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5p1r17 says

Hi folks,

can someone clearly describe me, what is the difference between interlaced footage which was rendered directly from AE and same footage, but rendered without fields and later converted to Lower\upper field order ?

Let’s say I have project X. I rendered it to DV PAL 720×576 LowerField. Then I rendered this project to 720×576 progressive and converted to Lowerfiled.

What’s the difference between them ?

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felt_tips Volunteer moderator says

The footage that’s rendered progressively remains progressive. It’s pretty much irrelevant whether it’s interpreted lower or upper field first, since both fields per frame are identical.

The footage rendered in fields intrinsically has two pictures per frame, spread out over alternate lines, so when you read this back in it’s crucial that it’s correctly interpreted. Otherwise you will get the following field cadence

2 1 4 3 6 5 8 7 etc.

This will make you freak out. Then your eyes will explode. :)

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5p1r17 says

Haha thanx ;)

It means that interpreted progressive footage will flicker (slightly) on screen, right?

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felt_tips Volunteer moderator says

edit: It depends on what screen and whether the digital file’s metadata is being read.

Most of the time, it makes not a jot of difference. Most digital players apparently don’t interpret fields for anything other then mpeg2 anyway.

An old school TV scans fields, regardless of how the picture is. Upper first for broadcast, lower for DV (and DVD , IIRC).

If you’re dealing with footage in fields, it’s very important to get AE to interpret the fields right. If you don’t interpret them at all, moving the footage down by one pixel in a comp effectively reverses the fields and gives you that funny cadence. If the fields are being dealt with by AE, then it sorts all that out for you, by breaking the image into double the number of half-height frames internally.

A modern TV is switchable, but whether it picks up progressive / non-progressive from the media or via a hard switch inside is anyone’s guess.

100Hz progressive gives a pretty similar effect to 50Hz interlaced anyway.

I wouldn’t worry too much about it. Just leave fields off if you’re rendering progressively.

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5p1r17 says

I’m rendering project for broadcasting. Broadcast machine accept PAL DV Avi.

Usually I render with basic AE setting for PAL DV , but now I figured out that rendering with fields is twice longer than progressive, because renderer need to calculate Upper and Lower fields separately.

That’s why I started to think about rendering without fields and after that convert to interlace. it’s much faster. But is it worth ?

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felt_tips Volunteer moderator says

I’d just render progressive if it’s a progressive look that you want. It won’t make any difference to the picture. The lower field interpretation will just be a flag in the meta-data. I think that if it’s missing, most systems for laying off to tape will force the default lower-field first.

As I said, if both fields are the same, it doesn’t matter which way round you render them.

On the other hand, if you want an interlaced look (i.e. more videoy than filmy) then you should render interlaced… that means that you get an effective frame rate of 50 instead of 25 for PAL . That’s why it’s called 50i.

Be careful… if you mixed up interlaced and progressive footage it can look rather odd.

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felt_tips Volunteer moderator says

Interlacing is a much misunderstood thing. Most seem to assume that it’s just some esoteric thing that gets done with your picture.

But it’s actually really simple. Interlacing gives you 50 pictures per second and progressive only 25. Interlacing achieves this by cramming two pictures with lower vertical resolution into one frame, then scanning the alternate lines with a 1/50th second time gap between them.

Old TV engineers did this to fake 50 images a second when the bandwidth wasn’t high enough to actually send that many per second.

Modern TVs don’t need any such fakery, but it still exists for backward compatibility (exactly like overscan / action safe).

The big visual difference is that 50fps is closer to how we see than 25fps, so interlaced feels more direct, immediate, real, video-ish…. like TV news or sport. 25fps feels more detached from reality, more epic, story-like, archetypal, filmic…. like a feature film.

Modern computer screens and TVs can scan above 100hz progressive, so you can achieve similar effects by scanning a higher progressive frame rate. 50 frames progressive feels remarkably like 50 fields interlaced…. just a little less flickery.

Modern TVs are also able to interpolate up frame rates. Indeed this seems to be their default setting. Walk into any TV showroom and watch all the high-budget feature films being given the charming quality of a low-budget soap opera with this interpolation effect.

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5p1r17 says

Yep,

Now I’m feelin myself more smarter, than 2 hours ago ;) Thanks Felt for your tips, I appreciate that!

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