First it was 120 Hz. Now it’s 240 Hz. Already manufacturers are talking about 480 Hz. I mean, 60 Hz is so 20th century. Are there really any advantages to these faster refresh rates? If so, how do they work?
Well, to find out, I’m gonna bring back Norman.
You remember Norman. Norman was our friendly visual aid for my modestly titled Definitive Guide to Video Processing. Sit, Norman, sit. Good dog.
Let’s start with the simplest scenario. Norman was recorded on video at 60 Hz (Hertz, as in the number of times per second). In Figure 1, you can see how he was recorded: moving from the lower left of the screen to the upper right. He’s moving rather quickly (a good trick for a sitting dog).
A video camera is just like a regular camera: It takes a series of still images. These are played back by your TV fast enough so they seem like motion to your brain. At 60 times per second, this is above your flicker-fusion threshold, and as such appears smooth. The “persistence of vision” you were probably told about in school is, as far as TVs are concerned, nonsense. More on this later.
So as the camera cycles (60 pictures per second), Norman has moved. He’s moving faster than the camera can capture him, so when you slow down the series of images, he appears to jump from frame to frame. Not a big deal, as it all gets kind of muddled up in your brain and it appears to be smooth.
There are multiple ways of dealing with twice the number of frames with 120 Hz TVs. The simplest solution is to just double each frame (Figure 2). All 120 Hz TVs, though, have motion interpolation.
This can go by different names (MotionFlow, Auto Motion Plus and so on), but they all basically do the same thing: Create new frames to insert between the old ones. They look at the original Frame 1 and Frame 2 and create a brand new Frame 1+2.
What varies is how many frames each system looks at and how “intrusive” the interpolation is (this can be adjusted on many TVs). For example, Figure 3 illustrates a mild version of the interpolation; the TV creates a frame only a little different than Frame A. Stronger interpolation (Figure 4) has a frame very different from either—a more exact hybrid of the two original frames.
Not So Fast
At first glance, this may seem like a great thing, and in one way it is. The smoother motion, as far as video is concerned, is actually a happy byproduct of the real reason LCDs moved to 120 Hz (We’ll get there in a bit). Strong motion interpolation, on video at least, isn’t that big of a deal. For most people, the added smoothness won’t look very artificial. In fact, it may be pleasing. The problem is with film.
If you remember from the video processing feature, film is 24 frames per second. To show a movie on a 60 Hz display, you need to create a 3:2 sequence (more accurately, 2:3). The first film frame is shown twice, the second frame thrice, the third frame twice and so on.
This entire sequence can be doubled again for 120 Hz displays. More often what happens is the TV will interpolate new frames to fill in the 3:2 sequence. This is where it gets rather cerebral.
Film has a different motion than video. Think of how a movie on DVD looks compared with how your evening news or a soap opera looks.
It is so engrained into our collective psyche that nearly everyone equates the motion of film with fiction, and the motion of video with “real life.” This is why Blair Witch Project seemed different, and why Coverfield was stupid. Blair Witch tries to portray a real situation, like a documentary, by using video. It comes across like something you would have shot in your backyard (yet, with worse acting and writing). Cloverfield tried to do the same thing… but somehow that cheap camcorder was outputting a flawless 24 frames per second. Riiiiight…
Motion interpolation messes with this film/video aesthetic. It makes film look like video. I, and most of the staff here at HE, notice this and are distracted by it. In its most extreme forms, it even makes me a little queasy. Others I’ve spoken to don’t mind it and some even like it. The fact that it takes out the jutter inherent in the 3:2 sequence is a benefit that many point to.
Now, if you’ve never noticed the difference in film and video then this probably doesn’t matter to you. (Though now that I’ve described it, I’ve probably ruined it for you. Sorry.) For those of us who do notice these things and are film purists (read: snobs), we are violently opposed to motion interpolation. Thankfully, nearly all 120 Hz TVs let you turn this feature off.
Other factors do contribute to the film/video look, but this is one of the biggest and the one most likely to be “adjusted” by a television.
OK, great, but why?
Now that you know how it works, its time to dive into why. It comes back to that flicker-fusion threshold thing I mentioned earlier. There is a point where individual images shown rapidly stop being individual images and become smooth motion: flicker fusing to motion. (I love it when it’s that easy.)
The number of frames per second this takes varies depending on a number of factors: the species, how much the image takes up in your field of view, ambient light, light from the image and so on. In a movie theater, the 48 Hz (each frame is shown twice) is fine as it’s fairly dark and the movie screen isn’t very bright. (Maybe 15 foot-Lamberts, but probably a lot less).
At home, with a flat panel pumping out 70 ft-l, 48 Hz looks kinda jumpy. At least, it does to me. Those wacky Europeans didn’t seem to mind Pal (50 Hz). To each their own.
LCDs, though, mess with your flicker-fusion threshold. LCDs function on what’s called sample and hold. The liquid crystal material twists and holds the frame until it’s time to put up the next frame. In a standard 60 Hz TV, 16.68 milliseconds are available for each frame (60 Hz TV is technically 59.97 frames per second). With CRT TVs, the electron gun would paint the screen from top to bottom. By the time it reached the bottom, the phosphors on the top of the screen were already fading out. The gun would start over, constantly updating the image.
With LCDs, that frame stays on screen for pretty close to the entire 16.68 milliseconds. What happens is your brain expects that image on screen to move in that amount of time. Your brain/eye starts trying to track where the movement is going, and when it doesn’t, it gets confused and blurs it.
