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GMU IT 212 - Spring 20071 Displays, I/O, Portable Computers, Multimedia IT 212 002 How Computers Work April 26, 2007.

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Presentation on theme: "GMU IT 212 - Spring 20071 Displays, I/O, Portable Computers, Multimedia IT 212 002 How Computers Work April 26, 2007."— Presentation transcript:

1 GMU IT 212 - Spring 20071 Displays, I/O, Portable Computers, Multimedia IT 212 002 How Computers Work April 26, 2007

2 GMU IT 212 - Spring 2007 2 Lecture Agenda Chapter 16: How a Computer Display Works Chapter 17: How Data Gets into Your PC Chapter 18: How Scanners Capture Images and Words Chapter 19: How Portable Computers Work Chapter 21: How Multimedia Sound Works Chapter 22: How Multimedia Video Works

3 GMU IT 212 - Spring 2007 3 Chapter 16: How a Computer Display Works

4 GMU IT 212 - Spring 2007 4 Overview Various types of display technologies can be found in the market today: CRT LCD Plasma A monitor displays millions of colors on the screen by combining 3 fundamental colors: Red, Green and Blue (RGB) A single pixel of an image displayed on the screen is typically comprised of 3 smaller dots, one for each color component Pixel = picture element

5 GMU IT 212 - Spring 2007 5 CRT: Cathode Ray Tube Cathode Ray Tubes (CRT) monitors have been largely replaced by Liquid Crystal Displays (LCD) nowadays A CRT monitor comprises a cathode ray tube with an electron gun that emits 3 electron beams The electron beams in turn strike a screen coated with 3 types of phosphors Each electron beam is used to produce a single color component Phosophors are materials that fluoresce when electrons are impinged on them Each electron beam strikes a single dot on the screen making it glow The intensity of each electron beam is varied to control the amount of fluorescence of each dot Different colored pixels are created by varying the intensity of each electron beam

6 GMU IT 212 - Spring 2007 6 CRT The electron beam scans the phosphor screen in a raster scan format which means that after the beams make one horizontal sweep across the screen, they go back and make a second horizontal sweep across the screen This is called raster scanning The persistence of the phosphor glow enables the eye to see an uninterrupted image The screen is repainted about 60 times per second (or higher) This is called the refresh rate Some displays can paint only every other line on the screen at one pass and then go and paint the other lines during the second pass This is called interlacing and creates a higher resolution image with less expensive components This however causes the screen to flicker The beams are deflected using a component called a magnetic deflection yoke which uses EM fields to bend the path of the electron beams The video graphics adapter card sends signals to the electron gun to produce the electron beams The intensity of each beam is controlled by the signals issued by the adapter

7 GMU IT 212 - Spring 2007 7 CRT The adapter card receives the digital information from the imaging software and converts the digital information into analog signals using a DAC converter These signals are then fed to the CRT to control the intensity of each electron beam There are various display standards for PCs including: VGA (Video graphics Array) SVGA (Super Video Graphics Array) XGA (Extended Graphics Array) UXGA (Ultra Extended Graphics Array) They mainly differ in their resolutions

8 GMU IT 212 - Spring 2007 8 CRT

9 GMU IT 212 - Spring 2007 9 CRT The main problem associated with CRTs is their bulkiness owing to the electron beams that must span across the screen and their high power consumption To scan the screen, the electron gun generating these beams must be positioned at a certain distance away from the screen  bulkiness This drawback may be overcome by using a combination of phosphor dots and individual, smaller electron guns striking these phosphor dots Each phosphor dot may have its own electron beam striking it This technology is called Surface-Conduction Electron-Emitter Display (SED) and is a new technology that provides a flat display but is much cheaper than plasma displays It also requires fewer materials compared to LCDs

