TUTORIAL
1080P — Q&A — August 22, 2005
1080p continues to be a “hot button” story, judging by reader emails. This article addresses specific questions about 1080p display and signal formats.
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Before I go any further, it's important to note that the original article was written as a cautionary piece for prospective buyers of new HDTV sets, who are no doubt hearing a lot of misinformation about 1080p programming, signal processing, and imaging systems from manufacturers these days.
The important thing to remember is that at present, there is no 1080p acquisition, production, and broadcast formats using a fast picture refresh rate (50 Hz or 60 Hz) equivalent to the 720p standard. None. And there isn't likely to be any time soon, as the current crop of 1920x1080 cameras, switchers, graphics generators, processors, and title generators all work in an interlaced format.
None of them support 1080p50/60 at present, although there are some expensive 1080p/50/60 products coming to market, such as Sony's $100,000+ camera shown at NAB 2005. This camera cannot be integrated into an existing 1080i production environment unless its output signal is format-converted to 1080i first.
The only 1080p production format is 1080p/24/25, used to achieve a "film look" for prime time TV shows and some theatrical motion pictures. Programs shot in 1080p/24 in Hollywood are edited and finished in this format, then transferred to either 720p/60 or 1080i/30 for broadcast in the United States.
On to the questions:
Q. You keep referring to 1080i/30. There isn't any such standard! It's 1080i/60!
A. Classic misinformation. The refresh (frame) rate in any SDTV or HDTV format describes the total time interval it takes to completely refresh a new picture. In the 720p/60 system, a new progressive-scan image (i.e. a "frame") with 1280 visible pixels on 720 lines is displayed every 1/60 of a second.
In the 1080i system, there are two fields in a frame. One has all of the odd-numbered scan lines (1,3,5,7,9, etc.) and the other has all of the even-numbered scan lines (2,4,6,8, etc.) Each field is presented in 1/60 of a second. So, in the first 1/60, we have 1920 pixels on 540 odd-numbered scan lines, and in the next 1/60 of a second, we have the remaining 1920x540 pixels presented on the even-numbered lines.
Do the math: 1/60 + 1/60 = 2/60, or 1/30. So, the frame rate for a complete 1920x1080i image is 1/30 of a second, not 1/60 (that would imply the interlaced fields are each being presented at 1/120 of a second). In the interests of technical accuracy, the actual frame rates are 1080i/29.97 and 720p/59.94 in the United States.
Q. 1080p/24 programming can be shown on some TVs that have fast refresh rates. What's wrong with broadcasting 1080p/24 HDTV in its native format?
A. True, some HDTVs can handle a 1080p/24 signal by refreshing it three times in a given time interval. That works out to a 72 Hz frame rate, and many plasma and LCD TVs support it. But the vast majority of TVs in use are still CRTs, and many HDTV sets use CRT technology. None of these sets support a 72Hz or even a 48 Hz (double) scan rate.
That's because, to save a few bucks, manufacturers standardized on a 33.8 kHz scan rate, which is what is required to show 1080i/30. 720p/60 (44.9 kHz) often isn't supported, or if it is, the signal is scan-converted down in resolution to 540p. In fact, your 1080i content is often converted to 540p by simply throwing away one of the fields (usually the even one). So you aren't getting full resolution from the 1080i signal to begin with!
Another point to consider: At present, a 72Hz refresh rate is not supported in the DVI HDCP and HDMI formats for DVD players and set-top receivers. 720p/60 is, as is 1080i/30. (But 1080p/60 is not, nor is 1080p/24.) This may change in the future.
You wouldn't want to watch HDTV programs refreshing at a native 24Hz rate on a CRT set - the flicker would drive you crazy! And 1080p/24 is totally inappropriate for broadcasting sports. No sports fan would tolerate the motion blur and loss of fine detail in fast-moving objects. Even 1080i/30 would be a better choice.
As it stands now, converting 1080p/24 to 1080i/30 for broadcast is a winning combination. Picture quality is quite good and the "film look" holds up well even when converted to interlaced scan. On some of the best HDTVs, picture quality is outstanding.
Q. You mention "wobbulation" as the trick used by DLP RPTV set manufacturers to achieve more horizontal resolution on the screen from half-resolution chips. Not all manufacturers use this system.
A. The current digital micromirror device (DMD) used in 1080p RPTVs does not have full 1920x1080 pixel resolution. Instead, it has 960 horizontal pixels. A technique that resembles segmented-field scanning presents 960 horizontal pixels in a brief time interval, then the remaining 960 pixels in the next time interval. Shifting the tiny mirrors slightly at super-fast speeds does the trick.
The total time interval is fast enough that you see what appears to be a smooth image with lots of horizontal picture information. HP is credited with implementing this "wobbulation" technique in their RPTVs, but Samsung, Toshiba, and Mitsubishi all have variations of segmented-field picture scanning using different brand names.
It's important not to confuse segmented-field scanning with interlace scanning. There are no motion artifacts with SF pictures, only with interlaced pictures converted to progressive scan. It's also important to note that some RPTVs use full-resolution 1920x1080 LCoS and LCD imagers, although some of those TVs need help in terms of image quality.
Q. Isn't 1080p required for viewing HD on large screens? The pixels from a 720p or 7680 set would be distracting.
A. This might be the best argument for a 1080p display, but it all depends on your viewing distance. The rule of thumb for seating distance, depending on whom you speak to, is 3x or 4x the screen height. If you have a 50-inch screen with 1366x768 pixels, the viewing distance using the 3x rule would be about 75 inches (assuming a picture height of about 25 inches) and with the 4x rule, it would be 100 inches.
At a SMPTE presentation a couple of years back at Panasonic's Advanced Television Labs in New Jersey, discussion was made of the development of the 720p standard and that a 4x screen height viewing distance was recommended for display with that resolution. That would mean a viewer should be about 8.3 feet from that 50-inch screen, which is not at all unusual for a viewing distance in most American homes.
Switching to a 1080p display and using the 3x recommendation would allow you to sit six feet from the same 50-inch screen. In my house, that would mean I could just barely use my reclining couch without hitting the TV cabinet or stand. Would a larger screen make a difference? With a 60-inch 720p set, I'd need to be 120 inches, or 10 feet away. With a 1080p 60-inch set, that distance could be optimally cut to 7.5 feet.
I've have 61-inch plasma TVs in my studio for review on more than one occasion, and really haven't noticed the pixel structure at distances of 8 feet or more. So, I guess it depends on the individual. (Keep in mind that RPTVs with screen sizes of 60 inches or more are a very small part of the market at present.)
Q. So why are manufacturers pushing people to buy 1080p HDTV sets?
A. Because the prices of 720p and 768p RPTVs are dropping to a point where profit margins are becoming very slim. You can thank the aggressive pricing from plasma and LCD manufacturers for that. 1080p isn't feasible in plasma TVs smaller than 70 inches as of now, and 1080p LCD TVs are too darned expensive.
1080p provides a nice market niche for manufacturers of RPTVs — for now. Problem is, prices of these 1080p sets are already being discounted, pushing 720p and 768p prices even lower. At some point, RPTV manufacturers will likely opt out of 720p and 768p altogether in favor of 1080p products. Let's hope that the "half resolution" problem, SDTV (480i) scaling issues, and other issues such as clipped bandwidth are resolved by then!
