Radio broadcast .. (1922-30)

do not overload on the lower frequencies but fail to reproduce satisfactorily and in their proper proportion the overtones which lend the distinctness to different instruments of the orchestra and are of especial importance in obtaining clear articulation of speech. A comparison test is excellent indica- tion of the relative audio-frequency range of two receivers. Judgment, however, should also be passed upon the ease with which the low notes are handled as well as the frequency range of the audio system. Freedom from overloading under conditions of normal reception can be detected readily by an experienced ear which perceives manifestations of overloading as tendency to rattle and resonate. More than one receiver has an audio system which tends to oscillate at the low notes, giving it a slight effect of persistence which is even pleasing with certain types of music, such as organ and cello, but is decidedly detrimental to good articulation and brilliance. Considerable assistance may be had in this process if a milliammeter is placed in the plate supply of either the output stage or the entire plate supply. If the milliammeter tends to fluctuate while low notes are being repro- duced with good volume, it is definite indication that overloading is ex- perienced. Another method, which has been used successfully, is to connect a fixed conden- ser of one microfarad or more across the loud speaker terminals. The higher frequencies pass more readily through such a ct idenser than the lower. With some re- ceivers, a one-microfarad condenser so connected makes little difference even in the articulate- ness of speech, while with others, having faith- ful amplification not only of the lower frequencies but of the high frequen- cies as well, there is a marked falling off in clarity. To one not trained in detecting the contribution of the higher frequencies and overtones, this procedure presents a simple, if crude, method of by-passing them so that their effect can be observed by comparison. Com- pensating adjustment of volume control should, of course, be made before and after connecting the condenser in order that the comparison be made under similar conditions. Speech is the most ready source for determining the presence of the higher frequencies. Although most of the volume of speech is given by frequencies below 1500 cycles, understandability or articulation is very largely a matter of the higher frequencies. The violin and the soprano voice are also excellent means for determining the presence or absence of amplification at the higher frequencies. The fidelity tests suggested outline means not only for making a general judgment of the reproduction but for HENRY W. JOKES * MOIO SALES »NO SERVICE , hj „ .peered stations coverec (bj I Selectivity (1) Compare dial ranges < C) ^Sare volume o, distant stations through dial ™, •*« reception conditions--- chec ker and correct line voltage input Power Supply (1) Measure voltage supplied Dustmen, oltagtt- A. A r coding of «r,ng and .m inaccessibility to user. (7) Rigidity and bondin 8 of shielding. (8) Accessibility of hum balancer (9) Absence of exposed delicateparfc-.- Dealers will find a chart of this type helpful in studying the merits of a new receiver. determining the total range of tone amplified by the receiver, checking overloading, and comparing the relative amplification of the lower and higher frequencies. No systematic examina- tion of the receiver's performance fails to include all of these factors. Tests for Selectivity TESTING FOR selectivity is too easy and simple to be worthy of detailed description. So long as a standard antenna system is used and the convenience of a comparison switch is available, the procedure is obvious. In almost all locations, high-powered stations are available so that, the dial range covered by a strong signal is readily observed. The antenna system used with a receiver should be a normal one and no valid test can be made with an antenna of excessively long or short dimensions. A short antenna gives satisfactory results even with a receiver having unsatisfactory selectivity, while a long one handicaps even the best of re- ceivers. An average an- tenna, 75 to 100 feet in length, should be used. Every location has its individual possibilities for testing selectivity, such as the reception of a particular distant station on a channel neighboring one assigned to a local, which gives an index to the performance of a re- ceiver. One who is called upon to pass judgment on receivers should be familiar with these char- acteristics of his location by frequent observation so that judgment may be passed with any re- ceiver. For example, in the New York area, re- ception of WLW through WOR is usually an indi- cation of excellent selectivity, but only familiarity with the par- ticular points and dis- tance at which the test is made makes this a specific criterion of a receiver's selectivity. The three points upon which definite judgment as to selectivity should be based are as follows: first, the dial range covered by the nearest high-powered transmitter; second, the nearest channel upon which a distant station may be heard neighboring on the highest-powered local signal; and third, with the receiver set for maximum volume, determination of the loudest station, restricted sufficiently by the selectivity of the receiver so that a distant station may be heard on the neighboring channel without interference. Comparison tests offer an easy method of judging sen- sitivity. It must be remembered that tested and measured tubes must be used in sensitivity tests. Also conditions must be observed simultaneously when comparing two receivers be- cause periodic changes take place in the effectiveness of the transmission medium. The advantage of familiarity with 82 JUNE -1929