10 GMU IT 212 - Spring 2007 10 LCD: Liquid-Crystal Display LCD monitors use liquid crystal trapped inside small cells instead of phosphor dots painted on the screen liquid crystals are liquid chemicals whose molecules can be aligned precisely when subjected to electrical fields (PCWorld) An illuminating bulb resides at the back of the cells and sends light onto the array of cells A polarizing filter polarizes the light sent by the illuminating bulb—it aligns light vertically The light passes through liquid crystal layer The molecules of the liquid crystal can change alignment by applying electrical signals to each liquid crystal cell 3 cells make up a single color of the display After the liquid crystal layer is a color filter A red color filter will allow only red light to pass through and will block other colors of light Similarly, blue and green color filters only allow blue and green light to pass through Finally there is a horizontal filter

11 GMU IT 212 - Spring 2007 11 LCD Molecules in liquid crystals can become twisted by varying degrees No electric field  molecules naturally “twist” the light to make it horizontal Electrical field  molecules straighten and light is not bent The twisting of the molecules affects the polarization angle of the light passing through them Control this by strength of electrical field The light that passes through the cells then strikes the filters Each filter only passes light of a specific color The color light from the filter passes through another polarizing filter and the 3 colors combined make up the unique color of each pixel Horizontal filter only lets horizontal direction light through

12 GMU IT 212 - Spring 2007 12 LCD In summary: LCD Operation Works by controlling polarization of light Liquid crystals can rotate polarization of light. By varying the voltage on a single cell, one can: Rotate light controllably between 0 and 90 degrees Angle of polarization passing through polarization filter controls the intensity of the light of the color which is seen

13 GMU IT 212 - Spring 2007 13 Plasma Displays A plasma display contains many tiny cells filled with a mixture of xenon and neon gases Each cell acts as a tiny bulb that emits different colors of light Each pixel on the screen is made up of 3 of these tiny bulbs By applying varying levels of electricity to each cell, the gas inside the cell may be turned into a plasma that emits light at different intensities One side of each cell is coated with a type of phosphor Cells that are meant to produce green light are coated with a phosphor material that gives off green light Similarly, cells that are meant to produce blue and red light are coated with a phosphor material that gives off blue and red light respectively By varying the amount of electricity applied to each cell, the plasma display creates different colors of light for each pixel A combination of thousands/millions of these pixels creates an entire image More power than LCD

14 GMU IT 212 - Spring 2007 14 Plasma Cross Section

15 GMU IT 212 - Spring 2007 15 Digital Light Processing A digital light processing (DLP) projector projects bright and sharp images onto the screen It works by shining a light through a spinning wheel divided into red, green and blue filters The light that goes through the colored filters strike millions of mirrors on the surface of a DLP chip Each mirror corresponds to a single pixel on the screen and is attached to a hinge to tilt the mirror When the mirror tilts, it reflects the light through a lens and onto the screen As the red filter passes over the light source, it only passes red light The red light that has passed through the red filter reaches the mirrors Each mirror is held tilted for a specific duration controlled by the electricity applied to it The duration that the mirror is held tilted determines the level of red color within each pixel This process is repeated as the filter spins to pass only green and only blue light respectively The persistence of human vision causes the eye to see pixels with various colors

16 GMU IT 212 - Spring 2007 16 Chapter 17: How Data Gets into Your PC

17 GMU IT 212 - Spring 2007 17 Overview The Keyboard is one of the primary device to input information into the PC Mouse is also another popular device The basic way that a keyboard works has not changed significantly since its introduction

18 GMU IT 212 - Spring 2007 18 Keyboard and Scan Codes Pressing a key on the keyboard causes a change in the amount of current flowing through a circuit associated with a specific key The microprocessor on the keyboard constantly scans the circuits leading to the keys It detects the increase/decrease in current from a key that has been pressed Each key has a different code that can be recognized by the processor Depending on the key pressed, the processor detects the change in current and generates a number called a scan code There are two scan codes for each key, one when the key is depressed and another when it is released The processor stores the number in the keyboard buffer for the BIOS to read KEYBOARD MICROPROCESSOR ( KEYBOARD MATRIX

