"Retina Display" in the next iPad?
Part 2: Technological Constraints
PART II: What are the technological constraints behind the Retina Display?
Obviously, upgrading to a "Retina Display" increases significantly the number of pixels that have to be processed: as shown in the table below (see Part 1) this would involve 4 to 6.3 times more pixels compared to the original iPad. This increase in pixel count may require a significant boost in the CPU and GPU capabilities both in terms of speed and memory.
1024 x 768
12" < 17"
45º > 32º
2048 x 1536
2560 x 1920
The iPad uses a new A4 CPU, a custom-optimized Cortex A8 core with a clock speed of 1GHz that is at least 70% faster than the ARM Cortex A8 600 MHz CPU found in the iPhone 3GS, paired with a PowerVR SGX 535 graphics processor (GPU), already used in the iPhone 3GS. When bringing the "Retina Display" to the iPhone 4 and iPod touch 4G, Apple brought the same CPU and GPU (1GHz A4 CPU, 100MHz bus, PowerVR SGX535 GPU) as the iPad. However despite the significant 4-5 times increase in pixels number (from 480 x 320 to 960 x 640 and 1024 x 768), there was however no apparent change in the graphics processor, the PowerVR SGX535 GPU, suggesting that this GPU was powerful enough to deal with the significantly improved resolution of the iPhone "Retina Display". Moreover only the iPhone 4 had a significant increase in memory 512 MB versus 256MB for the iPod touch 4G and iPad, suggesting that the amount of RAM is not related to the display resolution (the iPad has about 30% more pixels than the iPhone 4). Given its technical specification, it is also unlikely that this GPU could not handle higher resolutions (for example the 1280 x 960 resolution with twice more pixels the "Retina Display"). Moreover there is a 64-bit path between the A4 processor and the main memory that provides twice the memory bandwidth as in the previous generations of iPhone and iPod touch [EETAsia]. This architecture improvement was likely required to support the greater need for graphics bandwidth in the iPad, iPhone 4 and iPod touch 4G.
The new iOS devices to be released in 2011 are rumoured to pack an ultrafast, dual core SGX543 GPU said to offer twice the processing power at the same clock speed, and a Cortex-A9 dual core CPU running at 1.2GHz (to be called A5) much faster processor than the current Apple A4 processor with a single-core Cortex A8 architecture [AppleInsider]. This new mobile Application Processor has been rumoured to be manufactured by Samsung which is expected to quadruple its mobile chip production for Apple in 2011 [AppleInsider]. To fully take advantage of the GPU upgrade, the iPad 2 would need also a bigger memory bandwidth, and may get to get the same amount of RAM as the iPhone 4 (512MB RAM, i.e. twice as much as the original iPad) but running at higher clock 1,066 MHz (compared to the 800 MHz memory clock of the iPhone 4) [AppleInsider]. With such an upgrade, the iPad 2 would undoubtedly have the computational power to handle 4 times more pixels than the current generation, in particular a 2048 x 1536 resolution.
Interestingly, as illustrated below, the 2560 x 1920 resolution consists of exactly 8 "Retina Displays" (960 x 640 each) put side by side (leading to a 10" diagonal instead of the 9.7" iPad). One may think then that a "true" Retina Display is technically feasible for the iPad 2. If so, one could also wonder why the original iPad has not used the equivalent of 8 original iPhone displays (480 x 320) put side by side to shape its 9.7" display with a 1280 x 960 resolution at 165 dpi. If pixel count is unlikely to be a limiting factor, then other factors may have impeded the feasibility.
Some have argued that a "Retina Display" for the iPad is technically unfeasible so far because it would surpass the pixel count found in Apple LCD monitors [ArsTechnica]. Such an argument is somewhat fallacious: as discussed above, a 10" display with a resolution of 2560 x 1920 could be theoretically built from 8 "Retina Displays" found in the iPhone 4 or iPod touch 4G, and the pixel count is unlikely to be a limiting factor per se for the device GPU. Moreover the recommended minimum viewing distance to a computer monitor is 25" and the acuity-constraints (see chart in Part 1) would suggest an adequate pixel density to be anywhere between 140 and 230 ppi for a monitor. The table below lists the current Apple display-based products with their resolution and pixel density indicated. Note that Apple laptop and desktop displays have all a sub-optimal pixel density and could be actually twice as high to keep in with the visual ergonomics precepts.
