Technology product Focus: The World is Flat

Continuing to purchase CRT monitors because they're cheap is like holding onto your old LPs. The new world of computer displays is flat, digital - and cheaper than you thinkMatthew JC PowellIn an ideal world, you wouldn't be reading this. Because, in an ideal world, I wouldn't have to write it. Feature articles weighing up the relative merits of cathode-ray tube and flat panel displays should have stopped appearing last year, when the inherent superiority of flat-panels became most clearly apparent. Big, hot, bulky, glaring, flickering CRT monitors should have left your organisation roundabout the same time as your old SyQuests.

But they didn't, so here we are again. Why have so many organisations stuck to the old faithful instead of moving on up to the latest and greatest in visual technology? The bottom line comes down to price. While flat panels have become more competitive, CRT monitors have downright plummeted. You can now pick up a decent 15-inch flat panel for the same money you paid two years ago for a 17-inch CRT, but on the other hand you can get a new 17-inch CRT for pocket change.

The question is, does the initial cost saving in sticking with CRT reflect a false economy, or do the benefits of flat panels outweigh the extra cost? Can you, in fact, save money by going with a flat-panel display, even at twice the price or more? There are a number of factors to consider.

First up, don't be fooled by the size. It may seem that comparing the price of a 15-inch liquid crystal display to a 17-inch CRT is unfair, but consider that the diagonal measurement given by manufacturers of CRT monitors takes into account the curvature of the screen (unavoidable due to the physics of glass tubes) as well as areas near the edge of the screen that tend to be distorted due to that curvature.

The measurements given for the viewable area of a flat panel are much "truer", reflecting the actual display area. And, because of the lack of curvature, all of those pixels are usable. Therefore, it's fair to say that a 15-inch LCD screen gives a comparable usable viewing area to a 17-inch CRT.

In a CRT, the image is generated by a beam of electrons that scan across the screen. The electrons react with phosphorous "dots" on the surface of the screen and cause them to glow. Definition between the phosphorous dots is provided by a "mask" or metallic screen. Since the screen itself is not of any fixed resolution, CRTs are able to simulate multiple different resolutions, simply by scanning the screen faster.

The downside of this is that the pixels blur into each other, reducing the definition of details such as small text. What's more, the electron beam can only be in one place at any one time, so there is much reliance on the "persistence" of each phosphor dot. Immediately after the beam leaves, the dots begin to fade, and must be refreshed by the beam next time around. If the beam refreshes less than 72 times per second, this cycle of decay and refresh causes a noticeable flicker.

Different technologies

There are two main types of flat-panel display in common usage. Passive matrix LCD displays are constructed of row and column electrodes that overlap to define the pixels. The number of rows and columns is fixed, meaning that there is a one-to-one correspondence between the resolution generated by the computer (say, 1024x768) and the resolution that can be displayed on screen. The pixels are addressed by voltage from individual capacitors that are charged one row at a time. The pixels see this voltage during the entire time of the scan, and don't have a chance to decay. So, while a CRT beam has to land on more than a million phosphor dots per second to create a reasonably stable image, the scan signal in an LCD is responsible for only a thousand or so rows. Therefore, an LCD operating at as little as 40 cycles per second will produce an image more stable than a CRT at 70 cycles per second.

Active matrix displays substitute an array of data lines and scan lines for columns and rows. These data and scan lines address a switching element called a thin-film transistor. Voltage on the scan lines causes the switch connecting the TFT to the data line on each pixel, allowing the voltage at data lines to be applied to the pixels. This may sound terribly technical, but what it basically means is that the voltage to each pixel remains constant for the entire scan cycle, as if the pixels were being addressed individually. Refresh rates for active-matrix (also known as TFT) displays become immaterial -there is no potential for flicker.

What does this mean from an economic standpoint? Flickering displays cause eye strain and headaches. Either to avoid these effects or as a consequence of these effects, users cannot look at CRT monitors for an extended period of time. Productivity is lost, either because of frequent breaks or because of absenteeism.

Several manufacturers are producing LCD monitors in wide-screen panoramic formats, not merely to cash in on the DVD craze. Silicon Graphics claims, in a white paper, that these monitors create "a machine interface that more closely allies with the way the human cerebral cortex 'reads' information". The company claims that having access to extra information laid out horizontally can reduce fatigue because taking in complex vertical information requires movement of the head, while horizontal reading only requires movement of the eyes - much more efficient.

