Mobile Communications Displays
Maxi Displays and Mini Projectors
Pictures that are too small and connections that are too slowtwo reasons why Internet and multimedia applications for cell phones have yet to take off. GPRS and the new UMTS networks will increase data transfer rates, thus making cell phones suitable for videos. But the question is: which display best serves the depiction of multimedia content?
"Hi, I'm Claudia, your avatar!" Future cell phone users shouldn't be surprised to find computer-generated faces on their cell phone displays. They may also find themselves interacting with displays projected directly into their eyes (photo right)
Developers in electronics companies and researchers around the globe are working on ultra-thin plastic monitors based on organic light-emitting diodes (OLED), which could replace today's liquid-crystal display monitors (LCD). The principle behind this new technology is simple: A layer of small organic molecules or polymers is sandwiched between two electrodes, at least one of which allows light to pass through it. Light is created by electroluminescence. When a few volts of electricity are applied to the electrodes, the molecules absorb the electric energy and emit it again as visible light, the color of which depends on the material itself. Anode and cathode each consist of a multitude of parallel printed conductors, and are oriented at 90 ° to one another. The intersections of the conductors define the image pixels. Sealed airtight between two glass layers, the entire device is only a few millimeters thick. Experts believe the plastic monitors could become especially popular for mobile applications, as such monitors offer a higher level of luminance with lower power demand than backlit LCD models. Furthermore, they are characterized by significantly better contrast than non-backlit liquid-crystal displays, although in this case they do require more power. OLED monitors also boast a wide 170 ° viewing angle. After an introductory phase, prices are likely to become comparable with those of conventional LCDs. The resolution of OLEDs is, similar to that of LCDs, limited only by the spacing of the conductor grid. "We are now in the process of developing a display with more than 120 lines and a pixel size of approximately 0.1 mm² that can be reduced even further," says Dr. Gerhard Kuhn, a product engineer at Osram Opto Semiconductors.
The first (monochrome) miniature displays for cell phones (Motorola) and car radios (Pioneer) are already on the market. Osram is also setting up a production facility for OLEDs. There are a number of problems yet to be solved, however, before the amazing plastic monitors can completely replace their LCD counterparts. In principle, the color spectrum is complete, but the substances currently used for blue (and therefore for white) lose their luminosity after only a few weeks. Scientists are therefore working intensively on improving this relatively short lifespan. "It depends on the material used," Kuhn explains. "The best laboratory results are currently yielded by our Elegance Yellow, a polymer that shines a greenish-yellow color and is effective for more than 20,000 hours at room temperature." Osram engineers also want to replace the carrier glass with a flexible substrate. The crucial point is that the OLEDs must be encapsulated in an absolutely air- and moisture-tight environment in order to ensure a long lifespan. However, this is hard to achieve with flexible films. Potential applications for such micrometer-thin displays include having them wound up and inserted in a pen or laptop hinge and simply unrolled when needed. And newspaper readers would certainly appreciate a feather-light electronic version of their favorite daily.
Tomorrow's cell phones will make increased use of organic light-emitting diode displays. Not only are these new displays extremely bright and contrasty, they also save energy
The miniature format is also of great interest in the field of projection technology. Siemens has developed a matchbox-sized daylight projector that can be plugged into a cell phone with a corresponding interface. The projector can be used for presentations in small meetings or for surfing the Internet. This projection cell phone was recently presented at the CeBIT 2002 computer show in Hannover, Germany. An LED array in the module lights up a microdisplay through a beam splitter. The light is modulated with the image information by the display, then exits through the projection lenses. Full-color pictures can be generated by rapidly illuminating the monochrome display with the red, blue and green LED colors in sequence. "This mini-projector can project onto any surfaceeven a piece of paper or the back of an airplane seat," says Marco Werner from Siemens Information and Communication Mobile. The projector currently generates 1.1 lm, enough to illuminate a postcard-sized surface. "But the prototype still offers enormous room for improvement," says Werner enthusiastically. With optimized design and equipped with more intense light-emitting diodes, it should be possible to improve the luminous efficiency by a factor of ten over the next two years.
