50p vs. 50i

[Roland Achini]

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?

[Dave Baker]

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.

[Roland Achini]

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.

[Cary Knoop]

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

[Roland Achini]

Originally Posted by Cary Knoop

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

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.

[Cary Knoop]

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.

[Pete Cofrancesco]

Originally Posted by Roland Achini

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.

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.

[Steve Game]

Originally Posted by Pete Cofrancesco

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. ...

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.

[Ron Evans]

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.

[Rainer Listing]

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.

[Roland Achini]

Originally Posted by Rainer Listing

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.

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."

[Rainer Listing]

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.

[Cary Knoop]

Originally Posted by Roland Achini

Theoretically 50p clips should be doubled in size.

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

[Steve Game]

Originally Posted by Rainer Listing

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.

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.

[Pete Cofrancesco]

Originally Posted by Cary Knoop

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

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.