Published: Jan 1962
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Electroforming plays a particularly vital role in the manufacture of disk records. The sound groove, cut in relatively soft material, has a smoother surface than the finest mechanical finish. Less than 0.5 microinch rms surface roughness is possible—in fact is common at present. When some of the sound amplitudes are less than 7 microinches peak to peak, the surface has to be much smoother so that the signal—in this case sound—be distinguishable from the background noise caused by groove roughness. The problem is to make a negative of this recorded surface in sufficient detail to reproduce faithfully the sound groove and having sufficient strength to be used as a die to form a duplicate of the original sound groove in disk after disk. The particular value of electroforming is its ability to copy the surface in ultimate detail in a form which, when mounted in a plastic molding press, can mold thousands of records and in so doing reproduce 40 to 50 min of music in less than 40 sec. Some appreciation of the surface problem can be obtained from the fact that in attempts to cut a sound groove in crystalline materials even though microcrystalline, cleavage along the microcrystalline boundaries is sufficient to increase surface noise 2 to 3 decades in decibels. Using fusible alloys which have a low dimensional change in solidification and casting against a metal negative surface also result in a surface noise 10 to 20 db higher than the original surface, again due to the crystallinity of the alloy. By metallurgical criteria the crystal dimensions may be extremely small; however, to a record playback stylus these crystal dimensions are large enough to cause appreciable background noise to mask the signal. Let us take a closer look at the surface of a record. The program material is cut in a relatively soft material. A nitrocellulose lacquer coated on an aluminum disk is the common recording medium. The nitrocellulose lacquer is generally a low molecular weight highly plasticized grade to give a gel structure of relatively low strength. The groove is cut with a specially ground sapphire stylus to form a groove with approximately a 90-deg included angle and less than a 0.2-mil bottom radius. The stylus is moved inward as the disk rotates to give the groove a gradual spiral motion varying from less than 175 to more than 300 lines to the inch. In lateral recording, the sound signal is made to vibrate the cutting stylus sideways. This action results in a lateral modulation of the groove as shown in Fig. 1. It is also possible to modulate the groove vertically to give what is known as a “hill and dale” record. Since vertical and lateral modulations are 90 deg apart, the motion due to each can be independent of the other and, consequently, two independent sound tracks can be recorded in the same groove. This has led to the stereophonic groove record. In actual practice the present stereophonic groove is the 45-45 system which can be viewed as a vertical-lateral system rotated 45 deg from the vertical axis. The effect appears as a combination vertical lateral motion since if the frequencies are equal and in phase a vertical stylus motion results, and conversely, if the phase difference is 180 deg a pure lateral motion results. A close look at the stereo grooves in Figs. 2(a) and (b), however, shows that the two sidewalls have independent modulation.
Max, A. M.
Manager of the Chemical and Physical Laboratory, Radio Corp. of America, Indianapolis, Ind.