basilh
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United Kingdom
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Posted 31 Mar 2008 2:35 pm |
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No, I said check out this article:-
Stereophonic Sound, system of recording, transmitting, and reproducing sound in a way that captures the qualities of the original sound. Every sound has three basic characteristics: frequency, amplitude, and harmonic constitution. Sound is created when air vibrates. The frequency of a sound is the rate at which the air vibrates, measured in hertz (Hz), a unit of one cycle, or vibration, per second. Amplitude is a measure of the pressure of each vibration, which determines the loudness of the sound. The harmonic constitution of a sound measures how much of a sound is its main frequency and how much is exact multiples, or harmonics, of its main frequency.
The human ear can detect the three basic characteristics of a sound—frequency, amplitude, and harmonic constitution—as well as a fourth characteristic: the direction from which a sound is coming. While monophonic recording captures the three sound characteristics, it cannot convey direction. Stereophonic recording is able to provide all four characteristics of a sound, mimicking the way the human ear works.
In sound recording, a microphone translates sound into an electrical signal. Stereophonic sound is recorded with at least two different microphones. Each microphone captures a slightly different sound, depending on the microphone’s position relative to the sound’s source. During recording, transmission, and playback, the signals remain separate from one another. Systems that play back stereophonic sound use separate speakers for the different recorded signals. When the speakers are in the proper position relative to the listener, the sound from the speakers has the directional quality of the original sound.
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Stereophonic Microphone Techniques
Several techniques convey the directions of sound sources in a recording. A widely used technique for rock and popular music is multi-mono recording. Recording engineers isolate each instrument or voice and place a microphone close to it. The engineers then split the signal output of this microphone, evenly or unevenly, between two output signals, using a device called a panoramic potentiometer, or “panpot.” If the instrument is equally loud in both signals, it appears to be in the center. If it is present in only one signal, it appears to be all the way to the left or right.
Stereophonic recordings of classical music seek to convey a sense of the place in which the music was played. Unlike other recordings, classical music recordings attempt to stay true to the actual positions of each instrument. To capture the original sound of the performance, recording techniques for classical music capture the sound complete with its directional information and the qualities that the space in which the music is played adds to the sound. The floor, wall, and ceiling materials and shape of a recording area cause sound to bounce off or be absorbed by surfaces in the area. This is called the acoustics of a space. The acoustics of a large concert hall differ greatly from those of a small recording studio.
In the binaural technique, recording engineers place a pair of omnidirectional microphones, which pick up sound equally from all directions, where the listener’s ears would be. These microphones capture the sound wave that would have entered each ear so that the sound wave is recreated when played back over headphones. The brain then has the same directional information as if the listener had been present at the original event. This technique can be uncannily successful in recreating the original sound, but it never became very popular because headphones are required for playback.
The British engineer Alan Dower Blumlein developed the coincident pair technique of sound recording in 1932. This technique uses two directional microphones crossed at right angles. The sound sources range in position in a semicircle from along the axis of the left microphone to along the axis of the right microphone.
The coincident pair technique produces a recording with well defined positions of each source of sound. However, the most convincing image of the arrangement of the sound sources does not necessarily provide the most appealing musical balance. Engineers often place spot microphones close to the sources of sound. The producer can then mix the sound from the spot microphones with the sound from the coincident pair microphones to give a stereo recording with a better musical balance.
A simple and popular technique, called spaced omnis, uses a pair of omnidirectional microphones spaced apart. The spaced omnis technique does not provide a true stereo recording—-instead, sounds seem to clump near the speakers and sound sources often seem to shift positions. Recording engineers often use a third microphone between the two spaced omnis to improve the stereo quality. In the 1950s and 1960s some excellent classical stereo recordings were produced using this technique.
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Stereophonic Recording and Transmission
The technology for sound recording underwent a radical change, from analog to digital, in the 1970s. In analog recordings, the recording equipment uses the electrical signals from the microphones to produce changes in a substance, such as vinyl long-playing (LP) phonograph record, magnetic tape, or movie film, on which the sound is recorded.
An LP has a spiral groove cut into the surface of each of its sides. When a sound is recorded on an LP, a special sharp instrument translates the signal from the microphone and cuts tiny notches into each side of the groove. One side of the groove represents the signal from one microphone, and the other side of the groove represents the signal of a second microphone. For recording on magnetic tape, the recording device uses the electrical signal from the microphone to magnetize areas of the tape with different intensities. When recording sound on movie film, the electrical signal from the microphone is converted into light, which exposes areas of the film to different degrees.
The higher the magnitude of the original sound pressure that the microphone picked up, the larger the notches in the record groove, the tape magnetization, or the area exposed to light on film. All these methods can record the extreme swings of the audio signal between the softest whisper that a listener can detect and the loudest tone that the system can accommodate.
With all analog systems, the recording substance, or medium, is vulnerable to unwanted changes, in which the two signals can get confused or the medium can deteriorate, such as when LPs warp and scratch or magnetic tape and film stretch or scratch. These changes can produce noise and distortion in the final sound. In addition, every time a copy of the recording is made the signal degradation increases.
In the late 1970s companies in Japan, the United Kingdom, the Netherlands, and the United States developed a new type of audio recorder immune to the problems of analog recording. Instead of preserving a physical change of varying magnitude, the digital recorders store the signal as digital words, each of which describes the sound intensity at one instant of time. A digital-to-analog converter reconstructs the original signal from the digital information for playback. Digital recording media include magnetic tape, magnetic discs, and optical discs. By 1992 the compact disc (CD) had become the standard medium for consumer stereophonic sound, replacing LPs. Digital recording can reproduce the original sound without adding as much additional noise as analog recording adds.
Digital recording has two important drawbacks. If the recording equipment does not sample the sound often enough—that is, record the loudness and other characteristics of the sound—the resulting recording will be choppy and parts of the signal may be missing. To capture every frequency in a sound, the equipment should sample the sound at least twice the frequency of the highest frequency of interest in the sound. Since the highest frequency that humans can hear is about 20 kHz, standard sampling frequencies are 44.1 kHz for CD recording and 48 kHz for digital recording on magnetic tape (see Digital Audio Tape).
The second drawback of digital recording is the limit of the amount of information that can be stored in a single digital word. The recording equipment translates each sample into a digital number. The wider the range of numbers available to the recording, the more specific a recording can be. An eight-bit word or byte storage contains eight binary digits, which can only define 256 different levels. This is not enough for high fidelity sound reproduction and can result in an unpleasant hiss called quantization noise in the recording. CDs use 16-bit storage units, which encode 65,536 different amplitudes and reduce the quantization noise to just below the threshold of human perception. Beginning in 1996 the standard in professional audio recording became 20-bit storage units, which can describe 1,048,576 different levels.
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Playing Back Stereophonic Sound
In a stereophonic recording, separate loudspeakers play back the signal from the original microphone, called the channel. Listeners need to be equally distant from each speaker to get the best stereo sound. If the listener is closer to one of the speakers, then the stereo sound image will be pulled to one side, due to the Haas effect or the precedence effect.
Listeners can optimize their stereo sound by positioning their speakers correctly. The loudspeakers and the listener should form an equilateral triangle. The speakers should be away from the walls of the room, so that sound does not reflect from the walls and confuse the sound coming from the speakers. The wall to the side of each speaker should absorb or disperse sound, rather than reflect it. The listener should position the speakers so that the distances to the three nearest room boundaries—such as the floor, the side wall, and the wall behind the speakers—are all different. This will reduce the cumulative effect of reflections from these surfaces. |
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