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These days, we are so familiar with the wonders of technology that we tend to take them for granted. For example,how often do you look at the large crisp display on your computer screen and reflect on the tortuous path it took forscientists and engineers to create this marvel?

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Victorian Fax Machines?Displaying Pictures on a Toaster

Nipkow DisksCathode Ray Tubes (CRTs)Campbell-Swinton & John Logie BairdPhilo Farnsworth & Vladimir ZworkinVideo TubesColor Vision: One of Nature's Wonders

Video Display Units (VDUs)Memory-Mapped Displays

I/O-Driven Displays1-Bit, 8-Bit, 16-Bit, 24-Bit, etc. ColorAlternat ive Schemes: 12-bit , 17-bi t,. ..Modern/Future Display Technologies

An Alternative Sub-Pixel TechnologySo, Just what is a Pixel Anyway?

Victorian Fax Machines?

The whole basis of the DIY Calculator accompanying our book How Computers Do Math(ISBN: 0471732788) isthat its a virtual machine that exists only in your physical computers memory; and the only way to see the DIYCalculator is on your real computer's screen, or monitor. Displaying information on a screen is an incredibly efficientway for a computer to communicate with us. So where did computer screens come from? Well, as is often thecase, engineers employed an existing technology that was developed for an entirely different purpose ... television.

Television, whose name comes from the Greek tele, meaning "distant," and the Latin vision, meaning "seeing" or"sight," has arguably become one of the wonders of the 20th Century, so you may be surprised to learn thattelevision's origins are firmly rooted in the Victorian era. In fact one of the earliest examples of an image beingcaptured, transmitted, and reproduced by electromechanical means occurred in 1842, only five years after QueenVictoria had ascended to the throne, when a Scotsman Alexander Bain came up with a rather ingenious idea.

Bain created an image to be transmitted by snipping it out of a thin sheet of tin, placing this representation on amoveable base, and connecting it to one side of a battery. He then created a pendulum using a conducting metalwire and a weight ending in a sharp point, and he set this device swinging above the base. The base was slowlymoved under the pendulum, where the swinging weight made periodic contact with the metal image, therebycompleting the electrical circuit and converting the dark and light areas of the image (represented by the presenceand absence of tin) into an electrical signal.

Bain then used this signal to control a relay, which was moving back and forth in time with the pendulum. Whenactivated, the relay pushed a pencil down onto a piece of paper mounted on a second base moving at the samerate as the first, thereby reproducing the image as a pencil drawing.

Obviously, Bain's device had little application with regard to the transmission of moving pictures, but it certainlywasn't a wasted effort, because he had essentially created the precursor to the modern Fax machine.

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How to Display Moving Pictures on a Toaster

In 1878, Denis Redmond of Dublin, Ireland, penned a letter to the English Mechanic and World of Science

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publication. In his letter, Redmond described creating an array of selenium photocells, each of which wasconnected via a voltage source to a corresponding platinum wire. As the intensity of light on a particular photocellincreased, it conducted more current, thereby causing its associated platinum wire to glow more brightly.Redmond's original device contained only around 10 10 elements, and therefore was very limited as to what itcould represent. Having said this, it could apparently reproduce moving silhouettes, which was pretty amazing forthe time.

In fact Redmond's photocell-array concept was not far removed from today's semiconductor diode-array cameras,while his array of glowing platinum wires is loosely comparably to the way in which images are constructed ontoday's Liquid Crystal Displays.

Also, had Redmond continued to increase the size of his platinum-wire array to contain say 1,000 1,000elements, then it would have had the added advantage of being able to double-up as a toaster! Sadly, the largesize of Redmond's photocells drastically limited the quality of the images he could display, and the ability toreproduce his efforts using semiconductors and related technologies lay some 100 years in his future, so theinventors of yesteryear were obliged to search for another approach ...

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Nipkow Disks

In 1884, the German inventor Paul (Julius) Gottlieb Nipkow proposed a novel technique for capturing, transmitting,and reproducing pictures based on flat circular disks containing holes punched in a spiral formation:

Nipkow's idea was both elegant and simple. A strong light source was used to project a photographic image ontothe surface of the Nipkow Disk, which was spinning around. As the outermost hole on the disk passed through theimage, the light from the light source passed through the hole to hit a light-sensitive cell, such as a silver-caesiumphototube. The intensity of the light was modified by the light and dark areas in the image as the hole traveled past,thereby modulating the electrical signal generated by the phototube. The holes were arranged such that as soon asthe outermost hole had exited the image, the next hole began itstrek. Since the holes were arranged in a spiralformation, each hole traversed a different slice, or line, across the image.

At the other end of the process was a brilliant lamp and a second spinning Nipkow Disk. The electrical signalcoming out of the phototube was used to modulate the lamp, which was projected onto the second disk. Themodulated light passed through the holes in the second disk to construct a line-by-line display on a screen.

Although the resulting image was constructed as a series of lines, the speed of the disk combined with persistenceof vision meant that an observer saw a reasonable (albeit low-resolution) facsimile of the original picture. (Theconcept of persistence of vision is discussed in a little more detail later in this paper).


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Leaping ahead to the year 1895, the Italian electrical engineer and inventor Guglielmo Marconi extended the earlierresearch of such notables as the British physicist James Clerk Maxwell and the German physicist Heinrich Hertz byinventing the forerunner of radio as we know it today. In the early part of the twentieth century, engineersconstructed experimental systems that could transmit images using a combination of Nipkow's disks and radiosignals. The electrical signal coming out of the phototube was merged with a synchronization pulse (which indicatedthe start of a rotation), and this combined signal was then used to modulate the carrier wave from a radiotransmitter.

At the receiving end of the system was a radio receiver and a second spinning Nipkow Disk. The receiver first

separated the synchronization pulse from the video signal, and the synchronization pulse was then used to ensurethat the receiver disk was synchronized to the transmitter disk. Meanwhile, the amplified video signal was onceagain used to modulate a brilliant lamp, which was projected through holes in the receiver disk to construct aline-by-line display on a screen.

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Cathode Ray Tubes (CRTs)

Modern television systems are based on a device called a cathode ray tube (CRT). Primitive cathode ray tubeshad been around since 1854, when a German glass blower named Heinrich Geissler invented a powerful vacuumpump. Geissler then proceeded to use his pump to evacuate a glass tube containing electrodes to a previouslyunattainable vacuum. Using these Geissler Tubes, experimenters discovered a form of radiation which they called

cathode rays(and which we now know to consist of electrons).The idea of using a cathode ray tube to display television images was proposed as early as 1905, but practicaltelevision didn't really become a possibility until 1906, when the American inventor Lee de Forest invented avacuum tube called a triode, which could be used to amplify electronic signals. Even so, progress was hard foughtfor, and it wasn't until the latter half of the 1920s that the first rudimentary television systems based on cathode raytubes became operational in the laboratory.

The principles behind the cathode ray tube are quite simple (although actually building one is less than trivial). Thetube itself is formed from glass, from which the air is evacuated to leave a strong vacuum:


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In the rear of the tube is a device called an electron gun, which generates electrons. A positively charged gridmounted a little way in front of the electron gun focuses the electrons into a beam and accelerates them towardsthe screen. Thus, the name "cathode ray tube" is derived from the electron gun (which forms the negative terminal,or cathode), the electron beam (or ray), and the glass enclosure (or tube).

The inside face of the screen is lined with a layer of material called a phosphor, which has the ability to fluoresce.Hmmm, this is going to take a moment to explain. Phosphors are distinguished by the fact that when they absorbenergy from some source such as an electron beam, they release a portion of this energy in the form of light.Depending on the material being used, the time it takes to release the energy can be short (less than one-hundred-thousandth of a second) or long (several hours). The effect from a short-duration phosphor is known asfluorescence, while the effect from a long-duration phosphor is referred to as phosphorescence. Televisions useshort-duration phosphors, and their screens' linings are therefore known as the fluorescent layer.

The end result is that the spot where the electron beam hits the screen will glow. By varying the intensity of theelectron beam, it's possible to make the spot glow brightly or hardly at all. Now, this would not be particularlyuseful on its own (there's only so much you can do with an individual spot); but, of course, there's more.

