MultiMedia Systems/Multimedia Input & Output Technologies/MultiMedia Systems

Hardware for Multimedia

Input and Output Devices

Key devices for multimedia output

• • Monitors for text and graphics (still and motion)
  • Speakers and midi interfaces for sound
  • Specialized helmets and immersive displays for virtual reality
• Key devices for multimedia input
  • Keyboard and ocr for text
  • Digital cameras, scanners, and cd-roms for graphics
  • midi keyboards, cd-roms and microphones for sound
  • Video cameras, cd-roms, and frame grabbers for video
  • Mice, trackballs, joy sticks, virtual reality gloves and wands, for spatial data
  • Modems and network interfaces for network data

Monitors

• Most important output device
  • Provides all the visual output to the user
  • Should be designed for the highest quality image, with least distortion
  • Large vacuum tube with electron gun at one end aimed at a large surface (viewing screen) on the other end
  • Viewing screen is coated with chemicals that glow with di�erent colors; three different phosphors are used for color screens
  • Source of electron beam is electrically negative pole or cathode (hence the name Cathode Ray Tube, or CRT)
  • Two different sets of colors used in monitors ** rgb and cmy, with either set capable of full color spectrum
  • Electron beam strikes the screen many times per second
    • Phosphors are re-excited at each electron strike for a brief instance
    • Refresh rate, measured in Hz
    • Preferred refresh rate is 75 Hz or more
  • Electron beam sweeps across the screen in a regular pattern
    • Required to refresh phosphors frequently and equally
    • Raster scan pattern
    • Always strikes when going from left to right (trace), and turned on to go from right to left (retrace)
  • Three separate electron beams for three colors, for better focus and higher refresh rates
  • Screen divided into individual picture elements, or pixels
    • Each pixel is made of its own phosphor elements to give the color
    • Memory chip contains a map of what colors to display on each pixel
  • Bit map
    • Mostly used in context of binary images (black or white)

Hardware for Multimedia 20

• • • One bit per pixel to indicate whether pixel is black or white
  • Color maps, or pixmap
    • One byte for each color for every pixel (24-bit color)
  • Image changed in the memory map associated with screen
    • For realistic motion images and for flicker-free screen, bit-map must be modi�ed faster than the eye can perceive (30 frames/sec)
    • For a 640 � 480 screen, number of bits is: 640 � 480 � 24 = 7; 372; 800
    • To refresh the screen at 30 times per second, the number of bits transferred in a second is: 640 � 480 � 24 � 30 = 221; 184; 000 or 221 Mb
    • Larger screen requires more data to be transferred
    • Transfer rate limitation can be overcome by using hardware accelerator board to perform certain graphic display functions in hardware
    • Full-screen 30 image per second performance may not be possible even with graphics accelerator board
  • Physical size of monitor
    • Important factor in the quality of multimedia presentation
    • Typically between 11 and 20 inches on diagonal
    • Another important factor is the number of pixels per inch

� Too few pixels make the image look grainy � For best quality images, pixels should not be wider than 0.01 inches (28mm) in diameter � Latter quantity is used for marketing the monitors (25mm dot pitch)

• • Graphics display board
    • Used in addition to monitor to speed up graphics
    • Special hardware circuits for 2D and 3D graphics
    • Simple graphics boards just translate image data from ram into one usable by monitor
    • Complex boards can even speed up the refresh rate of screen
  • Qualities of a good multimedia monitor
    • Size, refresh rate, dot pitch
  • Other concerns about monitor include weight and ambient light
  • Liquid crystal display monitors
    • Flat screen displays
    • Crystals allow more or less light to pass through them, depending upon the strength of an electric field
    • Not appropriate for multimedia presentation as the view angle is extremely important
  • 3D monitors in the future
  • Human factor concerns

Speakers and midi interfaces

• • Production of sound

1. Digitized representation of frequency and sound transmitted at appropriate time to the loudspeaker (.WAV �les) ** common method 2. Commands for sound synthesis can be transmitted to a synthesizer at appropriate time (midi �les) ** used for the generation of music

