flathdtv.net Blog

There’s a lot of talk these days about frame rates in the new HDTVs. The frame rate is simply the number of distinct images a TV screen can display in a second, and is also known as the display rate, the refresh rate, or “Hertz”. Hertz is the scientific unit for cycles per second, and is abbreviated “Hz”. The latest buzzword is “120Hz”, which means a display rate of 120 images per second. This is twice as fast as most HDTVs are capable of (60Hz), so it ought to make the moving pictures look smoother. But, the human eye can only process about 20 to 25 distinct images per second, so why should it matter what the frame rate is, as long as it is faster than your eyes can see?

The answer comes when you realize that not all video is filmed at the same rate. Movies are usually filmed at a rate of 24 Hz, most TV is filmed at 30 Hz, and some sports events are now being filmed at 60 Hz.

Some simple math will show the problem and reveal the solution. When a TV set wants to display 60 images per second, and the cable TV company sends it 30 images per second, that’s easy to deal with: the TV shows each image twice in a row, each image will last exactly 1/30th of a second on the screen, and things will look perfectly smooth.

So what happens when your DVD player sends 24 images per second to the TV, but the TV still wants to show 60 on the screen? The images come in a little slower than the cable TV images, so if it simply shows each image twice and then waits for the next one, there will be a black gap between the images and the movie will look terrible. So, the TV needs to triple one image, double the next, and so on, giving it a total of 60 images to show each second. This is known as 3:2 Pulldown.

The 3:2 Pulldown trick works OK, but since half of the images are being shown 3 times in a row, for a total of 1/20 (or 3/60) of a second, and the other half are being shown 2 times in a row, for 1/30 (or 2/60) of a second, there is a problem. A 20th of a second is long enough for most people to tell that it’s really a static image, and when the camera pans around in a scene, things will seem to move unevenly. But if your TV is able to display 120 frames per second, it can take the 24 Hz input and simply show each frame 5 times in a row for a total of 1/24 (or 5/120) second per frame, with no variation from frame to frame. And when 30Hz video comes in, it displays each frame 4 times in a row, for a total of 1/30 (or 4/120) second per frame. Problem solved.

Now that frame rates are fast enough to display any kind of input smoothly, HDTVs not only have high resolution in the x and y (height and width) dimensions, but also in the t (”time”) dimension. The 120Hz frame rate won’t be very common until 2008, but a few already have it, like the Sony Bravia XBR4 series and the Sharp Aquos 92U series.

And finally, I’ll admit that the title of this article is a bit misleading. High definition has certainly entered the fourth dimension (time), but has skipped the third (depth) in the process. So far, no one seems to be broadcasting holographic TV shows. We’ve got a while to wait for true four-dimensional HDTV, unfortunately.

For more information, check out the FlatHDTV.net HDTV Tutorial.