April 18, 2005
24p in Final Cut Pro HD and the DVX 100
By Graeme Nattress
24p can be a very complex subject. 24p means a lot more than making video run at the same frame rate as film, so in this article, I will first provide a technical background to 24p, and then look at the practical considerations of using a 24p DV camera such as the Panasonic DVX100 in Final Cut Pro.
What is 24p
24p is more than just the latest “buzzword” in indie-filmmaking and video production. It’s a specification for shooting video with 23.98 progressive frames per second, instead of normal NTSC’s 29.97 frames per second, where each frame is made up of two interlaced fields, giving a total of 59.94 interlaced fields per second.
Fields and Frames, Interlaced and Progressive
Right from the very first commercial television systems, the video signal that was broadcast has been interlaced. Interlace is in effect an early form of signal compression that gives about 70% of the vertical resolution of a progressive system, and the same smooth motion, but at 50% of the bandwidth. Interlacing works by instead of the camera taking, say, 60 full frames of video per second, it takes 60 “half-frames” or fields, as they are called, per second, each representing alternating horizontal rows of picture information. This works through the persistence of vision and the high speed at which the interlacing occurs to allow us to see smooth motion at a good level of detail. Because of the nature of interlaced video, it is important that the camera softens the image vertically to stop what is called “line twitter” where thin horizontal lines will flicker annoyingly.
Progressive scanning is not a completely new phenomena as some of the very earliest television systems were progressively scanned. Progressive scanning is where the entire video frame is taken in one go, rather than in two interlaced fields. Progressive scanning has many advantages in that we do not have “line twitter” and to worry about “field ordering” and other complications. However, for equivalent smoothness of motion, you need twice as much bandwidth to broadcast a progressive signal, although, when played back on progressive equipment, the resolution would also be slightly higher than the interlaced equivalent. Also, for any kind of post production work, interlacing and interlace artifacts can get in the way of creating a seamless effect. It’s not impossible to do great effects on interlaced video, but extra care is needed.
In the 60p diagram above, we see that each frame has all the lines
that make up the picture being captured by the camera for at once.
|24p||23.98 progressive frames per second|
|50i||50 interlaced fields per second|
|25p||25 progressive frames per second|
|60i||59.94 interlaced fields per second|
|30p||29.97 progressive frames per second|
25 frames per second
To convert from film to video, the film is sped up by about 4% and then matched frame per frame to video. Sometimes this is called 2:2 pulldown because each frame of film gets matched to 2 fields (1 frame) or video.
29.97 frames per second
A method called 3:2 pulldown (sometimes called 2:3 pulldown or even 2:3:2:3 pulldown) is used to convert the film to video. This method is based around the fact that 60 divided by 24 is 2.5. That means that each frame of film needs to last for 2.5 fields of video. First the film is slowed ever-so-slightly to 23.98 frames per second to account for NTSC video not being precisely 30 frames per second. Obviously, we can’t have half fields, so instead we alternate between frames of film lasting 3 or 2 fields in duration, hence averaging out to be the necessary 2.5 fields. This pulldown method has a very characteristic look that NTSC television viewers are very familiar with, just as Europeans are used to the 4% speed up, which until modern audio technology necessitated a pitch change.
In Europe, film that was directly destined for television as also often shot at 25fps to match the video rate precisely and thus not incur the 4% speed up.
Better picture quality
When “normal” 2:3:2:3 pulldown is removed (in Cinema Tools, say) the DV video needs to be decompressed so that the pulldown fields can be extracted. Final Cut Pro can remove the “advanced” 2:3:3:2 pulldown without decompressing the video, and hence it’s “in camera” quality can be maintained.
Cinema Tools can remove “normal” 2:3:2:3 pulldown, but this is not automatic and cannot be done inside Final Cut Pro.
Less System Demands
Once the pulldown is removed, the data-rate of the remaining video is reduced from 25mbits to about 20mbits, and any rendering only needs to be done on 24 frames instead of 30, meaning that you’re going to render faster for the same time length of video.
After editing video at 24p, it can be turned directly into a 24p DVD which allows you to achieve a higher quality end result as the MPEG2 compression has less frames to compress per second, or you can add 3:2 pulldown back to the video to allow it to be delivered on conventional video tape.
When shooting in 24p Normal, the camera is adding normal standard 3:2 pulldown to the video, which results in 24p footage designed to work with any non-linear editing suite and it will play back and look good directly to any NTSC monitor. You can use 24p Normal footage just like normal video from any DV camera, and everything will work fine, but obviously, the footage will have a film look to it. If you’re just going straight back to NTSC video tape, then using 24p Normal is the simplest, easiest workflow. No special treatment of the footage is needed and you really can just edit as normal.
Before you shoot 24p Advanced, you should fully understand it’s workflow implications. If you watch 24p Advanced footage before you’ve removed it’s pulldown, then it will look a bit jumpy and jerky. This is totally correct, because 24p Advanced is not designed to be viewed as is.
