Audio Systems: Vinyl discs, syncronous motor, vinyl discs

Question
Hi,
This is a banal question and I hope I am troubling the right person: Why were
the speeds of 33.33, 45 and 78 chosen in their time (and not say, 30 rpm, 50
rpm and 70 rpm)?
Many thanks in advance and seasons wishes.
Chris  

Answer
Although the speeds of the earliest discs can vary considerably from 78 rpm they are still called "78s". The recording speed chosen was a compromise between playing time (faster = shorter playing time), groove and needle size, wear on the record and needle, and fidelity of reproduction. At first the speeds were not standardised but ranged between 65 and over 100 rpm, but in the 1900s an average speed was around 78rpm. The first disc recording machines were weight driven with the speed controlled by a governor and different calibrations between machines could lead to speed differences. When electrical recording was first used in 1925, it was used for the soundtrack of films and so a precise speed was needed to keep in sync with the picture. In America using 60Hz mains, the speed of electric motors is either 3600 or 1800 rpm. Using a 3600 rpm motor with a 46 : 1 reduction gives a speed of 78.26. A similar calculation for England using 50 Hz mains gives a speed of 77.92 rpm. These speeds became the standard for the rest of the 78 rpm era. The LP speed of 33.33 rpm and the vinyl single speed of 45rpm comes from a similar calculation of speed reduction from a syncronous motor.