Whether you accept that or not, wide-screen LCDs are becoming more commonplace. The reason for this is really that LCDs are not restricted in their aspect ratio the way CRTs are. They don't require an electron gun to be able to scan across the entire surface in a flash, and therefore they can extend horizontally or vertically - to display an entire A4 page of text at once, for example. Without knowing anything about the human cerebral cortex, you can see that displaying an entire page of text at once requires less clicking and scrolling, and this is obviously more efficient. Several manufacturers make monitors that can switch from a horizontal panoramic format to a vertical portrait format easily.

If your office is prone to frequent reshuffles, you might also consider the ease of moving a flat-panel display (typically a one-person job, even for a reasonably big screen) versus the Herculean task of shifting a CRT. Requiring two or even three people to help shift a monitor takes more people away from actual work. And, once you've placed an LCD where it needs to go, it works straight off. CRTs are notable for their sensitivity to interference from magnetic fields, and the electron guns are picky about being bumped. Moving a CRT into the next room can require a service call to get it right - that's money down the drain.

It is possible, of course, to reduce the flicker of CRTs by running them at higher refresh rates (this doesn't work for all of them). The higher refresh rates, obviously, require more power to run. In addition, they cause the monitors to generate even more heat. Compensating for the extra heat means running the air conditioning system in offices more heavily, and those costs build up.

Power trip

LCDs generate almost no heat at all, and their power consumption, according to various manufacturers' estimates are between 60 and 80 per cent lower than for CRTs.

If that doesn't mean much in itself, consider this: in the case of a blackout, would you rather have your uninterruptible power supplies feeding power-hungry CRTs, or would you rather get a few extra minutes for emergency saves thanks to your trusty flat panels?

Flat panel displays may be sounding like the answer to your every dream, but (as they say) every silver lining has a cloud around it. The cloud over flat-panel displays is the lack of any genuine standard for digital-to-digital display interfaces.

Flat panels are, by their nature, digital. Since the earliest applications of flat panels were in notebook computers, transferring the digital graphics information from the computer to the built-in display was not an issue - the digital information within the graphics frame buffer is simply sent, row by row and column by column, to the frame buffer in the display. There is, as I have already said, a one-to-one correspondence between the pixels generated by the computer and the pixels the monitor can display. However, when you want to connect a flat-panel display to a desktop computer, which tends to have an analog (VGA) display connector, this presents problems.

Signal loss

In a purely digital interface (as with notebooks, for instance), red, green and blue information for each pixel is transferred from the frame buffer in the computer's graphics subsystem to the frame buffer in the display. A "pixel clock" is used to ensure that the information for each pixel on one side if the transfer remains in perfect sync with the information for that pixel on the other end of the equation.

Before it can be sent across the analog interface, the digital information must obviously be converted to a serial analog stream. As with any conversion process, certain information is lost in the translation. The key part of the signal that is lost (because it is simply not supported by VGA) is the pixel clock. CRT monitors, since they are not comprised of pixels that can be individually addressed, don't require this information - they simply produce an approximate "guess" that looks good enough once it's been blurred by the shadow mask.

Flat-panel displays, however, cannot get away with such approximation. Once the analog stream reaches the display, the frame buffer within the flat panel attempts to reconvert the information to digital, calculating the pixel clock information based on other factors in the analog stream.

This calculation is prone to errors, meaning that the red, green and blue information for individual pixels can become out of sync. The error is small, but on monitors with high horizontal resolution (over 1024 pixels wide) the cumulative effect can lead to a distorted image at the right of the screen, and (worst of all) flickering.

The solution, obviously, is to purchase a flat-panel display with a digital interface.

Unfortunately, there currently exists no standard for these interfaces, so the various manufacturers offer proprietary and (generally) incompatible solutions. Your choice of monitor ends up dictating your choice of graphics card or even CPU. This is obviously a problem for organisations not wishing to be locked into buying from a single supplier.

Several different standards have been proposed by the Video Equipment Standards Association (VESA), as well as the Digital Flat Panel Group and the Visual Interface Consortium, International (VICI). In Japan, the Japan Electronics Industry Display Organisation (JEIDA) is working on its own standard. The barrier for all is that there are well over half a dozen different protocols in use, and within each protocol there is disagreement over factors such as connectors, pinouts and cables.

Without going into too much of the technical details of each proposal, suffice it to say that each presents compelling economic and technological advantages.

A clear winner may be a year or more away. Obviously, if the flicker-free, crystal-clear image quality of flatpanels is what compels you to buy, you'll want to avoid the VGA compromise. This leaves you with two choices: leap in now, ignoring the advantages of standardisation, or wait a while and see what standard is accepted.

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