Phone video: variable frame rate (VFR) and why the audio drifts
Phones often record video with a variable frame rate (VFR): the number of frames per second changes during the recording. In the edit this can make the sound drift from the picture. MultiSyncer Studio finds these clips right after the copy and flags them so they are checked first.
What variable frame rate is
A regular camera records exactly 25 or 30 frames per second for the whole clip. Phones often record video with a variable frame rate (VFR): the number of frames per second can change during the recording, for example when the light changes. Watching such video on a phone is no problem, but it gets in the way of editing.
Why the audio drifts with it
An editing app lays a clip out on the timeline frame by frame, assuming every frame has the same length. If the frame rate actually changed, the picture and the sound inside the clip can gradually move apart. The app puts it this way: “Editing programs may shift the sound in such clips.”
| What you see | Possible cause |
|---|---|
| Phone audio drifts from the picture and the offset changes through the clip | Variable frame rate (VFR) |
| The audio falls evenly behind by the end of a long take | Clock drift, see the article “Audio out of sync in editing” |
| One camera has different frame rates over the day | The camera settings were changed during the day |
How to find these clips before the edit
Opening every phone file and looking at its properties takes a long time. MultiSyncer Studio checks it itself, right after the card is copied.
Copy the phone footage
Like a regular card: the app recognises the device by its files and puts the footage into its folder.
Look at the day's issues
In “Footage”, among the card's issues, you will see “Variable frame rate (VFR)” with the note “Editing programs may shift the sound in such clips.” It also shows when the frame rate or frame size changed on the card, or when a camera had different frame rates over the day.
It shows on the clip itself
A variable frame rate clip has a VFR label next to its frame rate, for example “30p VFR”. The same label goes into the day report.
What to do with them
Check these clips first in the edit. MultiSyncer does not change the original files; proxies and dailies keep the frames as in the original, so their frame rate is variable too. The app finds these clips in advance so that the sync problem is not a surprise in the edit room.
On sync, short phone clips are found by sound near their own time. A clip with no shared sound is placed by file time and marked: check its position.
Synchronization complete
Needs review
F002 · IMG_0412.MOV · by file time
F002 · IMG_0415.MOV · by file time
Different recording speed
Recorder: 40 ppm, 96 ms by the end