Media Technology I

Lecture Notes and Tutorials on:

Web : www.staff.city.ac.uk/~raj

Aim of this Module

• Describe the theory and operation of the major technologies and equipment of relevance to the media and information industries

• Introduce you to the multimedia communications and its range of applications and networking infrastructures

• Learn about different media types (text, images, speech, audio and video) and applications (VoIP, multimedia electronic mail, interactive television, e-commerce, m-commerce and others)

Syllabus• Multimedia Communications

- Multimedia information representation- Multimedia Networks (Telephone, Data, Broadcast, ISDN,

Broadband)• Multimedia Communications

- Multimedia applications (interpersonal communications, interactive applications over the Internet, Entertainment applications)

- Application and Networking Terminology (Media types, communication modes, Network types, multipoint conferencing, network QoS, Application QoS)

Syllabus

• Multimedia Information Representation - Digitization principles (analogueue signals, encoder design,

decoder design) - Text (unformatted text, formatted text, hypertext). Images (Graphics, digitized documents, digitized pictures)

• Multimedia Information Representation - Audio(PCM speech, CD-quality audio, synthesized audio). - Video (Broadcast Television, Digital Video, PC Video, Video Content)

Syllabus• Text and Image Compression

- Compression Principles (source encoders and destination decoders, lossless and lossy compression, entropy encoding, source encoding)

- Text compression (Arithmetic coding, LZW coding)- Image compression (GIF format, TIF format, digitized documents,

digitized pictures, JPEG)• Audio and video compression

- Audio compression (Differential pulse code modulation, adaptive differential PCM, adaptive predictive coding, linear predictive coding, MPEG audio coders)

- Video compression (Video compression principles, H.261, H.263, MPEG-1, MPEG-2 etc..)

Syllabus• Standards for multimedia communications

- Reference models (TCP/IP, protocol basics)- Standards relating to interpersonal communications (circuit networks,

packet-switched networks, e-mail)

• Network management – Network operations centre Performance management, Configuration management, Billing, fault management,

security management

• Standards for multimedia communications - Standards relating to interactive applications (information browsing, e-

commerce, intermediate systems, Java and JavaScript)- Standards for entertainment applications (Movie/Video-on-demand, iTV)

Syllabus• Digital Communications

- Transmission media (two-wire open lines, twisted-pair lines, coaxial cables, optical fibre, satellites, microwave communications, signal propagation delay)

• New multimedia technologies - UpnP - Zigbee - Jini - smart homes - mobile web services - IMS (IP Multimedia Subsystems)

Introduction

• ‘Multimedia’ means the information transferred is composed of text, images, audio and video

- Text (Unformatted and Formatted) - Images (Computer-generated, Digitized, etc.) - Audio (Low-fidelity speech as in telephony and high-fidelity stereophonic music as in CD’s - Video (Moving images and complete movies/films)

Introduction

• Person-to-person (communication) – Two people communicate through suitable Terminal Equipment (TE)

• Person-to-system (Interactive ) – Using multimedia Personal computer or Workstation ( Located at home or in an office)

Voice and Data Networks

• Public switched telephone networks (PSTNs) – initially designed to provide speech services. However, due to the advances in Digital Signal Processing (DSP) hardware and software now can support multimedia applications

• Data networks that initially supported data applications (email and ftp) now support much complex multimedia applications

Multimedia Information Representation

• Text: Block of characters, each represented by a fixed number of binary digits (bits) known as codeword

• Digitized image: Two-dimensional block of picture elements represented by a fixed number of bits

• Audio and Video: Type of signal is known as an analogue signal and varies continuously with time (e.g: a telephone conversation can last for several minutes while a movie (audio + video) can last for a number of hours

Multimedia Information Representation

• Single type of media - basic form of representation of a specific media type used

• Mixed media – applications involving text and images or audio and video their basic form is used

• Integrated media (text,images,audio,video)- Must convert all the four media into a suitable digital form

Multimedia Networks

• Telephone Networks - Telephony

• Data Networks – Data Communications

• Broadcast Television Networks – Broadcast TV)

• Integrated Services Digital Networks (ISDN) – Multi service

• Boradband Multiservice Networks – Multi service

Telephone Networks

• PSTN – Now known as Plain Old Telephone Service (POTs)

• The term switched means a subscriber can make a call to any other telephone on the ‘total’ network

• PSTN (public switched telephone network) is the world's collection of interconnected voice-oriented public telephone networks, both commercial and government-owned.

