Constant frame rate
Also known as: CFR, fixed frame rate
Recording the same number of frames every second without exception, which is what editing applications assume and what cameras normally produce.
Updated
What it means
A constant rate means the interval between frames never changes. Thirty frames a second means a frame every thirtieth of a second for the whole recording, with no exceptions anywhere in the file.
That predictability is what makes editing possible. A timeline positions clips by counting frames, and an assumption that every frame occupies the same amount of time is built into every tool that touches video.
It is the opposite of Variable frame rate (VFR), which phones use to save data in dark scenes. One is designed for editing and the other for storage, and only one of them behaves on a timeline.
How it works in practice
Action cameras record it by default. A GoPro or a drone writes a constant rate without being asked, which is one of the quiet reasons their footage is easier to work with than phone clips.
Phone footage is the case that needs attention. Transcoding a variable-rate clip to a constant one on import removes a sync problem that otherwise appears forty minutes into a long edit.
Choosing one rate per session keeps it simple. Thirty or sixty throughout means no resampling anywhere, and a card shot at both still works, with one conversion somewhere along the way.
What to watch out for
The near-miss rates cause real trouble. Twenty-nine point nine seven and thirty are both constant and are not the same, and mixing them produces a drift that only shows up minutes later.
Some tools report a variable file as constant. The header says one thing and the frames say another, and the discrepancy is only visible once audio has drifted out of place.
Converting late wastes the work. A sequence built on a variable-rate clip and then fixed loses the trims, which is why Audio sync problems are worth ruling out before any cutting starts.
How RawClip handles it
Variable-rate material is normalised before analysis, so a phone clip and an action camera clip behave the same way once they are in. Everything about cut placement assumes an even frame interval, which is the reason that step happens first rather than later.