|Product||Resolution||Specification||Size (inches)||Density (ppi)|
|iMac||1920 x 1080||HD1080||21.5||102|
|iMac/Cinema Display||2560 x 1440||WQXGA||27||109|
|MacBook Pro||1440 x 900||WSXGA||15.4||110|
|MacBook/MacBook Pro||1280 x 800||WXGA||13.3||113|
|MacBook Air||1440 x 900||WSXGA||13.3||128|
|MacBook Pro (Hi-Res)||1680 x 1050||WSXGA+||15||132|
|iPad||1024 x 768||XGA||9.7||132|
|MacBook Pro||1920 x 1200||WUXGA||17||133|
|MacBook Air||1366 x 768||WXGA||11.6"||135|
|iPhone 3GS/iPod touch 3G||480 x 320||HVGA||3.5||165|
|iPhone 4/iPod touch 4G||960 x 640||WSVGA||3.5||326|
This table also shows that the larger the display is the lower its pixel density is, suggesting that it is far easier to create small displays with very high pixel density than large ones. The panel size, not the pixel count per se, appears to be the limiting factor: the manufacturing process is to be blamed for this since a new fab generation is typically needed to develop TFT LCD panels with larger glass substrates and increase production efficiency [TechEye].
Yield issues generally occur in the development of a new process for various reasons (in particular due to glass thinness, see also Manufactoring Logistics in AMLCD). The process is improved through fine-tuning of many parameters until the yield is good enough to allow mass production and see volumes ramping quickly as the process improves. Yield issues are generally near-term problems, but they directly lead to component shortage and impact panels price. Given the high cost of capital equipment involved in the development of high quality medium-sized LCD panels, high yield and throughput must be achieved to achieve low manufacturing cost. More importantly the panels manufacturer needs to acquire the key technologies behind the "Retina Display"!
LTPS and IPS, the two key technologies behind the "Retina Display"
Apple "Retina Display" in the iPhone 4 is a high quality 3.5" TFT-LCD (Thin-Film-Transistor Liquid Crystal Display) that relies on two key technologies: In-Plane Switching (IPS) that provides wide viewing angles and accurate colour reproduction, and Low-Temperature Polycrystalline Silicon (LTPS or p-Si) that provides higher TFT performance and low power consumption.
The IPS technology was developed originally by Hitachi Ltd. in 1996 to improve on the poor viewing angle and the poor colour reproduction of TN panels (Twisted Nematic, see note 1) by reducing the amount of light scattering in the LCD matrix, which gives IPS its wider viewing angle and better colour reproduction (with almost no off-angle colour shift). More specifically, IPS is a liquid crystal technology which aligns the liquid crystal cells in a horizontal direction, that is parallel to the front of the panel for an increased viewing angle, and an electrical field is applied through each end of the crystal. Hitachi's implementation required two transistors for each pixel instead of the single transistor needed for a standard thin-film transistor (TFT) display. This resulted in blocking more transmission area, and thus in requiring a brighter backlight that consumes more power. The IPS technology has since evolved through several generations to improve the initial shortcomings (slow response time and a low contrast ratio) and became cost-effective enough to be now widely used in the mainstream market, and not anymore in high-end monitors aimed at professional graphic artists. LG Enhanced IPS (E-IPS) introduced in 2009 has in particular improved this technology by providing wider aperture for light transmission, larger diagonal viewing angle, lower response time, and enabling the use of lower-power and cheaper backlights. This newer and lower power IPS technology can be found in the current lines of Apple iMac, Cinema Display, iPad, and iPhone 4 (all Apple laptops still use 6bit TN panels though much improved with their LED backlight).
The LTPS technology is the key technology behind the higher resolution in the "Retina Display". Silicon transistors in LCD panels are typically made from a thin film of amorphous silicon (a-Si) deposited on a glass panel. Transistors take up only a small fraction of the area of each pixel; the rest of the silicon film is etched away to allow light to pass through. The higher mobility of polycrystalline silicon (p-Si) means smaller geometry of transistors that are capable of providing sufficient charging power and higher capacitance than conventional amorphous silicon, which translates to higher TFT performance, larger effective area transmitted by light, higher aperture ratio and higher resolution. Whereas LTPS LCD panels are faster, denser and more integrated than those made with amorphous silicon substrates, the latter are still by far the most common due to their lower cost and easier production (see note 2). Polycrystalline silicon TFTs are more costly and difficult to produce and LTPS has been limited to LCD display smaller than 7" so far.
