But how do new televisions work? This overview should shed some light on the difference between past and current TV technologies.
CRT Technology
Although you can’t find new CRT TVs on store shelves anymore, a lot of those old sets are still operating in households. CRT stands for “cathode ray tube,” which is essentially a large vacuum tube, which is why CRT TVs are so big and heavy. To display images, a CRT TV employs an electron beam that scans rows of phosphors, line by line, to produce an image. The electron beam originates from the neck of a picture tube. The beam is deflected on a continuous basis that so it moves across lines of phosphors in a left-to-right fashion, moving down to the next needed line. This action is done so rapidly that the viewer is able to see what appears to be moving images. Depending on the type of incoming video signal, the phosphor lines can be scanned alternately, which is referred to as interlaced scanning, or sequentially, which is referred to as progressive scan.
DLP Technology
Another technology, used in rear-projection televisions, is digital light processing (DLP). This technology was invented, developed, and licensed by Texas Instruments. Although no longer available for sale in TVs, DLP technology is alive and well in video projectors. The key to DLP technology is the digital micro-mirror device (DMD), a chip made up of tiny tilted mirrors. The mirrors are referred to by their more common name, pixels. Every pixel on a DMD chip is a reflective mirror so small that millions of them can be placed on a chip. The video image is displayed on the DMD chip. The micro-mirrors on the chip tilt rapidly as the image changes. This process produces the gray-scale foundation for the image. The color is then added as light passes through a high-speed color wheel and reflects off the micro-mirrors on the DLP chip, rapidly tilting toward or away from the light source. The degree of tilt in each micro-mirror coupled with the rapidly spinning color wheel determines the color of the projected image. As it bounces off the micromirrors, the amplified light is sent through the lens, reflected off a large single mirror, and onto the screen.
Plasma Technology
Plasma TVs, the first TVs to have a thin, flat, “hang-on-wall” form factor, have been in use since the earlier 2000s. In late 2014, the last remaining plasma TV makers (Panasonic, Samsung, and LG) discontinued their manufacture. However, many are still in use, and you may still be able to find one refurbished, used, or on clearance. Plasma TVs employ a unique technology. Similar to a CRT TV, a plasma TV produces images by lighting phosphors. However, the phosphors are not lit by a scanning electron beam. Instead, the phosphors in a plasma TV are lit by superheated charged gas, similar to fluorescent light. All the phosphor picture elements (pixels) can be lit at once, rather than having to be scanned by an electron beam. 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.
LCD Technology
Taking another approach, LCD TVs also have a thin cabinet profile like a plasma TV. They are also the most common type of TV available. However, instead of lighting up phosphors, the pixels are merely turned off or on at a specific refresh rate. In other words, the entire image is displayed (or refreshed) 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 in LCD TVs is 60 or 120. Keep in mind that the refresh rate is not the same as frame rate. It must also be noted that LCD pixels do not produce their own light. In order for an LCD TV to display a visible image, the LCD’s pixels have to be “backlit.” The backlight, in most cases, is constant. In this process, 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, the backlight comes through. The backlight system for an LCD TV can be either fluorescent (CCFL or HCL) or LED. There are also technologies used in conjunction with the backlight, such as global dimming and local dimming. These dimming technologies employ a LED-based full array or edge backlight system. Global dimming can vary the amount of backlight hitting all of the pixels for dark or bright scenes, while local dimming 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. In addition to backlighting and dimming, another technology is employed on select LCD TVs to enhance color: quantum dots. These are specially “grown” nanoparticles that are sensitive to specific colors. Quantum dots are either placed along the LCD TV screen edges or on a film layer between the backlight and the LCD pixels. Samsung refers to their quantum-dot-equipped TVs as QLED TVs: Q for quantum dots, and LED for LED backlight.
OLED Technology
OLED is the newest TV technology available. It has been used in phones, tablets, and other small-screen applications for a while, but since 2013 it has been successfully applied to large-screen televisions. Manufacturers such as Samsung, Sony, Vizio and more all make televisions with OLED technology. OLED stands for organic light-emitting diode. To keep it simple, the screen is made of pixel-sized, organically based elements. OLED has some 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 TVs are subject to dead pixel defects. What OLED has in common with plasma is that the pixels are self-emitting — no backlight, edge-light, or local dimming is required. That makes for very deep black levels. In fact, OLED can produce absolute black. It can also provide a wide, undistorted viewing angle with smooth motion response. However, like plasma, OLED is subject to burn-in. There are indications that OLED screens have a shorter lifespan than LCD or plasma, especially in the blue part of the color spectrum. Current OLED panel production costs for large screens are very high in comparison to other existing TV technologies. However, OLED is considered by many to provide the best image of any TV technology. One stand-out characteristic of OLED 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 different ways, but a process developed by LG is the most common one. It is referred to as WRGB, and it combines white OLED self-emitting subpixels with red, green, and blue 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.
Fixed-Pixel Displays
Despite the differences between plasma, LCD, DLP, and OLED televisions, they all share one thing in common. They all have a finite number of screen pixels, which means 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 display that can produce an image 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 de-interlaced to 1080p for display on a 1080p TV, or de-interlaced and scaled down to 768p, 720p, or 480p, depending on the default pixel resolution of the TV. Technically, there is no such thing as a 1080i LCD, plasma, DLP, or OLED TV.
The Bottom Line
When it comes to putting a moving image on a TV screen, a lot of technology is involved, and each technology has its advantages and disadvantages. However, the quest has always been to make that technology “invisible” to the viewer. While you want to be familiar with the basics, which type of technology you should get almost always comes down to size, space, and price.
