided into two half-pictures by line interleaving

• Refresh rate of 25 Hz (PAL) for the full TV picture requires a scan rate of2 · 25 Hz = 50 Hz.


9. Temporal Aspect of Video BandwidthThe eye requires a video frame to be scanned every 1/25 secondScan rate and resolution determines the video bandwidth needed for transmissionDuring one cycle of video frequency (i.e. 1 Hz) at most two horizontally adjacentpixels can be scannedVertical resolution and frame rate relate to horizontal (line) scan frequency:

vertical lines (b) · frame rate (c) = horizontal scan frequencyHorizontal resolution and scan frequency relate to video bandwidth:

horizontal lines (a) · scan frequency / 2 = video bandwidth→ video bandwidth = a · b · c/2

(since 2 horizontally adjacent pixels can be represented simultaneouslyduring one cycle of video frequency)

A computer system with a resolution of a = 1312 and b = 800 pixels out ofwhich 1024 · 786 are visible and a frame rate c = 100 Hz needs:

• a horizontal scan frequency of 800 · 100 Hz = 80 kHz• a video bandwidth of 1312 · 80 kHz / 2 = 52.48 MHz

Digitization10

For image processing or transmission, the analog picture or video must be converted to a digital representation.

Digitization consists of:• Sampling, where the color/grey level in the picture is measured at a M×N array of

pixels.

• Quantizing, where the values in a continuous range are divided into k intervals.For a satisfiable reconstruction of a picture from quantized samples

– 100 or more quantizing levels might be needed– Very often 256 levels are used, which are representable within 8 Bits

A digital picture consists of an array of integer values representing pixels (as in a simplebitmap image format)

Video Controller Standards11

A B C = A · B

Television - NTSC12

Video format standard for conventional television systems in the USA (since 1954):NTSC (National Television Systems Committee)

• Picture size: 525 rows, aspect ratio 4:3, refresh rate of 30 frames/sec• Uses a YIQ signal (in principle nothing but a slight variation of YUV scheme)

Y = 0.30 . R + 0.59 . G + 0.11 . B

I = 0.60 . R - 0.28 . G + 0.32 . B

Q = 0.21 . R + 0.52 . G + 0.31 . B

Composite signal for transmission of the signal to receivers:• Individual components (YIQ) are composed into one signal• Basic information consists of luminance information and chrominance difference signals• Use appropriate modulation methods to eliminate interference between luminance

and chrominance signals

A Typical NTSC Encoder13

NTSC14

• Required bandwidth to transmit NTSC signals is 4.2 MHz, 6 MHz including sound• The luminance (Y) or “monochrome” signal uses 3.58 MHz of bandwidth• The I-signal uses 1.5 MHz of bandwidth, the Q-signal 0.5 MHz• The I-signal is In-phase with the 3.58 MHz carrier wave, the Q-signal is in Quadrature

(90 degrees out of phase) with the 3.58 MHz carrier wave.

Television - PAL15

PAL (Phase Alternating Line, invented by W. Bruch/Telefunken 1963)

• Frame rate of 25 Hz, delay between frames: 1000ms / 25 frames per sec. = 40ms• 625 lines, aspect ratio 4:3• Quadrature amplitude modulation similar to NTSC• Bandwidth: 5.5 MHz• Phase of the R-Y (V) signal is reversed by 180 degrees from line to line, to reduce

color errors that occur from amplitude and phase distortion during transmission.• The chrominance signal C for PAL transmission can be represented as:

Television Standards16

Television Standards17

-i interlaced, -p progressive (non-interlaced)

“More modern” Television Standards:• SDTV (Standard Definition TV): low resolution, aspect ratio not specified• EDTV (Enhanced Definition TV): minimum of 480 lines, aspect ratio not specified• HDTV (High Definition TV): minimum of 720 lines, aspect ratio of 16:9

Enhanced Definition TV (EDTV)18

EDTV systems are conventional systems which offer improved vertical and/or horizontalresolution by some “tricks”

• Comb filters improve horizontal resolution by more than 30% according to literature• Separate black and white from color information to eliminate rainbow effects while

extending resolution• Progressive (non-interlaced) scanning improves vertical resolution• Insertion of “blank lines” in between “active lines”, which are filled with information

from: above line, below line, same line in previous picture

Other EDTV developments is: IDTV (Improved Definition Television)• Intermediate level between NTSC and HDTV (High-Definition Television) in the U.S.• Improve NTSC image by using digital memory to double scanning lines from 525 to