• It's the aggregation of circuit-switching telephone networks that has evolved from the days of Alexander Graham Bell.

• Today, it is almost entirely digital in technology except for the final link from the central (local) telephone office to the user

PSTN

Telephone Networks• Telephones in the home or in a small business are connected

directly to their nearest local exchange/end office• Telephones in a large office are connected to a private switching

office known as private branch exchange (PBX)• PBX provides free service between two telephones that are

connected to it• A PBX is a telephone system within an enterprise that switches

calls between enterprise users on local lines while allowing all users to share a certain number of external phone lines. The main purpose of a PBX is to save the cost of requiring a line for each user to the telephone company's central office.

Telephone Networks• PBX is connected to the local exchange and this

enables phones connected to the PBX to make calls through PSTN too.

• Cellular phone networks – Provides service to mobile subscribers

• The switches used in a cellular phone network are known as Mobile Switching Centers (MSCs)

• International calls are routed to and switched by international gateway exchanges (IGEs)

Telephone Networks• Circuit mode – Telephone networks

operate in this mode in which a separate circuit is set up through the network for each call for the duration of the call

• Access Circuits – Link the telephone handsets to a PSTN or PBX and carry two-way analogue signals associated with a call

Telephone Networks

• Today with high bit-rate channels in addition to the voice using the same access networks high resolution audio and video can be downloaded from a range of entertainment related servers

Data Networks• Designed to provide basic data communication services such

as email and general file transfer• Most widely deployed networks: X.25 network (low bit rate

data) not suitable for multimedia and the Internet (Interconnected Networks)

• Communication protocol: set of rules (defines the sequence and syntax of the messages) that are adhered to by all communicating parties for the exchange of information/data

• Packet: Container for a block of data, at its head, is the address of the intended recipient computer which is used to route the packet through the network

Data Networks• Open systems interconnections (OSI)- is a

standard description or "reference model" for how messages should be transmitted between any two points in a telecommunication network

• Access to homes is through an Internet Service provider (ISP)

• Access through PSTN or ISDN (high-bit rate)

Data Networks

• Business users obtain access either through site network or through an enterprise-wide private network (multiple sites)

• Universities with single campus use a network known as the Local Area Network (LAN). However bigger universities with more than one campus use enterprise wide network

• If the communication protocols of the computers on the network are the same as the internet protocols then the network is known as an intranet (e.g large companies and universities)

Data Networks• All types of network are connected using a

gateway (router) to the internet backbone network

• Router - a router is a device or, in some cases, software in a computer, that determines the next network point to which a packet should be forwarded toward its destination

Data Networks

• Packet mode – Operates by transfer of packets as defined earlier

• This mode of operation is chosen because normally the data associated with data applications is in discrete block format.

• With the new multimedia PCs packet mode networks are used to support in addition to the data communication applications a range of multimedia applications involving audio video and speech

Broadcast Television Network

• Broadcast television networks support the diffusion of analogue television programs to a wider geographical area via a cable distribution network, a satellite network

• A cable modem integrated into the STB (set-top-box) provides both a low bit rate channel (connects the subscriber to the PSTN ) and a high bit rate channel (connects to the Internet) from the subscriber back to the cable head-end

Broadcast Television Network

• A set-top box is a device that enables a television set to become a user interface to the Internet and also enables a television set to receive and decode digital television (DTV) broadcasts. DTV set-top boxes are sometimes called receivers.

Satellite/terrestrial broadcast network

• In Satellite and broadcast networks by integrating an H-S modem into the STB a range of interactive services can be supported. This is the origin of the term “interactive television”

Integrated Services Digital Networks• Started to develop in the early 1980s to provide PSTN users the

capability to have additional services• Integrated Services Digital Network (ISDN) in concept is the

integration of both analogue or voice data together with digital data over the same network.

• ISDN is a set of ITU standards for digital transmission over ordinary telephone copper wire as well as over other media. Home and business users who install an ISDN adapter (in place of a modem) can see highly-graphic Web pages arriving very quickly (up to 128 Kbps). ISDN requires adapters at both ends of the transmission so your access provider also needs an ISDN adapter. ISDN is generally available from your phone company.

