Frames and fields
I grew up in my professional career with Broadcast Engineering magazine. Its pages have always given me information on both the old and new technology. When reading the sidebar accompanying David Birdy's “High-def video cameras come of age” article in the January 2008 issue, I found that I had to point out some errors.
Beginning with paragraph two, 59.94 was not used to reduce flicker. The field rate was 60fps until color was adopted in 1953. Because of the 3.58MHz that was chosen for the subcarrier, and its relationship to horizontal and therefore vertical, a slight adjustment to the frame rate was made to use the chosen subcarrier. Those of us that have worked in analog for years and had ground loop problems remember the hum bars slowly rolling up the picture, a true sign that the frame rate did not match the 60Hz power frequency.
Paragraph three states that film was shot at 23.97fps for television. This is not true. Film has always been shot at 24fps. A motion picture theater plays back film at 24fps, but displays 48fps by presenting each frame twice to reduce flicker. A telecine converts 24fps to television rates by using 3:2 pulldown. The film is running at 23.976fps (speed adjusted for 59.94 field rate). The first frame of film is presented to three fields of video. The next frame of film is presented to two fields of video. The process then repeats.
David Birdy responds:
When I wrote “to comply with broadcast standards, film is shot at 23.97fps” that was written within the context of how a telecine machine works to provide a usable format for television broadcast. The film is “shot” with a video camera as part of the telecine process. I can see how that may be confusing on its own, but given the context of the paragraph, I hope most readers understood the camera “shooting” to be a video camera, not a film camera.
I stand by the statement that 59.94 will avoid picture flicker on an electrical system that runs at 60Hz.
A ground loop is shown as a hum bar on the television display. The reason the hum bar is present has nothing to do with a frame rate. The hum is present due to a ground differential — two AC devices on different power sources that travel to a ground at different times, thus creating a potential visible as a hum bar.
Thanks for your constructive comments. I see we are both longtime readers of Broadcast Engineering and enjoy the magazine very much.
IT/broadcast engineer gap
Do you have any suggestions on bridging the gap between IT employees and broadcast engineers?
John Luff responds:
That is a HUGE question. I have some clients who feel that if there are “networks” involved, video people should stay away. Others feel that if the network carries content, then the IT folks should have an advisory role only. I applaud the SBE certification that provides for a bridge to educate video professionals in network fundementals. Unfortunately, the curriculum to do the educating is hard to come by. I am working with NAB to see if there is a role it can play in doing that, so stay tuned!
I think the key is to get someone to help educate the management on the complexity of carrying video over networks, and then get them to buy into training personnel in the crossover technology areas.
Glad to hear you started a forum (http://community.broadcastengineering.com/forums). CompuServe had one years ago before computer users got dumbed down through icons. Thanks for existing!
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