Download Video Monitor

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) – 6bit 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.
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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