3:2 Pulldown and Frame Rate Conversion

Three ways to change frame rate

Every frame rate conversion is one of three things, and each has a cost:

• • Change the speed. Play every frame, just faster or slower (24 → 25, 25 → 24). Motion stays clean; running time and audio pitch change.
• • Repeat fields or frames in a pattern. 2:3 pulldown (23.976 → 29.97) and its relatives. Running time stays the same; the repeats add a slight, regular judder.
• • Make new frames. Blending or motion-compensated interpolation, for rates with no clean pattern between them (25 → 29.97). Running time stays the same; quality depends on the converter and the motion.

How 2:3 pulldown works

Interlaced 60 Hz video shows 59.94 fields a second, two fields per frame. Film runs at 24 frames. To fit four film frames into the time of five video frames (ten fields), pulldown holds the film frames on the screen for two and three fields in turn. Call the four film frames A, B, C and D:

Pulldown keeps the running time and every original frame, but two of every five video frames mix two film frames. On a progressive display that shows as combing; on motion, as a regular stutter people call telecine judder. That is why masters are now usually kept at progressive 23.976, with any pulldown added downstream when a 59.94i version is needed. Netflix's IMF specification, for example, asks for the native frame rate and allows no 3:2 pulldown flags.

Removing pulldown (reverse telecine)

Because pulldown repeats fields rather than inventing them, it can be undone. A reverse pulldown (inverse telecine) finds where the cadence starts, drops the repeated fields, and reassembles the four original progressive frames from every five video frames, giving 23.976p again. Netflix's automated QC detects 3:2 pulldown in a delivery and asks for exactly this: an inverse telecine back to a progressive 23.98 or 24 fps master.

• • Find the A frame. The cadence has five possible starting points; step through until the two mixed frames line up.
• • Watch for broken cadence. Cuts, titles or effects made at 29.97 after the pulldown start a new cadence mid-sequence. Those sections need separate treatment.
• • Don't deinterlace instead. Deinterlacing pulldown material throws away half the vertical resolution and keeps the judder.

2:3:3:2 advanced pulldown

Panasonic's DVX100 recorded 24p onto 59.94i DV tape with a different cadence, which Adobe's software lists as 24P Advance (24Pa): the second and third film frames each take three fields, so the pattern is 2:3:3:2. That gives video frames AA, BB, BC, CC and DD, with only one mixed frame. Removing it is simpler than standard pulldown: drop the BC frame and the four originals remain. Editing software that supports 24Pa removes it on import.

24 to 25 fps: the PAL speed-up

50 Hz television runs at 25 frames a second. The usual way to show 24 fps film on it, and the one ITU-R BT.709 describes, is to play every frame at 25 fps: motion stays clean and nothing is repeated, but everything runs 4.17% faster. Final Cut Pro does the same when you put 23.98 or 24p clips in a 25p project: it conforms them and changes their duration.

From 23.976 the jump is slightly bigger: 104.27% speed and 95.9% of the running time. The sound has to follow: Pro Tools, for example, has pull-up and pull-down settings for exactly this 4% (25/24 and 24/25), plus the 0.1% step. Sped-up audio can then be pitch-corrected, or the mix can be conformed to the new length. Check music cues, lip sync and the overall timing after the change.

25 to 24 or 23.976

The reverse trip, for a film shot at 25 fps going to a cinema or a 60 Hz market, slows every frame down: 96% speed to 24, 95.9% to 23.976. A 96-minute film becomes 100 minutes, and the audio drops by about 0.71 semitone unless it is pitch-corrected. Conforming the 25 fps frames to 24 is the cleanest way to make a DCP from 25 fps material, as long as the sound is conformed with it.

50 Hz to 60 Hz: standards conversion

Between the two families there is no tidy ratio: 29.97 ÷ 25 = 1.1988. A converter has to create the missing frames. Frame repeat and frame blending are cheap and show as judder or ghosting; motion compensated conversion (often called optical flow in NLEs) estimates where each pixel moves and builds new frames, which looks best until it meets fast, complex or semi-transparent motion and leaves artifacts. For a high-value deliverable, budget for a dedicated converter and review the result on the hardest shots. Netflix asks for a professional hardware or software standards converter and rejects conversions that introduce artifacts.

Interlaced sources add a step: deinterlace first, or convert field-based (50 fields to 59.94 fields) to keep the full motion of interlaced sport and live footage.

Choosing a method

For any pair of rates, CineMath's Speed Change calculator gives the playback speed and the change in duration (23.976 → 25 is 104.27% speed and 95.90% duration), and flags conversions that mix fractional and whole-number rates; the free speed change calculator does the same sum in your browser. To fit a program to an exact slot instead, use Target Length.

Pulldown and conversion questions

Next: every speed change moves the sound too. The audio sample rates and sync guide shows how to pull audio down or up to match, and the frame rates guide covers which rate to start from.