1050.• One 1050-line image is displayed in 1/60 sec (60 frames/sec).• Digital separation of chrominance and luminance signals prevents cross-interference

High Definition Systems (HDTV)19

HDTV is characterized by:• Higher Resolution, approx. twice as many horizontal and vertical pixels as

conventional systems (1024×768, up to 1920×1080)• 24-bit pixels• Bandwidth 5 - 8 times larger than for NTSC/PAL• Aspect Ratio: 16/9 = 1.777• Preferred Viewing Distance: between 2.4 and 3.3 meters

Digital Coding is essential in the design and implementation of HDTV:• Composite Coding (sampling of the composite analog video signal i.e. all signal

components are converted together into a digital form) is the straightforward andeasiest alternative, but: cross-talk between luminance and chrominance in the composite signal composite coding depends on the television standard sampling frequency cannot be adopted to the bandwidth requirements of

components sampling frequency is not coupled with color carrier frequency


Alternative: Component Coding (separate digitization of various image components):

• the more important luminance signal is sampled with 13.5 MHz,

• the chrominance signals (R-Y, B-Y) are sampled with 6.75 MHz.Global bandwidth: 13 Mhz + 6.75 MHz > 19 MHz

• Luminance and chrominance signals are quantized uniformly with 8 Bits.

Due to high data rates (1050 lines · 600 pixels/line · 30 frames/sec) bandwidth is approx. 19 MHz and therefore substandards (systems which need a lower data rate) for transmission have been defined.


Worldwide, 3 different HDTV systems are being developed:

United States• Full-digital solution with 1050 lines (960 visible) and a scan rate of 59.94 Hz• Compatible with NTSC through IDTV

Europe• HD-MAC (High Definition Multiplexed Analog Components)• 1250 lines (1000 visible), scan rate of 50 Hz• Halving of lines (625 of 1250) and of full-picture motion allows simple conversion to PAL• HD-MAC receiver uses digital image storage to show full resolution and motion

Japan• MUSE is a modification of the first NHK (Japan Broadcasting Company) HDTV Standard• MUSE is a Direct-Broadcast-from-Satellite (DBS) System, where the 20MHz bandwidth is

reduced by compression to the 8.15 MHz available on the satellite channel• Full detail of the 1125 line image is retained for stationary scenes, with motion the

definition is reduced by approx. 50%


-i interlaced, -p progressive (non-interlaced)

Television - Transmission (Substandards)23

HDTV data rates for transmission• total picture elements = horizontal resolution · vertical resolution• USA: 720,000 pixels · 24 Bits/pixel · 60 frames/sec. = 1036.8 MBits/sec!• Europe: 870,000 pixel · 24 Bits/pixel · 50 frames/sec = 1044 MBits/sec!• HDTV with 1920×1080 pixel, 24 Bits/pixel, 30 frames/sec: 1.5 GB/sec!• Reduction of data rate is unavoidable, since required rates do not fit to “standard

capacities” provided by broadband networks (e.g. 155 or 34 MBits/sec)• Different substandards for data reduction are defined:

Television: Transmission24

Further reduction of data rates is required for picture transmission

Sampling gaps are left out (only visible areas are coded):• Luminance has 648 sample values per line, but only 540 of them are visible.• Chrominance has 216 sample values per line, but only 180 are visible.• 575 visible lines: (540 + 180 + 180) samples/line · 575 lines/frame = 517,500

samples/frame• 517,500 samples/frame · 8 Bits/sample · 25 frames/sec = 103.5 MBits/sec

Reduction of vertical chrominance resolution:• Only the chrominance signals of each second line are transmitted.• 575 visible lines: (540 + 90 + 90) samples/line · 575 lines/frame = 414,000

samples/frame• 414,000 samples/frame · 8 Bits/sample · 25 frames/sec = 82.8 MBits/sec

Different source coding:• Using an intra-frame working ADPCM with 3 instead of 8 Bits/sample• 414,000 samples/frame · 3 Bits/sample · 25 frames/sec = 31.05 MBits/sec

Compression Techniques25

Still very high data rate:

• Video should have about 1.5 Mbits/sec to fit within CD technology• Audio should have about 64 – 192 kbit/channel

→ need for further compression techniques, e.g.

