Stereophonic sound
Stereophonic sound, commonly shortened to stereo, is a method of sound reproduction that recreates a multi-directional, 3-dimensional audible perspective. This is usually achieved by using two independent audio channels through a configuration of two loudspeakers in such a way as to create the impression of sound heard from various directions, as in natural hearing.
Because the multi-dimensional perspective is the crucial aspect, the term stereophonic also applies to systems with more than two channels or speakers, such as quadraphonic and surround sound. Binaural sound systems are also stereophonic.
Stereo sound has been in common use since the 1970s in entertainment media such as broadcast radio, recorded music, television, video cameras, cinema, computer audio, and the Internet.
Etymology
The word stereophonic derives from the Greek + .Description
Stereo sound systems can be divided into two forms: the first is true or natural stereo, in which a live sound is captured, with any natural reverberation present, by an array of microphones. The signal is then reproduced over multiple loudspeakers to recreate, as closely as possible, the live sound.Secondly artificial or pan stereo, in which a single-channel sound is reproduced over multiple loudspeakers. By varying the relative amplitude of the signal sent to each speaker, an artificial direction can be suggested. The control that is used to vary this relative amplitude of the signal is known as a pan-pot. By combining multiple pan-potted mono signals, a complete, yet entirely artificial, sound field can be created.
In technical usage, means sound recording and sound reproduction that uses stereographic projection to encode the relative positions of objects and events recorded.
During two-channel stereo recording, two microphones are placed in strategically chosen locations relative to the sound source, with both recording simultaneously. The two recorded channels will be similar, but each will have distinct time-of-arrival and sound-pressure-level information. During playback, the listener's brain uses those subtle differences in timing and sound level to triangulate the positions of the recorded objects. Since each microphone records each wavefront at a slightly different time, the wavefronts are out of phase; as a result, constructive and destructive interference can occur if both tracks are played back on the same speaker. This phenomenon is known as phase cancellation. Coincident-pair microphone arrangements produce stereo recordings with minimal phase difference between channels.
History
Early work
demonstrated the first two-channel audio system in Paris in 1881, with a series of telephone transmitters connected from the stage of the Paris Opera to a suite of rooms at the Paris Electrical Exhibition, where listeners could hear a live transmission of performances through receivers for each ear. Scientific American reported:This two-channel telephonic process was commercialized in France from 1890 to 1932 as the Théâtrophone and in England from 1895 to 1925 as the Electrophone. Both were services available by coin-operated receivers at hotels and cafés or by subscription to private homes.
There have been cases in which two recording lathes were fed from two separate microphones; when both masters survive, modern engineers have been able to synchronize them to produce stereo recordings from a time before intentional stereophonic recording technology existed.
In 1925, engineer Heinrich Kluth-Nauen developed a device that created a spatial impression from a mono signal by means of a 180° phase difference. He called it the "Stereophone".
Modern stereophonic sound
Modern stereophonic technology was invented in the 1930s by British engineer Alan Blumlein at EMI, who patented stereo records, stereo films, and also surround sound. In early 1931, Blumlein and his wife were at a local cinema. The sound reproduction systems of the early talkies invariably only had a single set of speakers which could lead to the somewhat disconcerting effect of the actor being on one side of the screen whilst his voice appeared to come from the other. Blumlein declared to his wife that he had found a way to make the sound follow the actor across the screen. The genesis of these ideas is uncertain, but he explained them to Isaac Shoenberg in the late summer of 1931. His earliest notes on the subject are dated September 25, 1931, and his patent had the title "Improvements in and relating to Sound-transmission, Sound-recording and Sound-reproducing Systems". The application was dated December 14, 1931, and was accepted on June 14, 1933, as UK patent number 394,325. The patent covered many ideas in stereo, some of which are used today and some not. Some 70 claims include:- A shuffling circuit, which aimed to preserve the directional effect when sound from a spaced pair of microphones was reproduced via stereo headphones instead of a pair of loudspeakers;
- The use of a coincident pair of velocity microphones with their axes at right angles to each other, which is still known as a Blumlein pair;
- Recording two channels in the single groove of a record using the two groove walls at right angles to each other and 45 degrees to the vertical;
- A stereo disc-cutting head;
- Using hybrid transformers to matrix between left and right signals and sum and difference signals;
In the United States, Harvey Fletcher of Bell Laboratories was also investigating techniques for stereophonic recording and reproduction. One of the techniques investigated was the wall of sound, which used an enormous array of microphones hung in a line across the front of an orchestra. Up to 80 microphones were used, and each fed a corresponding loudspeaker, placed in an identical position, in a separate listening room. Several stereophonic test recordings, using two microphones connected to two styli cutting two separate grooves on the same wax disc, were made with Leopold Stokowski and the Philadelphia Orchestra at Philadelphia's Academy of Music in March 1932. The first, of Scriabin's Prometheus: Poem of Fire, is the earliest known surviving intentional stereo recording. The performance was part of an all-Russian program including Mussorgsky's Pictures at an Exhibition in the Ravel orchestration, excerpts of which were also recorded in stereo.
