Volume 11 , Issue 15

78 RPM Records

Though commonly referred to as 78's, ironically, none were designed to be played back at exactly 78 rpm. Even the term "78" is misleading, as the actual rpm later agreed on with the advent of the AC motor, is actually 78.26 rpm!

So how did the standard become 78.26 rpm instead of say a nice round 78 you're probably asking? And for that matter, where did the original 78 rpm standard come from in the first place?

Emil Berliner (the inventor of the gramophone) determined by subjective listening tests, the optimum speed for recording vintage records. The resulting optimum speed is dependent on a balance between the groove width, stylus and design of the cutter & reproducer. Best results were obtained at speeds ranging from a low of 70 to a high of 90 rpm, and Berliner and his British Gramophone Company, determined the best sound was obtained at 78 rpm. Since Berliner & his Gramophone established the record format to begin with, other manufacturers followed Berliner's lead, & 78 rpm became the defacto standard.

Later however, the standard was changed to 78.26 rpm - and is the standard that remains in effect to this day. The reason for the change is actually perfectly logical. The slight change came about with the introduction of the synchronous AC motor. All AC powered turntables in the US run off standard 60 Hz power, and use synchronous motors. A synchronous motor's speed is locked to the line frequency. This yields a constant rpm, even though line voltage may fluctuate, thus minimizing any power induced fluctuations in the playback speed. Thus a standard 60 cycle synchronous motor will run at 3600 rpm (60 rev/sec = 3600 rpm). The closest integer gear ratio to reduce the 3600 rpm down to about 78 rpm is a ratio of 46:1. Dividing 3600/46 , yields 78.26 rpm. Voila! The new standard was set.
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