DTV Primer

Chris Llana, Editor


Home
Transition?
FAQ












Links

1080p: Musings on Frame Rates

April 22, 2006

(After reflecting on what I had written for my HDMI article.)

I have concluded that my thinking bias goes back almost ten years when the big debate was whether 1080i or 720p was better. (re over-the-air HD broadcasts) 1080i had more than twice the pixels, but 720p had double the frame rate. 1080p was the thing that could give you both, but the technology wasn't there back then to do 1080p at 60 frames per second.

So 1080p has long been regarded as the holy grail of HDTV, unobtainable for so many years, the ultimate high-def format.

Well, after reflection, methinks 1080p as it is today is just another step to the top of the mountain.

Today we have the capability of 1080p at 24 frames per second (1080p/24), 1080p/30, and 1080p/60.

1080i after all is (in broadcast TV at least) 60 fields per second, equivalent to 30 full frames per second. Fixed-pixel displays work with progressive scanning, not interlaced, so 1080i signals are de-interlaced by those sets to progressive before the signal goes up on the screen. Which means those sets have been displaying 1080p video at 30 frames per second for years. (See below for an explanation of interlaced vs. progressive.)

Of course the video quality suffers a bit due to the de-interlacing. So 1080p sources will look better because no de-interlacing will be needed. But this would not seem to justify all the 1080p marketing hype.

I don't suppose that would bother the TV set advertising types, though, would it? If they say capable of displaying 1080p, that's no big thing. If they say capable of inputting 1080p, then that's something.

But what about frame rates?

For NTSC/analog broadcasts, it's 30 frames per second (fps), period.

For the ATSC digital broadcast standard, 24, 30, and 60 fps are available for 480p and 720p. For 1080i, it's only 24 and 30 fps.

But all the 1080p hype these days is not about broadcast TV, but about the new high-def DVDs, and that means MOVIES. Movies are shot on film at 24 frames per second, or lately many are being shot on high-def video cameras--again, at 24 frames per second--for easy transfer to film for distribution to conventional theaters.

Unless you add some sophisticated processing, going to 60 fps (or even 30) isn't going to add anything to movies on disc. No new picture information will be added, because it's not in the source material.

Standard def DVDs (which conform to NTSC TV standards) are 480i at 24 frames per second. The pictures are then typically processed for display on your TV. If you have a progressive scan DVD player and a digital TV, the interlaced signal is de-interlaced.

Second, some sets or DVD players will upconvert the resolution to match the TV's native display resolution (eg. 720 or 1080). No new data is added; the equipment interpolates (guesses) what the extra pixels will look like and then adds them.

Third, because movies are shot at 24 frames per second and NTSC TVs will only display them at 30 fps, either your DVD player or your TV will do that conversion (called 3:2 pulldown) to produce 30 frames per second from 24.

All of these conversions represent compromises to the original integrity of the movie. If they are done artfully, the picture will be a lot better than it would otherwise, but let's not confuse that with improvement.

Now with HD-DVD and Blu-ray (and ATSC/digital TVs), if you encode a movie at 1080p and your new optical disc player can output 1080p and your TV can input 1080p, then you get to skip the de-interlacing. A good thing.

And if your TV has a 1920 x 1080 display, then there is no resolution conversion. Also good. (If you've got a 720p TV, the conversion throws away more than half of the picture detail that's on the HD-DVD or Blu-ray disc!)

The third conversion problem disappears for digital TV sets that support the ATSC standard frame rates (24, 30, and 60). Presumably for movies, the TV will display them at 24 fps and there will be no need to do a 3:2 pulldown conversion.

So what frame rates do state-of-the-art digital TVs actually support?

I checked a couple of the best. The new HP 1080p DLP sets support 1080p at 24, 30, and 60 fps through the HDMI inputs, and 1080p at 24 and 30 fps using the component video inputs. The published specs for the Sony SXRD LCoS 1080p sets do not include supported frame rates, but one of the features is "reverse 3:2 pulldown." Presumably that means a broadcast movie that has been converted to 30 fps (for display on NTSC sets) can be unconverted back to 24 fps and displayed at that native frame rate.

I suppose all of this means that your new 1080p HD movie player and 1080p TV will show you a jaw-dropping picture, but you'll still be stuck with the lack of smooth fast motion inherent in 24 fps video.

I have read a report on a processing algorithm that interpolates the motion of objects on the screen (between each 24 fps frame) and adds in-between frames, all in real time. This was demonstrated on a computer, but could be implemented in a digital TV. It could produce the smooth high-speed motion that 720p/60 does now, but at 1080p/60 quality.

Waiting to take that next step up the mountain.

----

First reviewer reports of HD movies played on the new HD-DVD players talk about a dramatic improvement over regular DVD. Eg. a "jarring" difference.

Other reports says the new HD-DVD players and the first Sony Blu-ray player are not capable of outputting the latest audio standards (Dolby TrueHD and DTS-HD Master Audio).

---------------------

* Interlaced vs. progressive

A bit of techno-speak for the non-initiated: A 1080i or p display contains 1080 rows of pixels from top to bottom (rows are commonly called "lines" or "scan lines"). From left to right in each row are 1920 pixels, thus 1920 x 1080. Interlaced displays "scan" (or paint, or light up) the picture one line at a time--first scanning all the odd-numbered lines, top to bottom, and then the even-numbered lines. The odd-numbered lines form one "field" and the even-numbered lines form another field. The two fields together form one complete "frame."

(This assumes square pixels; some sets have pixels wider than high, and thus fewer than 1920 pixels from left to right.)

Progressive scan displays scan all of the lines sequentially, top to bottom.

Interlaced displays will not look as smooth because adjacent lines are scanned with a significant time delay. That is, for example, even lines 34-1080 and odd lines 1-31 will be scanned after line 32 is scanned and before line 33 is scanned. If there is motion in the picture, there will also be discontinuities.

With progressive displays, line 33 is scanned immediately after line 32. The time delay is insignificant.