Horizontal Lines on Old Footage: Interlacing Explained and Fixed
Old DV tapes and TV recordings look fine on the couch, then show comb teeth on every moving edge once they reach a computer. The file is not broken — interlaced video simply does not belong on progressive screens.
What interlacing is: a CRT-era inheritance
Interlaced video splits each frame into two fields — odd lines first, even lines second — and old CRTs blended them via phosphor persistence. It halved bandwidth in an era when the display itself was interlaced.
Every modern screen is progressive: one frame, complete. Play interlaced source on it and both fields show at once — moving edges sprout comb teeth while static shots stay clean.
Sources that hit you: tape-era DV exports, TV tuner and set-top captures, some old discs. Phone and mirrorless footage is progressive; it needs nothing.
Confirming your footage is interlaced
Two tells: lines appear only on moving edges, and a still frame looks clean while playback shows the teeth — the two fields take turns exposing themselves.
Run the codec checker and look at the spec: 1080i, 576i, 480i end in "i" for interlaced; "p" means progressive. Chinese TV-era material is commonly 576i (PAL).
Do not confuse combing with compression blocks: combing is fine horizontal teeth on motion; blocks are chunky squares visible even on stills. Deinterlacing does nothing for blocks.
Standard vs double: the trade-off
standard merges both fields into one frame — same frame rate, smallest output, best for content headed into compression and distribution. Fast pans lose a touch of detail; everyday viewing never notices.
double turns each field into its own frame — frame rate doubles (50i becomes 50p), motion is the smoothest possible. Worth it for archival-grade digitization of family tapes; costs more time and space.
When unsure, standard is the safe universal answer. Reserve double for big-screen replays where motion smoothness is the point.
The full digitization chain
Capture tape content to a file with your capture device first. On the computer, deinterlace before anything else — compressing interlaced footage bakes the combing into the picture permanently.
After deinterlacing, compress to H.264 MP4 for long-term storage and playback anywhere. The order is not negotiable: compress-first means your filter works on damaged frames.
Spot-check high-motion shots afterwards: waving hands, running kids, scrolling credits. That is where combing hides if anything survived.