Digital Subscriber Line (DSL):• DSL (Digital Subscriber Line) is a technology for bringing high-bandwidth

information to homes and small businesses over ordinary copper telephone lines.

• Assuming your home or small business is close enough to a telephone company central office that offers DSL service, you may be able to receive continuous transmission of motion video, audio, and even 3-D effects.

• Typically, individual connections will provide from 1.544 Mbps to 512 Kbps downstream and about 128 Kbps upstream. A DSL line can carry both data and voice signals and the data part of the line is continuously connected.

• Access circuit that allows users either two different telephone calls simultaneously or a telephone call and a data network

Integrated Services Digital Networks

• DSL supports two 64 kbps channels that can be used independently or as a single combined 128kbps channel (additional box of electronics). This is known as the aggregation function

Broadband Multi service Networks

• Broadband – Circuits associate with a call could have bit rates in excess of the maximum bit rate of 2Mbps – 30X64 kbps – provided by ISDN

• Broadband integrated services digital network (B-ISDN) – All different media types are converted in the source equipment into a digital form, integrated togeather and divided into multiple fixed-sized packets (cells)

Broadband Multiservice Networks

• Broadband – Circuits associate with a call could have bit rates in excess of the maximum bit rate of 2Mbps – 30X64 kbps – prvided by ISDN

• Broadband integrated services digital network (B-ISDN) – All different media types are converted in the source equipment into a digital form, integrated togeather and divided into multiple fixed-sized packets (cells)

Multimedia Applications

• Application that involve multiple media types:

- Interpersonal Communications: May

involve speech, image, text or video

- Interactive Applications over the Internet:

Browsing through sales, literature, newspapers,

etc.

- Entertainment Applications: Movie/Video on

demand, interactive television

Interpersonal Communications (Speech only) • Traditional interpersonal

communication involving speech was provided by using telephones connected to either PSTN/ISDN or PBX hub.

• Today multimedia PC equipped with a microphone and speakers can be used to make telephone calls. This technology is known as computer telephony integration (CTI)

Advantages of using CTI

• The users can create their own private directory of numbers and can initiate a call simply by selecting the desired numbers from the PC screen

• Provides access circuit to the network with more capacity known as the bandwidth

• Integration of the PC based network services with the telephony is possible

Additional services supported by the public and private networks

• Voice mail: Used in the event of the called party being unavailable. The voice mail is saved in the server mailbox and can be read by the owner next time they contact the server

• Teleconferencing: Involves multiple interconnected telephones/PCs. Each person can talk to all the others involved in the call. This is known as a conference call. A central unit called an audio bridge provides the necessary support to set up the call automatically

Additional services supported by the public and private networks

• Internet telephony initially supported computer-to-computer communications

• Today the technology is extend so that computer-to-telephony is possible

Telephony over the Internet

• To make a PC-to-PC telephone call the standard addresses that identify the PC on the network are used same as in a data transfer application

• However, since the internet operates in a packet mode necessary conversion software and hardware is mandatory in both the PCs. This type of telephony is known as Voice over IP (VoIP)

• To make a call using a PC connected to the Internet to a telephone connected to a PSTN/ISDN an interworking unit known as telephony gateway is necessary.

Principle of VoIP

• Initially the PC user sends a request to make a telephone call to a preallocated gateway using its internet address.

• If the user is registered the gateway will request the phone number to establish the call from the PC

• On receipt of this the source gateway will initiate a call with the gateway nearest to the called party.

• The called gateway then establishes the call to the recipient telephone using its telephone number and the call setup procedures

• If the called party answers then a signal is sent back by the recipient gateway to the PC user via the source gateway

Image only interpersonal Communication

• Fax: Exchange of electronic images or documents over PSTN/ISDN

• As shown above this requires use of a pair of fax machines, one at each termination point

• Both fax machines have an integral modem within them

Image only interpersonal Communication

• PC can also be used instead of a normal fax machine

• The PC can send an electronic version of a document stored directly within the PCs memory

• This requires a telephone interface card and associated software

• In addition it is possible to send digitised documents over other enterprise network (LAN interface card and software required)

