introduction to multimedia1. 2 introduction zmultimedia description zwhy multimedia systems?...

Introduction to Multimedia 1

Introduction to Multimedia


Introduction

Multimedia DescriptionWhy multimedia systems?Classification of MediaMultimedia SystemsData Stream Characteristics


Multimedia Description

Multimediais an integration of continuous media (e.g. audio,

video) and discrete media (e.g. text, graphics, images) through which digital information can be conveyed to the user in an appropriate way.

Multimany, much, multiple

MediumAn interleaving substance through which something

is transmitted or carried on


Why Multimedia Computing?

Application drivene.g. medicine, sports, entertainment, education

Information can often be better represented using audio/video/animation rather than using text, images and graphics alone.

Information is distributed using computer and telecommunication networks.

Integration of multiple media places demands oncomputation powerstorage requirementsnetworking requirements


Multimedia Information Systems

Technical challenges Sheer volume of data

Need to manage huge volumes of data

Timing requirementsamong components of data computation and

communication.Must work internally with given timing constraints - real-

time performance is required.

Integration requirementsneed to process traditional media (text, images) as well as

continuous media (audio/video).Media are not always independent of each other -

synchronization among the media may be required.


High Data Volume of Multimedia Information

Speech 8000 samples/s 8Kbytes/s

CD Audio 44,100 samples/s, 2 bytes/sample

176Kbytes/s

Satellite Imagery

180X180 km 2̂ 30m 2̂ resolution

600MB/image (60MB compressed)

NTSC Video 30fps, 640X480 pixels, 3bytes/pixel

30Mbytes/s (2-8 Mbits/s compressed)


Technology Incentive

Growth in computational capacityMM workstations with audio/video processing capabilityDramatic increase in CPU processing power Dedicated compression engines for audio, video etc.

Rise in storage capacityLarge capacity disks (several gigabytes)Increase in storage bandwidth,e.g. disk array

technology

Surge in available network bandwidthhigh speed fiber optic networks - gigabit networksfast packet switching technology


Application Areas

Residential Servicesvideo-on-demandvideo phone/conferencing systemsmultimedia home shopping (MM catalogs, product

demos and presentation)self-paced education

Business ServicesCorporate trainingDesktop MM conferencing, MM e-mail


Application Areas

EducationDistance education - MM repository of class videosAccess to digital MM libraries over high speed

networks

Science and Technologycomputational visualization and prototypingastronomy, environmental science

MedicineDiagnosis and treatment - e.g. MM databases that

provide support for queries on scanned images, X-rays, assessments, response etc.


Classification of Media

Perception MediumHow do humans perceive information in a computer?

• Through seeing - text, images, video • Through hearing - music, noise, speech

Representation MediumHow is the computer information encoded?

• Using formats for representing and information• ASCII(text), JPEG(image), MPEG(video)

Presentation MediumThrough which medium is information delivered by

the computer or introduced into the computer?• Via I/O tools and devices• paper, screen, speakers (output media)• keyboard, mouse, camera, microphone (input media)


Classification of Media (cont.)

Storage Medium• Where will the information be stored?• Storage media - floppy disk, hard disk, tape, CD-ROM

etc. Transmission Medium

• Over what medium will the information be transmitted?• Using information carriers that enable continuous data

transmission - networks• wire, coaxial cable, fiber optics

Information Exchange Medium• Which information carrier will be used for information

exchange between different places?• Direct transmission using computer networks• Combined use of storage and transmission media (e.g.

electronic mail).


Media Concepts

Each medium definesRepresentation values - determine the information

representation of different media• Continuous representation values (e.g. electro-

magnetic waves)• Discrete representation values(e.g. text characters in

digital form)Representation space determines the surrounding

where the media are presented.• Visual representation space (e.g. paper, screen)• Acoustic representation space (e.g. stereo)


Media Concepts (cont.)

Representation dimensions of a representation space are: Spatial dimensions:

two dimensional (2D graphics)three dimensional (holography)

Temporal dimensions:Time independent (document) - Discrete media

• Information consists of a sequence of individual elements without a time component.

Time dependent (movie) - Continuous media• Information is expressed not only by its individual value

but also by its time of occurrence.


