Projection engineering (Sept 1929-Nov 1930)

Projection Engineering, May, 1930 Page 23 Two-way Television Present State of Development of Bell Laboratories System By Herbert E. Ives EVER since the initial demonstration of television both by wire and by radio at Bell Telephone Laboratories in 1927, experimental work has been steadily pursued in order to learn the problems and the possibilities of this newest branch of electrical communication. The latest development to be demonstrated is that of two-way television as an adjunct to the telephone. As a result of our development work, there is now set up an experimental and demonstration system between the headquarters building of the American Telephone and Telegraph Company at 195 Broadway and the building of the Bell Telephone Laboratories at 463 West Street, New York City, two miles away. This system makes it possible to experiment with a method of communication in which the parties engaged not only speak with each other but at the same time see each other. Study of this system will serve to give information on the importance of the addition of sight to sound in communication and will give valuable experience in handling the technical problems involved. In principle the two-way television system consists of two complete systems of the same sort as those used for one-way transmission in the demonstration from Washington to New York City in 1927. In place of a scanning disc and set of photoelectric cells at one end for generating the television signals and a single disc and neon lamp at the receiving end for viewing the image, there are in the two-way system, two discs at each end and a bank of photoelectric cells and a neon lamp at each end. One of the discs, which in the system as constructed, is of 21-inch diameter, serves to direct the scanning beam from an arc lamp onto the face of one of the parties to the conversation. Three banks of photoelectric cells, making 12 in all, are arranged at either side and above the person's face and serve to pick up the reflected light and generate the television signals. The second disc, which is 30 inches in diameter, is placed below the sending disc and exposes through its holes the neon lamp, which the observer sees through a magnifying lens in a position slightly below that of the scanning beam. This neon lamp is, of course, actuated by the signals coming in from the distant end of the system, where there is a similar arrangement of two discs, photoelectric cells, and neon lamp. While we were talking and "looking" over the demonstration circuit, in New York City, of the Bell Laboratories' two-way telephone— television system, the wonder and satisfaction of the experience vividly called to mind the names of physicists and engineers who have contributed largely to the success of this development. The names of some of these men are : Dr. H. E. Ives, Dr. Frank Gray, Dr. Frank B. Jewett, John Mills, H. M. Stoller, Elam Miller, A. W. Horton, M. W. Baldwin, A. L. Johnsrud, D. G. Blattner and L. G. Bostwick — all of the Bell organization. The usually closed doors of present-day large research laboratories hide from general view the industrious workers engaged in making realities out of the wishes and desires of humans ever dreaming of "what next?" — Editck. Scanning Operation The two parties to the conversation take their places in sound-proof and light-proof booths where, sitting in front of the photoelectric cells, they look at the image of the person at the other end at the same time that the scanning beam plays over their faces. A problem of illumination is immediately encountered in that the scanning beam is of necessity intensely bright and tends to dazzle the eyes to the extent that the somewhat faint neon lamp image is hard to see. This difficulty is met by using light for scanning to which the photoelectric cells are extremely sensitive, but to which the human eye is relatively insensitive, that is, blue light. By interposing a filter in the path of the scanning beam, the spot of light in the lens which projects it is seen as a blue disc of light not bright enough to interfere with clear vision of the neon lamp which provides the image of the person located at the distant end In our original demonstrations of one-way television, scanning discs were used which had fifty holes arranged in a spiral. With this number of holes, it is possible to secure a definitely recognizable representation of the human face. It was decided, however, that for the two-way system a degree Dr. F. B. Jewett, President of Bell Telephone Laboratories, with Dr. Frank Gray of the Laboratories standing beside him in the booth of the television-telephone which permits each party in a telephone conversation to see as well as hear the other. of definition should be provided .such that faces were rendered in an entirely recognizable and satisfactory manner. Accordingly the number of scanning holes has been increased to seventytwo, which provides just twice the number of image elements. The transmission band is, of course, doubled by this change, requiring wire connections of considerably higher quality than heretofore. When a seventy-two hole scanning disc is used the component frequencies of the image signal encompass a range of from 10 cycles to 40,000 cycles per second whereas intelligible speech may be reproduced by a signal wave whose component frequencies cover a range of 2,500 cycles per second. This comparison indicates roughly how much more difficult it is to transmit high quality television images than it is to transmit ordinary speech. In general the electrical features of the apparatus are similar to those previously used, although in the interval improvements and refinements have been made in many directions. Glow Lamp Reproducer Light reflected into the photoelectric cells gives rise to an alternating electric current whose effective value is of the order of a ten-thousand-billionth ampere. The neon glow lamp on which the image is received at the distant station reproduces the image satisfactorily when the effective value of the alternating current is of the order of one-tenth ampere. This thousand millionfold increase in current variation, considerably greater than required for the earlier one-way system, is effected by amplifiers in which the vacuum tubes are coupled by condensers and resistances. The tubes, which operate at low energy levels, are shielded against electrical,