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Geek 101: LCD and plasma TV basics

If you've shopped for an HDTV, you've probably seen an array of display-technology terms bandied about in the fine-print specs listings.  Here's what you need to know about them.
Image: California State Energy Commission Votes On Banning Big Screen TV's
Buying a new TV can almost be as complex as buying a new car, with lots of terminology to consider.Justin Sullivan / Getty Images file
/ Source: PC World

If you've gone shopping for a new PC or HDTV, you've probably seen a dizzying array of display-technology terms bandied about in the fine-print specs listings on store shelves. You may have asked yourself, "What do these terms mean? Does any of it really matter?" We hear you. Here's our no-nonsense guide to what these specs mean, what you need to know about them, and what you should be looking out for.

LCD and plasma are the most common types of displays you'll see on the market. Plasma is predominantly used for HDTVs, while LCDs are common in both TVs and computer monitors. Several other interesting technologies are on the horizon, as well.

Some of the most important factors contributing to a display's quality are its color depth (how accurately the screen can reproduce colors), its viewing angle (whether any color shifting occurs when you view the screen from the sides), and its motion processing (how well the screen can handle fast-action scenes). We'll look at all of those topics in this primer.

Plasma vs. LCD
Until fairly recently, plasma was the preferred technology for HDTVs; it has since been overtaken by LCD. Plasma technology has several advantages and disadvantages in comparison to LCD.

Plasma sets have claimed a smaller percentage of HDTV sales lately. Some vendors, such as Samsung, make both LCD and plasma HDTVs, and Panasonic sells plasmas exclusively, but most manufacturers have ditched plasma technology. Pioneer discontinued its plasma HDTV line last year.

The biggest drawback to plasma is its power usage. Plasma HDTVs generally consume more power than comparable LCD TVs do, which of course means that you'll have to pay more on your electric bill with a plasma model than with an LCD set.

Plasmas are also more susceptible to screen burn-in and image ghosting than LCDs are, although that is less of a problem with current plasmas than it was with older models.

That said, there are real reasons to consider buying a plasma instead of an LCD, especially if you value image quality. Plasma displays do a much better job of handling dark scenes, and they have better viewing angles than most LCDs do.

Plasma HDTVs have historically handled fast motion more smoothly than LCDs have, too. In fact, it hasn't been until the past couple of years — with the advent of LED backlighting and faster refresh rates — that LCDs have become competitve with plasmas in this respect.

Liquid crystal display — LCD for short — is one of the most common screen types you'll find in a PC or TV.

LCD screens employ several different technologies, with twisted nematic (TN) and in-plane switching (IPS) being a couple of the most prevalent. Other higher-end LCD types include MVA (Multi-domain Vertical Alignment) and PVA (Patterned Vertical Alignment).

The differences among the display types tend to be arcane (having to do with how the liquid crystals inside the displays are structured), but there is one key difference that most people should care about: A TN LCD panel will generally have narrower viewing angles than an equivalent IPS, MVA, or PVA panel. Wider viewing angles mean less color shifting when you look at the screen from the side. HDTVs normally use the higher-performing IPS, MVA, and PVA technologies.

Color depth
Some LCDs are 6-bit panels, which are capable of displaying approximately 65,000 colors; others are 8-bit, and can display over 16 million colors. On the very high end, there are 10-bit LCDs that can display over a billion colors. A 6-bit LCD can mimic 8-bit color to some extent by using a technique called "dithering." This approach tries to approximate the true color of an image by using combinations of colors that the screen is capable of displaying.

For most computer users, a 6-bit display may not be ideal, but if all you're doing is Web browsing and word processing, it isn't the end of the world. On the other hand, if your work requires high color accuracy — professional photo or video editing, for example — you'll want to make sure that you get at least an 8-bit screen. HDTVs normally use 8-bit or better screens, so this is less of a concern for HDTVs.

The other big differentiator among LCDs is the backlight. Traditionally, LCDs used a cold-cathode fluorescent lamp (CCFL) backlight, a setup that basically uses a fluorescent tube to illuminate your screen. The biggest drawback to CCFLs is that over time the backlight dims somewhat, so as your HDTV or computer monitor ages, it'll become darker and less vibrant.

LED (light-emitting diode) backlighting fixes that particular issue, and offers some added benefits. LED backlighting won't dim as it ages; and unlike CCFL backlighting, which takes a few moments to warm up to full brightness when you turn on the screen, LED backlighting is at its full brightness from the moment you switch it on. LED backlighting is also more energy efficient, making it ideal for laptops and other portable devices such as smartphones or tablets. Market research firm DisplaySearch predicts that over half of the HDTVs sold in 2011 will ship with some form of LED backlighting.

You can find two variations of LED backlighting. Edge-lit LED backlighting, as its name suggests, places the LEDs along the edge of the display, whereas in full-array LED backlighting, the LEDs sit behind the panel itself.

Typically, full-array LED backlighting can do a useful additional trick: It can turn off the backlight in darker areas of a scene to improve the display's contrast — a technique known as local-area dimming.