Multimedia Systems

Qualitative and quantitative evaluation of multimedia systems Combination of media

continuous and discrete.

Levels of media-independencesome media types (audio/video) may be tightly

coupled, others may not.

Computer supported integrationtiming, spatial and semantic synchronization

Communication capability


Data Streams

Distributed multimedia communication systems

data of discrete and continuous media are broken into individual units (packets) and transmitted.

Data Streamsequence of individual packets that are transmitted in

a time-dependant fashion.Transmission of information carrying different media

leads to data streams with varying features• Asynchronous• Synchronous • Isochronous


Data Stream Characteristics

Asynchronous transmission mode • provides for communication with no time restriction• Packets reach receiver as quickly as possible, e.g.

protocols for email transmissionSynchronous transmission mode

• defines a maximum end-to-end delay for each packet of a data stream.

• May require intermediate storage• E.g. audio connection established over a network.

Isochronous transmission mode• defines a maximum and a minimum end-to-end delay

for each packet of a data stream. Delay jitter of individual packets is bounded.

• E.g. transmission of video over a network.• Intermediate storage requirements reduced.


Data Stream Characteristics

Data Stream characteristics for continuous media can be based on

Time intervals between complete transmission of consecutive packets

• Strongly periodic data streams - constant time interval• Weakly periodic data streams - periodic function with finite

period.• Aperiodic data streams

Data size - amount of consecutive packets• Strongly regular data streams - constant amount of data• Weakly regular data streams - varies periodically with time• Irregular data streams

Continuity• Continuous data streams• Discrete data streams


Classification based on time intervals

Strongly periodic data stream

Weakly periodic data stream

Aperiodic data stream

T

T

T1 T3T2

T1 T2

T


Classification based on packet size

TD1

D1

TD1D2D3D1D2D3

D1D2D3

Dn

Strongly regular data stream

Weakly regular data stream

Irregular data stream

t

t

t


Classification based on continuity

Continuous data stream

Discrete data stream

D

D1 D2 D3 D4

D

D1 D2 D3 D4


Broadband Multimedia Communications

Audio/Image/Video Representation


Introduction

Basic Sound ConceptsComputer Representation of SoundBasic Image ConceptsImage Representation and FormatsVideo Signal RepresentationColor Encoding Computer Video Format


Basic Sound Concepts

Acoustics study of sound - generation, transmission and

reception of sound waves.

Sound is produced by vibration of matter.During vibration, pressure variations are created in

the surrounding air molecules.Pattern of oscillation creates a waveform

• the wave is made up of pressure differences.Waveform repeats the same shape at intervals called

a period.• Periodic sound sources - exhibit more periodicity, more

musical - musical instruments, wind etc.• Aperiodic sound sources - less periodic - unpitched

percussion, sneeze, cough.



Sound TransmissionSound is transmitted by molecules bumping into each

other.Sound is a continuous wave that travels through air.

Sound is detected by measuring the pressure level at a point.

ReceivingMicrophone in sound field moves according to the

varying pressure exerted on it.Transducer converts energy into a voltage level (i.e.

energy of another form - electrical energy) Sending

Speaker transforms electrical energy into sound waves.


Frequency of a sound wave

period

amplitude

time

Airpressure

Frequency is the reciprocal value of the period.



Wavelength is the distance travelled in one cycle

20Hz is 56 feet, 20KHz is 0.7 in.

Frequency represents the number of periods in a second (measured in hertz, cycles/second).

Frequency is the reciprocal value of the period.Human hearing frequency range: 20Hz - 20Khz, voice

is about 500Hz to 2Khz. Infrasound from 0 - 20 Hz Human range from 20Hz - 20KHz Ultrasound from 20kHz - 1GHz Hypersound from 1GHz - 10THz



Amplitude of a sound is the measure of the displacement of the air pressure wave from its mean or quiescent state.

Subjectively heard as loudness. Measured in decibels.

0 db - essentially no sound heard

35 db - quiet home 70 db - noisy street 120db - discomfort


Computer Representation of Audio

A transducer converts pressure to voltage levels.

Convert analog signal into a digital stream by discrete sampling.

Discretization both in time and amplitude (quantization).

In a computer, we sample these values at intervals to get a vector of values.

