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The picture elements (pixels) are formed by the LCDs, and by grouping red, blue and green cells as shown, accurate colour images of good definition are produced. Each LCD cell is controlled by a thin film transistor (TFT). By addressing rows and columns these transistors are switched on and off, as required.

8.4 Countering cross-talk

With cables that are conducting data pulses, it is not always possible to separate them by a distance that would prevent 'cross-talk' (one signal interfering with another) and other measures are adopted, for example, screened cables and 'twisted pairs'.

Figure 8.5(a) shows a twisted pair of cables. Capacitive interference from a nearby cable is reduced because the two wires are kept close together and this helps to make the signal-carrying wires less susceptible to capacitive cross-talk. The twisted pair is also used to counter inductive interference from nearby cables. The theory is that the 'interference' effects cancel one another out over the length of the cable.

Figure 8.5(b) shows a screened cable (co-axial). Here the interference is reduced because the interfering current is mainly confined to the outer surface of the screen, where it does not distort the signal in the central conductor. The earthing of the screen is an important factor.

Plastic insulation

Conductors

Twisted pair

Plastic insulation

Conductors

Twisted pair

Metal screen

Fig. 8.5 Twisted pair and screened cable

Metal screen

Conductor

Screened cable

Fig. 8.5 Twisted pair and screened cable

8.5 Logic devices

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