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What Is the Difference Between 1080p and 1080i?

HDTV displays and HD media content are labeled with the designation 1080p and 1080i, but what exactly does that designation mean and how does it impact your purchasing and watching decisions?

Today’s Question & Answer session comes to us courtesy of SuperUser—a subdivision of Stack Exchange, a community-driven grouping of Q&A web sites.

The Question

SuperUser reader Avirk is curious about the designations he sees on HDTVs and their content as well as how it applies to computer screens. He writes:

I have seen many times resolution of 1080p and I know that mean 1080 pixels but sometime I also seen option on the HDTV is 1080i. So I want to know the exact difference between them and is 1080i video quality available for the laptop too?

I some Googled and got there are some time 1080P rather than 1080p is there any difference between them too or they represent the same?

Let’s dig into the SuperUser contributor answers to get the bottom of things.

The Answer:

SuperUser contributor Rsp goes into great detail explaining the differences between 1080p, 1080i, and when one is preferable over the other. He writes:

I see a range of problems in the answers and comments here (even in some highly-voted answers that provide otherwise very good information) that span from minor deficiencies that need explanation to some serious inaccuracies, so I think that some clarification is needed.

The question is specifically: What is the difference between 1080p and 1080i? so I will start by outlining the main similarities and differences, I’ll add some tips on how to choose the best format and then I will proceed to explain the problems that I found here.

Some of the information presented below is adapted from my answer to Interlacing on a computer monitor but is rewritten to strictly stick to the subject of the difference between 1080p and 1080i.

Resolution

Both 1080p and 1080i have 1080 horizontal lines of vertical resolution which with a widescreen aspect ratio of 16:9 results in a resolution of 1920 × 1080 pixels (2.1 megapixels). It is not true that 1080i has a lower vertical resolution than 1080p.

Frames vs. fields

1080p is a frame-based or progressive-scan video where you are dealing with frames. You have frame rate and it is expressed in frames per second.

1080i is a field-based or interlaced or interleaved video where you are dealing with fields. You havefield rate and it is expressed in fields per second.

A field contains half of the lines of the frame, either even lines or odd lines, and if one field is composed of even lines, then the next one will be composed of odd lines and so on.

Frequencies

1080p has a frame rate of 25 frames per second for TV in PAL countries, 30/1.001 frames per second for TV in NTSC countries and 24 frames per second for cinematography.

1080i has a field rate of 50 fields per second for TV in PAL countries and 60/1.001 fields per second in NTSC countries.

(Note that it is not 30 frames and 60 fields per second for NTSC but actually 30/1.001 and 60/1.001which is approximately 29.97 and 59.94 but the difference is important. Read about the NTSC color encoding on Wikipedia to see why.)

How to think about it

1080p at 25 frames per second: Imagine that you are shooting 25 pictures per second and storing them as bitmaps. Every frame is a full picture from the given instant. Every pixel in that frame was captured at the same time.

1080i at 50 fields per second: Imagine that you are shooting 50 pictures per second but storing onlyhalf of the bitmaps every time – sometimes you store the odd lines and sometimes the even lines. (Note that it is not the same as storing pictures with lower vertical resolution.) Every field is a half of a full picture from the given instant. Every pixel in that field was captured at the same time.

50 halves ≠ 25 full pictures

Contrary to some comments here, interlaced video at 50 Hz does not mean that 25 full pictures per second are shown. It means that 50 halves of pictures are shown but those are halves of 50 different pictures that were shot at 50 distinct moments of time in every second. You not only don’t have 50 full pictures per second - you don’t have any full pictures at all.

Problems with 1080i

Interlacing causes a lot of problems. For example you can’t easily:

  • scale the video
  • rotate the video
  • make video slow motion
  • make video fast motion
  • pause the video
  • grab a still picture frame
  • play video in reverse

without doing some tricks and loosing quality. You don’t get any of those problems with progressive video. In addition the video encoding is harder because the codec never has a full frame to work with.

