PDA

View Full Version : 50i with a 1/25 shutter transcode to 25p?



borax
03-31-2008, 11:34 AM
I have shot a bunch of 50i PAL footage using a shutter of 1/25 to get a better film look during filming. Now that I am starting post I have read that to convert 50i to 25p its better if you use the standard 1/50 shutter.

What are the possible side effects of using the 1/25 shutter?

During the shoot I thought it would be better to give the footage a slight motion blur so that it didn't look so much like interlaced video. But considering the ultimate transfer to progressive is it alway better to add a film look in post?

Will I get a higher quality transcode from an expensive post facility or will a standard software solution be sufficient?

Thanks

Barry_Green
03-31-2008, 11:46 AM
When you use a 1/25 shutter on a 50i camera, you're throwing away half your vertical resolution.

You'd be much better off to shoot 25P @ 1/50th, than you would be to shoot 50i @ 1/25th.

borax
03-31-2008, 01:15 PM
Thanks Barry,

Is that the only downside to using 1/25 on 50i when it comes to the transcoding to 25p?

I know that's a pretty big resolution compromise but I hope there aren't any further issues with artifacts or motion issues?

Ultimately the footage will then need to be transcoded to NTSC 24p, so does it make sense to transcode directly to 24p or edit in 25p or 50i and save that conversion till the final output is ready?

All of your help is greatly appreciated!

Barry_Green
03-31-2008, 01:26 PM
If your destination is 24p, why would you be shooting 50i in the first place? That's totally the wrong workflow.

With that said, the drawback to using 1/25 on 50i material is twofold: 1, it'll be half the resolution that 25p would have been, and 2) it'll have twice as much motion blur as 25p footage should normally have. It's just overall a bad, bad way to go.

If you can live with having too much motion blur and half the resolution, then yes you effectively have low-res blurry 25p footage already and you can transcode that to 24p.

As for when to convert, I'd do it after the entire program is done. Finish it in 25p and then when you're entirely done, transcode once to 24.

borax
03-31-2008, 04:07 PM
Yeah it was a production disaster that led to this situation.

What happened was two of the cameras that were sent to us were PAL and we didn't have enough time to replace them, so we ended up with 3 NTSC cameras and 2 PAL cameras that became our A cameras because they had phantom power for the shotgun mics.

One was a Canon A1 which did include 25p and was used for a few tapes of footage. The other was a Sony Z1 that was only capable of 50i so I had to switch the A1 to 50i to balance the look between the two. Now we were in interlaced land.

I can't tell you how frustrated I was with the entire situation, especially because I knew that we were shooting for a 24p project at the end of the day.

Now that we are in post I want to figure out what is the most logical way to proceed since I have more than 4 different HD codecs that we worked in (including a few timelapse and slowmotion shots from an HVX200).

The majority of the footage is 50i with a 1/25 shutter, then there is a little bit of 25p and a bunch of 24p that needs to be transcoded from the original 60i HDV that the Canon HV20 records in.

This is the worst post situation I have ever heard about let alone had to deal with.

So moving forward, Since the majority of the footage is in PAL 50i with a 1/25 shutter what is my first move?

-Do I transcode all the 50i and 24p to 25p and edit in a 25p timeline?
or
-Do I transcode the 25p and 24p to 50i and edit everything in 50i?
or
-Do I transcode all the PAL footage to 24p and edit in 24p?

There seems to be no good way to deal with this situation!

What seems to be the least destructive?

Barry_Green
03-31-2008, 04:11 PM
50i @ 1/25th is all going to be effectively 25p, so don't even think about 50i anymore. I mean, look at the footage and verify, but I'd guess that you could put that footage in a 25p timeline, tell it to process the footage as "progressive" and it'll just work.

So you should be able to mix all the 25p and the 50i@1/25 stuff together.

As for 25 and 24, that's a problem. You could string all the 25p stuff together and render it out as one big blob of 24p, and then bring that into a 24p timeline and cut it... I guess...

borax
03-31-2008, 04:35 PM
I have yet to capture the footage so I don't have anything in front of me to work with right now, so if I am understanding you correctly...

I can create a 25p HDV timeline and drop the 25p and 50i footage into it without the need to render or transcode in FCP 6. How does the 50i become deinterlaced without transcoding?

Does FCP do that on the fly?

If so that would be great!

The next step is the hard part because the director wants have access to all of the footage and be free to edit everything in the same timeline. So the 24p material would have to get transcoded to 25p?

Then after we are done transcode back to 24p while retaining the 25p for European markets?

Or should I tell him that in order not to compromise the quality of the 24p material we need to edit in 2 separate timelines 1 for 24p and the other 25p. Transcoding the final material from either timeline depending on which version we are making. Resulting in 2 versions 1 PAL and 1 NTSC.

This would save from further degrading the footage that was shot in its native format.

My thought on this is that converting from 24p to 25p then back to 24p would have a huge negative result on the 24p material or visa versa, and the only way to protect that from happening is to have 2 cuts of the film?

Its more work for sure but would that be worth it?

Barry_Green
03-31-2008, 06:34 PM
I could tell you how to do all this in about 10 seconds using Vegas, but for FCP I have no idea; some FCP expert is going to have to weigh in.

borax
03-31-2008, 07:11 PM
Maybe you should explain how to do it in Vegas, and someone could explain how that might work FCP.

I would love to hear any workflow I can at this point.

Maybe Vegas is the solution? (nothing has been captured yet)

Barry_Green
03-31-2008, 07:17 PM
Talkin' off the top of my head here, I haven't tried this, but I believe this is the way it'd work... In Vegas I'd set up a 23.976p timeline and import the all the 25p and 50i clips. I'd right-click and go to the clip properties for all the 50i clips and instruct it that it should treat this footage as "progressive scan" instead of "lower field first" or "upper field first". Voila, I now have converted the footage to 25p, losslessly.

To render out to 24p, I'd change the project properties to 23.976 and I'd change the "clip playback rate" to 0.959% (or whatever the accurate frame rate is) and disable resampling. That would cause the footage to play back 4% slower, which would turn 25 to 24. It's not a straight .96% because you want to account for the difference between 24 and 23.976. So, export all that footage (the 25p and the 1/25 50i that we've instructed to be "25p") as 24p. When that's done, all your footage is now ready for a 24p edit.