Re: Downconvert 720p 23.98 to 29.97 Betacam Problem

[Bill Paris] "Since each frame would be interlaced and scanned onto the screen, I thought this might even out some of the jitter associated with faster moving clips?"

Oh boy, you asked for it so let the good times roll.

I see how you could think that, but it doesn't work that way as the motion is still based of 24p and with 24p there are no fields and fields account for video like motion as each field as a distinct moment in time, even if the fields that make up that frame are different.

Okay since you now understand how we can see 720p23.98 on a monitor (really, it's 23.98 with pulldown added, so 59.94) we will take this a step further. Now if you were to record that 59.94 stream and look at it frame by frame, you would see that it would be 60 frames per second and have a repeating cadence of 3 duplicate frames, then 2 duplicate frames over and over. Using the ABCD frame cadence reference:

24p= ABCD
24p with 3:2 pulldown in a 60p environment = AAA BB CCC DD with each letter representing a whole frame

So when you down or cross convert to an interlaced 30 frame base band, each progressive FRAME gets mapped to an interlaced FIELD. Alright ... follow me on this one. You are going from 59.94 frames per second to 59.94 fields per second. So essentially each progressive frame gets segmented and turned into a field. Then when you combine those fields in a 30 frame per second wrapper each field follows the exact pattern of the the frames it was made up of. Since you have 3:2 pulldown, it would be 3 fields made up of the 3 prog. frames, then two fields made up of 2 progressive frames, but since they are fields and get combined/played back at 30fps, you have:

24p with 3:2 pulldown in a 30fps environment: AA AB BC CC DD with each letter representing a field.

SO, what the hell does all this mean? This means that the motion characteristics hold up with 24p material when down converting or cross converting. 3 out of every 5 frames are still 'progressive' whole frames, and 2 of those 5 are terlaced but made up the other 3. So you aren't getting any traditional interlacing where every field is a picture from a different moment in time which would equal:

AB CD

Now if you want to make your 24p material look more interlaced, we can talk about motion interpolation plugins that will attempt to recreate fields that are discreet moments in time. For the most part, you will have to retrain your eyes to get used to the motion. Pretty soon, after watching it day in and day out, you will get used to it. then you go back to a true 60i environment and say, "now how the hell can I get this to look like 24p?" And around and around we go.

If you can follow all that, I owe you the coffee and danish.

Jeremy