The United Kingdom patent 394325 'Improvements in and relating to Sound-transmission, Sound-recording and Sound-reproducing Systems' is a fundamental work on stereophonic sound, written by Alan Blumlein in 1931 and published in 1933.

[10] During World War I acoustic location was actively researched for military applications of air defense, artillery sound ranging and naval hydroacoustics.

[3] In the end of the 1920s researchers of American and British corporations approached the amplified stereo problem; basic strategy for solving it had taken shape in the early 1930s.

[14] Arthur C. Keller and Harvey Fletcher of Bell Labs and Alan Blumlein of Columbia Graphophone Company and EMI were the first to obtain practical results.

[1] It was adequate for recreating width, and even a limited sense of depth, but only for the listeners sitting close to the axis of the centre channel,[1] and too expensive for the consumer market.

[8] However, wrote Blumlein, a two-channel recording with carefully altered phase and level differences can deceive the listener with a lifelike spatial illusion.

[22] Practical experiments in 1931 were impossible due to the merger of Columbia and the Gramophone Company into EMI, subsequent restructuring and relocation of Blumlein's laboratory to the new building in Hayes.

[18] Blumlein proved mathematically that phase differences registered by pressure microphones and clearly heard via headphones will be inevitably lost when reproduced via loudspeakers.

[40] Finally, inversion of side channel polarity flips left and right signals, producing a mirror image of the original sound field.

[40] When an off-center, low-frequency sound source is registered with binaural pressure microphones, the resulting L and R signals have the same intensities and differ only in phase.

[50] Blumlein believed that this configuration simplifies construction of the stereo cutter, because only the critical lateral actuator must fully meet fidelity standards, including treble response to at least 10 kHz.

[58] Shellac was naturally noisy, and even more surface noise was added by the conductive graphite powder applied to the wax master prior to electroplating and making intermediate stamper discs.

[57] Keller and A. G. Russell proposed replacing graphite powder with a thin layer of gold, sputtered onto the master disc in a vacuum chamber.

[61] Vinyl pressings in the United States began in 1943, in response to wartime shortages of natural shellac, and were limited to propaganda programs for the troops.

[62] On 21 July 1932 Blumlein sent a detailed memo explaining the principles of stereophonic sound to his superior and mentor, technical director of EMI Isaac Shoenberg.

[64] First experiments were a failure: shellac test records and Western Electric pressure microphones were not fit for handling the subtleties of stereo sound.

[69] Blumlein's experimental cutting lathe could record the orchestra in stereo, but the available microphones could not capture and preserve true stereophonic image.

[70] In the beginning of 1935 Cecil Oswald Brown has built the first film camera with synchronous stereo sound recording on a single optical soundtrack.

[73] On 26 July 1935 Blumlein began shooting Move the Orchestra – a live action comedy short intended to be a marketing vehicle for his technology.

[78] According to Eric Nind, the EMI management initiated market studies, planning to supply at least a few experimental sound sets to the theatres, but the venture was terminated before any practical results could be obtained.

[78] Louis Sterling, co-founder and marketing director of EMI, felt that improvements in cinema sound could be worthwhile only after the introduction of colour.

[79] Traditional sound locators relied on the operator's hearing; Blumlein suggested augmenting it with a visual display that incorporated principles of binaural recording.

[79] The experimental Visual Indicating Equipment (VIE), presented for tests in October 1938, employed two cathode ray tubes for displaying of target bearing and elevation.

[82] The experimental radar installation in Lake Farm Country Park, operating at 66 MHz carrier wave, began trials in the end of 1939.

In February 1954 RCA made the first successful stereo recording of an orchestral performance (La damnation de Faust conducted by Charles Munch).

[89][33][92][93] Shuffling was intended to equalize high-frequency and low-frequency sound localization, however, it was a poor match to practical studio environment at Abbey Road.

[12] The British were enraged; even the conservative Gramophone chastised the Americans for "failing to discover Europe", as well as prewar work by Fletcher, Keller and Rafuse.

[33] The notion of stereo, which was once loosely applied to any manipulations intended to produce spatial effects, changed its meaning and became synonymous with two-channel sound.

[97] Popular music, intended for replay via cheap low-fidelity players, was pressed in mono throughout the 1960s, and was sold at lesser prices than "upscale" stereo records.

[92] In the 1980s Richard Kaufman, Michael Gerzon and David Griesinger independently proposed three binaural recording techniques based on the Blumlein shuffling, but their ideas did not gain wide acceptance, either.