Tutorials/Video/Interlacing

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Many digital video formats are interlaced, such as DV-PAL (576i50), DV-NTSC (480i60), HDV (1080i50, 1080i60). Some formats aren't interlaced, such as DV film-emulation e.g. 576p24, or high def 720p.

Because computer displays are progressive, movies that are interlaced need to be deinterlaced to become progressive. Movies that are progressive already should not be deinterlaced.

There is lots of information available on the web, and you might like to google for some tutorials. In brief, "i50" means 50 fields ('half frames') per second, equivalant to 25 frames (each composed of two fields). Deinterlacing turns "i" into "p", e.g. "i50" into "p25", ie. 25 progressive frames.

You have two options:

  • Deinterlace proper, to turn e.g. 576i50 into 576p25.
  • Discard one field. (Effectively this is a way of deinterlacing, a "poor mans deinterlace")

Let's start with turning 576i50 into 576p25. This requires you to deinterlace the two fields of each frame to make a single (progressive) field. Deinterlacing often is a destructive process: The most basic method would be to discard one field, and then interpolate the remaining lines from the remaining field. If you do this, you're effectively going down to 288 lines, and then interpolating back to 576 lines. Better methods would look for motion, and only deinterlace where required (either temporally, or within one frame). Particularly where you have low motion, high resolution content (such as slides with technical information) doing an advanced deinterlace is helpful.

Deinterlacing takes time, and it needs to be done carefully.

A simple way of deinterlacing is simply to discard one field. So you might want to go from 576i50 to 288p25. Assuming that your encoder works well, this should work without needing to apply a deinterlace filter. It should thus be quite fast, is fool proof, and gives you good image quality. Similarly of course for 1/4 of 576, ie. for 144p25.

1 A common pitfall: 320x240 from PAL footage[edit]

You're encoder probably offers you a setting for encoding to 320x240. And after all, many web movies are in that format. So encoding PAL video like this? I.e. what about 576i50 to 240p25? Of course you need to deinterlace: 576/240 = 2.4, so a typical encoder will be picking from both fields to generate the new 240 lines, and interlacing artifacts will be visible.

So why do many programs offer 320x240 as output? Well, it's 1/2 of NTSC, so 480i60 footage will be automatically deinterlaced by going to 240p30. In other words, those programs are aimed at the NTSC marked, but 320x240 for PAL footage just doesnt work (without deinterlacing). So, for PAL footage, if you want to go to 320x240, you'll need to switch the deinterlace filter on. Alternatively go to 384x288 (and you shouldn't need to deinterlace).

2 Other issues[edit]

Btw. you shouldn't deinterlace progressive formats. By the same token, don't interlace twice, ie. make sure that you automated workflow has only got one deinterlace stage in it.

What if you are going from one interlaced format to another? Do you need to deinterlace? These things can be tricky. Better encoders might offer to scale fields independently, thus preserving the field structure.

3 Links[edit]

Related reading on this site: Pixel Aspect Ratio, and also see the end of Multiformat Media Delivery.

Elsewhere: