A Frame Is a Frame Is a Frame

01 A frame is a frame

A clip is a numbered stack of pictures. The frame rate is not part of the pictures; it is an instruction for how fast to show them. Thirty seconds shot at 24 fps is 720 frames, and it stays 720 frames whatever you tell the timeline to do with it.

That gives you two very different ways to get to a new rate.

• Reinterpret. Keep every frame and change the instruction. Play those 720 frames at 25 and they last 28.8 seconds. Nothing is added, dropped or blended, so motion stays exactly as it was shot. The cost is duration, and sync.
• Convert. Keep the duration and change the frame count. To fill 30 seconds at 25 you need 750 frames, so 30 have to come from somewhere: repeated, blended or synthesised from their neighbours. Sync holds. The cost is motion.

Neither is the right answer in general. The right answer depends on one question.

02 The first question: is it sound sync?

The everyday version of this problem in Australia is an overseas commercial shot at 24 or 23.976 that has to deliver at 25. Before anyone opens a retime menu, we ask whether it is sound sync. By that we mean people speaking on screen, or diegetic sound, the sound of something happening in the picture, that has to stay matched to it. Music and voiceover do not need that precision. Nobody is checking a voiceover for lip sync.

If it is not sound sync, reinterpret. The picture runs 4.17% faster, which almost nobody can see, motion stays perfectly clean, and the 30 second spot now runs 28.8 seconds. In ad land that is rarely a problem, because you hold the end frame. Thirty frames at 25 on the pack shot or the end super gets you back to length, and an extra moment on the logo is not something a client has ever complained about. Sometimes it is an advantage.

03 What if they are talking?

Then the audio has to come with the picture, and there are three ways to go.

Speed the audio up with it

Run the sound 4.17% faster too and sync holds. Pitch rises by about 0.71 of a semitone. On a lot of dialogue that passes. It is most noticeable on a voice people know well, and on music, where the whole track has moved a little under three quarters of a semitone off its key.

Speed it up and correct the pitch

Keep the speed change and bring the pitch back down, either with a pitch-preserving time stretch or a pitch shift of the same amount in the other direction, in Fairlight or your DAW. A correction this small is usually clean. Listen for a slightly processed edge on sustained notes and sibilance.

Or a combination

In practice it is often both. Reinterpret the picture, take the pitch-corrected mix, and then look closely at the shots where sync is critical. Where a correction is audible, or a music edit lands somewhere it should not, those shots get individual attention. The rest of the spot gets the clean picture for free.

04 When the duration is locked

Sometimes reinterpreting is not on the table: a long-form programme with a fixed running time, a mix that is already locked, or a gap between rates too wide to hide. Then you convert, and the question becomes how the extra frames are made.

In DaVinci Resolve

Put the material on a timeline set to the delivery rate. Each clip keeps its own rate, and Resolve fills the difference using the retime process, set project-wide under Master Settings, Frame Interpolation, and overridable per clip in the Inspector under Retime and Scaling.

• Nearest repeats or drops whole frames. Every frame is sharp, but the irregular repeats read as judder, most of all on pans and steady movement. Going from 24 to 25, one frame in every second is shown twice.
• Frame blend mixes neighbouring frames. Smoother, but anything moving picks up a soft ghost.
• Optical flow estimates motion and draws new in-between frames. At its best it is invisible. At its worst it warps: fast motion, rain, fences, spokes, hair, and objects passing in front of each other are where it breaks. The motion estimation setting matters, and the Speed Warp modes in Resolve Studio are worth trying on difficult shots.

To reinterpret instead, change the clip itself rather than the timeline: Clip Attributes in the Media Pool sets the frame rate Resolve treats the clip as, and the frames then play one to one.

Outside Resolve

Dedicated motion-compensated standards conversion, in hardware or software, is still the reference for long-form and broadcast work, and it is worth the cost when a whole programme has to change rate and look untouched. For a short commercial, careful shot-by-shot work in the grade usually gets there.

05 What to watch for

• Detect the cuts first. Blending and optical flow build frames from their neighbours, and across a cut the neighbours belong to different shots. The result is a frame that is half one shot and half the next. We always run scene detect in Resolve on a flattened master before any conversion, so every shot is retimed on its own and nothing bleeds over the cuts.
• Check what 29.97 really is. Material labelled 29.97 is sometimes 23.976 with pulldown added. Step through it: a repeating pattern of clean and mixed frames gives it away. Remove the pulldown first, then decide what to do with the 23.976 underneath.
• Watch the pans. Slow, steady camera moves show judder and interpolation errors long before anything else does.
• Rebuild titles at the new rate. Interpolated type and motion graphics rarely look clean. Convert a textless master and rebuild the titles and graphics natively at the delivery rate, so they animate the way they were designed to.
• Retime everything that is timed. Captions, subtitles and any timecode-referenced deliverables move with the picture when you reinterpret. They need converting too.

06 Back to 29.97 to 50

The job was a business case study film. It premiered to a big audience on the huge screens at a large industry event, and it also needed a master for the website. It was shot and edited at 29.97, which suited the web, but the event screens needed 50.

Run it through the sum and reinterpreting is out straight away: 29.97 played at 50 is 66.8% fast, nearly nine semitones up, and a 30 second film becomes 18 seconds. But the deciding factor was not the maths. The film was one slot in a tightly run event schedule, so it could not get longer or shorter, and any change in duration would have flowed straight through to the sound mix. Once we knew the duration was fixed, the method followed: convert the finished picture, and leave the running time and sync exactly as approved.

It is an awkward conversion, because 50 divided by 29.97 is 1.67. There is no tidy repeat pattern, so repeating frames would judder in a cycle that never settles. The new frames had to be built.

Picture

We rendered the graded master without titles and converted it to 50 using motion-compensated interpolation, which builds new in-between frames so movement stays smooth on a screen that size. Each shot was converted on its own, so nothing blended across a cut, and we reviewed every shot and adjusted any that showed artefacts. The titles and graphics were then rebuilt natively at 50, so they animate cleanly instead of being interpolated.

Sound

Because the running time did not change, the audio needed no conversion and no new mix. The mix made for the 29.97 website master laid straight back against the 50 fps picture, in sync to the frame. One mix served both deliverables, which is the whole point of holding the duration.

The result was a 50 fps master that played smoothly at the venue and a 29.97 master for the web, both with the approved edit, grade and sound intact.

07 Quick reference

What reinterpreting does to a 30 second spot.

For any other duration or pair of rates, use the Reinterpret tab. For conversions that keep duration, the Frame Rate Conversion tab next to it maps timecode between rates.