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why 78, 45 and 33 1/3 rpm? Studio Monitors
Old 3rd January 2011
Gear Nut
dorisinger's Avatar

why 78, 45 and 33 1/3 rpm?

Moderator - not sure where to post, feel free to move. There's not a forum for too much time and not enough gear.

Anyone have any idea WHY vinyl rpm's were spun at the chosen rpm's: 78, 45, 33 1/3?

I understand the relative time/side issues, but why these particular rpm's.
Old 3rd January 2011
Gear Addict
rene-lemieux's Avatar
now this is just a guess that has only a hint of knowledge behind it, and a lot of assumptions, so don't take it as gospel... but the numbers may be in relation to the inches of the disc, but maybe im wrong (In Canada we use metric),

the amount of rotations is in direct relation to the optimum fidelity compared to the size of the disc... at 33 1/3, you can fit an average of 4-7 songs on a side, depending on groove depth, and maintain a Hi Fidelity. Deeper groove means less space but more fidelity/ volume/ volume to surface noise ratio... 7" 45's have a deeper groove, and a higher rotation rate most commonly (although if you make a shallow groove i think you could fit 4-7 songs on a 7") Thats why they run out of time so quick... Ever notice how 78's are almost 10" but only have a song per side, thats because its rotating so fast and the groove runs up quickly... You can put any speed of rotation to any size of record, depending on what speed its mastered at, you could even master it at a totally obscure number if you wanted, if you had a record player to play it back at the same mastering speed... Ive seen 33 1/3 records with 11 songs per side but the groove is so shallow you cant even see where the song changes.

I think that 33 1/3rd is a speed that was come up with probably by someone like EMI or RCA (just a wild guess) that was the median amount of rotations that could be deviated from slightly with cutting entire albums onto 12" vinyls in mind, it just a speed that works out for average length albums with average depth grooves with high enough fidelity and loud enough volume....

I suck at typing
Old 3rd January 2011
Registered User
Like most standards in life, they are fairly random. Somebody makes a product which becomes successful, and everyone else copies it.

The choice of angular speed would be firstly dictated by the choice of diameter, which has a huge impact on the peripheral speed. Who decided what diameter disk to make?

The use of synchronous AC motors would have dictated the choice of motor speeds available (depending on how many poles you choose to use). But the gearing in most turntables was done with pulleys - so they could have chosen any gear ratio to suit.

I don't believe there was any magic in the dimensions & speeds chosen. Just commercial compromises.
Old 3rd January 2011
Gear Head

Actually, 78.26 was chosen because it's what you get when you take a synchronous electric motor and apply a particular gear ratio to it -- and it can be gotten using either 50 or 60hz setups using different gear ratios.

45 rpm was chosen in the late 1940s by RCA Victor as the optimal speed for a disk -- unfortunately for them, the market insisted on more playing time.

33.33 was chosen probably because of gear-reduction considerations, and was first used for discs that duplicated the playing time of a reel of 35mm movie film -- early films synched disc sound to the projected images. It was also used for broadcast transcription discs and for early 1930s RCA and Columbia home discs, which didn't survive the heavy weights of early phonograph cartridges.
Old 3rd January 2011
Lives for gear
Early films synched disc sound to the projected images./QUOTE]

It was a nightmare when the record skipped!!
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