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Handing Off the PenTile Blog to Nouvoyance CEO – Candice Elliott

This blog will be my last as I hand this blog over to Candice Elliott, the founder of Clairvoyante and the principal inventor of the technology we refer to as PenTile.  PenTile technology has experienced a long and arduous evolution over which we have learned many important lessons.  Some have said that the technology was introduced too soon—business compelled us to introduce the technology into phones at 280 PPI and less, where some noticed pattern visibility.  Now that the standards have moved well beyond 300 PPI, to even 441 PPI for the Galaxy S4, there are few who can see any pattern visibility and are enjoying the benefits of PenTile technology.

Although Motorola and Nokia had developed phones based upon PenTile RGBW LCDs, PenTile LCDs have seen less adoption than has PenTile OLED.  However, now that resolution is creeping up for tablets to match that of phones, we again are seeing new opportunities to bring PenTile RGBW LCDs to the market to solve the battery life and brightness demands of tablets.  Samsung Display demonstrated at Display Week both the 10.1” WQXGA (2560 x 1600) “Green” display as well as the 13.3” 3200 x 1800.  Keep your eyes open for these in tablets this year.  The 10.1” WQXGA is already being used by one tablet provider in the USA and reviewers have noted the very long battery life that is spec’d.

Although I will be moving on to new challenges elsewhere, I remain a dedicated user of PenTile based products and foresee a continued path of adoption for PenTile technology as people continue to buy thinner, lighter and higher performance consumer electronics.  There is still no technical display solution that surpasses the benefits of PenTile for highly efficient, high resolution displays.

Cheers!

 

Joel Pollack

Just Look for Yourself

While it seems fashionable to criticize PenTile technology, increasingly we are reading blogs that refer to PenTile displays as  “oft maligned” but “looks fine to me”.   The reality is that as PenTile technology has moved to the higher end of resolution >300 ppi it doesn’t look less than sharp.  One example was this recent post regarding the RGBW in the Nokia E6.  Most people do not even realize that this display is a PenTile LCD.

http://bit.ly/HrspZI

This author, who never specifically noted that this displays was PenTile technology,  commented:

“…But we have no issues with the crispness and the brightness of the display as it works well with 640 x 480 …”

It comes down to looking at PenTile displays for yourself and not being misled by any negative blogs or claims of fuzzy text.

Opportunity to Learn More About PenTile Technology

For those who would like more information about PenTile OLED and PenTile RGBW technologies for mobile products, you may wish to attend the up coming talk by Nouvoyance CEO Candice Brown Elliott in Southern California, “AMOLED vs. Hi-Res LCD for Premium Cell Phones”.  She will be speaking at the Los Angeles Society for Information Display Chapter’s annual symposium on “Emerging Display Technologies” on February 3rd, 2012.  For more information regarding attending the symposium, visit the LA SID Chapter website:  http://www.sid.org/ConferencesExhibits/LAChapterEmergingDisplayTechConference.aspx

 

Ms. Brown Elliott will also be speaking in Northern California, at Stanford University on the 28th of February on the topic of “Reducing Field Sequential Color Break-Up Artifacts using a Hybrid Display with Locally Desaturated Virtual Primaries”.  This talk will cover the development of the PenTile Hybrid Multi-Primary Field Sequential Color display that was demonstrated by Samsung in their DisplayWeek 2011 booth last May in Los Angeles.  For more information regarding attending the talk, visit the Stanford Center for Image Systems Engineering website:  http://scien.stanford.edu/

Not all RGBW Displays are Created Equal

Perhaps you may have noticed that LG introduced an RGBW OLED TV at CES, using a white OLED process together with traditional RGB+Clear color filters.

http://www.oled-info.com/lg-55em9600

The purpose of this was ostensibly to facilitate the manufacturing of a large diagonal OLED, that is traditionally difficult with shadow mask deposition techniques.  My assumption is that they used RGBW to enhance brightness, taking advantage of their white OLED material to attain the longer 30,000 hours or more of  lifetime which is now expected for TVs.  At CES I was told that the layout was a QUAD layout, which uses 4 subpixels per pixel, unlike PenTile that uses, on average, 2 subpixels per pixel achieved through subpixel rendering.

