50p vs. 50i
 

Up to now I film in XAVC HD 1080/50i 50 Mbps with my Z90 camera.
I am considering to change from interlaced to progressive (XAVC HD 1080/50p 50 Mbps).
Both settings give Clips in 4:2:2 / 10bit. However, both Clips have almost the same file size.
This means, interlaced material has 2x higher quality than progressive.

When I import these clips into Avid Media Composer, I have to import 50i material using the DNxHD 185 X codec to obtain 4:2:2 10bit quality. For the Import of 50p material the DNxHD 365 X codec is needed to retain 4:2:2 10bit quality. Now, the file size of progressive is double as high as the one with interlaced, of course. I wonder what sense it makes to boost lower quality of 50p clips in such a way.

Or, simply said, is it worth to film in 50p or should I better stay with 50i?

Re: 50p vs. 50i
 

You seem to be muddling up "field" and "frame". An interlaced frame is made up from two fields, each field is half of one frame with half the bitrate which, when put together (interlaced), have the same bitrate as one progressive frame, so 50i is equivalent to 25p.

It depends on what you shoot as to whether one system seems better than the other for you. For example fast moving sport can look better shot interlaced as motion can appear smoother.

Re: 50p vs. 50i
 

Thank's Dave.
It is clear to me that 50i (25 fps) is equivalent to 25p (25 fps), and that for fast moving subjects or camera 50i is better than 25p. However my question is about 50i vs. 50p.

Re: 50p vs. 50i
 

It's obviously better to film 50p compared to 50i because while the temporal resolution would be the same.
the vertical resolution would double.

Re: 50p vs. 50i
 

That is theoretically true, however both 1-minute long clips filmed with my Z90 camera in (a) 1080/50i 50 Mbps and (b) 1080/50p 50 Mbps have practically the same file size. Theoretically 50p clips should be doubled in size. My only explanation for that is, the 50p clips overall quality is reduced.

Re: 50p vs. 50i
 

The 50p version compresses much better.

I would if possible stay away from interlaced.

It's a bad format that was introduced to solve a problem that no longer exists.

Re: 50p vs. 50i
 

I think you have this reversed. I live in the us world but the principle remands the same.
24p shutter is 48, 30p at 60, 60p at 120
when shot at the proper frame rate you lose a stop of light as you increase the shutter.

In my experience I can't tell the difference between 30p and 60p until you start using slow motion. Some of these prosumer cameras I'm skeptical of their specs especially when you consider the bitrate is the same for all these settings.

I always trust the eye test shoot using the different settings and look and see if there is any difference to your eye. Most likely you might not be able to see a difference. But generally speaking I prefer progressive.

Re: 50p vs. 50i
 

It's not necessarily as bad as it seems because with a higher frame rate, there's usually less change between successive frames, meaning that there's less difference data to be squeezed into the b & p frames.

Re: 50p vs. 50i
 

There is no need to follow the rule of shutter at twice frame rate when the frame rate is above the rate at which we as humans view the action as motion. That was assumed to be about 48fps. Using a fast shutter on slow frame rate makes the image too sharp emphasizing the judder so the shutter speed rule is an upper speed limit not a lower limit to create a blur and fool us into thinking there is motion rather than a series of still frames. So if you shoot at 60P there is no real need to use 1/120. I have shot 60i and 60P for years using 1/60. The image blur on each frame/field is then exactly the same as shooting 30P so if you really want the stuttering slow frame rate look you can get that directly from this 60P file, most NLE's will drop every other frame . You can also get interlace from the progressive file for DVD or Bluray too.

People get really confused about interlace and progressive. Partly because the timecode for interlace aligns with the interlace sync pulse every two fields. This sync pulse tells downstream equipment the start of the two fields sequence so made sense to use this for timecode. So timecode increments every two fields. It is half the sample rate that the camera captures images. Interlace 50/60i has exactly the same temporal motion as progressive 50/60P the only difference is that each field is only half the vertical resolution of the progressive sample. So if you want to get the same fluid motion of interlace 50/60i you need to shoot at 50/60P. 60i is not 30P it is 30fps very different. To be correct 59.94i is 29.97fps not 29.97P. To correctly deinterlace 50/60i one needs to interpolate the missing fields and get 50/60P and the really good de interlacers do exactly that. Simple de interlacers add fields together but of courses the fields are 1/60 sec apart ( for NTSC ) and there is movement between fields hence the jaggies one sees on PC screens that normally have really poor deinterlacers.

