PICTURE TRANSMISSION.

Otago Daily Times, Issue 20708, 4 May 1929, Page 26

PICTURE TRANSMISSION.

NEW , MARCONI SYSTEM. HISTORY OF TRANSATLANTIC SERVICE. Practically ever since the inception of the telegraph, the problem of picture transmission has fascinated electrical engineers. 'As early as 1842 Alexander Bain invented a highly ingenious system of picture telegraphy, and many others have followed more or less in his footsteps, with widely varying results. The first telegraphic picture service opened to the public, however, was that which has been in commercial operation between Radio House, the Marconi main telegraph office in London, and New York since Slay, 1020. Some hundreds of news pictures, cartoons, fashion plates, signatures, cheques, and plans have been transmitted commercially across the Atlantic by this system. The success of the Marconi short-wave beam telegraph services has opened the way to new-achievements in fasshnik transmission,; and the Marconi Research Department has, during the last three years, been developing a system adapted specially to short-wave wireless and to landline circuits which would give higher speeds and clearer reproduction. In fact, they arc preparing a true -facsimile service'that may in time replace Morse telegraphy on busy circuits and make it the normal procedure for telegrams to be received in the actual handwriting of the sender. ■ . ■ How far the research engineers have succeeded is to be demonstrated to-dayj given reasonable wireless conditions. A charming forecast of the happy future that may lie before the prosaic telegram of to-day may be found in the experience of recent .facsimile tests. Little Bryan" Davis, the son of the. English engineer in charge of the American end of the experiments, was able to send" a picture of himself in his new’hat with his New Year message to his grandparents at Chelmsford. Bryan is one of the proudest boys in the world to day, for lie was the first hoy to send his own New Year letter and picture by the Marconi facsimile system. He has also persuaded the- engineers to transmit favourite pages of his picture hooks and adventure stories across the Atlantic for test transmissions, and the engineers say that with their simple pictures and clear type these make ideal facsimilegrams—though perhaps rather too easy. . In order to produce a system commercially applicable for everyday telegrams, the Marconi engineers deliberately scrapped practically all that had previously been done in the field of picture transmission, and started afresh. The result is that the Marconi system differs greatly both mechanically and electrically from any other, and while there arc certain difficulties still to be overcome before practical perfection is attained, it is now possible to transmit two images each 8 inches by 10 inches in less than 20 minutes. These messages have the advantage of unimpeachable accuracy, and the possibility of including not only handwriting but the reproduction of anything that can he written or drawn on paper in the telegrams transmitted—a particularly valuable feature when complicated columns of figures and diagrams are required to be sent. During the tests the research engineers themselves have greatly appreciated the convenience of the facsimile messages, which have enabled them to receive without delay diagrams, graphs, and other valuable data in connection with the experiments. From the operating point of view, the most important novelty about the Marconi facsimile gear is that it runs continuously. It is not necessary to stop the machinery every time a new’ picture is put on the transmitter or taken off the receiver. As soon as one message is finalised another can be placed on the cylinder and transmission begins at once. This point alone effects a great economy in obviating the running-up of the machinery and synchronising the ,two ends of the circuit separately for-every picture. In addition, no" preparation o*f the message to he transmitted is required; and the reception is" perfectly direct and can be checked as if is taking place; enabling faults to be instantlydetected and corrected. - The apparatus being, also, ingeniously arranged to transmit two pictures at once, in commercial working one channel could be kept busy on “ ordinary ” telegrams and the other reserved for “urgent” messages. Essentially, tiic transmitter provides a means of translating the blacks and whites of a picture or of written matter into electrical impulses that may be transmitted by wireless. This is accomplished by focussing a very fine concentrated point of light in turn upon every point of tile image to be transmitted and conveying the .reflection, which varies in intensity with the degree of whiteness or blackness of the image, to a photo-electric cell. As a result of the Variation in the light -thrown on to the cell its internal resistance varies ami a current of varying intensity flows through it. This current is then amplified and transmitted by means of an ordinary beam telegraph transmitter. The process is as follows:—Themes-, sages to be transmitted arc placed face downwards upon a metal cylinder, being held in position by a flexible band. Around the metal cylinder is a narrow slit, across which the messages arc slowly drawn by a traversing mechanism. The point of- light, focussed through the slit, revolves and so lights up, successively, every part of the image as this travels along the cylinder. The light is provided by a powerful motor headlamp, which is focussed into a fine spot on the surface of a steel chopper disc, 1-t inches" in diameter, with 144 equally spaced holes near its periphery. The chopper disc i •: run at speeds ii]i to 5000 revolutions per minute, its purpose being to provide, a convenient carrier frequency which is modulated by the varying intensities of the image. The maximum carrier frequency given by this arrangement is 12.000 cycles per second, the normal rate being .0000 cycles per second. The interrupted light emerging from the chopper disc is collected by a con-densing'-lens and focussed down to give mi intense illumination in'"a 1 pin hole, from which .the. light is collected and converted into a parallel beam. This parallel beam of light travels down the hollow spindle of the motor driving the optical rotor—which is the means of revolving the point of light—to the inside of the transmitting cylinder, and is there thrown, by means of the prism lens of tlie rotor, through a slit in the cylinder on to (lie imago. to he transmitted. The rcliectcd light from the image varying according to the tone shades, is collected hy a lens and again converted into a parallel beam which 'is thrown on to the photo-electric cell. ; The ' voltage of the current flowing through the photo-electric cell, varied by the intensity of the light thrown upon it, is applied to the grid of the first of a number of magnifying valves, the output of which is utilised to modulate the beam transmitter. The receiver is similar in, its main features to the transmitter. It provides a means of reversing the process carried out in the transmitter, for it translates electrical impulses into terms of light. This is achieved by the application of a phenomenon discovered by Kerr in 1875, a Kerr cell acting as a light shutter, and allowing a spot of,light to fall on a sheet of sensitised paper only when an electrical current is passed through the cell. The arrangement of the receiver is as

