the audio quality of lps has increased greatly since their 1948 inception. while early lp recordings were monophonic, stereophony had been demonstrated in 1881 and alan blumlein had patented stereophonic sound in 1931. unsuccessful attempts were made to create stereophonic records starting in the 1920s, including emory cook's 1952 "binaural" lps using two precisely spaced tracks on the record (one track for each channel) which had to be played with two monaural pick-ups on a tuning-fork-shaped tonearm. the modern system ultimately released by audio fidelity records in november 1957 uses two modulation angles, equal and opposite 45 degrees from vertical (and so perpendicular to each other.) it can also be thought of as using traditional horizontal modulation for the sum of left and right channels (mono), making it essentially compatible with simple mono recordings, and vertical-plane modulation for the "difference" of the two channels.