Terms used with Multimedia

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Terms used with Multimedia
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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)

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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).

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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)

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Communication Modes
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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)

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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

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Circuit-mode- Operational Principle
Step1 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
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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

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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)

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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

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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)

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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.

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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)

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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

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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

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Multipoint Conferencing Decentralized mode

• The decentralized mode is used with
  packet-switched networks that support multicast
  communications
• E.g LAN, Intranet, Internet

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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Summary


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Summary


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Summary Multimedia Communication Network and
Services


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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

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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

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analogue Signals Signal Properties


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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

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Encoder Design

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

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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

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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

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Sampling Rate

• 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

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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

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Quantization Intervals

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

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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

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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)

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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

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Decoder Design



Encoderdecode Codec

• 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

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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

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Unformatted Text The basic ASCII character set

• Control characters
• (Back space, escape, delete, form feed etc)
• Printable characters
• (alphabetic, numeric, and punctuation)

• 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)

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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

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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

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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

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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

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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

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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

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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

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Graphics

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

• 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..

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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

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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)

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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

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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

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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

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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

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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

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Digitized Pictures Raster scan display
architecture






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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

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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

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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)

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Digitized Pictures Screen Resolutions

• NTSC 525 lines per frame (480 Visible)
• PAL,CCIR,SECAM625 lines ( 576 visible)
• Example display resolutions VGA (640x480x8), XGA
  (1024x768x8) and SVGA (1024x768x24)

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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)

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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