To use 24p Advanced and gain all it’s advantages, you should know that you cannot edit it as is (as this would leave it’s jerky looking pulldown intact), but you must first remove it’s pulldown. Final Cut Pro will do this for you, leaving you with the 24p footage without any of the extra “padding” fields that are added to make it’s frame rate 29.97fps. Now that your footage is 23.98fps, it must be edited on a 23.98fps timeline, and this can cause problems if you, for instance, want to include other footage, B-Roll, or stock footage, that comes from a different source. However, once you have your finished edit at 23.98fps, you can make a 24p DVD, which will allow you to compress your MPEG2 less than if you were making a normal 29.97fps NTSC DVD, and hence attain higher picture quality. Similarly, if you’re making a web movie, you will find it easier to get a higher quality result from 23.98fps media than normal NTSC media. 23.98fps movies are also easier to take out to film that 29.97fps movies.
If you are editing in a 23.98fps timeline, Final Cut Pro will add pulldown on the fly, over Firewire, so that you can see your movie on a normal NTSC monitor. Similarly, it will add pulldown when going back to DV tape. However, this will not work on a non-DV format output, say to Digital Betacam, and slower Macintoshes do not have the power to add 3:2 pulldown in realtime, falling back to lower quality pulldowns which although are not too bad while editing, will not make the final project look as acceptably good on television as a final product.
Just some of Final Cut Pro’s Easy Setups, which are accessed from the Final Cut Pro Menu.
24p Normal - Use DV-NTSC Easy Preset
Because 24p Normal uses “normal” 3:2 pulldown to embed the 24p in a normal NTSC DV video, you can capture and edit just as normal DV NTSC.
24p Advanced - Use DV-NTSC 24p (23.98) Advanced Pulldown Removal to Capture
Although we are capturing 24p footage to edit on a 24p timeline, we must remember that the “extra” pulldown frames were flagged by the camera as they were recorded, so Final Cut Pro must capture all the 29.97 frames per second over Firewire so that it can make sure that it sees and removes all the flagged frames correctly. Using this easy preset will leave you with 24p DV media.
24p Advanced - Use DV-NTSC 24p (23.98) to Edit
If you are not capturing footage, but want an easy preset that makes a NTSC DV 24p sequence, then this is the one to pick. Also, if you shot 24p normal and removed the pulldown in Cinema Tools, then this is the option to pick in Final Cut Pro for your easy setup.
None of the above 24p Easy Presets, have anamorphic widescreen variants. If you are shooting with the Canon XL2 for instance, which will shoot true widescreen and 24p, you will have to either create your own preset from the appropriate Easy Preset, adding in a tick in the Anamorphic checkbox, or modify your capture and timeline preferences to anamorphic afterwards. If you capture anamorphic widescreen video without the anamorphic checkbox checked, all is not lost as you can just open the movie properties in Final Cut Pro and set the video to be anamorphic.
Selecting the appropriate pulldown pattern for 24p playback.
2:3:2:3 This offers the highest quality of motion and is the choice to use for making a viewing master. It also requires the highest amount of processing power. This is the same as “Normal” pulldown.
2:3:3:2 This is the same pattern as used by “Advanced” pulldown on the DVX100. This does not offer the same quality of motion as 3:2, but uses less processor power, and also the highest quality if you were to recapture from a tape made using this mode and remove the pulldown. This mode would be the one to use if you were to make a 24p master tape with the intention that if ever you were to go back to the tape and recapture into Final Cut Pro you would be removing the pulldown to use the edit in a 24p project.
2:2:2:4 This is the lowest quality pulldown mode and is really only for use for viewing 24p on a slow Final Cut Pro System, or for maximizing real time playback of effects while editing.
One of the best ways to deliver your 24p DV project is on a 24p DVD. Because the DVD specification allows for the 23.98fps frame rate, we can produce a 24p DVD without having to add pulldown. Because a 24p DVD will have 20% less frames on it than the equivalent 24p media encoded with the addition of 3:2 pulldown, a 24p DVD should allow you to compress the media less and either produce a better quality picture or allow you to fit more high quality video on a DVD. The DVD player itself will add 3:2 pulldown on playback of a 24p DVD so that it produces a standard NTSC video signal for viewing.
When making a 24p DVD, it is important to do the compression to MPEG2 the correct way. If you export direct to MPEG2 from Final Cut Pro using the Quicktime MPEG2 compressor or bring the 24p media into DVD Studio Pro to compress, they will not treat the 24p media correctly and not produce a 24p DVD.
The best workflow for making a 24p DVD is to export your 24p movie from Final Cut Pro: File -> Export -> Quicktime Movie...
In the Export Dialogue box, leave “Setting” as “Current Settings”. You may, if you wish, “Make Movie Self-Contained”, but this will make a large file on disc.
Now, in Compressor you can select from any of the available MPEG2 presets. Pick the one suitable for your project length and whether it is widescreen or not. All of the MPEG2 presets in Compressor do not alter the frame rate of the media. This means that if you put in normal 29.97fps NTSC media to be compressed, it will leave the MPEG2 at 29.97fps. Similarly, if you put in PAL media at 25fps, the end result will be 25fps, and most importantly for us, compressing 23.98fps media will result in a 23.98fp MPEG2 file for importing into DVD Studio Pro.
24p for Web
|Graeme Nattress is the software developer at Nattress Productions Inc. where he enjoys researching cutting edge algorithms for the improvement of video quality. He is a frequent contributor to the kenstone.net, LAFCPUG, 2-Pop and Creative Cow websites and forums.|
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