Help with retiming calculations - Change Tempo (SOLVED)

[Shippo]

hey all
I'm creating a home made music video and am working to the following constraints

1. Each take of video can be no longer than 12 minutes (but I want to get it as close to 12 minutes as possible)
2. The video editor I am using only works to whole 1 percentages for speeding up/slowing down.

I have a 4 minute SONG that I want to make into a 12 minute SONG using Audacity.
The 12 minute (Slowed down version) will be mimed to and recorded to video.
Using Final Cut Pro X I will speed up the video and replace the audio with the original 4 minute song. The FINISHED video should then look like everyone is whizzing around at high speed but the guitar playing/lyrics look like they are being played/sung at the normal speed.

What calculation do I use to work out the percentage to slow a 4 minute song down to a 12 minute song?
What calculation do I use to work out the percentage of how much I need to then speed the 12 minute video up to turn it back into a 4 minute video?

My mind is melting

In Audacity I know how to use (Effect>Change Tempo) to change the tempo of the song.

My main issue is that Final Cut Pro X will only allow me to adjust retiming by a whole 1 percentage, so the retiming cant have any decimal places in it.

Thanks all ( I know this is half off/half on topic by the way)

[steve]

What calculation do I use to work out the percentage to slow a 4 minute song down to a 12 minute song?

("original duration" - "target duration") / "target duration"

for your example:

(4 - 12)/12
= -8/12
= -0.666666667

Then multiply by 100 to get it as a %
-66.66667

Tip: If you forget the formula, "Change Temp" works the same way, so if you go to use Change Tempo and enter the required length (but do not apply the effect), the plug-in will show you the required %.

What calculation do I use to work out the percentage of how much I need to then speed the 12 minute video up to turn it back into a 4 minute video?

That depends on how the video editor works. It may be the same as Audacity or different - check the Final Cut documentation.

When you stretch the audio in Audacity, get it as close as you can to the video length. When you speed up the video and sound together they should remain in synch even if Final Cut is rounding to the nearest 1%.

Alternatively, could you speed up the video first, then add the audio?

[Shippo]

What calculation do I use to work out the percentage of how much I need to then speed the 12 minute video up to turn it back into a 4 minute video?

That depends on how the video editor works. It may be the same as Audacity or different - check the Final Cut documentation.

When you stretch the audio in Audacity, get it as close as you can to the video length. When you speed up the video and sound together they should remain in synch even if Final Cut is rounding to the nearest 1%.

Alternatively, could you speed up the video first, then add the audio?

Thanks for the equation.
Like I said, I can adjust the VIDEO speed using Final Cut Pro by a whole 1% only.
I will simply replace the audio on the retimed video (in the end at 4 minutes) with the original 4 minute wav in the end.
So using your equations (with regards to video, or anything for that matter)
12 minutes of video to become 4 minutes of video uses the following equation:-

(12 - 4)/12
= 8/12
= 0.666666667

Then multiply by 100 to get it as a %
66.66667%

but speeding the video up by 66.66667% isnt going to work surely it needs to be 166.66667%?

I'm probably wrong though, but my brain hurts.

[steve]

So using your equations (with regards to video, or anything for that matter)
12 minutes of video to become 4 minutes of video uses the following equation:-

(12 - 4)/12

No, it's:
("original duration" - "target duration") / "target duration"

so that's (12 - 4) / 4 = 8/4 = 2
= 200%

[Shippo]

Great stuff.
So with my 4 minute song I slow it down by 66.66667%.
Video it and mime to it at that speed.

I then speed the video up by 200%
Replace my normal 4 minute audio over the top and alls hunky dorey?

[steve]

Whether Final Cut Pro uses ("original duration" - "target duration") / "target duration" or ("original duration" - "target duration") / "original duration" (or something else), I don't know, but this is how Audacity does it.

[Shippo]

Whether Final Cut Pro uses ("original duration" - "target duration") / "target duration" or ("original duration" - "target duration") / "original duration" (or something else), I don't know, but this is how Audacity does it.

Hi
How else could it work? I dont fully understand.
What actually happens with FCPX is you select "retime" and change the retime speed in percentages.
100% is the original speed, then you can retime faster or slower by stretching out the retime handles 1% at a time.
So if you retime it to 200% that means its twice as fast as the original.

I have done this previously with another video (But the other way around completely) where I sped the audio up by 100% (So it was twice as fast) and mimed to it at that speed.
I then simply retimed the video to 50% (100% being normal, so it was reduced by 50%) and everything was fine.

[steve]

How else could it work?

It could (and apparently does) work as you've just described.

"new duration" = "original duration" x ("% change" / 100)
which is possibly a more intuitive way than how Audacity does it.

[Shippo]

Great stuff.
So with my 4 minute song I slow it down by 66.66667%.
Video it and mime to it at that speed.

I then speed the video up by 200%
Replace my normal 4 minute audio over the top and alls hunky dorey?

Just to re-itterate, do the above, then in Final Cut Pro X set the retime speed to show 300% (which is 100% normal plus 200%)
Tried and tested, it works perfectly.
Thanks for all the help.
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