Aviation Electronics Technician 1 - Historic Naval Ships Association

electrical signal, which varies in amplitude with theamount of light that falls on each point of the image.A synchronizing signal is added to the electricalsignal from the camera. The resultant compositevideo signal is then transmitted to the monitor.In the monitor, the synchronizing signal causesthe beam to scan the picture tube (kinescope)faceplate in synchronism with the camera scanningbeam. The intensity of the beam is varied inaccordance with the picture signal. The varying beamcauses the image to appear on the face of the picturetube.The time required for one vertical scan of thepicture in broadcast television systems in the UnitedStates is 1/60 of a second (60 Hz) or a multiple orsubmultiple thereof. The rate of 60 Hz was chosenbecause most commercial electrical power sources inthe United States operate at a frequency of 60 Hz.Synchronization with the 60-Hz power frequencyreduces the visible effects of hum and simplifies theproblem of synchronizing film projectors withscanning.ScanningScanning is the process of breaking up the sceneinto minute elements, and using these elements in anorderly sequence. Reading a printed page is similar toscanning. You start at the beginning of one line andmove to the right, word by word, until you reach theend of the line. Then you jump back to the beginningof the next line and repeat the process until you reachthe end of the page. In television, this process is doneto the picture.number of lines. The present system of 525 lines waschosen for broadcast television as the most suitablecompromise between channel width and pictureresolution. This system is also used in manyclosed-circuit televisions (CCTV).NONINTERLACED SCANNING.— This is thesimplest method of scanning. It is also known assequential or progressive scanning. Noninterlacedscanning uses an electron beam that moves veryrapidly from left to right on an essentially horizontalline. It travels slowly from the top to the bottom ofthe picture. When the electron beam reaches the endof a line, a blanking voltage is applied, which shutsoff the beam. The period of time the beam is shut offis known as the horizontal retrace period or flybacktime. Similarly, when the beam reaches the bottom ofthe picture, the beam is again blanked, and returns tothe top of the picture. This period of time is known asvertical retrace or flyback time.INTERLACED SCANNING.— An importantvariation of the scanning method is known asinterlaced scanning. It is the method used bybroadcast television and most CCTV equipment.With interlaced scanning, it is possible to reduce thevideo bandwidth by a factor of two without reducingthe resolution or seriously increasing flicker.In the standard two-to-one method of interlacing,alternate lines are seamed consecutively from top tobottom. Then, the remaining alternate lines areseamed. This principle is shown in figure 5-23. InScanning MethodsThe number of scanning lines determines themaximum ability of the system to resolve fine detailin the vertical direction. Also, the number ofscanning lines is related to the resolution ability in thehorizontal direction. Resolution is determined by thenumber of scanning lines. For a given videobandwidth and frame time, horizontal resolution isinversely proportional to the number of lines.Therefore, as the scanning lines are increased innumber, the bandwidth of the system must also beincreased in the same ratio to maintain the sameresolution in the horizontal direction.Maintaining approximately equal values ofhorizontal and vertical resolution is ideal. Thebandwidth requirements increase as the square of theFigure 5-23.-Interlaced scanning.5-18