• MPEG for video and audio

Classification of Applications26

Dialogue Mode Applications

• Interaction among human users via multimedia information• Requirements for compression and decompression:

End-to-end delay lower than 150 ms End-to-end delay of 50 ms for face-to-face dialogue applications

Retrieval Mode Applications

• A human user retrieves information from a multimedia database• Requirements:

Fast forward and backward data retrieval with simultaneous display Fast search for information in multimedia databases Random access to single images and audio frames with an access time less than

0.5 second Decompression should be possible without a link to other data units in order to

allow random access and editing

Dialogue and Retrieval Mode27

Requirements for both dialogue and retrieval mode:

• Supporting scalable video in different systems Format must be independent of frame size and video frame rate

• Support of various audio and video data rates This will lead to different quality, thus data rates should be adjustable

• Synchronization of audio and video data Lip synchronization

• Economy (i.e. reasonably cheap solutions): Software realization: cheap, but low “speed” and low “quality” Hardware realization (VLSI chips): more expensive (at first), but high quality

• Compatibility It should be possible to generate multimedia data on one system and to reproduce

the data on another system Programs available on CD can be read on different systems

Encoding Mechanisms for Video28

Basic encoding techniques like used in JPEG

Differential encoding for video1. For newscast, video telephone applications, and soap operas

• Background often remains the same for a long time• Very small difference between subsequent images

→ Run-length coding can be used

2. Motion compensation• Blocks of NxN pixels are compared in subsequent images• Useful for objects moving in one direction, e.g. from left to right

Other basic compression techniques:

• Color Look-Up Tables (CLUT) for data reduction in video streams Often used in distributed multimedia systems

• Silence suppression for audio Data are only encoded if the volume level exceeds a certain threshold Can be interpreted as a special case of run-length encoding

MPEG29

Growing need for a common format for representing compressed video and audio for data rates up to 1.5 Mbit/sec (typical rate of CD-ROM transfer: 1.2 Mbit/sec)

→ Moving Pictures Expert Group (MPEG)

• Generic Approach → can be used widely

• Maximum data rate for video in MPEG is very high: 1,856,000 bit/sec• Data rates for audio between 32 and 448 Kbit/sec

→ Video and audio compression of acceptable quality

• Suitable for symmetric as well as asymmetric compression: Asymmetric compression: more effort for coding (once) than for decoding (often) Symmetric compression: equal effort for compression and decompression,

restricted end-to-end delay (e.g. interactive dialogue applications)

MPEG Today30

MPEG• Coding for VCD (Video CD) quality• Data rate of 0.9 - 2 Mbit/secMPEG-2• Super-set of MPEG-1: rates up to 8 Mbit/sec• Can do HDTVMPEG-4• Coding of objects, not frames• Lower bandwidth (Multimedia for the web and mobility)MPEG-7• Allows multimedia content description (ease of searching)MPEG-21• Content identification and managementMP3• For coding audio only• MPEG Layer-3

The MPEG Family31

First: MPEG32

Exact image format of MPEG is defined in the image preparation phase (which is similarto JPEG)

• Video is seen as a sequence of images (video frames)

• Each image consists of 3 components (YUV format, called Y, CB, CR) Luminance component has twice as many samples in horizontal and vertical axes

than other two components (chrominance): 4:2:2 scheme Resolution of luminance component maximal 768×576 pixels (8 bit per pixel)

• Data stream includes further information, e. g.: Aspect ratio of a pixel (14 different image aspect ratios per pixel provided), e.g.

1:1 (square pixel), 16:9 (European and US HDTV) etc. Image refresh frequency (number of images per second; 8 frequencies between

23.976 Hz and 60 Hz defined, among them the European standard of 25 Hz)

• The encoding basically is as in JPEG: DCT, quantization, entropy encoding, but with considering several “images”

Compression Steps33

• Subsampling of chrominance information - human visual system less sensitive tochrominance than to luminance information→ only 1 chrominance pixel for each 2×2 neighborhood of luminance pixels

• Image preparation – form blocks of 8×8 pixels and group them to macro blocks• Frequency transformation - discrete cosine transform converts an 8×8 block of pixel

values to an 8×8 matrix of horizontal and vertical spatial frequency coefficients→ most of the energy concentrated in low frequency coefficients, esp. in DC coefficient

