Raster scan

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Raster-scan display sample; visible gaps between the horizontal scan lines divide each character Raster-scan Display.jpg
Raster-scan display sample; visible gaps between the horizontal scan lines divide each character

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 raster comes from the Latin word rastrum (a rake), which is derived from radere (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. The data to be drawn is stored in an area of memory called the Framebuffer or Framebuffer. This memory area holds the values for each pixel on the screen. These values are retrieved from the refresh buffer and painted onto the screen one row at a time.

Contents

Description

Scan lines

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.

Scanning pattern

The beam position (sweeps) follow roughly a sawtooth wave. Raster-scan.svg
The beam position (sweeps) follow roughly a sawtooth wave.

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 cathode-ray tube (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.

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 (usually steady-rate) movements of the beam[s] are called "sweeps", and the circuits that create the currents for the deflection yoke (or voltages for the horizontal deflection plates in an oscilloscope) 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 (or an optical equivalent) 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 (dozens to hundreds) 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 (RIP).

Fonts

Computer text is mostly created from font files that describe the outlines of each printable character or symbol (glyph). (A minority are "bit maps".) These outlines have to be converted into what are effectively little rasters, one per character, before being rendered (displayed or printed) as text, in effect merging their little rasters into that for the page.

Video timing

In detail, each line (horizontal frame or HFrame) consists of:

The porches and associated blanking are to provide fall time and settle time for the beam to move back to the left (the voltage to decrease), and for ringing to die down. The vertical frame (VFrame) consists of exactly the same components, but only occurs once per image frame, and the times are considerably longer. The details of these intervals are called the video timing. See Video timing details revealed for a diagram of these. These are mostly not visible to end users, but were visible in the case of XFree86 Modelines, where users of XFree86 could (and sometimes needed to) manually adjust these timings, particularly to achieve certain resolutions or refresh rates.

Perception

Raster scan on CRTs produces both the impression of a steady image from a single scanning point (only one point is being drawn at a time) 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 (recall that on an analog display, "pixel" is ill-defined, as there are no fixed horizontal divisions; rather, there is a "flying spot"), 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 (not in a raster scan), 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 (a field being every other line, thus corresponding to a frame rate of 25 or 30 frames 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"[ citation needed ].

Theory and history

In a CRT 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 (at the pixel clock rate) to the digital-to-analog converters for each of the three primary colors (for modern flat-panel displays, however, the pixel data remains digital). 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. 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 (unblanked) 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 (typically 50 to 75 Hz). 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 (but with the beams blanked), 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 ("interlaced") between the odd-numbered lines. This is called interlaced scanning. (In this case, positioning the even-numbered lines does require precise position control; in old analog TVs, trimming the Vertical Hold adjustment made scan lines space properly. If slightly misadjusted, the scan lines would appear in pairs, with spaces between.) Modern high-definition TV displays use data formats like progressive scan in computer monitors (such as "1080p", 1080 lines, progressive), or interlaced (such as "1080i").

Radar

Raster scans have been used in (naval gun) fire-control radar, although they were typically narrow rectangles. They were used in pairs (for bearing, and for elevation). 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 (Plan Position Indicator, PPI) 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. [1] 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. [2] Similar terminology was used in German at least from 1897; Eder [3] writes of "die Herstellung von Rasternegativen für Zwecke der Autotypie" (the production of raster negatives for halftones). Max Dieckmann and Gustav Glage were the first to produce actual raster images on a cathode-ray tube (CRT); they patented their techniques in Germany in 1906. [4] 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 (in German): [5] "...als Rasterbild auf Metall in solcher Weise aufgetragen, dass die hellen Töne metallisch rein sind, oder umgekehrt" (...as a raster image laid out on metal in such way that the bright tones are metallically pure, and vice versa). 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: [6] "die Tönung der Bildelemente bei diesen Rasterbildern" and "Die Bildpunkte des Rasterbildes" ("the tone of the picture elements of this raster image" and "the picture points of the raster image"); and Schröter in 1932: [7] "Rasterelementen," "Rasterzahl," and "Zellenraster" ("raster elements," "raster count," and "cell raster").

