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Power-Conscious Display Design for a Great User Experience
Tuesday, October 10, 2017

by Paul Hooper –Display Group Manager, andersdx

 

Thinking creatively about the needs of the user interface can help meet challenging power and cost targets when designing products for use in environments ranging from homes or offices to cars or factories.

Introduction:

Power consumption and bill of materials are important factors in many design projects. If either is too high there may be pressure to compromise on some features. The user interface could become a target: a colour TFT-LCD can be resource-hungry, but the look and feel are critical if the product is to be popular and successful. Not all end-product applications need a TFT-LCD. The latest monochrome display technologies can provide good alternatives when applied thoughtfully, and can lower the demands on system resources while still ensuring a great user experience.

Close the Gap with Advanced Technologies

Monochrome LCD displays have improved in many ways since the end of the 2G-mobile era. Designers can choose from a wide variety of display types, and newer technologies such as Fast-Response (FRSTN) and film-compensated FSTN or FFSTN types, deliver faster response times, sharper images, higher contrast and wider viewing angles than older, standard STN displays. Dual-Scan DSTN displays have also been introduced, which achieve higher image quality by effectively doubling the line-refresh rate.

Vertical alignment displays with true black background are another exciting new development in monochrome LCDs. These can achieve a contrast ratio of more than 1000:1, and deliver sharp image quality as well as wide viewing angles. Almost any backlight colour can be used to deliver the required visual effect. Crisp white backlighting, for example, can be impressive and easy to read, and selective colour is also possible through special backlight design or with optical filters. Inventive use of selective colour can provide an efficient and cost-effective way of making a warning graphic stand out against simple and straightforward characters or graphics.

Figure 1 shows how this type of display can be engineered to deliver a powerful visual impact.

Figure 1 shows how this type of display can be engineered to deliver a powerful visual impact.

Although TFT-LCD still has the edge in terms of image quality, and faster response improves handling of moving graphics such as a mouse pointer, the performance gap between monochrome and TFT-LCDs has narrowed. By concentrating on meeting the needs of the application, such as the use of colours, likely viewing angles, or any need for fast-moving images or icons, a monochrome display can be engineered to deliver a high-quality user experience in applications ranging from consumer products to professional electronic equipment and automotive displays.

Simplify System Design

Today’s STN LCDs can also help save power consumption and bill of materials costs throughout the system as a whole. Whereas a TFT-LCD usually requires extremely bright backlighting to give punchy colours, the performance of monochrome displays is much less dependent on backlight brightness.

In addition, because there are usually fewer pixels to address, the demands on the display controller or system processor are also lower: whereas the resolution of a TFT-LCD can be 320×240 up to full HD, and with red, green and blue (RGB) channels, a good user experience can be achieved with a monochrome display having as few as 128×64 pixels depending on the application and size. This can ease controller selection, allowing selection of a lower-cost, lower-power device. Operating at lower clock speeds reduces power consumption and any thermal challenges. Also, less RAM is required to manage the display thereby helping save cost and reduce board size. The exact reduction in RAM demand or CPU loading that can be achieved by such a switch depends, of course, on the application, the size and type of display, and the controller.

Fig 2: Monochrome display use low power so ideal for metering applications

Fig 2: Monochrome display use low power so ideal for metering applications

It is worth noting that a low-resolution colour TFT-LCD may be a viable option for some projects, which can help reduce the number of pixels that need to be addressed and hence allow a lower-cost microcontroller or CPU. This could be implemented as a custom display, or by modifying a standard TFT-LCD. Various other enhancements are possible, such as re-optimising the backlight.

Customise Cost-Effectively

With careful design of the user interface and system electronics, a monochrome LCD can help product developers meet tight power and cost targets. Moreover, economic factors allow a custom STN-LCD to be developed at a competitive price, compared to a standard TFT-LCD panel. This gives product designers even more flexibility to specify exactly the right display size, special icons, dot arrays or characters. Fine-tuning of the materials and assembly processes can help to optimise environmental performance. Moreover, long-term availability can be assured. With the right kind of project management, a custom display can be manufactured in the far east at favourable cost. However, the team in charge needs the skills and experience to ensure that the display received from the factory will meet the specification and satisfy all requirements of the application.

Anders has helped many product design teams achieve an efficient and cost-effective user interface, in a wide variety of projects such as domestic appliances, coffee makers, security entry panels, and industrial controls.

Design from Experience

Some of the most important aspects in the design of a custom display are related to the operating environment. In one recent project; a graphical user interface for a high-end washing machine, the team helped to develop the display which included a direct heat-seal bonded flexi cable, creating a permanent electrical connection between the display and interface board to ensure superior reliability compared to the usual default construction comprising an elastomeric connector and compression bezel. The resulting display proved to be highly resilient when exposed to high humidity, variations in temperature, and motor-related vibration.

Fig 3: Design considerations include humidity, high temperatures and vibration

Fig 3: Design considerations include humidity, high temperatures and vibration

The performance of the rear polariser has a major influence on the overall visual effect given by the display. A small change in the polariser properties can be easily noticeable. Close interaction with the overseas manufacturer can help ensure consistent performance and quality, even if the specified materials must be changed. In another recent project, the specified polarising material was made obsolete by the factory’s supplier. Engineers at Anders helped to evaluate alternative materials to identify a replacement that would ensure consistent display performance when combined with the existing LCD and backlight.

Conclusion

A great user experience is a vital feature of any new product entering the market, but so, too, are power consumption and cost. Basing the user interface on a full-colour TFT-LCD could be excessive in most respects and not optimal for all end products. Today’s monochrome displays can deliver a good impression at lower cost and power, and the opportunity to customise cost-effectively within a well-managed project provides scope to create an even more outstanding result.

For further information please visit http://www.andersdx.com

 

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