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October 1930 Radio-Craft[Table of Contents]
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Radio-Craft was published from 1929 through 1953. All copyrights are hereby acknowledged. See all articles from Radio-Craft.
It really wasn't all that long ago when most people worked on computers with Color Graphics Adapter (CGA) that had just 16 colors (4-bit pixels). In the late 1980s (wow, maybe it really was a long time ago), the luxury of a 256-color (8-bit pixels) Video Graphics Adapter (VGA) monitor and video card would cost you around $300 each. I recall seeing ads for "16 million color" displays by ViewSonic that ran north of a kilobuck. My first 'real' monitor was bought in 1987 and was 4-bit monochrome. Televisions, as you know, began as black and white (actually a infinite number of gray levels between black and white). When TVs first arrived in people's homes, they were glad for any kind of display, but it wasn't long before marketing gurus convinced the masses that anything less than color would be unacceptable. This article from a 1930 edition of Radio-Craft describes one of the early attempts to satisfy consumer demands. It was long before the NTSC standard was adopted for a purely television electronic system. Hence, here you will find a combination of mechanical and electronic devices.
Ingenious novelties developed by European experimenters in the popularization of "sight-at-a-distance"
By Dr. Fritz Noack (Berlin, Germany)
The success of colored motion pictures, for which special films are prepared, suggests the application of somewhat similar methods to television. However, colored television necessitates either a special wavelength for each color employed, or else modulating the transmissions at twice or three times the normal image frequency. Either widens the waveband to an extent which is prohibitive.
In addition to this, the superposition of two or three pictures, each in a single color, does not give an absolutely natural effect; because the spectrum of visible light is much more complex.
Then, too, the methods of colored television heretofore published are in principle exactly like those used in black-and-white reproduction. The different image points, varying in illumination, are converted into corresponding electrical impulses at the transmitter; they must be faithfully and exactly reproduced at the receiver, to have a perfect picture. But there are many causes of faulty reception, the most unpleasant of which is fading; not only does this cause interruptions of reception, but it also suppresses parts of the transmission, corresponding to certain frequencies. In a loud speaker, this means that sounds are lost or changed in timbre; in a television, that details may disappear entirely.
The effect of fading may be overcome in telegraph work by employing such a modulation that the signal swings back and forth between zero and a fixed value. This, for instance, is done in the transoceanic short-wave work of the Telefunken Co.; a receiver of amplification so high that the signals never disappear entirely is connected to recording apparatus through an automatic volume control. This, of course, is impossible in telephony, where variation of loudness as well as frequency is part of the signal; and it is also unsuited to television, where we must reproduce different light-values. If a process of scanning should be adopted, in which pictorial points of the same intensity are always selected, then fading could readily be overcome.
Dr. Schroeter, of the Telefunken Co., has made the suggestion that the image points be distinguished not by reproducing them at equal intervals with varying intensity, but by giving them the same brilliancy for varying periods of time; just as in telegraphy dots and dashes of the same strength are sent out, instead of dots of varying strength. In order to make this practical, it will of course be necessary to invent a method of converting the image from points of varying brilliancy into dots and dashes representing the same light values in terms of length.
However, a television system has been announced, which overcomes these difficulties and makes color television possible without widening the waveband, without fading and with the greatest fidelity to nature. It is that of a Berlin engineer named Ahronheim, who has lately acquainted me with its details.
It is based upon the assumption that the colored image points show, not different intensities of light, but different color tones that dark red and light red are not merely reds of different intensity but, actually different colors. His methods of scanning the image are the same as in previous systems; the novelty lies in filtering the light before it enters the photo-cell, according to its place in the spectrum. The visible spectrum contains many gradations of color, but Ahronheim undertakes to reproduce it with twelve.
Then, if a revolving disc is arranged with colored glass sectors (Fig. 1) through which the light must pass from the scanning apparatus to the photo-cell, the ray of light can penetrate only the filters of appropriate color. The scanning mechanism operates as in other systems; and the filter disc only must revolve at higher speed, to make up for the fact that the light ray penetrates only one of its sectors at each revolution.
At the receiving end, a suitable system of scanning is used to build up the image; but here also a glass filter with twelve sectors is placed between the source of light and the eye of the observer. If the two discs are synchronized, a picture corresponding most exactly to the image at the transmitter will be seen.
Since the photo-cell receives just as many impulses as with black-and-white telegraphy, the modulating frequency required is no higher for colored television; and if the transparency of the filter is properly regulated, the image impulses in the input of the transmitter, and the signals it sends out are of uniform intensity.