Image only interpersonal Communication

Step1: Initially the caller keys in the telephone number of the intended recipient and a circuit is set up through the networkStep2: The two fax machines communicate with each other to establish operational parametersStep3: The sending machine starts to scan and digitized each page of the document in turn and is simultaneously transmitted over the networkStep4: After the final page has been sent/received the connection through the network is cleared by the calling machine

Text only interpersonal Communication

• An example of interpersonal communications involving just text is email

• The user terminal is normally a PC or a work station networked

• Associated with each network is a server/servers. Each is known as an email server and they contain mailboxes for each user connected to the network

Text and images

• An example of an application that involves both text and images integrated together is computer-supported cooperative working (CSCW)

• The network used is Intranet, Internet or LAN

Text and images

• A distributed group of people working on the same project can share each others display. This is known as shared whiteboard.

• The CSCW comprises a central “whiteboard program” and a linked set of subprograms in each PC/workstation with a shared window or workspace (shared whiteboard)

Speech and video• An example of this type is video telephony

• As can be seen from the figure the terminals/PCs incorporate a video camera in addition to the microphone and speaker

•The network must provide sufficient bandwidth to support the integrated speech and video generated

Speech and video• Desktop videoconferencing call – Many

interconnected PC users in geographically distributed sites can share speech and video between various locations

• To support video conferencing a central unit called a multipoint control unit(MCU) is used. This selects a single information stream to send to each participant hence reducing the communication bandwidth

Speech and video• Multicasting – In which all transmissions from any

of the PCs/workstations belong to a predefined group are received by all the other members of the group

• Using multicasting eliminates the need for an MCU unit

Note: Only possible when there are few participants involved

Many-to-many videoconferencing

• As group of people present at each location these rooms must contain audio and video equipments and are known as videoconferencing studios

• Each studio will have few cameras, a large-display, and associated audio equipment. These will be connected to a central unit called the videoconferencing system

Many-to-many videoconferencing• A multimedia email will consist of text, images, audio and video.

Examples of email applications consisting media types other than text are Voice-mail, Video mail and multimedia mail

• Voice-mail: With internet-based voice-mail, there is an associated voice-mail server.

The user enters a voice message addressed to the intended recipient and the local recipient’s voice-mail server then relays this to the local recipient the next time he logs in

Interactive applications over the Internet

• Anchor – The optional linkage points within documents are defined by the creator of the document and are known as anchors

• Hypertext – Web documents comprising only text are created using hypertext

•Hypermedia – Web documents comprising multimedia (Video, Sound) are created using hypermedia

• Browser – The client function that is used to explore the total contents of the web

Interactive applications over the Internet

• Applications such as homeshopping, homebanking, etc.. the user may want to pass on information back to the server.

• This information might contain credit card details and personal details and hence a rigorous security procedure needs to be in place

• This type of two way process is known as interactive application over the Web.

Entertainment Applications

• Entertainment applications are classified into:

- Movie/video-on-demand

- Interactive television

Movie/video-on-demand

• The entertainment applications require higher quality / resolution for video and audio since wide-screen televisions and stereophonic sound are often used

Movie/video-on-demand

• Normally the subscriber terminal comprises television with a selection deive for interation purposes

• The user interactions are relayed to the server through a set-top-box (STB) which contains a high speed modem

• By means of the menu the user can browse through the movies/videos and initiate the showing of a selected movie. This is known as Movie-on-demand or Video-on-demand.

Movie/video-on-demandKey features of MOD

- Subscriber can initate the showing of a movie from a library of movies at any time of the day or night

Issues associated with MOD

- The server must be capable of playing out simultaneously a large number of video streams equal to the number of subscribers at any one time

- This will require high speed information flow from the server (multi-movies + multi-copies)

Movie/video-on-demand

• In order to avoid the heavy load there is another mode of operation used. In which requests are queued until the start of the next playout time.