The table below summarizes the display technologies believed to be used by Apple in its different iOS devices, and shows that it has explored various combinations in terms of liquid crystal (LC) and thin-film transistor (TFT) types:
|Apple Products||LC Type||TFT Type|
|Previous iPhone/iPod touch||TN||a-Si/CG-Silicon|
|iPod touch 4G (See note 3)||MVA||LTPS|
|iPad (See note 4)||H-IPS||a-Si|
The iPhone "Retina Display" is based on the Advanced High-Performance IPS (AH-IPS) LCD developed by LG Display [WayBeta]. LG Display has a 10-year-old experience with the IPS technology with several generations of IPS technology developed over the years: S-IPS (Super IPS), AS-IPS (Advanced Super IPS), H-IPS (Horizontal IPS), E-IPS (Enhanced IPS), P-IPS (Professional IPS) and AH-IPS (Advanced High-Performance IPS). This AH-IPS technology brings to small-sized LCD panels several advantages typically found now in larger LCD displays and flat panel TVs: 16.7M colours, high contrast (500:1 and above), 72% of gamut (colour saturation), and wide viewing angle in both horizontal and vertical orientation (about 160°). Though not disclosed the LTPS technology has been certainly coupled to LGD's IPS technology to achieve the very high pixel density that characterizes the "Retina Display" [DigiTimes]. LG Display is believed to supply the majority of the "Retina Displays" in the Apple iPhone 4 through an exclusivity agreement, with Sharp being possibly the supplier for the lower-quality "Retina Display" found in the iPod touch 4G.
The first generation of the iPad uses a 9.7" LED-backlit IPS display with an XGA (1024 x 768) resolution corresponding to a pixel density of 132 ppi. This display was also developed by LG Display and is based on its H-IPS technology to achieve wide viewing angles (160 deg vertically and horizontally) which is one of its notable 2010 achievements [LG Display's SEC Form 6-K for May 2010]. This LCD panel, referred as LP097X02-SLA* (See note 4), has been mainly produced by LG Display in its Gen 3.5 and 5 fabs, which planned to produce 9.7" panels on a Gen 6 line starting in 2011 [TechEye]. The iPad 2 display is very likely to benefit from this migration with the recent adoption of thinner glass substrates (from 0.7 mm to 0.5 mm) in Gen 6+. The Q&A session of LG Display Q4 2010 Earnings Call Transcript on January 21, 2011 [SeekingAlpha] fuels some arguments in this direction since LGD expects a 5 times growth in tablet units and revealed that they are adjusting right now to a single customer by expanding their production as most of their displays go to them. Apple is undoubtedly this particular customer...
Other sources for the 9.7" XGA display are believed to be Samsung Electronics and Chimei Innolux Corp (CMI, a subsidiary of the Foxconn Group) [KoreaTimes] though it is not clear whether they have been using the IPS technology: CMI may have been using the advanced fringe field switching (AFFS) technology and Samsung its super patterned vertical alignment (S-PVA) technology instead, both being alternative LCD technologies that achieve high-contrast ratios and wide viewing angles.
A 9.7" LCD panel with a "Retina Display" pixel density (246 ppi and above, see Part 1) would certainly require a technology similar to the LTPS TFT that could be applied to larger glass substrate. Unfortunately, there is no evidence so far that LTPS can be currently used to produce a 9.7" IPS display with the "Retina Display" pixel density. It is worth noting that the LTPS technology is not the only way to achieve LCD panels with high pixel density: for example, Hitachi Displays still relies on the a-Si TFT technology to produce its 302 ppi 6.6" IPS tablet display (UXGA resolution of 1600 x 1200) or its very bright, high-resolution 329 ppi 4.5" IPS LCD display (HD resolution of 1280 x 720) for the smartphone market. Casio uses Hyper Amorphous Silicon (HAST) to produce its 546 ppi 2" TFT LCD (resolution of 960 x 540) and a 458 ppi 4.8" HAST LCD display (resolution of 1920 x 1080) through Ortustech, its joint venture with Toppan Printing. However, similar to the LTPS limitation, none of these high-resolution technologies has been applied to LCD display larger than 7".
