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. Cathode Ray Tubes Most common display type Phosper Vertical Deflection
Electron Beam
Horizontal Deflection
Grid

3. Raster Scan Beam directed in lines (scan lines) across screen.
Dots are created by turning the beam on & off.
During retrace, the beam is off.
Horizontal retrace is when beam returns to left edge
Vertical retrace is when beam returns to upper left corner.
Active lines
Retrace lines

4. 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.

5. Horizontal, Vertical Syncs
Horizontal oscillator
Horizontal amplifier
15.75 Khz
CRT
Vertical
Sync
Vertical oscillator
Vertical amplifier
60 hz
Video amplifier
Video
Input

6. 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.

7. The horizontal sweep controls the number of lines on the screen
Sweep across screen (slow), drives horizontal deflector
Horizontal Retrace (fast), moves beam back to left edge of screen.
The vertical sweep controls how fast each screen is displayed.
Sweep down the screen, (drives vertical deflector)
Vertical Retrace moves beam back to top left corner

8. 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)

9. How many lines on screen?
Let Horizontal sweep = 15.75Khz
Let Vertical sweep = 60 hz
Num lines = horizontal/vertical = 15750/60 = 262.5
These numbers are for standard North America broadcast television
Half line allows next field (next lines) to be offset by one half line (even & old fields). Two fields make one frame
Gives an interlaced display of 524 lines refreshed at 30 hz
Human eye can detect flicker at 45 Hz - don’t notice flicker on TV because of image types

10. How many Dots on screen? Video input controls whether beam on or not
How fast we can turn beam on/off during horizontal trace time
Monitors use internal clock to sample video signal
Monochrome - only one line for video signal
Digital RGB - three digital lines (Red, Green, Blue)
Gives 8 colors (a 4th line, an intensity signal, can be used to give 16 colors)
Analog RGB - three analog lines, each driven by 8-bit DAC - gives 256 * 256 * 256 = 2 24 colors

11. VGA Timing (640 dots x 480 lines)
Horizontal Sync = 31.5 Khz, Vertical Sync = 60 Hz
Internal Monitor clock (Dot Clock) for latching video signal is Mhz
#max dots per line = Dot Clock Freq/ Horizontal Sync
= Mhz / 31.5 Khz = 800 Dots
Only can use 640 dot times out of possible 800 for display because we need black areas on left/right edges and time for horizontal retrace.
#max lines per screen = Horizontal Sync/Vertical Sync
= 31.5 Khz / 60 hz = 525 lines
Only 480 lines usable, need blank areas on top/bottom, time for vertical retrace

12. Horizontal Sync Timing
31.75 us (31.5 Khz)
756
799
639
640
659
755
659
755
Horizontal retrace
Left Border
Right Border
Display
Next Line
800 dot times per line
Counter can be used to keep track of horizontal screen position.

13. Vertical Sync Timing 16.67 ms ( 60 hz) 525 line times per screen
495
524
479
480
493
494
493
494
Vertical retrace
Top Border
Bottom Border
Display
Next Screen
525 line times per screen
Counter can be used to keep track of vertical screen position.

14. 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)

15. Other resolutions
800 x 600
Dot clock 36 Mhz
Horizontal Sync Khz
Vertical Sync 56 hz
1024 x 768
Dot clock Mhz
Horizontal Sync 48.3 Khz
Vertical Sync 60 hz
Allow about 20% of horizontal trace time for borders, retrace
6% to 8% of vertical trace time for borders, retrace

16. 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)

17. What drives video signal?
Every dot clock time for visible pixels, need to determine value of video signal
For monochrome, 1 bit per pixel (black or white, on or off)
For Digital RGB, 4 bits per pixel (R,G,B, Intensity)
For Analog RGB, N bits per R,G,B value where each RGB value can have 2N distinct values
The memory that defines the screen contents is called the display memory or frame buffer.

18. Memory Calculations 800x 600 x 24 bit color – how much memory?
800 x 600 x 24 bits/pixel = bits
= bytes = 1406 Kbytes = 1.4 Mbytes
2 Mb video card fine for this resolution/color
1280 x 1024 x 24 bits = Mbytes
Need 4 Mbyte Video card

19. Display Memory Characteristics
Accessed at high data rates
Will need to accessed by two sources
CPU which will be doing read/writes to random locations
Video signal driver which will be reading memory locations in a fixed pattern (the scan pattern)
Can use either SRAM, DRAM, SDRAM or specialty graphics memory to implement the graphics memory.
Graphics memory usually on same board as rest of video logic

20. Memory Access Video Card Video Memory CRT
CPU
Write Mostly,
Random Access
Video Controller
Read Only,
Sequential Access
Video Memory
CRT
Video controller must arbitrate between its accesses and CPU accesses. CPU connection to card can either be via system bus (I.e. PCI bus) or dedicated connection (Advanced Graphics Port).

21. 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.

22. What type of Memory for Video?
Early cards used SRAM because of simple interface (no refresh, no DRAM controller needed), and because of low memory size requirements ( a few Kbytes)
As colors/video memory size increased, began using DRAM for density/cost reasons
Some special DRAMs appeared (called Video DRAMs) that had dual port access – parallel input/output for CPU, serial output for CRT. Died out because of cost reasons.
Memory choice today is Synchronous DRAM since 3-D graphics cards need 16 Mbytes and up
Some 3D cards performance is limited by speed of DRAM – even DDR-DRAM not fast enough.

23. 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

24. 2D Graphics 2D graphics operations are very simple
2D graphics operations required by ‘window’ type Graphic User Interfaces
Block moves – memory to memory copy of rectangular areas on screen
Implements window movement
Block Fills (fill rectangular area on screen)
Hardware support for cursor operation
As cursor moves over screen pixels under cursor are not damaged
Hardware support for Sprite movement/collision detection
Sprite is rectangular group of pixels representing an object
Used by 2-D games

25. Animation
To produce animation on a screen, successive ‘frames’ are displayed in which screen objects are in slightly different positions in each frame.
The area of memory that the video controller is currently displaying on screen is the active Frame Buffer.
Most cards support two frame buffers. The CPU writes to the 2nd frame buffer to prepare the next frame.
When the next frame is ready, the 2nd frame buffer becomes the active Frame buffer and the 1st frame buffer is used to create the next frame.
The rate at which new frames are produced is called the Frame Rate (not the same as Video refresh rate).

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

27. 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

28. Video Palettes
Video generation circuit is used to generate VGA video. Each color is generated with an 18-bit digital code (6 bits red, 6 bits green,6 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.
18-bit code stored in video display RAM VRAM is used to specify a color