Undersampling

Glenn_G wrote on 4/23/2012, 7:11 PM
I'm having a rather odd result and I was hoping someone would explain why this might be occurring.

I have short video taken from a moving car (video: 00:00:07.508, 29.970 fps interlaced, 1920x1080x12, AVC). I have matched my project settings to this.

I am trying to get a stop action video from this. So I adjust the undersampling rate to .1 and the playback in Vegas Pro (version 10) is fine.

When I render to an interlaced MXF format (60i) the video is ghosting and blurry but when I render to a progressive (24p) the video is sharp.

Smart, Force or Disabled Resampling is having no impact here (at least that I can see)

Why is this happenning and if I wanted my output to be interlaced, how could I avoid this ghosting?

Thanks

Comments

Chienworks wrote on 4/23/2012, 8:22 PM
I don't really think interlaced is something you'll want in this situation. Consider the original footage, and you'll have something like this: (u = upper frame, l = lower frame)

1u1l2u2l3u3l4u4l5u5l6u6l7u7l8u8l9u9l10u10l11u11l12u12l...

When you undersample you throw away frames and duplicate the ones you keep. If you undersample to .1 and keep it interlaced you'll end up with

1u1l1u1l1u1l1u1l1u1l1u1l1u1l1u1l1u1l1u1l11u11l11u11l...

So you'll be bouncing back and forth between two fields over and over again, then moving on to the next frame. That's gotta look strange and bad.
johnmeyer wrote on 4/23/2012, 10:50 PM
You should have no problem with rendering to interlaced. I just took some 29.97 interlaced footage, set the undersample to 0.1 and then rendered to 29.97 MXF interlaced (i.e., "60i"). I put that back on the timeline in Vegas and did an A/B between that and the original. At first I thought I saw a difference, but when I set the preview resolution to "Best-Full" I was unable to see any difference whatsoever. It is always a good idea to use the maximum preview resolution (which is Best-Full) whenever dealing with interlaced material because all the other resolutions potentially drop one of the fields in order to speed up previewing, and this can lead to all sorts of subtle quality issues that make you think you have a problem when in fact none exists.

I did my tests using Vegas 10.0e and then re-did them on Vegas 11 (latest build). I got the same results in both cases, and didn't see any blurring or other problems.

[edit] One thing to look at is whether the render template for your MXF file is using the same field order as your original footage. While, in theory, it shouldn't matter, some people have reported problems when rendering interlaced footage to interlaced output when the render field order is opposite of the source video in the project.
TheHappyFriar wrote on 4/23/2012, 10:52 PM
I can't get a MXF to play on my system but I'd imagine the issue is that in Vegas you're nut using Good/Best-Full preview quality and it's cutting the res in 1/2 so you loose the interlace. I'd suggest you set the project properties to 1/2 the vertical resolution, disable resampling & then render your video out to the full 1920x1080 res. Or, just render to 1/2 the vertical resolution (960x1080). Either way should give you the same results.
john_dennis wrote on 4/23/2012, 11:56 PM
" I can't get MXF to play on my system..."

You can dowload the XDCAM VIEWER SW V2.30 from the Sony Professional site.