video hardwares

7/27/2019 Video Hardwares

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VIDEO HARDWARES

Display Devices

2.1 Cathode Ray Tube (CRT)

Electron gun = cathode + control grid Pixel ratio = height of pixel width of pixel Aspect ratio = # rows in display __ # columns in display Resolution -- Number of pixels per linear distance (e.g., 640 400-pixel display).

COLOR -- 3 different phosphors + 3 different gusns (e.g., red, green, and blue)

2.2 Phosphore


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The electron beam causes the phosphors atoms to move into higher energy s tate The atoms give off energy as light when they return to their stable state

REFRESHMENT

Persistence -- Time for emitted light to fade by 90% of its intensity (normally 10 to 60microseconds).

Refresh -- Redrawing of an image to preserve it on a screen Refresh rate -- Number of times per second the image is redrawn (normally > 60 Hz) Flicker -- Develops in low refresh rate because the eyes can not integrate the individual

light impulses comming from a pixel.

Critical fushion frequency -- Refresh rate above which a picture stops flickering andfuses into a steady image. Depends on

o Phosphors persistenceo Image intensityo Ambient room lightingo

Wavelenght of emitted lighto The observer

2.3 Raster Display

SYSTEM COMPONENTS

The screen is subdivided into a matrix of pixels (smallest addressable units).

Raster scanline -- A line of pixels along the screen

Frame (refresh) buffer -- Block of memory used to store the screen pattern


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HOW IT WORKS

The DISPLAY PROCESSOR produces the raster image in the frame buffer from thecommands

The VIDEO CONTROLLER moves the beam row wise across the pixels setting it on andoff according to the content of the frame buffer

The display must be refreshed to avoid flickering (raster image redisplayed 30 to 60 timespers second)


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2-BIT BLACK-AND-WHITE GRAY LEVEL

00 0 3 of full intensity




FRAME BUFFER

Single-bit black-and-white frame buffer (monochrome, bitmap) N-bit black-and-white gray level frame buffer (pixmap) N-bit black-and-white gray level frame buffer with M-bit lookup table N-bit color frame buffer with M-bit look-up table (typically N = 8 and M = 24)


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WHY USE A LOOK-UP TABLE?

Reduced memory for full array of colors24 bit color * 1280 by 1024 resolution/ 8 bits per byte \\ 4MB

Color table animation

2.4 Vector (or Random Scan) Display

Images are described in terms of line segments rather than pixels. Display processor cycles through the commands

2.5 Advantages / Disadvantages

Raster Displays Random Displays

Accuracy of lines Poor good

Refresh time dependent

on image complexity

No Yes < 100,000

Filled & Patterned

Areas

Yes No


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Bit manipulation Yes No

Display Controller cheaper Costly

Memory Requirements cheaper

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