Remove 3:2 Pulldown in FFmpeg

This article provides a quick guide on how to perform Inverse Telecine (IVTC) in FFmpeg to remove a 3:2 pulldown pattern from a video. By using the fieldmatch and decimate filters, you can successfully reconstruct the original progressive frames and restore your video from 29.97 fps to its native 23.976 fps.

The Inverse Telecine (IVTC) Process

When film (shot at 24 frames per second) is converted to NTSC television format (29.97 frames per second), a process called 3:2 pulldown is used to stretch the frames across interlaced fields. To reverse this and restore the original progressive video, FFmpeg utilizes a two-step filter chain:

  1. fieldmatch: Reconstructs the original progressive frames by matching complementary interlaced fields.
  2. decimate: Identifies and removes the duplicate frames resulting from the field matching process, reducing the frame rate back to 23.976 fps.

The Basic FFmpeg Command

To remove the 3:2 pulldown pattern, run the following command in your terminal:

ffmpeg -i input.mp4 -vf "fieldmatch,decimate" -c:a copy output.mp4

How the Command Works

  • -i input.mp4: Specifies your source video file containing the telecined 3:2 pulldown video.
  • -vf "fieldmatch,decimate": Applies the video filtergraph.
    • fieldmatch analyzes the interlaced fields and combines them to recreate progressive frames.
    • decimate drops one out of every five frames (the duplicate frame created during telecine), restoring the video to 23.976 fps.
  • -c:a copy: Copies the audio stream directly without re-encoding to save time and maintain original quality.
  • output.mp4: The resulting progressive, 24fps (23.976 fps) video file.

Advanced Adjustments

In most cases, the default settings for these filters work perfectly. However, if your source video has specific properties, you can tweak the filter parameters:

  • Handling combed frames: If some interlaced artifacts remain, you can tell fieldmatch to pass stubborn frames to a deinterlacer like yadif by adding the combmatch option:

    ffmpeg -i input.mp4 -vf "fieldmatch=combmatch=sc,yadif=deint=interlaced,decimate" -c:a copy output.mp4

  • Variable Frame Rate (VFR) input: If the input video mixes progressive film and native interlaced video, you can configure decimate to output variable frame rate video to prevent judder in the purely interlaced sections.