In Favor of 1080i: The Last Word on the 1080i vs. 720p Format Debate

by Unnamed Correspondent
Friday, May 1, 1998

Progressive scan advocates generally dismiss interlace as quaint. Not so, say 1080i advocates. So, a debate has risen. "This issue will not be settled in our lifetime," groaned a highly placed 1080i expert.

What is in favor of the 1080i? Why should it prevail? We asked the best of the best of the best, who asked not to be identified and thus drawn further into an endless debate. So we hear his very last words on the subject. Undoubtedly, there are those not in agreement. They will rile against the conclusions drawn. That is the nature of a debate... especially an endless one.

Dale Cripps

Our expert offers the following:

1. First, the number of pixels per picture.

The 1080 format has 1080 vertical pixels and 1920 horizontal pixels in a 16:9 aspect ratio. That is 2,073,600 pixels per picture.

The 720 format has 720 vertical pixels and 1280 horizontal pixels in a 16:9 aspect ratio. That is 921,600 pixels per picture.

The 480 format has 480 vertical pixels and 704 horizontal pixels in a 4:3 or 16:9 aspect ratio. That is 337,920 pixels per picture.

Format Pixels
====== ======
1080 2,073,600
720 921,600
480 337,920

2. Second, the number of pixels per second in production (broadcast will be discussed later).

Look at this issue for a moment in the "common image format" manner. We wish to define pictures by the number of pixels, the aspect ratio, and the colorimetry. We will allow the application to specify the number of pictures per second. A still picture is 0 frames per second. Movies are 24 frames per second. European video is 25 frames per second. American video is 30 frames per second. High speed capture (for playback at a slower frame rate for smooth slow-motion) would be a much higher frame rate -- I have heard numbers between 180 and 240 frames per second.

The 1080 format for film originals (24 frames per second) would be
49,766,400 pixels per second.

The 720 format for film originals (24 frames per second) would be
22,118,400 pixels per second.

The 480 format for film originals (24 frames per second) would be 8,110,080 pixels per second.

Keep in mind that 75% of prime time originates on film.

Video origination does become more complicated because of the issue of interlace/progressive scanning, and the choice of frame rate.

The 1080 format for video origination (30 frames per second) would be 62,208,000 pixels per second. The video could be 30 progressive frames per second or 60 interlaced fields per second. The number of pixels per second would be the same. The 30 progressive mode would give very good looking pictures under most conditions but would result in motion artifacts with fast motion (like sports). The 60 interlaced mode would give better motion rendition for sports but would result in interlace artifacts.

Note that video at 30 frames progressive would look great if handled in a manner comparable to how 24 frame film is handled. After all, movies are only 24 frames per second. But, you rarely try to film a football game at 24 frames per second. In making a movie, you control everything, including the motion. On the other hand, video is generally used to capture real life.

Of course, if you used the 1080 format at 60 frames per second progressive scan, you would have remarkable pictures. That would be 124,416,000 pixels per second. At this time, there are no cameras that can do this, and it falls outside the MPEG parameters (and thus ATSC parameters) for digital compression.

With the 720 format, you could do video at 60 frames per second or 30 frames per second. Both would be progressive scan as interlace scan was not included as an option in the 720 format. At 60 frames per second, you have 55,296,000 pixels per second. At 30 frames per second, you have 27,648,000 pixels per second. As mentioned a couple paragraphs ago, you really want the 60 frames (or fields) per second rate for sports. The 30 frames per second rate would be sufficient for drama and situation comedies, but many feel it is not sufficient for sports.

And, not to leave out the 480 format, with 60 frames per second progressive scan you have 20,275,200 pixels per second. With 30 frames per second progressive or 60 fields per second interlace scan, you have 10,137,600 pixels per second. All my previous comments on motion artifacts and interlace artifacts continue to apply.

Format Rate Pixels per second
====== ==== =================
1080 60p 124,416,000
1080 60i/30p 62,208,000
720 60p 55,296,000
1080 24p 49,766,400
720 30p 27,648,000
720 24p 22,118,400
480 60p 20,275,200
480 60i/30p 10,137,600
480 24p 8,110,080

3. Third, the number of pixels per second in broadcasting.

Keep in mind that most of the frames of television pictures and movie pictures are very similar to the preceding frame and the following frame. In fact, that's the only reason that digital compression works in the first place. If sequential frames were completely unrelated, digital compression would fall apart. So, I must strongly disagree with those who say that "still picture pixel counts" don't matter in DTV. Often, most of the picture is still and only a small part is moving.

