What’s the deal with: 120 Hz and “dejudder”?

All the rage of the newest LCD TVs is supporting a few things like “120 Hz” and “dejudder”. What are these?

When a TV is marked as “120 Hz”, it means it is capable of redrawing the screen 120 times a second. Traditional tube TVs in the US refreshed around 59.94 times per second, and other kinds of TVs typically fell into providing 60 or 75 Hz refresh rates. Since most video in the US is encoded at either 24 (film), 29.97 (television), or 30 (VHS) frames of video per second, these refresh rates were more than sufficient to keep the video playing smoothly.

What does 120 Hz buy us? Well, first, it adds a lot of cost. More powerful video processors are needed to process the video twice as fast and to do fancier operations, and will, of course, cost more money and generate more heat. The video components, like the screen itself, will have to be higher quality.

And what does this cost give us?

Basically, double the frames it can display per second. The reason for this is two-fold: one is that LCDs traditionally have a lower refresh rate than CRT and plasma screens, leaving some models to display “ghosting” and other mysterious artifacts. Some people notice it, some don’t care. Doubling the refresh rate to 120 Hz provides, potentially, a more natural, smooth playback. Possibly.

The second reason has to do with “dejudder“. What does that mean?

Well, as I said above, video sources come in a variety of frame rates, typically 24 frames per second if they originated from film. If you can display only 60 frames per second, then there’s some arithmetic error: 60 ÷ 24 = 2.5. There way this is normally handled is by showing frames multiple times, and some more than others. This can cause an uneven jerkiness (”judder”).

There is also judder on 120 Hz screens, since the source is still usually 24 Hz. Some believe that it is still jerky, if it is just evenly.

So, now that there’s dejudder technology, that’s great right? Not quite.

It appears to be different with every person, and different TVs implement dejudder differently. The dejudder technology interpolates extra frames in between the real frames, which can appear to degrade the video source. Some may mind, some may not. Some people even like the jerkiness of the original source, and so the 120 Hz and dejudder technologies are actually bad things.

The moral of this story: Don’t leave the store that you are buying your 120 Hz, dejudder TV without playing with the dejudderer. Make sure that you will be satisfied with the quality, and that it looks better to you.