From the Bench

DUAL-LAYER TV SCREENS The latest form of LCD display is the best yet, says Alan Bennett

Since the demise of plasma TV screens in 2014 there have been two basic direct-view display technologies: LCD and OLED. Each has its pros and cons.

RESTRICTIONS

Both LCD and OLED displays can be made in large sizes and with very high defi nition. LCD is by far the most common type, used with an LED backlight. It gives the highest brightness levels, especially in the more expensive QLED and Quantum Dot forms. Its drawback is that even when its LCD light shutters are fully turned o there is slight leakage of illumination through them so that a true black cannot be achieved, though it can be reduced by ‘local’ and full-fi eld dimming of the backlight.

OLED screens, because each pixel is self-emissive, can achieve a true black: each diode can be fully turned o . The drawback here is that they are unable to reach to a very high brightness level – driving them too hard reduces their operational life. Viewing in high ambient light requires a bright screen display, and to fully exploit HDR (High Dynamic Range) systems, good black level and a high brightness capability are needed. Micro-LED screens (see our August 2019 edition, page 19) can achieve both but are not yet available in economic or mass-market form. DUAL-LAYER LCD Dual- layer screens are based on LCD technology, in which polarised light is ‘twisted’ in a multi-point liquid crystal display panel so that it is blocked by a downstream polarising fi lter working at right-angles to the fi rst. It is not fully e ective, and the brighter the backlight the more light leakage occurs. Dynamic local and overall dimming of the backlight – incorporated in more expensive models – helps, but is not targeted tightly enough to completely eliminate light bleed and blooming e ects in the image. A dual-layer screen has a second LCD array behind the main one; their pixels are aligned so that what little light escapes through a fully turned o rear pixel is intercepted by the outer one, reducing the black level to virtually zero, comparable to that of an OLED screen. The white LED backlight can be very bright and uniform in intensity over its full area. In

e ect the inner layer produces the black and white (luminance) component of the picture while the outer one provides the colour. It is very e ective, but expensive to produce. PANASONIC MEGACON Exhibited at IFA 2019 was Panasonic’s Megacon (Mega contrast) prototype, a 55 inch UHD screen with dual-layer technology. Both its LCD screen layers have UHD/4K defi nition so that individual pixels can be blacked out without any e ect on adjacent ones. This is a huge advantage over even the ‘tightest’ local dimming systems, in which the limited number of LED backlight clusters causes whole bunches of pixels to be darkened. The Megacon has a claimed contrast ratio of 1,000,000:1, with a maximum brightness of 1000 nits, the level currently achieved by top-end conventional LCD/LED screens with QLED and Quantum Dot technology. A wide angle of view is another attribute, according to Panasonic. This screen, as exhibited at IFA, is several centimetres thick, and at present likely to fi nd a role as a top-level studio reference monitor at a price expected to be in the tens of thousands of pounds. It is not yet envisaged for the home cinema market, but all such innovations have the potential to appear there in the fullness of time; it will need to slim down, perhaps physically and certainly price-wise, even to become a niche model for well-heeled movie enthusiasts in the future... “Viewing in high ambient light requires a bright screen display, and to fully exploit HDR (High Dynamic Range) systems, good black level and a high brightness capability are needed.”

Megacon screen layers

HISENSE U9E

Also featured at IFA 2019 (and already launched in China) was the Hisense duallayer TV model, expected here this year at a price comparable with currently available high-end TVs. It’s not up to the standard of the Panasonic Megacon! This 65-incher has a UHD outer screen layer but the inner one is coarser at 1080p defi nition, providing one LCD lightblocking cell for each four pixels in the image. This does not impinge on the UHD resolution as viewed, but provides less tight targeting, so to speak, and hence a contrast ratio around 150,000:1, aided by a new low-refl ection outer surface. It can, however, produce a peak brightness of almost 3000 nits. In terms of brightness, then, it is way ahead of all conventional TV screens and second only to the emerging Micro-LED type (August issue, page 19). This is more than adequate for all home cinema needs. In assimilating these brightness levels bear in mind that an LCDbased screen can achieve its peak level over the whole image area when required, while OLED ones can only manage their quoted top level over a smaller area of the picture.

Dual-layer screens, especially of the sort developed by Panasonic, will be highly priced but our industry thrives on innovation: remember that when colour TV started, and again at the launch of thin-screen TVs the sets were una ordable for most people. Now mass-production, competition between manufacturers and world-wide research, production, manufacture and marketing are driving prices down as never before, though not always to the advantage of the retailer...

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