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Hello.

I've sped a song up using Quicktime in order to create a neat slow motion effect in Final Cut Pro.

I set the speed of the song to 1.5x in Quicktime and filmed the video in normal time just playing along to the faster track.

However, if I set my video to play at 50% speed in FCP I'm still out of sync with the video.

What is the math here? I know it's nothing difficult but for whatever reason it's not behaving the way I'd like things to.

I just need to know what speed to set the video to in order to get the most sync possible.

Thank you for any advice.

Speeding up from 100% to 150% is a 50% increase, but slowing 150 back down to 100 is only a 33% decrease. Try setting to 67%.

I should have said “a 1/3 decrease”. Set as close to 2/3 (66.6666667%) as your video app allows.

Agreed.

Similarly you could increase to 133.33..% then slow down by 75%

If you needed more precision with the speedup or slowdown, you could use DP's pitch/time stuff.

Audacity is freeware and does pitch and time pretty well.

And my go-to is IRCAM TS

stubbsonic wrote: ↑Sat Jan 01, 2022 9:54 am Agreed.

Similarly you could increase to 133.33..% then slow down by 75%

If you needed more precision with the speedup or slowdown, you could use DP's pitch/time stuff.

Audacity is freeware and does pitch and time pretty well.

And my go-to is IRCAM TS

DP uses ZTX which is really good. I prefer using ZTX inside TwistedWave — same engine but far less convoluted to use when doing T/S math to match video.

I also have IRCAM TS. It and ZTX do not sound the same. Both are great but sometimes I prefer one over the other.

Thanks for the responses guys!

I realized that most of my problem was bad pantomiming to the original track. (Hard to appear to play correctly at such a speed!)

Either way, what is the breakdown of the math?

I get what the deal is but not 100% intuitively.

Wouldn't 150% be 50% faster than normal?

Thanks again.

LUG wrote: ↑Mon Jan 03, 2022 7:02 am Either way, what is the breakdown of the math?

...

Wouldn't 150% be 50% faster than normal?

Let's use a simple example. Let's say the current speed is 2 (with an arbitrary unit). So increase the speed by 150%. Now the speed is 3.

Your new speed as now the new value for what is 100%. That old value of 2 as "100%" is no longer valid.

So now if you went to reduce that new speed back to it's original speed, you can't just subtract 50%-- because obviously that would be half of 3 (1.5).

2 is no longer 100%, it is now 3 that is 100%. So to get back to a speed of 2. So what would be the formula to get back?

So what would be the formula to get back?

Hint... the answer was posted by me earlier in the thread.

Kurt Cowling wrote: ↑Mon Jan 03, 2022 1:30 pm

So what would be the formula to get back?

Hint... the answer was posted by me earlier in the thread.

Yes, we both arrived at the answer. The reason I asked a question about the formula was to create a little step-by-step process.

If a person had a tempo of X (in BPM), and they wanted to increase the tempo by a factor of Y (a percentage, perhaps), resulting in new tempo Z, then they want to revert tempo Z back to tempo X again, say by a factor of W (another percentage)-- is there a relationship between factors Y and W? Is there an elegant way to show all the ratios/relationships? My math skills are not great, but it's not for lack of trying.