• This mode of operation is known as the near movie-on-demand (N-MOD)

Interactive television (Cable network)

• The set-top box (STB) provides both a low bit rate connection to the PSTN and a high bit rate connection to the internet

• Through the connection to the PSTN, the subscriber is able to actively respond to the information being broadcast

Interactive television (Satellite/terrestrial broadcast network)

• The STB associated requires a high speed modem to provide the connections to the PSTN and the Internet

Terms used with Multimedia

Media Types• The information associated with the different applications can be either continuous or block-mode

• Continuous: The information is played out directly as it is received continuously (called streaming or real-time media) (E.g Audio and video)

•Block-mode: The source information is created in a time-independent way and is often stored at the source in, say, a file

• When requested it will be transferred across the network and displayed at a time specified by the requesting application (called downloading) (e.g. email consisting of a block of text)

Communication Modes

• Simplex: The information associated with the application flows in one direction only.

• Half-Duplex: Information flows in both directions but alternatively (two-way alternative).

• Duplex: Information flows in both directions simultaneously (Two-way simultaneous).

Communication Modes

• Broadcast: The information output by a single node is received by all the other nodes connected to the same network

• Multicast: The information output by the source is received by only a specific subset of the nodes (Latter form known as multicast group)

Communication Modes

Communication mode Examples

• In half-duplex and duplex communications, the bit rate associated with the flow of information in each direction can be equal (symmetric) or different (asymmetric).

•Video Telephony – Symmetric duplex communication

• Web browsing – Asymmetric half-duplex mode (as different bit rates for downloading and uploading)

Network Types (Circuit-mode)

• This operates in a time-dependent manner and comprises an interconnected set of switching offices/exchanges to which the subscriber terminals/computers are connected

Circuit-mode- Operational PrincipleStep1: The source must set up the connection first through the network

Step2: Each subscriber terminal has a unique network wide address and to make a call the source first enters this number of the intended communication partner

Step3: The local switching office uses this number to set up a connection. Depending on the availability of the destination the connection will be estabilished

Step4: Finally at the end of information exchange the call will be terminated by the source or the destination

Circuit-mode- Terminology• Signalling messages – The messages associated with the setting up and clearing of a connection

• Call/Connection setup delay – The delay associated with the connection procedures

•Examples of Circuit-mode operation – PSTN and ISDN

•PSTN – setup delay varies from fraction of a second to few seconds for international connections

• ISDN – setup delay ranges from tens of milliseconds through to several hundred milliseconds

Packet mode

• There are two types of packet-mode network

- Connection Oriented (CO)

PSE: Packet Switching Exchanges

• As the name implies a connection is established prior to information interchange

• The connection utilizes only a variable portion of the bandwidth of each link and known as virtual circuit (VC)

Packet mode – Operational Principle

• To set up a VC the source terminal sends a call request control packet to the local PSE which in addition to the source and destination addresses holds a short identifier known as virtual circuit identifier (VCI)

•Each PSE maintains a table that specifies the outgoing link to use to reach the network address

•On receipt of the call request the PSE uses the destination address within the packet to determine the outgoing link

• The next free identifier (VCI) for this link is selected and two entries are made in the routing table

Packet mode – Connectionless

• In connectionless network, the establishment of a connection is not required and they can exchange information as and when they arrive

•Each packet must carry the full source and destination address in its header in order for each PSE to route the packet onto the appropriate outgoing link (router term used rather than PSE)

Packet mode – Summary

• In both types each packet is stored in a memory buffer and a check is performed to determine if any transmission errors are present in the received message. (i.e 0 instead of a 1 or vice versa)

• If an error is detected then the packet is discarded known as best-effort service.

• All packets are transmitted at the maximum link bit rate

• As packets may need to use the same link to transfer information an operation known as store-and-forward is used.

Packet mode – Summary

• The sum of the store and forward delays in each PSE/router contributes to the overall transfer delay of the packets and the mean of this delay is known as the mean packet transfer delay.

• The variation about the mean are known as the delay variation or jitter

• Example of connectionless mode – Internet

• Examples of connection oriented network – X.25 (text) and ATM (multimedia)

Multipoint Conferencing

• Multipoint conferencing is implemented in one of two ways

- Centralized mode

- Decentralized mode

Centralized mode

• This mode is used with circuit switched networks such as PSTN and ISDN

Multipoint Conferencing – Centralized mode

• With this mode a central server is used

• Prior to sending any information each terminal needs to set up a connection to the server

• The terminal then sends the information to the server.

