1080p vs 1080i - HD Videos Progressive & Interlaced Scan Explained

I have an article on HD related website regarding the HD video functionality of the Sony SLT-A55 and A33 new DSLR/SLT cameras. In this article, the editor claims that Sony is going in the wrong direction, offering 1080i/60i movie recording, and not allowing cinematographers 60p (progressive scan) and 24p/25p frame rates. Well, this is kind of new to me, all of those terms, so I have decided to deep a little further to find an answer. This guide/review is not only about Sony, but rather an in-depth practical guide for new comers in the video/cinematography field.

First of all, Sony explicitly wrote that the Sony SLT cameras shoot videos in AVCHD format and in 1080/59.94i (60i), taken from 29.97fps sensor output. Dpreview.com has also explicitly written that the camera shoot Full HD videos on 1080p/60i. Confused?, me too. So before I start investigating the subject, let's see what 1080p/1080i/60i/24p all means?, What is interlaced scan and progressive scan?

All the comments on various HD related forums and blogs, have let me confused. The best thing to do, is to investigate things myself. So that's what I did.

So I have said hello to Wikipedia and some other useful websites, trying to find, what is the difference between 1080p vs 1080i? On Wikipedia 1080i definition page, it says that the 'i' means interlaced scan and 'p' means progressive scan. Furthermore, the EBU (European Broadcasting Union) prefers the use of resolution as frame rate, by specifying the frame rate after a slash.
For example:
In frame rate: 1080i/30
In fields per second: 1080i60

In frame rate: 1080i/25
In fields per second: 1080i50

OK, so far so good. However, what it the difference between 1080p & 1080i, which one is better?
In terms of the amount of details (according to 1080p vs 1080i guide on thebestplasmatv.com), 1080p transmits contains more data, and transmits more data when displays on a Plasma or LCD HD definition displays. You actually won't see a difference when watching it on your TV, because you actually see the same amount of pixels (resolution) - 1920x1080.  The advantage of progressive scan (p), is that because it contains more information, it gives sharper images when you shoot fast moving subjects.

1080i vs 1080p - The Difference is Quite Simple

I wanted more in-depth technical details, I have found a very good article on about.com, called "1080i vs 1080p". In this article, the author Robert Silva explains the differences between 1080im 1080p/60, 1080p/30 and 1080p/24. Let me summarize the main facts:

• In 1080i, each frame displays in alternative fields.
  That means that there are 1080/2=540 fields/horizontal lines, also called odd fields, which are displayed from top to bottom. And the other 540 fields/horizontal lines, also called even fields, which are also displayed from top to bottom, but displayed second, after the odd fields. In total, you get 540x2=1080 fields, which create the full frame. That means that there is a rotation between the odd fields and the even fields, every 30th of a second.
  
  
• In 1080p, each frame displays progressively.
  So instead of showing different odd fields or even fields alternatively every 30th of a second, both odd fields and even fields are presented at the same time. By using the progressive (p) format, you will get better looking projected images. So even if you are shooting fast moving subjects, you will not get motion artifacts.

Odd lines field + Even lines field = Complete Frame

It wasn't that hard, wasn't it?

Let's summarize it:

1080i (Interlaced) = Rotates between odd fields and even fields every 30th of a second

1080p (Progressive) = Display both odd fields and even fields at the same time (Better image quality and less motion artifacts)

1080p/60 vs 1080p/30 vs. 1080p/24

When I wrote "30th of a second" in the previous paragraph, I referred to "30", like in 1080p/30. But there are other common display options, in terms of fields/lines per XXth of a second. The common ones are: 60, 30 and 24.

Again, for the sake of simplicity, let me summarize the main things that you need to know and remember:

• 1080p/30 - All the fields (odd and even) are being displayed once, every 30th of a second
• 1080p/60 - All the fields (odd and even) are being displayed once every 60th of a second (or twice every 30th of a second 60/sec x 2 = 30/sec)
• 1080p/24 - All the fields (odd and even) are being displayed once, every 24th of a second

Now regarding 1080i, it means:

• 1080i/30 - One of the fields (odd or even) are being displayed once, every 30th of a second
• 1080i/60 - One of the fields (odd or even) are being displayed one, every 60th of a second (or twice every 30th of a second 60/sec x 2 = 30/sec).
• 1080i/24 - One of the fields (odd or even) are being displayed once, every 24th of a second

What 30th and 24th mean?
It means that it like splitting a second into 30 or 24 units.
So for example, when I say that the frame is being displayed every 30th of a second, It means that a single second is being split into 30 sub-second units, and the frame is being shown every time that 30th second comes by.

