Inverse Telecine

Film is shot at 24 frames per second, but NTSC video runs at 29.97. Telecine bridges the gap with 3:2 pulldown, which spreads every four film frames (A, B, C, D) over five video frames by repeating fields: AA, BB, BC, CD, DD. Two of every five frames are a mix of two film frames and look combed. Inverse telecine (IVTC) reverses this: it matches fields back into the original progressive frames and then drops the repeated one, giving back 24000/1001 fps with no interpolation and no loss.

That is why IVTC is the right fix for telecined film and deinterlacing is the wrong one. Deinterlacing would interpolate frames that never needed it and leave you at 29.97 fps with an uneven cadence.

Is it telecined?

Run idet over a few thousand frames:

ffmpeg -t 60 -i input.mkv -vf idet -f null -

Telecine shows up as repeated fields: the “Repeated Fields” line reports Top or Bottom repeats in roughly two of every five frames. Stepping through frames in a previewer shows the same thing, three clean frames and two combed ones in a fixed rhythm. True interlaced video combs every moving frame instead.

There are two kinds of telecine:

• Hard telecine has the pulldown baked into the frames. Most NTSC TV recordings and many anime DVDs are like this. This is what IVTC is for.
• Soft telecine stores 24p video with flags that tell the player to repeat fields. NTSC film DVDs are often like this. There is nothing to undo; ignore the flags and you already have clean 24p. Plain core.bs.VideoSource ignores them (its rff option is off by default, and you may need to set the frame rate to 24000/1001 with std.AssumeFPS), but the vs-jetpack BestSource wrapper applies them by default (rff=True) and hands you hard-telecined 30i that needs the IVTC below.

FFmpeg

fieldmatch pairs fields into frames, yadif deinterlaces the few frames that field matching couldn’t repair, and decimate drops one frame in every five:

ffmpeg -i input.mkv -vf "fieldmatch,yadif=deint=interlaced,decimate" -c:v libx264 -crf 18 output.mkv

deint=interlaced makes yadif touch only the frames fieldmatch still marked as combed, so clean frames pass through untouched. Field order is taken from the stream; set fieldmatch=order=tff or bff if the stream is flagged wrong. decimate only works on constant-frame-rate input, so a source that mixes telecined and genuinely 24p sections needs dejudder,fps=30000/1001,fieldmatch,decimate instead (from the FFmpeg documentation); anything more tangled belongs in VapourSynth.

FFmpeg is fine for clean, constant-cadence sources. When the cadence changes partway, or you want to check what each frame did, use VapourSynth.

VapourSynth

VIVTC’s VFM (field matcher) and VDecimate do the same job with more control. VFM only accepts 8-bit input, so field-match before converting to a higher bit depth. order is mandatory: 1 for top field first, 0 for bottom field first.

from vssource import BestSource

clip = BestSource.source("input.mkv", bits=None)
clip = core.vivtc.VFM(clip, order=1)
clip = core.vivtc.VDecimate(clip)

VDecimate removes one frame per cycle of five, whichever is the most nearly duplicate. vs-jetpack’s vsdeinterlace wraps both and can deinterlace frames VFM could not match, much like the FFmpeg chain:

from vsaa import NNEDI3
from vsdeinterlace import vfm, vdecimate

clip = vfm(clip, tff=True, postprocess=NNEDI3(double_rate=False).deinterlace)
clip = vdecimate(clip)

VFM’s mode trades safety against matching. mode=0 is the safest and never creates duplicate-frame jerkiness, but outputs combed frames where a good match existed in damaged sections (bad edits, blended or orphaned fields); mode=5 finds a match almost everywhere at the risk of jerky motion. The default is 1. vsdeinterlace also has sivtc, a minimal fixed-pattern IVTC for sources whose cadence never changes, and jivtc for the case where a normal IVTC leaves chroma blends on every fourth frame.

For difficult hard-telecined sources, especially anime with changing cadence, Wobbly is an assistant for fixing field matching and decimation scene by scene by hand, on top of VFM and VDecimate’s output.

Check the result

You should get exactly four fifths of the original frame count, at 24000/1001 fps, with the same duration so audio stays in sync. Scrub through motion and look for duplicate frames or sudden jumps. Check the credits and any inserted video too: sections that were not film (credits, titles, commercials, 30i video) don’t follow the pattern, and a fixed one-in-five decimation drops real motion from them. Those need separate handling; see decombing.

Don’t use -r 24 or fps=24 as a shortcut. They drop frames on a schedule without matching fields, so you keep combed frames and lose clean ones.