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By the use of a two-dimensional Fourier analysis of the transmitted picture a theory of scanning is developed and the scanning system related to the signal used for the transmission. On the basis of this theory a number of conclusions can be drawn: 1. The result of the complete process of transmission may be divided into two parts, (a) a reproduction of the original picture with a blurring similar to that caused in general by an optical system of only finite perfection, and (b) the superposition on it of an extraneous pattern not present in the original, but which is a function of both the original and the scanning system. 2. Roughly half the frequency range occupied by the transmitted signal is idle. Its frequency spectrum consists of alternating strong bands and regions of weak energy. In the latter the signal energy reproducing the original is at its weakest, and gives rise to the strongest part of the extraneous pattern. In a television system these idle regions are several hundred to several thousand cycles wide and have actually been used experimentally as the transmission path for independent signaling channels, without any visible effect on the received picture. 3. With respect to the blurring of the original all reasonable shapes of aperture give about the same result when of equivalent size. The sizes (along a given dimension) are determined as equivalent when the apertures have the same radius of gyration (about a perpendicular axis in the plane of the aperture). 4. With respect to extraneous patterns certain shapes of aperture are better than others, but all apertures can be made to suppress them at the expense of blurring. An aperture arrangement is presented which almost completely eliminates extraneous pattern while about doubling the blurring across the direction of scanning as compared with the usual square aperture. From this and other examples the degradation caused by the extraneous patterns is estimated.
Mertz et al. (Sun,) studied this question.