AMPEX demonstrated magnetic tape recording in 1947, based on the German magnetophone technology used during WWII. Shipments to radio stations began in 1948.
http://www.aes.org/aeshc/docs/company.histories/ampex/leslie...
http://www.aes.org/aeshc/docs/company.histories/ampex/leslie...
An interesting article about archiving of Dictabelt and other audio format recordings: http://thequietus.com/articles/18004-sound-library-british-l...