Radio annual (1938)

ihg each performance by operation of the "shading controls" which introduce compensating electrical waveforms into the television signal. With the mechanical scanner this manipulation is not required. The use of this scanner is particularly prevalent in Germany. The Fernsen A. G. ("Television Corporation"), where over one hundred men are engaged in research on all systems of television, have recently made known their high-definition mechanical film scanner. The work of the D. S. Loewe organization in this regard is also known. In England, "Television" reports that the mechanical film transmissions of the Baird System were more clearly received by the public than the present film pickup with the Emitron mosaic tube. The Scophony System utilizes a mirror drum for film transmission. In the United States the Bell Laboratories of the American Telephone & Telegraph Company have developed a mechanical film scanner for use in their coaxial cable work. The Don Lee Broadcasting System uses the mechanical method for film scanning and the mosaic tube for direct pickup. The modern mechanical scanner is, however, a far cry from its low-definition predecessors. Low-definition television, formerly in use throughout the world, may be defined as television systems employing a standard of from 30 to 120 lines. The present high definition television may be defined as any greater lineage than this, but. is usually taken to mean standards of from 240 to 441 lines. Without new principles of operation, the fundamental scanning disk or mirror drum would be incapable of transmitting an accurate image. The mechanical tolerances required transcend the art of the skilled machinist. The manner in which these limitations are overcome has not yet been disclosed by those who have developed this equipment. In order that the subject matter seen by the electric eyes or taken from motion picture film be reproduced at a distance requires the coaction of a host of cooperating devices. Scanning sources, which produce electrical waveforms of special shapes, are required to operate pickup tubes and also the cathode-ray tubes at the receiver. Amplifiers, developed from the type utilized in radio, but capable of amplifying a band of frequencies from thirty to over two million cycles per second are used at both transmitter and receiver. Television transmitters and receivers, or a coaxial cable handling this wide band width, must be utilized to transport the television signal from the point of origination to the distant point of observation. Finally, the receivers must be held in step with the transmitter by a process called synchronization, and the foundation upon which modern television has been built, the cathode-ray tube, is required to display the image. If sound is to accompany the visual performance, microphone, amplifiers, a transmitter, receivers, and loudspeakers must be provided for a second channel of communication. The perfection of several of these devices has been required in order to make present high-definition television possible. Considering these devices in order, we find that present-day scanning sources produce rectilinear (straightline) "sawtooth" waveforms,' to the end that the scanning spot in the camera and on the cathode ray tube screen moves from one side to the other fairly "slowly" (in one thirteen-thousandth of a second!) but returns to start the next trace in less than one-tenth that time. "High vacuum" thermionic tubes are now largely utilized, replacing the former gas triodes, or Thyratrons, although the latter may still be used in simplified receivers. The amplifiers are usually of the resistance — capacitance coupled type with compensating inductances or feedback to secure the wide frequency band. Transformer coupling, as widely used in radio, is not suitable. It falls to the amplifier, in cooperation with the scanning sources, to provide the various blanking, pedestal, and synchronizing waveforms which must be inserted in the television signal. The modern television signal is a composite of several waveforms, assembled in a manner hardly known ten years ago. It is universal practice to place the image signal on one side of the axis and the synchronizing pulses on the other side. In usual radio broadcasting, both sides of the axis are occupied by the more or less "sine wave" quality of speech and music. In television, the two sides of the axis are utilized for separate functions, one side for carrying the image variations of intensity, the other side for synchronization purposes. At the receiver the waveform is essentially "split in half" by the synchronizing equipment. The two parts are directed to the proper portions of the receiving apparatus to make the receiver operate as a whole. The synchronizing equipment is nearly human in carrying out this process. The television transmitters invariably operate on ultra-high frequency chan (I'lcase turn to page 451) 448