Bad compression? Interlace lines on progressive footage?

[Dave Haynie]

Sure hope you didn’t order 1,000 DVDs without proof approval!

If one knows what they’re doing, a good 1080p24 original should yield a fantastic DVD. First things first: how and where do you see interlace lines? Is this on a CRT, a modern digital TV, a PC, just what? And I take it you’re see this as a motion artifact.

Ok, so there are a few challenges in making a good DVD. First, it needs to be cleanly downscaled from 1080p to 480p. Most video editors will do this well. And going from a progressive source is best, hard to screw up. But you can.

Second thing is matching your video to the DVD format, usually done on render to MPEG-2. The proper thing here is to render to DVD’s “NTSC Film” format, which encodes a 24p source with flags, so a 24p capable player can play it at 24p, while a 60i player will know how to telecine the video to look good at 60i. Pretty much all major DVD films are done thus way, one reason DVD looks pretty good on a modern TV. If they did this, it should look good. And its otherwise hard to mess up. You can see pulldown judder on a 60i display if you know what to look for, but no worse than any other DVD.

So let’s assume a screw-up. Best way to diagnose a problem is to understand how things break. One way to screw thus up is to render the 24p to a real 60i output. Two things can happen. If the rendering allows interpolation, you’ll have your 24p video resampled to 30p, then interlaced. This will smear things a bit, but given the higher starting resolution, you may not notice too many terrible things.

Now, if the 24 -> 60i conversion happens without interpolation, each field of the video will be drawn from part of the 24p original. But tge program doesn’t necessarily do an intelligent telecine here. So you will see odd field transitions, uglier video.

But it gets worse. If they got the interlace field order wrong — very hard to do, particularly from a progressive source, but possible. With this, you can see backwards judder… the N+1th field is from the Mth frame, the Nth field from the M+1th frame. This is unwatchable, any motion will judder.

-Dave

[Stony Browder]

First of all, thank you so much Dave for sharing your expertise. Now, into the details a bit more: The interlace lines on the motion are much more apparent on my 2011 imac. When I play the DVD on my Flat screen through my PS3 it becomes less apparent. (Strangely some of the music soundtrack cuts in and out— I don’t know if that’s related but it’s definitely something I’ve never come across). From what you said it seems like we mistakenly gave him master pro res files at 1920×1080. Still, I’d assume that he would have corrected us.

I also noticed last night that on both the imac and the TV that there is usually a frame flash before transition to another shot— kind of like when an editor forgets to delete a sliver of a clip. Again, something I’ve never seen. Does any of this additional information provide any insight?

Thanks again,
Stony