Engadget Primed: all mobile displays are not created equal

Primed goes in-depth on the technobabble you hear on Engadget every day -- we dig deep into each topic's history and how it benefits our lives. Looking to suggest a piece of technology for us to break down? Drop us a line at primed *at* engadget *dawt* com.

The quality of a mobile phone's display is arguably the most important factor to consider when you establish a relationship with a handset. It's inescapable, really. Whether you're playing a rousing game of Robot Unicorn Attack or (regrettably) drunk-dialing an ex, it's the one interface element that you're consistently interacting with. It's your window to the world and your canvas for creation, and if it's lousy, it's going to negatively influence everything you see and do. Today, we're delving into the world of mobile displays, where we're aiming to entertain and edify, and hopefully save you from making regrettable decisions -- when it comes to purchasing new phones, anyway.

In this edition of Primed, we'll be examining the different qualities and underlying technologies of several displays, starting with the ubiquitous TFT-LCD and moving through the nascent realm of glasses-free 3D and beyond. We'll also be addressing the importance of resolution and pixel density. Finally, we'll be scoping out a handful of upcoming technologies -- while some are thoroughly intriguing, others are just plain wacky. Go ahead... buy the ticket, take the ride, and join us after the break. It's Primed time.

Table of Contents

Introduction
LCD fundamentals
Display technologies explained
A 3D detour
Why resolution matters
The touchscreen
Looking forward
Wrap-up

Introduction

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Generally speaking, two display types rule today's mobile phones: the Liquid Crystal Display (LCD), and the Organic Light-Emitting Diode (OLED). While each technology carries a set of strengths and weaknesses, a very important distinction can be drawn between the two. The LCD uses the light modulating properties of liquid crystals (LCs), but LCs don't emit light directly. As such, a light source is necessary for proper viewing. Conversely, the OLED uses organic compounds that illuminate when exposed to electric currents. As backlights aren't necessary for OLEDs, they're significantly thinner than traditional LCDs. All things equal, OLED phones should be slimmer than their LCD counterparts, but this isn't always the case. Take for example the MEDIAS N-04C, which uses a TFT-LCD and measures 7.7mm thin, versus the Galaxy S II, which uses the latest Super AMOLED Plus display and is 8.5mm thick.

The most desirable phone displays today are variants of these two technologies. In the LCD camp, there's the Super LCD (S-LCD) and the IPS display -- with the latter as the basis for the Retina Display and the NOVA display. Likewise, the OLED territory is filled with options such as Super AMOLED, Super AMOLED Plus and ClearBlack. We'll discuss the important distinctions between these competing display types shortly, but first let's develop a fundamental understanding of how these brilliant creations work and how they came to be.

LCD fundamentals

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The story of the LCD began in 1888 when cholesterol was extracted from carrots. Think we reached too far back? Not if you've ever wondered what liquid crystals are. You see, a botanist named Friedrich Reinitzer discovered this extract had two distinct boiling points and observed the molecule's ability to transmute from liquid to a crystalline structure in the interim. Even more shocking, the cloudy substance was able to reflect circularly polarized light and rotate the light's polarization. (This little tidbit will become important when we discuss how LCDs operate.) While liquid crystals appear throughout nature, it wasn't until 1972 -- when 5CB (4-Cyano-4'-pentylbiphenyl) was synthesized -- that they became commercially viable. A first of its kind, 5CB was chemically stable and entered its nematic phase at room temperature. While there's actually three phases of liquid crystals, we're most interested in the nematic one. This describes a state where molecules flow like liquid and self-align in a thread-like helix -- and coincidentally, are easily manipulated with electricity.

Now that you've got a little background about liquid crystals, let's examine how they're used in LCDs. Let's start by making a sandwich. As our bread, we'll take two polarizing filters, one which polarizes light on the horizontal axis and the other on the vertical axis. If we take the slices of bread and hold them up to a light source, nothing is going to pass through. Remember when we said liquid crystals have the ability to rotate light's polarization? Yeah, they're a critical ingredient in our sandwich because they determine light's passage. When nematic crystals are in their natural (or relaxed) state, they form a twisted helix. As light travels through the molecule structure, its polarization is rotated by 90 degrees and light is allowed to pass through the top filter. Conversely, when voltage is applied to the LCs, the helix is broken and light can't escape the polarizing filters. If you're keeping score, this is known as the twisted nematic field effect. Going back to the sandwich analogy, the nematic crystals are placed between two layers of transparent electrodes which apply voltage to the liquid crystals. It's a rather simplistic sandwich, but it describes the fundamentals of how LCDs work. For you visual learners, Bill Hammack does an excellent job of explaining these concepts in the following video.


