Radio Broadcast (May 1928-Apr 1929)

158 RADIO BROADCAST JANUARY, 1929 limited to frequencies below 5000 cycles is, in the writer's opinion, far from hopeless. It is far easier to ruin a picture with improper adjustments of the amplifiers at the transmitter or receiver. THE PROBLEM NOW we come to the problem of figuring out what kind of a disc the transmitting station is using by listening to it, and the writer has had some experiences along this line that may be of interest to recount. This form of "radio sleuthing" originated early in March, 1928, when an attempt was made to unscramble the signals sent by the Baird Laboratories in London to the Berengaria in mid-Atlantic. The details of this adventure were in the newspapers at the time and included such features as banging the characteristic notes on a piano and sending them over a telephone line to a piano tuner who was called out of bed to sound his tuning forks on the other end of the line to determine the absolute scan frequency. W2BU0, who assisted in this escapade, procured a fairly flat square brass disc of power-house flywheel proportions, in the haste to get the apparatus operating before the next nightly schedule, the corners were not even cut off. No further signals were transmitted, however, so it was never learned how accurately the number of holes and revolutions were calculated. A phonograph record was made of the signals as it is possible to preserve moving pictures this way. Recently a near-by station was secretly sending short television schedules that have been shrouded in a similar deep mystery. Trial on all discs and speeds produced nothing intelligible, yet the sounds apparently were genuine and had the characteristic variations of a person moving around or the scene shifting. The characteristic scan frequency was quickly found to be B above middle C on the piano, which, according to international pitch scale, would be 488 cycles. The pitch of the piano in question is according to standards that pianos assume that have not been tuned since radio became popular — usually lower. However wrny "tuned in" on G above middle C and their scan frequency is close to 360 cycles. (The keys now look like a log of Who's Who in Television.) Dust was blown off the old college physics book and the ratio of the two notes was found to be 4 to 5. The two musical notes on the piano still being good chords, both having lowered the same amount, the ratio was applied and a scan frequency of 450 determined. Now, as the scan frequency is the product of the number of holes and the speed of rotation per second, and neither of these factors were known, the problem was still quite a way from complete solution. Some slide-rule computations reduced the unlimited possibilities to the following probabilities: 60 holes at 75 r. p. s., 50 holes at 9 r. p. s., 45 holes at 10 r. p. s. or 30 holes at 15 r. p. s. Any of these cases would give the characteristic 450-cycle scan-frequency note. It was assumed that even speeds were used with no "fractional" holes or trick arrangement of holes. A NOVEL FREQUENCY COUNTER THE next step was to measure the picture frequency which is strong in cases where an unmodulated series of scan lines exist, as for example, the margin above the head of an individual being scanned, or other irregularities appearing once in each complete picture. In the case of the unknown signals in question they were too fast to count — one can count to almost 12 in a second — so a device was invented for the pur REAR VIEW OF TELEVISOR f~\NE of the most ingenious experimenters ^-s in radio to-day is Boyd Phelps. His work represents in our mind a proper example of genuine "amateur" experimenting. This article, describing experiments in sorting out television signals in which every factor but transmission frequency was unknown, will be found worthy of the reader's attention — not only because the work represents an extraordinarily ingenious procedure but because it indicates very definitely the difficulties of achieving results of any account at all in television experimenting. — The Editor. pose. The device consisted of a hand drill, a saucer and a bent nail. The gear ratio of the hand drill was such that the bent nail in the chuck made four taps on the saucer for every turn of the handle. The handle was turned at such speed as to have the taps on the saucer in step with the picture frequency, and the counting of the handle turns was easy. Thus, in a ten-second run a count of handle turns of 225, 25 or 37! would establish whether the picture frequency was 9, 10 or 15 per second or if not it would probably be a near-by value. The second time the signals were heard this was tried and every trial turned out very close to 37! so it was a safe enough assumption that the transmitter was using 30 holes in a disc running 15 r. p. s. (900 r. p. m.) A vibrating reed was used, and a variable-speed 48-hole disc produced stationary specs of the image at 1 5 r. p. s.; all checked the bent nail observation closely. Much has been written concerning the design of television discs so only the final data will be given here. A spiral inside an existing 48-hole spiral was laid out. In a 30-hole disc a maximum radius of slightly over 7" gives an image if" wide at the top. A picture height of i|" was convenient as this gave exactly 20 scan lines per inch. These scan lines, while 0.05" wide in theory, were made with a round drill of larger size calculated on circle overlap such that inscribed squares would be edge to edge. The sides of the theoretical square being 0.05", the diagonal (also circle diameter) figured 0.0706" and the nearest drill size was No. 50 having a diameter of 0.0700." THE RESULTS IT WAS the morning of the third day 1 when the disc was tried out and the interesting pictures watched with a thrill of one eavesdropping in on something unusual— like watching the antics of a comedian practicing in supposed solitude. This key-holing being absolutely a one-way affair added to the charm, due to security from detection. The question now arises, if I describe what I saw would I be violating my oath of secrecy sworn to on the back of my operator's license and the law not to divulge or publish the contents of any message not addressed to me or which I am not the authorized agent to forward? It was quite obviously not broadcast for public consumption, has had no advertising or publicity, and was preceded by a weak announcement, "Station 2X? conducting a test." The days of only a code operator being able to receive and divulge a radio message are over. Perhaps the oath of secrecy should be administered to the whole public and thereafter to all infants within 90 days of birth. The pictures on these pages show the apparatus used in the experiments described in this article. The front view of the televisor reveals that the container was once a phonograph cabinet — pioneering now in television as it did in broadcasting at 9 zt, when the writer sent its music to Minneapolis amateurs in 192 1. Below the speed-control knob and shaft may be seen the bias resistor for the power tube. This resistor, which is common to the plate supply and the grid returns, is employed to regulate the brilliancy of the picture on the neon tube which is connected directly in series with the plate circuit of the 210-type power tube. The double-throw switch behind the bias resistor makes possible a change from ear to eye " entertainment." An interesting feature of the televisor is the flivver speedometer which is mounted on the front panel. This instrument, having been taken apart and calibrated in r. p. m., is employed as a tachometer. The second picture shows the mechanical arrangement of the scanning disc assembly. A synchronous motor providing uniform speed is belt connected to a countershaft having friction drive to the back face of the scanning disc. The knob to the left turns a threaded brass rod that moves the countershaft assembly radially to frame accurately the picture and compensate differences in scanning speeds of various transmitters. The pulleys have additional small flanges (not shown) which quickly take a shorter belt for slower speeds. The neon tube is shown opposite the 30-hole spiral described in this article, but it can be raised easily to the level of the 48-hole spiral. Many methods of speed control have been tried but this system has provided the best results. 00 International}Pitch ' r^. o C\J O <T> O \o CO is id oi N *o \o f*» 00 o» FIG. I How Television Signals "tune-in" on the writer' s piano