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Re-interlacing footage at the broadcasters request: normal or not?
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Hi folks, chill down, it’s not so hard to do. But first some plain English:
Progressive has full spatial resolution, i. e. 720 x 576 in PAL, 720 x 480 or 486 in NTSC (all digital values), but it has half as much temporal resolution, because all lines come from the same moment in time (25 moments per second in PAL)
Interlaced has half the vertical resolution for any given moment, but twice the temporal resolution, since every field is coming from a different moment in time (50 moments in PAL). BTW, on any progressive display like computer screens, modern flats and chip projectors, it will need de-interlacing, reducing vertical resolution – tell that to the broadcaster. Plus, perceived vertical resolution of interlace on interlaced displays is only about 70% of the number of lines, called the Kell factor.
Progressive Segmented Frames (short: PSF) is used whenever progressive is shown on traditional displays (tubes), which expect interlace because of their technical construction. But the display doesn’t care if they come from the same moment in time, the players electronics are taking care of that. There is NO loss of spatial resolution!
Pseudo progressive is the so-called Film or Frame or Movie mode of some older camcorders, which don’t have true progressive imagers and just de-interlace in the camera electronics for that jerky look. Older Sonys were notorious for their weak de-interlacing, while older Canons did it quite a bit better.
So, it all boils down to missing temporal information in the case at hand. While it would have been better to shoot in 50 frames (i or p), it can be done in software. We do this all the time when we need to do slo-mo from ordinary video. The additional positions in time can be generated the simple way by blending frames (as FCP would do it) or in a much better (but slower) way with Flow Motion, a modern concept where the software will actually analyse motion in consecutive frames and generate inbetween positions.
Motion 3, Shake and After Effects CS3 can do this very well, while FCP can’t. I like to use After Effects, because it is very logical and clearly laid out for handling of different frames rates. Just put your 25p footage into a 50p sequence and activate the best quality for frame interpolation. Medium quality will only do blending, which makes the look softer. You may give it try, though, since it’s much faster – and show it to the broadcaster…
Once you have generated 50 fps progressive footage, re-interlace it to 25 interlaced and you are done. While extreme slo-mo done with flow motion can show strange artefacts (kind of warping), this approach will look very good, since only every other line is actually coming from temporal interpolation.
Best regards,
Uli
Director of the Institute of Media Research (IMF) at Braunschweig University of Arts
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Many thanks, Uli.
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[Paulo Jan] “Have you ever found a broadcaster refusing to air progressive material because they are broadcasting in interlaced? Do you think that the technical lead in that channel was being… erm, overlay fastidious, or that we’re the ones in the wrong?”
Yes, this is standard procedure at most networks here in the U.S. No matter what you shoot on, you need to deliver 1080i/29.97 masters for air.
We do this all the time using our AJA Kona 3 boards. I’ll edit projects in 720p in Final Cut Pro, but then let the Kona 3 do a realtime Cross-Convert to 1080i to master out to tape for the network. What’s sweet about this is that I can edit and keep and archive copy of the shows in 720p and satisfy the network’s requirement for 1080i at the same time. All in one timeline, no filters required.
Walter Biscardi, Jr.
Biscardi Creative Media
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Oh, that’s what we did too. What we delivered was an interlaced DVCPRO, but the guy at the station was still complaining that it wasn’t really interlaced, because both fields looked the same (as they had to, since the original footage was progressive).
In your case, the AJA Kona can cross-convert from 720p to 1080i, but the interlaced result… is it “real” interlaced? In other words, if one were to look at it field by field, would each field look different from the next one?
Paulo.
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[walter biscardi] “Yes, this is standard procedure at most networks here in the U.S. No matter what you shoot on, you need to deliver 1080i/29.97 masters for air.”
I’m not going to say Walter is wrong, just point out a few of the networks where they accept progressive masters. History Channel and PBS require 1080 psf 23.98 HDCAM masters. NOt sure what Discovery requires. They used to require 720p 59.94 HDCAM. But that was a progressive format as well.
But it is true that most broadcasters require 1080i29.97.
Shane
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If alternate scanlines of progressive converted to interlace do not look different you’re not looking close enough. True they don’t look radically different. That’s why it’s good. Don’t look for nasty interfield jitter, it’s not there.
And stop saying “because both fields looked the same (as they had to, since the original footage was progressive).”
Look more closely, and see that all the scan lines of a progressively shot image blend together nicely (to make the clearest possible picture of a moving object) – but they are not the same.
– Larry Asbell
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Paulo Jan,
My comments remain the same. I am not saying progressive is better or worse than interlaced nor did I say the interlace has more data. The amount of data is the same but during the conversion data can get lost because of the field vs frame method. If the station needs interlaced footage you should just shoot and edit with interlaced footage. You are fighting an up hill battle when you don’t need to. It would be like useing interlaced vido for a film project. It could be done but it would not be wise.
I think you have got some good info that will help you see the light.
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[Paulo Jan] “he told me that this (delivering progressive material for interlaced broadcast) was as if you were trying to sell somebody “a car with three wheels”.”
Just point him to this web page: https://www.3wheelers.com/enter.html
Arnie
Now in post: Peristroika, a film by Slava Tsukerman
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[Paulo Jan] “In your case, the AJA Kona can cross-convert from 720p to 1080i, but the interlaced result… is it “real” interlaced? In other words, if one were to look at it field by field, would each field look different from the next one? “
Yes, but the way it’s created is different then what you are trying to do. Using whole numbers instead of NTSC decimals, when going from 720p60 (which is 60 progressive FRAMES per second, not fields) to 1080i (which is 60 fields per second) each 720p frame gets split and mapped to a 1080i field, then the next 720p frame gets mapped to the next 1080i field. When you combine those two fields, you get one FRAME of interlaced material (which then becomes 1080i30). Therefore it is interlacing at it’s most traditional, 60 samples per second at half of the vertical resolution when combined make up one full frame of full resolution video. This works the same for PAL HD as well going from 720p50 to 1080i25.
In your case, Paulo, you don’t have double the frame rate (you have 25p, not 50p), so when you use Fields Kit, there will be some interpolation. But the master you submitted should be just fine and will meet all specs. It’s the guy running the Avid that needs to know all of this anyway.
Jeremy
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[Paulo Jan] “He showed us our show in an Avid, advanced field by field and, as it happens when you do that in progressive footage, there was only movement in one of the two fields.”
Which is the wrong way to evaluate progressive footage. No viewer at home is going to step through the show field by field (or frame by frame, for that matter).
The real question is how does it look when played back at normal speed? You may see some stuttering with fast pans, but most people would not otherwise be able to see a significant difference between 25i & 25P footage.
Arnie
Now in post: Peristroika, a film by Slava Tsukerman
https://www.arniepix.com/blog
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