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Sample Rate Changes after conform to 24fps
Hi guys,
I’m hoping someone can either put my mind at ease here, or let me know that I’m doing something wrong.
We’re shooting a project on film – 24fps – with neg telecined to Pal DV at 25fps. The audio is being synched at the TK house, played out onto the same DV tape with the picture, and flex files/ALEs are being sent to me for importing into Cinema Tools so that I can capture/conform and then edit at 24fps in Final Cut 6.0.4.
So far so good – all pretty standard procedure.
When working at 24fps in the past, I’ve used the sound recordists’ original WAV files to manually synch myself up in FCP. This is the first time I’ve relied on the audio coming from the telecine tape and used Cinema Tools to export audio EDLs for the sound editors. So what struck me when processing the first tape is that once the QTs are conformed to 24fps by Cinema Tools, the audio sample rate of the clips changes from 48khz to 46080Hz, meaning that I get a green line on the clip whenever I cut some footage onto my timeline (sequences set to 24fps, 48khz). There appears to be no problems playing the timeline back and there is none of the ‘beeping’ that tells you the audio needs rendering but it is clear that the settings are mis-matched.
Now, I vaguely understand the logic that changing the speed/frame rate of a clip would also alter the rate of the sound on that clip but I wanted to check if there is any way to prevent this from happening. Or if I decide to just carry on editing with the audio clip settings like this, are there going to be any knock-on effects of cutting the whole film in this way i.e. compromised sound quality, longer render times etc. I realize I could export WAV files for each clip at 48khz and do a simple version of ‘synching up’ but the whole point of using the audio from the tape and getting the telecine company to sync up was to avoid this process.
Am I doing something wrong here, have I missed a trick? Or is this just an inevitable and irrelevant side-effect of editing at conformed 24fps media in FCP that I shouldn’t be worrying about? Thought I’d better check before I go too far down the line on this one.
Thanks for any help/advice you can give
Paul
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2 Replies
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Hi Paul,
When you conform 25 to 24 you just make your movie play a 4% slower
With the audio happens the same. If in a second (25 footage) you had 48.000 samples, when you slow it down you get a 4% less of samples played every second. 48.000-1920= 4680. The calculation is perfect.
If you have a look at the data rate (Video and audio) of the whole p24 clip I guess that it will be a 4% smaller than the 25fps too.
rafael -
Before entering a world of pain, I suggest you do a test OMF and send it with a reference QT file to your sound post people.
When syncing the audio in telecine, the file would have been sped up 4% to sync. After Cinema Tools it has been slowed down again so pitch is correct. The trick is to hope that the OMF labels the files as 48khz.
If that all works, then apart from potential render issues to play a big fat timeline with lots of audio tracks, I see no issue.
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