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Variable Frame Rate
The Panasonic VariCam is the first truly variable frame rate Digital Video Camera. In order to understand the process, it's helpful to think of the VariCam as two separate machines working together. The first machine is the image capturing device, the camera part. It involves three high-density, 16x9 chips that capture the light focused by the lens and converts that light information into electrical information (analog into digital). The second machine is the VTR (Video Tape Recorder) which records this digital information onto a magnetic tape. When the operator selects the frame rate of the VariCam, the camera alters the rate at which the "camera part" captures images. When one selects 24fps, for instance, the three chips are capturing a new frame 24 times a second. As a result, the image blur and relative exposure are similar to changing frame rates on film cameras (because the exposure time is increased or decreased). That's pretty straightforward. In fact, the concept is exactly the same as a film camera. The VTR part of the Panasonic VariCam, however, never changes speed. It's always laying down 60 frames every second, regardless of what rate is selected in the camera menu. Why? It doesn't need to change because the chip does it for you. In a film camera, we change the speed of the film through the gate, because it's analog—each frame records exactly what it sees as it passes through the gate. In order to over or undercrank, you change the speed at which the passes film through the gate. In the VariCam, the three chips are the recording device, taking the analog image and converting it into digital information. That information is then recorded onto the digital tape. So, if the tape is constantly recording 60fps, then how do we capture 24 frames? Simple. The VTR records the 24 frames (assuming 24fps is selected) plus 36 "filler" frames, identical to the originals, to make it all fit. While doing this, the VTR is also "flagging" the original 24 frames so the fillers can be thrown out in post leaving us with the original 24 frames per second. When another frame rate is selected, the VTR simply changes the number of filler frames, always flagging the true frames (i.e. 30fps=30 filler frames, 18fps=42 filler frames). Should we wish to undercrank, we could set the frame rate all the way down to 4fps. The chips begin making nice, long exposures (complete with motion blur and increased light sensitivity) and the VTR makes 15 copies of each of the 4 original frames to fit it into its 60 frames, flagging the originals. In post, 60 frames go in, only 4 come out, and when played back at 24fps environment you get the traditional film style undercrank. To overcrank for slow motion effects, we can set the camera up to 60fps, the chips capture 60 frames each second, and the VTR writes 60 completely unique progressive frames which can later be played back at 24fps. Voila Film style slow motion! No need for de-interlacing or otherwise fudging your footage because all the information is there, like with film. One result of this process is that, when the original tape from the camera is played back in the field, all 60 frames will be played, complete with both flagged and filler frames. Therefore, everything will appear in real time (until the filler frames are disposed of). You will, however, see the other effects of under/overcranking such as the blurriness and the exposure changes.
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