Note the two plates referred to as vertical deflection platesin the above illustration. If an electrical potential isapplied across these two plates, the resulting electric field will deflect the electron beam. If the upper plate is morepositive than the lower, it will attract the negatively charged electrons forming the beam and the spot will move upthe screen. Conversely, if the lower plate is the more positive, the spot will move down the screen. Similarly, twomore plates mounted on either side of the tube can be used to move the spot to the left or the right of the screen(these horizontal deflection platesare not shown in the above illustration so as to keep things simple).

By combining the effects of the vertical and horizontal deflection plates, we are able to guide the spot to any pointon the screen. There are several ways in which we can manipulate our spot to create pictures on the screen, butby far the most common in modern CRT-based displays is the raster scantechnique as illustrated below (see alsothe note on Vector Displaystoward the end of this topic):

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Using this technique, the electron beam commences in the upper-left corner of the screen and is guided across thescreen to the right (the top-most blue line). The path the beam follows as it crosses the screen is referred to as aline. When the beam reaches the right-hand side of the screen it undergoes a process known as horizontalflyback, in which its intensity is reduced and it is caused to "fly back" across the screen (the top-most mauve line).While the beam is flying back, it is also pulled a little way down the screen. (This description is something of asimplification, but it will serve our purposes here.)

The beam is now used to form a second line, then a third, and so on until it reaches the bottom of the screen. Thenumber of lines affects the resolution of the resulting picture; that is, the amount of detail that can be displayed.

As An Aside: Standard (pre-high-definition) British television is based on 625 lines [the British system wasoriginally 405 lines (from 1936), but this was phased out in the early 1970s in favor of the 625 line format]. Bycomparison, standard (pre-high-definition) American television is based on 525 lines. In fact, with regard to

pre-high-definition television systems, there are three main formats used in the world (there are also a numberof derivations of these formats along with a few totally weird formats):

PAL

This stands for Phase Alternation Linesor Phase Alternating Line(PALis jocularly said to standfor Pictures At Lastor Pay for Added Luxury). The majority of countries with a 50 Hz mains powersupply use the PAL broadcast/video standard.

NTSC

This stands for the National Television Standard Committee, which established the originalAmerican TV broadcast standard in 1953 (NTSCis often and unfairly said to stand for NeverTwice the Same Color). The majority of countries with a 60 Hz mains power supply use the NTSCbroadcast/video standard.

SECAM

This stands for SEquential Couleur Avec Memoire, which is French for "Sequential Color withMemory" (SECAMis also said to stand for System Essentially Contrary to the American Method).This was designed by the French primarily for political reasons, including protecting theirmanufacturing industries. This format is also commonly used in Eastern Block countries so as to beincompatible with the majority of Western transmissions.

When the beam reaches the bottom right-hand corner of the screen, it undergoes vertical flybackin which itsintensity is reduced, it "flies back" up the screen to return to its original position in the upper left-hand corner(diagonal black line), and the whole process starts again. Thus, in a similar manner to Nipkow's technique, we cancreate pictures by varying the intensity of the beam as it scans across the screen. For example, consider how we'dconstruct the image of a simple triangle:


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Note that this small group of lines represent a tiny area located somewhere in the middle of a much larger screen.In the real world, the lines forming the picture would be very close together, so this would actually be a very smalltriangle, but it serves to illustrate the concept. When they are first introduced to this technique for creating pictures,many people wonder why they can't see the lines being drawn and why the image doesn't appear to flicker. Theanswer has three parts:

1) The electrons forming the electron beam travel at a tremendous speed, and the beam itself can bemanipulated very quickly. The beam used in a television set can scan the entire picture in a fraction of asecond, and the entire picture is actually redrawn approximately thirty times a second (for American/NTSCformat televisions) or 25 times a second (for European/PAL systems).

2) The phosphor lining the inside of the screen is carefully chosen to fluoresce for exactly the correct amount oftime, such that any particular point has only just stopped fluorescing by the time the electron beam returns tothat point on its next scan.

3) The combination of our eyes and nervous system exhibit persistence of vision, which means we continue tosee an image for a fraction of a second. For example, if you look at a bright light for a short time and then turn

your head, an after-image of the light persists for a while.

All of these effects combine to form a seemingly substantial picture. However, if you ever look at a televisionprogram where the scene contains a television set, youll often see bands of light and dark areas moving up ordown that television's screen. This is because the camera taking the picture and the television inthe picture are notsynchronized together, resulting in a kind of stroboscopic effect (much like wagon wheels appearing to rotatebackwards in old cowboy films). Thus, if producers of television programs wish to include a television in the scene,the engineers have to ensure that the systems are synchronized to each other.

As Another Aside: This topic has focused on Raster Displays, because this is the most common technique inuse for today's computer displays. As we've discussed, this form of display means that we take the image wewish to display, we convert that image (the official term is to renderthe image) into a bitmap, and we then scanthe electron beam across the display row-by-row turning it on and off for each pixel on the screen.

One reason we use this technique for modern computer displays is that the bitmap images discussed abovehave to be stored in memory (either in the computer's main memory or in dedicated memory located on aspecial graphics card/subsystem) and memory is cheap these days, but this wasn't always the case. In the1960s and 1970s computer memory was very expensive, so the standard form of computer display was knownas a Vector Display(these little scamps were also known as Calligraphic Displaysor Stroker Displays).

The idea here is that the computer is used to wield the electron beam like a pen, controlling it's location in thehorizontal and vertical axes so as to draw lines and curves directly onto the screen (this explains the "vector"moniker, because lines are often referred to as "vectors" by engineers and scientists). In addition to the factthat they required relatively little memory, a big advantage associated with vector displays is that lines andcurves drawn on the screen using this technique look much "sharper" and "cleaner" that their rasterizedequivalents.

The reason vector graphics can be more visually appealing is easy to understand if you imagine a diagonal linedrawn from the bottom-left-hand-corner of the screen to the top-right-hand-corner. This will be reproduced as a


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perfectly smooth line on a vector display, but it will end up as a somewhat jagged "staircase" in a bitmap/vectordisplay (we can use "anti-aliasing" techniques to make the raster representation look smooth, but thesetechniques are beyond the scope of our discussions here).

As one final point of interest the reason we use the phrase "television set" derives from the early days of radio. Thefirst radio systems for home use essentially consisted of three stages: the receiverto detect and pre-amplify thesignal, the demodulatorto extract the audio portion of the signal, and the main amplifierto drive the speaker. Eachof these stages were packaged in individual cabinets, which had to be connected together; hence the user had topurchase all three units which formed a "wireless set." The term "set" persisted even after all of the radio'scomponents were packaged in a single cabinet, and was subsequently applied to televisions when they eventuallyarrived on the scene.

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A.A. Campbell-Swint on and John Logie Baird

There are two primary requirements for a functional television system: a technique for capturing images and a wayto display them. Following Nipkow's experiments, other inventors tried to move things forward with limited success.The history books mention several names in this regard, but one who seems to have "slipped through the cracks"and rarely gets a mention in texts on the origin of television was the Scottish electrical engineer Alan Archibald(A.A.) Campbell-Swinton(1863-1930).

In 1908, Campbell-Swinton wrote a letter to the magazine Naturein which he described an electronic technique forimplementing a television system. Three years later, in 1911, he expanded on his original proposal and described a

complete system using a special cathode ray tube to capture images and another to display them. Campbell-Swinton's idea for the "camera" cathode ray tube was to use a lens to capture an image and project it onto the flatend of the tube. Meanwhile, inside the tube, the glass on the flat end would be covered by a sandwich ofphotoelectric material, an insulating layer, and a layer of conducting metal.

Photons of light associated with the image projected onto the tube would result in areas of positive electricalcharge in the photoelectric material lighter areas would have more charge; darker areas would have less charge

the insulating layer would stop the charge leaking away.

By scanning the electron beam row-by-row in a raster pattern (see the previous topic) across the metal layer, itwould be possible to "read" the areas of charge. By this means, the image could be converted into an electricalsignal that could be sent to a "display" cathode ray tube where it would be reconstructed and presented toobservers as discussed in the previous topic.

Campbell-Swinton was a man ahead of his time. At a high-level his scheme was near-perfect, but his plans omittedmany of the fine details that would be required to make the system actually work. In fact, it took another 20 years


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before a fully electronic television system was realized as discussed in the next topic.