• The server then distributes this information to all the other terminals connected in the conference

Multipoint Conferencing – Decentralized mode

• The decentralized mode is used with packet-switched networks that support multicast communications

• E.g – LAN, Intranet, Internet

Decentralized mode Operation

• The output of each terminal is received by all the other members of the conference/multicast group

• Hence a conference server is not required and it is the responsibility of each terminal to manage the information streams that they receive from the other members

Hybrid Mode

• This type of mode is used when the terminals are connected to different network types

• In this mode the server determines the output stream to be sent to each terminal

Network Qos

• Network Quality of Service parameters: Operational parameters associated with a communication channel through a network that determine the suitability of the channel in relation to its use for a particular application

• Circuit-switched network: Bit Error Rate (BER) is the probability of a bit being corrupted during its transmission in a defined time interval. The transmission delay is determined by the bit rate used plus the codes (network interfaces) and propagation delay of the digital signal

•Packet-switched network: Mean packet transfer rate is a measure of the average number of packets transferred per second. Mean Packet Error Rate (PER) is the probability of a received packet containing one or more bit errors

Network Qos

• Most networks (circuit and packet switched) provide an unreliable service which is also known as a best-try or best-effort service

• If the application accepts only error free blocks then it is necessary for the sending terminal to divide the source information into blocks of a defined maximum size and the destination to detect any missing blocks

•When a block is missing then the destination must request for a copy of the block from the source. The service is then called a reliable service

•

Application Qos

• Transmission of a constant bit rate stream over a packet switched network

• The startup delay defines the amount of time that elapses between an application making a request to start a session and the confirmation being received at the destination

Application Qos

• To transfer a large file from the server to your home computer using the packet switched (PW) and circuit switched (CS) networks

- PSTN (28.8kbps) and ISDN (64/128kbps) operate in CS mode and provide constant bit rate channel

- Cable modem operate in PS mode and the bit rate of the shared channel is 27Mbps

Application Qos

• Assuming the file size is 100Mbits, the minimum time to transmit the file using the different Internet access modes is:

- PSTN and 28.8 kbps modem: 57.8 minutes

- ISDN at 64 kbps: 26 minutes

- ISDN at 128 kbps: 13 minutes

- cable modem at 27 Mbps: 3.7 seconds

Application Qos

• The application quality of service is different from the network QoS

• For example in an application involving images the parameters may include a minimum image resolution and size while a video may include the digitization format and the refresh rate

Application QoS - Parameters

• The required bit rate or mean packet transfer rate

•The maximum startup delay

•The maximum end-to-end delay

•The maximum delay variation/jitter

• The maximum round-trip delay

Application Qos

• To overcome the effect of jitter a technique known as buffering is used

•The effect of jitter is overcome by retaining a defined number of packets in a memory buffer at the destination before playout of the information bit stream is started

Application QoS - Summary

• In order to determine whether a particular network can meet the QoS requirements of an Application a number of standard application service classes have been defined

• Each service class has an associated set of QoS parameters defined

• For networks that support different service classes ( i.e internet), the packets relating to each class are assigned a different priority

• Real time streams have higher priority than packets relating to email

Summary

Summary

Summary – Multimedia Communication Network and Services

Multimedia Information Representation

• Multimedia Information is stored and processed within a computer in a digital form

• Codeword: Combination of a fixed number of bits that represents each character, in the case of textual information

• analogue signal: Signal whose amplitude (magnitude of the sound/image intensity) varies continuously with time

• Signal encoder: Electrical circuit used for the conversion of an analogue signal into a digital form

• Signal decoder: Electrical circuit that converts stored digitized samples into time-varying analogue form

analogue Signals

• As mentioned earlier the amplitude of the signal varies continuously with time

• The Fourier analysis can be used to show that any time varying signal is made up of infinite number of single-frequency sinusoidal components

• The range of frequencies of the sinusoidal components that make up the signal is called the signal bandwidth

• Speech bandwidth: 50Hz – 10kHz

• Music Bandwidth: 15Hz – 20kHz

analogue Signals –Signal Properties

analogue Signals –Signal Properties

• To transmit an analogue signal through a network the bandwidth of the transmission channel should be equal to or greater than the signal bandwidth

• If the bandwidth of the channel is less than the signal bandwidth than channel is called the bandlimiting channel

Encoder Design

• The Encoder consists of bandlimiting filter and an analogue-to-digital converter (ADC) ( comprising sample and hold + quantizer)