Now let's apply this knowledge to the modern TFT-LCD that you're familiar with. It's the basis for twisted nematic (TN) and in-plane switching (IPS) displays, and both technologies rely upon the thin film transistor (TFT) for the quick response time and image clarity that we take for granted. Fundamentally, the TFT is a matrix of capacitors and transistors that address the display pixel by pixel -- although at a blistering speed. Every pixel consists of three sub-pixels -- red, green and blue -- each with its own transistor, and a layer of insulated liquid crystals are sandwiched between conductive indium tin oxide layers. Shades are made possible by delivering a partial charge to the underlying LCs, which controls the amount of light that passes through the polarizing filter, thus regulating the intensity of each sub-pixel.

Display technologies explained

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The most common LCD display is based on TN technology, which has been successful due to its relatively inexpensive production costs and fast refresh rates. Many of you will remember the shadow-trail that plagued early LCDs, and faster refresh rates reduce this effect and make the displays better suited for movies and games. Unfortunately, TN displays are famous for exhibiting poor viewing angles and most aren't capable of showing the entire 24-bit sRGB color gamut. In attempt to mimic the full range of 16.7 million colors, many screens implement a form of dithering to simulate the proper shade. Basic TN screens are hardly fantastic, but they're also good enough to survive the day without eliciting too many complaints.

IPS displays were created to resolve the long-standing problems of poor viewing angles and color reproduction of their TN counterparts. The fundamental difference between the two technologies is that liquid crystals run parallel to the panel rather than perpendicular. This alignment allows for wider viewing angles and more uniform colors, but at a loss of brightness and contrast. Traditionally, IPS panels were significantly more expensive than TN alternatives, but recent advances have lowered the production cost and improved the brightness and contrast issues. This technology is the basis for Apple's Retina Display and the NOVA display -- both of which are manufactured by LG.

Another technology that's gotten plenty of airtime is the Super LCD (S-LCD), which is a display that's manufactured by a joint-venture between Sony and Samsung. It employs an alternate method to IPS and TN that's known as super patterned vertical alignment (S-PVA). Here, the liquid crystals have varying orientations, which help colors remain uniform when viewed from greater angles. S-LCDs also feature improved contrast ratios over traditional TN displays, which exposes a greater amount of details in dark images. Further, these displays feature dual sub-pixels that selectively illuminate based on the brightness of the screen. As you can imagine, this provides power-saving benefits, along with refined control of colors on the screen.

Now, let's take a look at OLEDs, which are a staple of many high-end phones today. As we've mentioned, these displays operate without a backlight. Instead, they use electroluminescent organic compounds that emit light when they're exposed to an electric current. The main advantages of OLEDs include deeper black levels (because there's no backlight), enhanced contrast ratios, and excellent viewing angles, while drawbacks include reduced brightness and colors that are often over-saturated. OLED screens also suffer an awkward aging effect, where the red, green and blue sub-pixels will deteriorate and lose efficiency at different rates, which causes brightness and color consistency to worsen over time. While improvements are being made, it's important to understand that this display technology is still relatively immature.

You're most likely familiar with the active-matrix OLED (AMOLED), which relies on a TFT backplane to switch individual pixels on and off. Coincidentally, active-matrix displays consume significantly less power than their passive-matrix OLED (PMOLED) counterparts, which makes them particularly well-suited for mobile devices. These displays are typically manufactured by printing electroluminescent materials onto a substrate, and that relatively simplistic process suggests that OLEDs will ultimately become cheaper and easier to manufacture than LCDs. Shockingly, the most challenging step is the creation of the substrate itself, which remains a difficult and expensive endeavor. Currently, the limited supply and high demand of AMOLED screens has restricted their availability, and you're most likely to find them in high-end smartphones.

While all screens suffer from reduced visibility in direct sunlight, the original AMOLED screens were particularly vulnerable to this drawback. To resolve this, Samsung introduced the Super AMOLED display. With this new technology, the touch sensors were integrated into the screen itself. Naturally, this allowed for a thinner display, but this also improved brightness by eliminating the extra layer. Additionally, the screen's reflection of ambient light and power consumption were significantly reduced. While colors were now bright and vibrant -- and acceptable in direct sunlight -- the displays still couldn't match the crispness and clarity of LCD screens, particularly with respect to text. Samsung's PenTile matrix is to blame, which is a hallmark of its AMOLED and Super AMOLED displays. Here, a single pixel is composed of two sub-pixels, either red and green, or blue and green, and the green sub-pixel is significantly more narrow than the other two. While the scheme works fine for images because the human eye is more sensitive to green, it makes the anti-aliasing of text rather imprecise, and the end result is a bit blurry. Like Super AMOLED, Nokia's ClearBlack display was created to make the AMOLED screen more visible in direct sunlight. This was accomplished by adding a polarized filter to the display, which allows the viewer to see through the screen's reflection and view the images as they would appear under more ideal conditions.

