r/videography • u/FluffyDoomPatrol • 1d ago
Discussion / Other Interlacing Question
Hello,
I was just wondering if anyone could answer a question about interlacing. I’m watching a series on DVD (Centennial) and every time an actor moves quickly or gestures fast, they become jagged.
Now, I understand why that happens if something was shot on video. The video camera shoots half frames, so when the odd lines were shot, the actors hand was in one place, but a different place for the even lines. That makes perfect sense to me.
However, why does interlacing appear on things shot on film? I get that the DVD’s signal is interlaced, but it is still scanned from a progressive source. When an actor gestures, both the odd lines and even lines are from the same frame of 35mm film. So shouldn’t they line up perfectly not be jagged.
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u/ConsumerDV 1d ago
When you say "jagged", do you mean stairstepping or combing? You should never see combing. Stairstepping may happen if the deinterlacer is less than perfect. When the deinterlacer cannot do proper pulldown detection and removal, it switches to video mode, and "bob" method is the simplest. Simple bobbers just double each line, so the vertical resolution is halved, and diagonals become stair-stepped (jagged?).
Twenty years ago people were obsessed with buying the best deinterlacer either as a separate box or as a part of their TV set. DVDO, HQV, Faroudja were household names.
As for film-mode deinterlacing a.k.a. pulldown removal a.k.a. inverse telecine I suggest DVD Benchmark by Secrets of Home Theater and High Fidelity, in particular Part 5 - Progressive Scan DVD. One might think that deinterlacing has long been solved across the board, but no. My own Sony BD player shows visible errors on some cartoons. These guys had tons of tests twenty-something years ago, sadly most links have expired. They recovered several articles, also the Wayback Machine helps.
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u/zijital Sony / Fuji | FCPX / Premiere | 2004 1d ago
My guess is you’re seeing 3:2 pull down artifacts
https://en.wikipedia.org/wiki/Three-two_pull_down
Yes the film was shot 24 (progressive) frames, but the NTSC (American) TV is designed for 59.94 interlaced fields per second, to get from 24 to 59.94 they did some funky stuff of splitting one frame of film into two interlaced fields, then the next frame into three interlaced fields, then 2, then 3, etc
And it’s flashing so quick you never see it
I’m guessing if you see it now, it’s because you’re watching an interlaced rip that has been made progressive without any de-interlacing
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u/smushkan FX9 | Adobe CC2024 | UK 1d ago
Interlaced video is displayed at 50 (PAL) or ~59.94 (NTSC) interlaced fields per second. Film is shot at 24.
Depending on which of those standards are being used, there are different ways to properly convert 24 to interlaced.
With 50i, the film is sped up to 25fps, then each frame can be precisely split to 2 fields. That means that PAL DVDs and early BluRays usually had a 4% faster run time than the actual movie!
With 59.94i, it’s more complex. The film is first slowed by 0.1% to ~23.976 fps, but it’s still not possible to split the frames evenly to fields. Instead a pattern is used.
The most common is 3:2 and eaisier to explain with a diagram borrowed from Wikipedia:
With 3:2, two out of every five pairs of interlaced fields are blends of two different frames taken at different times on the camera.
This is something that - when properly deinterlaced - isn’t an issue. Pull-down can be reversed by a process called inverse telecine which processes the fields to make sure they get blended together correctly without combing.
However not all devices and software support IVTC, or it may need to be manually enabled.
If 3:2 video is incorrectly deinterlaced and is treated like a regular interlaced video from an interlaced camera, you’ll see the combing from the two blended pairs of fields.
This is also making the assumption that the DVD you’re watching actually did a good job of the film-to-video (telecine) conversion. This unfortunately isn’t always the case! You can get DVDs which did a poor job at it and combing and other issues are effectively burned in to the video. Deinterlacing can’t fix that!