• Quantization – for suppressing high frequencies• Variable length coding - assigning codewords to values to be encoded

Additionally to techniques like in JPEG, the following is used before performing DCT:• Predictive coding – code a frame as a prediction based on the previous / the

following frame• Motion compensation - prediction of the values of a pixel block by relocating the

block from a known picture

Macro Blocks34

Still images - temporal prediction yields considerable compression ratio

Moving images - non-translational moving patterns (e.g. rotations, waves, ...) requirestorage of large amount of information due to irregular motion patterns

• Therefore: predictive coding makes sense only for parts of the image***** → division of each image into areas called macro blocks *****

• Macro blocks turn out to be suitable for compression based on motion estimation• Partition of macro blocks into 4 blocks for luminance and one block for each

chrominance component; each block consists of 8×8 pixels. Size of macro block is acompromise between (storage) cost for prediction and resulting compression

luminance Y chrominance

��

Motion Compensation Prediction35

• Motion Compensation Prediction is made between successive frames

• Idea: coding a frame as prediction to the previous frame is useless for fast changingsequences

• Thus: consider moving objects by search for the new position of a macro block fromthe previous frame

• Code the prediction to the macro block of the previous frame together with themotion vector


How to find the best fitting position in the new image?• Search only a given window around the old position, not the whole image!• Consider only the average of all pixel values, not detailed values!• Set a fault threshold: stop

searching if a found macroblock fits „good enough“

• Search Pattern can be a spiral:

this procedure in generaldoes not give the best result,but is fast


Alternative search procedure:

• Store motion vector for a macro block of the previous image• Move the search window for the next search by the old vector• Start searching the window by a coarse pattern• Refine the search for the “best” patterns found

Left: lighter grey blocks are the searched blocks on coarse scale. Best matching fields are examined more detailedby also searching the 8 neighbored blocks. Right: when we would refine all blocks, such a picture would result; thelighter a block, the better the matching; the “lightest” block is taken for motion prediction. [Note: in reality not allblocks are refined, only the “lightest” ones; by this maybe the best block isn’t found.]

Types of Frames38

4 different types of frame coding are used in MPEG for efficient coding with fast randomaccess

• I-frames: Intra-coded frames (moderate compression but fast random access)• P-frames: Predictive-coded frames (with motion compensation, prediction to the previous

I- or P-frame)• B-frames: Bi-directionally predictive-coded frames (referencing to the previous and the

following I- or P-frames)• D-frames: DC frames (Limited use: encodes only DC components of intraframe coding)

B-frame can be decoded only afterthe subsequent P-frame has been decoded

Types of Images39

Bi-directionalprediction

I : P : B = 1 : 2 : 6

Prediction

Time

I-Frames40

• I-Frames are self-contained, i.e. represent a full image→ Coded without reference to other images

• Treated as still images→ Use of JPEG, but compression in real-time

• I-Frames may serve as points of random access in MPEG streams

• DCT on 8×8 blocks within macro blocks + DPCM coding of DC coefficients

• Typically, an I-frame may occur 3 times per second to give reasonably fast randomaccess

• Typical data allocation: I-frames allocate up to 3 times as many bits as P-frames P-frames allocate 2 – 5 times as many bits as B-frames

→ In case of li�le mo�on in the video, a greater propor�on of the bits should beassigned to I-frames, since P- and B-frames only need very low number of bits

P-Frames and B-Frames41

P-Frame• Requires information about previous I-frame and/or previous P-frame• Motion estimation is done for the macro blocks of the coded frame:

The motion vector (difference between locations of the macro blocks) is specified The (typically small) difference in content of the two macro blocks is computed

and DCT/entropy encoded• That means: P-frames consist of I-frame macro blocks (if no prediction is possible) and

predictive macro blocks

B-Frame• Requires information about previous and following I- and/or P-frame

→ B-frame = difference of prediction of past image and following P-/I-frame• Quantization and entropy encoding of the macro blocks will be very efficient on such

double-predicted frames• Highest compression rate will be obtained• Decoding only is possible after receiving the

following I- resp. P-frame

Group of Pictures (GOP)42

MPEG gives no instruction in which order to code the different frame types, but can bespecified by a user parameter.

