The Perfect Vision
Samsung LN-T4681F Additional Notes and Technical Ratings
Under the Hood | Adjustment Notes | Test Discs | Technical Ratings
Under the Hood
The LN-T4681F’s LED backlight is its most important feature by far, but to understand it, you need to know something about how LCD TVs work. All LCD flat-panel displays have a light behind the actual pixel array; this so-called “backlight” shines through the pixels much like sunlight shines through a stained-glass window. Each pixel can be made more or less transparent, letting more or less light through it, which is how different parts of the picture can be bright or dark or anything in between.
The problem is that LCD pixels cannot be made completely opaque, so a little light leaks through even in the darkest scenes. As a result, the blacks of most LCD TVs are not as deep as they are with other technologies.
Many manufacturers try to get around this by dynamically changing the brightness of the backlight according to the average brightness of the image on the screen—the backlight is lowered during dark scenes and raised during bright scenes. But changing the brightness of the entire backlight can be obvious and distracting, which is why I tend to turn this feature off and live with less-deep but constant blacks.
Up to now, the backlight has always been a set of fluorescent bulbs, much like the fluorescent lights in most offices and an increasing number of homes. The LN-T4681F is among the first LCD TVs to use LEDs (light-emitting diodes) instead of fluorescent bulbs for the backlight.
Why switch from fluorescent to LED backlighting? For one thing, LEDs have no mercury, making them more environmentally friendly. Also, the brightness of the LEDs can be controlled independently in small groups, unlike fluorescent bulbs, whose brightness can only be varied as a whole. This means the LEDs behind dark portions of the image can be dimmed while the LEDs behind bright portions of the image can be brightened, a technique called “local dimming.”
In essence, this creates a low-resolution, black-and-white version of the image in the LEDs behind the high-resolution, color version of the image in the LCD pixels. The end result is deeper blacks and greater contrast (the difference between the least and most amount of light the screen can produce) without varying the entire backlight as in traditional dynamic contrast.
The LED backlight makes it possible to implement another feature that purports to reduce motion blur—and it’s not a 120Hz refresh rate as with many other LCD TVs these days. Because the LEDs can be controlled independently, Samsung developed something it calls Motion Plus, which momentarily turns off groups of horizontal rows of LEDs in sequence from the top of the screen to the bottom once per frame.
Motion Plus is designed to reduce motion blur by momentarily turning off the backlight behind small slices of the image. LCD pixels turn on quickly, but they turn off relatively slowly, resulting in ghostly trails behind moving objects. By turning off the backlight behind the pixels momentarily, the visible effect of motion blur is said to be reduced. I tried turning Motion Plus on as I was looking at test discs, and it did seem to reduce motion blur slightly, but it also gave the picture a somewhat garish quality, and the slight reduction in motion blur was not worth the tradeoff.
Adjustment Notes
Contrast could be maxed out without clipping. Sharpness seemed to do nothing; it did not induce ringing at any setting, nor did it seem to sharpen or soften the picture, so I left it at the default setting for the Movie mode. I set Size to Just Scan for 1:1 pixel mapping, but even at this setting, one pixel was cropped from the right and three were cropped from the top. According to Samsung, the image is shifted up and to the right slightly to eliminate distorted edges that can often be seen in upconverted material.
With SmartLighting off and backlight at minimum, light output measurements were very respectable. With SmartLighting on, the Minolta LS-100 light meter measured a black level of 0.000fL! In order to avoid an infinite contrast ratio, I decided to specify a value of 0.001fL, which is the lowest accurate measurement of which the LS-100 is capable. With backlight at minimum, peak white level was roughly the same as with SmartLighting off, leading to a calculated peak contrast ratio of about 28,000:1, though it must actually be quite a bit higher because the black level was lower than I used in the calculation. With backlight at maximum, the black level remained the same, while the peak white level was almost 80fL, leading to a peak contrast ratio of nearly 80,000:1! (Again, the actual contrast ratio was undoubtedly quite a bit higher, but I felt it necessary to use the meter’s lowest possible reading in the calculation.) ANSI contrast was similar in both cases; the light level from the white and black squares both rose with the backlight level, leading to slightly better ANSI contrast with backlight at maximum.
At the default setting of the Gamma control (0), gamma measured around 2.5 in the low end of the brightness range. At a setting of +3, it was closer to 2.2. A value of 2.2 was the standard for CRT-based NTSC, but 2.5 is generally considered the standard in the high-def digital world. Thus, setting the Gamma to 0 yields the standard modern gamma, though I prefer the brightness to rise out of black more quickly than that, so I kept the Gamma setting at +3.
Grayscale and color primaries were about the same (that is, extremely close to accurate) with SmartLighting on or off. This means you can turn up the backlight for bright-room viewing without sacrificing blacks with SmartLighting on.
Adding Motion Plus into the equation increased the overall light levels at both ends, and the grayscale shifted slightly toward green. Motion Plus pegs the backlight level at maximum, whereas most of my measurements were taken with the backlight at minimum, which is why the overall levels increased. Comparing Motion Plus on and off with the backlight at maximum resulted in slightly reduced levels when it was on. Color primaries were close to the same in all cases.
Test Discs
Starting with the HQV Benchmark DVD at 480i, detail was very good, but jaggies were very evident on all three tests, including the waving flag. Digital noise reduction didn’t seem to do much; there was a slight improvement, so I left it on its Auto setting. Motion Plus did seem to reduce motion blur a bit in the roller coaster sequence, but the picture took on a slightly garish look, and the improvement in motion blur wasn’t worth it, so I left it off for most of my testing. The set’s processor picked up 3:2 pulldown at 480i quickly.
On the HQV Benchmark HD DVD, the video resolution loss test was solid as a rock, as was the film resolution loss test, though the latter had a moment of shimmering at the beginning and occasionally when the pattern changed direction. There were no jaggies to be seen.
Microsoft engineer Stacey Spears’ test HD DVD revealed that the set’s video processor picked up 3:2 pulldown at 1080i almost immediately. In the montage of clips, there was a bit of shimmering along moving diagonal lines, but it wasn’t bad overall. There was no moiré in the screen-door shot, though there was some shimmering in the gently curving yellow trim on the sailboat.
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