19 GMU IT 212 - Spring 2007 19 Keyboard and Scan Codes The BIOS reads the scan code and sends a signal to the keyboard to delete the scan code from the buffer The BIOS then writes to the RAM to maintain a record of which key was pressed The BIOS also translates the scan code into an ASCII code The key is then displayed by the OS or application software

20 GMU IT 212 - Spring 2007 20 Keyboard and Scan Codes QWERTY layout Layout chosen to slow down manual typist to keep keys from jamming

21 GMU IT 212 - Spring 2007 21 Alternate: Dvorak Simplified Keyboard (DSK) – 1930’s

22 GMU IT 212 - Spring 2007 22 Keyboard Connectors Use USB or PS/2 connector IBM Personal System/2 Pin Pin Description (Signal Name) 1 +KBD Data 2 Not used 3 Ground 4 +5.0 volt dc 5+KBD CLK 6Not used

23 GMU IT 212 - Spring 2007 23 The Mechanical Mouse As the mouse moves, a protruding ball turns in the direction of the movement As the ball rotates it turns two rollers mounted 90 degrees relative to each other One is for horizontal movements the other is for vertical movements Each roller is attached to a wheel called an encoder On the rims of the encoder are tiny metal contact points Two pairs of contact bars also extend from the housing of the mouse Each time a contact bar touches a contact point an electrical signal is generated Or there is an infrared LED and sensor. The encoder has holes on the rim to break the beam of light into the sensor. How often the signal is generated is translated into how fast the mouse moves on the screen Tapping either of the buttons on the mouse sends a signal to the PC which passes it on to the software Based on the number of clicks and the exact position of the mouse during the click, the software performs the task you want to accomplish

24 GMU IT 212 - Spring 2007 24 The Optical Mouse As you move an optical mouse, and LED (Light Emitting Diode) lights up the surface the mouse is on and bounces back to hit a CMOS sensor A digital black and white camera at the bottom of the mouse takes hundreds of photos a second looking for differences in the images that indicate the speed and direction of the mouse Signals from the camera are fed to the digital signal processor of the mouse The information from the processor is fed to the computer which uses the data to reposition the mouse on the screen Optical mice are usually more expensive than mechanical mice LEDs confused by polished or glass surfaces Wireless mice are also very popular Wireless mice send signals using radio waves to a receiver attached to one for the computer’s USB ports Some mice even use Bluetooth connections

25 GMU IT 212 - Spring 2007 25 Touchpad Is comprised of layers of electrodes row in horizontal row in vertical Layers do not touch positive charge builds up on one row negative charge builds up on the other Circuitry monitors capacitance between layers 100 times each second Finger entering the area changes that points capacitance circuitry determines finger’s center and computer horizontal/vertical intersection point

26 GMU IT 212 - Spring 2007 26 Pointing Stick Looks like an erase head (miniature joystick) Four force sensing resistors forward,backward, left, right - directions Finger pressure on the resistors changes resistance to allow more current flow Microcontroller monitors amounts of current flowing through sensing resistors and uses that information to translate the finger pressure into on-screen cursor movements

27 GMU IT 212 - Spring 2007 27 Pointing Device Tips Mechanical mice need care and feeding periodic maintenance will wear out use mouse pad whenever possible

28 GMU IT 212 - Spring 2007 28 Speech Recognition A person who is going to use speech recognition software must go through a process called enrollment which consists of a person dictating text that is already known to the software for 10 minutes to an hour From this, the software creates vocal references which are ways in which the speaker’s pronunciation of phonemes varies from models of speech based on a sample of hundreds to thousands of people Phonemes are the smallest units of sound that combine into words such as “duh,” “aw”, “guh” in dog. There are 48 phonemes in English After enrollment, the speaker dictates the text that needs to be converted into text As the speaker speaks, ADC circuits sample the analog sound waves and convert the audio signal into a binary stream Which is then compressed and adjusted by the speech engine to correct the phonemes