Finally Samsung Mobile Display has recently announced its new Super Plan to Line Switching (PLS) displays aimed at pocket-sized devices and whose production is planned for early 2011. The Super PLS technology is claimed to be better than the IPS technology in terms of viewing angles, brightness, production cost, and to support up to WXGA (1280 x 800) resolution [engadget]. This technology will certainly be soon incorporated in Samsung's line of tablet displays (which are probably used by Apple's competitors as shown in the table below) to improve on brightness, gamut and response time. Apple may very well consider the Super PLS technology as an alternative to IPS in the near future as Samsung can already mass-produce 7" panels with a resolution of 1024 x 600 (i.e. at 169 ppi) and 10.1" panels with a resolution of 1280 x 800 (i.e. at 150 ppi).
Even if one of these technologies was successfully applied to produce a medium-sized LCD display, its production would still be initially limited due to the need to migrate to a higher generation fab, a poor yield rate of panel making and manufacturing volume constraints. These initial problems would both limit the supply and increase the cost of the displays. These issues would impede Apple's product launch and make harder to compete against the new tablet entrants. It is likely that Apple would prefer to benefit from lower costs resulting from improved yields associated with the production of the IPS LCD panel found in the original iPad, so as to become even more competitive in terms of pricing rather than in terms of features.
So what improvements can we reasonably expect?
Whether the iPad 2 will get an improved display is still an open question, but we will find out soon enough given the growing number of reports from technology analysts and the approaching release of the product thought to be already in production. On its announcement, the iPad 2 will be inevitably compared to the plethora of already released or announced tablet competitors [RedSn0w]. The table below summarizes what is currently known about the display specifications for these tablets.
|Tablet Products||Resolution||Size (inches)||Density (ppi)||LC Type||TFT Type|
1024 x 768
|Samsung Galaxy Tab
1024 x 600
1280 x 800
|Apple iPad 2
1024 x 768
1280 x 800
|LG Optimus Pad
(March 2011 ?)
1280 x 768
|8.9"||168||LG Display ?|
(April 2011 ?)
1024 x 600
(Summer 2011 ?)
1024 x 768
A small improvement in resolution would be welcome, for example from 1024 x 768 (132 ppi) to 1280 x 960 (165 dpi), that is a 25% increase in vertical and horizontal resolutions or 56% increase in pixel number. A pixel density of 165 ppi is the same as the one found in the previous generations of iPhone and iPod touch (3.5" 480 x 320 LCD panel) for which the fabrication process is expected to be mature enough to be expanded to large display size. This resolution in landscape mode would be also equivalent to 2 "Retina Displays" put side by side, which may have some practical benefits when running iPhone/iPod touch only apps on the iPad. Even with such a minimal upgrade, Apple would trounce the tablets competition in terms of display size, resolution and pixel density.
This small improvement would be nevertheless a step toward the iPad "Retina Display" as it may then be more easily achievable by doubling the pixel numbers both vertically and horizontally (i.e. 2560 x 1920 at 330 ppi) like it has been done for the iPhone and iPod touch. If the iPad 2 gets a 1280 x 960 resolution then one should expect the iPad 3 to get a 2560 x 1920 resolution.
The high-resolution, 9.7-inch LED-backlit IPS display of the iPad is remarkably crisp and vivid with excellent color and contrast, and has a wide, 178° viewing angle thanks to its IPS technology (in-plane switching). Nevertheless this beautiful display has a so glossy and reflective surface that some people could not stand its glare and have to install some non-glare LCD protective film to prevent eye strain when using the iPad for a long period of time. However most available 3rd-party anti-glare protective films also introduce some amount of visual noise/ pixeling that look like some grainy texture, though generally quite small, to the display. With the anti-glare film the iPad display may also lose some image contrast and some brightness (~10%). Hopefully the iPad 2 will provide a solution to the glare problem, either as a matte option or some integrated technological solution like an anti-reflection display that could also provide a better reading experience under sunlight conditions.