• • Musical Instruments Digital Interface (midi)
    • Standard to permit interface for both hardware and control logic between computers and music

synthesizers

• • • Adopted in 1982

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• • • Consists of two parts

1. Hardware standard � Speci�es cables, circuits, connectors, and electrical signals to be used 2. Message standard � Types and formats of messages to be transmitted to/from synthesizers, control units (keyboards), and computers � Messages consist of a device number, a control segment to tell the device the function to be performed (turn on/o� a speci�ed circuit), and a data segment to provide the information necessary for the action (volume of sound, or frequency of basic sound)

• • • An entire piece of music can be described by a sequence of midi messages
  • midi interface
    • Required in the computer to communicate with midi instruments
    • Circuit board to translate the signals

Alphanumeric keyboards and optical character recognition

• • Used for textual input
  • Pressing a key on a keyboard closes a circuit corresponding to the key to send a unique code to the cpu
  • Printed text can be input using ocr software
    • ocr software analyzes an image to translate symbols into character codes
    • Systematically checks the entire page, searching for patterns of dark and light recognizable as alphabetic,

numeric, or punctuation characters

• • • Choose the best match from a set of known patterns
  • Quality of scanned page as well as output

Digital cameras and scanners

Real image and Digital image (Representation of real image in terms of pixels)

• Still image
• Snapshot of an instance
• Motion image
• Sequence of images giving the impression of continuous motion
• Graininess in real images (Individual dots observed when a photograph taken by conventional camera and enlarged)

Digital image capture

• Light is focused on photosensitive cells to produce electric current in response to intensity and wavelength of light
• Electric current is scanned for each point on the image and translated to binary codes
• Codes correspond to pixel values and can be used to rebuild the original picture

Scanners scan an image from one end to the other

• Scanning mechanism shines bright light on the image and codes and records the reflected light for each point
• Scanner does not store data but sends it to the computer, possibly after compression of the same

Quality of images

• Depends on the quality of optics and sharpness of focus
• Perceived by sharpness of resulting image
• Accuracy of encoding for each pixel depends on the precision of photosensitive cells
• Resolution of scanner/camera (number of dots/inch)
• Amount of storage available

Hardware for Multimedia

• Preferable to scan at the highest possible resolution under given hardware and storage space constraints to get the most detail in the original image

Video Camera and Frame Grabbers

Standard video camera contains photosensitive cells, scanning one frame after another. Output of the cells gets recorded as analog stream of colors, or sent to digiting circuitry to generate a stream of digital codes

• Video input card
  • Required for use of video camera to input video stream into computer
  • Digitizes the analog signal from camera
  • Output can be sent to a file for storage, cpu for processing, or monitor for display (or all of them)
• Frame grabber
  • Allows the capture of a single frame of data from video stream
  • Not as good resolution as a still camera
  • Typical frame grabbers process 30 frames per second for real time performance

Microphones and MIDI keyboards

These are used to input original sounds (analog)

• Microphone has a diaphragm that vibrates in response to sound waves
• Vibrations modulate a continuous electric current analogous to sound waves
• Modulated current can be digitized and stored as standardized format for audio data, such as .WAV �le
• Microphone plugs into a sound input board
  • Developer can control the sampling rate for digitizing
    • Higher sampling rate gives better fidelity but requires more space
    • Sampling rate for music ** 20,000 Hz
    • Sampling rate for speech ** 10,000 Hz
• Editing digital audio files (cut and paste)using Audio softwares like Cooledit,Audacity etc

Mice, Trackballs, Joy sticks, Drawing tablets

These are used to enter positional information as 2D or 3D data from a standard reference point

• Latitude, longitude, altitude
• Common to de�ne a point on the computer screen
• Mouse de�nes the movement in terms of two numbers ** left/right and up/down on the screen, with respect to one corner
• Movement of mouse is tracked by software, which can also set the tracking speed
• Trackball works the same way as the mouse
• A joystick is a trackball with a handle
• Pressing the button of mouse/trackball/joystick sends a signal to the computer asking it to perform some function
• Multimedia software should be able to determine the positional information as well as the signal context (mouse press)

CD-ROMs and Video Disks

This is a Popular media for storage and transport of data. Data written on disk by burning tiny holes, interpreted as binary 0 and 1 by software. These days Flash drives are USB devices which are gaining popularity. Features

• Read-only devices; data can be written only once
• CD-roms can typically store about 600MB of information
• With time, the speed has improved (4X in 1995 to more than 50X now)
• DVD-roms allow a few gigabytes of data on a single disk
• Ideal media for distributing multimedia productions (low cost)