How to avoid aliasing/interlacing-like artifacts in 24p?

[David Collins]

I'm using a Canon XL2 and I'm having trouble with what I believe is called aliasing -- 'blocky' horizontal lines appearing at the edges of objects, especially when they move or the camera does, and 'buzzing' diagonal lines in areas of fine detail that look like zebra stripes (though obviously aren't).

The weird thing is, the problem seems to be most pronounced when shooting in the 24p mode, which I thought was supposed to be free of any interlacing and provide a solid, film-like image. I've done a couple of tests with changes to the settings, and when the camera is set to 30p, this problem seems to mostly go away -- though when I capture the footage to Final Cut (version 7) it reappears looking just like the 24p footage. (When reviewing directly from the tape, the problem is visible in the 24p footage but not in the 30p shots...)

I've seen the same thing a lot in digital footage on line, but I've also seen some examples of footage shot on the XL2 that doesn't have any trace of this problem and looks very clear. What can I do to avoid it? Is there a setting on the camera I need to change? Might I not be capturing it to Final Cut correctly? Does drop/non-drop frame, 24p vs. 24pA, capture settings, shutter speed (I'm using 1/48), etc. have anything to do with this? Any ideas why this is happening most prominently with 24p, when that's the setting I assumed was least likely to have this problem? I can fix this to some degree by applying both Final Cut's 'anti-alias' and 'de-interlace' filters, but this reduces the sharpness of the image, and I'd like to avoid that if possible...

[Steve Marshall]

I just had the same experience shooting in 60i. I see no rhyme or reason for this issue, especially since I just paid Canon almost $400.00 to adjust a back focus problem and go over the entire camera...twice!! I haven't transferred all of the footage from the camera to PC as yet; this is one of over 200 clips. I'm now fearful of what other horrors await me on the remaining 2 hours of footage.

Here's a still frame of the problem I've experienced: http://img831.imageshack.us/img831/7...tifacting1.jpg

I'm confused as the offending clip appears fine on viewfinder/external monitor playback (yes I realize it's an analogue signal as such) yet in my NLE it looks abysmal. I don't know whether I have a problem with the camera, my capture/video card on my PC or my NLE program.

Any suggestions?

Thanks,
Steve M.

[David Collins]

The jagged edges (I don't know if the proper term is "aliasing", "stair-stepping" or "jaggies") in the highlighted areas of your image are pretty similar to the problem I was describing. The weird thing is that they're more noticable in footage shot with my camera in 24p mode, and less noticable in 60i footage from my camera, though that isn't a solution to me, since it doesn't have the look I'm wanting to (try to) get from 24p, which is why I bought this camera in the first place...

In my footage, these jagged edges are more pronounced in brighter lighting situations, and on diagonal and curved edges ... if lines are either perfectly horizontal or perfectle vertical in the frame, they look fine. I've also noticed this problem seems to occur most often when there's movement, either of the camera or of the subject.

I'm surprised no one else has replied, since I've seen this a fair bit on a lot of DV footage I've seen on-line...

[Ryan Chaney]

I THINK the jaggies you're seeing are due to how the DV codec itself handles highly saturated colors, rather than a problem with your XL2 specifically (or with 24p or 60i specifically). If I understand correctly, the 4:1:1 color space means used in DV recording means that the color information captured is at only one-fourth the resolution of the other visual details recorded... and thus the jaggies, which are simply lower resolution color info. I see the same issue all the time since I shoot a lot of events under bright DJ lights and highly saturated reds especially show the jaggies you're describing (and also highly saturated blues can too... probably any color I'd assume).

Final Cut has a filter called Color Smoothing - 4:1:1 which can help smooth them out in post, but the only way to actually solve the problem is to not shoot as saturated of colors. Good luck!

[Tom Blizzard]

Me too. I've had this problem with the GL2 for years. Not everytime, not everywhere. It still occures and it looks bad. Cure? Solution?? Cut down the color saturation in the camera???

[Andrew Hughes]

Definitely looks more like compression artifacts than interlacing issues to me. We use the XL2 at work all the time and I've never noticed any problems like that, but we shoot mostly 60i, not 24p. As Ryan said, DV is 4:1:1, so basically color information is compressed by averaging across 4 horizontal pixels. I wonder if you wouldn't be better off shooting in 60i and resampling in After Effects to 24p.