Frame Rates

North America and many other countries of the world use a television delivery system that has 30 television frames per second, each comprised of 525 lines. Motion picture film for theatrical or television display is usually photographed at 24 frames per second.

This difference in frame rates is the predominant difficulty in photographing television sets as part of a scene. The artifact that is most visible is the appearance of horizontal bars on the photographed TV image. This is caused by double exposure of some parts of the television screen. To understand what causes this double exposure and the horizontal bars it is necessary to understand several other facts about the television signal.

With 30 frame television there are 525 lines scanned each 30th of a second. But to avoid flicker in the display a method is used that is somewhat analogous to the two bladed shutter in the film projector. This 30th of a second television frame is further divided into two television fields. Each of these television fields lasts for a 60th of a second. The displayed television image is "refreshed" or scanned now at 60 times per second and the result is no flicker. This is accomplished by starting the scanning beam (a single point of light or energy) in the upper left corner of the television screen and moving it left to right a single line at a time. When this beam of light reaches the right side of the screen, it jumps back to the left side of the screen during a period when it has been "blanked" or turned off. This is called the horizontal blanking period. This occurs every television line or 525 times per television frame.

In order to provide the refresh rate of 60 times per second, this beam skips every other line of the 525 lines that comprise a television frame of a 30th of a second. In other words, the scanning beam scans line 1, skips over the position that would be occupied by line 2 and scans line 3. This continues to the bottom of the TV image until all of the odd-numbered TV lines have been scanned.

At this point a 60th of a second has passed. The scanning beam is now at the lower right corner of the screen. The beam is "blanked" and is moved to the upper left corner again — ready to start scanning again. This time period of the beam moving from the lower right corner to the upper left corner is called the vertical blanking period or vertical interval. This happens 60 times per second — twice per television frame. This scanning beam now starts its scanriing process over one line at a time, but during this 60th of a second the beam is positioned to scan lines 2,4, et cetera — all the even-numbered lines are now scanned.

Now let us look at how the film camera views this television image. The camera that is chosen for this example has a 180-degree shutter. If we run this camera at 30 frames per second with a shutter opening of 180 degrees, the camera is exposing the film every 60th of a second. From the television scanning explanation above it can be observed that the film camera is "blind" to one of the television fields and is only photographing half of the 525 lines that occur in a television frame. The resulting TV screen image on the film will be good (with no "shutter bars") because the film camera and the television scanning are occurring at the same frame rate. When the film camera and the television system are operating at different frame rates the result is double exposure to portions of the television screen image.

Best results are obtained when the shutter opening coincides with the beginning of the scanning of one of the two television fields. In other words, the shutter is open for only one complete television field — not part of one field and part of the next field. In order for this precise phasing (shutter open vs. closed) to occur, external specialized equipment is used in conjunction with the film camera and the video equipment.

There are four combinations of film rates and television rates that are possible. These are outlined below:

1. 30 Frame Video and 30 Frame Film: This combination features standard NTSC 30 frame video (US Standard) and the film camera also operating at 30 frames. This approach is appropriate if the film is going to be used for a 30 frame per second telecine transfer, but if used for 24 frame projection there will be a 20% "overcrank," and if there is sound the pitch will be altered. Any US television monitor can be used. Shutter phasing and synchronization are required and the camera shutter angle is optimum at 180 degrees.

2. 25 Frame Video and 25 Frame Film: This requires the video signal to be the European PAL-625 line system and also the VTR and monitor to be capable of operation on this standard. If the film shot is projected at 24 frames there is only a 4% "overcrank," and the sound pitch change is usually considered undetectable except to musicians. Shutter phasing and synchronization are required and 180 degrees is the preferred shutter angle. This is the system that is chosen for most TV monitor filming in Europe and much of the rest of the world that operates on 50 Hertz power.

3. 30 Frame Video and 24 Frame Film: This features standard 30 frame NTSC video and a camera specially designed to have a fixed 144 degree shutter or a camera whose shutter can be precisely set to 144 degrees. This specific shutter angle allows the film camera to only photograph one set of scan lines per film frame but is extremely difficult to adjust. Anything mechanical that causes the camera to vary in speed or cause drag on the shutter will result in inconsistent results. Also camera panning and zooming will cause portions of the TV image to be double-exposed or not exposed at all, resulting in small black or white bars to be present in the TV image. Again, shutter phasing and synchronization are required and a very precise 144 degree shutter angle must be maintained.

4.24 Frame Video and 24 Frame Film: This video/film combination requires a specialized video format, but the film camera is run at a standard speed and the resulting film is standard in all ways. The choice of shutter angle should be 180 degrees and there is a one-to-one relationship between TV frames and the preferred film rate of 24 frames. Shutter phasing to the TV signal should be used. Most TV sets and monitors can be adjusted to operate at this 24 frame rate, but caution should be used with an unknown model. Live video cameras and computers have been modified to run at this 24 frames, offering a wide choice of source material.

24 Frame video was first used for feature production in about 1960. Since that time steady progress has been made in sophistication and choice of the tools for this one-to-one relationship with 24 frame film. Because 24 frame video is a modification of standard NTSC television equipment, the TV image has the same scanning frequencies as 525 line television. This results in the 24 frame image having a total of 655 television scan lines per 24th of a second. Thus, the precise vertical scan rate or frame rate of the television signal is actually 24.01 frames per second.

The synchronization between the film camera and the video system can be achieved in two ways. This is the shutter phasing that was referred to above. The first method is to obtain a shutter signal from the film camera and have the video system follow the film camera. This allows the film camera to operate on its internal crystal and to "pull down" the video system to exactly 24 frames. With this method no connection is made to the sound recorder. The disadvantage of this method is that the video source is limited to videocassette playback. In recent years this method is almost never used. One major drawback is that only one film camera can be rolling simultaneously.

The second mode of operation is tire preferred method and offers the greatest flexibility of operation. In this mode the film camera is driven by a signal from video/film camera synchronization equipment. A signal is still received back from the film camera, used to phase the camera shutter opening to the TV signal scanning. A major advantage of this method is that any number of film cameras can be operating in sync and the choice of 24 frame signal sources is unlimited. As the film and television equipment are operating at a slightly higher frequency (24.01 frames per second), a 60.02 hertz frequency should be sent to the sound recorder to keep the sound in sync on long takes. Without this signal the sound will fall behind the picture about one frame every 45 seconds.

Both the above modes of operation can accommodate process or rear screen projection with the appropriate connections.

No attempt will be made here to describe the equipment available to synchronize the film and video equipment. This equipment is constantly changing and is avail able from many camera manufacturers and specialists in the field of video playback for film shooting.

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