How did turntable speeds develop into 33 1/3 and 45?

September 13, 2009 12:33 AM

Why are standard turntable speeds 33 1/3, 45, and 78 RPM?

Why did the industry standard for RPM develop from 78 to 45 and 33 1/3? Why not 81 RPM, 42 RPM, and 34 1/6 RPM? Is there some math or logic or was it just arbitrary?

posted by kensington314 to Technology (6 answers total)

History of 33 1/3 and other speeds.
posted by rokusan at 12:39 AM on September 13, 2009

According to this site:

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.

There's some more info on the site about 78s; apparently early systems were not well-standardized.

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.

Not the most satisfying explanation; I wonder if there isn't more to the stroboscopic angle. That would be one of the easier ways of checking that the platter is spinning at the correct speed, so it would make sense to pick a speed that can be easily tested.

The 45 rpm speed was the only one to be decided by a precise optimization 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.

Again a little handwavey, but it does seem to have been intelligently chosen rather than arbitrary.
posted by Kadin2048 at 12:43 AM on September 13, 2009

Much better and more technical explanation here. Supposedly, the 78 RPM (actually 78.26 and change) speed is an artifact of the original gear-drive systems that were used to transmit power from a 60Hz/3600RPM synchronous motor:

The 78,26086957. . . speed is obtained with a gear ratio of 46 to 1 between the synchronous motor and the recording turntable. This can be achieved with a one-tooth worm gear mating with a 46-tooth driven gear. This is the most direct and simple system preferred for precision work. This speed can also he obtained by a spur-gear train of two 12-tooth pinions, one intermediate 46-tooth gear and 144-tooth turntable drive gear. For 78 r/min exactly, the gears are 13-tooth pinions. 39-tooth intermediate, and a 200-tooth final turntable drive gear. There is not a practical simple worm gear drive to obtain 78 r/min exactly from a 60-hertz synchronous motor.

Interestingly, the speeds on pre-war "78s" made for Europe are slightly different than those made for the U.S., owing to the 50/60Hz power system differences. No idea if this would end up being audible or not, though.

The 33-1/3 RPM speed is a combination of a need for long playing time (to sync to early motion pictures) and also again to easily be driven from a synchronous motor operating at 60Hz:

What would be a convenient speed approximately one-half of 78 that could be easily locked to the 60-hertz line? The answer was 33-1/3. A simple worm drive with a one-tooth worm and a 54-tooth driven gear or a spur-gear train with 18-tooth pinions, a 108-tooth intermediate, and a 162-tooth final turntable drive gear did the job perfectly.

It also says that the 45 RPM speed was chosen as a direct result of the physical size and desired playtime, pretty much falling directly out of marketing/design decisions on the other two factors.
posted by Kadin2048 at 1:05 AM on September 13, 2009 [3 favorites]

And just to complete the picture— until the advent of cheap quartz oscillators, the 50/60Hz power line was the most readily available stable timebase for household use. Power utilities used to (and probably still do) pay close attention to the long-term frequency accuracy of the power, because otherwise the whole city's clocks would be off. (Short-term frequency variations, OTOH, were (are?) used as a form of load regulation… probably messed up turntables' sound on occasion.)
posted by hattifattener at 1:49 AM on September 13, 2009

Personally, I think Cecil Adams has it right
posted by CodeMonkey at 3:33 PM on September 13, 2009

opps: try this:
Cecil Adams
posted by CodeMonkey at 3:34 PM on September 13, 2009