Explained: What does 1080p/24 actually mean?

One of the problems of a technology like high-definition evolving at such a rapid rate is the constant changing of standards and the terminology that accompanies them.

First we had 1080i and 720p HD video formats and then we moved to so-called Full HD or 1080p, which provides the highest resolution using progressive scan as used by both Blu-ray and HD-DVD players.

But the confusion doesn’t end there as 1080p can now be distinguished between two formats – 1080p/60 and the latest 1080p/24. The final numbers refer to the frame rate that each one uses, but what does it all mean?

The easiest way to explain this is by understanding how movies are shot and then adapted to video. Theatrical films are usually shot on 35mm film running a frame rate of 24 frames per second, which means that when they are projected back at the same rate, images appear with natural movement.

In contrast, some media such as NTSC video runs at a rate of 60 frames per second. For consistency, US HDTVs continue to use a 60Hz rate either as interlaced or 60 complete frames if using a television with progressive scan. US broadcasts are still transmitted at 60Hz and early high-definition disc players of both formats output video at the same rate.

Like with movies, video shot specifically at 60Hz and then played back at the same rate appears perfectly natural and accurate – so the key to cohesive movement is that the display playback speed must match the original.

The problem arises when transferring movies shot in 24fps to 60Hz video. Speeding up the picture to match the faster frame rate can cause distracting sound and visual changes, which obviously isn’t the best solution.

To overcome this, a process called ‘3:2 pulldown’ was developed. This takes the original 24fps movie frames and alternates them in a pattern of 2s and 3s – first frame is displayed three times, the second frame twice, the third frame three times and so on. This repetitive sequence basically stretches four movie frames into 10 video frames, allowing the original 24fps content to be played at 60Hz without appearing sped up.

However, the problem with 3:2 pulldown is that it can cause picture judder or jerkiness in the movement. The effect is negligible and most viewers never notice it as it’s usually only slow camera pans that visibly suffer.

So, the only solution for home cinema perfectionists is to restore the video to its original 24Hz playback rate. This would be timely and expensive withstandard-definition content but for high-definition formats like Blu-ray and HD-DVD it’s considerately easier.

This is because most high-definition films are naturally encoded as 1080p/24 video frames and players use their own 3:2 pulldown for their standard 60Hx video output. But now some players also offer a raw 1080p/24 output that ignores 3:2 pulldown and transmits video over HDMI using 24Hz (it’s actually 23.97 but it’s impossible to tell the difference).

For this to work, the signal must be transmitted to a television that can properly synch with the 24Hz frame rate, or convert it to an even multiple such as 42Hz, 72Hz, 96Hz, or 120Hz. Not all televisions are compatible so even if your high-definition player can output 1080p/24, your television might not be able to display the benefits.

And, in most cases, even if you have a television that is compatible with the 1080p/24 signal, the improvement over 1080p/60 is very subtle – so it’s not an essential specification. Also, 1080p/24 only improves content that was originally shot at 24fps and that doesn’t include extras which can look extremely poor if converted – so you may have to change your player back to its standard rate unless it does it automatically.

But, if you’re a perfactionist viewer who demands to see the film as it was originally intended then 1080p/24 should provide generally smoother motion without any audio defects.

Source
High-Def Digest

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