What is 120Hz? Why Do I Need it?

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 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 frames 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 is built to display 60 frames per second, and a DVD player, gaming system or broadcast sends it 30 frames per second, that's easy to deal with: the TV shows each frame twice in succession, creating one image made up of two consecutive frames. Each image will last exactly 1/30th of a second on the screen, and things will look perfectly even.

So what happens when your DVD player sends only 24 frames per second (or, one frame every 1/24th second) to the TV, but the TV still wants to show 60 on the screen? This is a little slower than the cable TV frame rate, so if it simply shows each frame twice and then waits for the next one, there will be gaps and the movie will look odd. So, the TV needs to triple one frame, double the next, triple the following one, and so on, giving it a total of 60 frames to show each second. This is known as 3:2 Pulldown.

The 3:2 Pulldown trick works OK, but there is a problem. One image (made of 3 frames) lasts a 20th of a second, then the next (made of 2 frames) lasts a 30th. 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 one image to the next. 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. As in all kinds of digital media, higher resolution means smaller units of data, and that means a more lifelike reproduction of reality.

120 Hz is the latest "ultimate" feature to hit the HDTV world. As of this writing, it is only available in the newest models, but look for it to spread in 2008. Initial reports from owners of 120Hz sets are quite positive.