Another key player in the annals of television history was John Logie Baird, a Scotsman who used a derivation ofNipkow's disks for capturing and displaying pictures during the latter half of the 1920s and the early 1930s.

The British Broadcasting Corporation (BBC) allowed Baird to transmit his pictures on their unused radio channels inthe evening. By 1934, even though he could only transmit simple pictures with a maximum resolution of around 50lines, Baird had sold thousands of his Televisorreceivers around Europe in the form of do-it-yourself kits.Meanwhile, on the other side of the Atlantic, the Radio Corporation of America (RCA) experimented with a systemconsisting of a mechanical-disk camera combined with a cathode ray tube display device. Using this system, RCAtransmitted a picture of a model of Felix the Catendlessly rotating on the turntable of a record player in the early

1930s.

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Philo Farnsworth (a Man Lost in History) and Vladimir Zworkin

Strange as it may seem, relatively few reference sources seem to be aware of the real genius behind television aswe know it today a farmboy named Philo T. Farnsworth from Rigby, Idaho. In 1922, at the age of 14, withvirtually no knowledge of electronics, Philo conceived the idea for a fully electronic television system. Flushed withenthusiasm, he sketched his idea on a blackboard for his high school science teacher, a circumstance that was toprove exceedingly fortuitous in the future as we shall see.

Over the years, Philo solved the problems that had thwarted other contenders. He invented a device he called an

Image Dissector, which was the forerunner to modern television cameras, and he also designed the circuitry toimplement horizontal and vertical flyback blanking signals on his cathode ray tube, which solved the problems ofghosting images. By the early 1930s, Philo could transmit moving pictures with resolutions of several hundred lines,and all subsequent televisions are directly descended from his original designs.

The reason Philo has been lost to history is almost certainly attributable to RCA. The corporation first attempted topersuade Philo to sell them his television patents, but he informed them in no uncertain terms that he wasn'tinterested. In 1934, RCA adopted another strategy by claiming that the Russian migr Vladimir Zworkin, who wasworking for them at that time, had actually invented everything to do with televisions back in 1923.

Eventually the case went to the patent tribunal, at which time Zworkin's claim was shown to leak like a sieve. Thefinal nail in the coffin came when Philo's old science teacher reconstructed the sketch Philo had drawn on theblackboard back in 1922. This picture was recognizably that of the television system that Philo had subsequentlydeveloped, and the tribunal had no hesitation in awarding him the verdict.

Unfortunately, by this time the waters had been muddied to the extent that Philo never received the recognition he

deserved. Almost every standard reference continues to cite Zworkin (and his camera called an Iconoscope) asinitiating modern television. In fact it was only towards the end of the 1970s that Philo's achievements began to betruly appreciated, and, although it's still rare to find references to him, Philo's name is gradually coming to the fore.

As video historian Paul Schatzkin told the authors of this paper:

"Many engineers and scientists contributed to the emergence of the television medium, but a carefulexamination of the record shows that no one really had a clue until Philo Farnsworth set up shop inSan Francisco at the age of 20 and said:We'll do it this way!"

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Video Tubes

The tubes used in television sets and computer monitors (which we might call video tubes) are very similar tocathode ray tubes with some additional refinements. First, in place of the deflection plates discussed above, videotubes tend to use electromagnetic coils, but the end result is much the same so we don't really need to go into thathere. More importantly, video tubes have a second grid called the shadow mask, which is mounted a fraction of aninch from the screen's fluorescent coating:


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Like the grid, the shadow mask is positively charged, so it helps accelerate the electrons forming the electronbeam, thereby giving them more energy which results in a brighter picture. More importantly, the shadow maskhelps to focus the beam, because any electrons that deviate even slightly from the required path hit the mask andare conducted away, thereby producing a sharper image. (The shadow mask also has a role with regard toprotecting the image from the effects of magnetic fields such as the Earth's, but this is beyond the scope of thispaper.) Note that the illustration above is greatly magnified and not to scale; in reality the shadow mask is onlyslightly thicker than aluminum foil and the holes are barely larger than pin-pricks.

If you approach your television at home and get really close to the screen, you'll see that the picture is formed fromindividual dots (much like our "Christmas tree" in the above illustration). A "black-and-white" television contains onlyone electron gun, and the phosphor lining its screen is chosen to fluoresce with white light. In this case, each dot onthe screen corresponds to a hole in the shadow mask, and each dot may be referred to as a picture element, orpixelfor short.

By comparison, in the case of a color television, you'll see that the picture is composed of groups of three dots,where each dot corresponds to one of the primary colors: red, green, and blue. Each of these dots has its ownhole in the shadow mask, and each dot is formed from a different phosphor, which is chosen to fluoresce with that

color. In this case each group of three dots would equate to a single pixel:

A color television also contains three electron guns, one to stimulate the red dots, one for the green, and one forthe blue. The three electron beams scan across the screen together, but the intensity of each beam can be variedindependently. Thus, by making only one of the beams active we can select which color in a group will bestimulated (we can also specify how brightly the dot should glow by varying the strength of that electron beam).

Now this is the clever bit. We might decide to make two of the electron beams active and stimulate two of the dotsin the group at the same time: red-green, red-blue, or green-blue. Alternatively, we might decide to make all threeof the beams active and stimulate all three of the dots. The point is that as discussed in the following topic wecan form different colors by using various combinations and intensities of these three dots.


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Color Vision: One of Natures Wonders

As fate would have it, this topic grew in the telling to the extent that it became a full paper in its own right. In thisColor Visionpaper you will discover all sorts of interesting information on the visible spectrum, the discovery ofinfrared and ultraviolet light, the way in which color vision works, and the evolution of our visual systems.

For the purposes of this paper we need only note that what we refer to as "light" is simply the narrow portion of the

electromagnetic spectrum that our eyes can see (detect and process), ranging from violet at one end to red at theother, and passing through blue, green, yellow, and orange on the way (at one time, indigo was recognized as adistinct spectral color, but this is typically no longer the case.):

The point is that white light is a mixture of all of the colors in the visible spectrum. Furthermore, by mixing differentquantities of red, green, and blue light, we can trick our eyes into seeing just about any color. Thus, in the contextof our color television, if all three of the electron beams are active when they pass a particular group of dots, theindividual dots will fluoresce red, green, and blue, but from a distance we'll perceive the group as a whole as beingwhite. (If we looked really closely we'd still see each dot as having its own individual color.) Similarly, if westimulate just the red and green dots we'll see yellow; combining the green and blue dots will give us cyan (agreen-ish, light-ish blue); while mixing the red and blue dots will result in magenta. (The color magenta, which is asort of purple, was named after the dye with the same moniker; in turn, this dye was named after the battle ofMagenta, which occurred in Italy in 1859, the year in which the dye was discovered.)

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Visual Display Units (VDUs)


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Today's computer monitors have much smaller pixels and many more pixels-per-inch than a television set, whichmeans that they have a higher resolution and can display images with greater precision. (Note that the concept ofpixels is somewhat slippery when we come to computers we'll discuss this in more detail in the What is a Pixel?topic later in this paper).

Modern computers typically contain a circuit board called a graphics cardor a graphics subsystem, which containsa substantial amount of memory and its own on-board processor(s). In this case, all your main system has to do isto send an instruction to the processor on the graphics card saying something like "Draw me a purple circle with'this'diameter at 'that'location on the screen."The graphics processor then slogs away completing the task, whilethe system's main processor is left free to work on something else.

The combination of high-resolution monitors and graphics cards endow today's computer screens with millions ofpixels, each of which can be individually set to thousands or even millions of colors. We require this level ofsophistication because we often wish to display large amounts of graphical data, such as three-dimensionalanimations. This wasn't possible until recently, because it requires a huge amount of computing power andmemory. In fact it's only because today's computers are incredibly fast and memory is relatively inexpensive thatwe are in a position to display images at this level of sophistication.

By comparison, computers in the early 1960s were relatively slow, memory was extremely expensive, and thereweren't any dedicated computer monitors as we know them today. On the bright side, very few people had accessto computers, and those who did were generally only interested in being able to see textual data, which wastypically printed out on a Teleprinteror some comparable device. At some stage, however, it struck someone thatit would be useful to be able to view and manipulate their data on something similar to a television screen, and thusthe first visual display unit (VDU)(sometimes known as a video display unit (VDU)) was born.

