After World War II, two new competing formats came onto the market and gradually replaced the standard "78": the 33 1⁄3 rpm (often just referred to as the 33 rpm), and the 45 rpm (see above). The 33 1⁄3 rpm LP (for "long-play") format was developed by Columbia Records and marketed in June 1948. RCA Victor developed the 45 rpm format and marketed it in March 1949, each pursuing their own r&d in secret. Both types of new disc used narrower grooves, intended to be played with smaller stylus—typically 0.001 inches (25 µm) wide, compared to 0.003 inches (76 µm) for a 78—so the new records were sometimes called Microgroove. In the mid-1950s all record companies agreed to a common recording standard called RIAA equalization. Prior to the establishment of the standard each company used its own preferred standard, requiring discriminating listeners to use pre-amplifiers with multiple selectable equalization curves.
Who developed the 33 1/3 rpm LP?
Columbia Records