17EC741 MMC Module 2 - Electronics and communication notes for final year PDF

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MODULE-2

Multimedia Information Representation Topics: 2.1 Introduction 2.2 Digitization Principles 2.3 Text 2.4 Image 2.5 Audio 2.6 Video 2.7 Recommended Questions

2.1 Introduction All types of multimedia information are stored and proc They can be integrated together and transmitted ove

puter in a digital form. ommunications network.

2.2 Digitization Principles  A signal whose amplitude varies con  Techniques involved in analog-to  The range of frequencies of bandwidth.  Any signal transmitted o

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is known as an analog signal. include sampling and quantization. mponents that make up a signal is called the signal have a signal bandwidth less than the channel bandwidth.

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Figure 2.1 Signal Propert Signal bandwid

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g analog signal; b) Sinusoidal frequency components; c) Effect of a limited bandwidth transmission channel

Nyquist sampling theorem: The am litude of a signal must be sampled at a minimum rate that is equal to or greater than twice the bandwidth of the signal.

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Figure 2.2 Signal encoder design: a) Circuit components b) Associated waveform set  Quantization is the process that confines the amplitude of a signal into a finite number of values.  The difference between the actual signal amplitude and the corresponding nominal amplitude is called the quantization error.  The ratio of the peak amplitude of a signal to its minimum amplitude is known as the dynamic range.

Quantization interval: Where n is the number of bits used and Vmax is the maximum positive and negative signal amplitude.

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Figure 2.3 Quantization procedure: a) Source of errors; b) Noise polarity

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Example 2.1:

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Example 2.12:

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2.3 Text There are 3 types of text that are used to produce pages of documents: 1. Unformatted text:(=plaintext) Comprise strings of fixed-size characters from a limited character set such as ASCII code set. e.g. .txt file

basic ASCII character set. 2. Formatted text: (=richtex Comprise of strings of character e.g. word .doc file

erent styles, size, and shape with tables, graphics, and images.

Figure 2.7: Formatted text: a) an example of formatted text string b) printed version of the string

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3. Hypertext: Integrated set of documents (each comprising formatted text) to be created which have defined linkages (hyperlinks) between them. e.g. .htm file

2.4 Images  Images include computer-generated images (referred to as computer graphics) and digitized images of documents and pictures.  All images are displayed in the form of a two dimensional matrix of individual picture elements called pixels.

2.4.1 Graphics  Graphics are composed of all kinds of visual objects such as lines, arcs, squares, circles and so on, as well as any form of hand-drawn objects.  Each object has a number of associated attributes such as color, pe, size, shadow and so on.  There are 2 forms of representation of a computer graphic:  The representation of a graphic is analogous to the st gram written in a high-level programming language, which consists of a set of co ecessary to draw the different objects that make up the graphic.  Another form of representation is the actual pix phic (bit-map format).

Figure 2.8: Graphic principles: a) example screen format; b) some simple object example; c) effect of changing position attribute; d) Solid objects

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2.4.2 Digitized pictures  A whole spectrum of colors can be produced by using different proportions of the 3 primary colors red, green and blue.  There are 2 mixing techniques: additive color mixing and subtractive color mixing.

Figure 2.9: Color derivation principles: a) Additive color mixing; b) Subtractive color mixing.

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Raster-scan principles:

Figure 2.10: TV/Computer monitor principles: a) Schematic b) raster scan principle  The frame refresh rate must be high enough to ensure the eye is not aware the display is continuously being refreshed.  A low refresh rate leads to what is called flicker.  The number of bits per pixel is known as the pixel depth and determines the range of different colors that can be produced.  The aspect ratio is the ratio of the number of pixels per scanned line and the number of lines per frame.  (common ratios: 4:3, 16:9)

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 Example display resolutions and memory requirements:

Figure 2.11: Color image capture: a) schematic; b) RGB signal generation alternatives

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2.5 Audio  There are 2 types of audio signal: speech and music quality audio.  Audio can be produced either naturally by means of a microphone or electronically using some form of synthesizer.  Output of all digitized audio signals the stream of digitized values must be converted back into its analog form as loudspeakers operate using an analog signal.  Summary of characteristics

Example 2.3:

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2.5.1 PCM  The bandwidth of a speech circuit in a PSTN was limited to 200Hz through to 3.4 kHz. Sampling rate 8 kHz.  The digitization procedure is known as pulse code modulation (PCM) and the international standard relating to this is defined in ITU-T Recommendation G.711.  North America & Japan: 7 bits per sample use m-law compression-expansion characteristics.  Europe: 8 bits per sample use A-law compression expansion characteristics.

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Figure 2.12: PCM principles: a) signal encoding and decoding schematic; b) compressor schematic c) expander characteristic.

