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 ', which is derived from ; 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.
Description
Scan lines
In a raster scan, an image is subdivided into a sequence of 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, but does not have discrete pixels – it instead varies the signal continuously over the scan line. Thus, while the number of scan lines is unambiguously defined, the horizontal resolution is more approximate, according to how quickly the signal can change over the course of the scan line.Scanning pattern
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, 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", with a slope of approximately –1/horizontal resolution, while the sweep back to the left 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 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.
In fact, spikes do occur, both horizontally and vertically, and the corresponding horizontal blanking interval and vertical blanking interval give the deflection currents settle time to retrace and settle to their new value. This happens during the blanking interval.
In electronics, these movements of the beam are called "sweeps", and the circuits that create the currents for the deflection yoke are called the sweep circuits. These create a sawtooth wave: steady movement across the screen, then a typically rapid move back to the other side, and likewise for the vertical sweep.
Furthermore, wide-deflection-angle CRTs need horizontal sweeps with current that changes proportionally faster toward the center, because the center of the screen is closer to the deflection yoke than the edges. A linear change in current would swing the beams at a constant rate angularly; this would cause horizontal compression toward the center.
Printers
Computer printers create their images basically by raster scanning. Laser printers use a spinning polygonal mirror to scan across the photosensitive drum, and paper movement provides the other scan axis. Considering typical printer resolution, the "downhill" effect is minuscule. Inkjet printers have multiple nozzles in their printheads, so many of "scan lines" are written together, and paper advance prepares for the next batch of scan lines. Transforming vector-based data into the form required by a display, or printer, requires a Raster Image Processor.Fonts
Computer text is mostly created from font files that describe the outlines of each printable character or symbol. These outlines have to be converted into what are effectively little rasters, one per character, before being rendered as text, in effect merging their little rasters into that for the page.Video timing
In detail, each line consists of:- scanline, when beam is unblanked, and moving steadily to the right
- front porch, when beam is blanked, and moving steadily to the right
- sync pulse, when beam is blanked, and moves rapidly back to the left
- back porch, when beam is blanked, and again moving steadily to the right.
Perception
Raster scan on CRTs produces both the impression of a steady image from a single scanning point through several technical and psychological processes. These images then produce the impression of motion in largely the same way as film – a high enough frame rate of still images yields the impression of motion – though raster scans differ in a few respects, particularly interlacing.Firstly, due to phosphor persistence, even though only one "pixel" is being drawn at a time, by the time the whole screen has been painted, the initial pixel is still relatively illuminated. Its brightness will have dropped some, which can cause a perception of flicker. This is one reason for the use of interlacing – since only every other line is drawn in a single field of broadcast video, the bright newly-drawn lines interlaced with the somewhat dimmed older drawn lines create relatively more even illumination.
Second, by persistence of vision, the viewed image persists for a moment on the retina, and is perceived as relatively steady. By the related flicker fusion threshold, these pulsating pixels appear steady.
These perceptually steady still images are then pieced together to produce a moving picture, similar to a movie projector. However, one must bear in mind that in film projectors, the full image is projected at once, uninterlaced, based on a frame rate of 24 frames per second. By contrast, a raster scanned interlaced video produces an image 50 or 60 fields per second, with each field being drawn a pixel at a time, rather than the entire image at once. These both produce a video, but yield somewhat different perceptions or "feel".
Theory and history
In a cathode ray tube display, when the electron beams are unblanked, the horizontal deflection component of the magnetic field created by the deflection yoke makes the beams scan "forward" from left to right at a constant rate. The data for consecutive pixels goes to the digital-to-analog converters for each of the three primary colors. As the scan line is drawn, at the right edge of the display, all beams are blanked, but the magnetic field continues to increase in magnitude for a short while after blanking.To clear up possible confusion: Referring to the magnetic deflection fields, if there were none, all beams would hit the screen near the center. The farther away from the center, the greater the strength of the field needed. Fields of one polarity move the beam up and left, and those of the opposite polarity move it down and right. At some point near the center, the magnetic deflection field is zero. So, therefore, a scan begins as the field decreases. Midway, it passes through zero, and smoothly increases again to complete the scan.
