Projection engineering (Sept 1929-Nov 1930)

Page 24 Projection Engineering, May, 1930 PHOTO ELECTRIC CELLS Two-way television is essentially the same in principle as the television demonstrated three years ago. A beam of light from an arc light is thrown by a scanning disc on the speaker's face, and reflected light is picked up by photoelectric cells and transmitted electrically to the distant end. The incoming image is seen by means of the lower scanning disc and a neon tube. A concealed microphone and loudspeaker act as speech terminal elements to complete the television-telephone system. mechanical, and acoustical interference. For the transmission of images between 463 West Street and 195 Broadway, the appropriate stages of the amplifier system are coupled by special transformers to telephone cable circuits equipped with special distortion correcting networks which are capable of transmitting the extremely complex current variations without distortion. The amounts of distortion inherent in other parts of the system are either kept small by design or annulled by means of correcting networks. Synchronizing Arrangement An indispensable part of a television system is the means for holding several scanning discs accurately at the same speed. For the two-way television, a simplified and improved synchronizing arrangement is used. The discs at the receiving and. transmitting ends, which rotate at a speed of 18 revolutions per second are synchronized by means of a vacuum Two scanning discs, one for sending (above), and one for receiving (below), are required at each terminal. Behind the upper edge of the receiving disc is the watercooled neon tube used to form the television image. tube oscillator located at one end of the line and delivering a frequency of 1275 cycles per second at a low power level. This frequency is transmitted over a separate pair of wires. At the receiving end this frequency, through vacuum tube means, controls the field strength of the motor and thereby holds its speed exactly proportional to the frequency. In the same way, the speed of the motor at the transmitting end is controlled by a similar vacuum tube circuit so that its speed is also proportional to the frequency of the same oscillator, and thus the motors driving the scanning discs at both ends of the lines are held in synchronism. By using a frequency of 1275 cycles per second, the degree of synchronization is held within sufficiently close limits to keep the picture at the receiving end central within its frame within a small fraction of the picture width. Novel features of this synchronizing system are the use of mechanically damping couplings between the discs and the motor shafts to improve the steadiness of the image, and of an electrical phase shifter for framing the images. Face to Face Conversation The acoustic portion of the two-way television system is unusual in that -it permits simultaneous two-way conversation without requiring either person to make any apparent use of telephone instruments. It is obviously desirable to ai'range the acoustic system in this way because the ordinary telephone instrument conceals part of the face and would thus prevent the system from approximating to the conditions of ordinary face to face conversation. The elimination of telephone instruments is accomplished by the use of a microphone sensitive to remote sounds and a loudspeaker concealed near the television image at each station. The microphone at one station is connected through suitable vacuum tube amplifiers and a telephone circuit to the loudspeaker at the other station. This permits conversation in one direction while a similar connection between the other microphone andloudspeaker permits conversation in the other direction. The persons using the system then communicate as if face to face and with no telephone system apparently involved. In order that the transmitted sounds may be familiar and natural, distortion in the sound transmission system has been reduced to a minimum. The microphones are of the condenser type used extensively in radio broadcasting and sound picture recording. Being of small size, they are readily concealed near the television image in the most advantageous position for picking up the voice. The loudspeaker, also of small size but capable of reproducing a broad frequency range, is likewise concealed near the television image, so that the sounds produced appear to emanate from the image itself. This loudspeaker is of the moving coil type with a small piston diaphragm. "Howling" Avoided In any system such as that described, the microphone is not capable of distinguishing between the sounds from the local speaker or from a speaker at the remote end of the circuit reproduced locally by the local loudspeaker. If the sounds from the local loudspeaker should be impressed upon the local microphone in sufficient magnitude, "singing" would result, and the system be no longer operable. To prevent this the microphone and the loudspeaker are installed in carefully chosen positions and the inner surfaces of the sound-proof booths are specially treated to prevent as much as possible the reflection of sounds from the walls into the microphone. Under these One of the water-cooled neon tubes used for the reception of television images. While in operation the central rectangle glows with a pinkish light. conditions, the attenuation of sounds transmitted is of about the same magnitude as would be experienced if the listener were say 10 or 12 feet away but in the same room. This acoustic illusion of distance is in harmony with the visual appearance of the television image. In addition to the television synchronizing, and acoustic circuits, others are provided for signaling and monitoring purposes. Matters are so arranged that an operator can see both the outgoing and incoming image, and by means of movable lens and prism systems can insure that the scanning beam is properly directed to correspond to the height of the observer and that the magnifying lens in front of the receiving disc directs the image to the observer's eyes. Operating arrangements are made so that the two parties to the conversa(Conclnded on page 26)