2.5.2 CD-quality audio  Sampling rates 44.1 kHz  16 bits per sample  2 channels Example 2.4:

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2.5.3 Synthesized Audio  Synthesized audio is often used in multimedia applications since i. The amount of memory required can be 2 to 3 orders of magnitude less than that required to store the equivalent digitized waveform version, ii. It is much easier to edit synthesized audio and to mix several passages together.  The computer takes input commands from the keyboard and outputs these to the sound generators, which, in turn, produce the corresponding sound wave forms to drive the speakers.  A standardized set of messages have been defined for both input and for output to the corresponding set of sound generators.  These are defined in a standard called Music Instrument Digital Interface (MIDI).

Figure 2.13: Audio/sound synthesizer schematic

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2.6 Video The quality of the video required varies considerably from one type of application to another.

2.6.1 Broadcast television:  Interlaced scanning: Each frame is divided into 2 fields. One comprises the odd scan lines and one comprises the even scan lines. They are transmitted one by one and integrated together to form a complete frame in the television receiver.  The refresh rate can be halved with interlaced scanning.  Progressive scanning: Frames are not split into fields and each of them is transmitted as a whole.  Two types of system (based on line resolution): 1. NTSC: 525 lines per frame 2. PAL/CCIR/SECAM: 625 lines per frame  For historical reason, a different set of color signals from R, G a d B were selected for color television broadcasts.  Luminance and chrominance components are used to desc f each pixel.  Luminance components describe the brightness of pix  Chrominance components carry the color informati PAL system: Y = 0.299R + 0.587 G + 0.114 B U = 0.493 (B-Y) V = 0.877 (R-Y) NTSC system: Y = 0.299R + 0.587 G + 0.1 I = 0.74 (R-Y) – 0.27 (B-Y Q = 0.48 (R-Y) + 0.41 (B-Y)

2.6.2 Digital video    

These are intended for use with standard television receivers. The 3 component signals are usually digitized separately prior to their transmission. The resolution of the eyes is less sensitive for color than it is for luminance. The International Telecommunications Union - Radiocommunications Branch (ITU-R) - formerl y known as Consultative Committee for International Radiocommunications (CCIR) - defined a standard for the digitization of video pictures known as Recommunication CCIR-601.  Variants of this standard have been defined for use in other application domains such as digital television broadcasting, digital telephony and videoconferencing.  Collectively these are known as digitization formats.

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4:2:2 Format  Original digitization format used in Recommendation CCIR-601 for use in television studios.  Line sampling rate: 13.5MHz for luminance and 6.75MHz for chrominance signal.  The number of bits per sample is 8 for all 3 signals.

Figure 2.14

with 4:2:2 format.

4:2:0 Format  Used in digital video broad  Interlaced scanning is u  Absence of chrominanc

rnative lines is the origin of the term 4:2:0.

Figure 2.15: Sample position with 4:2:0 format.

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SIF (Source intermediate format) Give a picture quality comparable with that obtained with VCRs

Figure 2.16: Simple p

nd CIF

CIF: common intermediate format     

Used in videoconferencing applica 4:1:1 digitization format Spatial resolution: Y=360x2 Temporal resolution 30H Derivatives of CIF: 4

QCIF: Quarter CIF  For video telephony appli

44 e scanning

ns.

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2.6.3 PC Video PC video digitization formats:

All PC monitors use progressive (non-interlaced) scanni

aced scanning

2.7 Recommended Questions 1. 2. 3. 4.

Explain code word, analog signal, signa ecoder?[06] Define ―bandwidth‖? explain ―ban ?[05] Explain Nyquist sampling theore 04] Define the meaning of term rval & how this influences the accuracy of the sampling process of an an 5. Explain a) unfor 6. Formatted/rich text. 7. Hypertext. [06] 8. Differentiate formatted t nformatted text? Explain origin the acronym WYSIWYG? [05] 9. Explain briefly: visual obje , freeform object, clip art, 3-D objects.[04] 10. Explain scanning, pixels, digitization principles with reference to facsimile machines?[08] 11. Define the aspect ratio of a display screen. Give two examples for current widely used screen sizes? [05] 12. Derive the time to transmit an image with each type of display assuming a bitrate of 56 kbps, 1.5Mbps?[06] 13. Define text & image.[03] 14. Define audio & video.[03] 15. Compare formatted & unformatted text.[08] 16. What is rendering & clip art?[02] 17. What is flicker & frame refresh rate?[04] 18. What is NTSC & PAL?[10] 19. What is sample & hold, Quantizer?[06]

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20. Define aspect ratio & pixel depth.[06] 21. What is composite video signal?[04] 22. Define Run – length encoding & statistical encoding.[06] 23. What is synchronization?[02] 24. Identify the main features of the MIDI standard & its associated message?[08] 25. State & explain three main properties of a color source that the eye makes of?[08] 26. Why is the chrominance signal transmitted in the form of two color different signals?[08] 27. Derive the bit rate that results from the digitization of a 525-line and a 625-line system using the 4:2:0 digitization format and interlaced scanning. Hence derive the amount of memory required to store a 2-hour movie/video?[10]

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