-
Cinema Tools: Reverse TK of 29.97 DVCAM transfer of 23.98 HDCAM original
Hi everyone,
I’m wondering if someone with knowledge of the internal workings of Cinema Tools can clear something up for me.
We’re going to be taping a feature with the CineAlta F900 at 23.98fps and recording sound with a Sound Devices 744T set to 48kHz and 24fps timecode, 4 channels of WAV mono, and are having unexpected results with the sound with respect to sync in our tests.
The idea was to do our offline on Final Cut Pro at 23.98 (will go into more detail below), relying on Final Cut to do automatic pulldown to the audio. It turns out Final Cut, as expected, IS applying automatic pulldown to the audio as our sequence preset is 23.98 (and all our video is 23.98), but it is as if it has DOUBLE pulldown as the audio is .1% slower than the image. This is probably all because of a perhaps wacky workflow I developed that I wanted to get some feedback on.
At first I though that we must be recording at some other speed, at 48048HzF (48048 stamped as 48kHz) as it didn’t make sense to me that the sound doesn’t need pulldown, either done in ProTools or automatically by Final Cut. But I just checked settings with the sound recordist and he’s recording straight 48kHz and 24fps timecode.
Now I’m pretty sure the problem is we’re having the HDCAM tapes transferred to DVCAM 29.97 for our offline, and the fact we’re doing a reverse TK to 23.98 instead of 24fps in order to produce a CMX3600 that we can use with the original tapes and have QT files that run at the same speed as the 23.98 online (at least in theory).
My logic in developing a workflow using Cinema Tools’ reverse TK feature on material originating from video was the following:
1. We weren’t set up to handle HDCAM on our Final Cut system, so we had to transfer to DVCAM for our offline.
2. Instead of dealing with the conversion to a 24 base timecode that matched the original tapes at the end, and since we had the original HDCAM timecode burned in on our transfers, why not reestablish the original frames and end up with a CMX 3600 with the original tape timecode?
3. In order to do this, why not use Cinema Tools, even though it is not intended for that purpose, and reverse to 23.98 so that we’re at the same speed as the eventual online?Now why did I think reverse TKing to 23.98 would work? I assumed that Cinema Tools, in reversing to 23.98, leaves the speed of the TK (which is also 23.98) alone and simply restores the original frames. Reversing to 24fps, on the other hand, would restore the original frames AND do a pullup in speed, I thought.
But perhaps it does not work that way? Am I somehow doing a pullup to my video by reversing material that’s originally 23.98 and then converted to 29.97 to 23.98?
I can’t think of what else could be happening.
Thanks,
Nayeli -
20 Replies
-
I think the sync issue is the audio TC. Audio recording should be 48K, 29.97 NDF TC to match the HDCAM at 23.98. When CT does a reverse TK, split fields are removed and the timebase is changed. So 60 fields/30frames becomes 48 fields/24frames, but 1 second still equals 1 second of duration.
Sincerely,
OliverOliver Peters Post Production Services, LLC
Orlando, FL
http://www.oliverpeters.com -
My guess is that Final Cut is the problem, not CT. When you import any A/V quicktime into Final Cut, the current Easy Setup dictates what sample rate the audio is interpreted as. Doesn’t matter what your sequence settings are. If you import something with NTSC timecode into a project and your Easy Setup is one that uses 24.00, it will resample any 23.98 audio accordingly. This is not a filter or anything that can be changed. It’s evident by a colored line in the audio track when you drag it into a sequence. This works the opposite way as well (importing 24.00 media when the Easy Setup is 23.98), and it also does this with PAL.
The trick is to change your Easy Setup to the rate your audio is supposed to be, quit FCP and relaunch a new project. Then import the audio into the new project.
I don’t know if this is related to your problem, but I suspect it is. Another important tool to help you troubleshoot this is the “Modify->Timecode” option in Final Cut. You may wish to change the TC format of your media clips to 29.97 Drop Frame. Then quit FCP and re-import.
Sean
-
[Oliver Peters] “I think the sync issue is the audio TC. Audio recording should be 48K, 29.97 NDF TC to match the HDCAM at 23.98. “
I disagree.
