Premiere CC and '25i'. What is 25i?

[Geoffrey Cox]

I'm editing some footage that I did not shoot myself. It comes from a Canon c100. This shoots at 25p, 50p and 50i (as far as I know) in PAL land. Yet when I import the clips into Premiere CC it says they are 25i. What exactly is 25i? The sequence that is automatically created when dragging and dropping the clips says it is upper field first so not progressive is is this in fact 50i material? I'm confused especially as it doesn't actually look much like 50i more like 25p (cross movement is quite blurred).

Just noticed that the manual says this: 'Shooting at 25.00 frames per second, progressive; recorded as 50.00i'. I find this confusing - is this progressive or interlaced?? (I understand that 50i is 50 half frames)

[Alan Craven]

It sounds as though what you have is "Progressive Segmented Frame" video. Several years ago I had a Canon prosumer camera that produced this form of video. I did not realise that it was still around, with faster processing allowing proper 25p, or even 50p on SD cards with HD.

[Geoffrey Cox]

So what does that mean in practice and what would I set the Premier timeline too? I tried changing to Progressive scan in Sequence settings but that just seems to mess everything up (very jittery movement and then also my TV monitor changes to 30Hz for no apparent reason). Do I just ignore it and leave Premiere as 25 fps UFF? The other funny thing is that the c100 shoots full 50p no problem.

When I export should I export as 25p?

Thanks Alan.

[Alan Craven]

PsF records the entire picture as 2 separate fields but differs from interlaced because both fields are of the same image taken at the same instant in time. Pictures recorded in PsF mode are compatible with interlaced pictures and can be fed directly into machines used to seeing interlaced video as they still offer 2 separate (but identical) fields per frame.

If you drag a clip from the project panel in Premiere onto the new item icon, it will automatically create a sequence with suitable settings. I think this is true for all versions of Premiere

[Geoffrey Cox]

OK thanks. I'll leave things as Premiere has set them then. Thanks for info about 25i, a new one on me. Normally I shoot 50p.

[Jeff Pulera]

Hi Geoffrey,

25i and 50i are exactly the same thing, just a different way of naming them. Across the pond here we have 30i or 60i. Basically saying "25 interlaced frames" or "50 interlaced fields".

There is only one flavor of "interlaced" HD video and that is 1080i. Of course the frame rate differs for PAL or NTSC, but within PAL there is but one kind of 1080i then. No different frame rate options that you would see with 1080p.

It does sound like perhaps the camera recorded the "25p as 50i" as mentioned elsewhere.

Thanks

[Geoffrey Cox]

Thanks Jeff. I'm still a little confused to be honest. I used to shoot interlaced all the time and that would always be recognised as 50i i.e. 50 half frames (so yes 25 full frames per second but separated into two different fields).

But if Alan is right this isn't the same as 25i as in that case each frame is duplicated and made into two fields making 50i which in effect becomes 25p. At the end of the day what I need to know is, is 25i interlaced in any meaningful sense or 25p? If each frame is identical then surely that makes it 25p? The specs of the camera are saying it is 25p but recorded as 50i. It looks like 25p to my eye as I am very familiar with what 50i looks like, so in that sense the end results are in fact quite different.

[Ann Bens]

It is filmed progressive and recorded as interlaced
Some people call this type of footage: malignant PsF.
Premiere does not know what to make of it and calls it 25i, so
Import the footage interprete it as 25p then drop it in a 25p timeline.

[Alan Craven]

PsF masquerades as progressive, but is equivalent to interlaced video. With PsF the top and bottom fields of each frame are identical - they are taken at the same time.

With conventional interlaced the top and bottom fields are taken half a frame time difference apart.

As a result PsF can have a "stuttering" look to it, which is not there with traditional interlaced.

I can empathise with Ann's "malignant", when I had a camera which produced these files they were a pain.

It was a contrived solution for an era when much equipment could not handle true progressive, but a consumer demand for it it had beeen created.

[Geoffrey Cox]

Yes cross movement is a bit stuttery especially when rendered out and viewed on the computer screen. Is there anything I can do to mitigate this, say when rendering the final version e.g. do I set the renderer to be progressive (I use Compressor)? I found when I changed the timeline preset to progressive the stuttering is actually much worse, unusably so in fact.

Ann what do you mean by 'interpret it as 25p'? Not sure what interpret means in this context.

I guess I will do some more tests...

[Gareth Watkins]

Hi there
This has been covered on a few forum.. the C100 shoots 25p but, as previous poster says, it's in a 50i stream. As Premiere does not know how to treat this you need to point it in the right direction or have horrendous artifacts.

>Import your sequences into Premiere project.
>Select the clips in the Project window and right click
> drop down to 'Modify' in the menu and select 'Footage'
> click Field Order...'Conform to' ... Progressive

The clips will now behave as regular 25p in the project... no artifacts or choppiness.
cheers
Gareth

[Geoffrey Cox]

Thanks Gareth. That worked though I kept the timeline as interlaced as that seemed to mess things up badly for some reason (maybe due to my hardware perhaps - Matrox mini). I exported as progressive though and it looks OK on the computer. Not keen on 25p I realise as the movement always looks a bit odd to me.

[Steve Game]

Surely, the correct terminology for an interlaced HD video stream is 1080i25, as opposed to 1080p25 which is progressive. It's only terminology but the EBU, (who by virtue of PAL and 50Hz based scan systems being originally introduced in Europe) define all such systems with a common format, i.e.:

1) No. of lines per frame
2) frame scan sequence
3) frame frequency

Thus, broadcast SD was defined as 576i25
most live broadcast HD is defined as 1080i25, - cinema and 'cinematic' films are usually broadcast as 1080psf25. Progressive segmented frame is just a method that broadcasters use for getting true 25fps (as in speeded up 24fps cinema film) through the existing broadcast chain which for HD is set up for 50 frames per second of 540 lines.

The practice of suddenly changing the scan frequency description from frame to field is illogical, especially since the demise of analogue scan TVs where each field had a half line timing to ensure a different screen position for successive fields. The TV and video camera industry have created and prepetuate this anomaly, possibly thinking that some buyers are impressed with bigger numbers, (there is a lot of history of similar cynical practices in AV equipment marketing).

As for how to handle the 25i footage is concerned, I don't use Premiere but it should be possible to confirm the exact nature of the footage by zooming right into the timeline and checking whether the frames show fast moving features as moving in every 'frame' (25i) or every other frame (25psf).

[Cary Knoop]

Both 'frames per second' and 'fields per seconds' are used for interlaced footage.
One is not more right or correct than the other, that would be like arguing that mph is more correct than km/h.

[Steve Game]

Originally Posted by Cary Knoop

Both 'frames per second' and 'fields per seconds' are used for interlaced footage.
One is not more right or correct than the other, that would be like arguing that mph is more correct than km/h.

I said:"The practice of suddenly changing the scan frequency description from frame to field is illogical" not not "more right or correct". If 1080p25 means progressive 25 frames per second, and 1080p50 means progressive 50 frames per second, then the p means progressive. Now change the p to i and 1080i25 means interlaced 25 frames per second. Fields aren't being mentioned here. If they are, then it refers to the '25' or '50' number and it would signify as fields and not frames. The 'i' or 'p' would then be unnecessary. So as I said, jumping around between numbers that are frame and field frequencies without reference to just what they are is at best illogical and at worst, confusing, - particularly for those not old enough to understand the history of interlacing as a method of spatial and temporal compression inherited from analogue television.