Encoder Design

• Bandlimiting filter: Removes the selected higher frequency components from the source signal

• Sample and hold Circuit: Samples amplitude of the filtered signal at regular intervals and holds the sampled amplitudes between samples

•Quantizer: Converts the samples into their corresponding binary form

Encoder Design – Data representation

• The most significant bit of the codeword represents the sign of the sample

• A binary 0 indicates a positive value and a binary 1 indicates a negative value

• The signal must be sampled at a much higher rate than the maximum rate of change of the signal amplitude

• The number of quantization levels should be as large as possible to represent the signal accurately

• Nyquist sampling theorem: To obtain an accurate representation of a time-varying analogue signal, its amplitude must be sampled at a minimum that is equal to or greater than twice the highest sinusoidal frequency component that is present in the signal

• Nyquist rate is represented either in Hz or more correctly in samples per seconds (sps)

• Antialiasing filter: Another name for bandlimiting filter. Since it passes frequencies that are within the Nyquist rate

Sampling Rate

Alias signal generation due to undersampling

• In reality the transmission channel used often has a lower bandwidth

• To avoid distortion the source signal is first passed through the BLF which is designed to pass only the frequency components that are within the channel bandwidth

• This avoids alias signals caused by undersampling

• Representation of the analogue samples require an infinite number of digits

Quantization Intervals

• Three bits are used to represent each sample ( 1 bit for the sign and two bits to represent the magnitude)

• If Vmax is the maximum positive and negative signal amplitude and n is the number of binary bits used then the quantization interval, q, is defined as

q = 2Vmax/ 2n

• A signal anywhere within the quantization interval will be represented by the same binary codeword

• Each cordword is at the centre of the corresponding quantization interval

• Therefore a difference of q/2 from the actual signal level is present. This difference is known as the quantization error

Quantization Intervals

Quantization noise polarity

• Quantization error is the difference between the actual signal amplitude and the corresponding nominal amplitude (also known as quantization noise since values vary randomly)

Dynamic Range

• With high-fidelity music it is important to be able to hear very quiet passages without any distortion created by quantization noise

• Dynamic range is defined as the ratio of the maximum signal amplitude to the minimum.

D = 20 log10 (Vmax/Vmin) dB

Decoder Design

• A signal decoder is an electronic circuit that performs the conversion prior to their output back again into their analogue form through a digital-to-analogue converter and a low pass filter

• Low-pass filter: Only passes those frequency components that were filtered through the bandlimiting filter in the encoder

Encoder+decode= Codec

Text

• Unformatted text: Known as plain text; enables pages to be created which comprise strings of fixed-sized characters from a limited character set

• Formatted Text: Known as richtext; enables pages to be created which comprise of strings of characters of different styles, sizes and shape with tables, graphics, and images inserted at appropriate points

• Hypertext: Enables an integrated set of documents (Each comprising formatted text) to be created which have defined linkages between them

Unformatted Text – The basic ASCII character set

• The American Standard Code for Information Interchange is one of the most widely used character sets and the table includes the binary codewords used to represent each character (7 bit binary code)

• Control characters

(Back space, escape, delete, form feed etc)

• Printable characters

(alphabetic, numeric, and punctuation)

Unformatted Text – Supplementary set of Mosaic characters

• The characters in columns 010/011 and 110/111 are replaced with the set of mosaic characters; and then used, together with the various uppercase characters illustrated, to create relatively simple graphical images

Unformatted Text – Examples of Videotext/Teletext

• Although in practice the total page is made up of a matrix of symbols and characters which all have the same size, some simple graphical symbols and text of larger sizes can be constructed by the use of groups of the basic symbols

Formatted Text

• It is produced by most word processing packages and used extensively in the publishing sector for the preparation of papers, books, magazines, journals and so on..

• Documents of mixed type (characters, different styles, fonts, shape etc) possible.