At this point, every LCD I’ve ever reviewed is saying in unison: “It’s not me, it’s you.”
One way to combat the sample-and-hold problem (which I’m going to abbreviate SnH because it’s easier to type), is to flash the backlight (Figure 5). If you drop out the backlight during the hold part of SnH, the LCD will work your brain more like how a CRT does. The problem is, this also drops the TV’s brightness, and that is the biggest no-no in the business. The brightest TV in a store is like the prom queen. Everybody flocks to her. It’s also the TV most likely to go home with the ogler, at which point the comparison either breaks down or stays true depending on how things went down in your high school.
The other way to solve the problem is to just put in more frames and holy crap, we’re back at the beginning. More frames means lowering the amount of “hold” in SnH. This means less apparent motion blur, which was actually all just in your head anyway. But then, isn’t everything? Oh yeah, deep thoughts with me and Norman.
At this point, you’re probably thinking: “What about response time?” Back in the day, LCDs’ response time was the big concern as far as motion blur is concerned. In the past few years, though, response time has gotten so good that it really isn’t the issue anymore. Most of the big name manufacturers don’t even talk about response time these days. Yet even when they were, there was no standard measurement, so they could publish pretty much whatever the marketing department told them to (like contrast ratio!). For what it’s worth, some are saying their response time is down to 2 milliseconds.
OK, so that explains 120 Hz. Why are companies pushing 240 and beyond? Well, one-upsmanship is certainly a big factor. When Company A gets word that Company B is working on 240, Company A of course has to follow suit and/or try to do even better.
But there’s more than that. While 120 was a definite reduction in motion blur over 60, so is 240 over 120. With 120, your total time for “hold” is down to 8.3 ms. Factor in the response time, and the images are still held for a few milliseconds. Increase the refresh up to 240, and now you’re down to a little over 4 milliseconds for each frame. This is impressive on several levels, as it wasn’t too long ago LCD’s response times were in the 8 ms range.
At this speed, SnH is less of an issue, because the changes are happening too fast for your brain to see that things are being “held.” Would 480 Hz be even better than 240? Could be, and I’m sure we’ll see it, but you start entering into the law of diminishing returns. You could make a subjective judgment that 120 Hz is twice as good (read: having less blur) as 60 Hz, but 240 Hz isn’t going to be twice as good as 120. Better yes, but not 2x. Again, all subjective numbers, but you get the idea.
But wait! Not all 240 Hz displays are really 240. Remember earlier how I mentioned flashing the backlight to alleviate the SnH problem? Turns out, that’s how several manufacturers are creating their 240 Hz models. It’s their normal 120 Hz TV with a flashing backlight. Is this really 240 Hz? Well, that’s debatable. Technically your eye is seeing 240 frames, but it’s not exactly the same. Will it be close enough to fool your brain and to seem better than 120? Probably. Could a “true” 240 Hz model look a little better? It’s certainly possible. The Samsung 8000 series I reviewed is a “true” 240 Hz model.
A backlight can also “scan,” as in the top portion is black at one moment, then the middle goes dark, then the bottom, then the top part goes dark again and so on. The number of sections that scan varies, but all act more or less the same and are functionally similar to having the whole screen go dark (though with potentially less of a light output penalty). There are some other tricks you can do with a scanning backlight, but to prevent this article from becoming epically long and epically boring, we’ll just lump them in with “flashing” and call it a day.
There’s no way to tell if the model you’re looking at has a flashing backlight or a true 240 Hz refresh. We’ll tell you when we can, but it’s doubtful every manufacturer is going to be forthcoming.
What about film?
With 120 you get this great number that is twice 60 and five times 24. So for those who want to get rid of the jutter of 3:2 (me!) but don’t want motion interpolation (hey, me again!) you can do 5:5 pulldown, and just repeat each film frame five times. Not all TVs offer this, but I heart those that do. As is pretty obvious, 240 allows the same simple math. So you get the benefit of the faster refresh and additional frames without the ugliness artifacts side effects of motion interpolation.
Bring on 960!
I have been a long and vocal critic of motion blur. I find it ironic that after years of LCD manufacturers saying there wasn’t a motion blur problem, they come out with a product that proudly features “No motion blur!” I think I’d live longer if I had a shorter attention span.
My narcissism hobby aside, 120 and 240 Hz displays are really a fantastic leap for LCDs. The 240 Hz displays especially have a noticeable reduction in motion blur. For those like me who notice this artifact, this is fantastic. One more step in the quest for the perfect display.
NO SPECIAL CABLES
I’ve seen a few advertisements for cables that imply you need special HDMI cables for your 120 Hz TV to work. I’ve even been told this by salespeople. In no way is 120 Hz a transmission format. The conversion is being done internally, and every source you have is either 24 or 60 Hz. Any manufacturer or salesperson who says that you need 120 Hz cables for your 120 Hz TV is either lying or… no, they’re just lying.
And for what it’s worth, HDMI cables either work or they don’t. You either get a perfect image, or it breaks up and/or has very noticeable artifacts (as in, it’s not working). If the picture looks fine, it is. It is physically impossible to get a better picture or sound quality with more expensive HDMI cables over short runs.
Over long runs, the impedance of the cable can more easily affect whether you get any signal at all, in which case better made cables will work where poorly made cables won’t. Note the language: “better made” doesn’t necessarily mean “more expensive.”