International photographer (Feb-Dec 1929)

Forty-four The INTERNATIONAL PHOTOGRAPHER October, 1929 mechanical element as regards resonance. If the inductance and capacity of such a circuit are so adjusted as to have a certain definite relation to each other, it will he found that the circuit actually offers less impedance, that is resistance in a loose sense, to the flow of an alternating current. In other words the circuit becomes a more efficient transmitting medium at that frequency. Of course, as the frequency changes the efficiency falls off very sharply as in the case of a mechanical element. The curves in both the mechanical and electrical cases are similar. Such a circuit may be placed across a transmitter. Assume the transmitter operates most efficiently at 1000 cycles. The object is to cut down the output at that frequency so that the efficiency will be no better there than at any other frequency. If the circuit placed across the transmitter is adjusted so that at 1000 cycles it conducts most efficiently, then the output of the transmitter at that frequency will be practically wasted in going through this short circuit. This shunt circuit does not affect frequencies far from resonance so that the output of the transmitter as actually fed into an amplifier may be adjusted so that it is uniform for all frequencies. The same thing ma}' done with a loudspeaker, recorder or reproducer or anything that vibrates. Electrical circuits that act selectively on certain bands of frequencies are well known and in this manner not only can mechanical distortion be neutralized but the distortion in any part of the electrical system may be neutralized. It is this feature that makes possible such faithful translation of sound into a record and back again into sound. In connection with eliminating mechanical distortion in an acoustic system, it is very interesting to consider how the Western Electric Co. and Bell Telephone Laboratories solved the problem. It is a very curious thing but the mathematics and theory of what happens in an electric circuit when an alternating current passes through has been much more highly developed and understood than in a mechanical system which is conducting vibratory energy. In electricity things happen in accordance with certain simple laws and the departure of the actual from the theoretical is very small. Measurements may be made with greater accuracy, ease and sensitivity than in mechanical systems. To simplify the mathematics in mechanics, many idealisations are necessary and these are very often quite a departure from the actual happening. At any rate the fact remains that mathematical analysis of an electrical circuit and its properties has been developed to a higher degree than in mechanics. A certain similarity in the fundamental properties of electricity and mechanics was observed. Thus mass in mechanics was considered to play a similar role as inductance in electricity. Elasticity corresponded to capacity. In fact an entire system of analogues between electricity and mechanics has been built up. The well known equations of telephone circuits were transformed into corresponding mechanical equations and a mechanical system built in conformity with those equations. Thus considering a phonograph reproducing mechanically, the needle is vibrated by the sound groove and transmits the energy to the diaphragm. The needle was made to have a predetermined elasticity, moment of inertia, mass so that it transmitted the energy at a uniform efhciencv for all frequencies. The diaphragm was made to have the same properties. The horn was so made that it acted as a pure conductor, like in a telephone line. All this was done by making the constants to conform to the equations. Artificial loading by means of springs, weights, leaves of paper was used to get these constants to the desired value. The same thing is old in telephony where the line may be loaded with inductance at various points to overcome the capacity of the line. This principle was applied to transmitters, receivers, recorders, reproducers, horns and in fact any element of an acoustical system. The Orthophonic Victrola is the result of this work. All this is disclosed in greater detail in British patents 230,876; 231,216; 231,409; 231,410 and others to the Western Electric Co. The most common amplifier is of course the three electrode vacuum tube. However there are two others which, though not used at all in practice, are interesting. One is a mechanical amplifier and consists of a rotating cylinder of some porous substance like chalk which is moistened with some electrolyte. A brush bears on the rotating periphery and this brush is mechanically connected to a diaphragm. A weight or spring tends to bias the brush and diaphragm in one direction. Telephone currents are conducted so that they pass from the moistened peripheral surface to the brush. The friction between the brush and surface is altered according to the intensity of the current and direction as well, and the result is that the brush is dragged a little ways forward and slips back and vibrates back and forth in that manner. Another patent shows an electromagnet fed by telephone currents which acts as the retarding force to a rotating magnetic element. Quite a clever scheme is where a dynamo is driven at a constant speed. The currents to be amplified are generated in the armature. All these amplifiers are impracticable for ordinary use but show that considerable ingenuity was displayed to develop one before the advent of the present tube amplifier. After the sound waves have been transformed into electric currents and suitably amplified, the different methods of making records diverge. The wax record will be considered first. The Wax Record It is well known that there are two different kinds of grooves used in wax recording. One is the Edison or hill-anddale groove in which the width of the groove is constant and of uniform curvature but the depth varies. The other is where the depth is constant, but the center line of the groove describes a sinuous curve which is the record. In the Edison record, it is customary in reproducing to have a screw feed the reproducer across the record since the side pressure of the needle on the inside wall of the groove appears to prevent proper operation of the needle. In the sinuous groove record, friction is relied upon to feed the reproducer across, the rapid vibration of the needle back and forth apparently preventing excessive wear on the inside wall of the groove. In both kinds of records, a screw feeds the recorder across. Thus it will be obvious that in the sinuous groove record, there is a distinct limit to the amplitude since if the recording needle vibrates too great a distance from the center, the separating wall between two tracks may be destroyed. In the Edison record groove, that problem is not present. However in that record, as the stylus digs deeper into the wax, it has to do much more work and thus distortion due to nonuniformity of load may be present. By making the recorder very powerful, this type of distortion may be eliminated. During the cutting of the record, the wax composition must be kept at a certain predetermined temperature and viscosity. The nature of these compositions as well as the conditions for proper recording are generally secret and are the result of long experience and experimentation. In the sinuous groove record, it is important that a predetermined maximum of current be set so that no groove will be ruined because of the lack of intervening wall. Ordinarily a person listens in on a head set or loud speaker or watches a meter and controls the maximum current by some sort of a resistance device. But things are apt to slip up and a defective record may be made. Thus the Victor record No. 6648-B "Pomp and Circumstance" made by the Chicago Symphony has one spot where the grand organ is recorded in the lower registers and the amplitude is so great there that the walls are very thin. The result is that after a few times of playing, the needle breaks down the wall and skips a whole track. A British patent No. 269,978 discloses a recorder whereby such a mishap is impossible, if the recorder is adjusted. It depends upon the differential action of one field superposed on another. Thus an increasing subtractive component brings the effective magnetic pull up to a certain predetermined maximum and any currents greater than desired have no effect at all, being registered merely as the maximum permissible. There are a great variety of reproducers and in general it may be stated that any transmitter will work by changing the structure so that a needle vibrates the current controlling or generating means rather than having an ordinary diaphragm do it. The reproducers, as a rule, are highly damped with rubber cushions so as not to have any resonance distortion. Many patents show very light reproducers for the fine groove records. In reproducing from wax records, considerable annoyance is caused by socalled "scratch". Much of it is due to dust in the grooves but some of it seems to have been put in the record during recording. Considerable effort has been made to get rid of it. A common way is to put in an electric condenser across the electrical reproducer so that all vibrations of 5000 cycles per second and above are not heard. The theory is that the "scratch" occupies that frequency range and by getting rid of these frequencies, reproduction is improved. Of course some of the record itself is lost that way but the net result is an improvement. (To be continued.)