The Use of Two Saw-Toothed Sweeps

Author: J.B. Hoag

In Fig. 22 J,saw-toothed voltages are applied simultaneously to the two sets of deflecting plates of the cathode-ray tube by means of the separate linear sweep-circuits 1 and 2.

Figs. 22 J, K, and L. Double linear-sweeping

When the frequency of 2 is greater than that of 1, and is an exact multiple, say six times, the pattern of Fig. 22 K will result. Starting in the upper left corner, the beam is deflected horizontally by 2, flies back, and repeats, the while it is slowly drawn downward on the screen by 1. Just before the end of the sixth horizontal sweep, the fly-back of 1 brings the spot back to the upper left corner at just the right moment to repeat the movements along the original paths. Then a stationary pattern of horizontal lines is seen on the screen. The entire process is called scanning.

While the scanning process just described is going on, let the control grid voltage of the cathode-ray tube be altered by a video or television signal. Changes of grid voltage cause changes in the number of electrons emitted from the electron gun, and hence in the brightness of the moving spot on the screen. The intensity of light can then be made directly proportional to the brightness of the spot scanned by the transmitter. If timed to start at the same instant and to sweep both horizontally and vertically in synchronism with the scanning at the transmitter, an image will be properly reassembled and in proper intensities over all the screen. It is customary in the United States to use 431 or 525 lines, where 6 are indicated in Fig. 22 K, in order to show the details of the picture. This complete set of lines (525) is called 1 frame and requires 1/30 of a second. In other words, 30 frames or complete pictures, each of 525 lines, are produced on the screen each second. Because of the persistence of vision of the human eye (approximately 1/10 of a second), the successive frames blend from one into the next and the semblance of continuous motion is established.

Another use of the double-sweep circuit is in the study of atmospherics, or static, produced either by nature or man-made machines. In other words, we propose to examine the makeup of a single " crackle " heard in a radio receiver. Frequently these consist of a rapid succession of short time impulses. If a cathode-ray tube is connected to the output of a receiver, and uses only a single horizontal sweep, it would have to be a very large tube indeed in order that the total time axis would be spread out long enough to show all of the individual impulses which occur during the single crackle. On a small tube, one might use a high-speed sweep, but then the individual pulses would be lapped back on each other and cause confusion. On the other hand, with a double sweep of the type discussed above, it is possible to realize the equivalent of a very long-time axis on a small tube. While sweeps 1 and 2 of Fig. 22 J are producing the pattern shown in Fig. 22 K, the series of pulses are applied across the resistor R (Fig. 22 J), yielding the pattern shown in Fig. 22 L. Starting at the upper left-hand corner of the screen, the beam is swept to the right-hand side, folded back to the left side and displaced downwards, swept to the right, and so on, as though a very long line were folded back repeatedly to condense the great length into a small area. With this folded pattern, a total time of sweeping, from upper left-hand to lower right-hand corner, equal to or greater than the total time of one crackle is attained on a small tube. Details of each of the separate pulses in the crackle can then be seen because they are suitably extended.