International photographer (Feb-Dec 1929)

October, 1929 The INTERNATIONAL PHOTOGRAPHER Forty-three ELECTRICAL RECORDING (Continued from Page 20) the natural resonance of the elements of the system about three thousand cycles, or even higher, most of the sound will be translated at a uniform efficiency. But the trouble is that this uniform efficiency is so low that practically all of the sound energy is lost. Thus the horn would have to be very small, the diaphragm would have to be very stiff and taut, the recording stylus would have to be tensioned tightly. Intuitively most people would realize that such highly strung parts would only respond to very powerful impulses. With electrical recording and reproduction of sound, the energy may be amplified so that there is enough to record, the distortion due to mechanical resonance may be completely neutralized and a thoroughly faithful recording and reproduction of the sound is possible. When the records as now made are played on a high grade electrical phonograph, the average person will fully agree that the new way is much better than the old. Electrical recording has some further advantages and possibilities that are peculiar to it and have no counterpart in the mechanical processes. The recording instruments can be any distance from the source of sound so long as wire or wireless telephony is possible between the two places. Each player of an orchestra may sit as he ordinarily does and in fact the recording may be carried on with an audience enjoying the music at the same time. A transmitting instrument may be placed near each player and the electrical currents combined later. One patent, 1,540,317, goes even further. Between each transmitter and the common junction is placed an electrical network that has the property of slowing up the speed of transmission of the currents. This is equivalent to lengthening the acoustic path between each player and the recording needle. In this way, the actual grouping of the players may be transformed to any desired virtual grouping and objectionable interference between sound waves that are out of phase is eliminated. A person may switch on a loudspeaker to any circuit and see how any one instrument is feeding into the circuit or may listen to the ensemble and determine the quality or volume. The operator may increase the volume of any instrument relative to the others or decrease it; he can do the same for the entire orchestra if necessary. The patent to Wier, No. 1,617,428, discloses tricks possible with electrical methods that the ordinary person would not even imagine. Thus several records may be combined into one. Filters may be switched in or out so that portions of the scale may be emphasized or suppressed and thus brilliance is possible in reproduction where the record itself lacks it. It is even possible to make a secret record that can be reproduced only by special apparatus. Probably one of the most practical and desirable possibilities is the fact that a wax record can be made so that it will play for over an hour where a mechanical record will only last for three minutes. The patents to Meissner 1,604,130 and to Dyer 1,570,297 disclose this. The ordinary wax records have about 100 grooves to the inch with a groove width of about five-thousandths of an inch. By increasing the pitch to several hundred grooves to the inch, decreasing the groove width to about one-thousandth of an inch, a track about five times as long may be obtained. Furthermore, by decreasing the speed of rotation to a fifth of the normal speed, thus making the sound groove curves sharper, five times as much may be put on a unit length of track. The net result is that about twenty-five times as much matter may be recorded on a wax record of standard dimensions as is the case now. It is perfectly possible therefore to record a book on a double record or two so that the blind may enjoy it; it is just as possible to record an entire opera or a play instead of having a small portion of it. This type of a record is only possible with electrical recording and reproduction. The ordinary type of reproducer would break the grooves and would not produce audible sound. By employing a very delicate electrical reproducer and amplifying the currents, a loud speaker may be used. From a commercial point of view, such records would be very desirable and greatly appreciated. It is understood that such records are on the market already but due to the fact that their reproduction cannot be effected by mechanical means, the field is somewhat restricted. Three General Classes and Apparatus Common Thereto Electrical sound recording divides itself into three sharply defined classes. The same is true of the reproduction. There are methods of recording and reproducing that do not fit into these classes but most of the work is along these three classes. What has been stated to be the advantages incident to the electrical processes applies to all electrical processes excepting where the advantage or improvement is specific to wax records. The three classes are wax records, records obtained in general by photographic means on a film, and magnetic records. All the electrical processes of recording have the initial step of transforming the sound waves into electrical current waves. The electrical reproducing processes have as their final step, the conjugate of the initial step in recording, the transforming of electrical current waves into sound waves. Amplification of electric currents is also the same in all processes, or may be the same. Before considering each system, the problems and devices that they have in common will be disposed of. Sound waves may be transformed into electrical variations by three general types of apparatus. The most common is the microphone type and depends upon its operation by resistance variation. The ordinary telephone transmitter has loose carbon granules, which are alternately compressed and loosened up by a diaphragm, and which varies the intensity of an electric current supplied by an outside source. In the same class is the electric arc where the sound waves impinge upon the arc between two electrodes and cause a variation in current. Then there is the type where an ionized field is produced as by X-Rays and an acoustically vibrated magnet varies the ionization, or a hot filament emits electrons and the distance between the filament and other elec trode is acoustically varied. The carbon granule type is the most common example of the microphone class of transmitters and is the most widely used. The advantage of this device is that a comparatively large output may be obtained, since an outside battery serves as the source of electrical energy, the sound waves merely modulating the current. The next type is the electromagnetic, in which current is generated in the same fundamental way that it is generated in a dynamo. This may take a variety of forms and any one of the three elements of this, the magnet, armature, or coil, may vibrate. The third type is the condenser in which the capacity of a condenser is altered by acoustically vibrating one or more of the plates that form one side of the condenser. Both the electromagnetic and condenser type have an extremely feeble output and are useless without some means of amplifying the current. Both these types may be made extremely sensitive and are capable of very faithful translation, much more so than the microphone type. However for ordinary commercial recording, any one of these devices may be made to give very satisfactory service. The ruggedness and simplicity of the carbon granule microphone keeps that in greater use than any other type. In addition to these types, there is the thermal transmitter in which a hot wire has its resistance varied by the sound waves cooling it. This is a species of microphone but is more of an oddity than anything else. The means for transforming electric currents back into sound are fundamentally the same as the transmitters. The microphone type is generally irreversible but the electromagnetic and condenser type can be reversd so as to emit sound. Most radio enthusiasts are familiar with the various types of loudspeakers, in fact almost everybody is to some extent, so no more need be said on the subject. It will be noticed that both the transmitters and loud-speakers have vibrating mechanical parts in most cases. These parts have their resonant periods and unless means are used to overcome or eliminate this distortion, electrical methods would have nothing over mechanical methods. A variety of means is possible. As was stated above, the parts may be so made that their resonant periods are far above audibility. In that case most of the distortion is absent. Since the power may be amplified, the actual efficiency of the transformations is not important, providing the efficiency does not really become too low. Another way is load the element by making it work against some uniform load. This may be accomplished by rubber packing inserted between some vibrating part and a stationary part. A sponge filled with oil so that a dashpot action in the pores of the sponge results may be used; an air column, leaves of paper or anything that imposes a load that is independent of the frequency may be used. This tends to take out any resonance in the vibrating part and makes the response uniform. There is also an electrical method for accomplishing the same result. This method depends upon the fact that a circuit carrying alternating current behaves in the same way as a vibrating