In its most recent incarnation, the Super AMOLED Plus features a traditional three sub-pixels of equal proportion within one pixel, along with an increased sub-pixel count and density. Both of these measures create a display that's much more crisp, especially when it comes to text. Further, the tighter spacing between pixels results in better visibility under direct sunlight. The new Super AMOLED Plus screens are also thinner and brighter to boot.

A 3D detour

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By now, you've probably had the chance of viewing a glasses-free 3D screen for yourself. Whether you think the feature is cool, gimmicky or annoying -- or, all of the above -- it's clear that autostereoscopic displays are moving into the mainstream. If you've ever wondered what makes this marvel possible, today is your lucky day. First, let's start with stereoscopic imaging itself. This merely refers to a technique that creates an illusion of depth by presenting two offset images separately to the right and left eye of the viewer. Traditionally, glasses were required to complete the effect, but a creation known as the parallax barrier has done away with that. Essentially, it's a layer of material placed atop the screen with precision slits that allows each eye to view a different set of pixels. As you've likely observed (or at least read about), you're required to position the display at a very specific angle to properly view the 3D effect. Also, because the parallax barrier effectively blocks half the light emanating from the screen, the backlight is forced to shine twice as bright -- which really kills the battery. Granted, it's an infant as technology goes, but researchers are already making refinements. For example, MIT's HR3D is a promising project that touts better viewing angles, brightness and battery life -- largely by increasing the number and varying the orientation of the slits.

Why resolution matters

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So far, we've discussed the underlying technologies of mobile displays, but these options are merely one factor for consideration as you select your next phone. Screen resolution is another very important topic, as it determines the amount of content that can be displayed at any given time. Many of you are likely aware of this, but the physical size of a screen conveys nothing about the content that it can display. For example, a 4.5-inch screen with an 800 x 480 resolution actually displays less information than a 3.5-inch screen with a 960 x 640 resolution. These numbers are simply measures of the physical number of pixels positioned vertically and horizontally across the screen. Taking it a step further, the 800 x 480 screen is capable of displaying 384,000 pixels worth of information, while the 960 x 640 screen is capable of displaying 614,400 pixels worth of information. Put simply, a low-res screen simply can't convey the same amount of content as a high-res alternative.

The most common displays today are generally based around the Wide VGA (WVGA, 800 x 480) standard, and lower-res options include Half VGA (HVGA, 480 x 320) and Quarter VGA (QVGA, 320 x 240). Another variation of this is Full Wide VGA (FWVGA, 854 x 480), which is common to Motorola's Droid family. Quarter HD (qHD) is an up-and-comer in the mobile industry, with a 960 x 540 resolution, which is one quarter the pixel count of full 1080 HD (1920 x 1080). Lest we not forget Apple's Retina Display, which measures 960 x 640. As you've seen in our reviews, we're particularly fond of high-res screens, and HVGA really is the minimum that you should accept when purchasing a new phone.

Another component of screen resolution is pixel density, which is the total number of pixels within a physical constraint. It's calculated in pixels per inch (ppi), which is fundamentally a measure of how tightly pixels are squeezed together. This element was somewhat of an afterthought until Apple introduced the Retina Display, but it has important ramifications for the overall crispness of text and images. While the iPhone 3GS came with a 3.5-inch screen with an HVGA resolution, the iPhone 4 kept this same screen size yet boosted its resolution to 960 x 640. The result was a massive increase in pixel density, which grew from 163ppi in the iPhone 3GS to a staggering 326ppi with the iPhone 4. Of course, these numbers are merely academic until you examine the impact that a high pixel density has upon the overall legibility of small text and clarity of images. As you'd expect, other manufacturers aren't letting Apple have all the fun in the pixel density war, and we're seeing particularly exciting developments from Toshiba and Samsung (more on that a bit later).

