American television directory (1946)

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PTM — A NEW TRANSMISSION SYSTEM (Continued from page 33 1 part is contained in an even smaller cabinet. Through the use of the “cyclophon,” the number of vacuum tubes required is very materially reduced. Coaxial cables and parabolic reflec¬ tors 8 feet in diameter serve to beam the 1300 me carrier. The performance of the system meets the strict require¬ ments of high quality modern telephony. These systems could also carry a number of telegraph channels in place of each of the telephone channels. The importance of such a system for long distance telephony is at once evident: no wires, little maintenance, little haz¬ ard of interruption by flood, hurricane or ice storm, and simplified operation even for direct dialing of long distance numbers across the continent. The possibilities of PTM are not lim¬ ited to telegraphy and telephony. In fact, high grade transmission of music has been successfully demonstrated and its application to broadcasting offers so¬ lutions to many pressing problems. With the ever increasing demand for broadcasting facilities and the already congested situation in the present broad¬ cast band, there is a trend toward utili¬ zation of high frequencies for radio broadcast purposes. Steps have been taken in the United States toward the establishment of an extensive system of broadcasting employing frequency mod¬ ulation. In view of the number of SO THEY SAY: “I had the novel experience of appearing in television before I did in motion pictures. I found tele¬ vision a great help when I made my first picture, ‘Goodbye Mr. Chips.’ The camera technique is much the same and teaches, most importantly, naturalness, restraint and poise. . . I discovered that, before either a television or movie camera, the worst fault a player can be guilty of is overacting. Fine acting before a motion picture or television camera isn’t really act¬ ing at all. It is the art of being natural.” — Greer Garson “I can now look forward to a long layoff in a new medium, tele¬ vision.” — Solly Violinsky “Fortunately for screen actors there is no deep dark mystery about acting in television. Any competent actor who has performed before the motion picture camera is ready to perform before the tele¬ vision camera. Those of us who have participated in a number of television productions know that acting in television is just about the same as acting in motion pic¬ tures. It is acting, nothing more and nothing less.” — Robert Shayne license applications filed with the FCC, it seems that the channels made avail¬ able in the new FM band 84-102 me, will in the near future prove insufficient to meet the total demand, and it is ex¬ pected that broadcasting will be shifted toward higher frequencies. In any case, wide-band FM, a com¬ paratively newer and superior type of modulation, has inherited the older con¬ cept of “simplex” broadcasting which does not permit full advantage of the possibilities of ultra-high frequency transmission for a number of stations. A Common Transmitter Most of the difficulties experienced in uhf transmission and, in particular, in densely populated areas, result from the “line of sight” requirements. To overcome these difficulties, the trans¬ mitting antenna must be located where it can be “seen” by the greatest number of receiving antennas. All transmitting stations serving a given area should be located at this optimum place. For space reasons only, this is not practical as a multitude of transmitters and antennas cannot be accommodated on top of the highest structure located centrally among the receivers. The use, for sev¬ eral programs, of a common transmit¬ ter and antenna is obviously attractive. In a “multiplex” broadcasting system, several programs would be transmitted from the optimum location on a common carrier frequency. The various studios originating the programs could be lo¬ cated at convenient places and convey their programs to the transmitting point by wire line or other means. Each of these programs would mod¬ ulate the common RF carrier system in a manner similar to the multiplex tele¬ phone and all the stations and listeners would benefit from the optimum loca¬ tion. The cost of installation should be greatly reduced as would also be the cost of operation and maintenance. Important advantages are also given to the broadcast listener; fixed tuned radio frequency receivers may be used since a common frequency is used for all stations; furthermore, the antenna problem is also simplified since the re¬ flection from nearby buildings is virtu¬ ally eliminated as all transmission originates from a single point — thus, a single efficient fixed directive antenna can be used for receiving all stations. Other advantages such as simplicity of relaying inherent to the multiplex broadcasting contribute to make this system a particularly attractive one. IT&T Laboratories have already dem¬ onstrated high quality transmission of music through PTM and are engaged in the construction of a UHF multi-chan¬ nel broadcasting system. In the television field, a common fre¬ quency for high definition or color televison as well as high quality sound is made possible by PTM. Since television transmitters require a wide frequency band to carry the picture channel, they may also be used to trasmit the sound channel in the form of very short pulses without any additional complica¬ PTM antenna Installation at Nutley, N. J., in Federal's multiplex radio relay system. tion in the transmitting station itself. A series of pulses, generated according to the principles of the PTM system, is added to the normal synchronizing pulses in the control room. The equip¬ ment required to do this is simple and makes use of receiving tubes only, since it operates at very low power levels. The resulting signal is sent to the picture transmitter in the usual manner and radiated. At the receiver, the pulses for the sound channel are separated from the synchronizing pulses by a “width selector” circuit, for example, and fed to the sound demodulating and reproducing circuits. Actually, the com¬ plexity of the receiver is reduced since the pulse-separating circuits are simpler and use fewer components than the corresponding sound I-F amplifier and detector of the conventional system. The distortion, noise level, and fre¬ quency response are all satisfactory for high quality sound reproduction, because in the high frequency television spectrum the line scanning rate is suf¬ ficiently high to permit transmitting a very large number of these sound pulses per second. The pulses do not interfere with the picture signal because they are transmitted during the time the scan¬ ning beam is reduced to zero as it re¬ turns to the start of each line. Because of the complete elimination of the sound transmitter and the fact that the receiver construction is simpli¬ fied, it is anticipated that this will prove to be one of the most important applications of the PTM system. It is expected that actual field tests under normal operating conditions will be made in the early part of 1946. 123