29 GMU IT 212 - Spring 2007 29 48 Phonemes

30 GMU IT 212 - Spring 2007 30 Speech Recognition The acoustic recognizer compares the corrected phonemes to known phonemes For each phoneme the recognizer finds the phoneme in a database that most closely matches the phoneme To form words, the speech engine compares groups of successive phonemes to a database of known words and selects the most appropriate word For words that sound the same but are written differently such as “their” and “there”, they are turned over to a natural language component which compares the sounds with grammatical rules and selects the most appropriate one If the speech engine cannot make a selection, it may ask the speaker to select the appropriate word Cell phones have simplified systems without training

31 GMU IT 212 - Spring 2007 31 Chapter 18: How Scanners Captures Images and Words

32 GMU IT 212 - Spring 2007 32 Scanners The basic element of any computer scanner is the CCD or charge coupled device A CCD is an array of small cells called photodiodes A photodiode is a device that is capable of generating electricity when light strikes it The amount of electricity that can be generated by the photodiode is dependent on the intensity of the light striking it A flatbed scanner has a light source that illuminates the piece of paper laying flat on the glass surface of the scanner A scanner works by relying on the reflection of this light from the different regions of the paper Blank or white spaces reflect light more than dark regions do A scan head attached to a motor moves over the paper and captures the light reflected from each individual area of the page The light from the page is reflected through a system of mirrors

33 GMU IT 212 - Spring 2007 33 Scanners A lens focuses the beams of light onto the photodiodes of the CCD that translate light levels into levels of electricity The more light that is reflected, the greater the electricity generated by the photodiodes If the scanner is a color scanner, the reflected light is directed through red, green and blue filters positioned in front of individual diodes An ADC circuit converts each analog electricity reading into a digital value and a digital number is generated for each pixel This digital vale is then sent to the software where it is stored in the appropriate image format

34 GMU IT 212 - Spring 2007 34 Scanners Scanner Issues: Personal (inexpensive to own) High Quality Copyright violations (easy to do) books, writing works, etc. Forgeries documents currency IDs Manipulation/alternations

35 GMU IT 212 - Spring 2007 35 Optical Character Recognition In order to implement character recognition, the document is scanned by a scanner The scanner creates a bitmap image of the document with pixels that are either black or white The Optical character Recognition (OCR) software reads the bitmap and averages out the zones The whitespace between lines of text within a block defines each line’s baseline, an essential detail for recognizing the characters in the text The software then tries to match images to text by trying to match each character though a pixel by pixel comparison to character templates that it holds in memory If any character is unrecognized then the OCR software further passes the character through a process called feature extraction In feature extraction, the software calculates the character’s height and analyzes it further If there is still no match then a special characters such as # is placed in place of it

36 GMU IT 212 - Spring 2007 36 Optical Character Recognition Many OCR programs also employ a spell checker to correct any mistakes Most OCR programs also give the option of saving the document in a variety of popular file formats recognized by word processors and spreadsheets

37 GMU IT 212 - Spring 2007 37 Optical Character Recognition Scanned page for OCR

38 GMU IT 212 - Spring 2007 38 Optical Character Recognition OCR’d text The (ult of the Internet W-. e write ”electricity” with a small ”e,” but the fact that the ”I” in ”Internet” is written in capital letters is symptomatic of the cult that surrounds it. The high-flown metaphors that are used to name this universe (from ”cyberworld” to ”virtual world”), as well as the routine expressions (the in- evitable ”information society”), clearly tell us that we are dealing with some- thing quite different from a simple tool. Veneration of the Internet leads to misuse of language: new techniques are called ”new technologies.” As a sociol- gist of communication, Philippe Breton calls into question the ”cult of the In- Telnet in a short book that is lively and stimulating: le Cults de l’Internet. Une menace pour le lien social? (The Cult of the Internet. A Threat to the Social

