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Chapter 10:
Overview of CRT Raster Display
Today’s modern computer display and HDTV rely on the same fundamentals that materialized from the first experimental television designs. The technology at the time, i.e., vacuum tubes, dictated the limits of performance that could be achieved. Early mainframe computer terminals, known as ASCII terminals, mimicked television refresh rates, with the United States at 60 Hz and Europe at 50 Hz. This required long, persistent phosphors to minimize flicker, which are typically not very efficient. Phosphors that were efficient with reasonable persistence were green and amber. With the desire to emulate the printed page in appearance came the realization that flicker that could be ignored on a B&W TV could not be ignored when reading black text on a white field. Increasing the refresh rate (vertical scan rate) to minimize flicker also increases the demand on the horizontal rate. The first major increase was from 15 kHz (TV 15,750 Hz actual) to 31.5 kHz, and this was considered leading-edge technology. Today, a 5-megapixel display operates at around 180 kHz and a 2-megapixel landscape at up to 105 kHz. In portrait orientation, a 2-megapixel display is at 120 kHz. In the early 1980s this would have been thinkable only on paper for commercial displays. Raster-scanned displays require timing information for the horizontal and vertical rates so that the video appears at the correct location on the screen. All three can be combined into what is referred to as composite video, a single connection typical of procedural playback monitors. The display then separates them into their respective components through tuned circuits that strip them apart. The next level up is separate video and composite sync. Isolating the video from the sync is better for maintaining a clean video signal. At the higher frequencies, separate sync and video with quality shielding is best.A term called sync on green applies to both monochrome and color displays that can also strip sync out of the green input channel. Typically there is a logic chip that will look for separate sync at their inputs and if none is detected, it will look on the green channel; the hierarchy would first default to separate syncs. The industry-standard high-density 15-pin VGA connector on the PC and/or the display provides the respective signals separately. The twist-and-lock (bayonet) type is called a BNC and is by design a single-wire connector. The BNC type will provide maximum shielding and isolation.
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