^C^1IBM-PC VIDEO PRIMER
^Cby:  Joel Ellis Rea

^CPart Two:  Fundamentals of Video Display 

Chances are that right now you are looking at a cathode ray tube.  Oh, you may 
call it a "monitor," a "TV" or even a "CRT," but what it really is is a cathode 
ray tube.  In fact, "CRT" stands for "cathode ray tube."  Despite all the newer 
technologies in video display, such as liquid crystal displays (LCD), gas 
plasma displays, electro-luminescent displays, etc., CRTs are still the primary 
means of presenting a visible display.  Even most projection displays use one 
or more CRTs as their image source. 
   
What exactly is a CRT?  It is a glass tube that contains a cathode 
(electrically-negative electrode) at one end, which emits a beam, or ray, of 
electrons.  This ray is called a "cathode ray". 
   
CRTs are used for many purposes. When used as a display device, the end of the 
tube opposite the cathode (commonly called "electron gun") is flared out so 
that there is a large, somewhat flat surface at that end.  The inside of this 
surface is coated with one or more chemicals that phosphoresce, or glow, when 
struck by the cathode ray.  These chemicals are called "phosphors."  Different 
phosphors glow in different colors.  Some commonly used phosphor colors include 
at least two Reds, several Greens, at least two Blues, Amber, and a phosphor 
which emits both Blue and Yellow light, thus producing a near-white.  This is 
the phosphor used in "Black-and-White" TV's and monitors.  A newer phosphor 
provides a warmer, easier on the eyes white that has been called "paper white." 

If a CRT screen is coated with one kind of phosphor in a solid sheet, and is 
driven by a single gun, it is called a "monochrome" display.  Monochrome liter- 
ally means "one color."  Monochrome displays are far less expensive than poly- 
chrome, or "color" displays because they are much simpler.  A monochrome 
display needs only one cathode ray, and has its one phosphor applied in a solid 
sheet over the display area of the tube.   A color tube needs multiple cathode 
rays, and multiple phosphors (usually three, in Red, Green and Blue) arranged 
in an small pattern of dots or stripes.  It also needs a precisely-machined 
"shadow mask," a grid of holes or slots just behind the tube that allows each 
cathode ray to strike only its proper phosphor dots or stripes. 
   
For now, we will concentrate on monochrome displays.  What I say here ap- 
plies to color displays as well.  The cathode ray is negatively-charged 
electrically, thus it can be diverted by a magnetic or electrostatic field.  
Near the cathode itself is a set of electromagnets (or electrostatic devices 
in more expensive tubes) that can bend the ray.  One set controls left-to-
right (horizontal) movement, and the other controls up-and-down (vertical) 
movement.  The ray can thus be "aimed" at any point on the phosphor-coated 
surface.  Only one small dot can be lit up at a time.  The intensity, or 
brightness, of this dot varies proportionately with the electrical intensity 
of the cathode ray.  The more electrons in the ray, the brighter the dot 
glows.  Since the ray can be moved rapidly in any direction, lines and curves 
can be displayed.  The phosphors "persist" (or keep glowing) for a brief time 
(depending on the kind of phosphor) after the ray leaves them. 
   
Controlling the horizontal and vertical movements directly is called "vector 
scan."  This is most commonly used in oscilloscopes and in some arcade games.  
It provides for smooth lines without jagginess, but does not permit large 
solid areas to be displayed.  To facilitate the display of large areas, most 
monitors contain a "raster-scan generator."  This device automates the control 
of the ray.  It causes the ray to cover most of the surface of the display by 
starting at the upper left corner (from the viewer's point of view) and moving 
to the right until it reaches the right edge, then shutting the ray off briefly 
as it sweeps it back to the left and very slightly down until it reaches a 
point just under where it began the last line (this is called "horizontal 
retrace,") then switches it back on and repeats the process.  It does a few 
hundred of these "scan lines" until it reaches the bottom right corner of the 
screen, then shuts the ray off for a longer time as it moves it back to the 
upper-left corner (this is called "vertical retrace.")  A monitor with such a 
raster-scan generator is called a raster-scan monitor.  All monitors currently 
used on the IBM-PC are raster-scan monitors. 

   By precisely controlling the intensity of the ray as it sweeps across the 
phosphors under control of the raster-scan generator, virtually any black-and-
white image can be displayed.  In part three, we will learn how computers in 
general and the IBM-PC in particular use CRTs.  We will focus on how the IBM 
Monochrome Display Adapter (MDA) generates the required signals to control a 
monochrome CRT display monitor. 

