LCD Or Plasma - What's Your Pleasure? Understanding Modern Flat-Panel TV Technologies

Addressing LCD Matrices

Addressing passive-matrix LCDs is done very similarly to the way it's done in plasma displays. A front electrode, common to the entire column, conducts the voltage. The rear electrode, common to the entire row, serves as ground.

The disadvantages of old-fashioned passive matrixes are numerous and well known: the panels are slow, and the display is not sharp. There are two reasons for that. The first has to do with the fact that the pixel, once it has been addressed, slowly begins returning to its normal state, creating a blurred image. The second is due to the capacitive coupling between the control lines. This coupling makes the propagation of voltage imprecise and contaminates the neighboring pixels.

To remedy these problems, manufacturers have adopted active matrix technologies.

The trick here is to add a transistor to each pixel, which operates like a switch. When the switch is closed (on), data can be written to the storage capacitor. When the switch is open (off), the data remains in the capacitor, which then operates like an analog memory. This technology has numerous advantages. When the switch closes, the data remains stored in the capacitor, and thus the liquid crystal will continue to have voltage across its terminals when the lines address another pixel. That means that it won't return to its initial state as was the case with the passive matrix. The writing time to the capacitor is much shorter than the crystal's rotation time, which means that the data can be written and another pixel addressed immediately without waiting.

This technology, known as "TFT" for the thin film transistors it uses, has become so popular now that its name has come to be associated with all LCD monitors.

The voltages used are much lower than for plasma displays. To operate a TFT pixel, voltages of around -5 to +20 volts are needed, which is a far cry from the hundreds of volts plasma panels require.

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