The number of pixels per second is important, but let's not get carried away with the numbers. For example, let me define a new format, a super low definition format. It will have 48 pixels vertically, 96 horizontally, 2:1 aspect ratio, and 100,000 frames per second progressively scanned. As you can imagine, it will have wonderful motion rendition. But, its resolution will leave something to be desired. However, it does have 460,800,000 pixels per second, about four times as many as 1080 with 60p!

So, I also must strongly disagree with those who say that the number of pixels per second is the most important thing in terms of how good a picture looks. According to that logic, 480 lines at 200 progressive frames per second (67,584,000 pixels per second) would be better than 720 lines at 60 progressive frames per second (55,296,000 pixels per second) because it is higher bandwidth.

When you change one parameter only, it is clear that the format with the highest set of numbers is best (assuming, of course, that it is still possible to broadcast the resulting bit stream). But, when you change two parameters, you have to apply a bit of intelligence.

For example, 1080x1920x60p is better than 1080x1920x30p is better than 1080x1920x24p. Also, 720x1280x60p is better than 720x1280x30p is better than 720x1280x24p. Those are no-brainers. But, how would you compare 720x1280x30p with 720x640x60p with 720x320x120p with 720x160x240p? They are all the same number of pixels per second.

Also, the amount of digital compression you can do varies depending on your selection of parameters. You know that if you are taking 100,000 frames per second the human eye cannot possibly follow that, so you can make different compromises in your digital compression algorithm than you can if you are only taking 24 frames per second.

To a first approximation, though, the number of pixels per second is an indicator of what can be broadcast. It may not tell you directly what the quality is, however. Note that 480x704x60p (337,920 pixels per picture and 20,275,200 pixels per second) is about the same number of pixels per second as 720x1280x24p (921,600 pixels per picture and 22,118,400 pixels per second). They will probably end up with about the same number of bits per second after digital compression for broadcasting. But, there is no question that the 720 set of numbers will provide far superior pictures under all cases except rapid motion. Then, it is hard to predict.

What compromise are YOU willing to make between spatial resolution and motion rendition and artifacts? Some may have a preferred set of compromises, and I may have my preferred set. But, what will be the preferred set of consumers?

I go for resolution. I love 1080x1920x24p. It is absolutely super. Everyone should see a demo at Sony Hi Definition Facility comparing projection of a 35mm theatrical release print with 1080x1920x24p broadcast of the same movie the same size on the same screen. I would have no trouble whatsoever accepting the interlace artifacts to have 1080x1920x60i sports. For me, that is a better compromise than 720x1280x60p for sports.

My preferred picture has about the same number of pixels per second, but it has 50% more horizontal resolution 100% of the time; not just when it is a still picture, but even with the most rapid motion. And, it has 50% more vertical resolution some of the time and rarely, if ever, has less vertical resolution. But, if you are talking heavy graphics, or non-Nyquist limited computer generated pictures, the 720 picture would generally be superior. The 50% extra horizontal resolution may not be an acceptable compromise.

As for those who believe that 1080x1920 is only 1035x1440. They are wrong. It may be that some people are only doing 1440 horizontal, but that is a minority. The Advisory Committee's tests were at 1920. The Model HD Station using Grand Alliance prototype equipment is 1920. Sony HD Camcorder tape format is 1440. The Panasonic HD-D5 is 1920. The ATSC Standard is 1920. There is only a small difference between 1035 and 1080. Most equipment is 1035, but that is changing daily. The Panasonic HD-D5 and the Sony HDW-500 can handle 1035 or 1080. The Sony HD cameras are 1035 today but will be 1080 by the end of the year.

For film, even if it is recorded on a 60i machine, it is still 24 frames progressive. And it absolutely is a downconversion to get from 1080/24p to 720/24p. As far as live video, in the horizontal direction it is absolutely a downconversion to get from 1920 to 1280. No matter what the motion. The vertical resolution can vary according to the motion, but it is rare that 720/60p has more vertical resolution than 1080/60i.

The bigger issue is computer graphics. When we do special effects, interlace is a pain. You end up treating each field as a frame with every other line missing. So, you end up working with 60 pictures for each second of material, not just 30 pictures for each second of material.

Compromises. That's what life is about.