Unfortunately, in spite of its simplicity or because of it, a passive matrix LCD has its
disadvantages.Themajordisadvantageisitsrefreshspeedorresponsetime,asitiscalled
on LCDs. Another is that the grid system delivers electricity imprecisely. The charge sent
to open (untwist) a liquid crystal pixel may also affect the pixels around it, which partially
untwist, causing a fuzzy display and image contrast problems.
Passive Matrix Liquid Crystal Types There are three types of liquid crystals used in passive
matrix displays:
Twisted nematic (TN) This type of liquid crystal has a 90-degree twist. It is
used in low-cost displays and provides a black on gray or silver background.
Not too common on recent computer products but still used on consumer
electronics and appliances.
Supertwisted nematic (STN) Despite its ominous name, this is the type of
liquid crystal used in most portable PCs and personal digital assistants (PDAs).
It has 180-degree or 270-degree twists, which means it has more tolerance
against the electricity radiating from nearby pixels and provide more degrees
of color shading. It is used for both monochrome and color displays.
Dual-Scan STN (DSTN) This type of LCD divides the display into two
halves, which are scanned individually and simultaneously, which doubles
the number of lines refreshed and cuts in half the time to refresh the display.Active Matrix LCD
Active matrix LCDs use thin film transistors (TFT), which are switching transistors and
capacitors, arranged in a matrix on a glass substrate. Three transistors are used to support
each liquid crystal pixel in the display, one for each of the RGB colors. For example, a
color VGA 640 × 480 display uses 921,600 transistors, and a 1024 × 768 display uses
2,359,296 transistors, which are etched into the substrate glass. If a transistor has a prob-
lem, it creates a bad pixel that may not be able to display one or more colors. It is common
for TFT displays to have some bad pixels.
Apixelisaddressedsomewhatliketheprocessusedinapassivematrix,withtheex-
ception that when a certain row is addressed on an active matrix display, the other rows are
switchedoffandthesignalisthensentdowntheappropriatecolumn.Sinceonlythead-
dressed row is active, there is no danger of energizing any pixel besides the one addressed.
The capacitor in the TFT is able to hold the charge until the next refresh cycle recharges it.
By controlling the amount of electricity that flows to the pixel, the amount that the liquid
crystal untwists is also controlled, along with the amount of light that is allowed to pass. In
fact, most active matrix screens are able to display 256 levels of brightness per pixel.
The active matrix display produces a sharp, clear image with good contrast. However,
active matrix displays are more expensive than passive matrix displays due to their
higher manufacturing costs.
Chapter 16: Monitors and Displays^393