But: each stream of MPEG frames shows a fixed pattern, the Group of Pictures:

• It typically starts with an I-frame• It typically ends with frame right before the next I-frame• “Open” GOP ends in B-frame, “Closed” in P-frame

Very flexible: GOPs could be independently decoded, but they also couldreference to the next GOP

• Typical patterns: I B B P B B P B B I I B B P B B P B B P B B I

• Why not have all P and B frames?→ It is clear that with the loss of one frame, a new

full-image (I-frame) is needed to allow thereceiver to recover from the information loss

D-Frames43

Intraframe encoded, but only lowest frequencies of an image (DC coefficients) are encoded• Used (only) for fast forward or fast rewind mode

• Could also be realized by suitable order of I-frames

• Slow rewind playback requires a lot of storage capacity:

Thus all images in a "group of pictures" (GOP) are decoded in the forwardmode and stored, after that rewind playback is possible

Coding Process44

Layers of MPEG Data Streams45

1. Sequence Layer Sequence header + one or more groups of picture Header contains parameters like picture size, data rate, aspect ratio, DCT

quantization matrices

2. Group of Pictures Layer Contains at least one I-frame for random access Additionally timing info and user data

3. Picture Layer I-, P-, B- or D-frame, with synchronization info, resolution, range of motion vectors

4. Slice Layer Subdivision of a picture providing certain immunity to data corruption

5. Macro block Layer Basic unit for motion compensation and quantizer scale changes

6. Block Layer Basic coding unit (8×8 pixels): DCT applied at this block level

Hirarchical Structure of Data46

SequenceLayer

GOPLayer

PictureLayer

SliceLayer

MacroblockLayer

BlockLayer

Audio Encoding47

Audio Encoding within MPEG

• Picture encoding principles can be modified for use in audio as well

• Sampling rates of 32, 44.1 and 48 kHz

• Transformation into frequency domain by Fast Fourier Transform (FFT) similar to thetechnique which is used for video

• Audio spectrum is split into 32 non-interleaved subbands (for each subband, the audioamplitude is calculated); noise level determination by psychoacoustical model

“Psychoacustical model” means to consider the human brain, e.g. recognizing onlya single tone if two similar tones are played very close together, or not perceivingthe quieter tone if two tones with highly different loudness are playedsimultaneously

Each subband has its own quantization granularity Higher noise level: rough quantization (and vice versa)

• “Single channel”, “two independent channels” or “Stereo” are possible (in the case of“stereo”: redundancy between the two signals is used for higher compression ratio)

Audio Encoding48

Audio Encoding49

3 different layers of encoder and decoder complexity are used

• Quantized spectral portions of layers 1 and 2 are PCM encoded

• Quantized spectral portions of layer 3 are Huffman encoded –

MPEG layer 3 is known as “mp3”

• 14 fixed bit rates for encoded audio data stream on each layer minimal rate: 32 Kbit/sec for each layer maximal rate: 448 Kbit/sec (layer 1), 384 Kbit/sec (layer 2),

320 Kbit/sec (layer3)

• Variable bit rate support is possible only on layer 3

Audio Data Stream50

General Background on the MPEG Audio Data Stream

• MPEG specifies syntax for interleaved audio and video streams – e.g. synchronizationinformation

• Audio data stream consists of frames, divided into audio access units composed of slots

• Slots consist of 4 bytes (layer 1, lowest complexity) or 1 byte (other layers)• A frame always consists of a fixed number of samples• Audio access unit: smallest possible audio sequence of compressed data to be

decoded independently of all other data• Audio access units of one frame lead to playing time between 8 msec (48 Hz) and 12

msec (32 Hz)

MPEG-251

Why another MPEG standard?• Higher data rate as MPEG-1, but compatible extension of MPEG-1• Target rate of 40 Mbit/sec• Higher resolution as needed in HDTV• Support a larger number of applications - definition of MPEG-2 in terms of extensible

profiles and levels for each important application class, e.g. Main Profile: for digital video transmission (2 to 80 Mbit/sec) over cable, satellite

and other broadcast channels, digital storage, HDTV etc. High Profile: HDTV Scalable Profile: compatible with terrestrial TV/HDTV, packet-network video

systems, backward compatibility with MPEG-1 and other standards, e.g. H.261• The encoding standard should be a toolkit rather than a flat procedure

