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TOPIC: Shooting Progressive for Interlaced

Shooting Progressive for Interlaced 25 Nov 2019 14:10 #102808

  • foxytom
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Hello all,

It's been some time since I've been on this forum. I've missed you all.

I am currently doing job where I am following a public figure around with an a7rii and a gimbal for output to social media.

Due to a last minute glitch, I have been asked by the people hiring me to film about 45 seconds of this public figure for (UK) broadcast.

All my gear is setup for Progressive video and the broadcast spec all requires 1080i/25 delivery.

Would it be safe to to shoot in in 1080p25 (or 50) and then convert using compressor, or would that be a major Nono?

One thing I do have is a choice (of sorts) of camera. An a7rii, an A7riii, an a7s, and an FS5 (which I hired for this purpose but doesn't seem to have much better interlaced features than the a7's

I assume the FS5 is still the better choice as it has 4:2:2 colour.

Any advice? pitfalls?
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Shooting Progressive for Interlaced 25 Nov 2019 16:53 #102809

  • JarrodMFay
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I've never had any issues sending progressive footage (that's been converted to interlaced) to news stations before. It has a different look than interlaced but they've never bounced it because of any incompatibility. I'd say as long as it still looks good then go for it.
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Shooting Progressive for Interlaced 25 Nov 2019 21:08 #102819

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Transcoding from progressive to interlace is almost always easy and clean. You start with the full, original image, cut it into fields.

Transocing from interlace to progressive gets tricky, cause you're trying to marry two fields that were never engineered to match up in the first place.

Start with progressive, transcode to interlace, you'll be fine. 1080p25 should work out really well for you.
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Shooting Progressive for Interlaced 25 Nov 2019 21:22 #102822

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Thanks peeps.

Would it be better to shoot in 50p? Would it somehow divide the frames into fields?
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Shooting Progressive for Interlaced 25 Nov 2019 22:13 #102824

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The difference between 25Fps interlace and 25 progressive is merely a temporal difference how a single frame is captured and transmitted/displayed.
Interlaced only means that 50 half resolution frames are send in a serial way. First the odd lines (1-3-5..)of frame one, then the even lines (2-4-6...), repeat for the next frame. So an interlace signal has a temporal difference between field one and two for a single frame. Progressive capture/display shoots a frame as a whole with all lines at once (bit more complicated than that but this is the principle)
So when you shoot progressive your 25 frames just get transmitted/displayed at a rate of 50 fields per second, but every 2 fields are the same frame in a temporal sense. Technically this isn't any difference to an interlaced captured image, it just looks different because of the way time is captured.
When you shoot 50 fps progressive and convert this to 25 interlace then it really depends how the conversion is done. Either take every 2nd frame and use those to make up your 25 FPS, then divide them in 2 temporal equal fields. The result would be same as shooting 25 progressive. But you could also take half the lines of frame one and combine this with half the lines of frame two to a new frame 1 in your 25 FPS sequence. The result will be the same temporal different 50 fields 25 FPS interlace produces.
So in summary technically there is nor problem whatsoever to convert either 25 FPS or 50 FPS into 25 FPS interlaced. The resulting clip is technically 100% acceptable. The difference is only the look. Interlaced with 50 fields of temporal resolution looks like 'TV' and 25 FPS progressive more like 'Film'. It effects things like pans where progressive stutters more because it captures less moments in time.
Hope that helps a little :-)

PS you might ask why the heck is there in a difference in the first place? Interlace only exists because of the technical difficulties to transmit images electronically at the time television was invented. Europe TV is 25 FPS because Europe works on 50 Hz power (American is 30 FPS because they 60 Hz power) Technically the simples clock to use is the freq of the power grid = 50 Hz. But it would have taken too much bandwidth to transmit 50 full frames. The human eye stops seeing single frames above 20 FPS (I think) so film before used 24. Knowing this TV guys decided 25 would be enough. So they just transmitted half the size every 50th cycle (50 Hz). The total lines per frame was then limited buy how much information they could send over the available bandwidth. This is a very simplified explanation, but I always like to understand the background of things, to understand why they work the way they do :-)
Last Edit: 25 Nov 2019 22:23 by cofe.
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Shooting Progressive for Interlaced 26 Nov 2019 08:13 #102833

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If you are going to do more work for broadcast the Intensity Shuttle might be worth buying. It is best to see your editing process on broadcast compliant equipment. The Intensity Shuttle works with FCPX but not as good as it should. Motion graphics look better at 1080i VS 1080 30P. The color space for broadcast will be slightly different as will the composition because of the over-scan area.

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Shooting Progressive for Interlaced 26 Nov 2019 12:59 #102838

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Now I remember how much I love you guys :)

Actually I won't be dealing with post (so maybe, erm, I'm using the wrong forum) but I have the answer from a camera operator perspective, which is actually that I don't need to worry.

Thank you all,

T
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Shooting Progressive for Interlaced 27 Nov 2019 13:24 #102871

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I'm production manager for a TV station. We broadcast interlace, we shoot progressive, works out great.
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Shooting Progressive for Interlaced 27 Nov 2019 22:11 #102883

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I also work for a network that only accepts interlaced, but we shoot 25p and deliver from a 25i project. no problem.
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