Abstract
THERE are various methods now in use in different countries for transmitting photographs, printed matter and line drawings by means of telegraph or telephone wires or by radio. In certain respects the circuits must meet more stringent requirements than those used by speech or music. The sending apparatus scans the picture in closely spaced lines and converts the light and dark portions into electric currents proportional in strength to the light and shade of the picture. The transmission along the line causes a weakening of the current, but this is got over by amplification at the receiver. In a circuit for telephotography the successive portions of the signal are spread out side by side so that all are seen by the eye at the same time. In telephoning, the ear hears them one after the other and any slight blurring is scarcely noticed. Any little irregularity in the time sequence of the signals is at once seen in the tele-photograph and remains as a permanent record. In the Bell Laboratories Record of February, P. Mertz describes how this defect can be remedied by equalising the ‘time delay’ of the signals (which is done in the new Western Electric telephotograph system). For good pictures this time delay of the various rays in the transmission band must not exceed the two-thousandth part of a second. Photographs are given showing the great improvement produced by using this device on the transmission line. It is also shown how ‘random noise’, telegraph interference, high-frequency noise and ‘echo’ distort the photograph. The first of the new telephoto systems to be installed connects Miami, Washington, New York, Boston, Chicago, San Francisco, etc., and includes 7,400 miles of main circuit. In a single connexion between sender and receiver, there may be as much as 3,500 miles of cable and 2,500 miles of open wire.
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New Telephoto Systems in the United States. Nature 137, 609 (1936). https://doi.org/10.1038/137609c0
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DOI: https://doi.org/10.1038/137609c0