October 05, 2003
Shooting 24p and Editing 24fps
I recently worked on a project where the Director of Photography had a Panasonic DVX-100. We shot the short using 24p (24 progressive frames per second as opposed to 29.97 interlaced). This gave me the opportunity to learn a lot about shooting on 24p and how DV records this information. I scavenged the internet and the information is bottomless. But there wasn't one web page that tied everythnig together for me. I've attempted to put all the information that I wish had been on one page... on one page.
I've attached links to the sites where I have gotten my information and where I have gotten the diagrams from. They are all excellent sites! Please follow the attached links for more information and visit their sites.
If you have any corrections to this posting, please reply in the comments section.
How Interlacing works
First, how a TV works. Most television sets and monitors have a phosphor coating on the front of the TV. An image is produced by a Cathode Ray Tube which focuses electrons into a beam and scans down the front of the monitor. It starts at the top and works it's way down. When the electrons hit the phosphor, light is emitted. But only for a moment. The phosphors holds the light and it slowly starts to fade out. Your eyes do the same thing. Think of a time when a bright light hit your eyes, like a camera flash bulb. You were blinded for a time until they adjusted to the light. The phosphors at the front of the television works the same way.
Cathode Ray Tube(CRT)
Because of this delay time when the phospor's light is decaying, if a television scanned every line, the first line that was scanned would fade out before the CRT could scan down all the lines in the TV and scan the first line again. This causes noticeable flicker. The monitors and TV's of today may be able to get around this because the refresh rate (the amount of time it takes for the CRT to scan down the TV screen) is fast enough that the first line hasn't had enough time to decay. But when TV's first came out in the 1950's the refresh rate wasn't very fast. They were limited to 30 progressive frames per second. A television station has a certain amount of bandwidth to broadcast a show.
The bandwidth is limited to 6MHz a second. There are 480 horizontal lines in a TV program, scanning these lines progressively limits the number of pictures sent to 30 progressive frames. The eye needs anywhere from 40 to 60 pictures a second, otherwise flicker will be noticable.
To get around this, frames were Interlaced The number of lines in a TV screen were split by half. And scan twice as fast. So the CTR scans the first line in a screen, then the 3rd then the 5th... it then goes back up and scans the 2nd, 4th, 6th. It does this 60 times a second. Two times for every frame. Two fields for each frame. An odd field (1st line, 3rd line) and an even field (2nd line, 4th line). This method produced a better picture on a television than scanning a complete 30 frames in a second. Because the number of lines being broadcast is now 240, 60 images can be sent in a second.
www.NTSC-TV.com
http://www.netbored.com/classroom/Proscanexplained.htm
So why does American TV operate at 30-fps instead of 25-fps? In America we use an Alternating Current that operates at 60 Hz in our electrical system. It was easy for the TV companies to develop a standard based on this cycle. NTSC was born! (National Standards Television Committee). Because of the 60Hz alternating current used in our power grid, it was easy to adapt the 30-fps video in an interlaced format because 30 frames splits up to 60i (interlaced fields). This also reduced the bandwidth requirements for sending out TV shows.
The difference between progressive and interlacing
When we watch a film, it is capturing entire pictures. Film runs at 24-fps, each frame is a complete picture. So how do we get 24 complete pictures a second to fit into the TV's 30-fps or 60i (interlaced) fields a second. By a process called Telecine.
Let's start with the interlacing. Each film picture is split into 2 interlaced fields.
| Full frame | First field or half frame |
 |  |
http://nickyguides.digital-digest.com/interlace.htm
That will give us 48 fields (2 x 24).
Now we need to stretch out 24 progressive frames into 30 interlaced frames. Or stretch out 48 fields to 60 fields. For every 24 frames that we have on the film, they need to be stretched out to 30 frames. This can be shown as a ratio, 24/30. Which can be broken down to 4/5. For every 4 frames of film we need to stretch it out to 5 frames. Each frame has two fields, so it could also be said for every 8 fields we need to stretch it to 10 fields.
To get the 4 frames to 5 frames we run the frames through a process called pulldown. There are 2 pulldown cadences that I'm aware of, 3:2 (which is sometimes expressed as 2:3) and 2:3:3:2. The 3:2 pulldown is what is used by most cameras and telecine processes that convert films for TV. So we will focus on the 3:2 pulldown and explain the 2:3:3:2 pulldown later.