That said, local-area dimming is coming to some edge-lit LCDs as well. At this year's Consumer Electronics Show, Samsung announced edge-lit LED-backlit HDTVs that also have the local-area dimming function.

Refresh rate
A display's refresh rate describes how quickly it can show a new video frame. The rate is usually expressed in hertz (Hz), which effectively means how frequently the screen refreshes every second (for instance, a 60Hz screen refreshes itself 60 times per second). LCDs generally come with 60Hz, 120Hz, or 240Hz refresh rates.

The refresh rate is an important indicator of how well a display can handle fast motion — an important thing to consider if you watch sports or action movies. In general, the higher the refresh rate, the smoother the motion. This is more of a consideration for HDTVs, but some 120Hz computer monitors are on the market too.

The refresh rate's impact on display performance is a controversial issue in home theater circles. In PCWorld Labs tests, we found that, in general, 120Hz is currently the sweet spot for HDTVs; you'll get smoother motion than you would with a 60Hz set, and the prices for 120Hz HDTVs are coming down. The jump between 120Hz and 240Hz wasn't as drastic in our testing, so getting a 240Hz HDTV may not be worth the extra cost.

Keep in mind that a higher refresh rate will not necessarily guarantee improved motion performance. We saw some 120Hz HDTVs fall flat in our motion tests, so other factors — such as the electronics behind the screen — may be in play. See Melissa J. Perenson's story "HDTV Motion: The 120Hz Difference" for more on our testing.

Your best bet, of course, is to look at HDTVs in person before you buy one.

Contrast ratios are meaningless
Many manufacturers make a big deal about contrast ratio, which is supposed to specify how wide a range of light shades and dark shades an HDTV or monitor can display. Overall, however, this is a meaningless measure of a screen's quality. Currently there's no standard way of measuring it, which explains to some extent why one company might list its products as having contrast ratios in the 20,000:1 range, while another may brag about ratios of 1,000,000:1.

Until the electronics industry settles on one way of measuring and expressing contrast ratio, you should probably ignore this metric altogether. Instead, judge a screen using your own eyes. You'll want to look for screens that have dark, inky blacks, and avoid ones whose blacks more resemble gray.

3-D displays generally work by showing two video streams at one time: one for the left eye, and one for the right. The frames from these video signals are interleaved, so you need to wear glasses to filter out the two signals (that is, the shutter in the left lens will filter out the image meant for the right eye, and vice versa).

Both the display itself and the underlying electronics play a role in 3-D technology. A 3-D HDTV or monitor needs a fast refresh rate (120Hz or faster), but it doesn't matter whether the screen is LCD or plasma. In the case of polarized 3-D displays, a film placed over the screen facilitates the 3-D effect (you'll still need glasses to filter out the two video signals, though). And since a 3-D TV requires two video signals for the 3D effect, it needs two HD tuners.

An upcoming 3-D technology is "autostereoscopic" 3-D — in other words, displays that don't require any glasses. Autostereoscopic 3-D is coming in the not-too-distant future for handheld gadgets such as smartphones and the Nintendo 3DS, but it will be a while before you can get an autostereoscopic 3-D TV.

For more on 3-D HDTV, see our earlier Geek 101 story on the topic.

What's next: OLED
Organic light-emitting diode (OLED) displays are a relatively new technology; they emerged first on mobile devices and are now making their way into PC monitors and televisions. Unlike LCD, OLED requires no backlight — the individual pixels light themselves. This allows OLED displays to be very thin.

At the moment, OLED remains impractical on larger devices, mainly due to manufacturing costs. For example, LG's upcoming 15-inch OLED TV is expected to cost 2,000 euros, or roughly $2,500 at current exchange rates. This makes OLED TV sets more of a proof-of-concept for the time being.

Life span has been a concern for OLEDs. Paul Semenza, an analyst with market research firm DisplaySearch, tells us that two problems exist: the degradation of the screen as a whole, and the life span of the individual color pixels. If one color pixel has a shorter life span than another, it'll result in inaccurate color as the screen ages. According to prior reports, the blue pixels on OLED displays may die out relatively quickly, though engineers have been hard at work to correct the issue. Semenza says that OLED life span has improved over the past few years, but that a lot depends on the manufacturing process.

For now, an OLED screen should work fine for a cell phone that you'll own for a couple of years and then replace, but it's hard to say how an OLED screen will hold up to ten years of use, as many monitors and TVs are expected to do.

Also, Semenza says that making bigger OLED screens in large amounts will require new factories and new manufacturing methods, so don't hold your breath waiting for a 40-inch OLED HDTV.

Despite those issues, OLED is still a technology worth following and looking forward to, even outside the living room. Last year, Sony demonstrated a flexible OLED display, which could one day be used in all sorts of gadgets.

For more technical details on flat-panel display technologies, see PCTechGuide's overview. And for more on the future of 3D TV, see David Ayala's 3-D HDTV overview.