William Wong
Electronic Design
Display technologies have always competed with each other, but there has never been such a wide variety of approaches. For example, many displays now incorporate touch technology, expanding the possible number of solutions.
The challenge lies in delivering the best viewing experience while meeting a range of requirements that depend on the application. Today’s displays must offer great color, high resolution, and fast refresh rates. They also have to be rugged, thin, and inexpensive. Size and viewing distance come into play as well. And, they can’t consume that much power.
On The Market
Apple’s iPad led the way to high quality (Fig. 1). With a resolution of 264 pixels/in., its Retina display provides non-pixelated images, assuming 20/20 vision and a viewing distance of at least 15 in. or a tablet’s typical viewing distance. Apple’s iPhone 5 has a higher resolution of 326 pixels/in. with a typical viewing distance of only 10 in.
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| Figure 1. | Apple’s latest iPad (right) leads in the use of high-definition displays in mobile devices, requiring a closer look to see the difference between its Retina display and its predecessor (left). |
A measurement that takes viewing distance into account is pixels per degree (PPD). The retina display starts at about 53 PPD. The actual resolution of the screen becomes less of an issue for mobile devices once this limit is reached.
On the other hand, large displays tend to be limited by resolution and standards. HDTV 720p (1280 by 720 pixels) and 1080p (1920 by 1080 pixels) displays now dominate. The p stands for progressive scan versus the interleaved scan utilized by old-style televisions and some HDTVs. In the past, computer monitors with resolutions higher than television were common. These days they tend to be the same with high-resolution monitors being the exception rather than the rule.
The next step up will be 4K Ultra High Def (UHD or Ultra HD) with a resolution of 3840 by 2160 pixels, which is twice the resolution of 1080 (Fig. 2). Double 4K and you get 8K UHD. The 7680- by 4320-pixel 8K Ultra HD is great for displaying images from a 33.2-Mpixel camera.
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| Figure 2. | 4K Ultra HD more than doubles the resolution of today’s 1080p displays. |
The problem for 4K and 8K in the consumer space will be content delivery, but that’s another discussion. Keep in mind that the RED EPIC digital camera that’s used to shoot many of the latest films has a DCI 4K output with 4096- by 2160-pixel resolution (see “Prometheus Takes Flight With Cutting-Edge VFX Technology”).
Display Technologies
LCDs have become the dominant display technology, though there are challengers like organic LEDs (OLEDs). LCDs come in a variety of forms, but they require a light source. Reflective LCDs use an external source like the sun. Transreflective displays reflect and transmit light so they can employ backlighting technology. Reflective and transreflective displays are useful in sunlight where a backlight source would be washed out. Backlighting allows viewing in the dark and provides better control of the display quality.
Most mobile and HDTV LCD displays are backlit. There are several ways to do this, but basically a light source is placed behind the display. Cold cathode fluorescent (CCFL) lighting used to be common, though it has been replaced by LED backlighting. LEDs are more expensive, but they are more reliable. They also provide improved control and a wider color gamut. And, they are very small.
The LEDs normally are placed behind the display or on the edge (Fig. 3). The different approaches are used based on the thickness of the display. Edge-mounted LEDs allow a very thin display, and this approach is often used on smaller displays. Larger displays that are thicker allow placement of the LEDs behind the display.
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| Figure 3. | Backlit LCDs are typically lit using an array of LEDs behind the display (top) or from the side (bottom). |
The simplest implement keeps the backlight on at a consistent level. This works for either approach. The other alternative is dynamic backlighting, also called local dimming. In this case the LEDs are individually controlled and changed based on what is being displayed. The LED intensity affects an area unlike other displays such as OLEDs where individual pixel control is possible.
This is a challenge because the display controller needs to account for lighting from both the LED backlight perspective and the LCD perspective since the LEDs affect overlapping regions of the screen. Likewise, the controller needs to account for the speed at which the LCD and the LEDs can change. Dynamic backlighting is possible regardless of the LED placement.
Dynamic backlighting reduces power consumption and improves color quality because it allows darker areas on the screen. However, it requires significantly greater controller complexity and produces visual artifacts like halos around bright objects. The type and impact of artifacts depends on the quality of the control algorithm, the hardware, and the image being displayed. The worst case would be a bright star field on a black background.
LCDs have been getting thinner and lighter while using less power. They have also been getting larger, with displays over 80 in. readily available.
OLED displays provide their own light. Each pixel is individually controlled, and they provide brighter, higher-contrast displays compared to conventional LCDs. OLEDs come in passive matrix (PMOLED) or active matrix (AMOLED) versions. Samsung’s Galaxy S4 will ship with a 5-in. AMOLED 1080p display (Fig. 4).
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| Figure 4. | Samsung’s Galaxy S4 will utilize a 5-in. AMOLED display. |
OLEDs are even more power-efficient than LED LCDs. Plus, they have a wider field of view that exceeds 165°. They are brighter with higher contrast than LCDs, which are designed to block light. They’re used in small, battery operated devices like smart phones because they’re brighter and require less power.
