Technical Computer Peripherals Monitor

The Monitor

A CRT is coated on the inside with a matrix of thousands of tiny phosphor dots. Phosphors are chemicals which emit light when excited by a stream of electrons. Different phosphors emit different coloured light. Each dot consists of three blobs of coloured phosphor, one red, one green and one blue. These groups of three phosphors make up what is known as a single pixel.

Also inside the CRT is an electron gun, which is composed of a cathode, heat source and focusing elements. Colour monitors have three separate guns, one for each phosphor colour. Combinations of different intensities of red green and blue phosphors can create the illusion of millions of colours. This is called additive colour mixing and is the basis for all colour CRT displays.

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:

the maximum resolution it will display
at what refresh rate
whether this is in interlaced or non-interlaced mode

Resolution is the number of pixels the graphics card is describing the desktop with, expressed as a horizontal by vertical figure. Standard VGA resolution is 640 x 480 pixels. The commonest SVGA resolutions are 800 x 600 and 1024 x 768 pixels.

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 output 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.

An interlaced monitor is one in which the electron beam draws every other line, say one, three and five until the screen is full, then returns to the top to fill in the even blanks (say lines two, four, six and so on). An interlaced monitor offering a 100Hz refresh rate only refreshes any given line 50 times a second, giving an obvious shimmer. Non-interlaced (NI) is where every line is drawn before returning to the top for the next frame, resulting in a far steadier display. A non-interlaced monitor with a refresh rate of 70Mz or over is necessary to be sure of a stable display.

The maximum resolution of a monitor is dependent on more than just its highest scanning frequencies. Another factor is dot pitch, the physical distance between adjacent phosphor dots of the same colour on the inner surface of the CRT. Typically, this is between 0.22mm and 0.3mm. The smaller the number, the finer and better resolved the detail. However, trying to supply too many pixels to a monitor without a sufficient dot pitch to cope causes very fine details, such as the writing beneath icons, to appear blurred.

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.

If the electron beam is not lined up correctly with the shadow mask or aperture grille holes the beam is prevented from being passed through to the phosphors, thereby causing a reduction in pixel illumination. As the beam scans it may sometimes regain alignment and so succeed in passing through the mask/grille to reach the phosphors. The result is that the brightness rises and falls, producing a wavelike pattern on the screen, referred to as moiré. Moiré patterns are often most visible when a screen background is set to a pattern of dots, for example a grey screen background consisting of alternate black and white dots. The phenomenon is actually common in monitors with improved focus techniques as monitors with poor focus will have a wider electron beam and therefore have more chance of hitting the target phosphors instead of the mask/grille. In the past the only way to eliminate moiré effects was to defocus the beam, but now a number of monitor manufacturers have developed techniques to increase the beam size, without degrading the focus.

FSTs improve on earlier designs by having a screen surface with only a gentle curve. They also have a larger display area, closer to the tube size, and nearly square corners. There's a design penalty for a flatter, squarer screen, as the less of a spherical section the screen surface is, the harder it is to control the geometry and focus of the image on that screen. Modern monitors use microprocessors to apply techniques like dynamic focusing to compensate for the flatter screen.

FSTs require the use of a special alloy, Invar, for the shadow mask. The flatter screen means that the shortest beam path is in the centre of the screen. This is the point where the beam energy tends to concentrate, and consequently the shadow mask gets hotter here than at the corners and sides of the display. Uneven heating across the mask can make it expand and then warp and buckle. Any distortion in the mask means that its holes no longer register with the dot triplets on the screen and image quality will be reduced. Invar alloy is used in the best monitors as it has a low coefficient of expansion.

One of the problems with completely flat screens is that they accentuate the problem of the shape of the electron beam being elliptical at the point at which it strikes the screen at its edges. Furthermore, the use of perfectly flat glass give rise to an optical illusion caused by the refraction of light, resulting in the image looking concave. Consequently, some tube manufacturers have introduced a curve to the inner surface of the screen to counter the concave appearance.

USB (The Universal Serial Bus) applies to monitors in two ways. First, the monitor itself can use a USB connection to allow screen settings to be controlled with software. Second, a USB hub can be added to a monitor (normally in its base) for use as a convenient place to plug in USB devices such as keyboards and mice. The hub provides the connection to the PC.

Digital CRTs
Whilst primarily of benefit to flat panel displays, which can now operate in an standardised all-digital environment without the need to perform an analogue-to-digital conversion on the signals from the graphics card driving the display device, the DVI specification potentially has ramifications for conventional CRT monitors too.

Most complaints for poor image quality on CRTs can be traced to incompatible graphics controllers on the motherboard or graphics card. In today's cost-driven market, marginal signal quality is not all that uncommon. The incorporation of DVI with a traditional analogue CRT monitor will allow monitors to be designed to receive digital signals, with the necessary digital-to-analogue conversion being carried out within the monitor itself. This will give manufacturers added control over final image quality, making differentiation based on image quality much more of a factor than it has been hitherto. However, the application of DVI with CRT monitors is not all plain sailing.

One of the drawbacks is that since it was originally designed for use with digital flat panels, DVI has a comparatively low bandwidth of 165MHz. This means that a working resolution of 1280 x 1024 could be supported at up to an 85Hz refresh rate. Although this isn't a problem for LCD monitors, it's a serious issue for CRT displays. The DVI specification supports a maximum resolution of 1600 x 1200 at a refresh rate of only 60MHz - totally unrealistic in a world of ever increasing graphics card performance and ever bigger and cheaper CRT monitors.

The proposed solution is the provision of additional bandwidth overhead for horizontal and vertical retrace intervals - facilitated through the use of two TMDS links. With such an arrangement digital CRTs compliant with VESA's Generalised Timing Formula (GTF) would be capable of easily supporting resolutions exceeding 2.75 million pixels at an 85Hz refresh rate.

Another problem is that it's more expensive to digitally scale the refresh rate of a monitor than using a traditional analogue multisync design. This could lead to digital CRTs being more costly than their analogue counterparts. An alternative is for digital CRTs to have a fixed frequency and resolution like a LCD display and thereby eliminate the need for multisync technology.

DVI anticipates that in the future screen refresh functionality will become part of the display itself. New data will need to be sent to the display only when changes to the data need to be displayed. With a selective refresh interface, DVI can maintain the high refresh rates required to keep a CRT display ergonomically pleasing while avoiding an artificially high data rate between the graphics controller and the display. Of course, a monitor would have to employ frame buffer memory to enable this feature.

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