Problems with 1080p

The drawback is that 1080p as currently in use has a frame rate that is only half of the field rate of 1080i so the motion is noticeably less fluid – in fact it’s exactly twice less fluid which is a lot. You can see it on large flat TVs that often deinterlace the video to be able to display it on their LCD screens (that, unlike CRT displays, are progressive in nature) which is the cause that they display picture of very high resolution but with jerky motion and some deinterlacing artifacts.

Another problem is that usually 1080i is required for TV broadcasting which means that 1080p is simply out of the question for some applications.

Best of both worlds

Using progressive 1080p with 50 or 60/1.001 full frames per second in the future has a potential to eventually solve the above problems but it will require a whole new range of studio equipment including cameras, storage and editing systems so it probably won’t happen anytime soon. The widely used SDI standard for connecting HD video equipment doesn’t have enough bandwidth.

Currently the only way to have a fluid motion with progressive scanning is 720p that has a frame rate that is two times faster than 1080p but the resolution of only 1280 × 720 pixels (instead of 1920 × 1080 pixels) which may or may not be a problem for some applications. There is no 720i.

Conclusion

There is no one clear winner here.

Update: Here are some general guidelines to choose the right format:

  1. Is it for high-definition TV? Use 1080i or whatever is required.
  2. Is it for standard-definition TV? Use 720p and then convert to 576i or 480i.*
  3. Is it for Internet and resolution is more important than fluid motion? Use 1080p.
  4. Is it for Internet and fluid motion is more important than resolution? Use 720p.

(It all assumes that 1080p has a frame rate of 25 or 30/1.001 frames/s, 1080i has a field rate of 50 or 60/1.001 fields/s and 720p has a frame rate of 50 or 60/1.001 frames/s as is currently the case. Hopefully a high resolution progressive format like 1080p with a frame rate of 50 or 60/1.001 frames/s or maybe even higher will make this recommendation obsolete in the future.)

*) For number 2 make sure that your 720p has the frame rate of 50 fps if your target format is PAL or SECAM and 60/1.001 if your target format is NTSC (unfortunately it means that there is no format that can be converted to both PAL/SECAM and NTSC). The reason I recommend using 720p for recording is to greatly simplify the edition process when every frame is complete with no interlacing (throwing out every other line at the end is easier than creating the missing lines if you need them) and you have some extra resolution to work with so you can for example zoom the image slightly without making the result look blurry. (If anyone has any bad experience of using 720p to prepare material for SD PAL or NTSC TV broadcasting then please comment so I could update this recommendation.)

Explaining problems

These are the parts that I found in the answers and comments here that I think need some explanation:

Progressive Scanning is more desirable in almost every case.

I think that progressive scanning is indeed better in every respect, but if we are not talking theoretically about the idea of interlacing but specifically about 1080p and 1080i standards as used today, then one has to take into account the fact that 1080i is often required for TV broadcasting and converting 1080p to 1080i would result in jerky motion.

P is better than I in most cases i believe, which is the important bit.

Again, yes, progressive is better than interlaced all other things being equal, but progressive video with frame rate that is two times smaller than the field rate of interlaced video (which is the case with 1080p and 1080i) is something very different, especially if interlaced video with high field rate is required for TV broadcasting and the high field rate cannot be reproduced from progressively recorded material with lower frame rate.

[In 1080i] all the odd lines are displayed, followed by all the even lines. This means that only 1/2 the resolution (540 lines or pixel rows) is displayed on the screen at any give time – in other words, only 540 pixel rows are displayed at any given time.

No. For LCD all 1080 lines are always displayed, for CRT displays usually much less than half of of the lines are displayed at any given time which is equally true for both 1080i and 1080p.

The phrase “only 540 pixel rows are displayed at any given time” is extremely misleading. All 1080 rows-of-pixels usually are displayed at once (and even if they weren’t, they’d still appear to be to the human eye), but only half of them will be updated in any given frame. It’s effectively the refresh-rate, not the resolution, that’s cut in half.

