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Why QHD Smartphones Might Burn Two Holes in Your Pocket

by Stuart Robinson | Mar 01, 2015

So here we are on the eve of another Mobile World Congress and my thoughts turn to what amazing new devices and technologies are going to be revealed this year.

One of the topics that I’m going to be following closely is displays, and I’m looking forward to getting my hands on the latest curved or even flexible displays, high colour-gamut displays, displays with built-in solar cells, and especially high-resolution smartphone displays. 

There are many smartphones on the market now with a QHD resolution display (2560 x 1440 pixels, often incorrectly referred to as 2K) including the Samsung Galaxy Note 4, Galaxy S5 LTE-A, LG G3 and Motorola Nexus 6. However, the first couple of smartphones to incorporate a QHD display were from much less well known vendors Vivo and Oppo, with their Xplay 3S and Find 7 models respectively. 

It’s unlikely that we’ll see an Ultra HD display in a smartphone this week. A 5.5-inch UHD (3840 x 2160 pixels) display doesn’t exist yet – not officially at any rate – and equates to a pixel density of around 800 pixels per inch. I don’t know any vendor that has a commercial grade 800ppi display yet, so I think we are going to have to wait a little longer for a UHD-equipped smartphone.

But there are enough challenges with a QHD display, let alone a UHD one, and even if a display with 800ppi was available and production ready now I don’t think we would see it in a 5.5-inch smartphone for another 6 to 12 months.

Going back to the title of this blog, why might QHD displays burn two holes in your pocket? Firstly the cost and secondly the heat. QHD displays are not cheap and the high resolution display necessitates other highly-specd components too.

As well as sporting an amazingly high resolution display many of the latest smartphones are also able to capture UHD video content at 30 frames per second or higher. But some users of the early devices were reporting thermal overload to the point where the device even shut down because it became too hot.

So, why are some smartphones with a QHD display and UHD video capture causing such thermal problems? There are several reasons:

- Number of pixels. A QHD display is made up of 3.69 million pixels, each of which has 3 sub-pixels (RGB) with 8-bit colour depth. That’s 88.47 million raw bits of information for each frame. 

- Frame rate. If the display panel is refreshed 30 times per second then the total number of bits of information could be as high as 2.65 billion bits per second. That’s without compression of course. 

- Codecs. Fortunately H.264 and HEVC (H.265) compression algorithms reduce the data flow to the display significantly. HEVC is approximately 50% more efficient in terms of bitrate than H.264.

- Hardware vs software implementation of the codec. The H.264 standard was ratified about 10 years ago and the most efficient implementations of it are done in silicon. However HEVC is still new and hardware implementations are only just coming onto the market now. Software implementations running on the CPU are not as energy-efficient as the dedicated hardware implementations.

Of course, I can’t finish without saying something about the latest Samsung/Qualcomm spat. Samsung reported last month that its upcoming Galaxy S6 would no longer be powered by the Qualcomm Snapdragon 810 processor due to thermal issues. It’s possible that the 810 has some power consumption challenges; for a start it’s based on the off-the-shelf ARM-Cortex A53 and A57 cores rather than Qualcomm’s own customised 64-bit architecture, which we are likely to hear more about this week and which normally offer significant power/performance improvements. However, Qualcomm has dozens and dozens of customers for its Snapdragon 810 processor, including the new HTC One M9 announced today, and none of them apart from Samsung is claiming thermal issues. Could it be that there is a hidden agenda here and that Samsung is using this as an opportunity to promote its own new octa-core Exynos 7420 processor instead? Time will tell.

So, the challenges with Quad HD displays will be all-but forgotten in a year’s time; the displays themselves will be much cheaper and the processors that drive them will be much more efficient and less power-hungry. Looking ahead to this time next year I predict that I’ll be talking about the challenges of UHD displays in smartphones and tablets, rather than QHD. Can’t wait.

Stuart Robinson

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