Digilent 410-258P-KIT - Manual (Page 6)

Anvyl Reference Manual
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CRT-based VGA displays use amplitude-modulated moving electron beams (or cathode rays) to
display information on a phosphor-coated screen. LCD displays use an array of switches that can
impose a voltage across a small amount of liquid crystal, thereby changing light permittivity through
the crystal on a pixel-by-pixel basis. Although the following description is limited to CRT displays, LCD
displays have evolved to use the same signal timings as CRT displays (so the “signals” discussion
below pertains to both CRTs and LCDs). Color CRT displays use three electron beams (one for red,
one for blue, and one for green) to energize the phosphor that coats the inner side of the display end
of a cathode ray tube (see Fig. 1). Electron beams emanate from “electron guns”, which are finely-
pointed heated cathodes placed in close proximity to a positively charged annular plate called a “grid”.
The electrostatic force imposed by the grid pulls rays of energized electrons from the cathodes, and
those rays are fed by the current that flows into the cathodes. These particle rays are initially
accelerated towards the grid, but they soon fall under the influence of the much larger electrostatic
force that results from the entire phosphor-coated display surface of the CRT being charged to 20kV
(or more). The rays are focused to a fine beam as they pass through the center of the grids, and then
they accelerate to impact on the phosphor-coated display surface. The phosphor surface glows
brightly at the impact point, and it continues to glow for several hundred microseconds after the beam
is removed. The larger the current fed into the cathode, the brighter the phosphor will glow.
Fig. 4. Cathode ray tube display system.
Between the grid and the display surface, the electron beam passes through the neck of the CRT
where two coils of wire produce orthogonal electromagnetic fields. Because cathode rays are
composed of charged particles (electrons), they can be deflected by these magnetic fields. Current
waveforms are passed through the coils to produce magnetic fields that interact with the cathode rays
and cause them to transverse the display surface in a “raster” pattern, horizontally from left to right
and vertically from top to bottom. As the cathode ray moves over the surface of the display, the
current sent to the electron guns can be increased or decreased to change the brightness of the
display at the cathode ray impact point.
VGA System Timing