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Implications of 4K Displays on Touch Sensing

Dr Andrew Morrison, Technical Director, Zytronic

The advent of 4K represents the latest stage in the ongoing progression of modern display technology. Over the last 18 months displays based on this ultra-high definition (UHD) video format have been introduced by a number of manufacturers. With 3840 x 2160 pixel resolution (four times the number in conventional HD displays), they enable crisper, brighter images to be rendered with improved color fidelity. It is already clear that 4K will not remain limited simply to the watching of broadcast television and DVDs in the home though, with users of commercial displays beginning to implement the technology in a range of other applications.
According to predictions made by industry research firm Markets & Markets, annual sales of 4K displays could reach approximately $370 billion by the year 2020, with a compound annual growth rate in the region of 36%. With such growth, the price point for 4K displays will inevitably decrease further encouraging commercial uptake of the technology.
As UHD displays are deployed in increasingly diverse applications it is inevitable that they will be integrated with touchscreens, to allow users to interact with the rich, appealing content they present and thereby derive more fulfilling user experiences. Retail spaces, entrance foyers of buildings, company board rooms, hotel lobbies and museums could all potentially gain considerable value from it. With the superior image quality offered, they will also be very well suited to all manner of high precision work, such as the display of architectural designs or engineering drawings. There is also the prospect of them being employed within the healthcare sector, allowing complex medical images to be viewed in ultra-high definition, aiding the diagnostic process. Adding touchscreen functionality to such displays allowing intuitive manipulation of the UHD images is almost certain to be deemed a very attractive option in the future.

Though there are a multitude of different touch sensor technologies available, projected capacitive (p-cap) has become the touch sensor technology of choice in everything from consumer electronics to industrial control. This is due not only to its resistance to damage, but also the ease with which it can bring reliable multi-touch interaction into the mix too. There are, nevertheless, a series of major technical hurdles that need to be overcome if 4K touch displays are going to truly deliver.
Firstly there is the fact that 4K displays currently generate higher levels of electromagnetic interference (EMI) as a consequence of the increased complexity of driving circuitry required to manage the much higher pixel density. This can result in the interference or ‘noise’ emitted from the displays being up to 3-4 times higher compared with normal HD displays. This can create problems for touchscreens and their control electronics in terms of identifying the signal (or touch) from the surrounding noise, i.e. decreasing the signal-to-noise ratio, and thereby impairing the identification of true touch events.

The delay (lag) or latency in the touch response of displays operating at 4K resolutions is probably the key issue to consider when planning combining touchscreen functionality with UHD displays. This is a challenge facing all touch technologies. Typical HD displays used with touchscreens today have a pixel refresh rate of around 120Hz. The data processing requirements necessary to control the extremely large number of pixels (>8 million) involved in a UHD display mean that most current 4K displays operate at 60Hz or less. This makes it challenging to deal with real-time touch events, such as dragging a cursor around a screen, since the display on board processing power is effectively being consumed by refreshing the background image. Consequently a moving touch event on a UHD display, like a drawn line, appears to trail the finger noticeably more than on a HD display. Even p-cap touchscreens reporting touch events at millisecond speeds will be caught up by this latency. Of course this delay is not noticeable during ordinary viewing (with no touch), as the viewer has no frame of reference by which to measure it. For dynamic touchscreen applications requiring drawing or the rapid dragging of objects, however, the 4K system’s delay compared to current HD based touchscreens will be obvious. As newer 4K displays enter the market with higher refresh rates this issue will diminish. But until then careful consideration must be given to the kind of touch application running on a UHD display and its likely effect on user experience.

To combat higher noise levels (EMI) on 4K displays it will be necessary for touchscreen manufacturers to improve the detection algorithms within the controller
firmware to optimize the signal-to-noise ratio. However, the above mentioned latency issue requires advancements by the UHD display manufacturers in respect of how the system handles external real-time inputs, possibly requiring faster processing technology to be utilized.

Zytronic, thanks to its expertise in p-cap sensing technology, has been able to engineer sophisticated proprietary firmware algorithms within its touch controllers that are optimized for UHD displays. Furthermore the company’s p-cap sensors ultra-low resistance copper matrix aid high signal-to-noise ratio in touch sensors up to 85 inches.