The result is actually a loss in high frequencies - an increase in distortion as the groove moves to the center. That 1.8 seconds at a 12” diameter is covering a lot more territory than at the minimum 4.75” diameter. The record is still turning at 33-1/3 rpm, but one revolution takes 1.8 seconds. Curvature isn’t usually a problem, near the outside edge of a 12” 33 1/3 record, but as the groove moves toward the center, its relative speed slows and curvature increases. In record mastering, the higher the recorded level and frequency, the greater the curvature of the grooves that get created when making master plates. Issues of Platter Size and Rotation Speed The 33-1/3 rpm, 12-inch format developed by Columbia was a compromise that attempted to fit more music on a single disc, accepting the limitations that will be discussed in this section on sound quality. Given vinyl groove dimensions and certain assumptions about bandwidth and tolerable distortion, a speed of 45 RPM came out of the formula. That's why a 7-inch single has a label 3 1/2 inches in diameter. Calculations showed that the optimum use of a disc record of constant rotational speed occurs when the innermost recorded diameter is half the outermost recorded diameter. The 45 RPM speed is the only one to be decided by a precise optimization procedure (by RCA Victor in 1948). ☞ Our " Sound Gallery" contains examples of the superior audio you can get from 45s, comparing them to the LP versions. Most of this material comes from companies that master records for a living and was used to explain to customers the differences between 7-inch and 12-inch records and between 45 RPM and 33-1/3 rpm speeds. The distance from the record's edge to its center, and the shape and configuration of the record's grooves play their part as well. As the discussion will make clear, speed isn't the only factor that comes into play. This page explains why music simply sounds better when mastered to 45 RPM than to 33-1/3 rpm, and the simple answer is that more of the original music is preserved at that speed. When using a master tape to create records, one of the primary goals is to preserve as much of the audio as possible.
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