Raster scan

A raster scan, or raster scanning, is the rectangular pattern of image capture and reconstruction in television. By analogy, the term is used for raster graphics, the pattern of image storage and transmission used in most computer bitmap image systems. The word comes from the Latin word (a rake), which is derived from (to scrape); see also rastrum, an instrument for drawing musical staff lines. The pattern left by the lines of a rake, when drawn straight, resembles the parallel lines of a raster: this line-by-line scanning is what creates a raster. It is a systematic process of covering the area progressively, one line at a time. Although often a great deal faster, it is similar in the most-general sense to how one's gaze travels when one reads lines of text.

In a raster scan, an image is subdivided into a sequence of (usually horizontal) strips known as "scan lines". Each scan line can be transmitted in the form of an analog signal as it is read from the video source, as in television systems, or can be further divided into discrete pixels for processing in a computer system. This ordering of pixels by rows is known as raster order, or raster scan order. Analog television has discrete scan lines (discrete vertical resolution), but does not have discrete pixels (horizontal resolution) – it instead varies the signal continuously over the scan line. Thus, while the number of scan lines (vertical resolution) is unambiguously defined, the horizontal resolution is more approximate, according to how quickly the signal can change over the course of the scan line.

In raster scanning, the beam sweeps horizontally left-to-right at a steady rate, then blanks and rapidly moves back to the left, where it turns back on and sweeps out the next line. During this time, the vertical position is also steadily increasing (downward), but much more slowly – there is one vertical sweep per image frame, but one horizontal sweep per line of resolution. Thus each scan line is sloped slightly "downhill" (towards the lower right), with a slope of approximately –1/horizontal resolution, while the sweep back to the left (retrace) is significantly faster than the forward scan, and essentially horizontal. The resulting tilt in the scan lines is very small, and is dwarfed in effect by screen convexity and other modest geometrical imperfections.

There is a misconception that once a scan line is complete, a CRT display in effect suddenly jumps internally, by analogy with a typewriter or printer's paper advance or line feed, before creating the next scan line. As discussed above, this does not exactly happen: the vertical sweep continues at a steady rate over a scan line, creating a small tilt. Steady-rate sweep is done, instead of a stairstep of advancing every row, because steps are hard to implement technically, while steady-rate is much easier. The resulting tilt is compensated in most CRTs by the tilt and parallelogram adjustments, which impose a small vertical deflection as the beam sweeps across the screen. When properly adjusted, this deflection exactly cancels the downward slope of the scanlines. The horizontal retrace, in turn, slants smoothly downward as the tilt deflection is removed; there's no jump at either end of the retrace. In detail, scanning of CRTs is performed by magnetic deflection, by changing the current in the coils of the deflection yoke. Rapidly changing the deflection (a jump) requires a voltage spike to be applied to the yoke, and the deflection can only react as fast as the inductance and spike magnitude permit. Electronically, the inductance of the deflection yoke's vertical windings is relatively high, and thus the current in the yoke, and therefore the vertical part of the magnetic deflection field, can change only slowly.

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