39 GMU IT 212 - Spring 2007 39 Chapter 19: How Portable Computers Work

40 GMU IT 212 - Spring 2007 40 Overview The size of portable computers is not necessarily limited by our ability to manufacture smaller and smaller chips It is due to the fact that a computer must be of a certain size for that we can interact with them on a human scale The size of our fingertips dictates that a useable keyboard must be at least of a certain size Our eyes also demand a display that cannot be very small in size

41 GMU IT 212 - Spring 2007 41 Overview Objectives Minimize size, weight, power consumption for portability Constraints: viewing area, keyboard, mouse, power capacity Realities Typically 2x cost of comparable PC lags desktop capabilities Upgrading is difficult, if not impossible economically practical?

42 GMU IT 212 - Spring 2007 42 Notebook PCs There are several components in notebook PCs that are different than desktop PCs and several components that are common to both Some of the components include: AC adapter Batteries Built-in modem RAM Touchpad Infrared Port Display Keyboard PC card slot External Bus Floppy drive and CD-ROM drive

43 GMU IT 212 - Spring 2007 43 Notebook PCs AC adapter Converts the AC voltage from wall outlet into DC voltage levels Batteries Usually constitutes the bulkiest and heaviest part of the notebook Some notebooks provide a slot to include a secondary battery for longer lifetime Built-in modem Allows easy connection to the Internet Sometimes is a software modem RAM Designed to fit into specialized slots Off the shelf desktop DIMMS and SIMMS cannot be used with notebooks Touchpad Takes the place of a mouse

44 GMU IT 212 - Spring 2007 44 Notebook PCs Infrared Port Allows file transfers between a laptop and a desktop Display LCD Keyboard Has smaller keys than a regular keyboard Reduced keys Tighter pitch Shorter travel PC card slot Allows other peripherals to be added easily Can serve the function of a hard drive, modem, network connection or connection to external CD-ROM drive Many laptops have 2 PC card slots

45 GMU IT 212 - Spring 2007 45 Notebook PCs External Bus Used to connect notebook to docking station where it can access ordinary desktop monitor, keyboard, mouse, etc Floppy drive and CD-ROM drive Often combined into one component or they can take turns using a drive bay

46 GMU IT 212 - Spring 2007 46 Notebook PC (Intel Processors) Celeron family Intel Celeron M Pentium family Intel Pentium M Core family Intel Core Solo Mobile Intel Core Duo Mobile Intel Core 2 Duo Mobile M or Mobile means lower power than desktop processor Intel Centrino combines an Intel mobile processor with wireless LAN capabilities MORE POWERFUL

47 GMU IT 212 - Spring 2007 47 Notebook PCs Heat Dissipation Is a Problem Battery heats up during use Fan cools electronic components Components switched off or clock speed lowered to reduce power consumption Sound Card, Video Controller, Modem, all Built Into Motherboard (not upgradeable) PCMCIA/PC Cards Allow Moderate Upgrade Memory, modems, NIC, storage, High Tech I/O

48 GMU IT 212 - Spring 2007 48 Notebook PCs Notebook Tips: Buy new current version versus upgrade of older machine RAM upgrades may be practical/economical Disable Infrared (IR) port (security) Portability invites theft Secure data Encryption Physical (off-line storage if possible)

49 GMU IT 212 - Spring 2007 49 PC Cards PC cards were earlier called PCMCIA cards They are used to expand the capability of the laptop and plug into a slot at the side of the laptop Most PC Cards contain non-volatile memory which stores configuration information about itself [called Card Information Structure or CIS] Used by the device driver to provide information about the card A device driver is software that is an extension to the operating system to provide access to the hardware Sometimes a PC Card contains a battery to maintain data stored within (i.e., volatile storage) Memory and I/O memory registers are digital scratch pads where the card holds the data that can be accessed by the laptop