While it is true that the phrase “only 540 pixel rows are displayed at any given time” is extremely misleading, it is not true that the refresh-rate is cut in half, because in 1080i the refresh rate is two times faster than with 1080p so it is actually the other way around.

1080i60 means that you’re getting 60 half frames (alternating lines) per second, so only 30 complete frames per second.

With 1080i60 you actually get less than 60 fields (or “half frames”) per second, but it doesn’t mean that you get 30 (or almost 30) complete frames per second. In fact you don’t get even a single complete frame per second.

More resources

This is what I consider the best resource on the subject of field-based (aka interlaced or interleaved) and frame-based (aka progressive-scan) video:

See also the following articles on Wikipedia:

I hope it somewhat clarifies the subject.

 


Have something to add to the explanation? Sound off in the the comments. Want to read more answers from other tech-savvy Stack Exchange users? Check out the full discussion thread here.

Jason Fitzpatrick is warranty-voiding DIYer and all around geek. When he's not documenting mods and hacks he's doing his best to make sure a generation of college students graduate knowing they should put their pants on one leg at a time and go on to greatness, just like Bruce Dickinson. You can follow him on if you'd like.

  • Published 01/22/13

Comments (30)

  1. tecn0tarded

    so i’ve wasted my childhood watching tv on a Zenith black and white tv with tubes and an antenna? and when I watch turner classic movies on my brand new 72″ 1080p tv the graininess looks even more awesome. Technology rules…

  2. epeace

    Hi

    I have read many texts on this subject and I must say that this text clear my doubts about progressive and interlaced.
    Thanks for this explanation, very simple and clear definitions of the terms!

  3. Brian

    What about TVs at 120Hz? Does that address the update speed problems with 1080p?

  4. Cindy

    Really nice explanation. Thanks. Now I know what to buy next time.

  5. George

    Pity the writer couldn’t re-read what he wrote to get rid of the obvious mistakes in grammar and English. Never mind. It wasn’t a bad attempt.

  6. Stewart

    Really good, thank you for taking the time to sort it out.

  7. john3347

    Is there an explanation that common folks can understand? Rsp has written what is likely a very accurate assessment of the differences between i & p that only the highly technically versed reader can possibly understand. Quite probably the reader who can understand this highly technical report already knows the answer and an article written on a laymans level would have been many times more effective and appreciated. After attempting to read the reply, I have no idea what the difference between 1080i and 1080p is. Thanks lots for the information anyway. Isn’t it the responsibility of an author to understand his audience

  8. Raven

    @george:

    Huh? The article is very well written concerning content, grammar and language. While one could debate an occasional comma might be used, please share with us your insights as to how YOU would correct it. Pity that you would even post such a rude, childish and totally unneeded comment!

  9. Jim

    A true 1080p signal requires (essentially) twice the bandwidth of 1080i, so it will cost more for a long time to come. Truth is: most people can’t see the difference.

    We could really use better *programming*, not just another evening TV show trying to have more sex and violence than the similar program on the competing network.

  10. Czarek

    This article is quite messy and is not written for the casual reader.
    I’m disappointed with this article…

  11. Michelle

    Very thorough explanation, well written and informative. Great information for when I go to buy. Thanks

  12. heinrich

    @George.

    Shut your @#&%* mouth unless you can comment constructively.
    Sick and tired of the grammar girls.

  13. Dale Vincereo

    “a frame rate that is two times faster”.
    Very poor English, and very misleading mathematically.
    You should say “a frame rate which is twice the speed of…”

  14. Ronny

    ARGH!
    It was so good, and then I got to the “LOOSING quality” part.
    I can’t read past spelling and grammar errors.
    Annoying.

