Playing back interlaced video on computers

If you are playing back video clips directly to a projector from a PC or Mac, and the clips are interlaced video, then you have probably seen the ‘jaggies’ effect where fast moving edges take on a comb effect, due to the two fields of the video capturing the motion at slightly different times. Here’s some suggestions for how to get rid of that distracting effect.

Interlacing is an analogue compression technique used in video to reduce the amount of ‘data’ required to show a video image. In short it shows every other line every 1/50th of a second, and relies on the persistence of the screen (and your eye) to smooth out the two fields.

This is fine for analogue video but a pain for playing video clips on computers and projectors that use progressive (all lines one after the other) scan, not interlaced scan. In general this produces the artefacts mentioned above, and can be a real distraction, especially on a large screen.

So, how to deal with it:

Use your DVD player software

First of all, let me say that this really only applies to playing video clip files, not DVDs. If you are playing back from a DVD on your computer using DVD software it’s almost certain that the software is de-interlacing the video from the DVD before you see it.

So one way to deal with it is to play back your video clips using your DVD software. This would work for any MPEG1 or MPEG2 video clips and in most cases you can point your DVD software to a file or folder on the hard drive.

Of course the downside of this approach is that most consumer PC DVD player software apps have HIDEOUS user interfaces that are almost unusable, and whose only purpose is to frustrate you from doing the most simple tasks while looking pretty. (or is that just me?)

Use de-interlace feature in video playback software 

The second option is to use the de-interlace feature of your video player software.

Some apps like Quicktime player pro (at least on a mac – haven’t checked PCs), VLC, etc. have a menu item to de-interlace the video clip. They may have different settings and options, which will have different CPU implications, so it’s best to experiment.

There’s a very detailed explaination here of the various methods used.

One option, if you are dealing with HDV video clips, is to  ‘throw away’ one of the fields and still keep a decent resolution image to show.

Here’s a description of how to do it in Quicktime on a Mac.

 Hugo van Rhijn says:

However, when showing the videos in Keynote on a regular business beamer, you are likely to encounter a nasty little problem. 1080i is a so-called interlaced format that produces jaggies (disturbing and unpleasant artifacts in the picture with a lot of horizontal motion) when viewed on a beamer. So what you need to do is to de-interlace the video. In QuickTime Player, you can open the HDV QuickTime file, then go to Window: Show Movie Properties, and then click on the Video Track.

In the lower right hand corner, you’ll see a de-interlace box. That is useful as it will de-interlace the video and offer perfect playback of the HDV on a beamer. In fact, it clearly and visibly improves the video’s picture quality. However, it will only do so within the QuickTime player application. As soon as you export the video to Keynote, this adjustment does not “travel” with the QuickTime file as it is imported into Keynote. So it appears to be of no use for your Keynote purposes.

Ticking the ‘Single field’ box, however, will do the job nicely. That is, it will not offer the exact same eye-pleasing picture improvement, but the video will play nicely and as it already has the sharpness and brilliance of HDV, it will look great. If your Powerbook or G5 is fast enough, you may also like to tick the ‘High Quality’ box.

Hugo then goes on to mention a tip for handling video clips in Keynote. 

 One additional tip: Videos in Keynote tend to freeze for a split second before starting to run. In Keynote, many templates have a lot of white in them. If your have choosen one of them, you can produce a very smart looking way of starting your videos within Keynote. When editing a video meant to be used in Keynote, start with a few frames of “white” (an entirely white screen). Then make a cross-dissolve between the white and the first (or last) picture.

Now, in Keynote, use a similar cross-dissolve as a transition between your slide and the video (and between the slide with the video and the next slide). It will give you a fluent and very slick and professional-looking presentation, where you and the HDV content will get all the well-deserved compliments.

There you go – very useful Hugo.

So watch out for similar de-interlace ‘tricks’ in your playback software. I haven’t tried this much on PCs – anyone care to share if you know?

De-interlace clips in NLE

 The other, more ‘traditional’ method of de-interlacing your video clips is to do it in an NLE application, like FCP or Premiere.

You can use the de-interlace filter, which lets you choose one of the fields to show, but of course that loses you half the resolution of your original video clip. Fine in HD, but not so good in SD.

You may have read that you can layer two versions of the same clip on top of each other, then apply the de-interlace filter to both, but for one use ‘odd’ field, and the other use ‘even’ field. Then set the top one to 50% opacity (transparency) to ‘blend’ the two clips and gain some of the resolution back again. This will then have to be rendered and exported as a new version of the clip.

This will work, but may not give you the best results compared to other more sophisticated methods. There are also various de-interlace plug-ins that you can use, which offer more ‘clever’ methods. (don’t have details to hand – anyone want to suggest any??)

Also remember that most compression tools like Cleaner or Compressor (part of the Final Cut Studio suite) have a de-interlace option so you can do the de-interlacing ‘on the fly’ while exporting your video clip to WMV or MP4 or M2T.

 Finally note that if you are playing clips from a PC back out through a scan converter then the video is going to get re-interlaced again at that point, so there’s not much point in going through this process – better to keep them as interlaced and keep the full resolution.

 OK, that’s all – anyone want to share any other tips or comments about de-interlacing ?

Just leave a comment below.



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