Re: 50p vs. 50i
 

Roland, That your 25i (to use the technical rather than the marketing terminology) takes up the same space as your 50P. seems to indicate it has around twice the bitrate. Unlikely, but, check it out with MediaInfo. If that is the case and your main final destination is TV broadcast, you are better off shooting 25i. Otherwise, shoot 50P, it gives you the maximum flexibility for final delivery. If you need 25i most NLE's will interpret 50P correctly.
Shooting shutter at around twice the frame rate for standard frame rates is a reasonable rule of thumb in most circumstances. People who apply it rigidly don't have the knowledge or experience to understand the relationship between frame rate, exposure and varied motion or to make appropriate creative choices.

Re: 50p vs. 50i
 

I checked with MediaInfo: My 25i and 50p clips have approximatly the same bitrate (around 52Mbps).
Steve's explanation sounds reasonable to me: "It's not necessarily as bad as it seems because with a higher frame rate, there's usually less change between successive frames, meaning that there's less difference data to be squeezed into the b & p frames."

Re: 50p vs. 50i
 

Only problem is for any scene 25i and 50p record the same amount of motion (one field per frame), but each field is half the frame resolution, so with 25i you're seeing twice the bitrate.

Re: 50p vs. 50i
 

Absolutely not, that's not how compression works, especially not compression with long GOP enabled!

Re: 50p vs. 50i
 

Not really. With 50p the sampled video has double the number of samples per second as 25i (or 25p), i.e. 1920x1080x50 vs. 1920x1080x25.
However, with interframe compression, only the 'I frames' have twice the amount of data. The 'b frames' and 'p frames' have data content proportional to the difference between samples, so 25 fps will have differences in the scene represented by samples taken 40ms apart encoded whereas with 50 fps, if all other parameters are equal, the differences will be less owing to half the time between the samples.
1080 25p means: 1080 lines of video at 25 frames per second, progressive scan
1080 25i means: 1080 lines of video at 25 frames per second, interlaced scan sent as 2 fields (i.e 50Hz)
1080 50i means: 1080 lines of video at 50 frames per second, interlaced scan sent as 2 fields (i.e 100Hz)*

*as far as I know not used in any mainstream broadcasting system and presumably, apart from generating slow motion video, not used anywhere else. The letter 'i' stands for 'interlaced' just as the 'p' in 25p stands for 'progressive', and relates only to the type of scan. It doesn't change to represent anything else just because the frame figure is above a particular value.

Re: 50p vs. 50i
 

What I’m saying is with theses cameras the data rate is the same typically 50mbs This means no matter how you slice that pie (24p/30p/60p/60i) its the same pie. I can notice a more film like of 24 vs 30 and I prefer not getting the interlaced jaggies. But beyond that... I do know other cameras when you go above 60p they raise the compression to maintain the data rate resulting in actually worse quality.

Re: 50p vs. 50i
 

Useful reference https://en.wikipedia.org/wiki/Interlaced_video

Re: 50p vs. 50i
 

It looks like you got it completely wrong, the temporal resolution of 50p and 25i are exactly the same, the only difference is the vertical resolution, 25i is half the vertical resolution of 50p.

Re: 50p vs. 50i
 

Many years ago it was simple when almost everything was interlaced. Then 50i and 60i refereed to interlaced fields. However in both cases the timecode incremented every two fields so 25 fps for PAL and 29.97fps for NTSC. Conventions were changed to refer to timecode. Very confusing since the temporal motion is the same as the field sample rate not the timecode. It is just that each sample ( field not frame ) is half the vertical resolution. So if now you want to shoot video with the same temporal motion as old TV most people were used to then in PAL it would 50P and in NTSC 60P ( 59.97 fps )

Re: 50p vs. 50i
 

Slightly wrong as this line is in error:
1080 25i means: 1080 lines of video at 25 frames per second, interlaced scan sent as 2 fields (i.e 50Hz)
That is what 25psf is, i.e. 25p interlaced to satisfy transmission channel capabilities or standard HD Blu-ray specifications.*
With 25i, the odd and even fields are sampled at 25Hz each so for any given line or individual sample, the repetition rate is 25Hz. Although not always practiced, shooting with a 180 degree shutter for both 25i and 25p (20ms) gives the same amount of motion flow because the motion is only updated at the full 1080 resolution every 40ms. Using a narrower angle (10ms) because every 20ms half of picture is updated would give a less smooth motion and grittier image detail..

