Radio Broadcast (May 1929-Apr 1930)

1 STRAYS from THE LABORATORY ■ » 1 " " 1 » LISTENERS on short Distortion of waves who occasionally run Broadcast into telephone conversation Harmonics or broadcasting which they »■» > »-» cannot "clear up" may wonder what kind of stuff it is. The material is probably a harmonic of some broadcasting station and the "sour" quality is the result of the following phenomenon. The oscillator of the broadcasting station generates harmonics as well as the fundamental frequency. These harmonics are modulated as well as the fundamental, but the chances are that the amplifiers following the oscillator do not have linear characteristics as regards the harmonics. Let us consider only the second harmonic. If the fundamental is modulated with a frequency of 1000 cycles and at a modulation percentage of 60 per cent.; the second harmonic will have sideoands corresponding to the original modulation, i. e., 1000 cycles, and in addition a carrier of twice the original frequency with sidebands of double the original modulating frequency. This in itself would not account for the horrible garble that may often be identified as the harmonic of an otherwise wellthought-of broadcasting station. The additional distortion results from the fact that the percentage modulation on the second harmonic is doubled — and if the fundamental is modulated 60 per cent, the second harmonic will be overmodulated and, of course, distortion is inevitable. The mathematics on this subject will be found on page 95 of February Experimental Wireless and the Wireless Engineer in a communication from A. B. Howe of the British Broadcasting Corporation. „,;„,„,:„,„:„„„„,,.,.,::,:,„ «■„«-„,„ WHERE does a moving Cut-off of coi] loud speaker in a baffle Dynamic board cut off? In other Reproducers words what is the lowest . an »n« __i.it.mn frequency to which it will respond with any degree of efficiency? Consider Fig. 1. A sound wave originates at the rear of the baffle as well as at the front. If these two waves come together at the correct phase, they will interfere, and the resultant sound to the listener will be less than if the radiation from the rear of the cone were suppressed. The purpose of the baffleboard is to increase the path through which the air waves must travel from front and back before they can interfere. The distance from front to back via the shortest mechanical path must be at least one T" -V4A~f Cycles o quarter the wavelength of the lowest tone desired. Sound in air travels at about 1110 feet per second. A frequency of 110 cycles, then, has a wavelength of 10 feet from the familiar formula — useful at radio or audio frequencies — that the wavelength is equal to the velocity divided by the frequency. Thus, if the shortest mechanical path is one quarter the wavelength, 10 -5 4 = 2.5 feet, and so the distance from the center of the hole in the The following are among the subjects discussed in "Strays" this month : Distortion of Broadcast Harmonics Cut-off of Dynamic Reproducers Simplifying Filament Transformers Argument on Power Vs. Energy Series-Filament Circuits Data on Carborundum Crystals Movie Men Make a Discovery Power Output Requirements baffle around to the back must be at least 2.5 feet. This means that if the moving coil is in the center of a square board, this board must be 2.5 feet across. For 55-cycle reproduction the board must be 5 feet on a side and so on. A board three feet on a side will give good reception to all tones now being broadcast from the majority of stations. INTRODUCTION of the Simpiifing 2.5-volt power tube, the Filament ux-245, will make unneces Transformers cary more than one winding 1 "" » " on filament transformers. This is one step in doing away with the 1.5volt winding and the center-tapped 5-voIt winding. Thus, the introduction of a single tube standardizes and simplifies the construction of filament transformers. The winding for the rectifier tube, however, must be distinct from the filament winding. Will not someone develop a rectifier tube which can be operated from this same 2.5-volt winding? The solution seems to be a heater-type tube — but can someone develop a tube which will not break down under the comparatively high voltages? An Argument on Power Vs. Energy Fig. 1. MR. HUBERT WOODS, of Riverside, California, takes us to task in the following vein: •mm "Under 'Strays from the Laboratory' in your issue of February, 1929, your first remarks are concerned with power, energy, and efficiency, and are well taken, in general. "You define power as the 'rate of doing work.' Accepting this definition, how can you possibly 'consider a tube, such as a 171, feeding power into a load, etc.,' as you attempt to do in a subsequent paragraph? Do you mean that the tube feeds 'a rate of doing work' into a load? "What the tube really feeds into the load is electrical energy, not power, nor work. Work is the final result. "It seems to me you are guilty of the same lack of discrimination against which you properly protest. "The units of energy and work are the same, but are not those of power, which is a rate, according to your definition. My company purchases electrical energy, which we convert (most of it) into work immediately, by use of motors. We do not purchase power (per your definition) although each motor has a certain power rating, because it can do work at a certain rate. " If you had not burned your bridges behind you by defining power in one way only, you might possibly be justified in your usage of the term, since a common definition of power makes it synonymous with energy (ability to do work)." '"" """" 1 1 1 '" READERS who have writData on ten this office for data on SeriesFilament series-filament receivers will Circuits be interested to know that '"»'»-'""" "I--™ -I a collection of blueprints has been prepared by the Raytheon Company on how to wire well-known makes of receivers for series-filament operation. These diagrams may be had by applying to the Raytheon company, but it is earnestly requested that something more than curiosity be the basis of writing to this company for them. The diagrams will tell a serviceman how to rewire a receiver for series-filament operation, or will give him service data on a receiver of a given type that is already wired in this manner. ™ THE curves in Fig. 2 were Sensitivity of taken in the Laboratory by Carborundum measuring the current Crystals through a Carborundum « »»'"»«"'» Dmmmm Company detector as the voltage across the crystal was changed. Several readings were taken in an effort to show the effect of varying the pressure of the con -70 -60 -50 -40 -30 -20 . -10 5 0 +10 +20 +30 +40 +50 +60 > / / / / — Y y 5 -4 -3 -2 -1 0 +1 +2 +3 +4 +5 +6 E VOLTS Fig. 2. • may, 1929 page 13 •