Interlaced and non-interlaced frame Different color subsampling modes e.g., 4:2:2, 4:2:0 Flexible quantization schemes – can be changed at picture level Scalable bit-streams

MPEG-2 - Profiles and Levels52

Scalable Profiles53

A signal is composed of several streams (layers):

• Base (Lower) layer is a fully decodable image

• Enhancement (Upper) layer gives additional information

Better resolution Higher frame rate Better quality

→ Corresponds to JPEG hierarchical mode

Scalable Profiles54

Scaling can be done on different parameters:

• Spatial scaling:

Frames are given in different resolutions Base layer frames are used in any case Upper layer frames are stored as prediction from base layer frames

→ Single data stream can include different image formats (CIF, CCIR 601, HDTV ...)

• SNR scaling: The error on the lower layer given by quantization is encoded and sent on the

upper layer

MPEG-2: Effects of Interlacing55

Prediction Modes and Motion Compensation

Frame prediction: current frame predicted from previous frame

Dual prime motion compensation:• Top field of current frame is predicted from two motion vectors coming

from the top and bottom field of reference frame• Bottom field of previous frame and top field of current frame predicts

the bottom field of current frame

16 × 8 motion compensation mode• A macroblock may have two of them• A B-image macro block may have four

MPEG-2 Audio Standard56

• Low bit rate coding of multi-channel audio

• Up to five full bandwidth channels (left, right, center, 2 surround) plus additional low frequency enhancement channel and/or up to 7 commentary/multilingual channels

• Extension of MPEG-1 stereo and mono coding to half sampling rates (16 – 24 kHz)→ improving quality for bit rates at 64 kbits/sec per channel

MPEG-2 Audio Multi-channel Coding Standard:

• Backward compatibility with existing MPEG-1 Audio Standard

• Organizes formal testing of proposed MPEG-2 multi-channel audio codecs and nonbackward- compatible codecs (rates 256 – 448 Kbits/sec)

MPEG-2 Streams57

• MPEG-2 system defines how to combine audio, video and other data into single ormultiple streams suitable for storage and transmission→syntactical and semantically rules for synchronizing the decoding and presentation of

video and audio information and avoiding buffer over- or underflow

• Streams include timestamps for decoding, presentation and delivery

• Basic multiplexing step: each stream is added system-level information and packetized→ Packetized Elementary Stream (PES)

• PESs combined to Program or Transport Stream (supporting large number ofapplications): Program Stream: similar to MPEG-1 stream

→ error-free environment, variable packet lengths, constant end-to-end delay Transport Stream: combines PESs and independent time bases into single stream

→ use in lossy or noisy media, packet length 188 bytes including header→ suited for digital TV and videophony over fiber, satellite, cable, ISDN, ATM

Conversion between Program and Transport Stream possible (and sometimesreasonable)

MPEG-458

• Originally a MPEG-3 standard for HDTV was planned→ But MPEG-2 scaling was sufficient; development of MPEG-3 was cancelled

• MPEG-4 initiative started in September 1993→ Very low bit rate coding of audio-visual programs

• February 1997: description of requirements for the MPEG-4 standard approved

• Idea: development of fundamentally new algorithmic techniques→ New sorts of interactivity (dynamic instead of static objects)→ Integration of natural and synthetic audio and video material→ Simultaneous use of material coming from different sources→ Model-based image coding of human interaction with multimedia environments→ Low-bit rate speech coding e.g. for use in GSM

• Basic elements: Coding tools for audio-visual objects: efficient compression, support of object

based interactivity, scalability and error robustness Formal methods for syntactic description of coded audio-visual objects

Core Idea of MPEG-459

Object based Representation

Representation of the video scene is understood as a composition of video objects with respect to their spatial and temporal relationship (same with audio!)

