News broadcasting is the broadcasting of various new events and other information via television or radio. The producer who assembles the newscast must build a collection of taped stories, live material and commercials that will fit the time slot exactly. The timing is very important, listeners and viewers will notice if there is an awkward split-second of dead air. Producers live with stopwatches hanging around their necks. A television newscast, in its entirety, will be 30 or 60 minutes long. The time slot is rigid, just as the "news hole" in a newspaper can’t be stretched. The newspaper news hole varies from day to day, depending on how much advertising has been sold. The newscast length remains the same. In a voice-over story, the anchor begins to read the story on camera. About 10 seconds into the copy, the director in the control room switches the picture from the anchor to videotape of what the anchor is talking about. The anchor continues to read, live, while we see videotape. The tape may have sound with it, played very low. This is called natural sound. The anchor voice is over the picture and any background sound on the tape. In the voice-over-to-sound formula (V/O to SOT), the anchor begins the story on-camera, just as before. Then the anchor stops reading. The sound on the tape is turned up, full volume. For this kind of story, everything must be timed precisely. After the copy is written, a producer with a stopwatch takes it to the anchor, who reads at a normal pace. They time from the point where the live voice-over begins, to where the actor in last night’s ceremony will speak on videotape. The voice-over section of tape is edited to run exactly as many seconds as it takes for the anchor to read the copy. When the anchor reads the copy during the newscast, the director in the control room punches a stopwatch as the voice-over tape begins to roll. The anchor must read for exactly16 seconds.If the reading is too fast, there will be a hole of silence between the anchor’s voice and the actor’s. Read too slowly, and the anchor will drown out the beginning of what the actor says. The director works with a microphone in the control room. The anchors wear hidden earphones.The floor crew all have headsets. By flipping a selector switch, the director can talk to anyone in the studio, or to all of them. As the anchor reads the voice-over section of the story, the director watches a stopwatch and begins a countdown to the floor manager. The floor manager, listening on a headset, drops a finger each second as the director counts. When the countdown reaches five seconds, the anchor will hurry a little, or slow down, to finish the voice-over just as the sound-on-tape (SOT) begins.
http://en.wikipedia.org/wiki/Newscasts
http://www.winning-newsmedia.com/newscast.htm
video newscast link