Slow motion and my poor math skills

Alex H.

Alex H.

Moderator
Staff member
Okay, folks. This section of algebra always tripped me up, and years later I still get confused.

Shooting at 30p (DVX100A) and conforming to 24p effectively reduces the playback by 20%, right?

I'm trying something new this week... shooting a music video. I'd like to slow-mo a couple of the shots. So here's where I'm totally lost...

To shoot slow-mo, the music has to be sped up prior to playback for the shoot, so that when the picture is slowed down it matches the original music PB speed. So, to what percentage should the music be sped up so that, when decreased by 20%, it is back to it's original playback in Final Cut? (By the way, I know that I'll be using the original, unaltered music track in post, but it still has to calculate out this way...)

I know that, if I were to cut the video PB to 50%, the music would have to be sped up to 200% so that when cut in half it is back to, effectively, 100%. If I'm calculating this correctly, it's 125% to match the 80% playback for video. Any math majors in the house that can verify this?

Thanks,
Alex
 
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i can verify that you still have poor math skills. and you don't need to change the audio with the video, you can slow the video alone.

as for what you want to do, you haven't said what you want to do.

how about going through the scene or shot and tell us what you want to do first, then ask about the math and how it can be achieved.

To shoot slow-mo, the music has to be sped up prior to playback for the shoot, so that when the picture is slowed down it matches the original music PB speed
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why?what gave you that idea?

if the music is or is not connected directly to the scene it would either slow down with the scene or keep at speed while the video is changed, why would you speed the audio to slow it down, it's cool if it's the speed you want, you don't have to change it.if you have the final speed of the audio correct from the start why do you think you need to change it and then re change it to go back to the original speed.
 
amrrahmy said:
why?what gave you that idea?

if the music is or is not connected directly to the scene it would either slow down with the scene or keep at speed while the video is changed, why would you speed the audio to slow it down, it's cool if it's the speed you want, you don't have to change it.if you have the final speed of the audio correct from the start why do you think you need to change it and then re change it to go back to the original speed.
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I know that the speed of playback doesn't matter if the video isn't directly connected. In fact, for scenes of that nature, there is no need for on-set playback at all.

The video is directly connected to the audio... these will be shots of the band. Shot with higher-speed music track, slowed down in post to keep them in sync with the original track for graduated movement from the musicians (i.e. simulated overcrank). In order to do that, they need to be miming/playing/lip-syncing along with the music at a faster rate.

amrrahmy said:
if you have the final speed of the audio correct from the start why do you think you need to change it and then re change it to go back to the original speed.
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As I stated in my OP, I know I won't use this sped-up track in post. Speeding up and then slowing down will create some nasty artifacts, and is just senseless considering I have the unaltered track at my disposal. But they have to have something to hear on-set so that the slowed video will match the original track.
 
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you haven't described the scene yet again, why do you want to show slow motion of the characters with normal speed lip sync and real time music. do you know how difficult that would be for the characters to do.how many seconds are you planning on doing this. there are probably better ways of doing what you have in mind than that way, if you describe what you want the final scene to look like, someone might help you better.
 
Bingo, Jeff! Thanks so much. I knew I had the right principle... just thought that my math was backwards, but according to the chart there I was right on: shoot 30p and play song at 125% speed which plays the song in 80% of the original duration.
 
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Yeah its weird math I'm famous for screwing up or at least not knowing if I did. Inverse something divide this and multiple by this something... Thankfully we have Barry Green to verify :)
 
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