*Note, in 50Hz regions, HD Blu-ray video is generally distributed as 25i or 25psf as there is no 25p in the standard. More recent TVs can display 23.976/24Hz framerates, but distributors generally only produce one version for 50Hz markets.

Re: 50p vs. 50i
 

Also 50i nowadays typically (sadly) is taken to refer to 50 (interlaced) fields per second, i.e. still 25fps.
Getting back to Roland, at the same bitrate each single frame captured at 25p is going to be higher quality than the single frame captured at 50p. If you are delivering 25i, it shouldn't matter whether you shoot 25i or 50p (your observation that 25i and 50p files are the same size). OTOH If you are delivering 25p and shooting 50p, your NLE drops every second frame so you need to shoot higher bitrate than you would at 25p to get the same quality (but I suspect you already know that).

Re: 50p vs. 50i
 

As I mentioned above the confusion is that with interlace the timecode is updated every two fields a full interlace sequence. It is the same for PAL as NTSC. sample rate for PAL is 50hz and for NTSC 60hz conveniently the mains power frequency for each source country. Each field is sampled at these frequencies. There is no difference in temporal motion between 50i and 50P or 60i and 60P just the picture vertical resolution. You expose both the same way as the camera is effectively shooting progressive frame and only recording a field. However convention says timecode for 50i is 25 and for NTSC 60i is 29.97fps. I believe now quoted as 25i for interlace and 29.97 fps interlace. Unfortunately interlace NTSC has the same timecode as 29.97P but they definitely do not look the same. On a good TV 60i looks just like 60P. On my upsampling Sony TV I cannot tell the difference as the TV interpolates the missing scan lines of the interlace signal to display a progressive image anyway.

Interlace is not a progressive frame split in two fields then recorded or transmitted one after the other. Interlace is the same sample rate as progressive but only fields of the progressive image are recorded or transmitted. That is why it is not possible to add them together since in PAL area the fields are 1/50 second apart and in NTSC 1/60 second apart. Initially done this way since all equipment was mains powered and the sync to mains frequency was easier than expensive oscillators that we have now.

Re: 50p vs. 50i
 

You are completely wrong and you now cause, presumably unintentionally, more confusion by adding PsF to the discussion.

It is very simple:

NTSC has a temporal resolution of 59.94, PAL has a temporal resolution of 50. The only difference between progressive and interlaced is that interlaced has only half the vertical resolution and that both fields are packed into a single frame.

In other words if you deinterlace an NTSC video and the result is not 59.94p but 29.97p you are doing something wrong.

Exactly!

Re: 50p vs. 50i
 

Uh uh. Other way round.

Re: 50p vs. 50i
 

Nope!

...

Re: 50p vs. 50i
 

Gary, sorry, missed the p on the 59.94, the way I read your post it seemed to say that the purpose of deinterlacing 59.94 fields, NTSC being 29.97i, is to result in 59.94 progressive frames, which of course is incorrect. .

Re: 50p vs. 50i
 

What I mean is exactly what I wrote:

In other words if you deinterlace an NTSC video and the result is not 59.94p but 29.97p you are doing something wrong.

If the result is 29.97p you just threw away half of the temporal resolution and merged fields from two different time events.

Here is an example of a properly deinterlaced PAL source, it's 50p and if you download the video and inspect each frame you will see that each frame is unique.


And here is an example of a properly deinterlaced NTSC source becoming 59.94p:

Re: 50p vs. 50i
 

OK, figured it out, probably showing my age, NLE deinterlacers, Resolve, Vegas, PP, blend or merge fields, 50 interlaced fields per second results in 25 progressive fields. In VirtualDub, QTGMC, rerender, you can uprez the fields to frames and seems nowadays that's also called deinterlacing. I guess either way is "proper", just need to be clear about terminology.

Re: 50p vs. 50i
 

The merge fields etc in the NLE is their way of creating the full frames. Do they double the nearest etc. Interpolation is the correct way to do it and needs a more elaborate process. It means taking several fields and creating the missing fields to complete the progressive frames . The interpolating TV's do this. I believe my Sony uses 7 fields and of course from this information can annoy the purists using 24P it can interpolate extra frames to smooth out the motion. Easily making 30P look like 60P. But the same process is used to replace the missing fields from an interlaced source to create the 60P that the TV needs for display.

Re: 50p vs. 50i
 

Even better is to use motion adaptive techniques.

QTGMC with Vapoursynth (or AviSynth) is your friend!