  15. john3347

    Everyone who is so critical of the spelling and grammar errors of the technical (and other category) writers need to realize that the technical writers have not changed. What is different now from previously is that many, many publishers have eliminated their proof-reading departments (so they can have a few more million dollars to pay their execs who don’t care what level the writing is). Proof readers used to catch the spelling and grammatical errors and correct them so the piece looked professional. Them days is gone now and we have to accept the writing ineptitude of the technical writers and accept their technical expertise and ignore their lack of writing skills. I do however sympathize with the grammar police because a technical article filled with poor spelling, grammar and composition is harder to read and takes considerably away from the professionalism of the author and from the comprehension of the writings. Bad, bad publisher move.

  16. imanoldgoat

    @John3347- right on! I know several proof readers who lost their positions and are now “Independent Consultants” (translation: they’re on their own and work when/if needed, which continually decreases). Don’t blame the Techie, blame the Wharton graduate. As for me, I’ll continue to use my Heathkit Color TV that I built many years ago. Got me a supply of tubes, a manual, and a soldering gun. I won’t give ‘em up ’til they pry ‘em out of my cold, dead fingers! Baa!

  17. Your bugger

    heinrich: Shut your @#&%* mouth unless you can comment constructively.
    Sick and tired of the grammar girls.
    Who do you think you are?

  18. A.D.Wheeler

    ALL word processors and even browsers have spelling and grammar checkers that work for free.

  19. Cody

    and you think someone who is trying to provide someone who is to lazy to look something up for themselves is going to go through the hassle of running a program just to check spelling because a bunch of wining babies can’t deal with a few miss spelled words. GROW UP. the world does not revolve around spelling and grammar anymore. I bet if we checked your text messages half of those words wouldn’t even be spelled right even though there’s word processors on smart phones these days that have spelling and grammar checks. GET A LIFE

  20. rKiller

    My 2560X1600 Monitor is fluid enough for me !

  21. Cody

    heck I just had my 8 year old son read the article and he had no problems interpreting any of it (other than the few larger words which were spelled right, he just never seen them before) so what does that say about you grammar babies that can’t understand an article due to grammar.

  22. colruss

    So not surprisingly a great number of anti-grammarites about which reflects the one missing element in society, DISCIPLINE.
    From the top levels of society and down the line, NO DISCIPLINE. Idle-backs!!

  23. Joe

    @Rsp – Nice job. You put a lot of time into an effort to explain something that confuses many people, and did it better than I could have. You effort is appreciated. Forget the idiot comments.

  24. JohnC

    Thanks for the article. Keep up the good work.

  25. Dic

    Poor grammar usage and mis-spelled words are tolerable in feedback (such as in most of those here), but I agree with those here who maintain that technical writers should ensure their offerings are free of them. They have to accept that text-editing is a thing of the past, and read over what they have written before attaching their names to articles they submit for publication.

  26. Dic

    Although I don’t consider myself particularly knowledgeable in the subject, as far as the technical content of the article was concerned, I found it well presented and easy to understand. Thank you for your effort, SuperUser contributor Rsp.

  27. heinrich

    YOUR BUGGER:

    “heinrich: Shut your @#&%* mouth unless you can comment constructively.
    Sick and tired of the grammar girls.
    Who do you think you are?”

    I’m your daddy… That’s right my boy:-)

    Seriously now people: if the grammar and spelling are so annoying, what are you still doing here?

    Stop complaining about a service that you receive free of charge. Be gratefull.

  28. heinrich

    YOUR BUGGER:

    “heinrich: Shut your @#&%* mouth unless you can comment constructively.
    Sick and tired of the grammar girls.
    Who do you think you are?”

    I’m your daddy… That’s right my boy:-)

    Seriously now people: if the grammar and spelling are so annoying, what are you still doing here?

    Stop complaining about a service that you receive free of charge. You should be thanking the author. Comment’s are supposed to be about the content of the article, as well as constructive.

    if we carry on like this the comments function will probably be removed in future.

  29. heinrich

    Sorry for the double post. Using mobile device.

  30. rajzoo1

    nice info

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