United Kingdom patent 394325

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. The work exists only in the form of a patent and two accompanying memos addressed to Isaac Shoenberg. The text is exceptionally long for a patent of the period, having 70 numbered claims. It contains a brief summary of sound localization theory, a roadmap for introduction of surround sound in sound film and recording industry, and a description of Blumlein's inventions related to stereophony, notably the matrix processing of stereo signals, the Blumlein stereo microphone and the 45/45 mechanical recording system. In 1933–1935 Blumlein built experimental stereo recording equipment and recorded two sets of stereo recordings using mechanical and optical media. Commercial implementation of his invention became a reality in the late 1950s, when the patent had expired. Blumlein's 45/45 system became a worldwide standard for stereo LP records, and Blumlein himself was proclaimed "the inventor of stereo".

Background

In 1881 Clément Ader presented the Théâtrophone – a working system for live delivery of opera performances over telephone lines. The théâtrophone was a one-to-one network, employing one carbon microphone to energize one remote telephone receiver, and required rows of microphones placed along the stage. While experimenting with parallel lines, Ader accidentally discovered stereo effect. By placing pairs of microphones at either side of the stage, near the footlights, Ader achieved strong binaural sound localization, simulating the effect of sitting at the edge of the stage, hearing actors and instruments as if they were spread in front of the listener. Ader himself explained the effect as the result of the differences in apparent loudness registered by the ears of the listener. Binaural théâtrophone, advertized as "auro-stereroscopic" or "binaural audition" failed to attract customers due to the need to have two telephone lines per subscriber, and overall low fidelity. Conventional, monaural théâtrophone operated successfully in France, Hungary, Italy and the United Kingdom until the end of the 1920s. In the 1900s Lord Rayleigh formulated the scientific sound localization theory. In Rayleigh's model, human hearing localizes low-frequency sounds based on phase difference between the signals registered by left and right ears (interaural time difference, ITD); high-frequency sounds are localized based on relative loudness of two signals (interaural level difference, ILD). Rayleigh's duplex (two-factor) model remains valid in the 21st century, with the addition of a third mechanism, the analysis of spectral cues provided by mechanical filtering of incoming soundwaves by human torso, head and pinna. During World War I acoustic location was actively researched for military applications of air defense, artillery sound ranging and naval hydroacoustics. Valve amplifiers that emerged at about the same time allowed reproduction of sound via loudspeakers. Early experiments with amplified reproduction of binaural signals ended in failure: binaural effect that was easily reproduced via stereo headphones was either weakened or completely absent. 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. Stereophony could not bring immediate financial gains: the main potential customer, cinema, was content with crude monaural sound film equipment; the Great Depression ruled out investments in new sound systems. However, the corporations were eagerly accumulating patent portfolios in anticipation of economic recovery, and continued financing research. 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. Each of the three inventors had a different objective, and followed a different course of research. Fletcher followed Bell Labs strategy for the improvement of traditional telephony. He concentrated on transmission of sound field images of the original for binaural reproduction via headphones. His experimental equipment successfully recreated high-quality spatial sonic imagery, but like all binaural systems did not work well with loudspeakers. Keller was primarily interested in amplified public address sound; he placed arrays of microphones on the stage and transmitted parallel audio signals to arrays of amplified loudspeakers in a remote listening hall, trying to capture and recreate the original "sound front". Best results were obtained with expensive two-dimensional arrays, capable of recreating both width and depth of the original; linear (one-dimensional) arrays could produce almost perfect sense of width, but not depth. The minimal working configuration required three channels (left, centre and right). 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, and too expensive for the consumer market. A cheaper two-channel stereo setup could not reproduce the sound field; the sound inevitably broke up into left and right point sources with a "hole in the middle". Blumlein envisaged introduction of surround sound in the film industry. He rejected the binaural model from the start. Instead of recreating spatial sound fields or "sound front" radiated by the orchestra, he settled on recreating the sound that is heard by a listener sitting in a concert hall, or by a camera operator on a film set. He reasoned that the microphone array should mimic human hearing apparatus, thus the two microphones must be placed close together (and close to the film camera). The resulting binaural signals cannot be used directly in an amplified stereo setup. However, wrote Blumlein, a two-channel recording with carefully altered phase and level differences can deceive the listener with a lifelike spatial illusion.

Publication

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