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Shahrad Rezaei Tehrani |
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Who Computer Monitor Works In an industry in which development is so rapid, it is somewhat surprising that the technology behind monitors and televisions is a 100 years old. The cathode-ray tube, or CRT, was developed by Ferdinand Braun, a German scientist, in 1897 but wasn't used in the first television sets until the late 1940s. Although the CRTs found in modern monitors have undergone modifications to improve picture quality, they still follow the same basic principles. Despite predictions of its impeding demise, the CRT looks set to maintain its dominance in the PC monitor market. Whilst competing technologies, such as liquid crystal displays (LCDs) and gas plasma screens, are establishing themselves in specialist areas, it looks as though it will be well into the new millennium before flat screens outnumber CRTs on our desktops. Anatomy
Images are created when electrons, fired from the electron gun, converge to strike their respective phosphor blobs (triads) and each is illuminated, to a greater or lesser extent. When this happens, light is emitted, in the colour of the individual phosphor blobs. The gun radiates electrons when the heater is hot enough to liberate electrons (negatively charged) from the cathode, which are then narrowed into a tiny beam by the focus elements. The electrons are drawn toward the phosphor dots by a powerful, positively charged anode, located near the screen. The phosphors in a group are so close together that the human eye perceives the combination as a single coloured pixel. Before the electron beam strikes the phosphor dots, it travels thorough a perforated sheet located directly in front of the phosphor layer known as the "shadow mask". Its purpose is to "mask" the electron beam, forming a smaller, more rounded point that can strike individual phosphor dots cleanly and minimise "overspill", where the electron beam illuminates more than one dot. The beam is moved around the screen by magnetic fields generated through a deflection yoke. It starts in the top left corner (as viewed from the front) and flashes on and off as it moves across the row, or "raster". When it impinges on the front of the screen, the energetic electrons collide with the phosphors that correlate to the pixels of the image that's to be created on the screen. These collisions convert the energy into light. Once a pass has been completed, the electron beam moves down one raster and begins again. This process is repeated until an entire screen is drawn, at which point the beam returns to the top to start again. The most important aspect of a monitor is that it should give a stable display at the chosen resolution and colour palette. A screen that shimmers or flickers, particularly when most of the picture is showing white (as in Windows), can cause itchy or painful eyes, headaches and migraines. It is also important that the performance characteristics of a monitor be carefully matched with those of the graphics card driving it. It's no good having an extremely high performance graphics accelerator, capable of ultra high resolutions at high flicker-free refresh rates, if the monitor cannot lock onto the signal. A monitor's three key specifications are:
Resolution and refresh rate Refresh rate, or vertical frequency, is measured in Hertz (Hz) and represents the number of frames displayed on the screen per second. Too few, and the eye will notice the intervals in between and perceive a flickering display. The world-wide accepted refresh rate for a flicker-free display is 70Hz and above, although standards bodies such as VESA are pushing for higher rates of 75Hz or 80Hz. A computer's graphics circuitry creates a signal based on the Windows desktop resolution and refresh rate. This signal is known as the horizontal scanning frequency, HSF, and is measured in KHz. Raising the resolution and/or refresh rate increases the HSF signal. A multi-scanning or "autoscan" monitor is capable of locking on to any signal which lies between a minimum and maximum HSF. If the signal falls out of the monitor's range, it will not be displayed. Interlacing Masks and dot pitch There's more than one way to group three blobs of coloured phosphor - indeed, there's no reason why they should even be circular blobs. A number of different schemes are currently in use, and care needs to be taken in comparing the dot pitch specification of the different types. With standard dot masks, the dot pitch is the centre-to-centre distance between two nearest-neighbour phosphor dots of the same colour, which is measured along a diagonal. The horizontal distance between the dots is 0.866 times the dot pitch. For masks which use stripes rather than dots, the pitch equals the horizontal distance. This means that the dot pitch on a standard dot-mask CRT should be multiplied by 0.866 before it is compared with the dot pitch of these other types of monitor. The difficulty in directly comparing the dot pitch values of different displays means that other factors - such as convergence, video bandwidth and focus - are often a better basis for comparing monitors than dot pitch. Dot trio Because the distance between the source and the destination of the electron stream towards the middle of the screen is smaller than at the edges, the corresponding area of the shadow mask get hotter. To prevent it from distorting - and redirecting the electrons incorrecctly - manufacturers typically construct it from Invar, an alloy with a very low coefficient of expansion. This is all very well, except that the shadow mask used to avoid overspill occupies a large percentage of the screen area. Where there are portions of mask, there's no phosphor to glow and less light means a duller