stereophony

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A classic application of stereophony is a stereo or hi-fi system for listening to musical works in the private sector.

With stereophony (from ancient Greek στερεός stereos , German , hard, rigid ' and φωνή Fone , Loud', 'sound') are referred to techniques that create a spatial sound impression in natural hearing using two or more sound sources.

history

The earliest forerunners of stereo technology go back to the French aviation pioneer and inventor Clément Ader . During the International Electricity Exhibition in 1881 , he had opera performances transmitted electrically to a remote room using multiple channels, where these signals could be listened to via telephone receivers. Ader called his system “Théatrophone”.

In 1925, the engineer Heinrich Kluth-Nauen developed a device that created a spatial impression from a mono signal through a 180 ° phase difference. He called it "stereophone".

Alan Dower Blumlein , electrical engineer and inventor in the research department at EMI , began experiments in 1931 with what he called “binaural” recording techniques, initially with speech. In the course of these experiments he applied for a number of patents, including several stereo microphone methods that are now considered standard: AB, XY and MS. The coincident system of two angled microphones with figure-of-eight characteristics, known today as the “Blumlein process”, is named in his honor, but does not seem to have been conceived by him. Blumlein also played a key role in the development of the cutting stylus in order to be able to record binaural signals on shellac records. At the beginning of 1934, EMI's "Abbey Road Studios" with "binaural" technology were operational and the first experiments with music were carried out on January 11th and 12th, 1934. A few days later, on January 19, 1934, a rehearsal was also recorded there under the direction of Sir Thomas Beecham with the London Philharmonic Orchestra on a total of nine records. Rehearsed Mozart's Symphony No. 41 "Jupiter" (KV551).

Walt Disney brought out his animated feature film Fantasia (1940) in "Fantasound", an early stereophonic sound process, which at that time only very few cinemas could reproduce.

In 1944 the Reichs-Rundfunk-Gesellschaft in Berlin made the first stereo recordings on magnetic tape using a magnetophone. The first recordings were made in collaboration with the conductor Herbert von Karajan .

It is possible that Blumlein's use of the term “binaural” in his patents prevented the further spread of this term in order to evade patent claims. After the rapid implementation of magnetic tape technology in industrial sound carrier production in the 1950s, the term “stereo”, derived from the visual technology of stereoscopy , became established.

Sidney Frey, head of the record label Audio Fidelity Records , brought out the first stereo record in 1957: On one side the noise of the train could be heard, on the other side Dixieland Jazz with the Dukes of Dixieland . In the first few years there were considerable problems with standardization and also with quality assurance in plate production.

The Beatles, for example, initially recorded their songs with a kind of “stereo technique” that, if you look more closely, is twice “mono”. The instrumental accompaniment was put on one channel and the vocals on the other channel. This has very little to do with stereo, except that something different came out of each speaker. The term “ping-pong stereo” is also known from this period and refers to a recording technique that allows instruments to move from left to right - or vice versa. This was particularly in demand in the early days of stereo recording technology for popular music: a stereo system was a status symbol in the early 1960s - and after all, people wanted to hear the effect clearly and proudly demonstrate it to friends.

The radio provided the increasingly attractive programs offered by the television the technical innovation of the Bilingual contrary. FM stereophony was introduced for VHF transmitters ( FM ) in Germany at the 25th Great German Radio Exhibition in Berlin on August 30, 1963 . In 1967/68 every state broadcaster operated at least one stereo FM transmitter network. This enabled radio to be broadcast in a better sound quality than was possible with television. Radio achieved further qualitative advantages in 1973 with the introduction of binaural sound recording . This " dummy head stereophony" could not prevail.

At the beginning of the 1970s, mono recordings were “stereo-phoned” for reasons of sale, that is, an artificial stereo signal was generated, which was referred to as pseudostereophony or “electronic stereo”.

Basics

Stereo (2.0 sound) mark on DVD cases
Schematic representation of the stereophony

In the simplest case, the horizontally distributed mapping takes place solely through level differences Δ L or through transit time differences Δ t of the two loudspeaker signals . The mapping of the depth gradation is based on the utilization of early reflections and on sound discoloration by Blauert bands in order to work out “diffuse and present” in the mapping and to emphasize spatial depth mapping from the ratio of direct sound components  D and room sound components  R as well as level differences .

While listening is psychoacoustic phenomenon exploited that man with his ears due to interaural level differences ( interaural Level Difference , ILD ) and run-time differences ( interaural time difference , ITD ), the direction of sound sources can locate. Setting up in a stereo triangle provides good listening conditions for stereo loudspeaker reproduction . The individual ear distance of man playing with loudspeaker stereophony not matter, but probably in the binaural recording technology ( dummy head stereophony ), which is suitable only for headphone playback: The dummy head stereophony with the interaural signal differences could not prevail as to its reproduction mandatory headphones required are. Normal stereo signals can be reproduced using headphones, but not artificial head recordings using loudspeakers; The latter leads to discolouration of the sound and a faulty or missing stereo impression, since the internal ear signals obtained with the artificial head microphone cannot be equated with the loudspeaker signals. That means: ILD and ITD are not equal to Δ L and Δ t . The ears of the listener form their own ear signals from the signals from the stereo loudspeakers with the personal HRTF (Head Related Transfer Function).

