Topics.

Screen output

1. Video monitor serves as output device for receiving messages from the computer.

2. Early computer monitors were designed to display characters: text, numbers, graphic symbols.

3. Today: must display graphics, photographic images, animation, and video.

4. Video Cards.
   • Monitors are connected to computers via video cards.

   • This is a separate circuit board inside the computer.

   • contains a video adapter (a circuit that converts signals from the processor) and video memory or VRAM . Special portion of memory dedicated to holding video images.

   • The more VRAM, the more colors and the higher the resolution.

   • Typical computer: 2-8MB VRAM.

5. Monitor size.
   • measures as the length of a diagonal line across the screen.

   • Typical sizes: 15", 17", 19", 21"

6. How monitors display images.

   • Images composed of tiny dots called pixels for picture elements.

   • A square inch of an image on a typical monitor is a grid of dots about 72 pixels on each side.

   • Such a monitor has a resolution of 72 dots per inch (dpi).

   • The higher the resolution, the closer together the dots.

   • Another measure of resolution: total number of pixels displayed on the screen.

   • For same monitor size, the monitor that displays the dots closest together displays more pixels. Sharper, clearer display.

   • To describe resolution this way: number of columns and rows of pixels.

   • Examples: 640X480, 832X624, 1024X768. For 1024X768 total number of pixels is 786,432.

7. Color depth. Number of different colors that a computer can display.
   • Each pixel is represented in VRAM by bits. (1 byte = 8 bits).

   • To represent two colors, black and white, need 1 bit for each pixel.

   • If allot each pixel 8 bits of VRAm, can have up to 256 different colors.

   • If give 16 bits/pixel, can have thousands of colors.

   • If give 24 bits/pixel, can have millions (> 16 million) of colors. This give photo realistic pictures.

   • Some computers can display different combinations of resolution and color depth on the same monitor (multisynch).

   • Since there's a fixed amount of VRAM, an increase in color depth means a decrease in resolutino.

   • Example: a computer may be able to switch between 800X600X16M and 1280X1024X 256.

8. Monitor classes.
   • Two classes : CRT (cathode ray tube) and LCD (liquid crystal display). Most common on desktops is the CRT, most common on portables and in projectors is LCD.

   • Larger monitors need more VRAM for color depth and resolution.

   • CRT Monitors refresh or update their images many times per second.

   • If the monitor refreshes its image fewer than 70 times per second (70 hertz), get flicker. Can cause eye strain, headaches, nausea.

   • Many monitors slow down their refresh rates if the resolution is increased.

9. CRT operation.

   • video card sends signals to 3 electron guns located at the back of the monitor's cathode-ray tube (CRT).

   • Each gun shoots out a stream of electrons, one stream for each primaty color.

   • The intensity of each stream is controlled by the signals from the video card.

   • The video card also sends signals to a mechanism in the neck of the CRT that focuses and aims the electron beams.

   • Part of the mechanism, a magnetic deflection yoke, uses electormagnetic fields to bend the path of the electron streams.

   • The signals sent to teh yoke help determine the monitor's resolution and refresh rate.

   • The beams pass through holes in a metal plate called a shadow mask . The mak keps the electron beams precisely aligned with their targets on the inside of the CRT's screen.

   • The CRT's dot pitch is a measure of how close the holes are to each other. The closer the holes the smaller the dot pitch, and the sharper the image.

   • The electrons strike the phosphors coating the inside of the screen. These glow when struck. Three different phosphor materials are used, one for red, blue and green.

   • The stronger the beam that hits the phosphor, the more light the phosphor emits.

   • if each red, green, and blue dot is struck by equally intense electron beams, get white.

   • To create different colors, vary the intensity of each of the beams.

   • After a beam leaves a phosphor dot, the phosphor continues to glow briefly (persistence). For an image to remain stable, the phosphors must be reactivated by repeated scans of the electron beam.

   • After the beams make one horizontal sweep across the screen, the electron streams are turned off as the electron guns refocus the paths of the beams back to the left edge of the screen at a point just below the previous scan line. Called raster scanning .

   • Upon completing a screen, the beams return to the upper-left corner to begin a new screen.

   • The screen is normally redrawn, or refreshed, about 60-70 times a second.

   • Some video cards scan only every other line when they scan over a screen. Called interlacing . Allows the card to create higher resolutions (more lines) with cheaper components. But also causes flicker. Don't want to buy this type of monitor.

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Last Modified: 8 September 2000

THIS PAGE MAINTAINED BY:
John Barr, Ithaca College