Radio Broadcast (May 1928-Apr 1929)

RECENTLY the radio listener has heard many peculiar sounds from his loud speaker, and, if he plays around on short waves, he probably is familiar with an unusual noise which may be identified as a television signal. To the untrained ear these transmissions all sound alike — a terrible racket; to the experienced television experimenter the sounds often give a fair idea of the picture, even to permit recognizing the "sound" of faces of the immediate laboratory staff. For the somewhat less experienced, a little practice with the speaker and televisor operating simultaneously will enable him to pick out important characteristics of the, signal, as, for example, an abrupt change in tone quality when the picture contains two figures, as two individuals side by side. The experimenter who intercepts a television program of unknown origin has before him the intensely interesting problem of deciphering these signals and determining the number of scanning holes and the speed of the disc, for this may be obtained from laboratory tests. In this connection this article relates the author's experience in unscrambling mysterious television signals which were heard regularly on Long Island. The lowest frequency in a television signal cannot be classed as a musical tone; it is a rapid series of thumps coming at the rate of l\, 10, 15 or 20 times per second. This represents the number of complete pictures per second. A picture rate below 1 5 generally causes a noticeable flicker to the eye, like moving pictures projected at a slower speed than normal. THE SCAN FREQUENCY THE audio tone which seems to be the loudest single frequency in a complex television signal represents the scan frequency which is the mathematical product obtained by multiplying the number of holes or scan lines by the number of complete pictures per second. This product is usually 360, 450, 480 or 720 cycles per second, or intermediate values. This pitch seems especially loud because it is in a sensitive range of the ordinary amplifier and speaker, and because it is pure, regular and continuous. It corresponds to a musical note near the middle of the piano key-board somewhat above middle C. In television amplifiers, contrary to speech and PHELPS" EXPERIMENTAL TELEVISOR Unscraniblin Television By BOYD PHELPS music amplifiers, all audio tones below the scan frequency can be eliminated in many cases and quite good quality will remain. This feature is useful if a B-power unit is used to supply plate power for the amplifier, as a scarcely noticeable power-frequency hum in the speaker manifests itself on the picture as light and dark bands. When a power frequency of 60 cycles is used, four light bands appear if the disc is running 900 r.p.m. (15 pictures per second) and eight bands appear if 450 r.p.m. pictures per second) is the disc speed, for example, with 3XK and wrny, respectively. If these bands creep slowly up or down while the picture is held correctly framed it is an indication that synchronizing by the direct mounting of the disc on a synchronous motor would not be feasible, as was discussed in last month's article by the writer. Therefore, if the amplifier has a sharp low-frequency cut-off above 120 cycles, the interference caused by the a.c. hum is eliminated. The only exception to this statement would be a case where the a.c. modulation varies the overall efficiency of the amplifier, for if the desired signal frequencies are choked off 60 times per second a high-pass filter will not help this. An example would be low-current filaments operated on a.c. much below their correct temperature where the emission varies rapidly with small changes in filament voltage. But a good amplifier underloaded should amplify weak or strong signals proportionately, whereas stray a.c. hum picked up in any stage, if passed to the next tube at high loss, does not assume great magnitude or appreciable nuisance in the television amplifier considered above. The writer has recently thrown together a three-stage transformer-coupled amplifier in which practically all the iron in the cores has been removed and the flat section of the curve moved up considerably, which seems promising although curves have not been run as yet. We now come to the complex picture frequencies of a television ig^l signal which are the result of the detail of the image. If the picture were divided vertically into one light and one dark section we know the frequency would be 720 cycles in the case of the better forms of common television, therefore, it does not take much imagination to appreciate the fact that the details of a face — eyes, nose, moustache, etc. — may produce frequencies that will run well into the thousands of cycles For example, impulses crosswise of the picture, equivalent to the none-too-good detail represented by 48 vertical lines, at speed near the flicker point would be represented by a frequency of 48 x 48 x 15 which works out to be 34,560. Although decently recognizable faces can be produced without such high frequencies, the change from light to dark at the sharp contrast points, as the pupils of the eyes or edge of coat sleeve, is quite a ways from instantaneous, and in the received image the shaded gray area at these points may be several scan holes in width, although the photo-electric cell at the transmitter may be making its maximum change in a onehole width of the picture. But, in photography, portraits usually have their sharp harshness removed by an intentional diffusion. Television '57