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Volume 8 (Aug 25, 2003)
LCD Driver ICs-Keeping pace with the ever-changing mobile phone (1/2)
A glance back at the history of mobile phones
It wasn't all that long ago that satellite communication came to mind when thinking about a means of communication that could be employed regardless of location. Now, however, mobile phones probably first come to mind for the majority of us. The history of the mobile phone in Japan, where it has literally become an essential part of life, began with the launch of cellular phone services in 1979. At that time, the calling area was limited to Tokyo and monthly fees totaled nearly 100,000 yen ($850), which seems unbelievably high compared with mobile phone service fees today.
The advent of the "shoulder phone" in 1985 marked the beginning of the use of the term "mobile phone." However, the shoulder phone weighed about three kilograms, with the battery accounting for the majority of the weight, and only provided 40 minutes of talk time. After that, tremendous progress was made in reducing the size and weight of mobile phones. In 1987, mobile phones weighed in at 900 grams, and 1991 marked the introduction of the 220-gram "Mova® " (an NEC-brand mobile phone), which was very similar to the mobile phones now on the market (photo 1).
Mobile phone services, which started as an analog system, made the switch in 1993 to a digital system that used the same number of electromagnetic waves and yet made it possible for a larger number of people to become mobile phone service subscribers. Around the same time, the lease system that required security deposits for mobile phone service was phased out and the paid system introduced. This new system paved the way for widespread diffusion of mobile phone service among ordinary users.
It can be said that the emergence of i-mode® in 1999 determined the direction of mobile phone culture in Japan. The i-mode system, which made it possible for users to send or receive e-mail, view various types of content and perform normal call functions, quickly became a hit product by winning the hearts of young people in Japan who were already familiar with using pagers to send messages.
Right now, one of the most advanced mobile phones on the market is "FOMA™ N2051" (photo 2), an NTT DoCoMo mobile phone that complies with third-generation mobile telephone standards. One of the great features of FOMA N2051, which comes with a function called "i-motion mail" that allows users to send video clips taken with the phone's built-in camera, is its ability to smoothly and clearly send and receive video clips. Behind the realization of features like this is a 176 x 240 dot, 2.21-inch LCD. Thanks to this LCD, it has become possible to produce clear and realistic images even with a mobile phone.
The parallel between the evolution of the mobile phone and display technology
Mobile phone LCDs have evolved from dot character displays like those of pagers to the graphic displays we know today. At the same time, screen size has increased considerably, thus making mobile phones with large, high-definition color screens the norm. It is truly hard to believe that just three years ago the future of colorization was seen as doubtful because of the lack of color content available.
Super-twisted nematic (STN) color-reflective LCDs were used primarily in the initial phases of colorization. STN, also referred to as a simple matrix system, can be described as a system in which the electrodes placed vertically and horizontally on the screen enable the image pixels to be lit (Figure 1). While the downside to this system is that an increase in the number of image pixels causes image quality and contrast to worsen, its upside is that production costs are very low, which is why at the beginning all of the makers adopted this system. At the time, the majority of color content, including mobile shopping information, simple maps and games, could be viewed in the range of 256 colors, so no one really felt that the system in place was inconvenient.
However, as it became possible to display high-definition and moving images, users were no longer satisfied with the STN system. Consequently, thin-film transistor (TFT) LCDs, which can display moving images with higher speed and more colors, began to gain widespread attention. With TFT LCDs, the target pixels can be lit by applying voltage to each of the elements making up the image pixels; this design is also referred to as an active-matrix system (Figure 1).
To display data on an LCD, it is essential to have a mechanism that can activate the LCD by converting the display data to voltage volume. LCD controller/driver ICs are the mechanisms used to perform this task.
TFT system-compatible driver ICs are configured with a source driver that controls the voltage volume, a gate driver that turns the display elements on and off and a power source (Figure 2). TFT LCDs have been used in a variety of sectors such as PC displays and portable TVs. In the case of LCD controller/driver ICs for mobile phones, size and power consumption, which did not have much effect on large displays like those used for PCs, are a major concern. NEC Electronics has made great efforts to meet the ever-changing needs of mobile phone designers by developing advanced LCD driver ICs.
* mova, i-mode and FOMA are trademarks or registered trademarks of NTT DoCoMo, Inc. in Japan and other countries.
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