→Individual objects in a scene can be coded with different parameters, at differentquality levels and with different coding algorithms

MPEG-4: Objects and Scenes60

A/V object• A video object within a scene• The background• An instrument or voice• Coded independently

A/V scene• Mixture of objects• Individual bitstreams are multiplexed and transmitted• One or more channels• Each channel may have its own quality of service• Synchronization information

Objects of a Scene61

Scene Graph

• Graph without cycles• Embeds objects in a coordinate system (including synchronization information)• MPEG-4 provides a language for describing objects (oriented at VRML – Virtual

Reality Modeling Language)• Usable for as well video as animated objects

An Example MPEG-4 Scene62

MPEG-4 Stream Composition and Delivery63

Linking Streams into the Scene64

MPEG-765

• Objectives

A flexible, extensible, and multi-level standard framework for describing (notcoding!) multimedia and synchronize between content and descriptions

Enable fast and efficient content searching, filtering and identification Define low-level features, structure, semantic, models, collections, creation, etc. Goal: To search, identify, filter and browse audiovisual content

• Description of contents Descriptors

• Describe basic characteristics of audiovisual content• Examples: Shape, Color, Texture, …

Description Schemes

• Describe combinations of descriptors• Example: Spoken Content

Simple Description66

MPEG-2167

• MPEG 21

Solution for access to and management of digital media E.g. offering, searching, buying, Digital Rights Management, …

Digital Video Broadcasting68

•1991 foundation of the ELG (European Launching Group)Goal: development of digital television in Europe

• 1993 renaming into DVB (Digital Video Broadcasting)goal: introduction of digital television based on satellite transmission (DVB-S) cable network technology (DVB-C) later also terrestrial transmission (DVB-T)

DVB Container69

DVB transmits MPEG-2 container• High flexibility for the transmission of digital data• No restrictions regarding the type of information• DVB Service Information specifies the content of a container

NIT (Network Information Table): lists the services of a provider, contains additionalinformation for set-top boxes

SDT (Service Description Table): list of names and parameters for each servicewithin a MPEG multiplex channel

EIT (Event Information Table): status information about the current transmission,additional information for set-top boxes

TDT (Time and Date Table): Update information for set-top boxes

DVB Worldwide70

Computer-based Animation71

To animate = “to bring to life”Animation covers changes in:

• time-varying positions (motion dynamics)• shape, color, transparency, structure and texture of an object (update dynamics) as

well as lightning, camera position, camera orientation and focus

Basic Concepts of animation are

• Input Process Key frames, where animated objects are at extreme or characteristic positions

must be digitized from drawings Often a post-processing by a computer is required

• Composition stage• Inbetween process• Changing colors

Composition Stage72

• Foreground and background figures are combined to generate an individual frame• Placing of several low-resolution frames of an animation in an array leads to a trail film

(pencil test), by the use of the pan-zoom feature (This feature is available for someframe buffers)

• The frame buffer can take a part of an image (pan) and enlarge it to full screen (zoom)• Continuity is achieved by repeating the pan-zoom process fast enough

Inbetween Process73

• Composition of intermediate frames between key frames• Performed by linear interpolation (lerping) between start- and end-positions• To achieve more realistic results, cubic spline-interpolation can be used

Interpolatedframes

Rather unrealisticmotion (in most

cases)

Key frames

Inbetween Process74

A function s is called cubic interpolating spline to the pointsa = X� < X� < ... < X�� = b, if

1. s is twice continuous differentiable2. for i = 0, ..., n it is a polynomial of degree 3

This line is smooth, because the polynomials have equal primary and secondaryderivatives at the points X� , X� ,... , X��

more realisticmotion achieved

by two cubicsplines

X� X� X�

Inbetween Process75

Calculation of successive cubic splines:

• ��(x) are polynomials of degree 3• Let ��(x) be given• Then ��(x) = ��·x� + ��· x� + ��·x + �� is constructed as follows:

��(x�) = ��·x�� + ��·x�

� + ��·x� + �� =� f(x�)

��(x�) = ��·x�� + ��·x�

� + ��·x� + �� =� f(x�)

�� (x�) = 3��·x�

� + 2��·x�+ �� =� �� (x�)

�� (x�) = 6��·x�+ 2�� =� ��

�� (x�)

4 equations for ��,��, ��, ��

Changing Colors76

Two techniques are possible

1. CLUT animationChanging of the Color Look Up Table (CLUT) of the frame buffer. This changes thecolors of the image.

2. New color information for each frameFrame buffer: 640 x 512 pixel · 8 Bits/pixel · 30 frames per sec. = 78.6 MBits/secdata rate for complete update

The first technique is much faster than the second, since changing the CLUT requiresthe transmission of only 0.3 - 3 Kbytes (here 2. is more than 300 times faster than 1.)

