Electronic Design

Subpixel Rendering For High-Res Mobile Displays

The exploding display industry is riding the wave of advances in cell phones, digital still cameras, digital video cameras, MP3 players, media players, portable DVD players, and portable GPS devices. These products are fueling the demand for higher-performance color LCDs, as OEMs increase functionality to differentiate their products from the competition.

Smart cell phones are more than phones. They can include theme calendars, imaging systems, Internet access, GPS, and even video. Already, 75% of cell phones integrate color displays. In some areas of the world, color displays have been standard for more than two years. While the slow emergence of 3G support for sending and receiving data has limited video capability, there has been a reluctance to add memory devices in North America.

Some parts of the world have seen inroads due to the bandwidth available for carriers, the introduction of video-capable processors, and the adoption of memory devices in camera phones that transfer image data. This evolution has pushed the envelope for display performance.

Where qCIF+ (176 by 220 pixels) was acceptable last year, users now expect qVGA (240 by 320 pixels). This year, we'll even see the introduction of 2.x-in. diagonal VGA panels in smartphones, enabling more information to be displayed with clarity and stunning colors.

We've also seen the resolution for CCD imagers outpacing the resolution of LCDs in digital still cameras and digital video cameras. MP3 players have become well established in the consumer market, led by the Apple iPod. Last year, these display screens moved from monochrome to color. Apple has even announced plans for storing photos on the iPod, combining enormous microdrive capacity with the convenience of portable viewability. It's easy to imagine that these devices will also demand higher resolution and superior color. Each of these applications not only demands higher resolution displays, they also must meet several other criteria:

  • Low power for long battery life while preserving the lightweight portability
  • High brightness for viewability in all ambients
  • Highly saturated color for realistic reproduction of photographs or video
  • Competitive costs

Yet there are still transmissivity constraints for panels, even with newer high-aperture-ratio amorphous silicon, polysilicon, or continuous-grain (CG) silicon technologies. One way to alleviate this is to further maximize the aperture ratio of the active-matrix LCD (AMLCD) structure. But at the extremes of these design rules, poor production yields can plague such measures.

Fortunately, there's an enabling method of achieving high yield, excellent aperture ratio, low power, and superior color saturation. Based on the knowledge of human visual perception, the PenTile Matrix technology from Clairvoyante Inc. uses subpixel rendering to reduce the required number of column drivers. As a result, the aperture ratio can be expanded by 50% to 100% using current design rules.

The PenTile methodology treats each of the R, G, and B subpixels as independent elements rather than as part of a fixed-pixel grouping of R+G+B. Subpixel rendering utilizes groupings of subpixels to form "logical pixels" that can overlap in regions, leading to a visual resolution that's intrinsically higher than standard RGB (Fig. 1).

Such a methodology permits the elimination of many pixels from a conventional display by capitalizing on the characteristics of a human vision system. In addition, it's possible to add clear (white) subpixels to further enhance brightness and dynamic range. By doing so, designers can design a high-resolution panel that's much brighter or requires lower power-or combines brightness with low-power requirements.

By choosing a higher NTSC color gamut, it's possible to take advantage of the higher aperture ratio to build an LCD with more saturated color. While this would adversely affect brightness or power in a conventional display, the brightness and power specifications may still be reached in a PenTile-enabled display (Fig. 2) (Fig. 3). The PenTile algorithms complement the displays and can be added to the driver IC without compromising performance or requiring changes to the panel.

A critical key in today's market is saving manufacturing cost. Displays that would have required four or five LEDs in the backlight now only require three for the same brightness. Driver chips have to drive fewer columns and require far less memory. Older processes on amortized production lines can still be used without change. Newer processes can be made more robust to improve yield without violating the design specifications for brightness or power. Overall, this leads to reduced cost for manufacturing high-resolution panels, with the added benefits of increased brightness and better color saturation.

Although PenTile technology is relatively new, its technique far better matches how the human eye sees images. It can be implemented through straightforward changes to the pixel layout and by using newly designed drivers now becoming available to the industry. In many cases, PenTile Matrix technology will be the only way to reach the tough design goals of cell phones, media players, and other consumer electronic products.

Early adopters of this technology will have first production products-2.x -in. VGA cell-phone displays-entering the market in the first half of 2005. This level of improved display performance is expected to fuel further acceptance and adoption of PenTile techniques for products in 2005 and 2006. It also will enable OEMs to incorporate VGA and above display technology into their next-generation products.

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