50 GMU IT 212 - Spring 2007 50 PC Cards A controller chip in the laptop links the PC Card to the laptop when the PC Card is inserted. When a PC card is inserted into a Laptop, the hardware detects that event and informs the operating system The Operating system access the PC Card’s CIS to determine which PC Card is it and what device driver to load and initialize The operating system assigns a memory address window and I/O register addresses and gives that to the card’s device driver This memory window links the laptop’s memory to the memory area in the PC Card Types of PC cards include modems, network cards, wireless LAN cards, cellular provider broadband cards, card readers, hard disks, etc.

51 GMU IT 212 - Spring 2007 51 PDAs Two Types Palm - Palm OS Pocket PC - Windows Mobile (formerly CE) Windows CE - compact version of Windows Color or Monochrome touch displays Ports (serial, USB, infrared, Bluetooth) Adapters for compact flash, secure digital, etc. Built-in functions (Word, Excel, Calendar, etc.) Volatile (RAM) & non-volatile memory (Flash) Programs and data storage Early models use volatile memory 1996 USR 2007 HP

52 GMU IT 212 - Spring 2007 52 PDAs Input not great handing writing recognition getting better small keyboard external peripherals available to augment I/O Host PC wired connection of syncing and data interchange still vital Wireless connectivity to host PC and services emerging Compatibility with desktop applications a key importance of PDA success New application: PDA + mobile phone

53 GMU IT 212 - Spring 2007 53 PDAs PDA Limitations: Battery operating life limited Battery loss means data loss if data in volatile memory Not easily upgradable quick obsolescence Less standardized than desktop PC Limited I/O Not meant to replace desktop! Which to Choose: PDA or Portable Computer?

54 GMU IT 212 - Spring 2007 54 Future of I/O (end of Part 5 in book) Major hurdle for today’s computers Man-machine interface (screens, keyboards, etc.) Key Evolutions Speech recognition (voice commands to computer) Computer visual recognition Computer as active companion (monitoring temperature, brain waves, screen phone calls, etc.) 3-D Holographic Displays Security Challenges Authentication Privacy

55 GMU IT 212 - Spring 2007 55 Chapter 21: How Multimedia Sound Works

56 GMU IT 212 - Spring 2007 56 Sound Cards: Input A sound card (also referred to as an audio adapter) receives analog electrical signals from the output of a microphone or other input device A microphone is a device that converts sound into a proportional signal, much like photodiodes that convert light into a proportional electrical signal The signals go to an ADC chip on the sound card that converts the analog audio signals into a stream of 1s and 0s A ROM chip on the sound card contains instructions for handling the digital data at the output of the ADC

57 GMU IT 212 - Spring 2007 57 Sound Cards A sound card also contains a chip called the Digital Signal Processing (DSP) chip The DSP chip processes the digital signal based on instructions that are stored on the memory chip Processing may involve compressing the digital information The compressed file is then sent to the CPU which in turn sends the data to the hard disk for storage in a format such as.MP3 or.WAV

58 GMU IT 212 - Spring 2007 58 Sound Cards: Output A sound card also converts digital audio files into analog electrical signals which it then feeds to the speakers of the computer To play an audio file, the CPU fetches the file from either the hard disk, CD or from wherever it is stored It then sends the file to the DSP for decompression The DSP in turn send the decompressed digital information to the DAC chip which converts the digital information into an analog signal The analog electrical signal is amplified and then sent to the speakers

59 GMU IT 212 - Spring 2007 59 MIDI and FM synthesis MIDI is short for Musical Instrument Digital Interface (MIDI) In a MIDI file, only instructions on how to play the audio file is stored, not the audio recordings themselves Instructions in MIDI files tell the DSP chip of the sound card which instruments to play and how to play them A sound card has the capability of reproducing sound from various instruments The different sounds of various instruments are stored in the ROM chip of the sound card The DSP then fetches these various sounds based on the instructions it receives

