i'm actually in a discussion with my sound engineer regarding a 25 to 24fps frame conversion. I do have an original project that has been edited in HD in a 25fps timeline in FCP and i need to convert it to 24fps for cinema broadcasting....
As i've read in several places over the net the easiest way to do that is to go through cinema tools or use the tools menu in FCP an do the conversion there. So i did, and i always find myself with a sequence that is now 36,5 sec (instead of my original 35 sec), and for me it is supposed to be that way.
Now the sound engineer keeps telling me that when i do a conversion i should end up wit a sequence that is still 35 sec. long but only playing in 24fps.....
Can anyone tell me which is right, as i don't get my head around it....
You are right, your soundguy is wrong.
Probably he thinks about going from 23.976 to 29.97,
or a standards conversion like from 25 to 29.97
But in your case, you want a 1:1 frame relation to keep the image as crisp as it can be, so you did the right thing.
(assuming your source was progresssive, or you did a de-interlace.)
Now you need to think about the sound. You can pitch shift it so you will have no quality loss, but that might be tricky with music of famous voices.
You can change the speed of the sound while maintaining the pitch, but that is a form of black magic.
(Meaning, it can be done but results are hard to predict. Good thing is your piece is short so you can experiment with different settings.)
Steve, two methods are available. Either slowing to maintain a frame for frame approach or a hardware standards conversion that maintains time as your sound guy says. He isn't wrong, just pointing out the more common alternative.
The question is does duration matter? In the case of most broadcast programs that I work on it does and so mostly a 25 to 24 conversion is done with hardware like a Teranex, which leaves the sound as original. The alternate approach is the one you have done and that requires changing the length of the audio whilst maintaining pitch. This is something that I do in my facility regularly and with the right software, the results are excellent.
With the approach you have taken, ask you sound guy if he has available iZotope Pitch/Time, MPEX3 or Pitch N Time software. All of them do an excellent job and are far preferable to the 4% pitch shift that happens otherwise. I am sure a pitch perfect famous voice would agree.
It isn't unpredictable with good software and is a routine practice in professional sound post facilities like mine.