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50p to 50i conversion

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Brian Cutts
50p to 50i conversion
on Nov 23, 2017 at 12:47:48 pm

Next summer we will be working on projects which are shot and edited in 1080/50p and need to be delivered in both 1080/50p and 1080/50i.

I understand that most transcoders will take 2 adjacent frames of the 50p source and create the two fields of the 50i destination frame from these two source frames (dropping alternate lines from each). So progressive frame 1 becomes output field 1 of interlaced frame 1 and progressive frame 2 becomes field 2 of interlaced frame 1.

My question is what happens with the situation where the two adjacent source frames are either side of a cut point. Do we end up with an output frame containing two completely different fields? This would be an issue if a client was to subsequently re-edit the piece (quite likely) in the 50i domain since they may end up with random odd fields in their edit if they selected the dodgy frame as either their in or out point. Given that many editing suites are set up with the video I/O card showing only field one when stopped, they would not notice until final playout.

Also, given the fact that TVs automatically de-interlace 50i content before displaying it on the panel, would a frame containing 2 radically different fields upset the process and cause unwanted artefacts?



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Bouke Vahl
Re: 50p to 50i conversion
on Nov 23, 2017 at 3:28:33 pm

First, please name 25i not 50i. It's Frames and Progressive or Interlaced.
(I know you're not the only one doing this....)

For two fields from different shots in one frame, that's how VHS worked, remember ?
It's not an issue, except indeed if you want to re-cut the stuff and want all shots at full length.

Now, you could avoid this to cut just on even frames.
Or, (would be my option) work on an NLE that takes care of the FPS conversion shot based. (So you switch the NLE settings to 50p or 25i for different outputs.)

hth,
Bouke

Bouke
http://www.videotoolshed.com


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Brian Cutts
Re: 50p to 50i conversion
on Nov 23, 2017 at 3:53:29 pm

Thanks for the response.

You're right about it correctly being referred to as 25i since that's the frame rate. I have similar issues with people who still refer to PAL & NTSC in the component digital world - and even with HD!

So from what you're saying, the de-interlacer in a modern TV will be fine and it's just an issue with future post prod?

I guess the ideal situation would be if you could edit in a 25 interlaced frame timeline with the even numbered frames of the source footage being treated as a 2nd field. I've noticed that this is how some EVS technology treats the format.

I'll look into the idea of doing the interlaced export directly from the timeline rather than through an external process and see if it handles it correctly.



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