By 1977, a few lucky souls were the proud possessors of rudimentary home computers equipped with simpleVDUs. A reasonably typical system at that time would probably have resembled the one shown below:

Even though these computers were slow and had hardly any memory by today's standards, their owners wereimmensely proud of them and justly so, because most of these devices were hand-built from kits or from theground up. Similarly, although their rudimentary VDUs could display only a few rows of "black-and-white" text,anyone who was fortunate enough to own one was deliriously happy to actually see words appearing on their

screen.

The professional VDUs (which we shall refer to as monitorshenceforth) typically offered between 20 and 24 rows,each containing 80 columns (characters). Why 80? Because there wasnt much point in displaying fewer charactersthan could fit on IBM Punched Cards, which were used to store a large proportion of the worlds computer data atthat time. Similarly, there didnt seem to be much point in being able to display morecharacters than were on thesecards.

By comparison, monitors for home use were less expensive, less sophisticated, and generally only capable ofdisplaying around 16 rows of 32 characters. A major consideration for the designers of these early monitors wasthe amount of memory they required. The reason for this was because unlike a television, which receives itspictures from afar a computer needs somewhere to store everything that it's displaying. From our discussions onvideo tubes earlier in this paper, you may recall that images are created as a series of lines containing "dots," andthis is the way in which a monitor displays characters. For example, consider how a monitor could be used to


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display the letter 'E':

One configuration that was common in early monitors was to use a matrix of 9 7 dots per character as illustratedhere (another common style was based on a 7 5 matrix, but the resulting characters were somewhat "clunky"and difficult to read). So, ignoring any extra dots used to provide spaces between rows and columns, eachcharacter would require 9 7 = 63 dots, and a display containing 16 rows of 32 characters would therefore use atotal of 32,256 dots. This means that if each dot were represented by its own bit in the computer's memory (wherea logic 0 could be used to indicate the dot is off and a logic 1 to indicate the dot is on), then remembering thatthere are 8 bits in a byte, the system would require 4,032 bytes of RAM just to control the monitor.

Although this doesn't seem like a tremendous amount of memory today, in 1977 you considered yourself fortunateindeed to have 2 kilobytes (2,048 bytes) of RAM, and you were the envy of your street if you had as much as 4kilobytes (4,096 bytes). Obviously this was a bit of a conundrum, because even if you were the proud possessor ofa system with a 4 kilobyte RAM, there wouldn't have been much point in running a monitor that required 4,032bytes, because you would only have 64 bytes left in which to store your programs and data!

In those days of yore, programmers prided themselves in writing efficient code that occupied as little memory aspossible, but this would have been pushing things above and beyond the call of duty. A measly 64 bytes certainly

wouldn't have been enough to have written a program that could do anything particularly useful with the monitor,which would defeat the purpose of having a monitor in the first place. What was needed was a cunning ploy, andelectronics engineers are nothing if not ingenious when it comes to cunning ploys (in fact, the term engineerisderived from the Latin ingeniator, meaning "a creator of ingenious devices"). The solution to the problem was aconcept known as a memory-mapped display, which we shall consider in excruciating detail in the next topic.

But before we move on, as a point of interest, when you purchase say a 32-inch television set, this distance ismeasured as a diagonal from the upper-left-hand corner of the screen to its lower-right=hand corner. The samething applies to computer screens. Traditional television sets and computer screens have an aspect ratioof 4:3,which means they are wider (4 parts) than they are tall (three parts). For example, consider a small screen that isfive inches across its diagonal; in this case, the width of the screen would be four inches and its height would bethree inches.

The reason we mention this is that the computer screen is wider than it is tall, but a piece of paper is taller than it iswide. Due to the fact that computers are often used for word-processing applications, a number of computermanufacturers have released taller, thinner screens with an aspect ratio of 3:4. However, the trick here is that themanufacturers didn't want to go to the expense of creating a completely new device. Instead, they simply rotatedan existing screen by 90 degrees and put it in a different cabinet. Of course, this meant the upper-left-hand cornerof the screen used to be the upper-right-hand corner and so forth; also that the raster scan would now beprogressing from left to right instead of from top to bottom; so the graphics subsystem would have to correct for allof this, but that really wasn't much of a problem at all.

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Memory-Mapped Displays

Purely for the sake of discussion, let's assume that we have memory-mapped display (we'll explain the "memory


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mapped" moniker shortly) that supports an array of 16 rows by 32 columns (characters). Also, since we aredealing with a hypothetical display, let's assume that each of our characters is going to be formed from a matrix of15 dots by 10 dots, which will give us much nicer-looking images (why suffer if you don't have to?):

When you magnify one of our imaginary characters (the letter 'B' in this example), it may seem that we'vecarelessly wasted a lot of our dots. But remember that we require some way to form spaces between the rowsand columns. Also, some of the lowercase characters such as 'q', 'y', and 'p' have "tails" that extend downward,and we have to reserve some space to accommodate these as well.

Be this as it may, the problem remains that weve got 16 32 = 512 characters, each of which requires 15 10 =150 dots. This would require 9,600 bytes if we used one bit to store each dot, yet we want to use the smallestamount of our computer's memory as possible. The solution to our problem lies in that fact that the patterns of dotsfor each character are pre-defined and relatively immutable (at least they were in the early days). To put thisanother way, if we wish to display several 'B' characters at different positions on the screen, then we know thateach of them will use exactly the same pattern of dots. This means that we can divide our problem into threedistinct parts:

a) We need some way to remember which characters are being displayed at each location on the screen; for

example, "The character in column 6 of row 3 is a letter 'B'."Due to the fact that we want to be able tochange the characters displayed at each location, this information will have to be stored in our RAM.

b) We need some way to store a single master pattern of dots for each character we wish to be able to display(for example, 'A', 'B', ... 'Z', and so on). Assuming that we don't wish to change the way our characters look,then these master patterns can be stored in some flavor of read-only memory (ROM) device.

c) We need some mechanism to combine the information from points (a) and (b). That is, if the system knowsthat the character in column 6 of row 3 should be a letter 'B', then it requires the ability to access the masterpattern of dots associated with this letter and display them on the screen at the appropriate location.

The first thing we have to decide is which types of characters we wish to use. Obviously we'll want to be able to

display the uppercase letters 'A' through 'Z', and it's not beyond the bounds of possibility that wed like to use theirlowercase counterparts 'a' through 'z'. Similarly, wed probably appreciate the numbers '0' through '9', along withpunctuation characters such as commas and semi-colons, and perhaps a few special symbols such as '$', '&', and'#'.

Just a moment, doesn't all of this seem strangely familiar? Are you experiencing a feeling of dja vu? (Didn'tsomebody just say that?) Well you can flay us with wet noodles if this isn't beginning to sound like the specificationfor theASCII Codethat we introduced in this document's companion paper (we love it when a plan comestogether).

As you may recall, ASCII is a 7-bit code. As we tend to store our information in 8-bit bytes, this means that we'vegot a spare bit in each byte to play with (but you can bet your little cotton socks that well find a use for these bitsin the not-so-distant future). The main point is that if we use ASCII codes to indicate the characters that we wish todisplay at each location on the screen, then we'll only need to reserve 512 bytes (16 x 32 characters) of our RAM


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for the entire screen, which is pretty efficient usage of our limited resources when you come to think about it.

For example, let's assume that the memory location at address $nnnn is associated with the character in the upperleft-hand corner of the screen (row 0, column 0); that the memory location at address $nnnn+1 is associated withthe character at row 0, column 1; that address $nnnn+2 is associated with the character at row 0, column 2; andso forth (note that the dollar '$' characters associated with addresses such as "$nnnn" indicate hexadecimalvalues). Furthermore, let's assume that locations $nnnn through $nnnn+4 contain the ASCII codes $42, $45, $42,$4F, and $50 as shown in the illustration below (where these codes correspond to the characters 'B', 'E', 'B', 'O',and 'P', respectively):

Apropos of nothing at all, the jazz style known as Bebopbecame highly popular in the decade following World WarII. Charlie Parker, Dizzy Gillespie and Thelonius Monk were especially associated with this form of music, which isknown for its fast tempos and agitated rhythms. One might wonder how many of the early computer scientistswere listening to the radio and clicking their fingers in time with a Bebop melody while pondering a particularlyperplexing problem. And we may only speculate if it was but a coincidence that many of the most significant ideasand discoveries in the history of computing occurred alongside the flourishing Bebop. But we digress...