Similarly, we are also seeing additional announcements of products by Sony using their Sony Whitemagic™ technology.

http://news.in.msn.com/technology/article.aspx?cp-documentid=5762843

The layout of RGBW for Whitemagic also has used 4 subpixels per pixel unlike PenTile’s 2 subpixels per pixel on average.  While Sony’s stated purpose for Whitemagic has been to enhance power efficiency, a 4 subpixel per pixel layout only take advantage of the improved light throughput of clear subpixels.  It is, no doubt, an improvement to power efficiency, but it forgoes the chance to improve aperture ratio, which a significant part of what is achieved with PenTile technology.  As one moves to increasingly higher resolution formats aperture ratio becomes a major limitation, even for low temperature polysilicon (LTPS) backplanes.  For PenTile, however, the contribution of improved efficiency in our highest resolution designs is equally attributable to improvements in aperture ratio as well as the improved throughput gained from clear color filters.

Competitive RGBW is a step in the right direction, but there is no substitute for genuine PenTile technology.

PenTile for OLED compare to PenTile for LCD

In reading a recent blog  http://bit.ly/sIanNL   I see that there still seems to be confusion on the value proposition for PenTile between OLED and LCDs.  It is very different between PenTile OLED and PenTile LCD.  While I have discussed this before in this blog, it is worth reviewing this one more time.

PenTile OLED – reduces current density to improve lifetime while maintaining display brightness

PenTile enables high resolution for OLED.  Due to limitations in the luminous efficiency of OLED materials there are tradeoffs between brightness and lifetime.  To maintain the brightness of an OLED display without compromising lifetime one has to reduce the current density, i.e the current per unit area of OLED material. OLED lifetime is very dependent upon current density.   PenTile eliminates one third of the subpixels and retains the same number of pixels, but when doing this one increases the fill factor of the display.  In other words, the ratio of emitting regions to nonemitting region of the OLED display is improved by having fewer subpixels.  The net effect is that the current density is reduced, improving the lifetime of OLED displays for comparable brightness.  Using subpixel rendering this is all possible without reducing the displays resolution as defined by modulation contrast ratio.

Since OLEDs only consume power where they are lit they are intrinsically lower power for all except images that are predominately white, which is the case even without using PenTile technology.  However, it consumes significantly more power to write white images with an OLED than an LCD.  On the other hand LCDs have a backlight that remains on all of the time and is only reduced in brightness for darker images based upon global dimming  techniques.  So, until you define the usage model you cannot say whether an LCD or an OLED panel will be more power efficient.

PenTile LCD – reduces power by about half in high resolution designs

The principal reason for using PenTile for LCDs is to save power.  Roughly half of the power can be saved as a result of

1. Improved open area (aperture ratio) which results from using fewer subpixels.  Transmissive regions increase in size while the area devoted to opaque transitors and capacitors remain as before, so the opaque regions are a smaller percentage of the total area of  display in a PenTile design.

2.  Clear (white) subpixels are regions where there is no color filter to absorb light.  Since photos, video, and ebook content is heavily weighted to white this becomes an important factor for saving power. ( Note:   I will discuss this in more detail in another blog soon.)

3.  A type of global dimming which we call Dynamic Backlight Control (DBLC ) looks at peak luminance of images as well as the presence of high luminance fully saturated color and uses this information to reduce backlight power without introducing color or clipping artifacts.

The net effect is that PenTile LCDs can save half of the power for high resolution designs.  This can improve both amorphous silicon as well as polysilicon LCD designs.

Will Increases in Processing Power Outpace PenTile RGBW LCD Savings in Display Power?

Recently I read with interest a string on PenTile technology at Howard Forums Mobile Community http://bit.ly/rDSNiJ where several good points were made and insightful questions were asked.  In today’s blog I will try to address a question that was raised here that jasaero rightfully said we have never addressed.

In the Howard Forums blog  rkeller62 making the point that as resolution continues to climb any pattern visibility becomes less apparent, much like the halftone in a magazine.  He also mentioned visual adaption, a process where the human vision system filters artifacts that do not do not contain data.  I had pointed out in a prior blog http://pentileblog.com/?p=879 that this adaption has happened with the jail bars of RGB stripe and is already happening for many with the checkerboard pattern of PenTile technology.