Of course, lasers have also been considered as a light source. Their use would make it possible to eliminate the focusing optics and to project onto curved surfaces. However, suitable miniature laser diodes are currently available only in red. The efficiency level of the blue and green solid-state lasers now on the market is still too low. In the distant future, Werner can also imagine 3D laser projectors that have no need of fixed projection screens. In such a situation, the image would then be created on a boundary surface like a patch of air whose density has been altered using ultrasound.
A mini video projector enlarges a cell phone's display to the size of a postcard. It accomplishes this by illuminating a microdisplay with red, green and blue LEDs in quick succession. The display receives the signals from the cell phone via an interface and uses a lens system to project the final image
Long before laser projectors appear, 3D cell phone images will be a reality. At CeBIT 2002, for instance, Siemens introduced an OLED cell phone complete with shutter glasses. The unit can be used to play interactive 3D multi-player games. With the help of infrared signals, the right and left lenses of the glasses are rapidly alternated between "black" and "transparent," giving images on the cell phone the impression of depth. The advantage of this system, as compared to red/green 3D images, is that the user sees a normal, two-dimensional picture when not using the glasses. "This technology yields such a perfect 3D impression of larger pictures that the viewer may even experience motion sickness," says Siemens researcher Thomas Riegel. In addition to mobile games, the 3D displays are suitable for special applications in the fields of medicine and architecture.
Conventional displays can also be improved; all they need is a few software tricks. These include sophisticated image transmission algorithms, such as those developed by Siemens engineer Dr. Uwe Rauschenbach, which can display large pictures despite limited room for data and a small screen. Rauschenbach, a computer scientist who received the Mannesmann Mobilfunk Stiftung Award for the algorithms, explains the basic idea as follows: "When viewing a map or a technical drawing, it's often the case that only a part of the picture is needed, such as the viewer's current location or a single component. We provide a large, detailed view of such a section, while the rest is pictured in much less detail." Depending on the specific application, the image can be divided into rectangles or into regions of any shape that are defined by the viewer or the author of the picture. If the user wants to shift the focus, only the differences are transmitted; picture elements already present are not transferred again. This so-called fisheye view uses modern mathematical wavelet compression to keep loading times down. The image is described by coefficients created by a set of filter functions. "The software must be running on the server and terminal in order to achieve optimal results," explains Rauschenbach. "The user then conserves some 75 % of the screen surface and requires only approximately one-fourth of the transmission bandwidth."
Using processes such as the so-called "fisheye view," it is possible to make optimal use of a small screen areasuch as this display of a map section
This kind of demand-driven image transmission is not only interesting for cell phones, but also for industrial automation. One example here is its use in combination with Siemens' compact PC MOBIC (MOBile Industrial Communicator), which can be wirelessly connected to a local company network and permits service engineers to "take the control room with them" anywhere in a factory.
The ability to define "regions of interest" in a display is possible only with the help of new data standards. JPEG 2000, adopted in 2001, offers not only better wavelet-based image data compression than JPEG, its nearly ten-year-old predecessor. More importantly, it also provides greater flexibility. It is thus possible, for example, to separate color and shades of grayan important feature for mobile applicationsand also extract various image sizes from the same compressed file, including the appropriate sizes for cell phone displays and computer monitors. The MPEG-7 metadata standard, which will make searching videos, audio files and image collections easier, will soon be available. This standard defines a format for saving and transmitting descriptions of multimedia content, such as the length of a recording, the names of actors, or even picture elements like "flower," "bicycle," "child," etc. It is also possible to save color distributions, textures or melodies. These can then be used to search for pictures with scenes of sunsets, for example, or to identify a specific music file by humming a melody.
Dörte Otten