Video Synchronization 823excites its appropriate phosphor? The answer is the shadowmask—a perforated, sheet-
steel barrier that masks the phosphors from the action of an inappropriate electron gun.
The arrangement is illustrated in Figure 28.8. For a color tube to produce an acceptable
picture at reasonable viewing distance, there are about half a million phosphor red, green,
and blue triads on the inner surface of the screen. The electron guns are set at a small
angle to each other and aimed so that they converge at the shadowmask. The beams then
pass through one hole and diverge a little between the shadowmask and the screen so
that each strikes only its corresponding phosphor. Waste of power is one of the very real
drawbacks of the shadowmask color tube. Only about a quarter of the energy in each
electron beam reaches the phosphors. Up to 75% of the electrons do nothing but heat up
the steel!
28.7 Analogue Video Interfaces ....................................................................................
Due to their wide bandwidth, analogue television signals are always distributed via
coaxial cables. The technique known as matched termination is universally applied. In
this scheme, both the sender impedance and the load impedance are set to match the surge
impedance of the line itself. This minimizes refl ections. Standard impedance in television
is 75 Ω. A typical interconnection is shown in Figure 28.9. Note that matched termination
has the one disadvantage that the voltage signal arriving across the receiver’s termination
is half that of the signal EMF provided by the sender. Standard voltage levels (referred to
earlier) always relate to voltages measured across the termination impedance.
Z SurgeZ Surge PD N/2 VoltsEMFN Volts
Coaxial lineVccFigure 28.9 : Video interconnection.