Motion Blur and the Cure: Frame Interpolation in Home Theater Projectors
Frame (creation) interpolation is something just starting to show up in the newest 1080p home theater projectors. I expect we’ll see this feature more and more, in future models. This article will discuss what it is, the benefits, and how important it is.
Before explaining exactly what frame interpolation is, perhaps it’s best to define the issue that it addresses.
The traditional movie has been shot with film, and traditionally that is done at a relatively slow 24fps (frames per second). 24fps is pretty much just fast enough to make viewing a movie seem to be continuous motion, as opposed to a series of individual frames. TV/Video, on the other hand, has been based on 30 frames per second, even faster, and TV broadcast at 1080i is actually shot at 60 fps, using two “fields” of 30 fps, the first doing half of the signal (alternate lines), and the second frame, handling the alternate lines. Together you get all frames refreshed in 1/30th of a second.
Now, either of these schemes may be fast enough to give you a continuously changing images, in most cases, but there are times when it just isn’t fast enough.
Further, this is complicated by the fact that home theater projectors produce a very large image, typically 92″ diagonal to 130″ diagonal, depending on screen size. Because we sit relatively close, to get the large screen/theater effect, this results in our greater attention to details. Those with home theater projectors have the image occupy a much larger percentage of their viewing field than looking at a far smaller screen (LCDTV, Plasma, conventional TV, Big screen TV), from a similar distance. As a result the relatively slow frame rates mean that fast moving objects in the content, cover a lot more ground (in terms of your field of view) than smaller sets.
The bottom line on that, is it’s much easier for fast moving objects to seem disjointed, not moving continuously.
Consider a televised tennis match. One person serves the ball. That person is standing close to the right side of your screen. The other player is near the left side of your screen. Now, I really have no idea how quickly the ball makes it from the serving racket to the other player’s, but let’s say it only takes 1/2 second. At 30 full frames per second, that means that the ball only shows up on 15 frames. It’s a very small thing, that tennis ball, so if you look at each frame, in turn, the ball has moved significantly. If your screen is 7.5 feet wide, the ball shows up approximately every 6 inches of screen (but the ball is less than 1 inch in size on your screen. (There would be some blurring of the ball from the shutter speed, that will stretch out the ball, but it wouldn’t be that great, and we will not worry about that.)
OK, so now we have a series of frames each with the ball at a new location.
Problem is, there is some blurring of the ball’s movement, generally referred to as motion blur. A faster frame speed (more frames per second) would obviously mean that the ball will have moved less from one frame to the next. The idea of frame interpolation, therefore, is to compensate for the “foolishly” slow frame rates of movies and TV.
The Fudge Factor: How do we solve this problem, of motion blur?
Right! more frames. Problem is, they weren’t there during content creation, which means the only solution is to fudge new frames.
And that’s just what some projector manufacturers are doing today.
The projectors with frame interpolation, basically do this. They look at one frame (#1) , and the frame following it (#2). They literally create a new frame which is inserted between them (let’s call it #1A).
The electronics will analyse the differences between frame 1, and 2, and identify fast moving objects. Only fast moving objects need to be dealt with. If our background is the tennis court, any minor motion (ie slow pan) can be ignored, but that tennis ball will be dealt with. The new frame – 1A, will move that tennis ball to a point approximately half way between its locations on frame 1 and 2. Instead of a 6 inch jump on your screen, there’s now an extra frame, and the jump is only 3 inches. This extra data, with the ball relecting its appropriate position on the new frame, will be picked up by the eye, as a smoother motion of the ball – eliminating most of the motion blur.
The same is true for movie viewing, and even more beneficial due to the even slower frame rate of movies (24fps).
In fact, some projectors are actually creating 3 frames between #1 and #2 (let’s call them 1A, 1B, 1C.
Thus, over a one second timeframe, instead of the original 24 fps, many projectors are now running at 4x that speed – 96 fps. Now some only duplicate the frames, but the ones using frame interpolation, can smooth out the motion blur of fast moving objects.
It must be noted, that often almost the entire frame is moving fast enough to cause blur – and I’m talking about something very common – a fast pan of a scene. Sweep that camera around quickly, and everything seems to blur.
In a side by side test at CEDIA, I witnessed two Panasonic projectors, the PT-AE2000U, against the PT-AE3000U, demonstrating the difference. The PT-AE2000U lacks frame interpolation, but the PT-AE3000U has it.
For a demonstration, instead of a movie, Panasonic picked some content that would very easily show the motion blur. Basically, it was a map with lots of printed text of locations on it. The faster the pan, the more blurry and hard to read the words were on the PT-AE2000U, but right next to it, the words all remained sharp, and easy to read.
It was a remarkable demonstration.
The good news is that in the real world things aren’t normally that obvious. We have all been getting along rather well without frame interpolation, but this technology does deal with a small problem, and seems to deal with it well. Let’s put it this way. All else being equal, between two projectors, one having frame (creation) interpolation, and the other not having it, having it, is a plus.
So far, the Panasonic PT-AE3000U, the Epson Pro Cinema 7500 UB, and it’s almost twin, the Home Cinema 6500 UB, and one of the high end Sony projectors have it. There might be one or two others, as well. More, certainly, will come.