LED LCDs surpass plasma
Technology is at last making LCD TVs equal to the best of the plasmas, and even surpassing them.
The great struggle of LCD TV makers has been to produce a good quality black output in their TVs.
Unlike plasma TVs and cathode ray tube (CRT) sets, the LCD panels do not in themselves produce a picture. Instead they control the transmission of light.
Behind the LCD panel is another panel, this one producing a clean and even white light. The pixels of the LCD panel then go opaque as necessary to block the light.
Unfortunately, they don’t go opaque enough. This means some light leaks through and the result is, at best, a dark grey appearance for blacks.
If watched under normal room lighting, this may not be obvious. LCD TVs shine, so to speak, under those circumstances. But if, late at night, you turn down the room lights, you will notice that the dark scenes in movies simply aren’t dark enough.
Aside from being somewhat distracting, this dark glow reduces the level of detail in dark scenes. This is because the different levels of near-black are crushed together.
Over the years, LCD TV manufacturers have attacked this from two angles.
The first has been to gradually improve the native performance of the panels themselves. This is bearing fruit, with the native contrast ratios increasing from under 1,000:1 to over 2,000:1. But it isn’t, in itself, good enough.
The more revolutionary step was to take control of the backlight and vary its level according to the picture content.
In dark scenes, and even more so when the screen was supposed to go fully black, the picture processing circuitry would turn down the level of the backlight. This had an impact, of course, on how the picture had to be delivered to the LCD panel in that the pixels that were shown had to be lightened to take account of the lower backlight.
This works reasonably well and does give a much better overall result. In fact, some TV brands have rated their contrast ratios using such a system (often called the ‘dynamic contrast’) up to a million to one, but more commonly 50,000:1 or thereabouts.
But it has one weakness: in a picture where much of the screen is dark, but a small area is very bright, the backlight has to remain on at full blast making the dark areas lighter than they should be.
That’s where light emitting diode (LED) backlighting comes in. LED’s have been making a splash since the development of white LEDs in recent years. Instead of one light which must be all bright, or all dark, some new LCD TV models are appearing with backlights made with an array of LEDs.
These work somewhat like having a low resolution TV behind the real TV.
One example of this technology is the Samsung LA46A950 LCD TV. At nearly $6,000 it is quite expensive for a 117cm model, but behind its 1,920 by 1,080 pixel resolution LCD panel it has a backlight consisting of 640 LEDs. Each of these can be turned on and off, or set to nine intermediate levels of brightness.
The advantage of this can be seen by thinking of a dark scene in a movie where a door opens at one corner of the screen, allowing a glaring light to enter. Often such scenes are meant to contrast the brightness of the doorway with the darkness of the room, but with a ‘traditional’, variable backlight, the whole room will brighten. With an LED backlight, only the part near the door will lighten.
You might worry that the ‘splash’ of the lower resolution backlight LEDs would leave a glow around bright objects on a dark background, but in practice that isn’t the case at all. It seems that your eyes take care of the contrast ratio when it is in such close proximity; if you look anywhere near the bright part, the pupils of your eyes contract, darkening its immediate surroundings.
LED backlighting technology has at last allowed LCD TVs to perform as well, or better, than plasma TVs when it comes to black levels.
LED backlighting technology is also used for a different purpose: to provide lighting in a very compact package. The purpose of this is to allow extremely thin LCD TV panels.
A typical LCD TV has a thickness of between 90 and 120mm. Some of the thinnest ones have managed to halve this to about 45mm. Sony’s new Bravia KDL-40ZX1 LCD TV reduces this to just 9.9mm!
That’s not quite fair. The panel is actually 28mm thick at its thickest, but much of the display stands clear of the slightly boxy rear section, and truly is less than one centimetre thick.
It’s difficult to overstate how different, physically, this TV looks to other ones. If seen from any angle at all, it looks utterly slim. While it comes with a desktop stand, the ideal use for it would be for on-wall installation.
But all this comes at a huge price premium: $7,399 for a mere 102cm TV.
One reason for physical bulkiness in a TV is the need to have lots of connections: primarily inputs for all the different signals, plus TV tuners. This TV avoids all that by offloading those elements to a separate ‘media receiver’ box.
This box talks to the TV by means of a wireless connection, avoiding the need for any wiring at all to the TV, aside from a power cable. Sony says that the connection can extend up to 20 metres.
However, all is not quite as good as it at first seems.
That’s because, as with Sony’s lower cost wireless series of TVs (the EX series), the wireless connection supports resolutions of 1080i, but not 1080p. Not even in the 24 frames per second 1080p24 format, which actually demands less resolution than 1080i!
Consequently, if you feed the TV (and it’s the same with the EX series) with a 1080p signal from a Blu-ray player, or a high quality upconverting DVD player, the media receiver will turn it into interlaced 1080i format for transmission to the TV. The TV, on receiving this, will have to deinterlace the signal again.
Unfortunately, while this worked reasonably well on 60Hz US material, Sony TVs tend to be fairly mediocre on 50Hz Australian material, including 1080i50 Blu-ray discs (yes, there are a few around – see Connected Home Australia May/June, page 36).
Sony’s wireless TVs have, so far, all included a work-around: a single HDMI input on the TV itself into which you can plug a physical cable. This input is a proper one, supporting full 1080p24, 1080p50 and 1080p60 signals, so picture quality need not be compromised.
But this might, of course, detract somewhat from the attraction of the wireless system.