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Fix mobile video woes with adaptive display

Posted: 18 Aug 2008 ?? ?Print Version ?Bookmark and Share

Keywords:video? imagery? mobile design? LCD displays?

Displaying high-quality imagery and video is an essential feature of mobile handset design. To provide true mobility, the viewing experience must be satisfactory under a variety of ambient light conditions, requiring adjustable display settings. Ideally, the handset itself will adapt its display to accommodate changes in ambient lighting.

Display quality is a key factor in the market success of image and video features in mobile handsets, particularly among emerging mobile TV services. Trial markets for mobile TV showed that image quality strongly influences user satisfaction with the service. Up to 25 percent of users discontinued service because of unsatisfactory viewing experiences.

For LCD displays, ambient lighting has an impact on perceived display quality and thus, the viewing experience. Perception of display quality depends heavily on the imagery's tonal dynamic range. The more subtle color and shading values that the display delivers, the more realistic and satisfying is the perceived image. The tonal dynamic range that an LCD display can deliver to the viewer depends on the intensity of the LCD backlight and on reflected ambient light.

Tonal range
Backlight intensity controls the maximum brightness that a display can deliver, which sets the upper limit of the mapping from image data value to light output. Under ideal conditions, a bright backlight provides a wide tonal range. And the lower the backlight, the more compressed the image's tonal range becomes. Too low backlight makes the image look muddy, degrading the user's viewing experience. Because backlighting consumes 40-60 percent of a handheld unit's power budget, many developers prefer to use low backlight.

While backlight intensity sets maximum display brightness, ambient lighting determines the minimum display brightness that the viewer can perceive. Any part of the image where the display generates less light than reflected ambient light simply cannot be seen. Thus, ambient lighting acts to fill in shadows, reduces perceived tonal range, and makes the image look washed out where it is supposed to be bright and featureless where it is darker. In effect, ambient lighting cuts off the lower portion of the display's tonal range.

One way to compensate for lost range segment is to ensure that it is only a small fraction of the image's total tonal range by increasing backlight intensity. This can be handled by providing the user with a display brightness control, but a need for manual adjustment can irritate a user when ambient lighting changes frequently. A better approach is to make the design adaptive, able to sense ambient conditions and respond appropriately.

Even if controlled by ambient levels, however, the approach of simply adjusting backlight intensity fails to address power concerns. A backlight cranked up to support daylight viewing can quickly exhaust the handset's battery. Handset designers need effective methods for adapting the display to changing light conditions.

Dynamic compression
Technology that has recently become available provides dynamic range compression for LCD displays, addressing both power and display quality challenges. This technology, developed by Apical Ltd, is the foundation of the Visual Enhancement Engine (VEE), available as a system block in QuickLogic's configurable platforms. The VEE uses parameters that describe both the maximum light output the display can provide and the ambient lighting conditions, and remaps the image data to maximize viewability.

Dynamic range compression improves user viewing experience of videos in handhelds.

Adaptive dynamic range compression works by emphasizing tonal variations of an image based on the available dynamic range and the surrounding data. Unlike global image processing operations such as gamma correction, the VEE's operation varies from pixel to pixel so that subtle variations in the image's shadow regions are accentuated for greater visibility without simultaneously washing out the image's highlights. It improves viewing experience regardless of lighting conditions. Designers can use this adaptive range compression as an alternative to raising backlight levels under high-ambient light, or drop backlight levels in lower-ambient light and use adaptive range compression to keep the image from getting muddy. Either way, users get a high-quality viewing experience, while the handset uses less power.

The ability to maintain consistent image quality under changing lighting conditions is what will make a handheld device truly mobile suitable to service users on the move. Adaptive dynamic range compression is a design approach that will provide that consistency while offering designers power-saving options.

- Tim Saxe
CTO and Senior Engineering Executive
QuickLogic Corp.

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