Understanding Fields and Interlacing in Final Cut Pro

[ This article was first published in the April, 2007, issue of
Larry’s Final Cut Pro Newsletter. Click here to subscribe.
Updated June, 2007. ]

This article got it’s start in an email from a reader asking why their still frames were “vibrating” on the TV set, but not on the computer screen. The answer lies in interlacing.

All NTSC and PAL video is interlaced.

A video clip is made up of frames, or individual distinct images (30 per second for NTSC and 25 per second for PAL). Each frame is composed of a series of horizontal lines. These horizontal lines are divided into two sets, called Fields, where the first field is composed of all the even-numbered lines, and the second field contains all the odd-numbered lines.

Interlaced means that these two fields are not shown at the same time, but one after the other — in the case of NSTC video, they are 1/60th of a second apart.

(Now I realize that there are other frame rates and that some video can be shot progressively, which means the contents of both Field 1 and 2 are shot at the same time. However, for this discussion, let’s assume we are working with traditional interlaced video.)

The problem that interlacing creates is that it takes both fields to create a single image, but the two fields were not created at the same time — they are actually 1/60th of a second different.

Now most of the time, this interlacing is not a problem — in fact, we’ve been watching it on TV all our lives. Where things get weird, however, is when you create a still-frame from a frame containing lots of motion.

On your computer monitor, everything looks great. This is because your computer monitor displays all the lines in your image progressively — that is, all at once from top to bottom. However, on your Aunt Martha’s TV set, when that still frame pops-up, the image vibrates so badly that poor Aunt Martha has to lie down and recover from motion sickness.

The reason is that when you use Final Cut to create a still frame, it freezes BOTH fields, even though they are showing action which is 1/60th of a second different.

What we need is a way to see this interlacing — that way we can decide if it’s a problem. (This is also a REALLY good reason to buy a video monitor that is not an LCD or plasma display, because LCD and plasma monitors don’t display interlacing properly, either.)

Fortunately, Final Cut Pro displays interlacing; if you know where to look.

When you look at a rapidly moving video image, like the one below, portions that are moving will look blurred.

However, if you change your View setting, in either the Viewer or the Canvas, to 100%…

… you’ll see thin horizontal lines radiating off all moving objects in the frame. These horizontal lines are from the two different fields. And when they are wide apart, as they are in this example, the “vibration” you’ll see in a still frame will be severe!

So, before you create a still frame, switch your View magnification to 100% and see if the interlace lines are visible. If they are, you should pick a different still frame.

To fix bad interlace artifacts, where the image is rapidly vibrating, you can apply Effects > Video Filters > Video > De-interlace to your still frame. However, deinterlacing a clip removes half the lines in an image which means that the vertical resolution of your image also gets cut in half; therefore, it will not look as sharp or detailed as the rest of your clip.

Interlacing is a fact of life for NTSC, PAL, and half of the HD formats (those ending with the letter “i”). The key is to find out whether your still frame has excessive interlace artifacts before you finalize your project, rather than trying to revive Aunt Martha on her couch.

If you want to learn more about interlacing, here’s a Wikipedia article you can read.

UPDATE – June 2007

After writing this article, Heidi Stone wrote in with:

Thank you, Larry, for the tip on de-interlacing a still frame. I had a DVD that needed a label using a frame from the video. I grabbed a still and imported it into PhotoShop. It didn’t have a lot of motion, but still had interlace lines of course. I use the PS version of de-interlace and realized it had a few more options than FCP. You can choose Odd/Even fields and (the best part) Interpolation instead of Duplication. Since I wasn’t marrying this image with the original footage I don’t know if the freeze frame would have looked any better than the FCP image. Since I was making the image a duo-tone, it worked just fine for print – and was able to use this image for the closing credits as well.

Larry replies: Heidi, thanks for writing!

In NTSC video, fields are shot 1/60th of a second apart (PAL fields use 1/50th of a second). So, when you select Odd or Even, you are choosing between the image shot first (Even) or second (Odd). Because de-interlacing removes every other line, this missing information needs to be replaced by something in order for the TV to display the signal.

Duplication simply doubles every line. This is fast and simple, but it makes your image look soft. It is, however, very fast.

Interpolation compares the line above with the line below, figure out what’s changed and fill in the missing line with it’s best guess at what is missing. This tends to be a better choice when you are concerned about image quality.

There are also third-party deinterlacers you can buy from companies like: Joe’s Filters, Digital Anarchy, and Nattress.com.

Steve Lyons, of Lyons Den Video, then wrote in with a comment:

In addition to what was mentioned in your monthly newsletter, when deinterlacing in Photoshop using interpolation, you can limit the area to be deinterlaced by selecting only the areas where there is noticeable movement to be deinterlaced. This results in a more detailed still.

Larry replies: Steve, you were the first of several people to suggest this. This is a great tip to retain as much image quality as possible.

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