What are the implications for the display in a virtual reality headset design? Until recently, refresh rate has barely been an issue for embedded system developers, but virtual reality is starting to change that.
The majority of display controllers refresh the screen at 50 or 60Hz – roughly once a second, and that’s been plenty fast enough even for most video content. Higher refresh rate 90Hz or 120Hz monitors are available, but typically only gamers need their screens refreshing more frequently to keep up with rapidly changing game scenarios presented in real time at high resolution.
Virtual reality headsets, though, will be a professional as well as a consumer electronics product. Currently simulators used to train pilots, operators of nuclear plant, surgeons and others typically use very large screens to present content in real time. Giving the user a headset would clearly be a great deal cheaper – but does have implications for the display.
Though simulators present fast-changing scenarios developing in real time, on a screen just a few centimetres from the wearers face. A refresh rate of 60Hz is no longer enough to present the motion crisply without blurring, and we are starting to see interest in 90Hz or 120Hz refresh rates.
Emerging technologies like LTPS and IGZO offer higher electron mobility and potentially support faster refresh, but these technologies aren’t readily available on the industrial market. They are expensive and associated with high MoQs . Fortunately, there is no need to move to these technologies to achieve 120Hz refresh. It is perfectly possible to obtain a fast refresh with high resolution amorphous silicon screens.
Doubling the speed doubles the amount of data that the IC has to deliver to the screen. Most driver ICs are designed to work with up to 60Hz refresh – but faster refresh can be achieved using multiple driver ICs in tandem to deliver the required data rate. A good architecture is to use three LCD display driver ICs – two source drives and one gate drive.
For these headsets it often makes a lot of sense to specify a semi or full custom display, as the combination of size and performance needed will be very specific.
It can be perfectly economic to do this, as often you are able to remove unwanted elements to reduce or even eliminate the cost premium over a catalogue display. You’re also guaranteed long term availability of the product.
Having run through some of the most important parameters in display selection, our next couple of knowledge base posts will explore the design decisions associated with choosing the processor board or module.
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