Radio Broadcast (May 1928-Apr 1929)

60 RADIO BROADCAST ADVERTISER I The Choice of the Leaders Junior Rheostat When the leading manufacturers of the country choose the Yaxley Junior Rheostat for their finest radio sets you are safe in following their good judgment. The Junior Rheostat is made to the highest Yaxley standards — a master instrument, with a smooth movement and lots of service stability. Diameter, lj\ inches; mounts in single T\ inch panel hole. Junior Rheostats, with Bakelite Knob, up to 400 Ohms, 75c; 1,000, 2,000, 3,000 Ohms, $1.00. Junior Potentiometers, 25c extra. Switches for Junior Rheostats, Self-Contained and easily attached, 40c. Resistance Units Absolutely dependable. Run true to rating. Have convenient screw eye and soldering lug for easy mounting and wiring. Space' wound, 1 to 60 Ohms . . 15c 100 to 400 Ohms 25c Tapped resistances, 6 to 64 Ohms 30c 100 to 400 Ohms 40c Grid resistances, 100 to 500 Ohms 25c 600 to 3000 Ohms .... 35c Your radio dealer or Jobber has them in stock YAXLEY MFG. CO. Dept. B, 9 So. Clinton St. Chicago, Illinois No. 238 Radio Broadcast Laboratory Information Sheet November, 1928 A Hook-up for ShortWave and Broadcast Receivers A METHOD FOR SWITCHING OVER TT IS general practice in constructing short-wave adapters to arrange them with extension leads so that they may be plugged into the broadcast set in the detector socket in place of the regular detector tube. This practice is all right when one is building an adapter that perhaps will not be used continually, but when both the broadcast and the short-wave tuners are going to be used frequently, it is better to arrange the circuit as indicated in the diagram on Sheet No. 239, which permits one to change from broadcast to short waves by a simpler means than taking out a tube and plugging in an adapter. The diagram shows the detector of the broadcast receiver and the detector of the short-wave receiver. They are both wired to the same A and B voltages, and either set is thrown in or out of operation by simply turning the proper filament switch, Si or &; Si turns on and off the broadcast receiver and & similarly controls the short-wave set. The two plates are permanently wired together, and for this reason the arrangement we have indicated should only be used when the two sets are located close to each other (which is usually the case) so that the plate lead running from one set to the other is not more than 1 or 2 feet long. Most of us have available only one antenna to use with both sets. To use it with both receivers a single-pole double-throw switch, S , can be placed in the antenna circuit; thrown to one side it connects the antenna to the broadcast set, and thrown the other way it connects the short-wave receiver. An easier arrangement, which works well in practically all cases ihow well it works depends upon the characteristics of the two receivers) is indicated by dotted lines. The antenna is connected directly to the broadcast set and through a 50-mmfd. condenser, C, to the short-wave set. This small condenser will block the broadcast signals from the short-wave set but permit these latter currents to pass quite readily. No. 239 Radio Broadcast Laboratory Information Sheet November, 1928 Circuit for ShortWave and Broadcast Reception BROADCAST RECEIVER mm B*Del SHORT-WAVE RECEIVER No. 240 Radio Broadcast Laboratory Information Sheet November, 1928 Television data on the bell telephone laboratories METHOD THE demonstrations of television given by the Bell Telephone Laboratories, associated with the American Telephone and Telegraph Company, rank higher, in our opinion, than any of the other demonstrations so far given, in quality of the results. In the following paragraphs are summarized some of the most important elements of the apparatus used by these Laboratories. (a) The scanning discs contained 50 holes and revolved at a speed of 1062.5 revolutions per minute, giving 17.7 pictures per second. 03) Thejoutput voltage of the photo-electric cells at the transmitter was about 10 microvolts. (c) The range of frequencies decided upon as being essential for good quality extended from 10 to 20,000 cycles. Overall measurements on the final amplifier indicated a frequency characteristic constant within plus or minus 2 tu over this range. (d) The signals from the transmitter were ampli fied and delivered to the transmission line at a level of 10 milliwatts. The amplification from the photoelectric cell to the line was 130 TU. (e) Synchronization was accomplished by the use of synchronous motors containing 120 poles and having a synchronous speed of 1062.5 r.p.m. The angular phase displacement was above 0.07 degrees. This magnitude of phase displacement corresponds roughly to the angular twist in a steel shaft 6 feet long of 1 inch in diameter, operated at full load. (f) With regard to the effect of extraneous currents due to noise, it was found that satisfactory results were obtained if the average picture currents were 10 times greater than the average noise currents. This corresponds to 20 tu, or a power ratio of 100. In ordinary sound broadcasting the noise in the telephone lines is kept at a level 60 tu below 10 milliwatts, giving a power ratio of 1,000,000. It is evident that it is permissible to have the noise level much higher in television reception than in sound reception.