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Video Monitor

Video Monitor. Uses raster scanning to display images

noelani-robertson
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Video Monitor

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  1. Video Monitor • Uses raster scanning to display images • Beam of electrons illuminates phosphorus dots on the screen called pixels. Starting at the top of the screen, the gun fires electrons from the left side to the right in a horizontal row, briefly shuts off, and returns to the left side of the screen to begin a new row. • Horizontal retrace – the time period when the gun is off between rows. • After the last row is drawn, the gun turns off and moves to the upper left corner of the screen to start all over again (vertical retrace) • Interlaced monitor scans every other line until it reaches the bottom, returns to the top and scans all other lines. Non-interlaced – has less flicker

  2. Raster line – horizontal line of video information that is displayed on screen • 640x480 display has 480 raster lines, 640 pixels per line. Pixel is smallest subdivision on line.

  3. Video Monitor • Clarity determined by several factors: • Dot pitch – distance between adjacent pixels • .26 average – higher numbers >more distance • Horizontal and vertical frequencies – the speed at which horizontal lines are drawn and the time it takes to draw all lines on the screen.

  4. Color Monitors • Composite video signal (much like tv signal) • Available resolution is low for most apps • TTL RGB Monitor • TTL voltage signals (0 or 5 V) • Can display a total of 16 colors (3 RGB lines, 1 intensity line) • Used in CGA systems in older computer • Primary video colors (red, green, blue) • Secondary (cyan, magenta, yellow) • 9-pin connector (gnd, gnd, red, green, blue, intensity, normal video, horizontal retrace, vertical retrace)

  5. Color Monitors (cont) • Analog RGB Monitor • Analog signals (0-0.7v) • Infinite number of colors available (generally, finite # are generated – 256K, 16M, 24M) • Typically a DAC (digital-to-analog converter used) – 6-bit DAC for each video signal to generate 64 different voltage levels between 0V and 0.7V (64 red, 64 green, 64 blue = 256k different colors available) • 15-pin connector (red,green,blue, red gnd, green gnd, blue gnd, horizontal retrace, vertical retrace, pin 9 blocked,color detect, monochrome detect) • Speed of DAC is critical – most displays require an operating conversion time of 25ns to 40ns max. • 7-bit converters can display 2M colors (128x128x128) • 8-bit converters can display 16M colors (256x256x256)

  6. Video Resolution • Set by software • Limited by the capacity of the video graphics adapter and the amount of video memory. • Expressed as the number of horizontal pixels, followed by the number of vertical pixels. • 640 x 480 (standard VGA) • 800 x 600 (super VGA) • 1024 x 768 (extended VGA) • 1152 x 864 • 1280 x 1024 • # of simultaneous colors supported is between 256(28 ) and 16 million (224)

  7. Video Resolution • Resolution determines amount of memory required for the video interface card. • 640 x 480 x256 colors(8-bits per pixel) • 640x480 bytes or 307,200 bytes required • 800 x 600 x 16M colors (SVGA) 24 bits/pixel • 800x600x24bits/8 or 1.4MB (2MB video card) • 1024 x 768 x 16M colors • 1024x768x3 or 2.5MB (use 4MB video card)

  8. Video Memory • The PC has a memory-mapped video display. Each screen position occupies a separate memory address. The video memory is special high speed VRAM (video RAM). • In color text mode, VRAM is at B8000 • In color graphics mode, VRAM is at A0000. • DOS applications typically write text and graphics directly to video display buffer – much faster than using built-in DOS subroutines. • Windows applications do not use direct video memory access because it corrupts the built-in redrawing of the screen by Windows directly.

  9. Video RAM • VRAM – most video adapters (can be a separate board plugged into an expansion slot, or it may be integrated on the motherboard) • 4-8MB standard • Optimized for storing color pixels • Dual-ported • One port can continuously refresh the display while the other port writes data to the display. Results in lower eye strain than with DRAM

  10. Windows RAM • Optimized for video graphics displays. • Generally outperforms VRAM, allowing screen to be refreshed more quickly.

  11. Synchronous Graphics RAM • Single ported RAM used on video accelerator cards. • Two video memory pages can be opened at the same time. • Able to clear memory quickly • Well-suited to 3-D applications

  12. Video Palettes • Video generation circuit is used to generate VGA video. Each color is generated with an 18-bit digital code (8 bits red, 8 bits green,8 bits blue) • 18 bit code is applied to the DAC. The address input selects one of the 256 colors stored as 18-bit binary codes. Thus 256 colors out of a possible 256K colors are allowed to be displayed at one time. • 8-bit code stored in video display RAM VRAM is used to specify a color

  13. Video Adapter • Controls the display of text and graphics on IBM-compatibles. • Two components • Video controller – separate microprocessor • VRAM typically 4MB or more • optimized for 2-D and 3-D graphics

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