So Good, It Hertz?

What is Reality?

The "Plato's Cave" allegory goes something like this: Imagine a deep underground cavern where prisoners have lived their entire lives chained to rocks, their heads immobile and facing one cave wall. Behind them is an illuminating fire. Between the fire and prisoners, statues of all sorts move back and forth. The prisoners only see the statues' shadows on the cave wall. The shadows are the only reality they know, and they accept the shadows as reality.

But after much effort, a prisoner breaks the chains, and leaves the cave. He is blinded by the sunlight, and cannot comprehend the shapes and colors he sees. They are so different from his shadow reality. But eventually he understands that the world outside the cave is the real world.

Now suppose the enlightened prisoner returns to the cave. He desperately wants to explain to the other prisoners that they're deluded, that the shadows are not reality. But after seeing the sunlight, he cannot see in the darkness of the cave. He stumbles, and cannot see the shadows anymore. The other prisoners believe that his trip outside the cave has ruined his eyesight, and made him useless in their world. The chained prisoners refuse to leave the reality of the cave.

Persistence of Vision and LCD's Lament

Reality is only as good as our senses that perceive it. There's a fair amount of leeway in how we interpret even the basics of reality. Example: If you take a notepad and, on each page, draw a stick figure walking across, moving it forward a bit on each page, and then you flip through the pages, you'll see a moving figure emerge from the static drawings. More than anything, it's a function of persistence of vision - essentially a chemical response-time limitation of our brains. Motion pictures take advantage of it; a series of still images are projected at a rate of 24 fps (frame per second) and the brain buys the series of stills as a moving image. Clever. Video does the same thing, at 60 refreshes a second. Regardless of the source rate, a TV display can show an image at any speed. For example, it can refresh at 120 or 240 Hz. That brings us to the heart of the technical question, and to a philosophical question as well.

LCD screens are terrific, providing a gorgeous picture even in brightly-lit rooms. But, they have traditionally fallen down when displaying motion - particularly in early displays, it all seemed like a blur. One reason for this "ghosting" was slow response time; the LCD elements couldn't change fast enough. That's gotten better as times of 8 milliseconds or less have come online. But LCDs still have an inherent problem: Each frame is held, for example, a full 1/60 of a second before it's abruptly replaced by a new image. This can look unnatural. Plasma and DLPs, though, don't have this problem: Images are briefly pulsed onscreen, and that looks more natural. One way to improve LCD motion reproduction is to increase the refresh rate. A rate of 60 Hz can look good, but does 120 or 240 Hz look better? The unequivocal answer is . . . maybe.

Faster refreshing is well and good, but it's only part of a larger technical problem, and there are very different approaches. In some 120-Hz sets, the 'pulse' thing is emulated by inserting a black frame between each bright frame. The result is terrific motion reproduction. But the black frames dim the picture, and consumers take a dim view of that.

One more technical issue: judder. The question of motion smoothness depends on the source material itself. When film is transferred to video, 2:3 pulldown is used to upconvert from film's 24 fps rate. A slight timing error occurs and slow, steady camera movements can appear slightly jerky. This visual artifact is called judder.

Motion interpolation, also known as smoothing, is needed to de-judder the signal. Thanks to a little processing power (well, a lot, actually) a midpoint morphed frame is inserted between each original frame. The quality of the result depends on the quality of the interpolation.

Kenneth, What is the Frequency?

So, what's going on here? The 120-Hz (or higher) thing is a hot selling point, but exactly what should you look for when buying an LCD TV? Let's consider some key points.

For starters, newer generations of 60-Hz refresh sets have minimal ghosting or blur problems with fast motion. And a set that only does a 120-Hz refreshing (no de-juddering) won't display motion tremendously better. And don't be fooled: In many cases, the blurring you see is in the content itself, and will look blurred on any TV. So, 120 Hz, by itself, is not a cure-all.

Many companies combine a 120-Hz refresh rate with de-judder smoothing. They call their algorithms different things: Samsung's Movie Plus, Sharp's TruD, Sony's Motion Flow, and Toshiba's Film Stabilization are all similar in concept. (Be aware that some TVs don't use de-juddering at all). When viewing film-based content, 120-Hz, de-juddering video processing can significantly improve the look of slow pans. Sony's Motion Flow has two settings. The standard setting yields a more stable image; the high setting looks even more solid. But be careful - both settings can introduce small, momentary motion artifacts of their own. More interestingly, de-juddering can sometimes cause the motion on a 120-Hz set to look unnaturally smooth compared to the images we're used to seeing.

Plato's Revenge

And there's the philosophical rub. As Plato taught, there's reality, and then there's reality. We generally accept the reality we're most accustomed to. If we're accustomed to shadows on the wall, we're fine with that. Same thing applies to our task of creating the appearance of motion out of still images. We'll probably accept the version of the illusion we're most accustomed to, or in any case, the reality that somehow seems "right" to us.

Judder that occurs during pulldown should be removed because it's clearly an artifact. But what about high refresh rates and de-juddering that makes a movie look smoother than the original projected 24 fps film? Film inherently has high judder, and that's one reason it has a different look from video. If you like the motion look of film, you'll probably not like some of the de-judder processing. On the other hand, if you like the motion look of video, you might like de-juddering a lot. Or maybe you're in between - you want less de-juddering for films and a little more for native video. Or maybe your preference varies with the actual images and scenes. In any case, there isn't really a right answer. Your choice is simply a question of which reality seems right to you.

Bottom line: Higher refresh rates will become standard on all TVs and that's okay; 120 Hz and higher can improve viewing when coupled with interpolation smoothing. But those smoothing algorithms will require some careful scrutiny on your part. For starters, they're works in progress, and I think they'll improve over time. For now, some work better than others, and in any case, some will appeal to you more than others according to their "look." I would personally strongly prefer any system that lets you vary the degree of processing, according to content and taste. They're all creating an illusion, and that's a tremendously subjective thing. In other words, perhaps more than ever, when buying a 120-Hz TV, you need to do careful evaluations. You don't want to put just anything up on the cave wall.