For a documentary project that I recently completed audio post-production on, I was asked about converting the native 25 frames per second (fps) audio mixes to a 24 frames per second (fps) version to make a 24 fps DCP that is a safe interop version for playback worldwide for festival screenings.
The normal process for converting from a 25fps to 24fps version is to simply play back all frames but 1fps slower effectively making for a 4% slower/longer 24fps version of the film. With this you then have the option of living with the pitch drop that goes with the speed change if using sample rate conversion, or else pitch shift the audio back up to compensate. It all seems a bit crazy to have to do this these days where pretty much everyone in the industry is all digital from start to finish, but it is still something that needs to be dealt with, at least until everyone decides we can all just agree on one global frame rate.
I mostly followed the procedure outlined in this article on Pro Tools Expert:
I didn’t bother with all the Premiere steps (but someone will need to check sync at some point), and I opted for a slight variation on the 4% To correct the audio, instead going with pitching up by 4.17% (0.71 semitones) which seems to be slightly more mathematically accurate. See the screenshot below of my Avid X-Form settings for reference.
I can’t say the process is perfect, and of course the pacing of the film slows down significantly and there are some artefacts from the slowing down and pitching up process, although from the sample I checked out they seem good enough given there is a compromise needed for this to work.
