Optimized displays address the power woes of mobile devices

Power management is a hot issue among designers of mobile devices for good reason. Internet, multimedia, navigation and Windows applications demand greater processing power. In addition, requirements for high-brightness, high-resolution displays with wider color ranges that are needed to achieve a more natural viewing experience put a greater tax on battery usage models. Higher luminance is especially needed if these devices are used outdoors where the ambient brightness is much greater and for devices that use MPEG video decoding, which is, by itself, a considerable power drain.



Unfortunately, higher display resolution and increased brightness draw more power, and multimedia applications – where the backlight needs to be bright and remain on for extended periods of time – exacerbate the existing problem of insufficient battery life. Therefore, the critical challenge to today’s designers is to find ways to cost-effectively achieve increased performance levels while continuing to manage power consumption in order to keep the device compact and lightweight and to offer longer runtime.



THREE PROMISING TECHNOLOGIES

The inefficiencies of current display standards are leading manufacturers to investigate new display technologies that optimize the display’s pixel layout, color filter or backlight system to improve power efficiency. Three promising technologies to help manage runaway power consumption are microelectromechanical (MEM) displays, organic light-emitting diode (OLED) displays and e-paper.



While legacy LCD technologies require constant refreshing to maintain or change an image, MEM displays only consume power when a pixel is changed. This varies depending on the type of data being displayed, so still images require no power and simple text draws very little power. MEMs seldom need additional lighting if they are used in areas where they can draw from ambient light sources. However, in dark environments, additional front lighting is required, adding to the power budget.



OLEDs do not require a backlight and their power consumption is content-dependent, so for some content, they may be more power efficient than today’s LCDs. In the future, with improvement in OLED materials and new manufacturing technologies, OLEDs could offer measurable power advantages over equivalent LCDs. OLEDs enable a greater range of color gamut, operating speed, and viewing angle than LCDs. OLED pixel colors appear unaffected, even as the viewing angle approaches 90° from normal. LCDs use a backlight and cannot show as dark a black state, whereas an “off” OLED element produces no light and consumes no power.



E-paper comes in several varieties, including electrophoretic types as well as cholesteric nematic phase transition (CNPT). Both of these have the ability to store images for extended periods, but each suffers from contrast reduction in indoor or darker ambients.

Unfortunately, MEMs, OLEDs and e-paper still have manufacturing and performance difficulties, and they lack the infrastructure of the LCD industry as a resource for funding improvements.



LCD innovations take the form of improvements upon the current state of the art. Methods such as dynamic backlight control (DBLC) are an option. Its primary function is to reduce power consumption in mobile LCDs. DBLC methods use sensors built into the display panel to automatically and simultaneously control the display transmissivity and LED backlight brightness to accommodate variances in image brightness. Depending on the image data content, these methods can enable a 15 to 30 percent reduction in power consumption, and they can be enhanced with complementary technologies such as sub-pixel rendering which can realize improvements independent of the image data content. When this feature is enabled, there can be quantization errors and other artifacts in maintaining luminance and dynamic range.



Sub-pixel rendering (SPR) technology, combined with a pixel layout that adds a white (clear) sub-pixel, can create high-brightness, low-power displays with wide- and standard-format qVGA and VGA resolution. Its use of larger sub-pixels with higher aperture ratio and the white color filter allows equivalent visual resolution using one-third fewer sub-pixels. Because this lets twice as much light pass through the LCD than traditional VGA stripe LCDs, display modules consuming the same power can be twice as bright, or they can offer similar brightness at half the power consumption. For equivalent brightness, the approach can cut in half the number of LEDs needed for a 2.4” VGA screen, for example, while saving costs by needing fewer LEDs, driver circuits and pixel transistors.



Sub-pixel rendering such as Clairvoyante’s PenTile RGBW technology, can enable a 50 percent power reduction for VGA displays, saving up to 250mW of power, or 100mW in a qVGA display which is very appealing in today’s mobile environment. (See Figure)



SPR technology is being used or considered by a growing number of display manufacturers as an alternative to standard RGB stripe LCDs. It also represents a way of boosting the inherent benefits of many types of display technologies such as amorphous silicon LCDs, low-temperature polysilicon LCDs and OLEDs.



Managing overall power consumption and increasing performance in a cost effective way will continue to be a formidable design challenge. By integrating highly optimized displays into mobile devices, engineers can realize greater overall device performance and better power management to accommodate myriad applications while meeting consumers’ demands for longer battery life and a better user experience at price points that will satisfy OEMs and users alike.



About the author

Joel Pollack is president and CEO at Clairvoyante Inc., 1601 S. DeAnza Blvd., Cupertino, CA 95014.

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