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

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.

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.

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)

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)

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(2 8 ) and 16 million (2 24 )

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)

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.

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

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

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

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

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