Aviation Electronics Technician 1 - Historic Naval Ships Association

this type of scanning, each of the two groups ofalternate lines is called a field, and the frame is madeup of two fields. Interlacing is accomplished bymaking the total number of lines in a frame an oddinteger. Thus, the number of lines in each of the fieldsis an even number plus one-half line. This results inconsecutive fields that are displaced in space withrespect to each other by one-half of a line. The totalnumber of lines is 525, the total lines per field is262 1/2, the vertical scanning frequency is 60 Hz, thenumber of frames per second is 30, and the horizontalscanning frequency is 15,750 Hz (60 x 262 1/2).Television SignalsThe standard television signal consists of thefollowing four elements:1. The picture information2. The picture blanking pulses3. The picture average dc component4. The picture synchronizing pulsesThese four elements are discussed in thetext.followingPICTURE INFORMATION.— The pictureinformation is the basic part of the signal. It is aseries of waves and pulses generated during activescanning of the camera tube. As the scanning linetravels across the tube, it is amplitude modulated inproportion to the brightness variation in the scene it isscanning. For commercial television, the amplitudevariations are such that the maximum video amplitudeproduces black, and the minimum amplitude produceswhite. Ordinarily, the maximum and minimum videoamplitude values represent 75 and 15 percent of themaximum carrier voltage, respectively.PICTURE BLANKING PULSES.— To preventundesirable signals from entering the picture duringretrace time, blanking pulses are applied to thescanning beams in both the camera tube and thereceiver picture tube (kinescope). Camera blankingpulses are used only in the pickup device. They serveonly to close the scanning aperture on the camera tubeduring retrace periods, and never actually appear inthe final signal sent to the receiver. In some systems,the same pulse that triggers the seaming circuit andblanks the kinescope also closes the camera aperture.The function of the kinescope blanking pulses isto suppress the scanning beam in the kinescope duringboth vertical and horizontal flyback times. Thekinescope blanking pulses are simple rectangularpulses, somewhat wider than the correspondingcamera blanking pulses. They have a durationslightly longer than the actual retrace time. Thereason for the slightly longer blanking time is to trimup the edges of the picture and to provide a clean,noise-free period during flyback. Figure 5-24 showsa complete video signal that contains pulses for theremoval of visible lines during horizontal retraceperiods only. The horizontal pulses recur at intervalsof 1/15,750 of a second. At the bottom of the picture,they are replaced by vertical blanking pulses. Theseare similar to the horizontal pulses, except they are ofmuch longer duration (approximately 15 scanninglines) and have a recurrence of 1/60 of a second.Note that the blanking pulses (and synchronizingpulses) are added at a relatively high-level point in thetransmitter because they are considered to beFigure 5-24.-The complete video signal for three scanned lines.5-19