•Format control characters are used

Hypertext – Electronic Document in hypertext

• Hypertext can be used to create an electronic version of documents with the index, descriptions of departments, courses on offer, library, and other facilities all written in hypertext as pages with various defined hyperlinks

Hypertext – Electronic Document in hypertext

• An example of a hypertext language is HTML used to describe how the contents of a document are presented on a printer or a display; other mark-up languages are: Postscript, SGML (Standard Generalized Mark-up language, Tex, Latex

Images

• Images include computer-generated images (referred to as computer graphics or simply graphics) and digitized images of both documents and pictures

• All types of images are displayed in the form of a two-dimensional matrix of individual picture elements (pixels or pels), but represented differently within the computer memory (file)

• Each type of these images is created differently

Graphics

• VGA is a common type of display that consists of a matrix of 640 horizontal pixels by 480 vertical pixels with for example, 8 bits per pixel which allows each pixel to have one of 256 different colours

Graphics

• All objects are made up of a series of lines that are connected to each other and, what appear as a curved line, in practice is a series of short lines each made up of a string of pixels

• Each object has a number of attributes associated with it. These include its shape, size in terms of pixel position, colour of the border etc..

• Colouring a solid block with the same colour is known as rendering

Graphics - Conclusions

• There are two forms of representation

- high-level representation (similar to a source code of a program) – requires less memory to store the image and less bandwidth for transmission

- actual picture image of the graphic ( similar to the low-level machine code and generally known as bit-map format) – e.g. GIF (graphical interchange format), TIFF ( tagged image format)

• A graphic can be transferred over the network in either form

• A software called SRGP (simple raster graphics package) - used to convert high-level form into a pixel-image form

Digitized Documents- Fax Principles

• The scanner associated with fax machines operates by scanning each complete page from left to right to produce a sequence of scan lines that start at the top of the page and end at the bottom

• Vertical resolution is either 3.85 (100 lines) or 7.7 mm (200 lines)

Digitized Documents- Digitization format

• Fax machines uses a single binary digit to represent each pel, a 0 for a white pel and a 1 for a black pel. Hence the digital representation of a scanned page produces a stream about 2 million bits

• Single binary digit per pel means fax machines are best suited for bitonal images

Colour Derivative Principles – additive colour mixing ( R + G + B)

•Black is produced when all three primary colours (R,G,B) are zero.

• Useful for producing a colour image on a black surface as is the case in display applications

Digitised Pictures- Subtractive colour mixing

• White is produced when the three chosen primary colours cyan,magenta and yellow are all zero

• Useful for producing a colour image on a white surface as is the case in printing applications

Digitized Pictures- Television/computer monitor principles

• The picture tubes used in most television sets operate using what is known as a raster-scan; this involves a finely-focussed electron beam being scanned over the complete screen

Digitized Pictures- Raster Scan

• Progressive scanning is performed by repeating the scanning operation that starts at the top left corner of the screen and ends at the bottom right corner follows by the beam being deflected back again to the top left corner

Digitized Pictures – Raster scan display architecture

Digitized Pictures-Pixel format on each scan

• The set of three related colour-sensitive phospors associated with each pixel is called a phospor triad and the typical arrangement of the triads on each scan line is shown

Digitized Pictures – Concepts

• Frame: Each complete set of horizontal scan lines (either 525 for North & South America and most of Asia, or 625 for Europe and other countries)

•Flicker: Caused by the previous image fading from the eye retina before the following image is displayed, after a low refresh rate ( to avoid this a refresh rate of 50 times per second is required)

• Pixel depth: Number of bits per pixel that determines the range of different colours that can be produced

• Colour Look-up Table (CLUT): Table that stores the selected colours in the subsets as an address to a location reducing the amount of memory required to store an image

Digitized Pictures

• Aspect Ratio: This is the ratio of the screen width to the screen height ( television tubes and PC monitors have an aspect ratio of 4/3 and wide screen television is 16/9)

Digitized Pictures – Screen Resolutions

• NTSC = 525 lines per frame (480 Visible)

• PAL,CCIR,SECAM=625 lines ( 576 visible)

•Example display resolutions: VGA (640x480x8), XGA (1024x768x8) and SVGA (1024x768x24)

Digitized Pictures – Colour Image Capture: Schematic

• Typical arrangement that is used to capture and store a digital image produced by a scanner or a digital camera (either a still camera or a video camera)

Digitized Pictures – Colour Image Capture: Schematic

• Photosites: Silicon chip which consists of a two dimensional grid of light-sensitive cells, which stores the level of intensity of the light that falls on it

• Charge-coupled devices (CCD): Image sensor that converts the level of light intensity on each photosites into an equivalent electrical charge