Henceforth, we'll refer to the group of 512 memory locations associated with the memory-mapped display as theVideo RAM. In reality, any contiguous set of 512 bytes would serve our purpose; however, due to the fact that wetypically place our programs in the lower-order memory locations, it would be common practice to locate the VideoRAM somewhere in the higher regions of the computer's memory map (the concept of memory maps is introducedin our book, How Computers Do Math).

It's important to note that the computer in the form of its central processing unit (CPU) doesn't know anythingabout any of this; much like a married man it just does what it's told. So if a program instructs the CPU to load avalue of $42 into memory location $nnnn, it will happily do so without any understanding that, in this case, we'retreating $42 as the ASCII code for the letter 'B', and that we're hoping the corresponding pattern of dots willsomehow wend their way to the correct position on the screen.

One of the clever things about all of this is that the video card (which is connected to the main circuit board by acable as shown in our discussions on VDUsearlier in this paper) performs most of the work. We can imagine thelittle scamp as slipping in to read locations in the Video RAM while the main computer's back is turned, and thendisplaying the appropriate characters on the screen.

In the case of our hypothetical video card, we can consider it as being "hard-wired" to understand that ourmemory-mapped display supports 16 rows of 32 columns, and also that address $nnnn is the start address of theVideo RAM. Thus, the video card knows that whichever ASCII character occupies address $nnnn is supposed toappear at row 0, column 0 on the screen; that the character at address $nnnn+1 is supposed to appear at row 0,column 1; and so on. Similarly, because the video card knows how many rows and columns our display supports, itunderstands that the character at address $nnnn+31 is supposed to appear at row 0, column 31; while thecharacter at address $nnnn+32 is supposed to appear at the beginning of the next line at row 1, column 0; and soforth.

The fact that specific memory locations are mapped to particular character positions on the screen is, of course,why this technique is referred to as "memory-mapped." As a point of interest, the first memory-mapped displaywas developed around 1976 by Lee Felsenstein. In addition to designing the Pennywhistle Modemand theOsborne One(one of the earliest successful microcomputers), Lee also found the time to moderate the famous


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Homebrew Computer Cluband to act as the system administrator of the pioneering Community Memorywide-area network (WAN). (Lee was also kind enough to write the foreword to one of our earlier books entitledBebop BYTES Back (An Unconventional Guide to Computers).)

But, once again, we digress... The final problem is to take the ASCII codes stored in the Video RAM, and to usethem to generate the patterns of dots that are displayed on the screen. In order to do this, the video card uses adevice called a Character ROM. Remember that read-only memory (ROM) is a form of memory containinghard-coded patterns of 0s and 1s; also that it remembers its contents, even when power is removed from thesystem. In the case of the video card's Character ROM, these 0s and 1s are used to represent the absence orpresence of dots on the screen, respectively. Let's assume that the video card is commencing a new pass torefresh the screen commencing at row 0, column 0 as shown below:

In the case of our hypothetical display, each character is formed from fifteen lines (rows), each containing 10columns of dots. One thing we have to remember is that the VDU/CRT's electron beam has to scan all the wayacross the screen to form each line on the screen. As we see in the above illustration, our Character ROM hastwelve input signals; eight of these inputs (char[7:0] ) are used to present an ASCII code to indicate whichcharacter we're interested in, while the other four (line[3:0]) are used to indicate a particular line (row) in thatcharacter.

Thus, the video card commences by peeking into location $nnnn of the Video RAM to see what's there (for thepurposes of this example we're assuming that it's going to find the ASCII code $42, which corresponds to the letter'B'). The video card passes this ASCII code to the character ROM's char[7:0] inputs, and it also sets the line[3:0]inputs to binary 0000 (thereby indicating that it's interested in line 0 of this character).

Using this data, the character ROM's ten outputs, dot[9:0], return the pattern of 0s and 1s that correspond to thefirst line of the character 'B' (binary 1111111000 in this case). This pattern is then loaded into a shift register, whichconverts it into a sequence of pulses that are used to control the electron beam (a 1 turns the beam on to form adot and a 0 turns it off to leave a space).

However, the video card can't complete the rest of this character yet, because the electron beam is continuing itsscan across the screen. So the video card peeks into location $nnnn+1 in the Video RAM to see what's there andfinds the ASCII code $45, which corresponds to the letter 'E'. The video card passes this new ASCII code to thecharacter ROMs char[7:0] inputs while maintaining the binary 0000 value on the line[3:0]inputs (therebyindicating that it's still interested in line 0 of the new character). Once again, the character ROM responds with thepattern of dots required to construct the first line of the letter 'E'; and once again, this pattern is loaded into theshift register, which converts it into the pulses required to control the electron beam.

The video card continues this process for addresses $nnnn+2 through $nnnn+31, at which time it has completedthe first line of the first row of characters forming the display. It then repeats the process for the same set of

characters in addresses $nnnn through $nnnn+31, but this time it sets the character ROM's line[3:0]inputs tobinary 0001, thereby indicating that it's now interested in line 1 of these characters. In due course, the video cardhas to repeat this process another fourteen times for the first row of characters on the screen (incrementing theline[3:0]inputs each time) until it finally manages to complete all of the lines required to form the first row ofcharacters.

Next, the video card has to perform another fifteen scans to construct the second row of characters from the ASCIIcodes stored in Video RAM addresses $nnnn+32 through $nnnn+63, and so on for the remaining fourteen rows onthe display. This may seem to be a dreadfully complicated process involving a lot of work, but it really isn't too bad.Transistors can switch millions of times a second, so what seems to be a horrendous amount of effort to usactually leaves them with a lot of time on their hands waiting around for something interesting to happen.

More importantly, as users we aren't really affected by any of this. All we really need to know is that assumingwe're working with a memory-mapped display if we create a program that stores an ASCII code into one of the


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RAM locations we've designated as the Video RAM, then the video card will automatically cause the correspondingcharacter to appear on the screen at the appropriate location.

As fate would have it, we don't use memory-mapped displays in our home computers anymore, because we'vegrown to expect (nay demand) sophisticated user-interfaces and high-resolution graphics (see also the discussionson Modern and Future Display Technologieslater in this paper). However, it would be a mistake to regardmemory-mapped displays as being only historical curiosities, because these devices are still found in some "cheapand cheerful" applications such as automatic teller machines (ATMs).

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I/O-Driven Displays

Following the memory-mapped displays presented in the previous topic, the next step up the evolutionary ladderwould be an equivalent input/output (I/O)-driven display. In this case, the Video RAM is a separate entity that ispart of the display (actually, it's located on the video card):

The idea here is that the main computer simply writes a series of ASCII characters to a certain output port that isbeing used to drive the video card. Special control logic on the video card keeps track of what's happening andstores each character in the appropriate location in its Video RAM. This control logic would also understand specialcodes that instruct it to do things like clear the screen (which would equate to loading all of the locations in theVideo RAM with ASCII space characters [$20]), returning a flashing cursor to its "home" position (at row 0, column

0), and so forth.Thus, assuming that the computer had already transmitted "clear" and "home cursor" codes to the video card,when the computer sent its first ASCII character, the control logic on the video card would automatically store thischaracter in the row 0, column 0 location in the Video RAM. Similarly, when the computer sent its next ASCIIcharacter, the control logic would store this little rascal in the row 0, column 1 location, and so forth. (The videocard's control logic would also understand special commands [control codes] such as "New Line," which wouldcause it to move the flashing cursor to the beginning of the next line on the display.)

This approach, which is the one used by the DIY Calculator's virtual Console (screen) as discussed in the releasedocumentation on the Downloadspage of this website, frees up the main computer's memory and leaves itavailable to store programs and data. The downside (at least, in the days when computer memory was veryexpensive) is that you need an additional block of memory to act as the Video RAM.