The numbers 3:2 are describing the number of interlaced fields that are being 'pulled' from the original frame.
Let's say you have 4 frames
1 2 3 4
The first step is to interlace them into two separate fields:
11 22 33 44
Now we run the 3:2 pulldown on those four frames and come out with 5 frames!
11 12 23 33 44
http://nickyguides.digital-digest.com/interlace.htm
The T & B stand for Top fields (odd lines 1,3,5) and Bottom fields (even lines, 2, 4, 6) and the (r) stands for a field that has been repeated. You'll notice that for some frames, they now share a half of completely different frames!
11 12 23 33 44
This isn't very noticeable, unless you are capturing something that is moving horizontally or if you are changing a scene.
http://nickyguides.digital-digest.com/interlace.htm
(This example has used letters to dictate the frame. So
11 12 23 33 44 is the same as AA AB BC CC DD)
The next picture shows what happens to a moving object. The yo-you on the left was captured interlaced and the yo-yo on the right was captured using progressive.
http://www.pcmag.com/article2/0,4149,1172157,00.asp
Here is a great example of showing the difference between progressive and interlaced during playback. (If these images are out of sync, refresh the page.)
| Progressive | Interlaced |
 |  |
http://www.aluminumstudios.com/digitalvideo/advanced/interlaced/interlaced.html
Right now you are thinking, why would anyone capture interlaced when you can capture progressive? Well that's were the plot thickens. DV cameras are using the same alternating current from the battery or AC adaptor to capture a progressive image. When a camera says that it is progressive, it is only capturing a full frame when SHOOTING! It then records the image to the DV tape, which is NTSC based. So it has to do a pulldown from the progressive scanned image to an interlaced image. Even though you are capturing progressive, when it gets to the tape it has undergone the 3:2 pulldown. When you capture the footage to your editing system, you have to capture it as 29.97-fps interlaced. This means your beautiful progressive image is now in this state:
http://nickyguides.digital-digest.com/interlace.htm
Which should be fine, unless you cut out the first two frames of the example and start your clip on frame 3. Suddenly you have a ghost image of frame 2 in your clip!
But fear not, there is a way to return your beautiful progressive image back to it's newborn state. It's called inverse telecine, or reverse telecine.
http://www.netbored.com/classroom/Proscanexplained.htm
You can then edit your footage in 24-fps. (It's actually 23.98-fps and I'll be using 23.98-fps for the remainder of this entry.)
If you have CinemaTools you can run this reverse telecine. If you bought Final Cut Pro 4, then CinemaTools came bundled with the application.
You should be aware though, that unless you are outputting to film or the web, you will have to do a 3:2 pulldown to re-interlace your project. Anything viewed on a conventional TV from a network or VHS or DVD must be interlaced and in 29.97-fps. There are some DVD players out there that can play back pure progressive footage, but you must have a TV that can scan progressively. And even if you have a progressive DVD player with a progressive TV you will still be watching your project at 60 fps. Only it won't be interlaced. This is because to avoid flicker the eye needs from 40 to 60 images per second. Why don't we see flicker in the movies if it is showing at 24-fps? Because there is a small rotor in the projector that is showing every frame 2 times a second. When we sit in a theater we are seeing 24 images, each image is doubled. Because you are sitting in a darkened theatre, the eye doesn't notice the flicker. But for a Television, in a lit room, the refresh rate needs to be a little higher. A progressive DVD player with a progressive scan TV takes the 3:2 interlaced pulldown, revearse telicine's it, and creates full progressive frames. It will show 10 progressive frames for every 4 progressive frames from the original film. It is still showing the same pattern of the 3:2 interlaced pulldown, only each image is a full image and not interlaced.
http://www.netbored.com/classroom/Proscanexplained.htm
At some point progressive DVD's and TV's will arrive that can scan 72 progressive images, which will give equal time for each frame. And you can watch films in the comfort of your own home as God intended. But I digress... back to our regularly scheduled program...
So now you are asking, what's the point of shooting progressive if it is going to be interlaced in the final media?
