Radio annual (1938)

MODERN TELEVISION SYSTEMS By HARRY R. LUBCKE Director of Television of the Don Lee Broadcasting System, Los Angeles Modern television systems may be divided into two groups: the electronic, and the mechanical. In the first group are found the Image Dissector tube of Philo T. Farnsworth, of Farnsworth Television, the Iconoscope of V. K. Zworkyin, of the Radio Corporation of America, and the cathode ray tube of antiquity. In the second group, recent advances have made the mechanical disk a superior means for scanning motion picture film, and the Scophony method of multiple light control is a competitor of the cathode ray tube for exhibiting the received image. The Image Dissector is a glass enclosed vacuum tube containing a uniform photoelectric surface at one end and a tiny aperture at the other. The scene to be transmitted, either live or film, is focused upon the photoelectric plate by a lens. Electrons are given off at each and every point on the surface according to the light intensity striking that point. The "electron image" of the scene thus created is caused to traverse the tube to the aperture, being focused to a sharp image thereat and being deflected systematically thereover to accomplish scanning, thereby producing a television signal. The traverse is effected by applying voltages to the tube electrodes, and the focusing and deflection by magnetic fields produced by current flowing through coils surrounding the tube. A device known as an "electron multiplier," a current amplifier as distinguished from the ordinary radio tube which is a voltage amplifier, has been developed and made a part of recent Dissector tubes. The Iconoscope is also a glass enclosed vacuum tube, but contains a special photoelectric surface in its principal enclosure and a cathode-ray "electron gun" in a narrow extension thereof. The special surface is known as the "mosaic" and is composed of an innumerable number of minute photoelectric globules, each insulated from the other, and forming in effect an innumerable number of separate photoelectric cells. The scene to be trans mitted is again focused upon the photoelectric surface by a lens. Electrons are given off as before, but this time each globule, since it is insulated, assumes a positive potential proportional to the light that falls upon it. Once each complete scanning of the image, an electron beam constantly emitted from the electron gun discharges each globule and thereby produces the television signal. The charging process takes place all the time, except the instant when the beam discharges the particular globule. This storage process is an important one, although its full possibilities have not been attained at the present time. These two devices are truly "electric" eyes. They are creations of the modern age. All credit is due the inventors, and the organizations behind them, in making these devices practical tools in the hands of present-day television engineers. In the transmission of film, certain characteristics of the mechanical arrangement cause the result to be accomplished in a particularly satisfactory manner. Although the modified motion picture projector required to run off the film is a mechanical device itself, the advantage of mechanical pickup is not because it coacts efficiently with another mechanical device, but because a true shading of the image is secured. The mosaic type pickup tube does not produce the electrical representation of the background of the scene being transmitted. This must be inserted dur 447