A computer measures the amplitude of the waveform at regular time intervals to produce a series of numbers (samples).


Computer Representation of Audio

Sampling Rate:rate at which a continuous wave is sampled (measured in

Hertz)• CD standard - 44100 Hz, Telephone quality - 8000 Hz.

Direct relationship between sampling rate, sound quality (fidelity) and storage space.

Question• How often do you need to sample a signal to avoid losing

information?Answer

• To decide a sampling rate - must be aware of difference between playback rate and capturing(sampling) rate.

• It depends on how fast the signal is changing. In reality - twice per cycle (follows from the Nyquist sampling theorem).


Sampling

samples

SampleHeight


Nyquist Sampling Theorem

If a signal f(t) is sampled at regular intervals of time and at a rate higher than twice the highest significant signal frequency, then the samples contain all the information of the original signal.

ExampleActual playback frequency for CD quality audio is

22050 HzBecause of Nyquist Theorem - we need to sample the

signal twice, therefore sampling frequency is 44100 Hz.


Data Rate of a Channel

Noiseless Channel• Nyquist proved that if any arbitrary signal has been run

through a low pass filter of bandwidth H, the filtered signal can be completely reconstructed by making only 2H (exact) samples per second. If the signal consists of V discrete levels, Nyquist’s theorem states:

max datarate = 2 *H log_2 V bits/sec• noiseless 3kHz channel with quantization level 1 bit

cannot transmit binary signal at a rate exceeding 6000 bits per second.

Noisy Channel• Thermal noise present is measured by the ratio of the

signal power S to the noise power N (signal-to-noise ratio S/N).

• Max datarate - H log_2 (1+S/N)


Quantization

Sample precision - the resolution of a sample value

Quantization depends on the number of bits used measuring the height of the waveform.

16 bit CD quality quantization results in 64K values.

Audio formats are described by sample rate and quantization.

• Voice quality - 8 bit quantization, 8000 Hz mono(8 Kbytes/sec)

• 22kHz 8-bit mono (22kBytes/s) and stereo (44Kbytes/sec)• CD quality - 16 bit quantization, 44100 Hz linear stereo (196

Kbytes/s)


Quantization and Sampling

samples

SampleHeight

0.75

0.5

0.25


Audio Formats

Audio formats are characterized by four parameters

Sample rate: Sampling frequencyEncoding: audio data representation

-law encoding corresponds to CCITT G.711 - standard for voice data in telephone companies in USA, Canada, Japan

• A-law encoding - used for telephony elsewhere.• A-law and -law are sampled at 8000 samples/second with

precision of 12bits, compressed to 8-bit samples.• Linear Pulse Code Modulation(PCM) - uncompressed audio

where samples are proportional to audio signal voltage.Precision: number of bits used to store audio sample

-law and A-law - 8 bit precision, PCM can be stored at various precisions, 16 bit PCM is common.

Channel: Multiple channels of audio may be interleaved at sample boundaries.


Audio Formats

Available on UNIX au (SUN file format), wav (Microsoft RIFF/waveform

format), al (raw a-law), u (raw u-law)…

Available on Windows-based systems (RIFF formats) wav, midi (file format for standard MIDI files), avi

RIFF (Resource Interchange File Format) tagged file format (similar to TIFF).. Allows multiple

applications to read files in RIFF format

RealAudio, MP3 (MPEG Audio Layer 3)


Computer Representation of Voice

Best known technique for voice digitization is pulse-code-modulation (PCM). Consists of the 2 step process of sampling and

quantization. Based on the sampling theorem.

If voice data are limited to 4000Hz, then PCM samples 8000 samples per second which is sufficient for input voice signal.

PCM provides analog samples which must be converted to digital representation.

Each of these analog samples must be assigned a binary code. Each sample is approximated by being quantized.


Computer Representation of Music

MIDI (Music Instrument Digital Interface)standard that manufacturers of musical instruments use

so that instruments can communicate musical information via computers.

The MIDI interface consists of:• Hardware - physical connection b/w instruments, specifies a

MIDI port (plugs into computers serial port) and a MIDI cable.• Data format - has instrument specification, notion of

beginning and end of note, frequency and sound volume. Data grouped into MIDI messages that specify a musical event.