Because of the way it is shot. Shooting 24 full pictures is going to capture motion better than shooting 60 interlaced fields. There will be less artifacts in your footage and less combing. The motion will be blurred instead of interlaced.
| Interlaced | Progressive | |
 |  |
However, shooting 24p will give you some stutter (for those of us who can notice) because you are doing the 3:2 pulldown. You are repeating fields when you go through the 3:2 pulldown. Each frame does not appear for the same amount of time. Back to our numbers:
11 12 23 33 44
Frames 1 & 3 show an extra 1/60th of a second longer than frames 2 & 4. I wouldn't worry about this too much. We watch movies on DVD and it is undergoing the same 3:2 pulldown to display it on your TV. I don't really notice it.
So the main reason to shoot 24p is the way that it captures the image. It will be interlaced when it is recorded onto tape, and interlaced in your final output unless you are transferring direct to film or onto the internet.
The beauty of the Panasonic DVX-100 and the Advanced Pulldown (2:3:3:2)
What's so special about the DVX-100? Some genius out there developed a different cadence for the pulldown method. Instead of pulling down 3 fields from a frame, and then 2 and then 3. The camera can be set to Advanced Pulldown mode where it pulls down 2 fields from the first frame, then 3 fields from the second then 3 fields from the third and then 2 fields. Why is this so cool? Read on.
Here's our original 4 frames of progressively shot video, split into 2 fields for each frame:
11 22 33 44
In the 3:2 pulldown (which can be expressed 3:2:3:2) it converts these 4 frames into 5:
11 12 23 33 44
http://www.adamwilt.com/24p/#24pRecording
In the 2:3:3:2 pulldown it converts the 4 frames into 5 like so:
11 22 23 33 44
This in and of itself isn't so spectacular, and doesn't really look that great. You have the 2 longest frames (frame 2 & 3) right next to each other. This will cause a lot of stuttering in the image. But, if you want to reverse telecine it, if you want to convert this to a 23.98 editing timebase, if you want to go back to the original progressive images, all you do is remove frame 3!
11 22 23 33 44
converts to
11 22 33 44
http://www.adamwilt.com/24p/#24pRecording
Remember how much of a pain it was to reverse telecine the 3:2 pulldown! You need a special program to untangle it! But this is so easy that you could do it yourself! If you captured 24pa (Progressive with Advanced Pulldown) onto your computer you could remove this yourself by deleting every 3rd frame! In fact, this is so easy that Final Cut Pro 4 has included this as a capture preset, saving you the trouble of deleting every 3rd frame! You don't need CinimaTools to reverse telecine!
Stepping through the process
There's been a lot of talk, now for the cold hard steps for each of the methods you may want to shoot in; 29.97 NTSC (Interlaced), 24pn (Progressive Normal 3:2 pulldown) & 24pa (Progressive Advanced 2:3:3:2 pulldown).
Shooting 29.97 NTSC
- Final Cut Pro -> Audio/Video Settings -> Sequence Preset
- Plack Checkmark on DV NTSC 48 kHz
- Final Cut Pro -> Audio/Video Settings -> Capture Preset
- Place Checkmark on DV NTSC 24 kHz
- Capture footage, edit and output
Nothing too special about that one. Now for the juice...
Shooting on 24pn (progressive normal with 3:2 pulldown)
- Final Cut Pro -> Audio/Video Settings -> Sequence Preset
- Plack Checkmark on DV NTSC 48 kHz - 23.98
I may have created this one but can't remember. If that Sequence Preset doesn't already exist:
- Select DV NTSC 48 kHz
- Click the edit button
- Rename Sequence to DV NTSC 48 kHz - 23.98
- Change the editing timebase to 23.98
- Click the save button
- Make sure there is a Checkmark next to the sequence preset
We are still capturing it as interlaced and we will use CinemaTools to remove the pulldown
- The Open Database Dialog appears - hit cancel
- In the Menu -> File -> Batch Reverse Telecine
- Browse to where you captured your footage (Capture Scratch directory)
- Select a file that resides inside your capture scratch folder & click the choose button
- Take a break while it processes
- When it is done, close CinemaTools
- Open Final Cut Pro
- In the Menu -> File -> Import -> Folder
- Browse to your capture scratch folder. There should be a folder there that CinemaTools created called Reversed. Choose this folder.