Every 30th of a second = 30 times (frames) a second
Every 24th of a second = 24 times (frames) a second

As you probably understand by now, 1080p is preferred over 1080i. The reason for that is quite simple: It displays all the 1080 lines (in case of 1080 HD resolution) at the same time. That leads to a better image quality overall, especially when shooting fast moving subjects.

1080p = Better Video Quality

What "Deinterlacing" Means?

Interlaced video is one that shows a sequence of two sub-fields. Yes, exactly like in 1080i.
Remember? - i = Interlaced.

Deinterlacing, is a process of converting Interlaced video (like in 1080i) into "Progressive scan" like video. So it supposed to combine two separate fields (odd and even), and create the full frame. Sound simple?, well, it is not.

The problem is, that when shooting videos in 1080i, you get less information in every frame. At a specific split of a second moment, the recorder captures only half of the information at a single moment. So in 1080i, odd lines from one frame, will need to be combined with the following even lines frame.  Because videos are a sequence of shots, mostly shooting moving subjects, the information found on the "odd lines" frame, most of the time, will be different than the "even lines" frame. So if you combine the two frames together, you mostly get a full frame with lines that are not lined up correctly (representing the same time frame).  The result is visible interlacing artifacts during motion, also known as "combing".

So when shooting subjects who move in fast speed, the results are visible artifacts. Yes, you get a full vertical resolution as in progressive scan (p), but the results are not the same, if you have shot the same footage with both "odd fields" and "even fields" at the same time (progressive). It also means that when shooting HD videos of subjects who are standing still and not moving fast, the "combing" artifacts will be less visible. This is the reason why you see lines when pausing an interlaced scan video. You actually see only one interlaced frame, odd or even (Finlay you know why it happens).

There are many algorithms (ex. weaving, blending, selective blending, inverse telecine, telecide)  that supposed to solve the problem with the deinterlacing process. However, it is very complex, and in most situations, it does not deliver good results. Interlacing  is an old technology, and was mostly used to save bandwidth.


OK, time to summarize things up:

Deinterlacing - The process of combining odd and even fields together in interlaced videos, in order to create a full vertical resolution frame. Feeling up the gaps.
(Hopefully, with a good algorithm, getting rid of the horizontal lines).

For those of you that still have a problem to grasp this info, here is a (progressive) YouTube video from InsideGeek, that explains the difference between Interlaced scan vs. Progressive scan:

Why 60i equals to 29.97fps (30fps)?

When talking about 60i, we actually are referring to 60 interlaced fields per second. Two fields (odd and even) make a frame. In 60i, the fields are separated in time by 1/60th of a second.  So if you do the math correctly, you get 30fps.

This will make it easier for you to understand (I hope):

1 Frame = 2 fields
1 Field every 60th of a second = Total of 60 fields a second
________________________________________
= Total of 60 fields a second
or 30 frames a second.

In other words, you are getting 60 half-frames in one second.

Sony SLT-A55 1080p/60i Confusion

Well, after summarizing all the information for all of us, its time to go back and try to solve the 1080p/60i mystery. I will also be adding a new term called "Progressive Segmented Frame" in a few moments. Don't be afraid, stay with me, and we will understand it all together, step-by-step.

So when looking at text like "1080p/60i", it's seems awkward. What, both p (progressive) and i (interlaced)?

The reason it is 1080p, it because the Sony SLT-A55 & Sony SLT-A33 DSLR cameras, has a sensor that records videos in 30p, yes P. Sony Alpha SLT-A55 and A33, both use Progressive Segmented Frame, to process the final video.

Now its time to clear thing out, so you can understand what all that means.

Sony SLT-A55, SLT-A55, VG10 and both Sony NEX-5 and NEX-3 digital cameras, all allow recording videos in 60i. All of them use a progressive sensor, which captures the frame in 30p. Those digital cameras utilize a Progressive segmented frame scheme (PsF in short), which was designed to distribute progressive-scan videos using interlaced equipment and media.

Progressive Segmented Frame
With Progressive segmented frame, every progressive (p) frame in the video, is divided into two separate segments (like fields in an interlaced format). It means that you get the same odd lines & even lines frame separation like in interlaced video.

So you probably asking yourself, why taking a high quality progressive video and interlace it?

Well, there is a huge difference between interlaced frames and progressive segmented frames (which were created from progressive recording). If you remember, interlaced videos have two separated fields, odd and even. You might also remember that the problem with interlaced videos, is that odd frames and even frames, capture different parts in time. So when shooting fast moving objects like racing cars, horse race or a football game, the odd frame will catch a moment, which is different than the moment that was captured in the even frame.

