Displaying Information

CRTs depict text and graphical information based on signals produced by the display controller. Two such signals are the horizontal and vertical sync signals. The display controller generates the vertical sync signal at a constant rate ranging between 60 and 100+ times per second (Hz). A vertical deflection generator phase locks on this signal and produces a signal that allows a vertical deflection amplifier to produce a strong synchronous sweep voltage. The sweep voltage varies from zero volts to a peak voltage of between 50 and 200 volts, depending on the monitor design.

Shortly before the vertical sync signal arrives, the peak voltage abruptly discharges to zero volts which causes the yoke to deflect the electron beam to the top of the monitor (taking between 250 to 800 [mu]s). The voltage then remains at zero volts for a brief period before beginning to rise constantly.

As the yoke vertical deflection component receives the increasing voltage signal from the vertical deflection amplifier, it begins to deflect the electron beam uniformly towards the lower part of the CRT when viewing the front of the monitor. The voltage peak deflects the electron beam to the bottom of the viewing area.

Monitors process horizontal sync signals similar to the way they process vertical sync signals. Horizontal sync signals occur hundreds of times more frequently than vertical sync signals and result in a horizontal deflection generator producing a signal that allows a horizontal deflection amplifier to generate a synchronous sweep voltage independent of the vertical sweep voltage. The sweep voltage increases from zero volts to a peak voltage, causing the yoke to deflect the electron beam from left to right when viewing the monitor screen from the front. Shortly before the horizontal sync signal arrives, the peak voltage abruptly discharges to zero volts which causes the yoke to deflect the electron beam to the left side of the monitor (taking between 2 and 20 [mu]s) where it remains for a brief period before beginning another left-to-right pass.

Between vertical sync signals, these combined vertical and horizontal rhythmic deflections allow monitors to deflect the electron beam in a pattern of parallel lines that slope slightly down from left to right before the beam whips back to produce the next line. The monitor generates a third signal internally from the vertical and horizontal sync signals which it passes to the blanking amplifier. The blanking amplifier controls whether the electron gun produces any electrons.

A final display controller signal for our purposes is the video output signal. In monochromatic and gray scale monitors, this signal continuously varies (is modulated), assuming various values from zero volts to a peak voltage of as much as 1 volt. A typical PC VGA video output signal provides 64 different output levels while the original bilevel IBM Monochrome Display controller produces only two.

The video amplifier in the monitor receives this signal and boosts it to a signal that varies between 10 and 60 volts depending on the monitor model. The electron gun control grid receives this variable signal, causing it to repel more or fewer electrons, depending on the voltage level. To accurately render an image, the video amplifier must react very quickly to video output signal changes and accurately amplify low voltage signals. A video amplifier’s slew rate indicates the time it takes to alter and stabilize its output signal.

As the deflection amplifiers operate, their effect is to drag the beam horizontally at a uniform speed from the monitor’s left edge towards the right. At a precise distance into the sweep, it reaches the point where the current control grid begins allowing electrons to escape the electron gun. When the beam reaches the right edge of the viewing area, the blanking amplifier shuts off the beam in anticipation of the horizontal retrace operation. If a beam sweeps uniformly across all horizontal lines in an identical fashion, the horizontal deflection circuitry is exhibiting linearity.

When the blanking amplifier allows electrons to escape the electron gun assembly and the video output signal is above zero volts, the escaping electron beam excites phosphors. Meanwhile, the vertical deflection amplifier drags the beam from the top of the viewing area until it reaches the bottom of the viewing area. At that point the blanking amplifier shuts off the signal and begins moving the beam towards the screen top. Eventually the deflection circuitry positions the beam at the upper left corner of the monitor and begins another vertical pass.