Terrible interlacing when exporting QT to custom sizes -- but not presets?

Question

Hiya,
Background: I've been capturing 1080i HDV footage, editing in iMovie. I export from iMovie to Quicktime to share with folks, using the 'Expert Settings' option to manage size and quality.
Problem: Movies look OK when I use some of the size choices in the menu, like NTSC "720x486 16:9". But if I try to enter in my own custom 16:9 dimension, e.g. 800x450, the video appears to be the same size on screen but looks lousy. Specifically, there is a clear interlace artifact in any areas with motion.
I'd stick with the NTSC outputs as they look OK when played in QT player, but the aspect ratio is always messed up when played inside a web browser. And I'd like to choose other sizes, like 400x225 for web playback etc.
Here's a screenshot:
http://homepage.mac.com/jasonacook/dvinfo/demo.png
Note that the settings (apart from size) are the same for both: I'm not using/changing deinterlacing, adjusting aspect ratios, etc. The only difference is that I used the drop-down 'NTSC 720x486' choice in the first, and manually entered 853x480 in the second. (As I understand it, 853x480 = 720x486 w/ the whole square/non-square pixel thing)
Any ideas what's going on? I'd like to use some 16:9 proportions other than the drop-down choices without having the picture look so interlace-y. How on earth does Apple make the deinterlacing look so good in preset sizes, that I can't seem to duplicate otherwise (not even in Compressor)?
Thanks!
Jason

Answer 1

Do you have QuickTime Pro? If so, one way to bypass the "Custom" size is to export with QuickTime Pro instead of iMovie.
Open the "Timeline Movie.mov" that's inside the iMovie project. (Control-click on the project icon and choose Show Package Contents from the popUp menu. The Timeline Movie.mov file is inside the Cache folder. Double-click on it to open in QuickTime Pro.)
Press Command-I (that's an eye) to open a window that shows the movie size. Drag the resize thumb in the lower right corner of the movie to the size you want.
Now do your export, but instead of configuring a custom export size, choose the "Current" size setting.
For testing, you may want to try this on a small chunk of the Timeline.mov. Copy a few seconds of the movie, create a new movie, and Paste. Export that.
Karl