To this jasaero raised the valid question of whether or not we really needed all of that extra resolution that would be needed to avoid pattern visibility.  He pointed out that these extra pixels add to computational power that could well outweigh the power savings of PenTile RGBW LCDs. He argues that such computation power would have to increase in proportion to the increase in the number of pixels.

Let me begin with the question of whether or not we need all of those additional pixels.  My reply is that it depends upon how you use your smartphone.  Judging from the various blog entries on this topic, there are many who seem to value more resolution, so I can tell that these people would appreciate formats with more pixels.  The question is, at what point would resolution exceed the capability of the human vision system to see such patterns?

For this, I have the chart below showing resolution in PPI as well as cycles/degree of human vision.  The human vision system (HVS) doesn’t care about PPI, but rather sees cycles/degree—how many black and white line pairs can be fit into one degree of view entering your eyes.  Displays at a given PPI will have various levels of cycles/degree depending upon how closely one is accustomed to viewing this display.  In my chart, I make some assumptions about viewing distance by application, but we know from reading blogs that there are some who can accommodate (focus) on a display from extraordinarily close range.  If these people have good visual acuity at this range they will see things like pattern visibility that were not intended by the designers of this produce.  That is true for RGB stripe as well as PenTile technology.

If you have somewhat aging eyes it may well be that the 3.65”, 316 ppi, Nexus One when viewed at 30 cm would be beyond the limits of your resolution, but for normal vision we would need to go to 50 cy/degree to exceed useful resolution.  For those with visual acuity of 50 cy/degree, PenTile pattern visibility will totally disappear for the new Nexus Prime when it is viewed at 32.5 cm.  For those with 50 cy/deg of acuity who prefer to view it still closer, we could use still a bit more resolution.

So let’s consider the question of increased power consumption for supporting all of those extra pixels.  Without debate, there is more power required to support more pixels, but the power of the AP (including the GPU) consumes at least two orders of magnitude less power than that required by the display .   Display backlights are one of the larger consumers of power in smartphones, only surpassed by the radio.  The power used by 4G LTE is considerable.  Dual core high power processors are using more power, but still this is orders of magnitude less power than the display backlight.

Why is this?  It is because TFT LCDs are horribly inefficient.  Even at modest resolution like TVs, LCDs only transmit about 10% of the light from the backlight to your eye.  In higher resolution displays one is doing well to get 5% of the light out the front.  Where does all of this light go?  Half is wasted by the light absorbtion of the polarizers, more is absorbed by the opaque structures in the backplane electronics array, more is absorbed in the color filter array, still more is lost though back reflection at various interfaces of the many layers in the display. Saving half the power in the display backlight is much more important than doubling the power of the AP that consumes orders of magnitude less power.  As displays grow in diagonal, if the brightness is maintained, the power will increase as the square of diagonal!

Even so, we are always trying to reduce the processing power for PenTile technology, as small as this currently has become.  Eliminating one-third of the gate drivers helps, but this is offset by gates needed to perform our adaptive filtering and our dynamic backlight control.  Overall the power consumed by logic that is only used by PenTile is roughly the same as the power saved by eliminating drivers.  At some point, however, PenTile logic will be placed into the AP.  At that point it will be possible to drop the data rate by one-third which will substantially decrease logic power and at the same time reduce EMI associated with the display.  In the meanwhile, Android users have benefited from having a tool to monitor power consumption to see exactly what is the culprit in their phone.

There is currently, and for the near term, no other technology that saves as much display power as PenTile technology.  If you enjoy high resolution PenTile technology continues to be a good companion technology.

 

Many Beginning to Appreciate the Value Proposition for PenTile

It has been interesting to read the wide variety of blogs that have opinions about PenTile technology.  We recognize that the technology is not a one-for-one replacement for RGB stripe, but rather a methodology to preserve resolution while achieving better than average brightness and long battery life.  Many have noticed that it works far better outdoors than other phones.  This is largely due to the white subpixel and improved aperture ratio.  Others have found that when they stop looking for the differences they stop seeing it.  There are, however, still a few who remain unconvinced.

