Aviation Electronics Technician 1 - Historic Naval Ships Association

Like the random interlace, this system does notprovide a special synchronizing signal; therefore, it issubject to the same synchronizing limitationsdiscussed previously. These limitations become animportant problem in the more elaborate installationswhere a series of cameras and monitors might beseparated by wide distances. For this reason,installations using these systems are usually thesmaller, less complex applications, where stabilityand reliability may not be an important factor.ODD-LINE INTERLACE, MODIFIED SYNCPtILSES.— The odd-line interlace method withmodified sync pulses provides further advantagesover the previous two systems. However, it has aconsiderable number of limitations. In this system,special synchronizing pulses have been added to thevideo waveform (fig.5-26, view C).Note that the synchronizing signal has been addedto the tip of each horizontal blanking pulse. The syncpulses also continue through the vertical blankinginterval. They provide synchronizing information forthe monitor horizontal frequency-locking circuits atall times. These circuits are no longer free-runningduring the vertical blanking interval. Addition ofthese special sync pulses greatly improves the lock-inability of the composite video signal under adverseconditions of noise and spurious signals.SLOW-SPEED SCAN.— A television system thatis being used frequently has a slow-speed scantechnique. This technique represents a radical departurefrom the nominal scanning standards. It permits a sceneto be picked up and transmitted successfully from onelocation to another, if the scene contains a limitedamount of action or movement and a great deal ofredundancy. It affords fair resolution and fidelity insignals transmitted over relatively economicalnarrow-band transmission facilities. For example, aslow-speed camera located in a bank, a message center,or a newspaper office can scan printed information andtransmit it to a distant location over ordinary telephoneline facilities. Some methods are able to transmitpictures having more action, such as a person talking,with reasonable clarity. Such methods, however,require a somewhat greater bandwidth.Most slow-speed scan systems use a much slowerscanning rate, with a correspondingly narrowerbandwidth, than present telecasting standards.Broadcasting systems transmit a picture every 1/30 ofa second, with a 4-MHz bandwidth. Slow-scansystems transmit a picture in 1/10 of a second to 2seconds, with a video bandwidth ranging fromapproximately 250 kHz to as low as 500 Hz.Most slow-speed scan systems are practicalwhere time is available for transmission. Forexample, the information contained in a 5-minutecommercial television program requires several hoursof time to be transmitted with comparable detail bythe average slow-speed scanning system. Theadvantages of the slow-speed system are greatlysimplified equipment and relatively inexpensivetransmission facilities. The disadvantages are that thescene content is limited to relatively immobileobjects, resolution is marginal, and the system isincompatible with standard television systems.Rather complex scan conversion equipment isrequired to make the two systems compatible. Exceptfor certain special applications, slow-speed scansystems are inferior in performance and cannot beused successfully where a high degree of resolutionand detail is required.COMPONENTSThe pickup devices and picture tube basics arediscussed in the following text.Camera TubesTo assure high-quality images at the picture tube, thecamera must resolve the scene into as many pictureelements as practical. The quality of the picture increasesas the number of elements increase. The pickup tubemust produce signals that accurately represent eachelement. The optical-electrical conversion must have asignal-to-noise ratio high enough to assure effectivepickup sensitivity at the lowest light level that may betransmitted. Ideally, when there is no light present, thereshould be no output signal.The type of camera tube used is determined by theintended use of the camera and the amount ofavailable light. The amount of light required by acamera tube is rated in candelas. The minimumnumber of candelas required by a camera tube is ameasure of the tube’s sensitivity. The following typesof tubes are in use today.IMAGE ORTHICON.— The image orthicon(fig. 5-27) is an ultrasensitive television camerapickup tube. The tube requires only 8 to 40 candelasfor light, and is used in modern conventional andCCTV systems. When this tube is used, a light imagefor the subject (arrow at extreme left in figure) ispicked up by the camera lens and focused on thelight-sensitive face of the tube. This causes electronsto be released from each of the thousands of tinyglobules in proportion to the intensity of the lightstriking it.These electrons are directed on parallel coursesfrom the back of the tube face to the target, from which5-22