|
In video, the illusion of streaming pictures (which in turn creates an illusion of movement) is created by a scanning process. The picture is formed on the imaging device in the following manner.
The video scanning process creates a series of 25 frames in 1 second. The video frame does not exist as a single, discrete frame like a motion picture frame. It is broken into 625 lines of information that are scanned one line at a time, line by line, as the electron beam moves across and down the image on the target areas. To minimize brightness variations and reduce flicker, each video frame is bI:6ken into two fields, each composed of half the 625 lines in the frame. The first field contains the 312.5 odd-numbered lines (1, 3, 5, and so on), and the second field contains the 312.5 even-numbered lines (2,4,6, and so on).
When the electron beam reaches the end of the first line (moving horizontally left to right), it goes into a blanking mode and then moves back across to the left edge to begin scanning the next line down, see Figs 2.4(a) and (b). During this horizontal blanking interval, the electron beam is momentarily shut off so that it cannot be seen as it moves back across the target to the next line. The process continues until the beam reaches the end of the last line in the odd-numbered field. The beam then goes into a vertical blanking interval while it moves back to the top of the even numbered field to begin the downward scan again.
The entire scanning process takes place at unimaginable speed. Two fields make one frame. In 1 second the electron beam scans 50 fields to create the 25 video frames. A process called interlaced scanning produces the full video frame in 1/25 of a second. Thus, 25 frames are produced in one second. This in turn lends movement to the image
|
Participants