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1-Bit, 8-Bit, 15-Bit, 16-Bit, 24-Bit, 30-Bit, or 32-Bit Color

As we previously noted, the simple displays presented in the previous topics are still to be found in some "cheapand cheerful" applications such as the automatic teller machines found in places such as banks and shopping malls.However, we don't use these displays in our home computers anymore, because we've grown to expect naydemand sophisticated user-interfaces and high-resolution graphics. [In the context of computer graphics andgraphics subsystems, the term resolutionrefers to the number of pixels (picture elements) that are used torepresent an image.]

Of course, a number of developments had to occur and technologies had to mature for us to reach the presentstate-of-play. First, manufacturing techniques improved, allowing computer monitors to have smaller pixels and(consequently) more pixels-per-inch than was previously achievable. This means that today's monitors support high


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resolution and can display images with great precision. Perhaps more importantly, the amount of memory we cansqueeze into a single silicon chip has increased enormously, while the cost of such devices has plummeteddramatically. Finally, modern computers are tremendously faster and more powerful than their predecessors.

As we previously discussed, modern computers typically contain a circuit board called a graphics cardor agraphics subsystem, which can contain a substantial amount of memory and its own on-board processor(s). Allyour main system has to do is to send an instruction to the processor on the graphics card saying something like"Draw me a purple circle with 'this'diameter at 'that'location on the screen."The graphics processor then slogsaway completing the task, while the system's main processor is left free to work on something else.

The combination of high-resolution monitors and graphics cards means that today's computer screens can have

millions of pixels, each of which can be individually set to thousands or even millions of colors. The fact that thegraphics card can individually address each pixel means we can create all sorts of sophisticated effects, such aschanging the size and font of characters on a character-by-character basis and dynamically varying the spacingbetween characters, because each character can be individually drawn pixel-by-pixel. Furthermore, thesehigh-resolution displays support today's graphical user interfaces, such as the one employed by the DIY Calculator,which both enhance and simplify the human-machine interface.

In this topic, we are going to consider some of the color schemes that may be employed by different softwareapplications and graphics subsystems. For example, if you right-mouse-click on your desktop (assuming you arerunning the Windows operating system), select the Propertiesoption from the ensuing pop-up menu, and thenselect the Settingstab from the resulting dialog, you can examine the options for Screen Resolutionand ColorQuality:

Some common resolution options are 800 600 (which means 800 pixels wide by 600 pixels deep), 1024 768,1152 864, and 1280 1024. (As we previously mentioned, the concept of pixels is somewhat slippery when wecome to computers well discuss this in more detail in the What is a Pixel?topic later in this paper). Meanwhile,some common color options are Low (8-bit), Medium (16-bit), and High (24-bit). But what does this actually mean?Well, we can explain it this way...

1-Bit "Color": Before we commence, we need to know that a modern graphics subsystem contains a block ofRAM (memory) known as the frame buffer. This stores a copy of whatever image is currently being displayed onthe computer's screen (where this image is constructed by the graphics processor). Another term of which weneed to be aware is color depth, which refers to the number of bits we use to represent the color of each pixel inan image.


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If we used only a single bit to represent each pixel, for example, then each bit could be in only one of two states either Off or On (logic 0 or logic 1) which would allow us to represent only two colors; for instance, black andwhite:

It's obvious that this illustration is not to scale and reflects a very limited number of pixels (if we tried to show themreal sized, we wouldn't be able to see anything at all). But if we assume a resolution of 1280 1024, then having ablack-and-white display boasting only one bit per pixel will still require our frame buffer to contain 1,310,720 bits (or163,840 bytes) of memory. And what of the other schemes? Read on...

8-Bit Color (A Teaser): The more colors that are available to describe an image, the more realistic will be the finalresult. Associating more bits with each pixel allows us to represent more colors. For example, two bits can be usedto represent 2^2 = 4 different binary values (00, 01, 10, and 11), which can in turn be used to represent fourdifferent colors.

Similarly, three bits can be used to represent 2^3 = 8 different colors, four bits can be used to represent 2^4 = 16different colors, and so forth. However, increasing the number of bits used to describe each pixel increases thecomplexity and cost of the graphics subsystem.

For this reason, low-end graphics subsystems tend to use as few bits per pixel as possible. A popular techniquefor these low-end cards is to assign eight bits to each pixel in the frame buffer, where these eight bits can be usedto represent 2^8 = 256 different colors. Of course a palette of only 256 colors is really quite limiting, so we often

use a cunning trick to get around this restriction. Can you guess what this ruse is? Actually, in order to understandhow this works, we need to understand some other concepts, so well first consider 15-bit, 16-bit, and 24-bit colorschemes, and then we'll return to the 8-bit scheme a little later in this topic.

15-Bit Color:One technique used in some mid-range graphics cards is to store 15-bits per pixel in the framebuffer. This requires almost twice the amount of memory as an 8-bit (256 color) approach, but in return it offers2^15 = 32,768 colors:


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In this case, the 15 bits associated with each pixel in the frame buffer are composed of three 5-bit subfields, whichare used to specify that pixel's red (R), green (G), and blue (B) color components, respectively (this is usuallyabbreviated to RGB). The value in each 5-bit subfield is used to drive an associated digital-to-analog converter(DAC), which transforms the digital data in the frame buffer into its analog equivalent as required by the monitor.(Until fairly recently, the vast majority of monitors were analog in nature.)

Note that there is some additional circuitry (not shown in the above diagram for simplicity) that scans through the

rows and columns of pixel data in the frame buffer. This circuitry commences with the pixel in the upper-left-handcorner and works its way across the first row; it then moves to the next row and repeats the process; it works itsway through the frame buffer until it's processed the last row, and then it returns to the upper-left-hand corner andstarts all over again.

15-bit color offers a reasonable tradeoff between memory requirements and the number of colors, but the resultingimages are not as realistic as those represented using the 16-bit or 24-bit color techniques as discussed in thefollowing sections.

16-Bit Color:Another technique used by mid-range graphics cards is to store 16 bits per pixel in the frame buffer.This requires twice the amount of memory as an 8-bit (256 color) approach, but in return it offers 2^16 = 65,536colors:


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Once again, the value in each subfield is used to drive an associated digital-to-analog converter (DAC), whichtransforms the digital data in the frame buffer into its analog equivalent as required by the monitor.

The 16-bit scheme illustrated here is called a 5-6-5 scheme because it uses 5 bits to represent the pixel's redcomponent, 6 bits for the green component, and 5 bits for the blue component. Due to the fact that the human eyeis more sensitive to variations in the green portion of the spectrum, using 6 bits to represent the green componentprovides a noticeable improvement over the 15-bit color scheme discussed above.

The following image goes some way to showing the differences between 1-bit (black-and-white), 8-bit, and 16-bitcolor with regard to representing some three-dimensional geometric shapes:

In reality, this image doesn't do justice to the 16-bit scheme, which would look much better on a display in the realworld.

24-Bit Color:Graphics cards that store 24 bits per pixel in the frame buffer can represent 2^24 = 16,777,216different colors. This is more than enough to accurately portray true-to-life images, so 24-bit color is often referredto as "true color."


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As usual, the value in each subfield is used to drive an associated digital-to-analog converter (DAC), whichtransforms the digital data in the frame buffer into its analog equivalent as required by the monitor.

High-end applications that require photorealistic images mandate the use of graphics subsystems that support24-bit true color, because lesser color schemes simply cannot reproduce images with the required fidelity.

8-Bit Color (Redux):And so we return to considering 8-bit color. As we previously noted, the eight bits associatedwith each pixel in the frame buffer can be used to represent 2^8 = 256 different patterns of 0s and 1s. The problemis that a palette of only 256 colors is really quite restrictive. One common way to mitigate this limitation is to usethe 8-bit fields in the frame buffer to index (point) into a set of lookup tables (color palettes):


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In this case, there are three lookup tables (LUTs) one each for the red, green, and blue color components eachof which is 8-bits wide and 256 words deep. The 8-bit value returned from each LUT (which is pronounced torhyme with "hut") is used to drive an associated digital-to-analog converter (DAC), which as usual transformsthe digital data into its analog counterpart as required by the monitor.

So now we have an interesting mix, because we can use only 256 colors, but each of those colors can be selectedfrom a 24-bit palette, which provides us with 16,777,216 different color possibilities.