image. The brightness of an image matters most for full-motion video and with multimedia becoming an increasing important market consideration a number of improvements have been made to make dot-trio mask designs brighter. Most approaches to minimising glare involve filters that also affect brightness. The new schemes filter out the glare without affecting brightness as much. Toshiba's Microfilter CRT places a separate filter over each phosphor dot and makes it possible to use a different colour filter for each colour dot. Filters over the red dots, for example, let red light shine through, but they also absorb other colours from ambient light shining on screen - colours that would otherwise reflect off as glare. The result is brighter, purer colours with less glare. Other companies are offering similar improvements. Panasonic's Crystal Vision CRTs use a technology called dye-encapsulated phosphor, which wraps each phosphor particle in its own filter and ViewSonic offers an equivalent capability as part of its new SuperClear screens. Aperture Grill Consequently, rather than use a solid perforated sheet, Trinitron tubes use masks which separate the entire stripes instead of each dot - and Sony calls this the "aperture grill". This replaces the shadow mask with a series of narrow alloy strips that run vertically across the inside of the tube. Rather than using conventional phosphor dot triplets, aperture grill-based tubes have phosphor lines with no horizontal breaks, and so rely on the accuracy of the electron beam to define the top and bottom edges of a pixel. Since less of the screen area is occupied by the mask and the phosphor is uninterrupted vertically, more of it can glow, resulting in a brighter, more vibrant display. With aperture grill monitors the equivalent measure to dot pitch is known as "stripe pitch". Because aperture grill strips are very narrow, there's a possibility that they might move, due to expansion or vibration. In an attempt to eliminate this, horizontal damper wires are fitted to increase stability. This reduces the chances of aperture grill misalignment, which can cause vertical streaking and blurring. The down side is that because the damper wires obstruct the flow of electrons to the phosphors, they are just visible upon close inspection. Trinitron tubes below 17in or so get away with one wire, while the larger model require two. A further down side is mechanical instability. A tap on the side of a Trinitron monitor can cause the image wobble noticeably for a moment. This is understandable given that the aperture grill's fine vertical wires are held steady in only one or two places, horizontally. Mitsubishi followed Sony's lead with the design of its similar Diamondtron tube. Slotted mask In order to allow a greater amount of electrons through the shadow mask, the standard round perforations are replaced with vertically-aligned slots. The design of the trios is also different, and features rectilinear phosphors that are arranged to make best use of the increased electron throughput. The slotted mask design is mechanically stable due to the criss-cross of horizontal mask sections but exposes more phosphor than a conventional dot-trio design. The result is not quite as bright as with an aperture grill but much more stable and still brighter than dot-trio. It is unique to NEC, and the company capitalised on the design's improved stability in early 1996 when it fit the first ChromaClear monitors to come to market with speakers and microphones and claimed them to be "the new multimedia standard". Enhanced Dot Pitch On a typical shadow mask CRT, the phosphor trios are more or less arranged equilaterally, creating triangular groups that are distributed evenly across the inside surface of the tube. Hitachi has reduced the distance between the phosphor dots on the horizontal, creating a dot trio that's more akin to an isosceles triangle. To avoid leaving gaps between the trios, which might reduce the advantages of this arrangement, the dots themselves are elongated, so are oval rather than round. The main advantage of the EDP design is most noticeable in the representation of fine vertical lines. In conventional CRTs, a line drawn from the top of the screen to the bottom will sometimes "zigzag" from one dot trio to the next group below, and then back to the one below that. Bringing adjacent horizontal dots closer together reduces this and has an effect on the clarity of all images.
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This page was last updated on 03-Mar-2002.
YAHOO ID: Shrezai@yahoo.com |
In the "bottle neck" of the CRT is the electron gun, which is composed
of a 
The vast majority of computer monitors use circular blobs of phosphor and
arrange them in triangular formation. These groups are known as 
In the 1960s, Sony developed an alternative tube technology known as Trinitron.
It combined the three separate electron guns into one device: Sony refers to
this as a Pan Focus gun. Most interesting of all, Trinitron tubes were made from
sections of a cylinder, vertically flat and horizontally curved, as opposed to
conventional tubes using sections of a sphere which are curved in both axes.
Rather than grouping dots of red, green and blue phosphor in triads, Trinitron
tubes lay their coloured phosphors down in uninterrupted vertical stripes.
Capitalising on the advantages of both the shadow mask and aperture grill
approaches, NEC has developed a hybrid mask type which uses a slot-mask design
borrowed from a TV monitor technology originated in the late 1970s by RCA and
Thorn. Virtually all non-Trinitron TV sets use elliptically-shaped phosphors
grouped vertically and separated by a 
Developed by Hitachi, the largest designer and manufacturer of CRTs in the
world, 
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