More on ‘judder’

The concept of picture ‘judder’, to which I referred two posts down, is very easy to show in a demonstration, but rather harder to explain in words. I’ve already received an email from a person along these lines. So rather than re-inventing the wheel, here’s a section of a review of a Philips TV I wrote in early 2000. The TV’s main innovation was to actually eliminate judder, so the review (I think, anyway) explains the issue rather well.

But I’ve saved the best for last: what it is that makes A Bug’s Life so different.If you have this DVD, I’d suggest you stop reading right now and go to your TV. Choose the full screen side of the DVD and start the movie. Right near the start there is a medium speed ‘camera’ pan up over some mud flats to the island upon which the starring ants reside. You will notice that the cracks in the mud seem to judder down your screen, jumping by discrete intervals with each frame of the movie. Sorry, but after the Philips demo of this, I’ve not been able to avoid seeing this myself, an effect of which I had previously been blissfully unaware, being so used to it.

The judder is not due to the PAL picture or TV representation, but to our own sense of perception being sensitive to a range of movement speeds that interfere with the 24 or 25 frames per second of film or PAL TV.

What I would like is for you to be able to watch this scene with the Philips TV under review. This has an option under its ‘Picture’ menu called ‘Natural Motion’. Select this and watch the same scene. The motion is silkily smooth. No judder at all. Quite incredible.

How is this done?

First, some background. A TV makes moving pictures by showing in succession 25 pictures (frames) per second. Each of these is made up of two pictures, interleaved line by line, so 50 separate pictures (called fields) are shown per second. This is your standard TV. An increasing number of high-end TVs are 100 hertz units. To reduce irritating screen flicker, these play each frame twice, so you end up with 100 pictures per second.

Now good old 50 hertz TVs had wonderfully fuzzy screens. Most nice new 100 hertz TVs have wonderfully crisp displays, as does this Philips TV. But the sharp focus means that the judder, previously concealed by screen fuzziness, is revealed. Philips new technology, ‘Digital Natural Motion’, instead of repeating each frame twice as in a standard 100 hertz set, calculates each intermediate frame as a new one, based on the preceding and the following ones.

This makes A Bug’s Life smooth. It works sideways as well, with one of the distant shots of the bikers in Easy Rider equally dropping all the judder (have a look around 19:12 into the movie).

This change is not subtle. But other effects are. First, a negative one. The processing seems to become a little confused when a sharply defined tan or black object moves across a diffuse green background, such as a person moving in front of foilage. This produces a subtle swirl around the edges of the foreground object, as though the air immediately around it is being heated, causing a lensing effect. An example of this is also in Easy Rider (see Dennis Hopper’s coat at 31:17). This appears only very occasionally, and is subtle.

Another subtle effect, but a more significant one in the longer term, has ultimately left me ambivalent about this processing. In short it improves the clarity of the film, making the visual representation ever so smooth. At some points it is breathtaking. In Easy Rider closeup shots of the characters taken in outdoor settings look, well, too clean, as though taken in the studio.

Why should this be? Well, consider the processing. Every second displayed frame is an average of the one before it and the one after it. Each real frame is a copy of the film frame. Each of these has film grain (especially on the 16mm film used for this movie), randomly distributed so the grain is different in each shot. The averaging of the intermediate frames removes the grain, so half the time the picture you’re watching is film-grain free. A welcome side effect is that DVDs telecined from poor quality prints, such as Blade Runner, lose a great many of the scratches and dust marring the film.

So why am I ambivalent? It isn’t the heat-haze effect. It’s the super clarity. Somehow it just seems too good.

Since then the technology has been improved and the ‘halo’ effect I referred to seems to have disappeared. Some Loewe TVs also use a similar system which also seems to work well. But they are still far from perfect. Sometimes they get quite confused with fine crosshatch patterns and dissolve the lot into a defocussed mess. But this seems to be rare now.

These days I don’t recommend high-end Philips CRT TVs because they have a huge flaw: they do not allow you to switch off all this digital processing. Nice as it is to have it available, I like to be able to switch it off if it is causing problems, or if I just want to see the movie as it was originally made, rather than as the TV ‘improves’ it.

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