Power-conscious display design to optimise user experience

Author : Paul Hooper, Display Group Manager at andersDX

15 August 2017

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

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. This piece explores how to approach designing power-conscious displays to optimise UX (user experience).

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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: colour TFT-LCDs can be resource-hungry, but look and feel are critical if the product is to be a success. However, not all end-product applications really 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.

Closing 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. 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.

Figure 2: Monochrome displays use lower power, so can prove ideal for metering applications

Simplify system design

Today’s STN LCDs can also help save power consumption and bill of materials costs throughout the system as a whole. While 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 320x240 up to full HD, with RGB channels, a good user experience can be achieved with a monochrome display having as few as 128x64 pixels, depending on the application and size. This can ease controller selection, allowing use 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 further helping save cost and reducing board size. The precise 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.

A low-resolution colour TFT-LCD might also 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 available, 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. Furthermore, 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.

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

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 a favourable cost. The team in charge, however, needs the skills and experience to ensure that the display received from the factory will meet the specification and satisfy all the application’s requirements.

andersDX has helped many product design teams achieve an efficient and cost-effective user interface – and 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 relate to the operating environment. In one recent project involving a graphical user interface for a high-end washing machine, the team helped develop a display which included a direct heat-seal bonded flexi cable, creating a permanent electrical connection between the display and interface board.

This was to ensure superior reliability compared to the usual default construction, which comprises 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.

The performance of the rear polariser has a major influence on the overall visual effect of the display, and a small change in the polariser properties can be easily noticed. 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 andersDX helped to evaluate alternative materials to identify a replacement, which 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 can sometimes be excessive and not necessarily 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 deliver an optimised user experience.


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