SloMo/SpeedUp Mathematics for Matching Audio???

[agentjonny]

hi guys-
this may have been covered numerous times, but the search engine here isn't helping me very much, so here goes...
let's say i need to shoot a music video. i'd like some of the footage to be slomo and some to be fastmotion. working in a 24 timeline. the mathematical questions are:
how much should i speed up the audio if i shoot in 30P and then slow it down to 80%?
how much should i speed up the audio if i shoot 60i and slow it down to 40%?
how much should i slow down the audio if i want the video sped up at these same increments?
i should note, when i say "slow down the audio" or "speed up the audio", i mean do that in Soundtrack to make a reference mix for someone to play along to, so the footage would have to be slowed down or sped up to match the actual tempo of the song..
someone has to know the math to this..
thanks!

 

[Barry_Green]

I'll ballpark it for you; you can experiment and fine-tune it.

One minute of footage at 30P would take 1:15 to play back at 24P.

So you would want to shoot 20% less time, so it takes 25% longer to play back.

So retime your audio to play back in 80% of its time, and it should all line up. If you have a 1:00 song, you want to speed it up so that it takes 48 seconds to play. Then you shoot 48 seconds worth of 30P footage. When you import that 48 seconds and tell it to play back at 80% speed, it'll play back in 60 seconds -- which is the speed of the original song.

Same for 60i, but twice as much -- so for a 1 minute song, you'd need to speed it up to where it plays in 36 seconds. Then shoot 60i sync'ed to that 36-second song. Put the footage on the timeline and tell it to play back at 40% speed, and it should now take 1 minute to play back, sync'd to the original 1-minute song.

 

[cheezweezl]

yeah, i just did a lot of this for a music video. for the shots at 30p that you will eventually playback at 80%, speed the music up to 125% when shooting. came out perfect to the frame for me. easiest way to do the 80% thing (on the mac) is use cinema tools. simply open the clip in cinema tools and reset the timecode from 29.97 to 24 fps. then it will play on a 24fps timeline in FCP at the right speed (80%) with no rendering necessary.