The odd frame rate of 29.97 exists because colour television had to fit alongside a signal designed for black and white. Adding colour information risked interfering with the sound, and the fix was to shift the frame rate down by a tenth of a per cent so the two no longer collided.
The compromise worked and it left a number that does not divide into a second. Every timecode problem in broadcast follows from that.
What are the standard rates?
Regional history, mostly, and each is still in use.
| Rate | Where it comes from |
|---|---|
| 24 | cinema |
| 25 | European television |
| 29.97 | colour television |
| 30 | early television, and modern digital |
| 50 and 60 | sport and fast motion |
| 120 and above | slow motion capture |
The European rate is 25 because the mains supply runs at 50 hertz, and the American rate started at 30 because its mains runs at 60. Early sets locked to the mains frequency, so the electricity supply decided the frame rate.
Cinema settled at 24 as the lowest rate that carried sound acceptably, which is why film has the motion character it does. Higher rates look smoother and, to many viewers, less like film.
What is drop-frame timecode?
A counting convention, and it drops numbers rather than pictures. At 29.97 frames a second, counting timecode as though there were 30 runs fast against a real clock — by about 3.6 seconds every hour.
Drop-frame fixes the count by skipping two frame numbers at the start of each minute, except every tenth minute. That works out to 108 skipped numbers an hour, which is exactly the 3.6 seconds of drift.
No frames are lost. The pictures are all there; only the labels skip, so that a timecode of one hour corresponds to one hour of wall clock. For broadcast, where a programme has to fill a slot exactly, that matters.
When should you use non-drop?
When frame counting matters more than clock time. Non-drop timecode counts every frame consecutively, which makes arithmetic on frame numbers straightforward and makes the timecode drift away from real time.
For editing, effects work and anything measured in frames, that is the more convenient behaviour. For delivery to a broadcaster it usually is not, and the delivery specification will say which is required.
The two are visually distinguishable: drop-frame timecode is conventionally written with semicolons between the parts, non-drop with colons. Mixing them in one project is a reliable source of clips that appear a few seconds out.
What happens when rates are mixed?
Something has to give. Converting 24 frames into a 30-frame stream traditionally repeats fields in a repeating pattern, spreading four frames across five — which works and introduces a slight judder on movement.
Slow motion is the same problem used deliberately. Footage shot at 120 and played at 24 runs at a fifth of speed with every frame intact, which is why high-rate capture looks smooth where slowing normal footage does not.
Anything that changes the rate after the fact has to either duplicate frames, drop them, or synthesise new ones. All three are visible on fast movement, which is why matching the rate at capture is worth planning.
Does any of this matter outside broadcast?
For anything cut to music or to a duration, yes. A sequence that must land on a beat or fill a slot exactly is being measured in frames, and a frame at 24 is a different length of time from a frame at 30.
It also matters for anything with burned-in timecode or a delivery specification. Those specify a rate and a timecode convention, and a file that ignores them is rejected rather than corrected.
For a video that will only ever be watched on a phone, none of it applies. Pick a rate, keep it consistent, and the question disappears.
Why is the shutter tied to the rate?
Because exposure time is expressed as a fraction of the frame. The convention of a 180-degree shutter means each frame is exposed for half its duration, which at 24 frames a second is a fiftieth of a second and is what gives film its familiar amount of motion blur.
Change the rate and the matching shutter speed changes with it. Shooting at 50 frames a second under the same convention needs a hundredth of a second, which is a stop less light and is why high frame rates need brighter scenes.
Departing from the convention is a deliberate look rather than a mistake. A very fast shutter produces the stuttering, hyper-detailed motion used in action sequences, and a slow one smears movement.
Questions people ask
Does 23.976 exist too? Yes, and for the same reason — it is 24 lowered by the same tenth of a per cent so it sits alongside the colour standard.
Is 30 the same as 29.97? No, and treating them as interchangeable is where the drift comes from.
Does a higher rate mean a bigger file? Usually, since there are more frames to encode. The bitrate article covers what actually drives the size.
Why does my export look juddery? A rate conversion somewhere in the chain, most often 24 into 30.
Can I change the rate later? Yes, and every method has a visible cost on movement.
Work in frames, then convert. The frame rate calculator handles duration and frame counts across rates, and the FPS timecode calculator converts between timecode and frames including the drop-frame convention.