High-Def FAQ: What's the Big Deal About 1080p24?

Posted Fri Sep 28, 2007 at 01:08 PM PDT by

Editor's Note: As part of his twice-monthly column here at High-Def Digest, from time to time, Josh Zyber answers frequently asked questions related to High-Definition and both HD DVD and Blu-ray. This week: Josh unravels the mysteries of 1080p24.

Commentary by Joshua Zyber

Along with all the advancements that the Blu-ray and HD DVD formats have brought to the area of home theater has also come a lot of confusing new terminology. Back before we could get High Definition content on disc, HD programming came in two different formats via television broadcast, 720p or 1080i, each with their own strengths and weaknesses. 1080i is higher resolution but interlaced, while the lower-resolution 720p offers the benefit of progressive scan. Nowadays, new TVs go all the way up to 1080p, combining the strengths of both prior formats, the highest resolution plus progressive scan. Catering directly to the latest technology are both HD DVD and Blu-ray, each predominantly offering content encoded in 1080p resolution.

But it doesn't end there. 1080p itself can be broken into two distinct formats. What we normally refer to as "1080p" could also be called 1080p60, and now there is also a variation called 1080p24. The final digits refer to the frame rate at which each runs. Several Blu-ray players offer 1080p24 video output over HDMI in addition to the usual 1080p60, and just recently Toshiba added 1080p24 output to their HD-XA2 and HD-A20 HD DVD player models as well. So what's the big deal? How does each work and is there really a visible difference between them? To explain, we must first understand the way that movies are adapted to video.

Theatrical motion pictures are usually shot on 35mm film, running at a speed of 24 frames per second (even those movies shot on other media use the 24 fps speed for theatrical compatibility). When projected back at the same rate, the image maintains a natural-looking representation of motion. People on screen walk and talk normally, without any 'Keystone Cops'-style sped up movement, unless done intentionally for effect. In contrast to this, NTSC video runs at a rate of 60 interlaced fields per second (59.94 to be more precise). For the sake of consistency, North American HDTVs continue to use a 60 Hz rate, though models with progressive scan will display 60 whole frames each second rather than interlaced fields. Television broadcasts are still transmitted at 60 Hz, and High Definition disc players of both formats also primarily output their video at that rate, either in fields or frames depending on which resolution output you choose.

Similar to what happens with film projection, video material specifically shot at 60 Hz (the evening news, for example) and then played back on TV at that same rate will look perfectly natural. The key to fluid motion is that the original capture speed must match the display playback speed. Unfortunately, we run into an obvious conflict when transferring 24 fps movies to 60 Hz video. If you were to simply speed up the picture to match the faster frame rate, you'd wind up with very distracting visual and audible changes to the movie, everything moving too fast and the soundtrack raised in pitch as it is also sped up to match. Clearly, that's not an acceptable solution.

To get around this, a process called "3:2 pulldown" was developed, in which the original 24 fps film frames are multiplied into an alternating pattern of 3s and 2s. The first frame is displayed 3 times, the second frame twice, the third frame 3 times, and so forth in series. This repetitive sequence effectively stretches 4 film frames into 10 video frames, allowing the original 24 fps content to play at the faster 60 Hz rate without appearing sped up. For a more detailed technical explanation of the process with visual illustrations, I recommend reading through articles at Secrets of Home Theater and High Fidelity and Wikipedia.

The downside to 3:2 pulldown is that it can leave the picture with an artifact called judder, where the repeated frames cause what should be smooth fluid motion to look slightly jerky. The problem is usually extremely subtle, and most viewers raised watching 60 Hz television have probably never noticed it. In fact, even when deliberately looking for it, judder can be difficult to see except during slow, steady camera movements. The opening credits sequence to the movie 'Sahara' provides a good example as the camera slowly tracks from object to object in the room. Nonetheless, for the most part it hasn't been an issue of much concern to most viewers.

We home theater fans can be nit-picky perfectionists, though. For those who demand nothing short of flawless video performance, what's to be done? The solution is to restore the movie to its original 24 Hz playback speed. In the Standard-Def DVD world, this normally requires an expensive external video processor to employ a function called "reverse telecine" that removes the extraneous frames from the video. Things are a little bit easier on Blu-ray and HD DVD, fortunately. The majority of movies on both High Definition disc formats are natively encoded as 1080p24 video frames. For their standard 60 Hz video output, the disc players themselves employ 3:2 pulldown by multiplying some frames 3 times and others twice. However, certain players also offer a raw 1080p24 output setting that bypasses the 3:2 pulldown step and transmits the video over HDMI at its 24 Hz encoding rate (technically 23.97, but the difference is indistinguishable from the original 24 fps film speed).

In order to get this to work, the signal must be transmitted to a compatible television that can properly sync with the 24 Hz frame rate, or convert it to an even multiple such as 48 Hz, 72 Hz, 96 Hz, or 120 Hz. Most HDTVs will not accept a 1080p24 input signal at all, and even among those that will, some simply convert the signal back to 60 Hz by applying their own 3:2 pulldown and re-introducing the judder. In other words, even if you can get the 1080p24 output of the disc player to work, your TV may still not be able to benefit from any improvement it promises.

In a best case scenario, when a player that offers 1080p24 output transmits the signal over HDMI to an HDTV that can accept and sync with the signal, the improvement over standard 1080p60 will still be very subtle. You may have to strain to find it, or do comparison tests of certain movie scenes at both frame rates. If you don't have equipment that will support 1080p24 video and you've gone this far in your life without ever noticing 3:2 pulldown judder, this may one of those things better not to worry too much about.

1080p24 output is only beneficial to content originally photographed at 24 frames per second. Any material shot at a different frame rate, such as most video bonus features found on HD DVDs and Blu-rays, will look very poor if converted to 24 Hz, so be prepared to change the player back to one of the standard 60 Hz resolutions if the player doesn't offer a "Native" mode that will do that for you automatically.

It should also be noted that the 24 fps photographic speed is a fairly slow capture rate that has its own inherent jerkiness in many situations that will not go away with 1080p24 output. The best you can do is eliminate the specific 3:2 pulldown judder, restoring the picture back to whatever level of jerkiness is present in the source. That said, 1080p24 should provide generally smoother motion that sensitive viewers will find a welcome benefit, and it brings a video image one step closer to recreating the original photography.

If you still have questions about the benefits or workings of 1080p24 video, we've set up a dedicated thread for discussion in our forums area.

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