60i, like in the Sony Alpha A55, utilizes Progressive segmented frame scheme. In PsF, odd lines frame and even lines frame, are separated from the same frame and from the same point in time. So in theory and in practice, there is not motion between the two frames. That means that when compared to native interlaced fields, you won't get the "combing" artifacts. The output will be smooth video, without ghosting.

Your video editing software will recognize your video as 25/30p progressive and not an interlaced video. So the video in Sony SLT-A55 is actually 1080p in 30 PsF (1920x1080) or 25 PsF (1440x1080). The reason why Sony used 60i, it's because AVCHD standard (also Blu-Ray) doesn't have 1080p/30 in its specification. On dpreview's Sony SLT-A55 specification page, you can see "1920 x 1080, 59.94i (from 29.97fps sensor output)", which explains what I have explained to you so far. That also means that it won't require extra work from your side.

So now let's summarize things up:

• Sony SLT-A55 sensor captures video in 30p
• Videos converted to 60i using Progressive Segmented Frame procedure
• Sony SLT-A55 will be recognized as 30p (progressive) when imported to video editing software

Why Cinematographers Prefer the 24/25p?

The difference in 30p vs 24/25p is quite obvious. Cinematographers have always preferred the cinematic-effect/film look that 24p videos create. It is mostly used in the filmmaking industry. Every film that you see in the cinema was shot in 24p.  Film cameras shoot movies in true 24p. Now DSLR photographers can achieve the same film-look effect with today's latest DSLR cameras. 30p is mostly used in broadcasting, like TV shows and sport games (Soccer, Football, Basketball, etc.).

There is another reason why cinematographers prefer to work with 24p, rather in 30p. 24p is suited for easy conversion to 50 Hz (PAL) systems and 59.94 Hz (NTSC) systems.  You can convert to 50Hz using 2:2 pulldown and scale 4% (faster rate), or convert to 59.94Hz using 2:3 pulldown and scale 0.1% (slow rate). This is a well known convention method, and it is widely used by cinematographers all over the world.

Usually when shooting HD videos in 24p, you should be careful not to pan your camera too fast, which will produce vertical banding across the frame.

Before we continue, I want you to watch this video called "24p Conspiracy". You might think twice before thinking of shooting 24p videos next time (or maybe not):

DSLR Cameras with HD Videos & Their frame rates

There several DSLR cameras, with which you can shoot HD movies. Here are some of them:

• Canon EOS 60D
  1920 x 1080 (HD 29.97/25/23.976 fps), 1280 x 720 (HD 59.94/50 fps), 640 x 480 (59.94/50 fps)
• Canon EOS 7D
  1920 x 1080 (HD 30/25/24 fps), 1280 x 720 (HD 60/50 fps), 640 x 480 (60/50 fps)
• Canon EOS 5D Mark II
  1920 x 1080 (HD 30/25/24 fps), 640 x 480 (30 fps)
• Canon EOS 550D (T2i)
  1920 x 1080 (HD 30/25/24 fps), 1280 x 720 (HD 60/50 fps), 640 x 480 (60/50 fps)
• Nikon D3100
  1920 x 1080 (HD 24p), 1280 x 720 (HD 30/25/24p), 640 x 424 (24p)
• Panasonic Lumix DMC-GF1
  1280 x 720 (AVCHD Lite, 30fps), 848 x 480 (30 fps), 640 x 480 (30fps), 320 x 240 (30fps)
• Panasonic Lumix DMC GH1
  1920 x 1080 (AVCHD, 30fps), 1280 x 720 (AVCHD, 30fps), 848 x 480 (30 fps), 640 x 480 (30 fps), 320 x 240 (30fps)
  

Summary

I have been crawling the Internet for many hours, trying to educate myself in a couple of hours. It was all started when I first posted my video frame rate confusion article. I was hopping to get some help from experts, but nobody has replied. So I had to give myself a 12-hours crush course. When you first encounter terms like: 60i, 50 Hz, NTSC, 24p vs 30p, PsF, you get really confused.


I am not a professional video producer/editor, and all I wanted, is to write a guide that will help you understand the basic terms in the world of cinematography. I also highly recommend buying book and reading information from reliable resources. I will continue my quest in learning more about video editing, because I need it for my next project. If you have anything to add/correct/comment, please comment on this blog. Comments are moderated, so if their is something that is not correct or not accurate, please comment about that.

In next posts, I will write about 1080p in YouTube, progressive scan in DVD/Blu-Ray players, how to edit a HD footage, AVCHD vs. other formats and other interesting subjects.

Hope you liked the article and find it useful. If you did, please share it.