The first use of raster specifically for a television scanning pattern is often credited to Baron Manfred von Ardenne who wrote in 1933: [8] "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" (In a lecture in January 1930 it was proven by demonstrations that the Braun tube was prototyped in the laboratory with point sharpness and point brightness for the production of a precise, bright raster). Raster was adopted into English television literature at least by 1936, in the title of an article in Electrician. [9] 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. [10]

CRT components

  1. Electronic gun:-
    1. Primary gun: used to store the picture pattern.
    2. Flood gun: used to maintain the picture display.
    3. Phosphor coated screen: coated with phosphorus crystals ("phosphors") that emit light when an electron beam strikes them.
    4. Focusing system: focusing system causes the electron beam to converge into a small spot as it strikes the phosphor screen.
    5. Deflection system: used to change the direction of electron beam so it can be made to strike at different locations on the phosphor screen.

See also

Related Research Articles

<span class="mw-page-title-main">Analog television</span> Television that uses analog signals

Analog television is the original television technology that uses analog signals to transmit video and audio. In an analog television broadcast, the brightness, colors and sound are represented by amplitude, phase and frequency of an analog signal.

<span class="mw-page-title-main">Cathode-ray tube</span> Vacuum tube manipulated to display images on a phosphorescent screen

A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen. The images may represent electrical waveforms (oscilloscope), pictures, radar targets, or other phenomena. A CRT on a television set is commonly called a picture tube. CRTs have also been used as memory devices, in which case the screen is not intended to be visible to an observer. The term cathode ray was used to describe electron beams when they were first discovered, before it was understood that what was emitted from the cathode was a beam of electrons.

<span class="mw-page-title-main">Video</span> Electronic moving image

Video is an electronic medium for the recording, copying, playback, broadcasting, and display of moving visual media. Video was first developed for mechanical television systems, which were quickly replaced by cathode-ray tube (CRT) systems which, in turn, were replaced by flat panel displays of several types.

<span class="mw-page-title-main">Interlaced video</span> Technique for doubling the perceived frame rate of a video display

Interlaced video is a technique for doubling the perceived frame rate of a video display without consuming extra bandwidth. The interlaced signal contains two fields of a video frame captured consecutively. This enhances motion perception to the viewer, and reduces flicker by taking advantage of the phi phenomenon.

In a raster scan display, the vertical blanking interval (VBI), also known as the vertical interval or VBLANK, is the time between the end of the final visible line of a frame or field and the beginning of the first visible line of the next frame. It is present in analog television, VGA, DVI and other signals.

Broadcast television systems are the encoding or formatting systems for the transmission and reception of terrestrial television signals.

The refresh rate is the number of times per second that a raster-based display device displays a new image. This is independent from frame rate, which describes how many images are stored or generated every second by the device driving the display.

<span class="mw-page-title-main">Display resolution</span> Number of distinct pixels in each dimension that can be displayed

The display resolution or display modes of a digital television, computer monitor or display device is the number of distinct pixels in each dimension that can be displayed. It can be an ambiguous term especially as the displayed resolution is controlled by different factors in cathode ray tube (CRT) displays, flat-panel displays and projection displays using fixed picture-element (pixel) arrays.

1080i is a combination of frame resolution and scan type. 1080i is used in high-definition television (HDTV) and high-definition video. The number "1080" refers to the number of horizontal lines on the screen. The "i" is an abbreviation for "interlaced"; this indicates that only the odd lines, then the even lines of each frame are drawn alternately, so that only half the number of actual image frames are used to produce video. A related display resolution is 1080p, which also has 1080 lines of resolution; the "p" refers to progressive scan, which indicates that the lines of resolution for each frame are "drawn" on the screen in sequence.

<span class="mw-page-title-main">576i</span> Standard-definition video mode

576i is a standard-definition digital video mode, originally used for digitizing analog television in most countries of the world where the utility frequency for electric power distribution is 50 Hz. Because of its close association with the legacy color encoding systems, it is often referred to as PAL, PAL/SECAM or SECAM when compared to its 60 Hz NTSC-colour-encoded counterpart, 480i.

<span class="mw-page-title-main">Scan line</span>

A scan line is one line, or row, in a raster scanning pattern, such as a line of video on a cathode ray tube (CRT) display of a television set or computer monitor.

Cromaclear is a trademark for CRT technology used by NEC during the mid to late-90s. This adopted the slotted shadow mask and inline electron gun pioneered by the 1966 GE Porta-Color and used by most then-current television tubes to computer monitor use. It was claimed that Cromaclear could offer the image clarity and sharpness of the Trinitron and Diamondtron aperture grille CRTs without the disadvantages e.g. expense and the horizontal damping wires.

