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(28 ) and 16 million (224)

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