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Topic: Varying Playback Speeds on Shellac Records (78 rpm, 80 rpm, 81 rpm, etc.)

13th May 2019, 7:22 PM
	xiphophilos Member since Dec 2013 3351 Points Moderator	There was a useful thread on this topic at http://vinylville.tripod.com/spindoc.html, a site that no longer exists, so I have moved it here: Explanations of why certain speeds were chosen for certain types of phonograph recordings... s p i n s d o c t o r e d ___________________________ q. Does anyone know why 78 revolutions per minute was chosen as the standard rotation speed of old-fashioned gramophone records, rather than a round number such as 75 or 80 rpm? And are there convincing explanations for the choice of speeds for later EPs and LPs of 33 1/3 rpm and 45 rpm? JAMES LEE Oisterwijk The Netherlands ____________________________________________________________ a. It was Emil Berliner, the inventor of the gramophone, who determined roughly how fast old disc records should spin. He avoided Edison's need for a stylus made from precious jewels by using points which could be made from steel sewing needles and pins. The size of the stylus effectively determined the size of the grooves in a record and the recordable frequency range limited by this groove size determined a speed between 70 and 90 rpm. Standardisation did not begin until 1912, when the British Gramophone Company conducted listening tests on their back catalogue. They settled on the average (or possibly the median) of these tests, which turned out to be 78 rpm. Other companies adopted this, but the process was not complete until the early 1930s. Even after this date rogue rpm records still appeared. After standardisation problems still occurred. Because of electrical mains frequencies differences on opposite sides of the Atlantic, stroboscopic speed testers and synchronous motors meant a nominal speed of 77.922 rpm in countries that used 50 hertz and 78.261 in countries that used 60 hertz. These were later fixed in national (but not international) standards. Records of 33 1/3 rpm were developed in conjunction with films. A 12-inch 78 with Berliner-type grooves could hold between 4 and 5 minutes per side. The first practical sound films produced in the US in the late 1920s had their sound on separate disc records and it was more important for the sound to be continuous. A reel of film might run for 11 minutes, so a rotational speed of about 32 rpm was required to make the sound match the picture. History doesn't tell us why precisely 33 1/3 was chosen, but in retrospect it was a very good choice because stroboscopic speed testers can be made for this speed which will work on both sides of the Atlantic. It seems CBS engineers (who developed the first LPs in 1948), simply experimented with one of the old machines hanging around in their workshop. They then developed new groove dimensions which gave an acceptable signal-to-noise ratio with the new plastic material "vinyl". The 45 rpm speed was the only one to be decided by a precise optimisation procedure (by RCA Victor in 1948). Calculus was used to show that the optimum use of a disc record of constant rotational speed occurs when the innermost recorded diameter is half the outermost recorded diameter. That's why a 7-inch single has a label 3 1/2 inches in diameter. Given the CBS vinyl groove dimensions and certain assumptions about the bandwidth and tolerable distortion, a speed of 45 rpm comes out of the formula. PETER COPELAND British Library National Sound Archive London a. From 1894 to around 1930, there were many different record speeds ranging from 65 to 90 rpm, each case being a compromise between playing time and the 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. Constant linear speed, or varying the rpm, was commercialised but did not prove to be a success (until the arrival of the CD). The speed of 33 1/3 was introduced in 1927 after theoretical analysis of the compromise between signal-to-noise ratio and playing time (3 minutes per radial inch) by J. P. Maxfield of Bell Laboratories for sound films produced on the Vitaphone system. And it was a professional de facto 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 records but, in fact, Maxfield's analysis still applies: the 45 "single" was RCA's equivalent to a 10-inch, 78 rpm record, only smaller. GEORGE BROCK-NANNESTAD Copenhagen Denmark