Radio Broadcast (May 1928-Apr 1929)

126 RADIO BROADCAST JULY, 1928 baird's first "televisor" This crude but workable apparatus, undoubtedly the result of much labor on the part of Baird, who, like most inventors dependent upon their own resources, had very little money with which to carry on his experiments, gives an idea of how simple is the essential apparatus required for the production of television signals. The various disks function to break up the object to be transmitted into many tiny dots so that the light finally reaching the photoelectric cell is broken up into many consecutive impulses each of which corresponds in intensity to one particular spot of the subject. This original model has been placed on view at the Science Museum, at South Kensington, London illuminated. These two figures determine the size and quality of the possible image. They indicate 50,000 dots to the picture as substantially the possible maximum and strong artificial illumination as essential at the receiver, unless some way is found to maintain the illumination of the dots beyond the period of their stimulus. Transmitting an image and transmitting a musical composition are accomplished in the same way. The music is sent note by note in ordered sequence. We enjoy it as it is produced. A picture is similarly subdivided into dots of light and shade and these dots sent in any sequence, but they must all be received and placed in proper relationship before there is any picture. There is nothing to see until the transmission is ended. In telephotography, time is no bar to transmission because each dot is permanently recorded as received, and when transmission is ended we have a complete record. In television, each image is fleeting. There is no record. It is all over in a tenth of a second and the next image is on the way. Time is of the essence of television. It is largely the problem of time that makes successful telephotography meaningless with respect to television. A small picture sent in five minutes is commercially perfect but to send 3000 pictures of the same size in the same length of time is another story. REQUIREMENTS OF A GOOD PICTURE NEGLECTING color and form, for the moment, all pictures and images differ from each other only in the distribution of light and shade. The range of light intensities is of the order of one to thirty, as we go from deepest shadow to brightest light. But all these intensities are not usually sharply defined. They may shade into each other abruptly, hov/ever, as in the case of a church steeple standing out against a white cloud. Draw an imaginary line across the steeple and follow in your mind the changing light and shade along that line. From the white of the cloud you may change suddenly to a very dark edge of the steeple, and then come a continual series of changes through all shades as the detail of the steeple is recorded. Across a peaceful landscape even greater variations may be found as you follow a straight line through clouds, trees, leaves and grasses, flowers, dirt, stones, pebbles, and whatnot. These changes in light and shade are the "modulation" of the picture. Long experience with half-tones has shown that to produce a really good picture, provision must be made to reproduce 130 to 150 changes in light and shade to each lineal inch. This same modulation may be required up and down a vertical line as well as sideways along the horizontal. In other words, the modulation figure for a square inch may be of the orderof 20,000 changes. If the figure is made up of dots, 20,000 of them have to be printed to give the detail of a fine half-tone. On cheap news print, where the surface itself is rough, as low as 2500 dots per square inch are used in the poorest of newspaper reproductions. Most of the New York papers use 3969 dots per square inch, while this magazine and other popular ones on good paper uniformly use 14,400 dots to the square inch. Even with the highest of all these figures, however, details of cloud effects cannot be reproduced and the beautiful lights and shadows of woodcuts are impossible. In our television screen image let us aim no higherthan thedetail of anews print photograph. For each square inch of the picture there must be 2500 dots transmitted. For an image one foot square, which wouldn't give much of a view of a spectacle such as a ball game, there would be 360,000 dots. The last dot must arrive within a tenth of a second after the first dot, so the rate of transmission over a single waveband would be 3,600,000 dots or impulses per second. Each dot would exist only that small fraction of a second /Television signals from '<. Radio Receiver, BAIRD'S TELEVISION RECEIVER This shows the arrangement of the revolving disc, neon tube and translucent screen used in one model of the Baird television receiver. The light from the neon tube, varying in accordance with the picture signals, passes through the lenses in the revolving disc (which must rotate in synchrony with the transmitting apparatus) which focus the light on the screen; the image is viewed from the opposite side of the screen. The general system used here is very similar to that used by Dr. E. F. W. Alexanderson of the General Electric Company in his recent demonstration at Schenectady. The only difference was that the lenses in the disc and the screen were dispensed with and the observer saw the image by looking at the neon tube directly through the revolving disc. The received image is red in color — a characteristic of all television reception using neon tubes, with their characteristic red glow THE BAIRD "TELEVISOR" This transmitter, the result of experiments by J. L. Baird, makes use of infra-red rays, invisible to the human eye, but capable of affecting the photoelectric cell which converts the varying light signals into corresponding electric impulses. The infra-red rays, reflected from the subject transmitted, pass through the cellular structure which breaks up the light into many small sections or dots. The light then passes through the two revolving discs which rotate in such a manner as to expose to the light-sensitive cell at any moment only one of the light beams from the cellular structure. The resulting electric impulses are then caused to modulate the radio transmitter. The use of infra-red rays is not essential to the operation of this camera. Electrically, the system will function satisfactorily with any type of rays to which the photoelectric cell will respond. The amount of illumination required is quite intense, however, and, if ordinary lights were used, one would not be able to endure the intense glare for very long