In each case a compromise between playing time and a need for a clean cut in the original wax, the Victor Company used 76 rpm for many years for its recordings but instructed buyers to reproduce at 78 rpm; the record's durability was improved that way. The standard of 78 rpm arrived by default, although the actual speed depended on the electrical mains frequency subject to the speed of 331/3 introduced in 1927. This speed of 331/3 which was introduced after theoretical analysis by compromising between signal-to-noise ratio and a playing time 3 minutes per radial inch was decided by J. P. Maxfield of Bell Laboratories for sound films produced on the Vitaphone system. It was a professional de facto vinyl groove dimensions standard before it became commercialised by CBS in 1948. It has been suggested that 78 minus 33 equals 45 was the reason for the emergence of 45 rpm. In fact, the Maxfield's analysis still applies: the 45 single was RCA's equivalent to a 10-inch, 78 rpm record, only smaller. When talking pictures first arrived in the late 1920s, the sound was recorded separately on discs. Recorded separated on discs and had to be synchronized by the projectionist at each showing. Every cinema projection room had a pair of projectors, each taking 1000-feet reels of film, whose running time was about 10 minutes, then the projectionist switched projectors after each reel. Ideally, this meant that the sound should last 10 minutes as well, as it would be impossible to synchronize a sound changeover in midreel. At the time, however, a 12 inch 78 rpm record lasted for only about 4 minutes, so the Vitagraph company simply slowed down the 78 until it lasted roughly about 10 minutes, this act encouraged Vitagraph to recorded all their masters at that set 10 minutes |
