By Charles E. Phelps
There is a yarn about a young amateur who tried to explain to his “sweetie” all about radio. He dilated upon radio and audio frequencies, upon resistances, upon induction and harmonics and what not, and then concluded, “Now to you understand it?” “Oh, yes,” she replied, “but what makes it talk?”
There are a million people more or less like this young woman to whom radio is simply a mystery and who are merely confused by the average explanation of it. It is for these folk that this article is written so those who know all about it may as well turn a page.
If I were to pick a violin string even those with a simple ear for music could tell by the sound which string was vibrating. As an actual fact however, sound does not travel any more than the fire in the grate does. Both sound and heat, however, vibrate the air and the waves travel. To make sound there must be a vibration. Every sound has a certain rate of vibration of its own so that the ear can tell from the pitch what causes the sound. If I tap upon a door there is a certain vibration and if I knock upon the door-jamb there is another. These different sounds or sound-waves, each with its own rate of vibration, impinge upon our ear-drums and we recognize the cause. So the human voice does not travel but merely sets up a vibration in the air which does the traveling.
When we were youngsters we used to build the simplest sort of telephone systems of our own by knotting the ends of a long rosined string and running it through holes in the ends of tin cans. When the string was drawn taunt we could talk in one can and the voice could be clearly heard and recognized through the other, even at distances beyond the ordinary reach of the sound. What happened was that our voice caused the bottom of the transmitting can to vibrate as a diaphragm and the string carried this vibration to the other can and set up exactly the same vibration there, which in turn reproduced the sound. Over any considerable distance, of course, this vibration would die out and the reproduction would be so faint as to be unrecognizable. It was only when the electric current was brought into play that this vibration could be carried far.
The telegraph was the first instrument for conveying messages to far distances although as a matter of fact it conveyed neither sound nor soundwaves. Samuel F.B. Morse discovered that the electrical circuit could be used to convey thought by drawing down a bar upon a magnet when the circuit was “made” and releasing it by a spring when the circuit was broken. Then by devising an alphabet of dots and dashes he was able to spell out words merely by depressing and releasing a key with his finger. Alexander Graham Bell reached the conclusion that if two diaphragms were connected into this circuit and one of them vibrated by the human voice, the other would vibrate in sympathy and would reproduce the same sounds exactly as in the case of our rosined string and tin can. Out of this discovery came the great telephone system of today.
The Phonograph
Thomas A. Edison in his turn experimented with the telephone and it is said that one day a sharp edge on the diaphragm priced pricked his finger when his voice was vibrating it. This led him to believe that if the vibrations of the diaphragm could be impressed upon some substance which would retain them, it would be possible to reproduce the sounds long after the cause had ceased by merely passing another diaphragm over them. He took common lead foil and wrapped it around a cylinder which he turned with a small crank. He attached a needle point to a diaphragm and spoke into it. He then retraced the path and miraculously it seemed, the diaphragm repeated the words he had spoken. This of course was the first phonograph and the father of the Edison wax cylinder machine which was used for so many years. The present day disc record is familiar to everyone. If we could cut one of those records in two exactly in the center of one of the groves and immensely magnify the track of the needle, we would find it composed of millions of tiny hills and valleys of varying heights and depths and with no single inch of the contour similar to any other except where the same letter or musical note had been sounded. It is the number of these hills to the inch and their height which “sets the pitch” or reproduces the sound.
Radio Develops
Radio then is merely a step in the long path of carrying the vibrations of the human voice. It is built upon principles which were learned in the telephone, the telegraph and the phonograph. We have seen that it is only necessary to find a medium of transmitting waves in order to transmit sound.
Many men had a hand in the development of transmitting radio waves. The discoveries upon which the present system is based go back far more than a hundred years. Among these men were the Italian Marconi and our own De Forest. It was finally found that if vibrations could be set up at a frequency far above the ability of our poor ears to hear them, they would carry for long distances. At first this discovery was used, like the electric current had been, to transmit only dots and dashes but with the development of various devices, particularly the present day tube, it was found that this high-frequency wave could be vibrated by the human voice and would, like the telephone, vibrate another diaphragm at a distance in sympathy.
The broadcasting system is a highly complicated electrical apparatus which has the power of sending out vibrations into the ether. These are called the carrier wave and you often hear it when a station first begins a program and before the voice of the announcer comes. To each broadcasting station is assigned a certain rate of vibration called frequency. The carrier wave with no sounds vibrating the diaphragm or microphone can best be represented by the following diagram.
Each complete vibration is called a cycle and a thousand such vibrations are a kilocycle. The carrier waves of the different states are the same in diagram except as to the number of cycles to the second or to the inch. As soon as the voice begins to vibrate the diaphragm this carrier wave is “Modulated” exactly as the bottom of the tiny furrow on the phonograph record was, and this modulation vibrates the diaphragm of your radio instrument and reproduces the sounds put into the microphone far away. The carrier wave thus modulated might be represented as follows:
Thus we see how it is possible for you to reproduce instantaneously in your home or ranch or farm, in the mountains or on the sea, the music and talks sent into the microphone of the broadcasting stations thought they may be a thousand miles away. It is necessarily merely that you tune your instrument to the frequency of the particular station you desire and your diaphragm in ear-pones or loud speaker then vibrates in sympathy with that of the station you are hearing. Just how it is possible for you do to this and thus pick the desired station from hundreds of others will be explained in a later article.
From the Radio Index “The Tuning Book”. The diagrams mentioned in the above article on page 40.
www.worldradiohistory.com/Archive-Radex/Radex%203-2%2026%2011.pdf
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