After one line has been created on the screen and the beams are blanked, the magnetic field reaches its designed maximum. Relative to the time required for a forward scan, it then changes back relatively quickly to what's required to position the beam beyond the left edge of the visible area. This process occurs with all beams blanked, and is called the retrace. At the left edge, the field steadily decreases in magnitude to start another forward scan, and soon after the start, the beams unblank to start a new visible scan line.
A similar process occurs for the vertical scan, but at the display refresh rate. A complete field starts with a polarity that would place the beams beyond the top of the visible area, with the vertical component of the deflection field at maximum. After some tens of horizontal scans, the vertical component of the unblank, combined with the horizontal unblank, permits the beams to show the first scan line. Once the last scan line is written, the vertical component of the magnetic field continues to increase by the equivalent of a few percent of the total height before the vertical retrace takes place. Vertical retrace is comparatively slow, occurring over a span of time required for several tens of horizontal scans. In analog CRT TVs, setting brightness to maximum typically made the vertical retrace visible as zigzag lines on the picture.
In analog TV, originally it was too costly to create a simple sequential raster scan of the type just described with a fast-enough refresh rate and sufficient horizontal resolution, although the French 819-line system had better definition than other standards of its time. To obtain a flicker-free display, analog TV used a variant of the scheme in moving-picture film projectors, in which each frame of the film is shown twice or three times. To do that, the shutter closes and opens again to increase the flicker rate, but not the data update rate.
Interlaced scanning
To reduce flicker, analog CRT TVs write only odd-numbered scan lines on the first vertical scan; then, the even-numbered lines follow, placed between the odd-numbered lines. This is called interlaced scanning. Modern high-definition TV displays use data formats like progressive scan in computer monitors, or interlaced.Radar
Raster scans have been used in fire-control radar, although they were typically narrow rectangles. They were used in pairs. In each display, one axis was angular offset from the line of sight, and the other, range. Radar returns brightened the video. Search and weather radars have a circular display that covers a round screen, but this is not technically a raster. Analog PPIs have sweeps that move outward from the center, and the angle of the sweep matches antenna rotation, up being north, or the bow of the ship.Television
The use of raster scanning in television was proposed in 1880 by French engineer Maurice Leblanc. The concept of raster scanning was inherent in the original mechanical disc-scanning television patent of Paul Nipkow in 1884. The term raster was used for a halftone printing screen pattern as early as 1894. Similar terminology was used in German at least from 1897; Eder writes of "die Herstellung von Rasternegativen für Zwecke der Autotypie". Max Dieckmann and Gustav Glage were the first to produce actual raster images on a cathode-ray tube ; they patented their techniques in Germany in 1906. It has not been determined whether they used the word raster in their patent or other writings.An early use of the term raster with respect to image scanning via a rotating drum is Arthur Korn's 1907 book which says : "...als Rasterbild auf Metall in solcher Weise aufgetragen, dass die hellen Töne metallisch rein sind, oder umgekehrt". Korn was applying the terminology and techniques of halftone printing, where a "Rasterbild" was a halftone-screened printing plate. There were more scanning-relevant uses of Raster by German authors Eichhorn in 1926: "die Tönung der Bildelemente bei diesen Rasterbildern" and "Die Bildpunkte des Rasterbildes" ; and Schröter in 1932: "Rasterelementen," "Rasterzahl," and "Zellenraster" .
The first use of raster specifically for a television scanning pattern is often credited to Baron Manfred von Ardenne who wrote in 1933: "In einem Vortrag im Januar 1930 konnte durch Vorführungen nachgewiesen werden, daß die Braunsche Röhre hinsichtlich Punktschärfe und Punkthelligkeit zur Herstellung eines präzisen, lichtstarken Rasters laboratoriumsmäßig durchgebildet war". Raster was adopted into English television literature at least by 1936, in the title of an article in Electrician. The mathematical theory of image scanning was developed in detail using Fourier transform techniques in a classic paper by Mertz and Gray of Bell Labs in 1934.
CRT components
- Electronic gun:-
- #Primary gun:-It is used to store the picture pattern.
- #Flood gun:-It is used to maintain the picture display.
- # Phosphors coated screen:-coated with phosphorus crystal that limit light when electronic beam strike on them.
- #Focusing system:- focusing system is the electron beam to coverage into a small spot as it strikes the phosphorus screen.
- #Deflection system:-It is used to change the direction of electron beam so that electronic beam can made to strike at different location on the phosphorus coated screen.