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Scanning Methods
PICTURE AVERAGE DC COMPONENT

Aviation Electronics Technician 1 (Organizational)
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this type of scanning, each of the two groups of alternate lines is called a field, and the frame is made up of two fields. Interlacing is accomplished by making the total number of lines in a frame an odd integer. Thus, the number of lines in each of the fields is an even number plus one-half line. This results in consecutive fields that are displaced in space with respect to each other by one-half of a line. The total number of lines is 525, the total lines per field is 262 1/2, the vertical scanning frequency is 60 Hz, the number of frames per second is 30, and the horizontal scanning frequency is 15,750 Hz (60 x 262 1/2). Television Signals The standard television signal consists of the following four elements: 1. The picture information 2. The picture blanking pulses 3. The picture average dc component 4. The picture synchronizing pulses These four elements are discussed in the text. following PICTURE INFORMATION.— The picture information is the basic part of the signal. It is a series of waves and pulses generated during active scanning of the camera tube. As the scanning line travels across the tube, it is amplitude modulated in proportion to the brightness variation in the scene it is scanning. For commercial television, the amplitude variations are such that the maximum video amplitude produces black, and the minimum amplitude produces white. Ordinarily, the maximum and minimum video amplitude values represent 75 and 15 percent of the maximum carrier voltage, respectively. PICTURE BLANKING PULSES.— To prevent undesirable signals from entering the picture during retrace time, blanking pulses are applied to the scanning beams in both the camera tube and the receiver picture tube (kinescope). Camera blanking pulses are used only in the pickup device. They serve only to close the scanning aperture on the camera tube during retrace periods, and never actually appear in the final signal sent to the receiver. In some systems, the same pulse that triggers the seaming circuit and blanks the kinescope also closes the camera aperture. The function of the kinescope blanking pulses is to suppress the scanning beam in the kinescope during both vertical and horizontal flyback times. The kinescope blanking pulses are simple rectangular pulses, somewhat wider than the corresponding camera blanking pulses. They have a duration slightly longer than the actual retrace time. The reason for the slightly longer blanking time is to trim up the edges of the picture and to provide a clean, noise-free period during flyback. Figure 5-24 shows a complete video signal that contains pulses for the removal of visible lines during horizontal retrace periods only. The horizontal pulses recur at intervals of 1/15,750 of a second. At the bottom of the picture, they are replaced by vertical blanking pulses. These are similar to the horizontal pulses, except they are of much longer duration (approximately 15 scanning lines) and have a recurrence of 1/60 of a second. Note that the blanking pulses (and synchronizing pulses) are added at a relatively high-level point in the transmitter because they are considered to be Figure 5-24.-The complete video signal for three scanned lines. 5-19







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