4K gets touchy

Ian Crosby
Sales and Marketing Director
4K HD displays
There are some technical hurdles to adding a touch interface to 4K displays

As 4K or UHD displays become more popular, they are inevitably being integrated with touch sensors. There is no reason why this shouldn’t be done successfully, but there are some technical hurdles that need to be overcome if 4K touch displays are going to truly deliver.

4K displays currently generate higher levels of electromagnetic interference (EMI) because 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 than normal HD displays. This makes it harder 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.

Improvements to the electronic design and touch detection firmware employed by the touch controller are needed to ensure that signal integrity is maintained at a high level. P-CAP touch technologies such as Zytronic’s proprietary Projected Capacitive Technology (PCT™) have an X-Y matrix of micro-fine electrodes, embedded within a laminated glass substrate, and use frequency modulation to detect minute capacitance changes within the conductive electrodes. One way to combat EMI is to implement a ‘smart’ frequency-scanning function in the touch controller. The operating frequency moves dynamically between 0.7MHz and 2.2MHz in order to avoid detected environmental ‘noise’ that would otherwise prevent the detection of 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 and potentially improved drive circuitry to reduce EMI these issues will diminish. But until then careful consideration must be given to the choice of touch controller and to the kind of touch application running on a UHD display and its likely effect on user experience.

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