Also, OLEDs can be used to make thin, flexible displays. Samsung demonstrated large and small flexible displays at January’s International CES in Las Vegas (Fig. 5). The displays are not available yet, but they will offer some interesting design options.
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| Figure 5. | Samsung demonstrated flexible displays at January’s International CES in Las Vegas. |
OLED displays can be built using three methods. Vacuum deposition or vacuum thermal evaporation (VTE) is one method, but it is expensive and inefficient. Organic vapor phase deposition (OVPD) also has been used. It generates a thin film using a low-pressure, hot-wall reaction chamber with a carrier gas to deliver evaporated organic molecules onto a cool substrate. Finally, OLED displays can be built inexpensively using inkjet technology, enabling the creation of very large displays. Sharp’s IGZO (indium gallium zinc oxide) OLED displays range from 4 to 32 inches. The company demonstrated a 32-in. 4K IGZO display at January’s International CES. Like OLEDs, IGZOs are more energy efficient than LCDs, have a faster switching speed, and deliver brighter, higher-contrast images.
Sharp’s thin-film transistor (TFT) technology employs a C-axis aligned crystal (CAAC) structure (Fig. 6). Normally, a crystal is aligned in the A, B, and C axis. The CAAC structures are only aligned in the C axis, allowing the crystals to be employed in films. A crystal typically would have a cubic structure. The technology may also have applications outside of the display realm.
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| Figure 6. | Sharp’s planar C-axis aligned crystal (CAAC) structure can be used in films. |
IGZO has some characteristics found in older CRT displays. The on-screen data is maintained for a short period of time even when power isn’t provided. This is different from but functionally similar to the CRT phosphors, helping to reduce power consumption. And when mixed with a touch interface, IGZO displays drastically minimize the noise related to touch input for a more accurate touch response.
Touching The Display
Touch is critical to the smart-phone and tablet market, with digital stylus activity taking off as well. A thinner display is useful for mobile devices, but accurate touch and stylus responses are key. Reduced noise from the display greatly improves accuracy and speed, providing a much better user experience.
Microsoft’s Surface tablet is one place where displays, touch, and stylus are coming together. The Surface uses a conventional LCD along with Windows 8 and an Intel Core i5 processor. The Surface Pro and Surface RT platforms compete with Apple’s iPad and the array of Android-based tablets like Amazon’s Kindle Fire (see “Will Apple Take On The Kindle Fire And Nexus 7 Tablets?”).
Capacitive touch systems dominate the smaller end of the display spectrum from smart phones through tablets and all-in-one PCs. They mesh well with LCD and OLED technologies. The Kindle Touch and a few other e-readers used infrared-based sensors, but they have been eliminated by the falling costs of capacitive systems.
Resistive touch systems are used on displays up to 17 in. The technology tends to be used where capacitive touch systems won’t always work such as with gloved fingers. Fujitsu’s Feather Touch technology addresses two of the issues that had prevented resistive touch systems from being selected: pressure and multitouch support.
The four-wire system reacts like capacitive systems instead of requiring a significant amount of touch pressure. It can also handle two-finger gestures like pinch and zoom. Resistive touch systems fit industrial, medical, and other demanding application areas.
At the high end, there are platforms like MyMultitouch’s $43,100 Alvero (Fig. 7). The 84-in., 4K Ultra HD display has a multitouch interface that can be tickled by up to 32 fingers at one time. It likely will find a home in museums, high-tech lobbies, or maybe the latest police TV show. It plugs into a 4K Ultra HD display card and has Windows 8 multitouch support. It can be mounted on the wall or as a table. The multitouch system is infrared-based.
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| Figure 7. | MyMultitouch’s Alvero 84-in., 4K UHD multitouch display is likely to find a home in museums or other high-traffic areas where people need to interact with high-res displays. |
The Next Generation
Transparent displays and 3D displays also are emerging. Planar is delivering transparent displays like the 32-in. open-frame LT3200, and Crystal Display Systems offers Samsung-based transparent displays up to 70 in.
Transparent displays may look great on TV shows, but their primary use now is in retail merchandising, trade shows, points-of-sale, and museums. They are expensive, and brightness and transparency make them challenging for general use. They’re designed to show what’s behind them, though seeing what’s behind the display can be rather distracting.
3D HDTVs using active and passive 3D glasses are generally available. The marginal success of the 3D aspect of these products is really due to the limited 3D content. There also is additional cost because of the glasses, which are getting lighter but can still be annoying. This would be less of an issue if the content were more compelling. 3D content on cable is almost non-existent and usually available at a premium.
3D display technology that doesn’t require glasses has been steadily improving and on display at trade shows for a few years. It may eventually reach the consumer realm, but that is probably a decade away for large screens. It’s more practical on small displays now. Hopefully the content will catch up by the time they are more common.
The Outlook
For now, LCDs will continue to dominate simply because of availability. OLEDs have crossed the horizon and represent a challenge to LCDs, making a designer’s choice more of a challenge since different technologies must be considered. The tradeoffs aren’t always apparent, and each has its strengths and weaknesses.