• An instrument that satisfies both is a MIDI device (e.g. synthesizer)

MIDI software applications include• music recording and performance applications, musical

notations and printing applications, music education etc.


Computer Representation of Speech

Human ear is most sensitive in the range 600Hz to 6000 Hz.

Speech Generation• real-time signal generation allows transformation of text into

speech without lengthy processing• Limited vs. large vocabulary (depends on application)• Must be understandable, must sound natural

Speech Analysis• Identification and Verification - recognize speakers using

acoustic fingerprint• Recognition and Understanding - analyze what has been said• How something was said - used in lie detectors.

Speech transmission - coding, recognition and synthesis methods - achieve minimal data rate for a given quality.


Basic Concepts (Digital Image Representation)

An image is a spatial representation of an object, a 2D or 3D scene etc.

Abstractly, an image is a continuous function defining a rectangular region of a plane

intensity image - proportional to radiant energy received by a sensor/detector

range image - line of sight distance from sensor position.

An image can be thought of as a function with resulting values of the light intensity at each point over a planar region.


Digital Image Representation

For computer representation, function (e.g. intensity) must be sampled at discrete intervals.

Sampling quantizes the intensity values into discrete intervals.

• Points at which an image is sampled are called picture elements or pixels.

• Resolution specifies the distance between points - accuracy.A digital image is represented by a matrix of numeric

values each representing a quantized intensity value.• I(r,c) - intensity value at position corresponding to row r and

column c of the matrix.• Intensity value can be represented by bits for black and

white images (binary valued images), 8 bits for monochrome imagery to encode color or grayscale levels, 24 bit (color-RGB).


Image Formats

Captured Image Formatformat obtained from an image frame grabberImportant parameters

• Spatial resolution (pixels X pixels)• Color encoding (quantization level of a pixel - 8-bit, 24-

bit)• e.g. “SunVideo” Video digitizer board allows pictures of

320 by 240 pixels with 8-bit grayscale or color resolution. Parallax-X video includes resolution of 640X480 pixels and 24-bit frame buffer.


Image Formats

Stored Image Format - format when images are stored

Images are stored as 2D array of values where each value represents the data associated with a pixel in the image.

Bitmap - this value is a binary digitFor a color image - this value may be a collection of

• 3 values that represent intensities of RGB component at that pixel, 3 numbers that are indices to table of RGB intensities, index to some color data structure etc.

Image file formats include - GIF (Graphical Interchange Format) , X11 bitmap, Postscript, JPEG, TIFF


Basic Concepts (Video Representation)

Human eye views video immanent properties of the eye determine essential

conditions related to video systems.

Video signal representation consists of 3 aspects:Visual Representation

• objective is to offer the viewer a sense of presence in the scene and of participation in the events portrayed.

Transmission• Video signals are transmitted to the receiver through a

single television channelDigitalization

• analog to digital conversion, sampling of gray(color) level, quantization.


Visual Representation

The televised image should convey the spatial and temporal content of the scene

Vertical detail and viewing distance• Aspect ratio: ratio of picture width and height (4/3 = 1.33 is

the conventional aspect ratio).• Viewing angle = viewing distance/picture height

Horizontal detail and picture width• Picture width (conventional TV service ) - 4/3 * picture height

Total detail content of the image• Number of pixels presented separately in the picture height =

vertical resolution• Number of pixels in the picture width

= vertical resolution*aspect ratio• product equals total number of picture elements in the image.



Perception of Depth• In natural vision, this is determined by angular separation

of images received by the two eyes of the viewer• In the flat image of TV, focal length of lenses and changes

in depth of focus in a camera influence depth perception.Luminance and Chrominance

• Color-vision - achieved through 3 signals, proportional to the relative intensities of RED, GREEN and BLUE.

• Color encoding during transmission uses one LUMINANCE and two CHROMINANCE signals

Temporal Aspect of Resolution• Motion resolution is a rapid succession of slightly different

frames. For visual reality, repetition rate must be high enough (a) to guarantee smooth motion and (b) persistance of vision extends over interval between flashes(light cutoff b/w frames).



