Talking pictures : how they are made, how to appreciate them (c. 1937)

Talking Pictures The cells which are exposed to much light, as from the white paper of our page, will release a large current impulse when swept by the stream, while the ones in the dark parts of the type will release only a small current, or none at all perhaps. Those in a part which is neither pure white nor pure black will release amounts of current in proportion to the light in which they lie, as described before. Since the images usually sent by television are not all black and white, but of different intermediate shades, the television impulses are not all equally strong, as they are in a telegraph system. They are more like those in a telephone signal, in which the current varies in strength with the loudness of the voice sounds. From this preceding description, we can also visualize the method used for transmitting photographs over telephone wires. It is quite similar, but, instead of sending the entire page in one thirtieth of a second, several minutes are required. Further, instead of being received by some device which makes the image immediately visible to the eye, it is received on a piece of photographic film. It is impracticable to send the picture faster than this over the telephone lines, for they can carry only about ten thousand impulses per second. Instead of the letter being composed of merely four or five dots as in the telegraph, a television signal of that letter might require as many as fifty dots to define its form accurately. If we consider the image to be broken up into the tiny dots like a newspaper half tone, it would require approximately two electrical impulses to transmit each of the dots — one for the black dot and one for [282]