Animation Languages77

Categories for Animation languages• Linear-list Notations

Events are described by starting and ending frame number and an action (event) 17, 31, C, ROTATE “HOUSE”, 1, 45 means:

Between frames 17 and 31 rotate the object HOUSE around axis 1 by 45 degrees,determining the amount of rotation at each frame from table C

• General-purpose Languages Embed animation capability within programming languages Values of variables as parameters to the routines that perform animation e.g. ASAS, which is built on top of LISP:

(grasp my-cube): cube becomes current object(cw 0.05): spin it clockwise, by a small amount

• Graphical Languages Describe animation in a more visual way than textual languages Express, edit and comprehend the changes in an animation Explicit descriptions of actions are replaced by a picture of the action

Controlling of Animation78

Techniques for controlling animations (independent of the language which describes theanimation):

• Full Explicit Control– Complete way of control, because all aspects are defined:

Simple changes (scaling, translation, rotation) are specified or key frames andinterpolation methods (either explicit or by direct manipulations by mouse, joystick,data glove) are provided

• Procedural Control– Communication between objects to determine properties– Physically-based systems: position of one object may influence motion of another

(ball cannot pass a wall)– Actor-based systems: actors pass their position to other actors to affect their

behavior (actor A stays behind actor B)


Constraint-based Systems• “Natural” way of moving from A to B is via a straight line, i.e. linearly. However, very

often the motion is more complicated• Movement of objects is determined by other objects, they are in contact with• Compound motion may not be linear and is modeled by constraints (ball follows a

pathway)

Tracking Live Action• Trajectories of animated objects are generated by tracking live action• Rotoscoping: Film with real actors as template, designers draw over the film, change

background and replace human actors with animated counterparts• Attach indicators to key points of actor’s body. Tracking of indicator

positions provides key points in the animation model• Another example: Data glove measures

position and orientation of the hand flexion and extension of fingers and fingerparts

From these information we can calculate actions, e.g. movements


Kinematics:

• Description using the position and velocity of objects.• E.g. at time t = 0 the CUBE is at the origin. It moves with the constant acceleration of

0.5 m/�� for 2 sec. in the direction of (1,1,4)

(0, 0, 0) → (1, 1, 4)2 seconds

b = 0.5 m/��

Dynamics:• Takes into consideration the physical

laws that define the kinematics• E.g. at time t = 0 the CUBE is in position

(0 meters, 100 meters, 0 meters) andhas a mass of 5 kg. The force of gravityacts on the cube (Result in this case: theball will fall down)

kinematical description ofthe motion of a cube

(0, 100, 0)

Display of Animation81

For the display of animations with raster systems the animated objects have to be scan-converted to their pixmap in the frame buffer. This procedure has to be done at least 10(better: 20) times per second in order to give a reasonably smooth effect.

ProblemFrame rate of 20 pictures/sec. requires manipulation, scan-conversion and display of anobject in only 50 msec. Scan conversion should only use a small fraction of these 50msec since other operations (erasing, redrawing, ... etc) have to be done, too

SolutionDouble-buffering: frame buffer is divided into twoimages, each with half of the bits of the overall framebuffer (“pipeline”). While the operation (like rotating) andscan-conversion is processed for the second half of thepixmap, the first half is displayed and vice versa.

Time

Transmission of Animation82

Symbolic Representation• Graphical descriptions (circle) of an animated object (ball) + operations (roll)• Animation is displayed at the receiver by scan-conversion of objects to pixmap• Transmission rate depends on (transmission rate is context dependent):

size of the symbolic representation structure, size of operation structure number of animated objects and of commands

Pixmap Representation• Longer times for data transmission than with symbolic representation, because of

the large data size of pixmap• Shorter display times, because no scan-conversion is necessary at receiver side• Transmission rate = size of pixmap · frame rate (fixed transmission rate)

Conclusions83

NTSC and PAL as television standards• Widespread, but only belong to Enhanced Definition TV systems• Needed for better quality: High Definition TV (HDTV)• Problem: compression is needed for HDTV systems

MPEG as standard for video and audio compression• High-quality video/audio compression based on JPEG-techniques• Additionally: Motion prediction between video frames• Newer versions (MPEG-4) achieve further compression by considering objects

Video Transmission• DVB as one standard for broadcasting SDTV, EDTV, HDTV or any MPEG content to

the customer

Animation• Technique for artificially creating “videos”

multimedia communication systems 1 - صفحه...

Documents