Tech Tips:
24P, What does it mean?
By Andy DunnEveryone is talking about 24P. They say shooting 24P video looks like film. Well, who are “They” and what are “They” talking about?
No, 24P is not just the word for the day. It describes the number of frames captured every second by the camera. One frame is a complete representation of a single video image.
Ok, let�s take a step back for a moment. Video cameras have always captured video at approximately 60 fields every second. Hey, what happened to frames? Rather than capturing a single moment in time and using the whole image to represent one frame, NTSC video cameras always capture 2 snapshots (or moments in time) for each frame. The camera separates each snapshot into odd and even lines. It then makes a complete frame by combining the odd lines from one frame (the first) with the even lines from the next frame (the second field). This way, you can capture the scene nearly 60 times every second and display nearly 30 frames every second. This technique is called interlacing. When you show this footage on an NTSC monitor, it scans interlace as well. First it scans the odd lines from the first field, then it goes back and scans the even lines from the second field to create one frame. It will do this continuously to display every frame of your footage. You�ll notice I said approximately 60 fields and nearly 30 frames. This is because NTSC standards dictate that we must use 59.94 fields to make up 29.97 frames. Many people use the terms �30 frames per second� and �29.97 frames per second� to mean the same thing but you must understand that they are different.
An alternative used today to capture the moving images is to take 24 snapshots every second. Rather than capturing nearly 60 moments in time, you only capture 24 moments in time every second. This is exactly how film is usually shot. The film camera feeds 24 frames of film across the lens opening (through the gate) every second. These film frames are exposed when the lens shutter is open and each frame represents a captured moment in time. To best emulate film, new video cameras allow you to capture 24 snapshots or moments in time just like the film camera. Since the frames are not made up of interlaced fields but rather by complete images, we use the term progressive to express the difference. The camera has captured 24 progressive frames every second.
Different cameras record these frames differently. Since almost all monitors display interlaced fields, you eventually need to modify your captured footage to accommodate them.
The Sony F900 HD camera takes each of the 24 frames and segments them into odd and even lines. It then records the odd lines from the first frame followed by the even lines from the first frame. This allows the use of interlace technology with progressive footage. This differs from standard interlace because you have only captured one moment in time and have separated it into odd and even lines. With standard NTSC interlace, you actually combine the odd lines from one moment in time with the even lines from the next moment in time to build a frame. This camera records or outputs what is called 24 progressive segmented frames (assuming you captured at 24 frames per second). The Sony camera allows you to capture your footage at 24fps or 23.976fps. The latter is easy to convert to the nearly 30 frames of NTSC so, if you plan on distributing the footage as NTSC video, you should capture at 23.976fps.
The Panasonic Varicam converts the nearly 24 progressive frames captured by the camera into nearly 60 progressive frames before recording or outputting it. It does this by copying frames as necessary to fill the required number in one second. For 24p it makes 3 copies of the first frame, 2 copies of the second, 3 copies of the third frame, 2 copies of the fourth frame, and so on until it has nearly 60 frames to show in one second. I say nearly because when the Varicam captures at 24P, it is actually capturing 23.976 progressive frames every second. It then converts those frames into 59.94 progressive frames. This will make it easier to convert into NTSC later. You can change this default so that 24P really does mean 24P but it is not recommended since almost everyone will be converting to NTSC at some time.
The standard definition cameras such as the Panasonic SDX900 or the DVX100 series also capture at 23.97 frames per second when it says it is capturing at 24 frames per second. It then converts the footage to 59.94 interlaced fields before recording or outputting. Since this is NTSC, no additional changes will need to be made later in order to display the footage on standard NTSC monitors.
