Addressing sub-pixels

In order to address a single shutter on the display, a series of electrodes is deposited on the plates on either side of the liquid crystal. One side has horizontal stripes that form rows, the other has vertical stripes that form columns. By supplying voltage to one row and one column, a field will be generated at the point where they cross. Since a metal electrode would be opaque, LCDs use electrodes made of a transparent conductor, typically indium tin oxide.
Since addressing a single shutter requires power to be supplied to an entire row and column, some of the field always leaks out into the surrounding shutters. Liquid crystals are quite sensitive, and even small amounts of leaked field will cause some level of switching to occur. This partial switching of the surrounding shutters blurs the resulting image. Another problem in early LCD systems was the voltages needed to set the shutters to a particular twist was very low, but that voltage was too low to make the crystals re-align with reasonable performance. This resulted in slow response times and led to easily visible "ghosting" on these displays on fast-moving images, like a mouse cursor on a computer screen. Even scrolling text often rendered as an unreadable blur, and the switching speed was far too slow to use as a useful television display.
In order to attack these problems, modern LCDs use an active matrix design. Instead of powering both electrodes, one set, typically the front, is attached to a common ground. On the rear, each shutter is paired with a thin-film transistor that switches on in response to widely separated voltage levels, say 0 and +5 volts. A new addressing line, the gate line, is added as a separate switch for the transistors. The rows and columns are addressed as before, but the transistors ensure that only the single shutter at the crossing point is addressed; any leaked field is too small to switch the surrounding transistors. When switched on, a constant and relatively high amount of charge flows from the source line through the transistor and into an associated capacitor. The capacitor is charged up until it holds the correct control voltage, slowly leaking this through the crystal to the common ground. The current is very fast and not suitable for fine control of the resulting store charge, so pulse code modulation is used to accurately control the overall flow. Not only does this allow for very accurate control over the shutters, since the capacitor can be filled or drained quickly, but the response time of the shutter is dramatically improved as well.