With claims of decreased motion blur, better detail, world peace, and smoother motion, LCD manufacturers are drastically increasing refresh rates. But why?

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.

To 120!

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).

Figure 1: A standard video camera takes a single still image 60 times each second (60 Hz). Here Norman is moving quickly from the lower left to the upper right. All the movement between the frames is done "unseen" by the camera.

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.

Figure 2: With no motion interpolation, 120 Hz TVs just double the frames. Frame 2 is identical to Frame 1 and so on. Frame 3 and 4 are the same as Frame 2 from the original video.

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.

Figure 3: With mild motion interpolation, the TV creates a frame only slightly different to the original. Here, the created frame (Frame 2) shows Norman about 15 percent toward where he will be in Frame 3 (which is the same as the original Frame 2 "source").

Figure 4: With strong motion interpolation, the created frame (Frame 2) is very different from either original frame, and shows Norman halfway between where he is/was in the 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.