Thus, this approach allows a graphics application (computer program) to load the LUTs with any set of 256 colorsappropriate to that application. One problem with this technique, however, is that whichever application is currentlyactive loads the LUTs with itspreferred set of colors, which makes any images currently being displayed by otherapplications elsewhere on the screen look somewhat strange.

All things considered, this 8-bit approach which is frugal in terms of its memory requirements can be useful forapplications that require only a limited number of colors; for example, user interfaces that employ areas of "solid"color. However, in the case of applications that are intended to process more complex images such asphotographs, an 8-bit approach will result in low-fidelity images that are unrealistic.

30-Bit Color:Some very special imaging and sensor applications employ a 30-bit color scheme, in which eachpixel in the frame buffer is represented by 30 bits; 10 bits each for the red, green, and blue color components.However, this type of thing is extremely rare and is outside the scope of this paper.

32-Bit Color:In the case of a 32-bit color scheme, each pixel in the frame buffer requires 32 bits to represent it. Infact, the term "32-bit color" is technically a misnomer, in that this scheme is actually a combination of 24-bit truecolor (as discussed above) along with 8 alpha bits.

These alpha bits offer 2^8 = 256 different levels of translucency, ranging from transparent to opaque. This isapplicable to a variety of applications, such as three-dimensional graphics in which a scene may contain manyobjects, some of which are in front of others.

As a point of interest, the term transparencyrefers to the quality of a material that allows the passage of light suchthat objects behind that material can be clearly seen. There are a number of different techniques for representingtransparency in computer graphics. In the case of the alpha blendingapproach, we blend the colors of the pixelsforming the transparent object with the colors of the pixels associated with any objects that are behindthat object.The alpha bits associated with the pixels forming the object in the foreground are used to tell the graphicsprocessor just how transparent those pixels are.


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Graphics folks also tend to drop the term translucencyinto the conversation. This refers to the quality of a materialof allowing the passage of light and diffusing that light so that objects behind the material can be seen, but notclearly. In the real world, for example, it's possible to look at a scene through a sheet of colored glass and anidentically colored bowl of gelatin and see objects behind both of them. In this case, we would refer to the glass asbeing transparent and the gelatin as being translucent, because any objects seen through the gelatin would tend tobe less clear.

And finally: One question that may have popped into your head when we were considering the fact that yourcomputer offers you different levels of screen resolution and different levels of color quality is: "Why wouldn't wealways want to use both the highest resolution and the highest color quality?"

That's a good question. Well, first of all, your graphics subsystem may contain only a limited amount of memory forits frame buffer. The designers of these subsystems tend to make these things very configurable, such that thememory can be allocated in many different ways. So, for example, there may be enough memory to support 24-bittrue color at a resolution of 1024 768; however, if you wish to use a higher resolution of 1280 1024, then yourgraphics subsystem may be able to accommodate only 16-bit color at this resolution.

Alternatively, even if your graphics subsystem can support the highest color quality at the highest resolution, youmay decide to use a lower resolution, such that objects (especially text) appear larger on the screen.

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Alternat ive Color Schemes (8-bit, 12-bit , 17-bi t, etc.)

With regards to the previous topic, do we really need 24 bits per pixel to achieve photo-realistic images, or can weget by with less? Now that computer memory is relatively inexpensive, this may not be tremendously important inthe case of large (desktop, for example) systems. However, it may be very significant in the case of embeddedsystems and portable, handheld, battery-powered systems such as cell phones, in which using fewer pixelsequates to less real estate on the silicon chip, a reduction in computational requirements, and lower powerconsumption.

The reason we've come to question "conventional wisdom" is a paper written by IBM Fellow Mike Cowlishaw.Dated from 1985, and entitled Fundamental Requirements for Picture Presentation, this paper demonstratesthat it should never be necessary to use more than 17-bits to reproduce optimal color (5 red, 7 green, and 5 blue),but that a 12-bit scheme (4 red, 5 green, 3 blue) does almost as well. In fact, the paper even shows a photographof a person looking very realistic using only an 8-bit scheme (2 red, 4 green, and 2 blue)!

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Modern and Future Display Technologies

For many decades, cathode-ray tube (CRT)-based computer monitors were the only game in town. But things don'tstay the same for ever; new display technologies have already emerged, with even more exciting possibilities forthe future...

Liquid Crystal Displays (LCDs):In the early 1990s, a number of different companies started experimenting withsubstances known as liquid crystals (LCs); eventually, liquid crystal displays (LCDs)became available to themarket.

In their early incarnations, these displays were very expensive compared to CRT-based techniques; also, thepicture quality wasn't as good and the response time of the individual pixels was slow enough that "ghosting" andblurring effects were seen on fast-moving objects in the images. These problems have now largely been solved,and sales of LCDs have risen dramatically over the last few years, to the extent that almost 85% of all new

displays sold are LCD-based.Interestingly enough, LCD technology has its roots in 1888, when an Austrian botanist called Friedrich Reinitzer(1857-1927) was studying cholesterol in plants. He ended up creating a material we now know as cholesterylbenzoate. This was a phase of matter of which we had never previously been aware, but which we now know aspossessing a "liquid crystalline" structure.

One problem with LCDs is that there many different variations on the theme. For example, we now know of morethan 50,000 compounds and mixtures that possess liquid crystalline properties. Thus, the following will be a generic(high-level) description intended only to give the "flavor" as to how this all hangs together. As usual, the easiest wayof summarizing how these things work is by means of a high-level diagram:


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The key point about liquid crystals (at least from our perspective) is that by default they will arrange themselvesinto a tight ("twisted") helix pattern, in which case they will block the passage of light. However, if we apply currentto the liquid crystals, they will "untwist" to the extent that they will pass light. Varying the amount of current willaffect the amount of light that is being passed.

The illustration above reflects a cutaway portion of the screen showing the tiny red, green, and blue filters forminga single pixel. Each of these filters has a bunch of liquid crystals associated with it, and each of these bunches hasan associated transistor (these transistors are not shown here for reasons of simplicity).

Behind all of the crystals is a backlight formed from some source of white light. When individual transistors areturned on, they will activate their c orresponding bunch of liquid crystals, which will transmit the light into theirassociated filter. It's possible to control the bunches of crystals in 256 increments which we might number from 0 to255; 0 means the crystals are twisted and won't pass any light; 255 means that the crystals are sufficientlyuntwisted that they will pass the maximum amount of light they can; and the other values correspond to the crystalspassing lesser or greater amounts of light.

The great advantage of LCDs over CRT-based displays is that they are very thin, very light, and very flat. Havingsaid this, CRT-based displays still have an advantage in terms of the brightness, contrast, and "vibrancy" of the

images that can be achieved. If only there were some other technologies...Plasma Display Panels (PDPs): You may have seen flat-panel plasma displays at television stores. Thesedisplays offer bright, crisp, high-contrast images. In this case, we can think of each pixel as being formed fromthree tiny fluorescent lights (like microscopic neon tubes). By one mechanism or another, these three tiny neontubes can be coerced into generating red, green, and blue light, each of which can be controlled to form the finalcolor coming out of that pixel.

Plasma displays are fantastic when it comes to presenting ever-moving images such as films. However, if they areinstructed to present the same image over and over again, they suffer from "burn-in" effects that leave "ghost"images on the screen. This means that plasma-based technologies do not make an ideal display for computerapplications (although there are always some folks who will try to do so).

Organic Light-Emitting Diodes (OLEDs):These are devices that are formed from thin films of organic molecules


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that generate light when stimulated by electricity. OLED-based displays hold the promise of providing bright andcrisp images while using significantly less power than liquid crystal displays.

At some stage in the future, it may be possible to use OLEDs to create displays that are only a few millimetersthick and are two meters wide (or more); these displays would consume very little power compared to othertechnologies, and in some cases the display could be rolled up and stored away when it wasn't in use (OLEDs canbe "printed" onto flexible plastic substrates).

But (despite some very exciting "proof-of-concept" demonstrations), this technology isn't ready for "prime time"usage just yet. OLED-based displays are sometimes used for small-screen applications such as cell phones anddigital cameras, but their widespread use for applications like large screen computer displays may not come for

another five or ten years at the time of this writing (in fact, they may not make it at all if the SED technologydiscussed below fulfils its promise).

