Stereo
On August 30, 1881, Clement Ader (1841-1926) received a patent in Germany for the first stereophonic system. Two groups of microphones were placed on either side of a stage. These microphones relayed the sound of a play or musical directly to two telephone receivers, which paying subscribers held to their ears. He displayed this invention the same year at the Paris Exposition.
During World War I a similar system was used. What was referred to as "binaural receiving trumpets," were set up to locate enemy aircraft. Two large horns, like those on early phonograph systems, were connected by rubber tubes from their thin ends to the ears of operators. By using these horns, the operator could more readily hear from what direction planes were coming.
In the 1930s, further progress was made in stereophonic systems. The Bell Telephone Laboratories, under the leadership of Harvey Fletcher (1884-1981) and others, including the renowned conductor Leopold Stokowski (1882-1977), was the prime investigator in the area. Researchers there set up a tailor's dummy, nicknamed Oscar, who had microphones built into his ears in an attempt to reproduce the conditions of human hearing as closely as possible. If the electrical signals were sent through separate amplifier s to spaced loudspeakers, the listener was able to tell where the original instrument had been located in the room where it was recorded. Headphones improved on this system: when left and right signals were fed to the left and right earpiece, the stereo effect was very good. In Germany today this idea is still used; it can reproduce a 360-degree sound field that seemingly surrounds the listener.
However, this system of using a dummy head was not very flexible. The British engineer Alan Dower Blumlein was responsible for using ordinary microphone arrangements for stereo recording. He found that good stereo could be created by either of two methods that depended on the intensity of the signal or the time of the signal's arrival. For the intensity method, he set up two microphones next to each other but turned in such a way that each aimed at the side of the stage. Thus, a sound located in the center would not register as intense, while a sound either to the left or right would be picked up more readily by whichever microphone was aimed that way. He also found that stereo could be created by placing two microphones far apart; they picked up signals at a slightly different time depending on where the sound originated. In 1933, Blumlein created the first stereo recording for the British firm EMI (Electric and Musical Industries). The record contained two sets of information: the left-hand signal was engraved on the inside of the groove, while the right-hand signal appeared on the outside of the groove.
Since the 1970s, stereo has become far more sophisticated. Magnetic recording tapes and compact discs are among the foremost developments affecting stereo sound. Broadcasters learned how to transmit stereo through multiplex systems that first put the two channels into a composite signal that was later restored to true stereo through a matrix circuit in a stereo receiver. Sound engineers have also created four-channel stereo (quadraphony), often called surround sound which consists of four microphones, four amplifiers, and four loudspeakers that together provide remarkably realistic sound reproduction. In the 1990s, a 5.1 channel surround sound system became popular. Consisting of a left, right, center, two surrounds and a subwoofer, this multichannel system provides realistic sound space. Though first embraced by home theater enthusiasts, the 5.1 system will inevitably take over home stereo use as well.
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