Key takeaways
- Speeding up 1.5× turns 1m 0s from 90 seconds; to reach 60 seconds you need 1.50×.
- Checked on real files: our speed tool made a 5 second clip at 0.5×, 0.75×, 1.5× and 2× run 10.23 s, 6.83 s, 3.47 s, 2.63 s where the formula says 10.00 s, 6.67 s, 3.33 s, 2.50 s.
Speeding up to a length
To fit a clip into a time limit, divide its length by the limit: 90 seconds into 60 needs 1.5×. Check the sound after a large speed change, since pitch correction has limits.
A speed change also changes how many frames you need per second of output; encoders either drop frames when speeding up or repeat them when slowing down.
Scenarios
| Scenario | Duration at | Speed needed | Time removed |
|---|---|---|---|
| 90 seconds at 0.5× | 3m 0s | 1.50× | 1m 30s |
| 90 seconds at 1.25× | 1m 12s | 1.50× | 18s |
| 90 seconds at 1.5× | 1m 0s | 1.50× | 30s |
| 90 seconds at 2× | 45s | 1.50× | 45s |
| 10 minutes to 60 seconds | 1m 0s | 10.00× | 9m 0s |
Measured: speed changes on a real clip
| Speed | Formula | Real result | Difference |
|---|---|---|---|
| 0.5× | 10.00 s | 10.23 s | 0.23 s |
| 0.75× | 6.67 s | 6.83 s | 0.17 s |
| 1.5× | 3.33 s | 3.47 s | 0.13 s |
| 2× | 2.50 s | 2.63 s | 0.13 s |
The small differences are the audio: encoders round the audio length to whole frames.