If you're interested in calculating pixel density for yourself, you'll need to start by knowing the display size and screen resolution. From there, you'll need to determine the diagonal resolution of the screen with a little help from our friend Pythagoras (famous for the Pythagorean theorem). For our purposes, his equation is best expressed as follows:

Diagonal resolution = square root of [ (width x width) + (length x length) ]

Using the example of a 4-inch display with a WVGA resolution, your equation should look like the following:

Diagonal resolution = square root of [ (800 x 800) + (480 x 480) ]
Diagonal resolution = square root of [ 640,000 + 230,400 ] = square root of 870,400
Diagonal resolution = 933 pixels

Now, take the diagonal resolution (in our case, 933 pixels), and divide that by the display size (4-inches). If you've done the math properly, you'll see this particular display has a pixel density of 233ppi. While most smartphones on the market today feature perfectly acceptable pixel densities, this little tidbit could come in handy if you're looking for the clearest possible display.

The touchscreen

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Now that we've examined display technologies and screen resolution, let's take a brief moment to discuss touch screens, which are crucial elements for modern smartphones. The dominant touchscreen technology is known as capacitive touch, which receives feedback from your body's ability to conduct electricity. When you place a finger on the display, the screen's electrostatic field becomes distorted, and the change in capacitance is registered by the underlying sensor. From there, software is used to react to your input. The beautiful part about a capacitive touchscreen is its ability to register multiple points of contact at the same time, which enables multi-touch functionality such as pinch-to-zoom.

Another type of touchscreen on the market today is known as the resistive touchscreen. It's generally less expensive to produce and responds to physical force. While there are multiple elements to a resistive screen, the most important are two electrically conductive layers that are separated by a narrow space. When you press on the display, the two layers come into contact with one another, which registers as a change in current. Unfortunately, these added layers reduce the overall brightness of the display and increase the amount of glare reflected from the screen. You'll generally find resistive touch screens in lower-end smartphones because they don't support multi-touch, although a few individuals appreciate its ability to receive input from a stylus, gloved fingers or fingernails.

Looking forward

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Hopefully we've given you a solid overview of the current state of mobile displays, but as you'd expect in an industry that's rapidly evolving, there's plenty of exciting possibilities on the horizon. Here's a few gems that are sure to whet your palate for the future.

Full HD resolution and crazy pixel density

Ortustech (a joint-venture between Casio Computer and Toppan Printing) has developed a 4.8-inch screen with full 1080p resolution and a stunning pixel density of 458ppi. While a touchscreen isn't in the mix, manufacturers understand the appeal of full HD, and we're seeing the industry continually advancing upon this holy grail. Likewise, Hitachi has announced a 4.5-inch IPS display with a 1280 x 720 resolution that supports glasses-free 3D to boot. Toshiba has introduced a 4-inch contender, also at 720p, with a stunning 367ppi resolution. Samsung isn't resting on its laurels, either, and is working on mobile displays that will push between 300 and 400ppi -- by 2015, anyway. While this announcement was specifically for tablets, we know Sammy's smartphones are bound to benefit.

Transparent displays


Manufacturers are finding a new take on our mobile phones being a window to the world, as transparent displays are now coming into the fray. TDK began production of a see-through OLED earlier this year, and while we'd be shocked to see this novelty crop up in smartphones, it's sure to give some added intrigue to the feature phone segment. Whether it can actually save SMS fiends from walking into oncoming traffic is still debatable.

Flexible displays

If you find your current smartphone far too rigid, 2012 could be quite a milestone, as Samsung is readying flexible AMOLED displays for production next year. While we plan to see smartphones with large screens that can be folded into a smaller form -- a definite improvement over current hinge-based designs -- we'd love to see an outlandish solution that fully incorporates the flexible spirit.

Electronic ink


Take one quick look at your smartphone's power consumption and it's painfully obvious that the display is the primary culprit. With projects such as Mirasol and E Ink Triton leading the way, we're hoping to see a day when color "electronic ink" becomes useful for smartphones. In addition to requiring only a fraction of the power of its illuminated brethren, these displays offer full visibility in direct sunlight. Of course, the need for a light source is a given, and current refresh rates would make for lousy gaming and video playback, but these alternatives are getting better with each new announcement. For those needing maximum battery life at all costs, these displays can't come soon enough.

Wrap-up

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While we're steering clear of crowning one display technology as king of the mobile empire, hopefully you've got enough information to make that decision for yourself. Granted, the quality of a screen is only one factor to consider when choosing a phone that's best suited for your needs, but it's an important consideration nonetheless. While LCDs typically deliver sharper and brighter images, more accurate colors, and perform better in direct sunlight, they cannot match the vibrant colors and excellent contrast ratios of their AMOLED counterparts. No matter which side of the fence you're sitting upon, you're certain to make a good decision by choosing one of the newer technologies. We've seen a dramatic improvement in mobile displays throughout the past few years, and if your budget allows, we wholeheartedly recommend that you leave older displays in the past -- where they belong. At the end of the day, your eyes will surely thank you for the consideration.