Bell Laboratories gave a demonstration of three-channel stereophonic sound on April 27, 1933, with a live transmission of the Philadelphia Orchestra from Philadelphia to Constitution Hall in Washington, D.C. over multiple Class A telephone lines. Leopold Stokowski, normally the orchestra's conductor, was present in Constitution Hall to control the sound mix. Five years later, the same system would be expanded onto multichannel film recording and used from the concert hall in Philadelphia to the recording labs at Bell Labs in New Jersey in order to record Walt Disney's Fantasia in what Disney called Fantasound.
Later that same year, Bell Labs also demonstrated binaural sound at the Chicago World's Fair in 1933 using a dummy with microphones instead of ears. The two signals were sent out over separate AM station bands.
Carnegie Hall demonstration
Utilizing selections recorded by the Philadelphia Orchestra, under the direction of Leopold Stokowski, intended for but not used in Walt Disney's Fantasia, the Carnegie Hall demonstration by Bell Laboratories on April 9 and 10, 1940, used three huge speaker systems. Synchronization was achieved by making the recordings in the form of three motion picture soundtracks recorded on a single piece of film with a fourth track being used to regulate volume expansion. This was necessary due to the limitations of dynamic range on optical motion picture film of the period; however, the volume compression and expansion were not fully automatic, but were designed to allow manual studio enhancement; i.e., the artistic adjustment of overall volume and the relative volume of each track in relation to the others. Stokowski, who was always interested in sound reproduction technology, personally participated in the enhancement of the sound at the demonstration.The speakers produced sound levels of up to 100 decibels, and the demonstration held the audience "spellbound, and at times not a little terrified", according to one report. Sergei Rachmaninoff, who was present at the demonstration, commented that it was "marvellous" but "somehow unmusical because of the loudness." "Take that Pictures at an Exhibition", he said. "I didn't know what it was until they got well into the piece. Too much 'enhancing', too much Stokowski."
Motion picture era
In 1937, Bell Laboratories in New York City gave a demonstration of two-channel stereophonic motion pictures, developed by Bell Labs and Electrical Research Products, Inc. Once again, conductor Leopold Stokowski was on hand to try out the new technology, recording onto a special proprietary nine-track sound system at the Academy of Music in Philadelphia, during the making of the movie One Hundred Men and a Girl for Universal Pictures in 1937, after which the tracks were mixed down to one for the final soundtrack. A year later, MGM started using three tracks instead of one to record the musical selections of movie soundtracks, and very quickly upgraded to four. One track was used for dialogue, two for music, and one for sound effects. The very first two-track recording MGM made was "It Never Rains But What It Pours" by Judy Garland, recorded on June 21, 1938, for the movie Love Finds Andy Hardy.In the early 1940s, composer-conductor Alfred Newman directed the construction of a sound stage equipped for multichannel recording for 20th Century Fox studios. Several soundtracks from this era still exist in their multichannel elements, some of which have been released on DVD, including How Green Was My Valley, Anna and the King of Siam, The Day the Earth Stood Still and Sun Valley Serenade which, along with Orchestra Wives, feature the only stereophonic recordings of the Glenn Miller Orchestra as it was during its heyday of the Swing Era.