Out-of-Sync Subtitle Fixing Guide: Frame Rate Deviation and Timecode Conversion Handling

Common source frame rates are divided into three categories: 23.976fps, 24fps, and 25fps. Frame rate differences will accumulate into time deviation as video duration increases. Taking a 90-minute feature film as an example, the cumulative deviation between 23.976fps and 24fps is approximately 8.1 seconds, and the cumulative deviation between 23.976fps and 25fps is approximately 36.9 seconds.

If the video duration reaches 180 minutes, the cumulative deviation between 23.976fps and 25fps will expand to approximately 73.8 seconds. An offset of more than 1 minute will cause subtitles in the second half of the video to completely detach from dialogue timing, and must be corrected via speed stretching.

The SRT format uses a comma to separate milliseconds, with a standard format of 00:00:00,000; the VTT format uses a dot to separate milliseconds, with a standard format of 00:00:00.000. Some editing software is incompatible with non-standard separators, which will cause parsing errors for the entire subtitle timecode.

During the conversion between SRT to VTT or VTT to SRT, it is necessary to follow the millisecond truncation rounding rule: do not round the original millisecond value, and keep 3 decimal places of precision. This avoids additional cumulative deviation exceeding 100 milliseconds after multiple conversions. The OKfmt format conversion tool follows this rounding rule when processing timecodes.

Whole timeline shift refers to uniformly adding or subtracting a fixed number of milliseconds to the start and end times of all subtitles. It is only suitable for scenarios with uniform offset throughout the video caused by opening offset, such as subtitle offset caused by cutting the opening of the source video, and does not modify the duration of individual subtitles.

Speed stretching refers to scaling and correcting the timecode of the entire subtitle timeline according to the frame rate ratio. It is suitable for cumulative offset scenarios caused by exporting with different frame rates. After correction, subtitles throughout the video can match the dialogue rhythm of the picture, and the duration of individual subtitles will be scaled proportionally.

For Adobe Premiere Pro 2020 and later versions, you first need to check that the timebase in sequence settings matches the native frame rate of the source video, then check the video frame rate in export settings, and confirm that it matches the value of the sequence timebase to avoid frame rate deviation caused by secondary transcoding.

For CapCut PC version 2.9 and later versions, you first need to check that the canvas frame rate in project settings matches the native frame rate of the source video, then check the frame rate parameter in export settings, and confirm that it matches the project canvas frame rate; disable the smart frame interpolation function to avoid modifying the timebase of the original source video.