noise-free at that level. The importance of noise-free blanking and synchronizing pulses should be emphasized. They determine the stability of the viewed picture or the degree to which a picture remains locked-in on the picture tube, even under the most adverse transmission conditions. This is important when considering the use of television signals for close-circuit applications. The extreme environmental conditions that may be encountered can seriously degrade the picture signal. This makes it difficult to synchronize or lock-in a picture unless the original blanking-to-picture and signal-to-noise ratios are high. PICTURE AVERAGE DC COMPONENT.— If a television picture is to be transmitted successfully with the necessary fidelity, it needs the dc component of the picture signal.    This component is a result of slow changes in light intensity. The loss of the dc component occurs in ac or capacitive coupling circuits. The loss is evidenced by the picture signal tending to adjust itself about its own ac axis. The dc component is returned to the video signal by means of a dc restorer or inserter circuit. PICTURE SYNCHRONIZING PULSES.— Synchronizing the scanning beams in the camera and the receiver must be exact at all times to provide a viewable picture. To accomplish this, synchronizing information is provided by electrical pulses in the retrace intervals between successive lines and between successive pictures (fig. 5-24). The retrace periods should be as short as circuit considerations will allow. These periods are in areas where synchronization pulses may be inserted without interfering with the picture. Synchronizing pulses are generated in the equipment that controls the timing of the scanning beam in the pickup device. They become a part of the complete signal that is transmitted to the receiver. In this manner, scanning operations at both ends of the system are always in step with each other. In general, synchronizing signals should provide positive synchronization of both horizontial and vertical sweep circuits. They should be separable by simple electronic circuits to recover the vertical and horizontal components of the composite sync signal. They should be able to be combined with the picture and blanking signals to produce a standard composite television signal. Most television systems produce synchronizing information that conforms to the basic requirements of synchronization. Figure 5-24 shows how the synchronizing signal waveform is added to the picture information and blanking signals to form a complete composite picture signal ready to be transmitted. Note that the duration of the horizontal sync pulses is considerably shorter than that of the blanking pulses. Vertical sync pulses are rectangular, but they are of much shorter duration than the horizontal pulses. Thus, they provide the necessary means for frequency discrimination. Synchronization presents a difficult problem as more failures occur from the loss of proper interlacing. Discrepancies in either timing or amplitude of the vertical scanning of alternate fields cause displacement in space of the interlaced fields. The result is nonuniform spacing of the scanning lines. This reduces the vertical resolution and makes the line structure of the picture visible at normal viewing distance. The effect is usually known as “pairing.” Another series of pulses is added before and after the vertical sync pulses to prevent the pairing problem and to maintain continuous horizontal synchronizing information throughout the vertical synchronization and blanking interval. These are equalizing pulses (fig. 5-25). The time between the last horizontal sync pulse and the first equalizing pulse changes from a full horizontal line interval to one-half of a horizontal line interval every other field. This is caused by the ratio between 15,750 Hz and 60 Hz. The ratio produces the necessary difference between fields to provide interlaced scanning. Since the horizontal oscillator is adjusted to the frequency of the horizontal sync pulses, it is triggered only by every other equalizing pulse or serration of the vertical sync pulse. Other Systems Commercial broadcasting and many closed- circuit installations adhere closely to the synchroniza- tion signal specifications just discussed. There are some noncommercial and closed-circuit installations that use less rigorous sync signal specifications. We will now discuss four categories of these signals. RANDOM INTERLACE, NO SPECIAL SYNC PULSES.— The random interlace is the simplest television method. It provides no special sync pulses and no fixed relationship between the horizontal and vertical scanning raster. Synchroni- zation information at the monitor is obtained from the horizontal and vertical blanking pulses contained in the video signal. Figure 5-26, view A, shows this type of video signal. Usually at the camera control 5-20