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(CRT) An electrical device for displaying images by exciting phosphor dots
with a scanned electron beam. CRTs are found in computer VDUs
and monitors,
televisions and oscilloscopes. The first commercially practical CRT was
perfected on 29 January 1901 by Allen B DuMont.
A large glass envelope containing a negative electrode (the cathode) emits
electrons (formerly called "cathode rays") when heated, as in a vacuum
tube. The electrons are accelerated across a large voltage gradient towards
the flat surface of the tube (the screen) which is covered with phosphor. When
an electron strikes the phosphor, light is emitted. The electron beam is
deflected by electromagnetic coils around the outside of the tube so that it
scans across the screen, usually in horizontal stripes. This scan pattern is
known as a raster.
By controlling the current in the beam, the brightness at any particular point
(roughly a "pixel")
can be varied.
Different phosphors have different "persistence"
- the length of time for which they glow after being struck by electrons. If the
scanning is done fast enough, the eye sees a steady image, due to both the
persistence of the phosphor and of the eye itself. CRTs also differ in their dot
pitch, which determines their spatial resolution,
and may also incorporate interlace.
As in all of
the flat screen designs, Diamond Flat CRTs apply a calculated polynomial curve to the
internal screen surface and aperture grille which, in essence, pre-distorts
the image. This compensates optically for the refraction of light passing through
the CRT glass which causes the concave
effect that other flat CRTs exhibit.
A measurement that indicates the diagonal distance between like-colored phosphor
dots on a display
screen. Measured in millimeters, the dot pitch is one of the principal
characteristics that determine the quality of display monitors.
The lower the number, the crisper the image. The dot pitch of color
monitors for personal
computers ranges from about 0.15 mm to 0.30 mm.
A measurement that indicates the distance between like-colored phosphor stripes on a display
screen. Measured in millimeters, the stripe pitch is one of the new principal
characteristics that determine the quality of flat display monitors.
The lower the number, the crisper the image. The stripe pitch of color
monitors for personal
computers ranges varies.
Very Fine Stripe Pitch
A voluntary labeling program of the U.S. Environmental Protection Agency (EPA)
and the U.S. Department of Energy that identifies energy efficient products.
Qualified products exceed the minimum federal standards for energy consumption by a
certain amount, or where no federal standards exist, have certain energy saving
features. Such products may display the Energy Star label.
(HSR) The measurement of how many scan
lines of pixels
a monitor can
display in one second, expressed in kHz (generally somewhere between 20 and 100
kHz).
The HSR is controlled by the horizontal sync signal generated by the video
controller, and is limited by the speed the monitor can scan the
electron beam horizontally across one line on the screen and then returning it to the beginning
of the next line.
A green standard published by SWEDAC (the Swedish Board for Technical
Accreditation), MPRII limits the maximum amount of ELF
and VLF
electromagnetic radiation a computer monitor
may emit. Most personal
computer monitors comply with this standard or the more stringent European
TCO requirement.
Hardware or software that can immediately be used after being installed is known
as plug & play. Non plug & play hardware or software will require configuration.
Referring to the sharpness and clarity of an image, resolution is the term most often used to
describe monitors, printers, and bit-mapped
graphic images. In the case of dot-matrix
and laser
printers, the resolution indicates the number of dots
per inch. For example, a 300-dpi (dots per inch) printer is one that is
capable of printing 300 distinct dots
in a line 1 inch long. This means it can print 90,000 dots per square inch.
For graphics
monitors, the screen resolution signifies the number of dots (pixels)
on the entire screen. A 640-by-480 pixel screen is capable of displaying 640 distinct
dots on each of 480 lines, or about 300,000 pixels. This translates into
different dpi measurements depending on the size of the screen. For example, a
15-inch VGA
monitor (640x480) displays about 50 dots per inch.
Printers, monitors, scanners,
and other I/O devices are often classified as high resolution, medium
resolution, or low resolution. The actual resolution ranges for each
of these grades is constantly shifting as the technology improves.
Video RAM Required for Different
Resolutions
Resolution |
256 colors (8-bit) |
65,000 colors
(16-bit) |
16.7 million
colors (24-bit, true color) |
| 640x480 |
512K |
1 MB |
1 MB |
| 800x600 |
512K |
1 MB |
2 MB |
| 1,024x768 |
1 MB |
2 MB |
4 MB |
| 1,152x1,024 |
2 MB |
2 MB |
4 MB |
| 1,280x1,024 |
2 MB |
4 MB |
4 MB |
| 1,600x1,200 |
2 MB |
4 MB |
6 MB |
"Vertical Refresh Rate" or "Vertical Scan Rate" is the maximum number of frames
that can be displayed on a monitor
in a second, expressed in Hertz.
The scan rate is controlled by the vertical sync signal generated by the video
controller, ordering the monitor to position the electron
gun at the upper left corner of the raster,
ready to paint another frame. It is limited by the monitor's maximum horizontal
scan rate and the resolution,
since higher resolution means more scan
lines. Increasing the refresh rate decreases flickering, reducing eye
strain, but few people notice any change above 60-72 Hz.
The on-screen display (OSD) module converts programmed character addresses and control information into digital color and
blanking outputs to display user defined characters on a television screen for on-screen programming and
closed-captioning applications.
Universal Serial Bus is a new external bus
standard that supports data transfer rates of 12 Mbps. Known as Daisy Chain, a
single USB port can be used to connect up to 127 peripheral devices,
such as mice, modems, and keyboards. USB also supports Plug-
and-Play installation and hot plugging. With USB, you can add and
remove devices from a computer while it is running. The operating system
will automatically recognize the change. Worrying about DIP switches, jumpers, and other configuration
element settings is eliminated.
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