Explaining 1080p Video and 24-Frame-Per-Second (FPS) Questions
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By Adrienne Maxwell
As you peruse the features list or read a technical evaluation of a Blu-ray player, you may encounter phrases like "1080p/24 playback," "24p film mode," and the like. The ability to output 1080p/24 is one of Blu-ray's distinguishing features, but what does it actually mean?
The "1080p" in 1080p/24 refers to resolution. A 1080p resolution is equivalent to a 1920 x 1080 resolution, which means that each frame of the Blu-ray signal consists of 1,920 columns by 1,080 rows of pixels, for a total of 2,073,600 pixels. (Compare this to the 1,280 x 720 signal broadcast on channels like ABC and Fox, in which each frame contains 921,600 pixels.) 1080p is currently the highest resolution supported by most TVs and display devices. When you output a 1080p Blu-ray signal to a 1080p TV, the TV does not have to perform any scaling to tailor the signal to match its own resolution, which means one less processing step that could degrade picture quality. Pretty much every new Blu-ray player can output a 1080p signal through its HDMI output.
The "24" is where things get interesting for Blu-ray. Most films are shot at a speed of 24 frames per second. However, most televisions have a refresh rate of 60 hertz (the picture refreshes itself 60 times per second). Showing a 24fps film source on a 60Hz TV requires a conversion process called 3:2 or 2:3 pulldown, in which the first film frame is repeated twice, the second frame is repeated three times, the third frame twice, etc., to reach 60. The problem is, 3:2 pulldown introduces judder or a stuttering effect that is especially evident in slow-moving pans. Take a close look at a film being shown on TV, and you can see this jerky or stuttering quality in pans and other motion. This is something we've lived with for a long time, and many people may not even notice or care. But, suffice it to say, film purists aren't fond of it.
The DVD format adds 3:2 pulldown to film sources, but Blu-ray supports the ability to output film sources at their native speed of 24fps, with no 3:2 pulldown added. In fact, the native format of most Blu-ray films is 1080p/24. Many early Blu-ray players did not support 1080p/24 output, however; instead, these players added 3:2 pulldown and converted the signal to 1080p/60. Most newer Blu-ray models have added the option to output the native 1080p/24 signal. Of course, not all HDTVs accept a 1080p/24 signal, so new Blu-ray players continue to offer a 1080p/60 output mode as well.
While 1080p/24 is nice in theory, its tangible benefits are really dictated by your display device. If your 1080p HDTV has a 60Hz refresh rate, then the TV still has to add 3:2 pulldown to the 1080p/24 Blu-ray signal, so you're still going to see judder. The benefit comes if you own certain 120Hz TVs or perhaps a projector that supports a 48-, 96-, or 120Hz output. These numbers are all evenly divisible by 24. The display device takes in the 24fps film source and doubles, quadruples or quintuples each frame to get to its refresh rate, which produces less judder than traditional 3:2 pulldown.
Another benefit of 1080p/24 is in the processing realm. If you've purchased a Blu-ray player that doesn't have very good video processing, you may see moiré and other digital artifacts when watching Blu-ray films output at 1080p/60. In such cases, the ability to output the native 1080p/24 Blu-ray signal can come in handy, as it allows you to bypass the player's internal processing. This could result in a more pristine, higher-quality viewing experience if your HDTV has better video processing than the player.
Keywords
1080p/24, 24p film mode, Blu-ray, resolution, 1080p, 24 frames per second, 24fps, refresh rate, 3:2 pulldown, judder, 120Hz, video processing
