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TV Technology Demystified

CRT, Plasma, LCD, DLP Television, and OLED Technologies Overview

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Buying a TV can be very confusing these days, especially with several different types of TV technology to choose from. Gone are the bulky CRT (picture tube) sets that dominated living rooms in the second half of the 20th century. Now that we are well into 21st century, the long awaited wall-mountable TV is now common.

However, I get a lot of questions as to how newer TV technologies (Plasma, LCD, DLP, an OLED) actually work to produce images. Here is an overview that should shed some light on the difference between CRT and current TV technologies.

CRT Technology

CRT televisions employ an electron beam scans rows of phosphors on a line-by-line basis in order to produce an image. The electron beam originates from the neck of a picture tube (essentially a large vacuum tube). The beam is deflected on a continuous basis that so it moves across the rows of phosphors in a sequential manner.

Depending on the type of signal, the phosphor rows can be scanned alternately, which is referred to as interlaced scanning or sequentially, which is referred to as progressive scan. For more details on the difference between interlaced and progressive scanning, check out my article: Progressive Scan - What You Need To Know

Plasma Technology

Plasma televisions, on the other hand, although employing phosphors similar to a CRT, the phosphors are not lit by a scanning electron beam. Instead the phosphors in a Plasma television are lit by superheated charged gas (similar to a Fluorescent light). All the phosphor picture elements (pixels) can be lit at once, rather than having to be scanned by an electron beam as is the case with CRTs. Also, since a scanning electron beam is not necessary, the need for a bulky picture tube (CRT) is eliminated, resulting in a thin cabinet profile.

For more technical details, check out How Plasma TV Works (How Stuff Works).

LCD Technology

Taking another approach, which also results in a thin cabinet profile, unlike a traditional CRT televisions, the images on an LCD television are also not "scanned" by an electron beam. The picture elements (pixels) of an LCD Television are merely turned off or on at a specific refresh rate.

In other words, the entire image is displayed (or refreshed) all at once every 24th, 30th, 60th, or 120th of a second. Actually, with LCD you can engineer refresh rates of 24, 25, 30, 50, 60, 72, 100, 120, 240, or 480 (so far). However, the most commonly used refresh rates used in LCD TVs is 60 or 120. Keep in mind that refresh rate is not the same as frame rate.

For more specifics on what refresh rate is, how it works, and how it is different that frame rate, check out my article: Video Frame Rate vs Screen Refresh Rate.

It must also be noted that LCD pixels do not produce there own light. In order for an LCD television to produce a visible image the LCD's pixels have to be "backlit". The backlight, in most cases is constant. What happens in this process is that the pixels are rapidly turned on and off depending on the requirements of the image. If the pixels are off, they don't let the backlight through, when they are on, they let the backlight through.

For a more technical look at how this process works, check out: How LCD Works (How Stuff Works).

It is important to note that there are new backlight technologies which enhance the pixel on/off process, such as Global Dimming and Local Dimming. These dimming technologies employ an LED-based backlight (either full array or edge light system) rather than traditional Fluorescent backlighting.

Global Dimming can vary the amount of backlight hitting all of the pixels for dark or bright scenes, while Local Dimming (not applicable to edge light systems) is designed to hit specific groups of pixels depending on which areas of the image need to be darker or lighter than the rest of the image. For a detailed look at Local Dimming and LED use in LCD TVs, check out an informative article from Home Theater Magazine.

DLP Technology

Still another technology used in televisions (rear projection televisions, that is) is DLP (Digital Light Processing), invented, developed, and licensed by Texas Instruments.

The key to DLP is the DMD (Digital Micro-mirror Device), in which every chip is made up of tiny tiltable mirrors. This means that every pixel on a DMD chip is a reflective mirror.

The video image is displayed on the DMD chip. The micromirrors on the chip (remember: each micromirror represents one pixel) then tilt very rapidly as the image changes.

This process produces the grayscale foundation for the image. Then, color is added as light passes through a high-speed color wheel and is reflected off of the micromirrors on the DLP chip as they rapidly tilt towards or away from the light source. The degree of tilt of each micromirror coupled with the rapidly spinning color wheel determines the color structure of the projected image. As the amplified light bounces off the micromirrors, it is sent through the lens, reflected off a large single mirror, and onto the screen.

For further technical explanations on DLP, check out my article: Rear Projection Televisions: DLP as well as the Texas Instruments DLP Website.

However, it must be pointed out that DLP technology, while still being used in video projectors, is not longer being used in TVs as Rear Projection TVs, as product class, has been retired (read my report). This means that while there are still many DLP TVs in use, they are no longer being produced for the consumer market.

OLED

OLED is the newest TV technology available for consumers. It has been used in cell phones, tablets, and other small screen applications for a while - but beginning in 2013 it has been successfully applied to large screen consumer TV applications.

OLED stands for Organic Light Emitting Diode. To keep it simple, the screen is made of pixel-sized, organically-based elements (no, they are not actually alive). OLED has some of the characteristics of both LCD and Plasma TVs.

What OLED has in common with LCD is that OLED can be laid out in very thin layers, enabling thin TV frame design and energy efficient power consumption. However, just like LCD, OLED is subject to dead pixel defects.

What OLED has in common with Plasma is that the pixels are self-emitting (no backlight, no edgelight, or local dimming is required), very deep black levels can be produced (in fact OLED can produce absolute black), provides a wide undistorted viewing angle, and compares well in terms of smooth motion response. However, like Plasma, OLED is subject to burn-in.

Also, indications are that OLED screens have a shorter lifespan than LCD or Plasma - especially in the blue part of the color spectrum. In addition, current OlED panel production costs for the large screen sizes need for TVs are very high in comparison to all other existing TV technologies.

However, going with both the positives and the negatives, OLED is capable of displaying the best images seen so far in a TV technology. Also, one stand out physical characteristic of OLED TV technology is that the panels are so thin that they can be made flexible - resulting in the manufacturing of curved screen TVs.

OLED technology can be implemented in several ways for TVs. So far, LG employs a process, referred as WRGB, with combines white OLED self-emitting sub pixels with Red, Green, and Blue color filters, whereas Samsung employs Red, Green, and Blue sub pixels with no added color filters. LG's approach is intended to limit the effect of premature Blue color degradation that seems to occur with Blue self-emitting OLED pixels.

For additional facts and perspective on the core aspects of OLED technology, read the overview from About.com Components.

Fixed Pixel Displays

Despite the differences between Plasma, LCD, DLP, and OLED televisions, there is one thing they share in common.

Plasma, LCD, DLP, and OLED TVs have a finite number of screen pixels, thus they are "fixed-pixel" displays. Input signals that have higher resolutions must be scaled to fit the pixel field count of the particular Plasma, LCD, DLP, or OLED display. For example, a typical 1080i HDTV broadcast signal needs a native display of 1920x1080 pixels for a one-to-one point display of the HDTV image.

However, since Plasma, LCD, DLP, and OLED televisions can only display progressive images, 1080i source signals are always either deinterlaced to 1080p for display on a 1080p TV, or deinterlaced and scaled down to 768p, 720p, or 480p depending on the native pixel resolution of the specific TV. Technically, there is no such thing as a 1080i LCD, Plasma, DLP, or OLED TV.

Digging Deeper

If this overview has wetted your appetite for more details on TV tech, read Display Myths Shattered: How Monitor & HDTV Companies Cook Their Specs posted by Maximum PC.

Also, as a supplement to this article, watch our Home Theater Video Tip: Proper Distance for Viewing Different TV Sizes.

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