Continuity of motion• Motion continuity is achieved at a minimal 15 frames per

second; is good at 30 frames/sec; some technologies allow 60 frames/sec.

• NTSC standard provides 30 frames/sec - 29.97 Hz repetition rate.

• PAL standard provides 25 frames/sec with 25Hz repetition rate.

Flicker effect• Flicker effect is a periodic fluctuation of brightness perception.

To avoid this effect, we need 50 refresh cycles/sec. Display devices have a display refresh buffer for this.

Temporal aspect of video bandwidth• depends on rate of the visual system to scan pixels and on

human eye scanning capabilities.


Transmission (NTSC)

Video bandwidth is computed as follows700/2 pixels per line X 525 lines per picture X 30

pictures per secondVisible number of lines is 480.

Intermediate delay between frames is1000ms/30fps = 33.3ms

Display time per line is33.3ms/525 lines = 63.4 microseconds

The transmitted signal is a composite signalconsists of 4.2Mhz for the basic signal and 5Mhz for

the color, intensity and synchronization information.


Color Encoding

A camera creates three signalsRGB (red, green and blue)

For transmission of the visual signal, we use three signals

• 1 luminance (brightness-basic signal) and 2 chrominance (color signals).

In NTSC, luminance and chrominance are interleavedGoal at receiver

• separate luminance from chrominance components• avoid interference between them prior to recovery of

primary color signals for display.


Color Encoding

RGB signal - for separate signal codingconsists of 3 separate signals for red, green and blue

colors. Other colors are coded as a combination of primary color. (R+G+B = 1) --> neutral white color.

YUV signalseparate brightness (luminance) component Y andcolor information (2 chrominance signals U and V)

• Y = 0.3R + 0.59G + 0.11B• U = (B-Y) * 0.493• V = (R-Y) * 0.877

Resolution of the luminance component is more important than U,V

Coding ratio of Y, U, V is 4:2:2


Color Encoding(cont.)

YIQ signalsimilar to YUV - used by NTSC format

• Y = 0.3R + 0.59G + 0.11B• U = 0.60R - 0.28G + 0.32 B• V = 0.21R -0.52g + 0.31B

Composite signalAll information is composed into one signalTo decode, need modulation methods for eliminating

interference b/w luminance and chrominance components.


Digitization

Refers to sampling the gray/color level in the picture at MXN array of points.

Once points are sampled, they are quantized into pixels

• sampled value is mapped into an integer• quantization level is dependent on number of bits used

to represent resulting integer, e.g. 8 bits per pixel or 24 bits per pixel.

Need to create motion when digitizing videodigitize pictures in timeobtain sequence of digital images per second to

approximate analog motion video.


Computer Video Format

Video Digitizer A/D converter

Important parameters resulting from a digitizer• digital image resolution• quantization• frame rate

E.g. Parallax X Video - camera takes the NTSC signal and the video board digitizes it. Resulting video has

• 640X480 pixels spatial resolution• 24 bits per pixel resolution• 20fps (lower image resolution - more fps)

Output of digital video goes to raster displays with large video RAM memories.

• Color lookup table used for presentation of color


Digital Transmission Bandwidth

Bandwidth requirement for imagesraw image transmission b/w = size of image = spatial

resolution x pixel resolutioncompressed image - depends on compression schemesymbolic image transmission b/w = size of instructions

and primitives carrying graphics variables

Bandwidth requirement for videouncompressed video = image size X frame ratecompressed video - depends on compression schemee.g HDTV quality video uncompressed - 345.6Mbps,

compressed using MPEG (34 Mbps with some loss of quality).



Multimedia Compression Techniques


Introduction

Coding Requirements Entropy Encoding

Content Dependent Coding• Run-length Coding• Diatomic Coding

Statistical Encoding• Huffman Coding• Arithmetic Coding

Source EncodingPredictive Coding

• Differential Pulse Code Modulation• Delta Modulation

Adaptive Encoding


Coding Requirements

Storage RequirementsUncompressed audio:

• 8Khz, 8-bit quantization implies 64 Kbits to store per second

CD quality audio:• 44.1Khz, 16-bit quantization implies storing 705.6Kbits/sec

PAL video format:• 640X480 pixels, 24 bit quantization, 25 fps, implies

storing 184,320,000 bits/sec = 23,040,000 bytes/sec

Bandwidth Requirementsuncompressed audio: 64KbpsCD quality audio: 705.6KbpsPAL video format: 184,320,000 bits/sec

COMPRESSION IS REQUIRED!!!!!!!


