The Different Types of CRT Monitors – From ShortNeck to FST

By the beginning of 1998 15in monitors were gradually slipping to bargain-basement status, and the 17in size, an excellent choice for working at 1024×768 (XGA) resolution, was moving into the slot reserved for mainstream desktops. At the high end, a few 21in monitors were offering resolutions as high as 1800×1440.

In late 1997 a number of 19in monitors appeared on the market, with prices and physical sizes close to those of high-end 17in models, offering a cost-effective compromise for high resolution. A 19in CRT is a good choice for 1280×1024 (SXGA) – the minimum resolution needed for serious graphics or DTP, and the power user’s minimum for business applications. It’s also a practical minimum size for displaying at 1600×1200 (UXGA), although bigger monitors are preferable for that resolution.

One of the main problems with CRTs is their bulk. The larger the viewable area gets, the more the CRT’s depth increases. The long-standing rule of thumb was that a monitor’s depth matched its diagonal CRT size. CRT makers had been trying to reduce the depth by increasing the angle of deflection within the tube. However, the more the beam is deflected, the harder it is to maintain focus. Radical measures deployed included putting the deflection coils inside the glass CRT; they normally sit around the CRT’s neck.

The result of this development effort is the so-called short-neck CRT. In early 1998 17in short-neck monitors measuring around 15in deep reached the market. The downside was that the new design had a tendency to degrade images, especially at a screen’s corners and edges. This was addressed by improvements in the technology the following year with the introduction of tube designs employing a 100-degree deflection tube – in place of conventional 90-degree tubes – and narrower electron gun assemblies. The consequent increase in the beam deflection angle allowed the gun to be placed closer to the screen without the penalty of any image distortion. The result was a new rule of thumb that short-necked monitors should be about two inches shorter than their diagonal size.

The shape of a monitor’s screen is another important factor. The three most common CRT shapes are spherical (a section of a sphere, used in the oldest and most inexpensive monitors), cylindrical (a section of a cylinder, used in aperture-grille CRTs), and flat square (a section of a sphere large enough to make the screen nearly flat).

Flat square tube (FST) is an industry standard term used since 1997 to describe shadow mask monitors that have minimal curvature (but still a curvature) of the monitor tube. 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 displayed images. 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 eventually 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.

By 2000, monitors that used alternative mask technologies were available with completely flat screens. The principal advantages of a truly flat surface is that they have minimal glare and display images that have a more realistic appearance. However, these benefits are gained at the cost of accentuating 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 gives rise to an optical illusion caused by the refraction of light, resulting in the image looking concave. As a result, many tube manufacturers employ a double layer glass surface, the inner surface of which introduces a curve that counters the concave appearance. The downside of this is that it reduces brightness – and sometimes contrast – and can give rise to warping at the screen’s corners.

Sound facilities have become commonplace on many PCs, requiring additional loudspeakers and possibly a microphone too. The multimedia monitor avoids lots of separate boxes and cables by building in loudspeakers of some sort, maybe a microphone and in some cases a camera for video conferencing. At the back of these monitors are connections for a sound card. However, the quality of these additional components is often questionable, adding only a few pounds to the cost of manufacture. For high quality sound nothing beats decent external speakers which can also be properly magnetically shielded.

Another development which has become increasingly available since the launch of Microsoft’s Windows 98, which brought with it the necessary driver software, is USB-compliant CRTs. 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.