Here is an interesting string that shows how divergent the opinion is on this topic:

http://forum.xda-developers.com/archive/index.php/t-1262518.html

Recently I have noticed some bloggers going back to the statement that PenTile technology has being dropped by Samsung in favor of RGB stripe http://www.tested.com/news/droid-bionic-vs-htc-sensation-4g-vs-samsung-galaxy-s2/2855/That simply is not the case.  Samsung continues to manufacture many PenTile OLED and PenTile RGBW LCDs.  The reason for the choice of RGB stripe for the OLEDs in the Galaxy S2 are because a WVGA display in larger than 4-inch diagonal goes below the dpi that is recommend for PenTile in phone applications and may even look pixelated for RGB stripe http://pentileblog.com/?p=444.  For higher dpi you may see more new PenTile OLED and PenTile RGBW products from Samsung in the future where PenTile display still remain an enabler.  However, we fully expect that for future products, where the dpi is below our recommended minimum, that RGB stripe displays will be used.

 

 

Highest Resolution PenTile Panel in a Smartphone

I just got my hands on what is currently the highest resolution PenTile panel with pixels at 325 dpi. This resolution is very similar to the dpi in an iPhone 4.  It is  in a smartphone from a major brand supplier that is not Motorola.

As it turns out, this display is a PenTile RGBW LCD rather than a PenTile RGBG OLED.  I measured the white luminance at about 400 nits.

Any guesses from the phone experts which phone this might be?

Saving Power with Your Smartphone Display

When I was at my local Verizon today looking at phones I couldn’t help but notice that many of the choices for wallpaper were not optimal for saving power, and were chosen for reasons that had more to do with the graphics designer’s idea of what looked good.

Most people now understand that for an OLED display it only consumes power when and where the screen is lit, so a good screen saver would be one that is mostly black.

When it comes to an RGB stripe LCD the best way to save power is to turn the display off when the panel is not in use since the backlight is otherwise turned on.  However, for images that do not have lots of white brightness, Content Adaptive Backlight Control (CABC) in today’s RGB stripe displays will also turn down the backlight and increase the transmission of the LCD.

How about for a PenTile RGBW LCD?  In these displays there is a clear (white) subpixel, which together with improved aperture ratio increases light transmission to save power.  In addition there is an algorithm which we call Dynamic Backlight Control (DBLC) which continually analyzes image data for highly saturated color with high luminance.  The reason for this is that all RGBW displays must pay attention to an effect known as Simultaneous Contrast Error.  Simply stated, if you put a bright and highly saturated color on the screen at the same time as a bright white, it is necessary to supply more power for the display so that the eyes and brain will perceive the correct shade of saturated color.  This is especially true for yellow.   For PenTile displays, to combat this effect, the backlight brightness is turned up to increase the luminance of the colored subpixels while the white level is turned down tomaintain the desired perception of color to make a perfect looking yellow.  In other words the worst case power consumption for a PenTile display is when you are displaying more than a minimal amount of highly saturated colors, regardless of whether it is red, green, blue or a combination of these colors.  This is true even if the image is largely dark.  The best power consumption, other than a totally black screen, would be to use unsaturated colors or simply black and white.  The worst case power consumption for OLED  is the best case power consumption for a PenTile RGBW display.

Saturated Color Wallpaper

Unsaturated Color Wallpaper

 

 

 

 

 

 

 

So if you have one of the new Motorola smartphones with a PenTile RGBW LCD display, you might want to replace the  wallpaper featuring saturated colors with one that is either black and white, or colors of less than 50% saturation.  The power benefits should soon be apparent.

What is Pixelation?

Often, I am reading blog comments that say that PenTile is pixelated.  While this is a perfectly valid word, it has traditionally been used to describe the situation where information is being display on a display of too low resolution causing blocky looking fonts and lines.  Pixelation has, for example, been used to obscure or make anonymous faces, license plates and logos as in the photo from Wikipedia below

Pixelized Photo (Wikipedia)

 

For fonts however it generally degrades the appearance of crispness of edges.  It can be remedied or reduced by either going to a higher resolution display or by some type of anti-aliasing technique such as ClearType (Microsoft).

An example of text that is pixelated is show below along with a section that is magnified for better illustration.

Subpixel rendering generally smooths such edges, much like ClearType.  If anything it reduces pixelation.  What people are seeing that has been described as granularity or screen door effect would more correctly be described as pattern visibility.  Such pattern visibility will become less apparent as PenTile is applied to increasingly higher dot pitch screens such as HD in small formats for WQXGA on 10.1″ diagonal tablet screens.

Pixelated Text on RGB Stripe LCD

Magnified image of the the word "The" from above.