Acoustical aspects

There are several methods of recording stereo signals for loudspeaker stereophony . When it comes to miking , a distinction is made between runtime stereophony and intensity stereophony (level difference stereophony ). Mixed forms are called equivalence stereophony .

In the case of intensity stereophony, two microphones are used, the directional effect of which is used in such a way that the level difference between the signals on the two channels causes a certain direction of the auditory event when played back on the stereo loudspeaker base . More precisely, it is only the sound pressure differences that are effective here and that determine the direction of the phantom sound source heard . If a sound signal is distributed identically to two stereo speakers, the listener perceives a phantom sound source from exactly the center of the stereo. If the level of the signal on a loudspeaker is increased, the phantom sound source moves in the direction of this loudspeaker; at a level difference of around 18  dB (16 dB to 20 dB), the listener perceives a complete deflection from the direction of the loudspeaker. The simplest main microphone setups here are X / Y stereophony (two cardioid microphones with the corresponding axis angle to each other, but close to the same place) and MS stereophony (omnidirectional microphone for the mono sum (M) and figure-eight microphone for the left / right difference signal (p ), mixed to left and right via an electronic matrix). See direction mixer and panpot . The level difference stereophony produces the greatest localization sharpness of the phantom sound sources during loudspeaker reproduction.

In transit time stereophony, two microphones are set up at a certain distance from each other, the microphone base , but also at a certain distance from the sound source, so that sound events are recorded depending on their position at different times on the two channels as the transit time difference Δ t . If a sound signal is distributed identically to two stereo speakers, the listener perceives a phantom sound source from exactly the center of the stereo. If the signal on one loudspeaker is now changed by delay , the direction of the hearing event moves in the direction of the other loudspeaker. With a transit time difference of approximately Δt = 1.5 ms (1 to 2 ms), the listener perceives a complete deflection (100% direction of the auditory event) from the direction of a loudspeaker. Due to the different distance between the microphones and the sound source, there is always a certain level difference, at least with the instruments close to the microphone . The best known main microphone setup is A / B stereophony. These are two omnidirectional microphones at a defined distance, the microphone base, from one another. A distinction is made between “small A / B” for a base smaller than 35 cm (quasi double head) and “large A / B” for a correspondingly larger base. This value is not generally determined.

The transit time stereophony gives a better spatial impression of the resulting sound signal, but has the disadvantages compared to the intensity stereophony that the localization sharpness of the phantom sound sources is lower and the sound of the signal can be reproduced less compatibly via mono playback devices, since it increases due to the time shifts Interference phenomena (cancellation of certain frequencies) can occur, which falsify the sound impression (mono incompatibility).

Mixed forms which, according to the principle of equivalence stereophony, contain localization information about both runtime and level differences, try to combine the advantages of both methods. Well-known microphone setups are, for example, ORTF and NOS .

As a rare recording method, the artificial head recording technique should be remembered here. The shape of a human head is reproduced and the microphones are attached in place of the two ears . This recording technique records the sound signal in the same way as humans hear it directly on their eardrums. If this recording is listened to with headphones as intended (head separation), the listener receives the original sound event at the ears again and can localize the directions of the noises , although the exact pre-localization is not always successful. The reproduction of artificial head recordings can be improved by adapting to the individual HRTF , i.e. the head transfer function of the listener, as well as so-called head tracking, in which the movements of the head are included in the calculation of the head transfer function. A special form of this process is real head stereophony, in which a person wears microphones in their ear canal. Binaural recordings, i.e. artificial head recordings, are generally not intended for playback in the form of the usual loudspeaker stereophony.

It is easy to see that recording methods that represent hybrid forms between loudspeaker and headphone stereophony cannot work satisfactorily in practice. However, some of them, such as the Jecklin disc (microphone spacing now 35 cm), are sometimes used by amateurs.

If a sound recording (which was not made with an artificial head) is played back via two stereo loudspeakers, which are located in a plane in front of the listener, a sound field overlay occurs in the room, which creates a stereo sound impression on the center line between the loudspeakers. This is the equilateral “60 ° stereo triangle ”. The listener should ideally be in the “sweet spot” in the middle in front of the stereo speakers.

If several loudspeakers are arranged next to one another, each of which reproduces a defined mix of the right and left signals, the area in which the listener can perceive the spatial auditory impression can thereby be enlarged. This can be the case, for example, in the cinema with larger speaker distances.

