Every NLE will happily place 23.976, 25, 29.97, and 59.94 clips side by side in one timeline - and every one of them pays for that freedom somewhere downstream, in juddered motion, drifted audio, or a conform that slowly slides out of sync. The anchor fact: non-drop-frame timecode at 29.97 runs 3 seconds and 18 frames ahead of clock time every hour - about 3.6 seconds - per Apple's own Final Cut Pro glossary, and the same 0.1% NTSC math is why 23.976 material played at 24 runs long. The definitions are in the frame-rate and timecode primer; this is the fix post: what each NLE actually does to mixed-rate clips, and the one rule that prevents the damage.
The one-frame-rate rule
Decide the timeline's frame rate before the first clip lands, and hold it for the whole project. Delivery specs, not camera specs, pick the rate: a 24-fps festival master, a 25-fps European broadcast spot, a 29.97 US web deliverable. Every clip from a different-rate camera gets conformed into that container rate exactly once, deliberately - by retime, by pulldown, or by proxy generation at the container rate - instead of silently, by the NLE, on every playback. Mixed edits are sometimes unavoidable (archival inserts, multicam from second systems); the rule survives by keeping them the exception and handling them explicitly, not by letting the timeline become a museum of camera formats.
How each NLE conforms mixed-rate clips
| NLE | What it does to a different-rate clip | What to watch |
|---|---|---|
| Final Cut Pro | Sets the project rate from the first clip, then conforms others by frame sampling; Rate Conform options: Fast (Floor), Fast (Nearest Frame), Good (Frame Blending), Better (Optical Flow) | Apple warns changing the project frame rate can shift every edit point in the project - pick before editing |
| Premiere Pro | Sequences have one Timebase; different-rate clips play at sequence rate. Documented correction paths: Modify > Interpret Footage ("Assume this frame rate") or per-clip Speed/Duration | Adobe notes changing a clip's frame rate changes the audio along with the video - pitch and length move too |
| DaVinci Resolve | "Mixed frame rate format" project setting conforms all clips to the timeline rate (23.98/29.97/30/50/59.94/60 clips all play at 24, for example); retimed to match | The setting locks once media is in the Media Pool - decide at project creation, not mid-edit |
Note what is not on the list: a documented "Premiere auto-retimes anything into any sequence" behavior. Premiere's documented mechanisms are Interpret Footage and Speed/Duration - everything else is playback approximation, which is precisely how mixed-rate timelines look fine in the edit and fall apart in the conform.
The drift math, once, properly
NTSC black-and-white television chose a scan rate of 30 × 1000/1001 (≈29.97) to fit color into the existing signal, and every 29.97-family rate inherits the 0.1% discrepancy. Apple's glossary states the consequence for non-drop timecode precisely: at 29.97, NDF timecode is "off by 3 seconds and 18 frames per hour" against elapsed clock time - 3.6 seconds, the number that turns a one-hour program's sync into a visible problem. Drop-frame timecode exists to fix the labeling (skipping frame numbers, never frames, per Apple's definition - and per Resolve's conform dialog, a drop-frame checkbox that only enables at 30 fps, which is why the DF concept belongs to the 29.97 family). The same 0.1% applied to 23.976 footage played at 24 means an hour of program takes 3.6 extra seconds of clock time - which is exactly what an audio sync readout shows after a long program.
Pulldown, briefly
When 23.976 footage must live on a 29.97 pipeline, the bridge is pulldown - distributing four frames of 24 into five fields-worth of 30. DaVinci Resolve documents the common cadence directly ("Add 3:2 pulldown: inserts or removes the 3:2 pulldown required to record or play 23.98 fps media to or from a 29.97 tape format"), and Avid's editing guide documents both the standard 2:3:2:3 and the advanced 2:3:3:2 cadence, the latter preserving complete progressive frames for clean removal later. The practical takeaway for a mixed-rate timeline: if your 24p material arrived with pulldown, remove it once on ingest (most NLEs do this automatically when told the cadence) and edit clean progressive frames - never edit pulldown that has to be removed again at conform.
Why conform is where it breaks
The edit plays; the conform drifts. An EDL exported at the wrong sequence rate is the classic cause - Resolve's EDL import exposes the failure directly with its frame-rate pop-up (choose the rate the EDL was actually exported at; it offers 30-to-24 conversion deliberately). Audio is the tripwire: video frames retime silently, but a music bed or sync-effects track drifts audibly, and the drift compounds with runtime. When a graded or mixed version comes back out of sync, check the rate chain first: sequence rate, export rate, conform import rate - one mismatch anywhere produces the slide.
The review loop
Mixed-rate projects also produce the review-side version of the problem: a player that counts time differently from the NLE, so a comment at "1:24:12" lands on the wrong moment. The defense is the same one-convention rule - pick one timecode convention per project, burn it in on review exports if the audience reads raw timecode (Premiere / Final Cut Pro), and let the review platform's frame-accurate player do the precision work for everyone else (what frame-accurate review means, and the review-focused Premiere / FCP / Resolve workflows).
FAQ
Why does my audio drift out of sync in long edits?
Almost always a 0.1%-family rate mismatch: 23.976 material on a 24 timeline (or 29.97 NDF timecode against clock time) accumulates about 3.6 seconds of drift per hour, per Apple's own documentation of the 29.97 discrepancy. Music and sync effects make it audible before picture makes it visible.
What timeline frame rate should I pick for mixed footage?
The delivery spec's rate, not the dominant camera's. Conform every other-rate clip into it deliberately on ingest. If nothing is specified, 23.976 or 24 for film-flavored work and 25 or 29.97 for broadcast regions - but let the destination decide.
Does Premiere Pro auto-retime different-rate clips?
Premiere plays them at the sequence rate, but its documented correction mechanisms are Interpret Footage (project-wide) and Speed/Duration (per clip). Adobe also documents that changing a clip's frame rate changes the audio with it - so retimes are decisions, not side effects.
Why does 23.976 footage run long against the clock?
It is the NTSC 1000/1001 ratio: 23.976 is 0.1% slower than 24, so an hour of program occupies 3.6 extra seconds of clock time - the same discrepancy that makes 29.97 non-drop timecode drift 3.6 seconds per hour.
What is 2:3:3:2 pulldown?
An "advanced" cadence for putting 24-fps frames on a 30-fps (59.94-field) pipeline, documented in Avid's editing guide. Unlike the standard 2:3:2:3, it keeps complete progressive frames intact so they can be removed cleanly later. Resolve documents the plain 3:2 cadence for 23.98-to-29.97 conversion.