Lines, Active Horizontal
Lines, Active Horizontal – In the scanning of a video image, the line number associated with the format is the total number of lines assigned to one frame. It is in fact a timing specification defining the conjunction with the field frequency the time interval allocated to each horizontal line (commonly measured in number of samples at the specified sampling rate or in microseconds). Some of these lines and intervals carry image information, some from the total assigned are dedicated to operational and control functions, including returning the scanning beam back to the upper left corner to begin the next field. Those allotted time intervals (lines) actually carrying image information or image-associated information such as captioning, image test signals, etc., are the active lines. In further reduction of time allocated to image information, some of each active line is dedicated to the horizontal interval to get the scanning beam to return to the leftedge starting point for the next line and to reaffirm color subcarrier, etc. In the U.S. 525/59.94/2:1/NTSC system, about 7.6% of the total field or frame time is assigned to the vertical interval, and about 16% to the horizontal interval. Thus, the 525 television lines per frame provide about 480 active lines. Correspondingly, each active line displays image data about 84% of its time interval. Image information is thus conveyed for only about 76.4% of the total time. In digital encoding, it may be possible to reduce the number of bits assigned to the vertical and horizontal intervals and achieve significant bit rate reduction.
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