Transmission technology

TV guide stereo icon

The first records with stereo recordings were available in Germany since 1958. In 1964 radio transmission began with stereo sound on FM / VHF ( pilot tone multiplex method ), and in the early 1980s TV broadcasts also received stereo sound. Appropriate procedures were used to ensure that the new signals were compatible with the mono signals so that they could continue to be played back with the old devices. In the meantime it has also been possible to carry out stereo transmissions in the long, medium and short wave ranges ( AM stereo ) and to make stereo calls.

Stereo in pop music

In pop music studio productions, the individual sound elements (vocals, instruments, ...) are recorded separately and one after the other in individual tracks . When mixing the material, the position in the stereo image is set for each track using the panorama controller (also called panpot , from panorama and potentiometer). For singing it is usual to position it exactly in the middle, i.e. H. the voice sounds equally loud from both speakers. In addition, an artificial surround sound ( reverberation ) can be mixed in. Sound engineers refer to this type of recording and mixing as stick stereophony .

Stereo in practice

See also base width .

In order to experience a good stereophonic sound impression, a number of conditions must be met:

  • Both speakers should be about the same distance from the rear wall and the side walls of the room. Floorstanding speakers in particular should not be too close to the wall.
  • The listener's seat should be at the top of an equilateral triangle formed by them and the speakers. In practical terms, this means that the distance between the loudspeakers should correspond to the distance between the listener and each of the loudspeakers. This gives a listening angle of ± 30 ° = 60 °.
  • The tweeters should be approximately at the ear level of the listener.
  • The speakers should be facing the listener. In addition, it makes sense to pay attention to the radiation axis, as there are different characteristics in the high-frequency volume of the speakers (30 ° zone).
  • The room should have a high level of attenuation (low reverberation, low reflections from the walls).

Although most households, schools or conference rooms now have a stereo playback option, the conditions mentioned are rarely met due to the spatial conditions. Often the loudspeakers are placed in an unfavorable position or unwanted reflections occur on walls, so that more or less compromises have to be made here.

Portable stereo devices ( radio recorders , also known as “ghetto blasters”) have the loudspeakers permanently installed at a distance of 20 to 30 cm from one another. Although the playback is stereophonic, the listener would have to hold his head directly in front of the device for a good stereo impression. For portable devices with a short speaker distance, an electronic base extension or base width enlargement (also called "3D stereo effect") is therefore often used. To do this, some of the high frequencies of each channel are mixed in out of phase (i.e. with reverse polarity) to the other channel. Due to the runtime discrimination of the ears, the acoustic impression arises that the loudspeakers are further apart, or directional hearing is improved even with a greater listening distance.

Many home cinema systems use the effect that the location of the radiation of low frequencies below 100 Hz is irrelevant for the stereo directional impression - but not for the sense of space. They only have a single loudspeaker ( subwoofer ) for bass reproduction, which often contains all amplifier channels at the same time and can be set up as desired. Only the boxes for the reproduction of medium and high frequencies (satellites) are set up at locations that are fixed relative to the listener.

literature

  • Gustav Büscher, A. Wiegemann: Little ABC of electroacoustics . 6th edition. Franzis, Munich 1972, ISBN 3-7723-0296-3 .
  • Thomas Görne: Sound engineering . 1st edition. Carl Hanser, Leipzig 2006, ISBN 3-446-40198-9 .
  • Gregor Häberle, Heinz Häberle, Thomas Kleiber: Expertise in radio, television and radio electronics . 3. Edition. Europa-Lehrmittel, Haan-Gruiten 1996, ISBN 3-8085-3263-7 .
  • Matthias Thalheim: Dramaturgical staging consequences of artificial head stereophony in funk-dramatic productions , diploma thesis, Humboldt University Berlin 1985, Section Cultural Studies and Aesthetics, Theater Studies, epubli Berlin 2016, ISBN 9783737597814

See also

Web links

Individual evidence

  1. ↑ Illustrated technology for everyone, Issue 11/1925, p. 10
  2. ^ Robert Alexander: "The Inventor of Stereo: The life and works of Alan Dower Blumlein" Oxford: Taylor and Francis, 2000
  3. ^ Robert Alexander: "The Inventor of Stereo: The life and works of Alan Dower Blumlein" Oxford: Taylor and Francis, 2000, p. 78
  4. ↑ Russell Sanjek: American Popular Music and Its Business: From 1900 to 1984 , 1988, p 360
  5. ↑ R. Sanjek: American Popular Music and Its Business: From 1900 to 1984 , 1988, S. 361f.
  6. ^ Robert Rotifer: George Martin (1926-2016). He gave Technicolor to the Beatles. Even if he did not understand stereo , orf.at, March 9, 2016, accessed November 29, 2016.
  7. ↑ March 22, 2009 - 45 years ago: WDR radio starts stereo broadcasts