Use of a Prism
However, Ahronheim proposes, not to build a mechanical filter system of the kind described above, but to utilize a prism which, as we well know, decomposes white light into its constituent colors. The entire spectrum will come out only when light has been directed into the prism; light of a single color will emerge unchanged. Since the light rays are dispersed at different angles, according to their wavelengths, we could arrange twelve photoelectric cells side by side behind a prism; so that one would receive all the dark-red light which entered the prism, another all the light blue, etc. In practice, only one cell will be used, however; and, by the use of a scanning device, only the color corresponding to the image point reproduced at the instant will be conveyed to the cell.
The model at present completed is arranged for but a few color tones; but it demonstrates the fundamental characteristics of the invention. It is especially well adapted for televising colored motion-picture film, and it is possible to transmit simultaneously from the sound track. It is understood that an international moving-picture organization is interested in the invention.
Universal Television Receiver
At the present time, the English and German television transmissions are extremely interesting to radio enthusiasts. However, the systems used differ in details; the German transmission are made at the rate of 750 thirty-line frames per minute, framed at the top of the disc; the image is a third wider than it is high. In the English (Baird) system, while the number of lines and rate of speed are the same, the image is viewed at the side of the disc, and it is higher than it is wide. Since this presents complications to the set owner, a method of adapting a radiovisor to different systems will be valuable.
In America, where several types of transmission are in use, it has been proposed to pierce the scanning disc with several spiral rows of holes, at different distances from the center; this also makes it necessary to shift the glow lamp and "window" for each reception. In addition to this, the images received nearest the center of the disc must be smaller.
The use of separate discs entail unpleasant labor in interchanging them, which would be too much of an annoyance for the set owner who purchases his equipment.
However, the new Fries system overcomes all these difficulties. Radiovisors using this method have already been produced commercially. The simplest, intended for the present transmissions by English and German broadcasters, have two picture windows and two rows of holes in the disc; the regular glow lamp is used. The holes for the English transmissions are on radii midway between those for the German; since there are thirty of each; but the pitch of the spirals is different. Only one glow lamp is required.
To prevent the two sets of holes from causing optical interference, a special disc is used to cover one of them. It has a set of 30 slits, and may be turned on the axis of the main scanning disc to uncover one or the other spiral; it is then clamped tightly to the main disc. The English image, received sideways at the top of the disc, is made to appear right-side-up by the use of mirrors.
Four Discs Required
However, the inventor has carried his idea to the point of making it possible to receive transmissions on other systems, with holes of different numbers and different sizes. He accomplishes this, as shown in Fig. 2, by the use of four discs - two pairs of which are alike. Two of these have spiral slits and two have radial slits; the latter pair 20 each, presumably the least number of pictorial lines which will afford an image.
One disc of each pair is keyed to the axle, while the other may be turned, as before, to open or close the slits, before clamping it to its mate. The result is, that the holes apparently produced by the passing of a radial slit over a spiral slit may be made of any size suited to the image being received. The spiral discs turn once while each frame of the image is being scanned the radial discs are rotated by a separate shaft, at a speed sufficient to provide the necessary number of lines in each frame.
For instance, in the case of the German and English transmissions, the spiral discs revolve 12 1/2 times a second; but since the number of lines in the received images is 375, the 20-line radial discs must turn at the rate of 18 3/4 times a second, or 1125 R.P.M. The width of the slots is, necessarily, related to the number of lines in the image: the more lines, the finer the slots must be.
In the Fries system, it is necessary to synchronize the two parts of the scanning system with each other, as well as with the transmitter. The simplest method of coupling the pairs of discs is by gears; but this would necessitate frequent gear shifting. For that reason, two electric motors are connected to vacuum-tube generators which modulate each other. As will be seen, a good deal of complexity is bound up in the operation of this device which, however, demonstrates a solution of the problem of universal television.
In the endeavor to overcome the difficulties caused by the low available light intensities, in practical television, Col. Schildenfeld, an Austrian inventor, has patented the idea of applying regeneration to a photocell system. It is well known that the amount of light required for successful television is often rather trying, to the subjects and, outside of laboratories, it is seldom available. By this invention, the light impulses are caused to modulate a light source which shines directly back into the photoelectric cell.
Many practical difficulties, of course, are yet to be solved, including those of time-lag and possible optical "oscillation." The idea is especially well suited to oscillograph work; but it is being tested for its possibilities in television.
Posted September 16, 2015