Deinterlacing

Posted on 11 January 2012

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We ended up having recordings at 1080i50 (1080i/25 as EBU would like me to write it) to be used in a 1080p25 video. I needed to de-interlace them before or while using them.

So, because this was about real 1920*1080 playback at a big screen it was important to get it as good as possible. I had shots with lots of details, few details, aliasing and rapid movement. This is the best test material one could think of.

De-interlacing

Final Cut Pro shows the 50 fields interlaced material in 25p on the Digital Cinema Desktop and in the viewer. After dragging it to my 25p timeline it plays back without problems. De-interlaced. I read that FCP will do de-interlacing by taking the odd fields (discarding the even fields) and then interpolating (or double?) the in between lines. The timeline shows a green (preview) status.

It’s also possible to add the de-interlace filter. Now you can select odd, even or flicker filter. Odd didn’t make a difference. Even was less sharp, which is strange!

Strobing / Stutter / Jerky playback

Among other things, some of these recordings had a strobe / jerky look when playing back. At first I was suspecting my computers, screens and playback software, but after testing on a lot of devices I found it was the video. When playing back on a 1080i/25 capable CRT it was looking great.

So I looked further into a still frame. Apart from the de-interlacing (dropping 50% of the vertical resolution) everything was sharp as a razor. Even though these were very fast moving tele-lens shots. So, this has to be a high-shutter speed problem. You don’t see it at 50 fields a second, but at 25 frames a second it looks jerky.

50p timeline – line doubling

I found this quote: ” A high recorded frame rate (60 fps) presented at the same rate (60/60) will deliver smoother fast motion with higher shutter speeds. So a fast moving action movie or sports shot recorded and presented at 60 fps will look perceptibly better than a 30/30 fps version. The more frames the brain views in a second, the less ‘in-betweening’ it has to do. In actual fact of course, due to the higher shutter speed, each still frame is sharper and has more perceptible detail at this higher rate. But the higher frame rate in presentation is necessary for the images to feel ‘smooth’ at this higher recorded shutter speed.”

This made me test a playback of the material on a 50p timeline (FCP added de-interlacing automatically). And indeed: it was still a bit jerky but a lot better. On the other hand: I could see more aliasing problems in the details, i think. But that could be the preview mode of FCP. I should test this with the frame controls of Compressor.

This article explains the difference between a few methods of de-interlacing. In short: most consumer flat TV screens do the above technique: line doubling (bob). They double the lines of each frame to convert the 50i signal to 50p. If it is a smart TV, it might autodetect static scenes and choose weave for those (joining the information of two fields) and blend (mixing two fields) or blur for scenes with sharp lines (field bob effect). The best TV’s might even do some smart interpolating for the in-between lines, maybe even based on previous or next fields, to get rid of the field bob effect.

I would think it could be done even better: mimic the analogue behaviour of a *real* CRT. But I guess that’s up to the studio-displays.

25p timeline – Motion blur

All this is nice, but what to do for 25p display? Discarding fields makes the motion jerky, while weave will give it the real interlaced look (combing). Blend is also not nice, because at fast movement it looks like ghosting. But it also makes it look less jerky, as it looks a bit like motion-blur. So why not use a weave (full resolution on still scenes) method and add a real motion blur or motion compensation effect to get rid of the mice-teeth at movements? The images will be less sharp but motion will be smoother.

Compressor

Compressor can preview the de-interlace filter, but the other option in compressor is  frame controls, which can’t be previewed. I have had very good results with this de-interlace filter (blur) before. But frame control is said to have even better de-interlacing results. But: you will have to render a preview to see the effect of your settings. And it’s very, very slow. I guess there is a lot to calculate (CPU’s are all very very busy).

Squeeze

Squeeze has the options to discard, blend or choose adaptive or auto-adaptive. I guess the adaptive types are a combination of blend and discard.

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