Large TFT-LCD Panels Shift into High Resolution

Pixel densities are increasing, but at different rates, depending on device, application, and end-user desire.

HIGH RESOLUTION has become a key feature for smartphones and other mobile devices that display information content approaching that of PCs and TVs. Because these devices are viewed at close range, users can appreciate the high pixel density. Mainstream TVs – even "high definition" ones – as well as notebook and desktop displays, are still typically at modest resolution. But there are two market segments in which TFT-LCD panel makers, as well as the device makers, are looking to move to higher resolutions: high-end TVs and tablets.

TV: High Definition Not High Enough

It is clear from discussions across the supply chain that the industry is moving toward 4K x 2K (3840 x 2160) resolution TVs, which have four times the information content of current 1080p products. These have been called quad-full-HD (QFHD), ultra-definition (UD), and other terms to represent that the resolution is four times that of full HD (1920 x 1080). Panel makers have all demonstrated prototypes, and almost all plan to mass-produce these panels in 2012 (Table 1).

From a broadcast or packaged media point of view, any increase in content format is still a long way off, with many markets still to make the move even to HD. It is also hard to justify in terms of image quality: If you watched TV from farther away than 3 m (10 ft.) you would need a screen size of at least 55 in. to notice the difference. Any smaller, and it would be beyond the resolution of the human eye.

However, 4K x 2K displays enable the possibility that a TV could simultaneously display four full-HD (1920 x 1080) inputs. Another possibility is 2K/HD zoom mode, which can accept 2048 x 1080 or 1920 x 1080 signals and scale them to the full screen by doubling the size horizontally and vertically. There are other reasons to introduce higher resolution, even where it is not viewable. The most obvious is for passive 3-D glasses. Doubling the number of lines is necessary to restore 1080 lines to each eye and would overcome the main objection to passive 3-D. However, the ability to produce a retarder film with such fine resolution has yet to be demonstrated. (For another viewpoint with regard to the passive-retarder approach to 3-D and perceived resolution loss, see the article "Resolving Resolution" in this month's issue.)

Achieving 4K x 2K TVs will not be cheap. 4K x 2K video requires four times the signal bandwidth and memory, and it will require significant post-processing if the up-scaling is going to be worth viewing. Pixel rendering or pixel simulation uses a processor to simulate the pixels and scale the standard-definition content into ultra-high definition. Toshiba has implemented such an approach with its cell processor technology. It uses a 1-TB storage system built into the cell processor's personal video recorder.

In the convergence enabled by connected and "smart" TVs, 4K x 2K can virtually enlarge the TV screen's desktop within the same panel size, so the screen appears larger. A larger desktop can offer space for multiple sources, such as broadcasting, HDMI inputs, apps, and the Web. Furthermore, video conferencing and IP cameras can be put onto the TV desktop side-by-side. Because of this, TV displays can serve as a large dashboard on the wall with home networking (broadband router connected, other TVs, PCs, and other CE devices).

Tablet PCs: A Bigger Smartphone?

While displays used in notebook PCs and desktop monitors have improved greatly over the past decade in terms of viewing angle, color gamut, and form factor, there has been very little change in pixel density. Mainstream notebook PC and desktop monitor formats such as 14-in. 1366 x 768, 15.6-in. 1366 x 768, and 21.5-in. 1920 x 1080 have pixel densities that fall into the range of 100–110 ppi (pixels per inch). Given the mostly fixed viewing distance of 50–60 cm for these applications, the market has decided that anything over 100 ppi is acceptable and that there are few benefits to going higher.

However, given the different usage modes for tablet PCs – closer viewing distance, content consumption over content creation – the requirement for pixel density is between smartphones and notebook PCs. Tablet PCs have a bigger screen than smartphones and better usability (size, user interface, startup time) than notebooks. At the same time, market competition between tablet PC makers and the need for product differentiation are pushing resolutions higher (Table 2).

This trend can be seen in the competition between 9.7- and 10.1-in. tablet PCs. Currently, nearly all 10.1-in. tablet PCs are 1280 x 800 formats (150 ppi); this is higher than the iPad's 1024 x 768 pixels (132 ppi). The next iPad, however, is likely to move up to 2048 x 1536 resolution or 264 ppi. Consequently, panel makers are now planning to increase 10.1-in. resolution to 2560 x 1600, which is over 300 ppi.

A similar trend can be seen in the other key size for tablet PCs, 7 in., for which current resolution is typically 1024 x 600 (169 ppi). Panel makers are now developing 1280 x 800 and 1366 x 768 resolutions for 7-in. panels (over 200 ppi); 1920 x 1080 resolution (314 ppi) is possible in the future. The Android OS platform seems to be encouraging higher resolutions in large screens (Table 3).

It will be a challenge for panel makers to develop 300-ppi panels. Higher pixel density means tighter design rules in panel fabs and makes process stability more difficult, due to glass substrate size and equipment limitations. For example, in Gen 5 and 6, design rules are typically 3–4 μm for amorphous-silicon (a-Si) and 2–2.5 μm for low-temperature polysilicon (LTPS). A 3–4-μm pixel design rule can achieve 200–250 ppi, enabling such products to be produced in existing Gen 6 a-Si fabs. Above 250 ppi, LTPS is necessary, and the largest existing fabs are Gen 4, typically used to produce panels smaller than 10 in. In addition, tablet PC panels require the use of wide-viewing-angle technologies such as IPS or FFS, which are not produced on all fab generations.

There are other challenges to the production of high resolutions; one is power consumption. Low power consumption is an important feature for tablet PCs, but higher resolution increases power consumption because transmission is lowered; at the same time, tablet PCs require high brightness for outdoor usage. Therefore, more LEDs are needed in the backlight unit, which entails higher cost, increased heat, and a thicker backlight unit, making it more difficult to achieve thinness and light weight. The typical strategy of panel makers is to design the panel for high brightness and make the brightness adjustable, which increases battery lifetime through the use of dimming. While the standard brightness of 9.7-in. panels used in the iPad is 400 nits, 200-nit panels are being used in white-box tablet PCs in China.

Another challenge is the panel interface. The most common interface, LVDS (low-voltage differential signaling), requires increased numbers of the cables and connectors to deal with high resolution, making it harder to achieve a thin panel design. The larger number of signals increases the transmitter and receiver pin counts, increasing cost, and can result in a frequency shift, making transmission more difficult and increasing electromagnetic interference (EMI). Thus, for formats with more than 1500 rows, eDP (embedded DisplayPort) may be necessary. The eDP interface enables the use of a simple connector and lower pin counts, but to date has only been used in mobile and all-in-one PCs.

Finally, the shift to higher pixel densities could impact the development of new display technologies. Innovations that enable higher pixel density – such as metal-oxide TFTs and non-RGB pixel architectures including PenTile (developed by Clairvoyante, now part of Samsung Electronics) – will be in demand. For other technologies, such as AMOLED technology, for which pixel density is currently limited by manufacturing technologies, the move to high pixel densities could serve as a barrier to entry. As seen in Table 1, oxide TFT is a leading approach for achieving high pixel densities on large panel sizes in a cost-effective manner. Oxide TFTs are being pursued for TFT-LCD as well as AMOLED backplanes, and could serve to level the playing field to some extent.

High Hopes for High Resolution

Both 4K x 2K and 300+-ppi tablet displays represent a chance for panel makers to differentiate themselves and for device makers to create added value. Both require technical innovations in panel manufacturing as well as in interfaces and video processing. In the near term, the costs will likely limit such high-resolution products to a niche, but if consumers can be convinced that there is a significant benefit in terms of the capabilities of these products, high resolution can enter the mainstream. ?