Coding Format Examples

JPEG for still images H.261/H.263 for video conferencing, music and

speech (dialog mode applications) MPEG-1, MPEG-2, MPEG-4 for audio/video

playback, VOD (retrieval mode applications) DVI for still and continuous video applications

(two modes of compression)• Presentation Level Video (PLV) - high quality

compression, but very slow. Suitable for applications distributed on CD-ROMs

• Real-time Video (RTV) - lower quality compression, but fast. Used in video conferencing applications.


Coding Requirements

Dialog mode applicationsEnd-to-end Delay (EED) should not exceed 150-200 msFace-to-face application needs EED of 50ms (including

compression and decompression).

Retrieval mode applicationsFast-forward and rewind data retrieval with

simultaneous display (e.g. fast search for information in a multimedia database).

Random access to single images and audio frames, access time should be less than 0.5sec

Decompression of images, video, audio - should not be linked to other data units - allows random access and editing


Coding Requirements

Requirements for both dialog and retrieval mode applications

Support for scalable video in different systems.Support for various audio and video rates.Synchronization of audio-video streams (lip

synchronization)Economy of solutions

• Compression in software implies cheaper, slower and low quality solution.

• Compression in hardware implies expensive, faster and high quality solution.

Compatibility• e.g. tutoring systems available on CD should run on

different platforms.


Classification of Compression Techniques

Entropy Coding• lossless encoding• used regardless of media’s specific characteristics• data taken as a simple digital sequence• decompression process regenerates data completely• e.g. run-length coding, Huffman coding, Arithmetic coding

Source Coding• lossy encoding• takes into account the semantics of the data• degree of compression depends on data content.• E.g. content prediction technique - DPCM, delta modulation

Hybrid Coding (used by most multimedia systems)• combine entropy with source encoding• E.g. JPEG, H.263, DVI (RTV & PLV), MPEG-1, MPEG-2, MPEG-

4


Steps in Compression

Picture preparation• analog-to-digital conversion• generation of appropriate digital representation• image division into 8X8 blocks• fix the number of bits per pixel

Picture processing (compression algorithm)• transformation from time to frequency domain, e.g. DCT• motion vector computation for digital video.

Quantization• Mapping real numbers to integers (reduction in precision).

E.g. U-law encoding - 12bits for real values, 8 bits for integer values

Entropy coding• compress a sequential digital stream without loss.


Compression Steps

Picture Preparation

Picture Processing

Quantization

Entropy Coding

CompressedPicture

UncompressedPicture

AdaptiveFeedbackLoop


Types of compression

Symmetric Compression• Same time needed for decoding and encoding phases• Used for dialog mode applications

Asymmetric Compression• Compression process is performed once and enough

time is available, hence compression can take longer.• Decompression is performed frequently and must be

done fast. • Used for retrieval mode applications



JPEG Compression


Introduction

Requirements on JPEG implementations JPEG Image Preparation

• Blocks, Minimum Coded Units (MCU)

JPEG Image Processing• Discrete Cosine Transformation (DCT)

JPEG Quantization• Quantization Tables

JPEG Entropy Encoding• Run-length Coding/Huffman Encoding


Additional Requirements -JPEG

JPEG implementation is independent of image size and applicable to any image and pixel aspect ratio.

Image content may be of any complexity (with any statistical characteristics).

JPEG should achieve very good compression ratio and good quality image.

From the processing complexity of a software solution point of view: JPEG should run on as many available platforms as possible.

Sequential decoding (line-by-line) and progressive decoding (refinement of the whole image) should be possible.


Variants of Image Compression

Four different modesLossy Sequential DCT based mode

• Baseline process that must be supported by every JPEG implementation.