Surface Emission Displays (SEDs):This is where things start to get very exciting. Prior to the mid-1980s,graphite and diamond were the only forms of pure carbon that were known to us. In 1985, however, a third formconsisting of spheres formed from 60 carbon atoms was discovered. Commonly referred to as "Buckyballs," theofficial moniker of this material is Buckministerfullerine, which was named after the American architect R.Buckminister Fuller who designed geodesic domes with a similar underlying symmetry.

Sometime later, scientists discovered a related structure that we now refer to as a carbon nanotube. Suchnanotubes can be incredibly small, with a diameter only one thousandth of one millionth of a meter. Furthermore,they are stronger than steel, have excellent thermal stability, and are tremendous conductors of heat andelectricity.

In addition to functioning as wires, nanotubes can be persuaded to act as transistors. Of particular interest to ushere is that they can also be coerced into emitting streams of electrons out of one end. Hmmm, tiny little electronguns; what wonders could we perform with these little rapscallions?

Imagine a screen that is thin and flat like a LCD, but is as bright and vibrant as a CRT-based display. Well, that'swhat you end up with if the screen is formed from a carbon nanotube-based SED. In this case, the inside of thescreen is covered with red, green, and blue phosphor dots (one of each to form each pixel), and each if these dots


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has its own carbon nanotube electron gun.

This technology has been skulking around in the background for some time, but it appears as though theoutstanding issues that had been holding it back have been resolved, and SEDs are poised to leap onto the centerstage. At the time this topic was first written, it was predicted that we would be seeing SEDs on the streets towardthe end of 2006 and the beginning of 2007. It was later announced, however, that the introduction of these devicesis being held back until around the middle of 2008 (to coincide with the Summer Olympics in Beijing).

Toshiba hosted the first public demonstration of a large-scale carbon nanotube-based SED at the consumerelectronics show (CES) in January 2006. Industry expert Dennis P. Barker attended the show, and as he told theauthors of this paper:

"High-definition television is incredibly realistic, but SED goes one step beyond. When I saw theToshiba demonstration, it gave me chills and the hairs on the back of my neck stood to attention. Ihave seen the future and to me the future is SED!"

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An Alternative Sub-Pixel Technology

In our previous illustrations, weve shown the red, green, and blue sub-pixels forming a full pixel as being circles orsquares. In reality, this was to some extent a case of artistic license that made it easier to get the conceptsacross. In order to fully wrap our brains around this current topic, however, we need to be aware that the red,green, and blue sub-pixels forming a pixel on a liquid crystal display (LCD) are rectangular. Combined, these threesub-pixels form a square (or close enough to a square for our purposes here).

And so we come to those clever guys and gals at Clairvoyante (www.clairvoyante.com), who have used theirexpertise in human vision to come up with an incredibly cunning new way of doing things. First of all, let's consider a4 4 array of standard RGB (red, green, and blue) pixels compared to a 2 4 array of Clairvoyantes RGBW(red, green, blue, and white) PenTile pixels. (To be fair, we should note that the underlying RGBW technologyhas been around for quite some time for specialist applications such as displays in aircraft. Until now, however,these displays have been unsuccessful when it comes to displaying natural images. In order to address this, thefolks at Clairvoyant have come up with a treasure chest of cunning tricks and techniques such as specialsub-pixel-based rendering algorithms that result in bright, crisp images with natural color.)


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Observe that each new row of RGBW pixels is shifted by two sub-pixels from the previous row. That is, the first,third, fifth, etc. rows have their sub-pixels ordered R, G, B, W, R, G, B, W, etc. By comparison, the second, fourth,sixth, etc. rows have their sub-pixels ordered B, W, R, G, B, W, R, G, and so on. We'll see how this comes intoplay shortly.

One key point to note here is that four RGB pixels in a row (in the horizontal direction) are formed from 4 3 = 12sub-pixels. By comparison, two RGBW pixels in a row are formed from 2 4 = 8 sub-pixels. Thus, the PenTileMatrix employs only 2/3 the number of sub-pixels required by the traditional scheme. In turn, this means that aPenTile-based screen requires a third fewer transistors, which improves reliability. Moreover, the remainingtransistors can be fabricated a tad larger, which makes them more robust.

So why is the smaller number of larger-sized RGBW sub-pixels important? Well, apart from anything else, botharrays occupy the same physical area. Now, if you look at any of the sub-pixels in the illustrations above andbelow, you'll see them shown as being surrounded by a black line. This represents the real world, where each

sub-pixel has an opaque periphery that blocks extraneous internal light coming out (and unwanted external lightgetting in).


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The term "aperture ratio" refers to the transparent area of a sub-pixel compared to the total area occupied by thatsub-pixel (including its opaque periphery). The fact that there are only two PenTile RGBW sub-pixels in the samearea as three of the standard RGB sub-pixels means that the aperture ratio of the PenTile sub-pixels is larger; inturn, this means that they pass more light per unit area and therefore are more efficient.

However, the real key to the excitement surrounding this new technology is its power efficiency. Although LCDs areconsidered to be energy-efficient, when it comes to handheld, battery-powered devices such as cell phones, sucha display can consume a large proportion of the device's total power budget. And things will only get worse as westart to use new features such as graphics-intensive games and streaming video on these devices.

In the case of today's cell phones using qVGA resolution (240 320), for example, the backlighting requires twohigh-efficiency white light-emitting diodes (LEDs), each of which consumes 50 mW and costs 25 cents (whenpurchased in extremely large quantities). By comparison, forthcoming cell phones boasting VGA resolution (480 640) will require 8 or 10 LEDs for the backlighting, with a combined cost of $2.00 to $2.50 and a combined powerconsumption of 400 to 500 mW. (Good Grief, Charlie Brown!)

Thus, another really important point about the PenTile arrangement is the fact that the white (W) sub pixels arebasically transparent, which means they propagate the backlight with minimal losses (as opposed to the RGBsub-pixels whose colored filters impose significant losses). The way all of this works is best described visually (apicture is worth a thousand words,as they say). Lets start by considering a small 3 4 array of traditional RGBpixels and the corresponding array of PenTile pixels; initially well assume that none of the sub-pixels are lit up.

Now, let's assume that we want to fully light up the traditional RGB pixel in row 2 column 2, which will require us to


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The fact that the RGBW sub-pixels have a larger aperture ratio than the traditional RGB sub-pixels coupled withthe fact that the W (white) sub-pixels propagate the backlight with minimal losses (as opposed to the red, green,and blue sub-pixels in which there are significant losses) makes the PenTile matrix much more efficient as a

whole.

And what do we mean by more efficient? Well, taking VGA resolution as an example, a PenTile screen will be100% brighter than a traditional screen using the same number of backlight LEDs. Alternatively, a PenTile screencan achieve the same brightness as a traditional screen using half the LEDs (and therefore consuming half thepower). ("But wait,"we hear you cry, how can we achieve a 100% increase in brightness or a 50% reduction inpower when the previous image showed the center group of RGBW sub-pixels at 62.5%?". Well, once again, ifyou instinctively understand this then please continue reading; otherwise check out our Brain Boggleraddendum.)

In addition to the concept of the PenTile matrix itself, the folks at Clairvoyante have developed correspondingsub-pixel image processing algorithms that take images intended for standard displays and convert them intoequivalent images for PenTile matrix displays (this extra processing consumes only a few milliwatts). Thesealgorithms can be made available in the form of intellectual property (IP) for folks to include on their customintegrated circuits, or as software algorithms to be run on a CPU/DSP.

But what about the fact that there are 1/3 fewer RGBW sub-pixels in the PenTile matrix as compared to the RGBsub-pixels in the traditional displays. Or, to put this another way, a single RGBW-based pixel replaces (occupiesthe same area as) two of the traditional RGB-based pixels in the horizontal direction. Let's remind ourselves as towhat this looks like:


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So, doesnt the fact that the PenTile matrix effectively has half the number of pixels (2/3 the number of sub-pixels)as compared to a traditional RGB-based display affect the resolution of the display. Well, it all depends what wemean by resolution, doesnt it? The conventional way of specifying resolution is to report the number of whole pixelsforming the display. Thus, if the arrays in the above image represented the total display, we would say that theRGB version has a resolution of 4 4 pixels, while the RGBW has a resolution of only 2 4 pixels. Is this b

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