MEMs screens speed up e-readers

SCREEN TECHNOLOGY is about the only reason that e-book readers exist as a category distinct from other mobile computers. Their displays, typically using an e-paper layer from E-Ink, draw so little power that batteries can last for days rather than hours between between charges.

But anyone who has used an e-reader knows their drawbacks. There is none of the fast interactivity of a normal computer and the nearest the screen gets to white is a parchment colour that offers poor contrast and so requires strong ambient lighting for reading. E-Ink has promised colour for this year.

The power drain is low because the screens are bistable, requiring power only when they change state. Stick an image on screen and it will stay there without power, as if you had drawn it on paper.

However these screens do draw significant power when they change state, and the overall drain is low because book readers turn the page at a leisurely rate. If you tried to run video on an e-reader screen (assuming it could switch fast enough) it could be thirstier than an LCD.

So the race is on to find a screen with the advantages of both E-Ink and the LCD, or at least an acceptable compromise between the two. Most people, for instance, would be happy to get a fast interactive device that could comfortably last a day, rather than several days, between charges.

As The Inquirer reported last year, one possibility is being touted by Qualcomm MEMs Technology (QMT). Its Mirasol displays use a matrix of tiny cavities that act as optical resonators that can be tuned to emit red, green or blue when light falls on them.

The cavities are opened and closed by micromechanical shutters and the system is bistable, though its voltage regulation circuitry draws around 1mw even when the image is static. QMT says the overall power drain is still far lower than that of an LCD, and even of an E-Ink display, given the way e-readers are used in the real world.

The company's figures on this are shown in the chart above: you may take them with a pinch of salt but they do make the valid point that people browsing magazines and newspapers turn pages far more often than when they are reading a book, and each turn draws power even on an E-Ink screen. This is important because e-readers, whether dedicated models or modules in general-purpose tablets, are expected to become the dominant publishing medium.

And if E-Ink machines make more use of animations, as expected, then their power consumption will rise

Mirasol displays have so far been used only in tiny handset screens capable of showing video at 15 frames per second. But QMT has been showing off a stylish e-reader (pictured top) with a 5.7in Mirasol screen that the company says will be launched later this year (this format, pocketable yet large enough for serious use, is interesting in itself).

The whites on the Mirasol display are muddy and the colours lack the vibrancy of LCD, though your eyes do adjust to them as you read. Qualcomm's public relations manager Beatrix Hernando put it rather more delicately: "It's a different aesthetic."

QMT says the screen can show video at 30 frames per second but a recent demonstration at Mobile World Congress in Barcelona could manage only a sluggish 15fps. Hernando said 30fps would be available in production models.

Nearby Philips-spinoff Liquavista was showing a 6in display based on a technology called electrowetting Each pixel contains drop of water lying above a layer of coloured oil on a transparent hydrophobic surface - ie one that tends to repel water. When a voltage is applied the oil is pushed to one side, so that the water ‘wets' the surface. The display can be reflective, backlit or both (transflective).

The screen is not bistable but the power drain compares favourably with backlit LCDs and OLED displays, drawing in reflective mode only just over a third as much when screening video, according to Liquavista (see chart above - iMoD refers to a Mirasol screen).

The colours in reflective mode looked similar to those on the Mirasol display and the white was muddy. Eric Derckx, Liquavista's vice-president of engineering, said this was simply the colour of the substrate on the demonstration screen and production models would give a better white.

The colours were more vibrant when backlit, but of course this would draw more current. The screen is surprisingly fast. Derckx was using a TI OMAP 4 system-on-a-chip to screen a video at 30 frames per second video but he said it could go even faster.

The other big advantage of the screen is that is can be largely made using existing LCD technology, Derckx said.

Another technology demonstration by TI literally added another dimension to mobile displays, delivering 3D video on a 3in handheld screen with no need for special glasses. It used a transparent layer with a corrugated surface that presents a different image to each eye.

The system again used a TI SoC to drive the screen at 60 frames per second - 30fps for each eye - to give 3D picture. The stereoscopic layer was announced last year by manufacturer 3M but the idea is not new: Sharp has used a similar system to create dual-image screens that could, for instance, display a map to a driver while a passenger watched a movie.

The 3M layer would not work out so well on TV screens because it has a narrow angle of view, but that is not an issue with handhelds which are held directly in front of the viewer. TI's demonstration system could be used to watch 3D video and even create it - twin cameras for stereo capture were mounted at the back.

But of course it had all the components necessary for a 3D videophone, an idea more feasible and useful than might appear at first. Compressed stereoscopic streams would require well below twice the bandwidth required for normal video. And, judging from a demonstration at CeBIT a few years back, you don't realise until you have experienced a 3D videophone link how much you miss with a 2D image. µ