What do 120Hz, 240Hz, 600Hz really mean for the picture quality of your new HDTV?
Original video frames (1 and 2) at 60 frames per second aren’t enough to fill 120Hz and 240Hz LCDs. Duplicating the original frames is one method. Alternatively, frames can be interpolated to fill the gaps. In this example, the TV’s processor creates frame 1a from the difference between 1 and 2. This (along with 2a, 3a, etc.) makes up the difference between 60Hz video and 120Hz TVs.
With 120Hz, 240Hz, and even 600Hz, the refresh rate gets a lot of attention in the marketing of new HDTVs.
What it is and how it works is interesting, but why it exists is even more so. And it can have a profound effect on the picture quality of your HDTV.
Let’s start with the basics. Television is a series of images, shown rapidly enough that your brain sees it as motion.
In the U.S., our electricity runs at 60Hz, so it’s only natural that our TVs run at the same rate (elsewhere, 50Hz is common). This is largely a holdover from the CRT days, but our entire system is based on it, so there’s no use changing it.
What this means is that modern HDTVs show 60 images per second (60Hz). For a refresher on progressive scan (720p, 1080p) and interlaced (1080i), check out “1080i and 1080p are the same resolution.”
Upping The Frame Rate
A few years ago, LCDs hit the market with higher refresh rates. These started at 120Hz, though now you’ll see 240Hz and beyond. In this case, higher is indeed better, but to understand why it’s better, we have to discuss why it exists in the first place.
All LCDs have a problem with motion resolution. Which is to say, when there’s an object in motion onscreen (or the whole image is moving), the image blurs compared with when the object/scenery is stationary. In the early days of LCDs, this was predominantly because of the “response time,” or how fast the pixels could change from light to dark. Response times on modern LCDs are quite good, and this isn’t a big issue anymore.
The top is full motion resolution. The bottom half is a representation of what motion blur looks like. Notice how the dolphin on the right is blurred compared with the other three and the rest of the image.
The issue is how your brain interprets motion. Because it’s your brain, everyone is going to see motion resolution somewhat differently. Some people don’t notice motion blur. Some people aren’t bothered by it. Some (like me) notice it quite often and are bothered by it. Others, like our David Katzmaier, know it exists but don’t notice it enough in normal program material to consider it a major factor in picture quality.
There are two primary ways to fool the brain into seeing better detail with LCDs: backlight flashing (also called backlight scanning) and frame insertion.
Backlight flashing is what it sounds like. The most basic version of backlight flashing is the backlight going dark in between video frames. This moment of darkness is much like how a film projector works: an image, then darkness, an image, then darkness, and so on. Done slowly, this can result in flicker. Done fast enough, and you don’t notice it. A more advanced version, called backlight scanning, dims sections of the backlight in sequence with the video. In either case, the side effect is a loss of light output (sometimes significantly), because there are sections of time where the backlight is literally off (or close to it). There is another way of doing this called black-frame insertion, which shows a black image in between the real frames, but that doesn’t actually manipulate the backlight.
Panasonic’s “1920bls” backlight scanning technology. It’s like a scrolling dimming of the LED edge lighting to darken rows of LEDs in sequence. It does this very quickly.
With 120 and 240Hz displays, there’s another option: frame insertion. This method, also called frame interpolation, actually creates entirely new frames of video to insert in between the “real” frames of video. With video sources, like live TV, sports, and video games, there’s very little downside to this method. You get excellent motion resolution, and you maintain the light output of the display. The image at the top of this article is an example of frame interpolation.
However, with film/24fps content (movies, most scripted TV shows), there’s an issue. The interpolated frames smooth out the inherent judder motion of 24fps content. On the surface, this may seem like a good thing, but the resulting ultrasmooth motion makes movies look like soap operas. Fitting, then, is called the Soap Opera Effect. We and many TV companies call it “dejudder.” Personally, I find motion-interpolated video annoying to watch. In some, it causes nausea. Some people don’t mind it, which I find rather shocking. Check out What is the Soap Opera effect? for more on this “feature.”
Most modern 120/240Hz TVs have one or both versions of this technology, and it’s completely selectable which to use (if at all). In some bizarre cases, like the Cinema mode in Panasonic’s WT50 series of LED LCDs, you’re locked into the motion interpolation. The trade-off, usually, is that if you don’t use motion interpolation or backlight scanning, you don’t get full motion resolution.
Personally, I find the step from 60Hz to 120Hz to be noticeable, and worth the additional money. The step from 120 to 240 is far more modest of an improvement.