Expanded Lossy DCT based mode• enhancements to baseline process

Lossless mode• low compression ratio• allows perfect reconstruction of original image

Hierarchical mode• accommodates images of different resolutions


JPEG Processing Steps

Block, MCU8bits/pixel

LosslessMode

ExpandedLossyMode

HierarchicalMode

BaselineSequentialMode

12 bits/pixel 2-16 bits/pixel

Layeredcoding

TransformationSource Codinglossy DCT

PredictiveEntropycoding

Switch betweenlossy DCT and losslesstechnique

Run-lengthHuffman

Pixel,Block, MCU

ImagePreparation

ImagePreparation

Quantization

EntropyEncoding

PredictionFDCT

Run-lengthHuffmanArithmetic

UncompressedImage

CompressedImage



MPEG Compression


Introduction

General Information about MPEG MPEG/ Video Standard MPEG/ Audio Standard MPEG Systems

• Multiplexing of Video/Audio Data Streams


General Information

MPEG-1 achieves data compression of 1.5Mbps.This is the data rate of audio CD’s and DAT’s (Digital

Audio Tapes). MPEG considers explicitly functionalities of

other standards,e.g. it uses JPEG. MPEG defines standard video, audio coding and

system data streams with synchronization. MPEG Core Technology

• includes many different patents• MPEG committee sets technical standards


General Information (cont.)

MPEG stream provides more information than a data stream compressed according to the JPEG standard.

Aspect Ratio - 14 aspect ratios can be encoded.• 1:1 corresponds to computer graphics, 4:3 corresponds to

702X575 pixels (TV format), 16:9 corresponds to 625/525 (HDTV format).

Refresh Frequency - 8 frequencies are encoded - • 23.976Hz, 24, 25,29.97, 50, 59.94, 60 Hz.

Other Issues with frame rateEach frame must be built within a maximum of

41.7(33)ms to keep display rate of 24fps(30fps). No need or possibility of defining MCUs in MPEG.

• Implies sequential non-interleaving order.For MPEG, there is no advantage to progressive display

over sequential display.


MPEG Overview

MPEG exploits temporal (i.e frame-to-frame) redundancy present in all video sequences.

Two Categories: Intra-frame and inter-frame encoding DCT based compression for the reduction of

spatial redundancy (similar to JPEG) Block-based motion compensation for exploiting

temporal redundancy causal(predictive coding) - current picture is modeled

as transformation of picture at some previous timenon-causal (interpolative coding) - uses past and future

reference


MPEG Image Preparation -Motion Representation

Predictive and interpolative codingGood compression but requires storage and informationOften makes sense for parts of an image and not the

whole image.

Each image is divided into areas called macro-blocks (motion compensation units)

Each macro-blocks is partitioned into 16x16 pixels for luminance, 8x8 for each of the chrominance components.

Choice of macro-block size is a tradeoff between gain from motion compensation and cost of motion estimation.

Macro-blocks are useful for compression based on motion estimation.


MPEG Video Processing

MPEG stream includes 4 types of image coding for video processing

I-frames - Intra-coded frames - access points for random access, yields moderate compression

P-frames - Predictive-coded frames - encoded with reference to a previous I or P frame.

B-frames - Bi-directionally predictive coded frames - encoded using previous/next I and P frame, maximum compression

D-frames - DC coded frames Motivation for types of frames

Demand for efficient coding scheme and fast random accessGoal to achieve high compression rate -

• temporal redundancies of subsequent pictures (i.e. interframes) must be exploited


MPEG Audio Encoding Steps

PsychoacousticModel

QuantizationBit/noise

Allocation

Filter Bank

Multiplexer

Entropy Coder Huffman Coding

If noise level is too low --> finer quantization is applied

If noise level is too high --> rough quantization is applied

Transformation from time to frequency domain

32 subbands

Compressed data


MPEG/System Data Stream

Video Stream is interleaved with audio. Video Stream consists of 6 layers

Sequence layerGroup of pictures layer

• Video Param - width, height, aspect ratio, picture rate• Bitstream Param - bitrate, bufsize• QT - intracoded blocks, intercoded blocks

Picture layer• Time code - hours, minutes, seconds

Slice layer• Type - I, P, B• Buffer Param - decoder’s bufsize• Encode Param - indicates info about motion vectors

Macro-block layer• Vertical Position - what line does this slice start on?• Qscale - how is the quantization table scaled in this

slice?Block layer

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