29.97 audio is an Avid thing….Always shoot with the Audio TC settings the same as they are for the frame rate the camera is shooting or you will be in trouble later.
you are correct however- audio does not have TC by default- it is measured in feet and inches (old style) or minutes and seconds (digitally)
gary adcock
Studio37
HD & Film Consultation
Post and Production WorkflowsInside look at the IoHD
https://library.creativecow.net/articles/adcock_gary/AJAIOHD.php -
Gary,
[gary adcock] “I disagree. 29.97 audio is an Avid thing…. “
My point was that he said the audio TC was 24fps. My thinking is that this is now the incorrect speed. It should have been 29.97 or 23.98 to match the “video rate” of the camera, not a true 30 or 24, which is what was implied.
In any case, I’m currently cutting a feature in FCP shot with an EX1 and EX3 plus double-system audio as a back-up. The video is all 23.98, but the audio TC is 48K/29.97 and sync is just fine. So 29.97 should be OK in FCP-land as well.
– Oliver
Oliver Peters Post Production Services, LLC
Orlando, FL
http://www.oliverpeters.com -
“When CT does a reverse TK, split fields are removed and the timebase is changed. So 60 fields/30frames becomes 48 fields/24frames, but 1 second still equals 1 second of duration.”
But that would depend if you reverse to 23.98 or to 24, right?
-Nayeli
-
Sean: “My guess is that Final Cut is the problem, not CT. When you import any A/V quicktime into Final Cut, the current Easy Setup dictates what sample rate the audio is interpreted as.”
Yes, I’ve run into this before. The sequence preset was and is set to 23.98, we were very careful about that. All audio imported has been under 23.98, which is why Final Cut applies automatic pulldown to the audio, which is recorded straight 48K with 24fps timecode. In this case, we WANTED Final Cut to apply the pulldown (though I do wish it gave you a choice, aside from what your sequence preset is), and it did. The strange thing is that the audio with pulldown is now running 1% slower than the video which is *supposedly* at 23.98. Audio without pulldown runs at precisely the right speed. Which makes me suspect that somehow the video is actually running at 24fps. But I can’t get my head around why that is….
-Nayeli
-
Oliver: “My point was that he said the audio TC was 24fps. My thinking is that this is now the incorrect speed. It should have been 29.97 or 23.98 to match the “video rate” of the camera, not a true 30 or 24, which is what was implied.
In any case, I’m currently cutting a feature in FCP shot with an EX1 and EX3 plus double-system audio as a back-up. The video is all 23.98, but the audio TC is 48K/29.97 and sync is just fine. So 29.97 should be OK in FCP-land as well. ”
Yes, audio TC is 24fps and it’s 48K. What I’m perplexed about is that it is perfectly in sync, without adding pulldown, with my 23.98 video, making me suspect that somehow my video is not running at the right speed.
I have a question about timecode, though. What happens when you have 29.97 or 23.98 timecode when you’re at 48K? Because as far as I know, TC is only base 30 or base 24. If you record at 48K but choose 29.97 timecode, is that like recording at 48048 and stamping it as 48K?
-Nayeli
-
[Nayeli Garci-Crespo] “Which makes me suspect that somehow the video is actually running at 24fps. But I can’t get my head around why that is.”
As a test, take one long clip that has been reverse TK’ed and use the Cinema Tools CONFORM function to make this clip have a timebase of 23.98. If it’s indeed 24, then this will change the QT file to be read as 23.98. If that fixes the problem, then you might have to do the same to all clips.
– Oliver
Oliver Peters Post Production Services, LLC
Orlando, FL
http://www.oliverpeters.com -
[Nayeli Garci-Crespo] “But that would depend if you reverse to 23.98 or to 24, right?”
not really since when audio is independent of the video 1sec of audio would still be added as 1 sec of audio
As long as there is no video associated to the audio clip on import to the timeline the time of the clip remains unchanged.
gary adcock
Studio37
HD & Film Consultation
Post and Production WorkflowsInside look at the IoHD
https://library.creativecow.net/articles/adcock_gary/AJAIOHD.php -
Oliver: “As a test, take one long clip that has been reverse TK’ed and use the Cinema Tools CONFORM function to make this clip have a timebase of 23.98. If it’s indeed 24, then this will change the QT file to be read as 23.98.”
Aaah! Thank you!
I’m about to try putting the audio against the original 29.97 clip and one that has been reverse TKed to 24 instead of 23.98 to see what happens. Had not even considered the conform function!
-Nayeli
Reply to this Discussion! Login or Sign Up