- You are now editing in 23.98 progressive frames!
I've heard that CinemaTools will clip your whites when it process the pulldown. If your not too concerned about this then it's a great tool to reverse the telecine.
http://www.adamwilt.com/24p/#24pRecording
Outputting back out to tape is a little different. I'll explain below after going through the steps for capturing 24pa.
Shooting on 24pa (progressive advanced with 2:3:3:2 pulldown)
- Final Cut Pro -> Audio/Video Settings -> Sequence Preset
- Plack Checkmark on DV NTSC 48 kHz - 23.98
I may have created this one but can't remember. If that Sequence Preset doesn't already exist:
- Select DV NTSC 48 kHz
- Click the edit button
- Rename Sequence to DV NTSC 48 kHz - 23.98
- Change the editing timebase to 23.98
- Click the save button
- Make sure there is a Checkmark next to the sequence preset
- Place Checkmark on DV NTSC 48 kHz Advanced (2:3:3:2) Pulldown Removal
- Click on the edit button
- Notice the frame rate is set to 29.97! This is correct! The source material was recorded to the DV tape as 29.97!
- Make sure there is a checkmark in the Remove Advanced Pulldown (2:3:3:2) From DV-25 and DV-50 Sources!
- Click on the OK button.
That's it! You are now editing progressive frames at 23.98!
Viewing your project through a NTSC monitor & outputting to tape
In order for you to view your project through an external monitor, you'll need to make sure the playback settings in Final Cut Pro are performing the 3:2 pulldown to interlace the progressive video.
In the Menu -> Final Cut Pro -> System Settings -> Playback Control -> Set the Pulldown Pattern to 2:3:2:3.
This is the same as the 3:2 pulldown because the 3:2 pulldown is really pulling down 3:2:3:2:3:2... The 2:3:2:3 pattern is starting with the 2 instead of the 3.
When you are ready to export your project to tape, export as you normally would and make sure your pulldown pattern is the 2:3:2:3 pulldown and the Quality is set to full.
Conclusion
Using the 24 progressive advanced pulldown is not intended for viewing on NTSC! The placing of the interlaced frames creates a great deal of stuttering because it puts the longest frames (each 3 fields) together.
11 22 23 33 44
The 3:2 pulldown is a standard pulldown for viewing progressively shot frames on NTSC because it alternates the 3 field frames.
11 12 23 33 44
Do not shoot in 24pa mode unless you are going to remove the 2:3:3:2 pulldown and edit in 23.98-fps. If you are going to be editing in 29.97-fps shoot in 60i or 24pn. The 60i will give you a nice fluid image capture but will capture motion interlaced. If you want the progressive look for motion (remeber, the final output will be interlaced) but will be editing in 29.97fps the use the 24pn. This will capture images progressively, and the 3:2 pulldown drags the interlaced images out better for 29.97-fps than the 24pa.
If you don't have a Panasonic DVX-100 and want to shoot progressive frames and want to remove the interlacing and edit in a 23.98-fps timebase, use CinemaTools to remove the pulldown and edit in 23.98-fps. If you don't have CinemaTools, I'd stick with shooting 29.97 NTSC. The frames will be spread out evenly and you won't have any weird ghosting images in your final footage.
Have Fun!
---Other Links:
http://www.100fps.com/
http://www.grand-illusions.com/percept.htm
http://www.uca.edu/org/ccsmi/ccsmi/classicwork/Myth%20Revisited.htm
http://www.wikipedia.org/wiki/Persistence_of_vision
--Bibliography
Cathode Ray Tube(CRT)
Interlaced
www.NTSC-TV.com
http://www.netbored.com/classroom/Proscanexplained.htm
Alternating Current
NTSC
http://nickyguides.digital-digest.com/interlace.htm
http://www.aluminumstudios.com/digitalvideo/advanced/interlaced/interlaced.html
http://www.student.virginia.edu/~fms-uva/professional/
http://www.adamwilt.com/24p/#24pRecording
CinimaTools
Final Cut Pro 4
http://www.pcmag.com/article2/0,4149,1172157,00.asp