Hardware Reference
In-Depth Information
they will remain at the smaller size no matter what settings you change.
Depending on their ability to see and read small text, many people will have difficulty seeing the text
and icons when the display is rated at 100 ppi or higher. If you are going to choose a display rated
over 100 ppi, you may need to either sit closer to the screen or use bifocals or reading glasses to read
it. Changing resolution to a lower setting is usually unsatisfactory with a flat-panel display because
either the display will simply be smaller (text and icons will remain the same size, but you can't fit as
much on the screen) or the display will attempt to scale the data to fit the screen. However, scaling
invariably results in a blurred and distorted image. The bottom line is that LCDs really work well
only at their native resolution—something you should strongly consider when purchasing.
As a consolation, even with their tinier text and icons, LCD and LED backlit screens are much
sharper and clearer than CRT-based monitors. So, even though the text and icons are physically
smaller, they are often more readable with less eyestrain because they are sharp and perfectly
focused.
Note
CRTs don't have a 1:1 relationship between resolution and pixels. Therefore, when you're
comparing CRTs, the smaller the pixel pitch, the sharper the images will be. As an example,
the original IBM PC color monitor from the early 1980s had a pixel pitch of .43mm, whereas
newer color CRTs have a pixel pitch between .25mm and .27mm, with high-end models
offering .24mm or less. To avoid grainy images on a CRT, look for those with a pixel pitch of
.26mm or smaller.
Horizontal and Vertical Frequency
Analog display connections such as VGA are designed to transmit signals that drive the display to
draw images. These signals tell the display to draw an image by painting lines of pixels from left to
right and from top to bottom. For example, if the display resolution is 1024×768, that means there are
768 lines that would be drawn, one after the other, from top to bottom. Once the 768 th line is drawn,
the entire image would be completed, and the process would repeat starting again from the top.
The speed at which this image drawing occurs has two components, called the horizontal frequency
and the vertical frequency . These frequencies are also called scan or refresh rates . The horizontal
frequency is the speed in which the horizontal lines are drawn, expressed as the total number of lines
per second. The vertical frequency (or vertical refresh rate) is the speed in which complete images
are drawn, expressed in the number of images per second.
Using a 1024×768 display as an example, if the vertical refresh rate is 60Hz, then all of the 768 lines
that make up the image would need to be drawn 60 times per second, resulting in a horizontal
frequency of 768 lines per image × 60 images per second, which equals 46,060 total lines per
second, or a frequency of about 46KHz. If the vertical refresh rate were increased to 85Hz, the
horizontal frequency would be 768×85 = 65,280, or about 65.3KHz. The actual figures are
technically a bit higher at 47.8KHz and 68.7KHz, respectively, because there is about a 5% overhead
called the vertical blanking interval that was originally designed to allow the electron beam to move
from the bottom back to the top of the screen without being seen. Although there is no electron beam
in LCD displays, the blanking time is still used for backward compatibility as well as to send
additional data that is not part of the image. The exact amount of vertical and horizontal blanking time
 
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