Horizontal scan rate, or horizontal frequency, usually expressed in kilohertz, is the number of times per second that a raster-scan video system transmits or displays a complete horizontal line, as opposed to vertical scan rate, the number of times per second that an entire screenful of image data is transmitted or displayed.

Horizontal blanking interval refers to a part of the process of displaying images on a computer monitor or television screen via raster scanning. CRT screens display images by moving beams of electrons very quickly across the screen. Once the beam of the monitor has reached the edge of the screen, the beam is switched off, and the deflection circuit voltages are returned to the values they had for the other edge of the screen; this would have the effect of retracing the screen in the opposite direction, so the beam is turned off during this time. This part of the line display process is the Horizontal Blank.

<span class="mw-page-title-main">Vector monitor</span> Type of display device

A vector monitor, vector display, or calligraphic display is a display device used for computer graphics up through the 1970s. It is a type of CRT, similar to that of an early oscilloscope. In a vector display, the image is composed of drawn lines rather than a grid of glowing pixels as in raster graphics. The electron beam follows an arbitrary path tracing the connected sloped lines, rather than following the same horizontal raster path for all images. The beam skips over dark areas of the image without visiting their points.

<span class="mw-page-title-main">Oscilloscope</span> Instrument for displaying time-varying signals

An oscilloscope is a type of electronic test instrument that graphically displays varying electrical voltages as a two-dimensional plot of one or more signals as a function of time. The main purposes are to display repetitive or single waveforms on the screen that would otherwise occur too briefly to be perceived by the human eye. The displayed waveform can then be analyzed for properties such as amplitude, frequency, rise time, time interval, distortion, and others. Originally, calculation of these values required manually measuring the waveform against the scales built into the screen of the instrument. Modern digital instruments may calculate and display these properties directly.

This is a subdivision of the Oscilloscope article, discussing the various types and models of oscilloscopes in greater detail.

<span class="mw-page-title-main">History of the oscilloscope</span>


The history of the oscilloscope reaches back to the first recordings of waveforms with a galvanometer coupled to a mechanical drawing system in the second decade of the 19th century. The modern day digital oscilloscope is a consequence of multiple generations of development of the oscillograph, cathode-ray tubes, analog oscilloscopes, and digital electronics.

A time base generator is a special type of function generator, an electronic circuit that generates a varying voltage to produce a particular waveform. Time base generators produce very high frequency sawtooth waves specifically designed to deflect the beam of a cathode ray tube (CRT) smoothly across the face of the tube and then return it to its starting position.

<span class="mw-page-title-main">Deflection yoke</span> Part of a cathode ray tube which moves the electron beam around

A deflection yoke is a kind of magnetic lens, used in cathode ray tubes to scan the electron beam both vertically and horizontally over the whole screen.

References

  1. Leblanc, Maurice, "Etude sur la transmission électrique des impressions lumineuses" (Study on electrical transmission of luminous impressions), La Lumière électrique (Electric light), December 1, 1880
  2. "Half-Tone Photo-Engraving". The Photographic Times. Scoville Manufacturing Co. 25: 121–123. 1894.
  3. Josef Maria Eder, Ausführliches Handbuch der Photographie Halle: Druck und Verlag von Wilhelm Knapp, 1897
  4. George Shiers and May Shiers (1997). Early Television: A Bibliographic Guide to 1940. Taylor & Francis. p. 47. ISBN   0-8240-7782-2.
  5. Arthur Korn, Elektrisches Fernphotograhie und Ähnliches, Leipzig: Verl. v. S. Hirzel, 1907
  6. Gustav Eichhorn, Wetterfunk Bildfunk Television (Drahtloses Fernsehen), Zürich: Teubner, 1926
  7. Fritz Schröter, Handbuch der Bildtelegraphie und des Fernsehens, Berlin: Verl. v. Julius Springer, 1932
  8. Manfred von Ardenne, Die Kathodenstrahlröhre und ihre Anwendung in der Schwachstromtechnik, Berlin: Verl. v. Julius Springer, 1933.
  9. Hughes, L. E. C., "Telecommunications XX-IV: The Raster," Electrician 116 (Mar. 13):351–352, 1936.
  10. Pierre Mertz and Frank Gray, "A Theory of Scanning and Its Relation to the Characteristics of the Transmitted Signal in Telephotography and Television," Bell System Technical Journal, Vol. 13, pp. 464-515, July, 1934