Journal of the Society of Motion Picture and Television Engineers (1950-1954)

latter requirements that are the hardest to meet. This is particularly true in instances of near-end crosstalk in cable circuits carrying video signals in opposite directions. As the length of cable to the nearest repeater, or terminal, is increased, two things occur: the level of the incoming signal is decreased and the steepness of slope of the equalization of the receiving amplifier is increased. The first increases the effective coupling between circuits and the second increases the highfrequency transmission of the crosstalk signal. The end result, for the particular case under discussion, is that the maximum length of circuit is limited by the 14-db requirement for 26-db/octave slope. The differences between the results for synchronous and nonsynchronous operation are not significantly great. As implied earlier, the term "synchronous" is a misnomer, and actually means erratic motion rather than uniform motion of the crosstalk image. For the flat crosstalk tests the difference was trivial; for the 12-db/octave case a difference of 4 db obtained. All the above results, it should be noted , were obtained for lumped couplings between circuits carrying the wanted and unwanted picture signals. This results in a reasonably clear unwanted picture, which aids in its detection and imposes rather severe coupling requirements. Where the coupling is distributed over a long distance, as in adjacent pairs, the crosstalk image will be less clear and probably somewhat less severe requirements would be imposed. Recapitulation Tests were made to determine how much video crosstalk can safely be tolerated in black-and-white television pictures. Artificial crosstalk coupling with four different loss-frequency characteristics was used. In using lumped, rather than distributed, coupling more conservative requirements were obtained. The degree of synchronization was con trolled to insure that approximately the most disturbing conditions obtained. The disturbed picture was one known to be sensitive to interference; the disturbing picture was a test pattern which probably was as disturbing as any picture for both the flat and sloping coupling tests. The rendition of the main picture received considerable care: the highlight brightness and contrast range were better than average, and the noise was barely noticeable. Frequent checks were made to keep the quality of the picture reproduction constant. The observers were experienced in judging impairments of television pictures, and were known to be stable in their judgments. The conditions of observation were controlled in the matter of ambient light and viewing distance. The results obtained from these tests and proposed as limits for the several types of crosstalk coupling are somewhat conservative. The crosstalk resulting from these values of coupling could not be detected by about half the observers and were not rated as objectionable by the most critical. Acknowledgments The author acknowledges his indebtedness to H. N. Christopher, who was a close associate in conducting the experiments; to J. M. Barstow, under whose direction the work was done; and to Pierre Mertz, technical advisor to the project, who suggested the testing technique. References 1. P. Mertz, A. D. Fowler and H. N. Christopher, "Quality rating of television images," Proc. I.R.E., vol. 38, pp. 1269-1283, Nov. 1950. 2. L. W. Morrison, "Television transmission in local telephone exchange areas," Jour. SMPTE, vol. 56, pp. 280-294, Mar. 1951. 3. L. H. C. Tippett, Random Sampling Numbers, Cambridge University Press, London, (Tracts for Computers, XV), 34 pp., 1927. 424 November 1951 Journal of the SMPTE Vol. 57