follows: —A source of light is passed through two Nicol polarising prisms, between which is placed, the Kerr cell, which is actuated by the incoming wireless signals. The light traversing the first Nicol prism is polarised and passes between the two small electrodes of the cell, which is filled with nitre-benzine. After traversing the cell, the light passes to the socond . Nicol prism, which is so arranged, that the light is completely cut oil from the receiving cylinder when no voltage due to an incoming wireless signal is applied to the electrodes of the coll. When a wireless signal arrives and causes a voltage to be applied to the electrodes of the cell the light is no longer cut-off by the second Nieal prism, but is able to pass to the receiving cylinder. This light is then collected in a parallel. beam by an optical rotor which throws a. fine spot of light tlirough the slit in the receiving cylinder on. to a piece of photographic bromide paper which is placed around.it and laterally pushed oyer the slit. In this manner the light and dark portions of the image on the transmitting apparatus are, faithfully recorded at the receiving end. The photographic paper on tlie receiving cylinder is held down by a thin flap of celluloid through xyhich the position of the received image may during reception be observed from the revolt ing streak of light which scans the paper through'the'slit' in the cylinder. It is obvious that in order to reproduce at the receiving end the exact image placed on the . transmitter, the transmitting and receiving optical rotors should synchronise in their revolutions, so that at any given moment the spot of light on the bromide paper on the receiver is focussed on a point exactly corresponding to, that illuminated by the spot of light on the original image. . Synchronous rotation is achieved by motors whose , speed is controlled by valve maintained tuning forks at the transmitting and receiving stations,, In order to obtain constancy in the rate of vibration the forks are enclosed in a heat-insulating box,, the air of which is kept at a steady temperature. If the temperature falls below a certain point, heating lamps are automatically switched on by means of a toluol regulator, and as soon as a highly pre-deter-mined temperature has been reached the lamps arc automatically switched off. Two fans in the box stir the.air in order to keep an even temperature. By this arrangement the temperature in the box is maintained constant to within 0.05 degrees Fahrenheit, as can he observed for checking purposes by' means of a thermqmcter which projects through the top of the box. When starting, up the machines the tuning fork is made to vibrate and either the natural frequency, 'which'is 300 cycles, or the second or fourth harmonic of the fork frequency can be selected amplified, and passed through the armature winding of the synchronous motor in the driving machine. The frequency selected- is also made to illuminate a Neon tube mounted over a stroboscope wheel on the: spindle of the motor driving the optical rotor, which may then be adjusted'so that a clear stationary image of the*, stroboscope is obtained. The direct current motor is then locked and the driving mechanism is automatically kept in step with the fork frequency or its harmonic selected. The new Marconi facsimile system is .in every way suitable for the transmis-. sion of. pictures over landlines, the speed with which pictures can be transmitted depending on the quality of the landlines.' The same quality which enables laiidiines to carry the highest musical, notes without serious diminution in volume allows the transmission of pictures at maximum speed. The Marconi apparatus is capable of taking full-ad-vantage of the quality of tile very high grade landlines such as are used , for relaying broadcast • programmes.