Display considerations for a handheld multimedia device
A multimedia mobile product requires the proper display to provide full functionality for its services and applications
BY JOEL POLLACK
Clairvoyante
Cupertino, CA
Consumers’ appetites are growing for mobile products featuring services and applications that enable them to make calls, take photos, watch video or TV, play games, access e-mail, surf the web, and view maps. Analysts foresee a promising market: IDC predicts around 24 million U.S. mobile users will pay for some form of TV/video content and services on their mobile devices by 2010, up from 7 million in 2006, and Infonetics forecasts worldwide revenue from mobile video and services will skyrocket from $45.2 million in 2005 to $5.6 billion in 2009, an astounding 11,997% jump in 5 years.
As applications become increasingly more visually-oriented, the mobile display selected for new devices is critical because it affects not only viewability, but also device runtime. Therefore, design engineers developing multimedia handheld devices need to consider several factors when choosing a display in order to balance power consumption without compromising other performance levels needed to accommodate next-generation applications.
The display in a mobile device is critical because it affects both viewability, and device runtime.
Applications
The main factor that will determine the brightness level required and acceptable battery life is the applications intended for use with a handheld device. Multimedia applications such as mobile video, gaming, navigation, and web browsing not only use substantially more processing power, but also need considerably more luminance for optimum performance. Ideally, a display for a data-centric video-enabled mobile phone, for example, needs a minimum resolution of 480 x 640, 500:1 contrast, and 300 to 400 nits of brightness. Yet at these levels, achieving adequate runtime may require compromises that are detrimental to performance.
Resolution and color
QVGA resolution is now standard, but the trend for video-enabled mobile devices necessitated by the more advanced content is fast moving toward more colorful displays offering widescreen QVGA and VGA resolution, 262K to 16M colors, and high color gamuts. However, higher resolution decreases aperture ratio and transmissivity while also using more power for equivalent luminance.
Luminance and viewability
Brightness is essential for viewing video and other multimedia applications. Insufficient luminance levels will not satisfy consumer expectation to clearly see all the fine details and images onscreen. Current portable device displays have an average of 200 nits of luminance, but for viewing multimedia, it is imperative to at least double brightness to 400 nits or higher.
However, multimedia content also significantly shortens battery life since the backlight must not only be very bright but also “always on,” and MPEG decompression further taxes power. The challenge then is to optimally balance brightness and power levels to satisfy expectations for both display performance and battery life.
Alternative display technologies
New display options such as advanced power sources and brightness enhancement films are promising, but many suffer from unacceptable tradeoffs to performance or cost. The best solutions are the hardest to achieve—those able to address the high-brightness, high-resolution, and low-power requirements for viewing multimedia applications without causing a host of other dilemmas or being extraordinarily complicated to implement. Some alternative approaches include OLEDs, LTPS and CG-Silicon displays, and more revolutionary advancements such as subpixel rendering technology.
Organic light-emitting diode (OLED) displays are gaining popularity because the organic materials require no additional backlight, so they’re bright, offer high response time and wide viewing angles needed for full-motion video, can consume significantly less energy, and are relatively easy to manufacture (thus potentially cheaper). But they have shorter lifetimes because their red, blue and green films degrade unevenly. In addition, they suffer from image burn-in and contrast reduction in bright ambients.
Low-temperature polysilicon (LTPS) and continuous-grain silicon (CG-Silicon) technologies have proven to be excellent manufacturing processes for high-resolution, high-brightness panels. Their high levels of integration and use of small transistors enable exceptional aperture ratio and brightness for high-resolution displays, but they still use a large number of backlight LEDs, which increases power consumption and cost, inhibiting their full potential in power-hungry, feature-rich handsets.
Subpixel rendering
Subpixel rendering technology may provide a comprehensive solution without causing difficult tradeoffs to performance, manufacturing complexity, or cost and can be applied to the aforementioned technologies to complement their inherent benefits. For example, Clairvoyante’s PenTile RGBW technology offers power savings over RGB Stripe displays without creating converse brightness problems, is also being applied to IC drivers and bridge chips, and has been adopted by display and IC manufacturers including Samsung, AU Optronics, Tomato LSI, and LG Innotek.
PenTile RGBW subpixel technology adds a white (clear color filter) subpixel and utilizes fewer required pixels, reducing the number of drivers by one-third over RGB Stripe without compromising visual resolution. A subpixel rendering approach of this nature also allows for a smaller footprint, making it ideal for thin compact designs, and an enhanced transmissivity and aperture ratio means the number of backlight LEDs can be dramatically reduced without compromising brightness. In addition, because data rates can be lowered using this technology, there is an overall improvement in EMI, which is highly important when integrating high-performance radiolike applications.
Complementing LTPS with PenTile RGBW technology achieves high brightness without taxing the power supply. LTPS uses narrower metal lines in the fabrication, resulting in smaller transistors, improving brightness by increasing aperture ratio, which is also boosted by PenTile RGBW technology. Due to the larger aperture ratio of the bigger subpixels and utilization of white subpixels, nearly twice the white brightness can be achieved without increasing power.
The future
As OEMs gear up for the continued demand for multimedia devices, and strive to differentiate their designs from the competition, it is imperative that they choose a display that will meet the resolution, luminance and power needs demanded by their applications. By choosing displays that offer the greatest design flexibility, performance tradeoffs are minimized and the overall bill of materials remains competitive. As a result, new generations of small multimedia devices will easily satisfy user demands for higher visual performance and longer battery life at a reasonable price point, driving sales for everyone along the supply chain.
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