Like the random interlace, this system does not provide a special synchronizing signal; therefore, it is subject to the same synchronizing limitations discussed previously. These limitations become an important problem in the more elaborate installations where a series of cameras and monitors might be separated by wide distances. For this reason, installations using these systems are usually the smaller, less complex applications, where stability and reliability may not be an important factor. ODD-LINE INTERLACE, MODIFIED SYNC PtILSES.— The odd-line interlace method with modified sync pulses provides further advantages over the previous two systems. However, it has a considerable number of limitations. In this system, special synchronizing pulses have been added to the video waveform (fig.5-26, view C). Note that the synchronizing signal has been added to the tip of each horizontal blanking pulse. The sync pulses also continue through the vertical blanking interval. They provide synchronizing information for the monitor horizontal frequency-locking circuits at all times. These circuits are no longer free-running during the vertical blanking interval. Addition of these special sync pulses greatly improves the lock-in ability of the composite video signal under adverse conditions of noise and spurious signals. SLOW-SPEED SCAN.— A television system that is being used frequently has a slow-speed scan technique. This technique represents a radical departure from the nominal scanning standards. It permits a scene to be picked up and transmitted successfully from one location to another, if the scene contains a limited amount of action or movement and a great deal of redundancy. It affords fair resolution and fidelity in signals transmitted over relatively economical narrow-band transmission facilities. For example, a slow-speed camera located in a bank, a message center, or a newspaper office can scan printed information and transmit it to a distant location over ordinary telephone line facilities. Some methods are able to transmit pictures 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 slower scanning rate, with a correspondingly narrower bandwidth, than present telecasting standards. Broadcasting systems transmit a picture every 1/30 of a second, with a 4-MHz bandwidth. Slow-scan systems transmit a picture in 1/10 of a second to 2 seconds, with a video bandwidth ranging from approximately 250 kHz to as low as 500 Hz. Most slow-speed scan systems are practical where time is available for transmission. For example, the information contained in a 5-minute commercial television program requires several hours of time to be transmitted with comparable detail by the average slow-speed scanning system. The advantages of the slow-speed system are greatly simplified equipment and relatively inexpensive transmission facilities. The disadvantages are that the scene content is limited to relatively immobile objects, resolution is marginal, and the system is incompatible with standard television systems. Rather complex scan conversion equipment is required to make the two systems compatible. Except for certain special applications, slow-speed scan systems are inferior in performance and cannot be used successfully where a high degree of resolution and detail is required. COMPONENTS The pickup devices and picture tube basics are discussed in the following text. Camera Tubes To assure high-quality images at the picture tube, the camera must resolve the scene into as many picture elements as practical. The quality of the picture increases as the number of elements increase. The pickup tube must produce signals that accurately represent each element. The optical-electrical conversion must have a signal-to-noise ratio high enough to assure effective pickup sensitivity at the lowest light level that may be transmitted. Ideally, when there is no light present, there should be no output signal. The type of camera tube used is determined by the intended use of the camera and the amount of available light. The amount of light required by a camera tube is rated in candelas. The minimum number of candelas required by a camera tube is a measure of the tube’s sensitivity. The following types of tubes are in use today. IMAGE ORTHICON.— The image orthicon (fig. 5-27) is an ultrasensitive television camera pickup tube. The tube requires only 8 to 40 candelas for light, and is used in modern conventional and CCTV systems. When this tube is used, a light image for the subject (arrow at extreme left in figure) is picked up by the camera lens and focused on the light-sensitive face of the tube. This causes electrons to be released from each of the thousands of tiny globules in proportion to the intensity of the light striking it. These electrons are directed on parallel courses from the back of the tube face to the target, from which 5-22