Sound recordings are made at one of two speeds, 78 rpm or 33 1/3 rpm; most recording, turntables are provided with a lever or other mechanism for changing over from one speed to the other. The use of the lower speed gives a longer-playing record, but at the same time results in somewhat poorer fidelity and a higher scratch or noise level. With a 33 1/3 rpm speed, 16-inch discs which will play for 15 minutes are generally used. With a 78 rpm speed, 12-inch discs playing for 5 minutes are standard, but smaller-diameter discs than these can be used in either case when desired.
Records cut at 78 rpm are always started at the outside of the discs, but with 33 1/3 rpm recordings, it is common practice to start the cutter at the inside, at least 3 3/8 Inches away from the center of the disc. The reason for this is quite practical, being based upon the fact that the needle used in playing a 15-minute record becomes considerably worn after a few minutes of playing, with consequent broadening of the point. The linear velocity of the needle in the groove is lower near the center of the record than at the outer edges, and therefore the sound track is more crowded (the distance between peaks in a groove is less) near the center of the disc. At the higher frequencies being recorded, the physical size of a worn-out needle point becomes comparable to the distance between peaks (the wavelength) on the disc, and these high frequencies are bridged over by the needle. A new needle is more likely to follow the high-frequency fluctuations at the center of the disc.
Although some sound recording systems can be secured with only a 78 rpm speed, I consider the extra cost of a dual-speed system well justified. A 15-minute disc is highly desirable when recording long programs, to avoid loss of part of the program while changing discs.
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