The playing time of a record depends not only upon the actual diameter of the disc, but also upon the number of lines per inch which are cut along the diameter. A feed screw which is driven by the turntable either through gears or belts moves the cutting head gradually outward or inward to space the grooves uniformly on the disc. Some systems provide means for changing the speed of the feed screw with relation to that of the turntable in order to secure three or four different pitches from 90 to 150 lines per inch, while other systems provide for only a single pitch of approximately 110 lines per inch. The lower the pitch, the greater will be the distance between grooves on the disc and the greater will be the permissible amplitude to which the cutting head may be driven without overcutting into an adjacent groove.
A low pitch would be used where it is desired to record with maximum amplitude a program in which low or bass frequencies are quite predominant. A high pitch, on the other hand, would be used where it is desired to secure the maximum possible playing time at some sacrifice in fidelity and with shallower grooves. A single pitch of about 110 lines per inch will generally be satisfactory for general recording purposes, however, so do not let the question of pitch influence your choice of a system.
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