Mobile devices are rapidly becoming multi-media platforms. One reason for this evolution is the increasing use of video over the Internet coupled with increasing Internet connectivity in mobile devices. Nearly a third of young American mobile users post photos and videos to social websites such as YouTube, and virtually every social website will develop a mobile component over the next two years.
Broadcast television for mobile devices is also expanding the use of mobile video displays. Field trials have demonstrated substantial consumer interest in receiving video on demand, news, sporting events, and the like on their mobile devices. A variety of networks to deliver mobile television are now arising, with developers chasing a market for subscription and advertising revenue expected to exceed $14 billion by 2011.
The rise of visual media poses several challenges for mobile device designers. The trend toward larger display screens and higher resolution formats is moving faster than corresponding improvements in battery technology, increasing the strain on power budgets and battery life. Strong ambient lighting conditions under which mobile devices must operate wash out a conventional LCD image without bright, and power-hungry backlighting. Further, diverse regional and national preferences create a need for products to offer a wide range of device and display form factors, e.g. clamshell, swivel, slider and candybard, making it difficult to design product families that can serve a global market.
Display limits image viewability
One of the most critical challenges, however, stems from LCD technology's limited dynamic range. Human vision requires strong contrast in order to perceive fine details. The LCD's dynamic range limits restrict the image contrast that the display can generate, thus limiting the perceptibility of image details. The result is a displayed image that appears to lack the clarity and tonal depth to which consumers have become accustomed with their televisions and computer screens.
This restriction to the viewability of images and video on LCD displays is a challenge that must be met if mobile visual media devices and services are to satisfy consumer expectations and succeed in the market. Field trials have proven that poor viewing experiences are the main reason consumers stopped using mobile television after trying it out. Nearly 24% of viewers in European trials turned off their mobile televisions because of dissatisfaction with image quality.
The roots of the viewability problem lie in the way that LCD displays work. In essence, an LCD display consists of a light source behind a controllable light blocker. At one end of the data range the light blocker is fully open and passing its maximum amount of source light. (The actual fraction of light transmitted varies depending on the light blocker's design and resolution.) At the other end of the data range the display blocks 99.5% of the source light. This forms a simple relationship between image data value and the fraction of light transmitted.
Mapping of image data
The mapping of image data value to visual perception, however, is not so straightforward. Three factors affect how the display translates image data into image visibility. One is the display's input range. A full-color image uses 8-bits each for red, green, and blue data, but the display may not accept all 24 bits. This input range restriction results in truncation of the image data. The output of different source values could be identical due to rounding errors, with a resulting loss in visibility of image details.
The second factor affecting viewability is the backlight's intensity. Light from the display at the maximum end of an image's data range can only be as bright as the backlight itself. Thus, backlight intensity sets the upper limit of an image's displayable dynamic range.
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