Lapas attēli
PDF
ePub

which of recent date much publicity has been given, operate on these same principles.

Some details have, of course, been much improved. Unquestionably, new principles, which will result in a practical device and which will be followed by at least the same eager adoption by the public that radio enjoyed, will come in the future. The large corps of inventors put to work on this problem will aid considerably in hastening this time. Perhaps we are on its brink today. But as yet the old principles still persist.

Basic Principles.

In order to explain the principles of operations, it would perhaps be better to first analyze the basic principle of any transmission of intelligence. When we have a thought or an idea which we desire to tell to some one, we have for our assistance in this, "language" consisting of thousands of words. These words can be combined in different combinations to express our thoughts. One particular combination will produce that thought or idea which we desire to relate. Speech, therefore, has as its unit the word and a thought is a selective combination of words. All signaling, as will soon be pointed out is exactly that-a selective combination of a predetermined unit.

When, however, we come to writing, it is necessary to further reduce the words to a simpler unit, the "sound" represented by letters. Twenty-six letters represent the units in English and these in different selective combination go to make up any word of the English language.

In telegraphy, however, it was found necessary to still further reduce the unit because it was impracticable to have twenty-six different kind of electrical impulses. Instead, there are two-the dot and dash and these in different selective combinations make up the twenty-six letters of the alphabet which in turn produce words and these in turn produce thoughts, ideas of intelligence. A telegraph message may, therefore, be said to depend for the thought desired to be sent on two components; name

ly, the combination of dots and dashes sent and the order in which they are sent.

Similarly, it is true in picture transmission, that if a unit can be found which as a whole produces the picture just as dots and dashes as a whole produces a telegraph message, such signaling would be equally as simple. And, of course, there is such a unit.

When a combination of various shades and degrees of light are reflected from a surface, the result is an optical effect called a picture. That is, a picture might be said to be made up of an infinite number of tiny areas each emitting a shade or degree of light, the composite effect being an orderly object.

If now each of these areas could be analyzed so that the light it reflects can be isolated from the other areas and each of these light beams observed, signals could be sent to represent them. This broadly, was in fact, the first system proposed.

[merged small][graphic]

Specifically the picture was composed of black and white under a transparent cross hatch paper so that

the effect was very much as that shown in the figure, the object showing through the lines. Each little section. was numbered 1, 2, 3, etc. The operator observed the first area and since it was white sent a signal to represent white, the signal having been prearranged. At the receiving end, a paper similarly prearranged had the same number and size of sections. On receipt of the signal, the first area was left white. If now the second area was black, the corresponding signal was sent and the receiver filled this area in with black.

This crude system had many obvious faults. First there was the important question of time. To transmit in this way meant the use of a good deal of time and line time is expensive. A picture two inches by three inches would take over an hour to transmit.

Then there was the human factor. The system depended for accuracy on the skill with which the transmitter operator analyzed the light and the skill with which the receiver filled in the areas with the proper shadings. Error at both ends crept in. Furthermore, the mesh used could not be very fine.

Optical Systems-Transmitter.

The next step came with the discovery of Selenium characteristics. Selenium is an element which has the property of changing its resistance when exposed to light. It at once becomes obvious then, that each of these areas, if successively brought before a selenium cell so as to reflect its light surface thereon, could be made to automatically translate the light into electrical energy.

The simplest example of this system is illustrated in the following view. The picture in the form of a film

[graphic][subsumed][subsumed][subsumed][subsumed][ocr errors][ocr errors][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed]

is laced on a cylinder. The cylinder is fastened to a shaft which turns on a worm screw. In the center there is a light source. The whole thing is placed in light tight chamber having a recess and in this is placed a selenium cell. The receiving end looks exactly the same except that on the cylinder there is a light sensitive film and the selenium is electrically connected to an electric lamp. Both worm screws are driven by motors. At the transmitter, a beam from the light source shines through that portion of the picture which at that instant is opposite the selenium. The intensity of light between the picture and the cell depends on the thickness of the film at that point, or in other words, the shade of the picture.

This intensity of light to which the selenium is exposed will then determine the amount of electricity which will flow in the line, because there will be either more or less resistance of the cell, depending on whether there is more or less light.

[graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed]

At the receiving end the electricity lights a lamp, the luminosity of the lamp depending, of course, as is obvious, on the amount of current in the line. This lamp throws a beam of light then through the aperture shown and effects that portion of the light sensitive film which at that instant is opposite the aperture.

We have then light to which a light sensitive film at receiver is exposed proportional to electricity in the line, the latter in turn proportional to the resistance of the selenium cell which in turn is proportional to the amount of light to which it is exposed from the picture. The in

« iepriekšējāTurpināt »