Apple devices have typically inaccurate and under-saturated colours: first, until the iPhone 4, all iOS devices had 18-bit colour depth (i.e. 6-bits per sub-pixel) and had to use temporal dithering to emulate 24-bit colours (see Apple's patent application #20100207959). On the contrary, the iPhone 4 display has a true 24-bit colour depth. Secondly, all iOS devices have LCD panels rather weak in colour saturation, their gamut (i.e. the range of colours that can be produced) being limited to 60% of the standard gamut (sRGB/Rec.709). Despite having an IPS LCD display, the iPad is also limited to a 18-bit colour depth as indicated in the product specifications available for display part LP097X02-SLA* (see note 4). Bringing the next iPad display on a par with the iPhone's true 24-bit colour depth would be a step in the good direction, but accurately calibrating it to the industry standard specifications (sRGB/Rec.709) would be another long overdue improvement.
March 2, 2011 - Update
The iPad 2 has just been announced! So what did it get? Unfortunately during its announcement by Steve Jobs, absolutely nothing was said about its display. One had to check its specifications on Apple web page to discover (unsurprisingly) that the iPad 2 has the exact same display as the iPad 1 (see note 6): a "9.7-inch (diagonal) LED-backlit glossy widescreen Multi-Touch display with IPS technology, 1024-by-768-pixel resolution at 132 pixels per inch (ppi)". This is a bit of a deception since LG has recently announced its own 8.9" tablet with a 1280 x 768 resolution (WXGA), i.e. with a pixel density of 168 ppi (see also note 5 for latest Samsung 300 ppi tablet display). Let's hope that the iPad 3 will get a significantly improved display next year...
Part 3 of this blog series will deal with the business constraints, cost and supply, that beyond the technological issues have undermined the use of a "Retina Display" in the new iPad, as well as the strategic investments to ensure that Apple’s requirements are met in terms of production volume and cost.
William H.A. Beaudot, PhD
Founder & Chief Scientist
KYBERVISION Consulting, R&D
Field Of View".
Visual Acuity XL" app for iPad for measuring "Far Visual Acuity". We hope that the improved resolution in the iPad 2 will allow our app to measure also "Near Visual Acuity".
1. The inexpensive twisted nematic display (TN) is the most common consumer display type. Nevertheless they can have a very fast response time (compared to IPS) which has been their main marketing message. However most TN panels represent colors using only 6 bits per RGB color, or 18 bit in total, and are unable to display the 16.7 million color shades (24-bit true color) available from graphics cards. Instead, they generally use a form of temporal dithering called Frame Rate Control (FRC), which cycles between different shades with each new frame to simulate an intermediate shade. Such 18 bit panels with dithering are sometimes still advertised as having "16 million colors".
2. The high-density layout characteristic of LTPS is also particularly suitable for OLED (Organic Light-Emitting Device) display which requires a relatively more complex current drive structure necessary to provide high brightness and high picture quality.
3. The "Retina Display" of the iPod touch 4G has a poorer viewing angle and a poorer color reproduction compared to the "Retina Display" found in the iPhone 4. It is also somewhat sensitive to touch pressure (which does not occur on previous generations). Moreover it does not seem to be able to display true 16.7M colors and looks like to be using some temporal dithering as it is usually done with TN panels that support only 6 bits per red, green and blue sub-pixels. However, its maximum brightness (400 cd/m^2) and color gamut are comparable to those of the iPod touch 3G.
4. Some putative iPad 2 display part has recently surfaced [9to5Mac]. The display part in the first iPad generation is referred as LP097X02-SLA* while this new part is LP097X02-SLN1 (LP: LG Phillips, 097: 9.7 inch, x02: XGA, SL: H-IPS) with the same 1024 x 768 resolution. There is however no direct evidence that this display is intended for the iPad 2. This panel could very well go to HP / Palm 9.7" TouchPad tablet with a display virtually identical to the iPad's one [NetBookNews] or to one of the numerous Android tablet clones of the iPad.
5. Samsung showcased its new high pixel density LCD panel for tablets at SID 2011, a 10.1-inch, PenTile RGBW-based (red, green, blue, white) screen with a resolution of 2560 x 1600, that is 300 pixels per inch [Electronista].
6. In its latest SEC Filing Form 6-K, LG Display disclosed the development and mass production of 9.7-inch LCD panels for the iPad 2 using its AH-IPS and slim LCD technology resulting in a decreased thickness by 20% and weight by 7% compared to the LCD panel found in the iPad 1.