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Mailbag (LCD monitors)

Toronto Star Fast Forward column for October 5, 2000

Copyright ©, Myles White, 2000

It has been ages since I answered reader letters. But this note from Bruce Murphy, via e-mail, gives me the perfect excuse.

Bruce writes: I read your latest article in the Toronto Star ("PC buyer's guide flawed," Sept. 21, 2000) and was most interested in the section on monitors. 

Missing from the discussion was LCD monitors on notebooks (probably because the article dealt with desktop systems) and how they compare to CRT models. (Cathode Ray Tube - mw) 

I have three 17" monitors, all of which are high quality. However, when I recently bought a Dell Inspiron 3800 notebook computer over the Internet, I was surprised that the image quality was far less than a CRT. Re-reading the specifications, the notebook has a 14.1" SVGA active-matrix screen. I presumed that the screen would look like a 15" CRT, but the colour depth (despite my fiddling with the settings in the Display properties) looks almost like it is in 256 colour mode (currently it is set for 16 million). Photos don't look as life-like and seem slightly posterized. My sandstone background that I use also looks rough compared to the CRT. 

Thus, my question would be, is there something wrong with the display or video card settings or is it normal for the LCD screen to appear "worse" than a CRT?

Myles replies: It helps if you understand the technology a bit. 

On a CRT, each pixel (picture element, defined as the smallest area of the screen that the computer can change) will contain at least three, but often many more, fluorescent dots on the inside of the monitor face. The distance between the dots (dot pitch on a shadow mask monitor with round holes, stripe pitch on an aperture grille model with a rectangular array), determines whether the picture is sharp or grainy. 

Measured diagonally between two dots of the same colour, the dot-pitch figure ranges from sharp (.26 mm) to grainy (above .28 mm). Measured horizontally between two rectangles of the same colour, an aperture grille monitor will have an effective equivalent of .24 mm or .25 mm. 

Your picture is made up of not only glowing dots (which when lit, appear bigger than they actually are), but also of the spaces between them. Because there are at least three colour sources in each pixel (otherwise we can't change a pixel's colour), but often many more, the separation between pixels is very hard for you to see. There's just enough blurring that your brain fills in the difference. And of course, this will be affected by the resolution (i.e., the number of pixels on screen) and the physical size of the screen.

Now we come to LCD monitors. The pixels on both types of LCD monitor (passive matrix, super-twist nematic, and active matrix, thin-film transistor units) are fixed in number. That number is expressed by the resolution available from the particular model. So, for example, if your screen will output 1024 pixels by 768 rows of them, there are 786,432 pixels available to it. 

Each pixel on an LCD monitor is composed of three liquid crystal cells, corresponding to red, green, and blue. The colour range each pixel can produce depends on the degree to which the liquid crystal can twist to let more or less light through it. If it can twist through a range of eight settings, you can produce 16.7 million colours. 

The main difference between active matrix and passive matrix screens has to do with how the cells get power. In an active matrix screen, each cell gets its own transistor and can be changed quite quickly. Typically, these screens provide fast redraw, bright images, and a wide field of view (you can sit with people on either side of you and all can see the image quite clearly). 

Passive matrix screens have a row of transistors across the top of the screen with a column of them down one side (although some models also have a row through the middle to speed up redraw). To change the image, the power signal has to be sent to the correct intersection in the matrix. Typically, these screens are not as bright, they don't redraw quickly (often, for instance, you'll lose your cursor because it moved too fast), and have a more limited field of view. 

Back to the nubbin of your question: LCD monitors have a pitch measurement, too, but the standards for the measurement are all over the map. Some companies measure the width of each pixel and call that the dot or pixel pitch. Others measure between two cells of the same colour (horizontally) and call the result the dot or pixel pitch. Still others actually measure the distance between pixels and call that the pixel pitch. 

In short, it's very hard for a consumer to figure out what the term means when reading the specs, and that's assuming the manufacturer reveals the information - something that's not always done. Unfortunately for direct marketers such as Dell, this confusion means there is no substitute for eye-balling an LCD screen before you buy it.

 Here's what I suspect is happening with your experience. Screen manufacturers (not necessarily the same company that makes the notebook computer or sells the desktop LCD monitor), don't often vary the size of the cells within each pixel on any given run. The number of pixels is fixed on screen (see above), so if they increase the viewable screen size, the only thing left for them to alter is the horizontal and vertical distance between the pixels (see illustration above). 

So, yes, it is possible that a 14.1-inch LCD screen with a resolution of 1024 by 768 will look grainier than a 12.1-inch screen with the same resolution, and may not appear to be as sharp as a CRT monitor with the same viewable surface area.

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