Educate me: Why/when is deinterlacing needed?

johnmeyer wrote on 7/18/2008, 10:21 AM

Lots of people regularly go through the step of converting video from interlaced to progressive

Question: Why do you do this?

I can think of a few reasons, but for me they are rare. For instance, if you have several cameras, and some record interlaced and some record progressive and you want the "look" to be consistent and you can't afford to rent or buy cameras which match, you would have to deinterlace (and probably change frame rates). I can also see how a person might want to deinterlace in order to achieve a certain "look."

But if I shoot with my DV or HDV camcorder, both of which record 29.97 NTSC and 29.97 HDV respectively -- and both record interlaced -- when would I ever want to deinterlace if I don't have the issues mentioned above?

I am really trying to learn here, not start some sort of argument.

The reason why I don't see the need is that interlaced video look just fine on my TV when I playback from my camera or from a DVD. Also, on my computer monitor, if I use the proper playback software, interlace looks fine and doesn't have any "herring bone" artifacts. It is true that fast motion action will show the interlaced "herring bone" if I use the wrong software that displays both fields at the same moment in time, but that is the fault of the software.

So help me out, when should I deinterlace?

[Edited for spelling]

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johnmeyer wrote on 7/19/2008, 10:15 PM

I had about a ninety minute phone conversation this evening with Roger Evans, the guy that invented the Workprinter film to video transfer device. Part of that call was a long conversation about his experience trying to show 24p film that had 3:2 pulldown added, and problems he was encountering trying to show it on various LCD and Plasma devices. There are all sorts of problems that interact that it becomes difficult to simply say that the lousy picture is due to any one defect, problem, etc.

The refresh rate of the display clearly is a factor, something that has been mentioned more than once in this thread. Interlaced material interacts differently with the different refresh rate simply because 60 is a higher multiple (or is it divisor?) of say, 120 Hz refresh, than is 24.

He also implied something similar to what you are stating, namely that some displays are not actually handling the interlacing correctly.

So, for me, things are at this point muddier, and not getting clearer.

GlennChan wrote on 7/20/2008, 6:58 AM

I think you should look at different mediums different.

TV:
Consumers TVs, theoretically, should be able to handle interlaced material because almost everything is broadcast that way (i.e. 24p will be broadcast with pulldown, 30p as psf, interlaced as interlaced).

On plasmas and LCDs, the TV will have a deinterlacer in it because the image has to be deinterlaced for display. A lot of these deinterlacers are crap (as bob mentions).

If you deinterlace the material beforehand, then the TV might deinterlace it again which can be a problem.

- HDMI: I don't know if HDMI might let you bypass some of these problems since the TV could figure out whether the signal is interlaced/progressive by the frame rate of the interface. I think it would, though I've not tried. e.g. 24fps and 60fps (frames per second) will definitely be progressive so the display should never deinterlace in that situation. (Though this won't really help for most people.)

Computer display:
All computer displays are progressive, so the material must be deinterlaced somewhere.

In some computer software combinations, the material can be flagged as interlaced/progressive and the player can do the right thing. Though... you should check that the material is flagged correctly and that the player won't read the flag incorrectly (because metadata tends to get screwed up).

If you want the highest quality, you can run your footage through a high-quality deinterlacer and encode the material as progressive.

Or you could encode to MPEG-2 and let the player do the deinterlacing. Some do a decent job... though not as good as a non-real-time deinterlacer (or specialized hardware like a Teranex).