Local Dimming LED

The greatest thing to happen to LCD, ever.

The coolest demo I saw at CEDIA 2007 was a demo I saw at CEDIA 2006. The original demo was at the Planar suite. Dolby now owns the company that was working with Planar, BrightSide Technologies, and the technology shown in these demos has a name—Dolby Vision. The short version is this: Using LEDs, you can dim specific areas of the backlight to go along with what is happening with the video. In other words, you can dim certain areas of the screen, while keeping other areas bright. In the simplest form, picture a split screen with black on one side and white on the other. Local dimming would allow the LEDs on the black side to be off and the LEDs on the white side to be lit. The result is a fantastic, legitimate contrast ratio, along with possible energy savings and a host of other potential benefits. But first, we have to understand the problem before we can talk about this solution.


Figure A: LED Backlight, LEDs (inset)

Always On
LCDs don't create light. The light is created at the very back of a display, usually by a series of CCFLs or Cold Cathode Fluorescent Lamps (see Figure B). These are similar, but not identical to, the fluorescent lamps in your office, bathroom, or wherever you want cheap, usually harsh, lighting. These CCFLs are always on and, in most cases, at a steady light output. From here, the light passes through a polarizing layer, a diffusion filter (which is why you see a reasonably smooth field of light and not 10 or more hot spots), and, eventually, color filters and the LCD material itself. The liquid crystal's job is to block all of this light.

How well it does this varies, but for the most part, none do it terribly well. Some are better than others, and they've gotten a whole lot better; but no matter how well an LCD performs, it's still going to leak.


Both: From Left to Right, LCD, Filter, CCFL

5-Gallon Leak, 4.9-Gallon Bucket
No matter what you do, LCDs leak light. Because of the physics of how they work, even the best LCD can't completely block the light coming from the backlight. Worse, they often leak different parts of the spectrum differently. This is why you'll often notice a blue tint to black while the rest of the image looks fine. LCD manufacturers realize this, which is why they offer adjustable backlights. The LCD panel is essentially a bandpass filter, and adjusting the backlight moves it up and down. So, if you want a bright image, you can have it at the expense of a good black level. If you want a good black level, you can have it at the expense of light output. You get pretty much the same contrast ratio in any case; you just choose how much light you want.

Thus, the problem, as you can see, is the backlight. If it wasn't pumping out such a constant torrent of light all the time, LCD could compete with the other technologies in terms of onscreen contrast ratio. Enter LEDs.

Cathodes Are So Negative; Give Me Diodes!
Light Emitting Diodes have been around for some time, but only recently have they been available in a wide enough variety of colors, performance, and at reasonable prices. For more info on LEDs, check the July 2006 GearWorks (also online). There are a number of ways to do LED backlighting for an LCD TV, from different-colored LEDs bunched together to single LEDs with coatings to appear white (there is no such thing as a pure white LED). Either way, there are many LEDs spread across the back panel (see Figure A). On most LCD-based displays, these act in the same way as CCFL backlights. But several companies, including Dolby, decided to move to the next logical step: Individual lights should be addressed individually.


Figure B: CCFL

Local Dimming
In my simple example earlier, I mentioned a split black-and-white screen. Real video is a little different. Here's how Dolby's version of the technology works: The video signal comes in, and the TV does all its normal processing (deinterlacing, scaling, and such). Then Dolby Vision takes over and turns the 8-bit video into a sort of 16-bit. This is smoothed out and analyzed frame by frame. It then sends both the LCD panel and the LED backlight array 8 bits of video data, for a combined 16 bits of gray scale. Each LED (or perhaps group of LEDs) is addressed individually, just like each pixel on the LCD side.

It's a lot easier to understand when you look at the pictures. On the left in Figure C, you have the Dolby logo as it looks with just the LEDs active. (It's blurry because the light still needs to go through the diffuser/ polarizer.) On the right is what you'd see on the screen. Figure D is a side-by-side with the Dolby Vision prototype on the left and a regular CCFL backlighted panel on the right. As you can see, the bright parts of the image are much brighter on the left, while having a better black level. Here's the kicker: It's the same LCD panel. The only difference is the type of backlight. There's no photo trickery here. I took this photo at CEATEC. It's one shot, not two different shots with different exposures. Figure E is the same screen image, with the LCD panel shut off.


Figure C

Ooooh, Ahhhh
The end result is nothing short of amazing. Even using a 2-year-old, 8-bit LCD panel, the Dolby Vision prototype had some of the best picture quality I've seen. The onscreen contrast ratio was stunning. Bright areas of the image popped, and dark areas were dark—more so than any LCD, ever, and even maybe on par with the Pioneer KURO PRO-110FD. So, imagine what this will look like when it's implemented with current-generation glass.


Figure D

Of course, how well this works in real products remains to be seen. The hand-built Dolby prototype was using 1,380 fairly costly LEDs. Before your wallet shrivels up, keep in mind that, once you take into account mass-production volume and the rapidly decreasing price of LEDs, the cost above a non-local-dimmed LED LCD will likely be marginal.

The same is true of the power consumption. The prototype drew enough current to give you the kind of buzz that Starbucks wishes it could market. But it was a prototype and built to just show off what can be done. LED power efficiency (referred to in lumens per watt) is getting better so quickly that it is approaching parity with CCFL. It will undoubtedly get even better. Also, because local dimming reduces the power in some part of the screen at all times, overall power is likely to be less than other non-local-dimmed backlight displays at the same brightness levels, perhaps even if there are more LEDs overall. Or you could keep the power consumption the same as the CCFL and drive the brightness even higher. Who wants 300 foot-lamberts?!


Figure E

Lastly, the Dolby technology is scalable, as is the concept as a whole. Just because an LED-backlighted display is locally dimmed doesn't mean it will look as good as what we've talked about here. A manufacturer could have 50 LEDs or 1,000; obviously, the more the better in terms of picture quality (to a point). So my guess is that we'll see both ends of this spectrum. There will be some displays with a minimum of LEDs for a small improvement in picture quality, reduced power consumption, and lower cost. Then there will be the more premium sets that could well mimic the superb picture quality seen in the Dolby prototype but at a slightly higher cost compared with non-local-dimmed competitors. We won't know by how much or how good until we see them.

Regardless, local dimming is an extremely promising technology with the potential to elevate LCDs to real videophile levels, and we're going to see it in real products soon. In fact, by the time you read this, Samsung will be shipping the 81 series (as in LN-T5281F), which features their version of local dimming. Look for more on that down the road.

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