60 GMU IT 212 - Spring 2007 60 MIDI and FM synthesis The MIDI concept is analogous to storing the notes of music instead of the music itself Instead of storing samples of the sounds, the sound card may also store just the characteristics of the musical instruments such as their pitch (FM synthesis) Based on these characteristics, the sound card may synthesize the sound This does not produce as good quality music as storing the individual sounds on the ROM chip and using them to reproduce the music

61 GMU IT 212 - Spring 2007 61 MP3 MP3 is a popular audio compression standard that has enabled the efficient sharing of music files The reason that MP3 is so popular is because it provides a high degree of compression, yet maintains a good level of sound quality MP3 perceptual coding: compression techniques discard the frequencies imperceptible to human the less prominent sounds that are “drowned out” perceptually by more dominant sounds In addition to this lossy compression approach, the MP3 format also applies a lossless compression algorithm exploits redundancies in information to further compress information Analogous to a “ZIP” file Conversion from a CD file to MP3 may be performed by a ripper (utility program)

62 GMU IT 212 - Spring 2007 62 iPod Music on an iPod is stored mainly in two formats: MP3 or AAC Both are compressed formats Stored along with each file is metadata that contains info about the artist name, album name, song category etc. The number of songs that you can store depends on the size of the hard disk and how much each song has been compressed You can store about 10,000 songs on an iPod with a hard disk of 40GB The iPod places the music file that is selected for listening in memory instead of directly playing it from the hard disk This is because the song might skip if jostled in any way The memory chip does not contain any moving parts so the music will not skip

63 GMU IT 212 - Spring 2007 63 Chapter 22: How Multimedia Video Works

64 GMU IT 212 - Spring 2007 64 Multimedia Video A camera and a microphone capture picture and sounds and send the analog signals to a video card or also called a video-adapter card Video is merely a sequence of images (frames) that are captured by a camera and shown in rapid succession together with sound An ADC on the adapter card converts the analog audio and video signals into a pattern of 1s and 0s A compression/decompression chip compresses the information Compression involves eliminating redundancy in each image that makes up the video More space is saved by interweaving the picture and audio information within the same file or format called.AVI (audio/video interleave)

65 GMU IT 212 - Spring 2007 65 Multimedia Video To replay the video, the data is sent to the decompression chip which restored the data that was eliminated during compression The combined audio and video elements are separated and both sent to a DAC chip The DAC translates the digital information into analog signals which are then sent over to the speakers and the screen Advanced compression techniques cut down on the amount of data that must be recorded One method, used by.AVI format saves a complete video frame and then records only the differences (called the delta) in the frames that follow Each frame is recreated by combining the delta data with the data for the frame that preceded it

66 GMU IT 212 - Spring 2007 66 Multimedia Video Video on DVDs are stored in a compressed format called MPEG (Motion Pictures Experts Group) MPEG compressed by storing frame as Intraframe: complete data image for that frame Predicted frame: the difference between current frame and last predicted frame or intraframe Bidirectional frame: using data from closest frames, interpolates to get frame

67 GMU IT 212 - Spring 2007 67 TiVo TiVo is essentially a self-contained computer with a hard disk that uses Linux The TV signal arriving from the coaxial cable feeds into the TiVo If the signal is in analog format, TiVo converts it into digital and compresses it using MPEG The digital signal is then sent over to the hard drive For viewing, the digital file is sent over to the decoder which converts it back into analog format for viewing If someone wants to pause live television, TiVo stops playing the image on the screen but still continues to write the TV signal onto the hard drive Puts this into the live television buffer When the play button is pressed, the stored file is retrieved from the hard disk and played on the screen TiVo also enables the user to record programs

68 GMU IT 212 - Spring 2007 68 Homework #7 Write a 2 page essay comparing DSL versus cable modems for residential use: Give a brief overview of each technology Discuss their advantages and disadvantages compared to one another

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