Radio Broadcast (May 1928-Apr 1929)

RADIO BROADCAST through from one groove to the next in the wax, or, if the pitch of the spiral were increased to allow for a wider swing, the amount of entertainment which could be put on a given disc would be much less. The loss in low frequencies introduced by adhering to constant velocity cutting only as low as 250 cycles may be compensated in reproduction, if desired, by means of corrective net-works, or in the characteristic of the electric pick-up employed. Of course the amplitude of the cut as a whole can be reduced, and the loss in output made up by increased amplification in reproduction. In this way the constant velocity relationship could be maintained down to a considerably lower frequency, but when this is attempted another limiting factor is encountered— needle scratch or surface noise, caused by the nature of the record material. It is desirable to keep the oscillations corresponding to the speech or music large in order to keep the disturbance caused by the needle scraping the record relatively low. Before the general amplitude of cutting can be reduced, therefore, improvements in record material must be effected. Frederick states that wax records can be made with special recorders flat to within 1 db from 250 to 7500 cycles, and not deviating more than plus or minus 4 db between 30 and 8000 cycles. Such a characteristic, or even the characteristic shown in Fig. 3, can be secured only by flat amplifier design and by constructing the cutting mechanism mechanically on principles analogous to the electrical ones now generally employed. One of these principles, in telephone transmission practice, is to terminate the system with an impedance of such a value as to avoid reflection of energy. In modern electrical recording this is accomplished, in the mechanical portion of the system, by not using the rather variable load imposed by the wax on the cutting stylus as a termination, but instead supplying a relatively large mechanical load in the form of a rubber rod, which dissipates the energy. The work done at the stylus is then incidental mechanically, although functionally it constitutes the whole purpose of the machine. The actual cutting takes place on what is essentially an accurate lathe with a vertical shaft. The groove cut is generally between five and six mils wide, and 2.5 mils deep. The space between grooves is of the order of four Fig. 1 — General view of the Simplex projector Fig. 2 — Schematic diagram of mechanism in a standard motion-picture projector with, sound adjunct mils. The number of grooves per inch varies between 80 and 100 in normal practice, with 90 as the average. This gives a pitch, or distance between turns of the smooth spiral, of 0.011 inch, (11 mils). Since the space between grooves is 4 mils, the permissible amplitude of oscillation is not over 2 mils. If this limitation, for the reasons stated above, is set at 250 cycles, then for a constant velocity cut (amplitude varying inversely with frequency) the amplitude will be 0.0001 inch (0.1 mil) at 5000 cycles. These are microscopic dimensions, and the work is, indeed, best done under a calibrated microscope, which moves across the disc with the recorder stylus. It is interesting to note that in wax recording and reproduction the linear speed of the record past the cutter or reproducing needle varies between 70 and 140 feet per minute, a figure of the same order as sound film speed (90 feet per minute). For a minimum linear speed and fixed groove spacing, there is an optimum relation between the size of record, rate of rotation, and playing time. This may be mathematically expressed and drawn as a family of curves, here reproduced as Fig. 4 By means of a special "playback pick-up," which is lighter and more elastic than the usual electrical reproducing pick-up, the newly cut wax may be played immediately through an amplifier and loud speaker system, with quality sufficiently close to that of a hard record to allow an accurate judgment of the performance. This is a valuable feature both in phonograph recording and soundmotion picture applications. The material of the finished record, or "pressing," has to be hard, in order to afford a sufficient number of playings, and it must contain enough abrasive material to grind a new needle quickly (in 10 to 30 seconds) to fit the groove. Most of this wear is in the starting spiral, in which no sound is recorded. Since pressure is force divided by area, the pick-up, with a weight of about 4.5 ounces, exerts an enormous pressure through the microscopic area of a new needle, and even after the needle is worn to a fit the pressure is of the order of 50,000 pounds to the square inch. Contrary to a general impression, the finished pressing is a faithful copy of the wax record and loses practically nothing in es sential frequencies. Its defect is in the addition of surface noise. By improvements in the material, as well as in the electro-plating process by which the wax is copied, Mr. Frederick states that a reduction of 3 to 6 db in surface noise has been secured during the last two years. Further improvements may be expected in this line, without change in the present size of records and amplitudes of recording. By an interesting calculation, Mr. Frederick shows that the diameter of the bearing portion of existing commercial needles used in electrical reproduction is not too great to follow the groove undulations at 7000 or even 10,000 cycles, with standard methods of recording, so that this factor is not a limitation in reproduction of high frequencies by this method. Advances in electrical pick-up design have been in the direction of reduced mechanical impedance at the needle point, elimination of resonances in the tranmission chain, and reduced weight. These factors combine to secure better reproduction of the higher notes and less wear on the record. The comment of an engineer who is not a specialist in the wax recording field, based on observation in theatres, would probably be that Mr. Frederick's paper shows interesting and important improvements, but that much still remains to be done before an entirely satisfactory technique is evolved. Mils, and What They Are THE theatrical papers are certainly the snappiest on earth, but in technical matters, however simple, they are about as ignorant as one can be without trying. Last summer there was considerable speculation among the Broadway savants regarding standardization of sound track width on talking films, and the trade papers found numerous occasions to write about 80-mil sound tracks, 100-mil sound tracks, and so on. But they hardly ever printed it "mil." Frequently it became "mile." Again it was "mill." And "millimeter." But a mil is neither a mile nor a millimeter. A mil is a thousandth of an inch, and nought else. 5000 10,000 Fig. 3 20 40 60 80 100 120 140 160 180 200 R.P.M. T l|t1 24 V„ Tm = Max. Playing Time, Mins. N = Grooves per Inch. R = Outside Groove, Radius.lnches. Vo = Min.Linear Speed-Ft.per Min. V Inside Groove, Radius Inches R = 2V Fig. 4 • april, 1929 page 386 #