CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [321]
Once all of the pixels are laid out, how do you charge the right spots to make an image? Early LCDs didn’t use rectangular pixels. Instead, images were composed of different-shaped elements, each electrically separate from the others. To create an image, each area was charged at the same time. Figure 19-17 shows the number zero, a display made possible by charging six areas to make an ellipse of sorts. This process, called static charging, is still quite popular in more basic numeric displays such as calculators.
Figure 19-15 Electrical charge, no light is able to pass
Figure 19-16 LCD pixels
The static method would not work in PCs due to its inherent inflexibility. Instead, LCD screens use a matrix of wires (see Figure 19-18). The vertical wires, the Y wires, run to every sub-pixel in the column. The horizontal wires, the X wires, run along an entire row of sub-pixels. There must be a charge on both the X and the Y wires to make enough voltage to light a single sub-pixel.
Figure 19-17 Single character for static LCD numeric display
Figure 19-18 An LCD matrix of wires
If you want color, you have three matrices. The three matrices intersect very close together. Above the intersections, the glass is covered with tiny red, green, and blue dots. Varying the amount of voltage on the wires makes different levels of red, green, and blue, creating colors (see Figure 19-19).
We call this usage of LCD technology passive matrix. All LCD displays on PCs used only passive matrix for many years. Unfortunately, passive matrix is slow and tends to create a little overlap between individual pixels. This gives a slightly blurred effect to the image displayed. Manufacturers eventually came up with a speedier method of display, called dual-scan passive matrix, in which the screen refreshed two lines at a time.
Figure 19-19 Passive matrix display
Although other LCD technologies have since appeared, dual-scan continues to show up on some lower-end LCD panels.
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NOTE You’ll also find passive matrix displays in smaller, handheld devices. See Chapter 21, “Portable Computing,” for details about handheld computing devices.
Thin Film Transistor
A vast improvement over dual scan is called thin film transistor (TFT) or active matrix (Figure 19-20). Instead of using X and Y wires, one or more tiny transistors control each color dot, providing faster picture display, crisp definition, and much tighter color control. TFT is the LCD of choice today, even though it is much more expensive than passive matrix.
Figure 19-20 Active matrix display
LCD Components
The typical LCD projector is composed of three main components: the LCD panel, the backlight(s), and the inverters. The LCD panel creates the image, the backlights illuminate the image so you can see it, and the inverters send power to the backlights. Figure 19-21 shows a typical layout for the internal components of an LCD monitor.
One of the great challenges to LCD power stems from the fact that the backlights need AC power while the electronics need DC power. The figure shows one of the many ways that LCD monitor makers handle this issue. The AC power from your wall socket goes into an AC/DC transformer that changes the power to DC. The LCD panel uses this DC power.
Figure 19-21 LCD internals
Note in Figure 19-21 that this monitor has two backlights: one at the top and one at the bottom. Most LCDs have two backlights, although many only have one. All LCD backlights use cold cathode fluorescent lamp (CCFL) technology, popular for its low power use, even brightness, and long life. Figure 19-22 shows a CCFL from an LCD panel.
Figure 19-22 CCFL backlight
CCFLs need AC power to operate, but given that the transformer converts the incoming AC power to DC, each CCFL backlight needs a device called an inverter to convert the DC power back into AC. Figure 19-23 shows a typical inverter used in an LCD.
Figure