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When subtitles timed for 25 fps footage are used unchanged on 29.97 fps video, they fall progressively later behind the picture. To convert them, multiply every cue’s start and end time by 25 ÷ 29.97 ≈ 0.834168. That moves the cues earlier; a 10:00.000 cue becomes about 8:20.501. The cited worked example reports roughly 9.95 seconds of drift per minute for this mismatch. DEV Community’s worked example supplies the figure; the factor and timestamp conversion follow from the frame-rate ratio.
Why 25 fps subtitles fall behind on 29.97 fps video
A subtitle cue is associated with a point in the video timeline. If the same frame count is interpreted at two different frame rates, its clock time changes: a frame number F occurs at F/A seconds on a timeline running at A frames per second, and at F/B seconds on one running at B frames per second. The conversion from source time to destination time is therefore the source rate divided by the destination rate.
For 25 fps subtitles going to 29.97 fps video, that ratio is 25/29.97, or approximately 0.834168. Since the factor is less than one, converted timestamps move earlier. Leaving the original timestamps unchanged makes the subtitles arrive increasingly late relative to the video. The cited article reports about 9.95 seconds of drift per minute for this specific conversion; it is a worked technical example, not a population statistic about how often the problem occurs.
Diagnose drift before changing timestamps
Compare subtitle cues with the corresponding dialogue near both the beginning and end of the video. The shape of the error helps distinguish a timing offset from a rate mismatch.
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- Roughly the same error at the beginning and end: investigate a constant offset. Shifting every cue by the same amount may be appropriate.
- The beginning is aligned, but the error grows toward the end: investigate a frame-rate or timeline-duration mismatch. A single offset will not correct an error that accumulates over time.
Telestream describes captions that are aligned at the start but drift by the end as captioning drift, and its CaptionMaker User Guide 8.3 documents a stretch/shrink timecode operation. First confirm that you are comparing the intended video version: a different cut or inserted material can complicate a global correction.
Convert 25 fps cue times to 29.97 fps
- Confirm the destination video and its playback frame rate. Check the actual version being played; do not infer frame rate from a timecode label alone.
- Confirm the subtitle timeline’s source rate. Apply this conversion only if the cues were authored for 25 fps footage and the destination is 29.97 fps.
- Scale every start and end timestamp by 25/29.97. Use approximately 0.834168 as the multiplier, consistently across the file. For example, 10:00.000 × 0.834168 is about 8:20.501.
- Validate cues near the start, middle, and end. Compare each converted cue with the actual picture and audio. If the remaining error grows over the timeline, reassess the assumed rates or whether the video is a different edit.
Scaling is not the right choice for every conversion. If subtitles are already synchronized and only their frame-rate or timecode representation is changing, preserve their clock times. If the video version itself plays faster or slower, cue clock times may need to change to match that timeline. Telestream’s guide treats these as distinct conversion intentions.
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Do not confuse frame rate with timecode mode
Frame rate describes how often the image updates. Timecode mode describes how frames are numbered or labeled. Telestream’s guide distinguishes them explicitly: “Video frame rate—the rate at which the image is updated” and “Time code mode—the numbering or labeling system used to uniquely identify each video frame.” A label can therefore represent a different clock time under a different timecode mode.
Drop-frame and non-drop-frame are timecode modes associated with 29.97 workflows; they are not different playback frame rates. Drop-frame changes the numbering convention to keep labels closer to clock time. It does not scale arbitrary 25 fps cue timestamps to fit 29.97 fps footage. Do not use a drop-frame toggle as a substitute for the 25/29.97 timestamp conversion.
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When one global scale may not be enough
A single multiplier assumes the subtitle timeline and video differ by a consistent rate relationship. If the picture has edits, inserted material, or comes from a different cut, the mismatch may not grow uniformly throughout the file. Check the start, middle, and end after conversion; if one region remains out of sync while another is correct, investigate the edit or timeline structure rather than repeatedly applying a global scale.
The 9.95-second-per-minute example applies to the stated 25-to-29.97 mismatch. It should not be confused with Telestream’s separate figure of 3.6 seconds per hour for conversions between 29.97 and 30 fps, documented alongside analogous 23.976/24 and 59.94/60 conversions in its 2020 guide. Those are different rate pairs and different drift magnitudes.
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