[Hartley118]

I believe that, as with so many innovations, it was the Western Electric engineers who introduced the 33 1/3 rpm speed in the 1920s when the first film sound tracks were recorded on disc. The obvious speed to use was of course 78 rpm, but the disc needed to play for 10 minutes, the length of a reel of film at that time. So the speed was reduced pro rata. A 12-inch 78 can play for a little over 4 minutes, so if a 10 minute playing time is needed, you end up with a speed a bit more than 30 rpm. Why was such a precise 33.333 rpm decided on? Does anyone know? Perhaps it suited available gear ratios on film projectors?

So when CBS introduced the long-playing record in 1948, they didn't need to reinvent the 33 1/3 rpm speed. They did, however, need to develop the microgroove recording process. Fortunately, sapphire styli were becoming established for replay: steel needles would have been an unlikely solution!

When CBS patented the LP, there was clearly panic at RCA. They decided to do something entirely different by going small, with the 7-inch disc. The public had by then got used to autochangers, so RCA designed the smallest and fastest changer ever (pictured below), hoping that the public could cope with short breaks in the music every 5 minutes or so. The player pictured is actually branded HMV, and was sold in the UK to compete with the Decca group LP introduced in 1950. HMV resisted the new-fangled LP until 1953!

Why 45 rpm? Perhaps, as suggested, in order just to be different. However, it's certainly arguable that, if the 7-inch disc is to be recorded to a smaller inner radius than the 12-inch LP, then audio quality dictates the higher rotational speed in order to maintain a minimum required surface speed.


Martin