Sound Recording and Reproduction
From Media Hex
IntroductionSound Recording and Reproduction">
Sound Recording and Reproduction, technology used to record, store, and play back sounds. Sound recording and reproduction are two separate processes. Sound recording uses microphones to pick up sound waves in the air. The pressure changes associated with the waves are converted into electrical signals, which can be coded and stored for future access. Sound reproduction, or playback, uses additional devices to retrieve the stored information and convert it back into electrical signals. The signals are then sent to a loudspeaker, which converts them back into sound.
To record soundTo record sound, a microphone changes the acoustic energy of sound waves in the air into electrical signals. Inside a microphone is a thin, flat, metallic surface, called a diaphragm, that is suspended in a magnetic field. When a sound wave reaches the microphone, the air pressure changes around the diaphragm, causing the diaphragm to move. This movement within a magnetic field creates an electrical signal. The signal is then moved through a signal chain and finally transferred to a storage medium, such as a computer hard disc, a multitrack tape, a compact disc (CD), or a phonograph record.
The signal chain for recording has theThe signal chain for recording has the signal move from the microphone through a microphone preamplifier to a mixer that can combine the microphone signal with others. It can also send the signal to one or more output channels for stereo playback, and can alter the sound`s quality by signal processing. The final signal is then controlled for loudness level and sent to the storage medium. This signal chain has the potential to greatly change the original live performance.
To reproduce soundTo reproduce sound, a playing devicesuch as a CD player, MP3 player, or phonographaccesses the stored data. The playing device reads the data and converts the information back into electric energy. The electrical signal is sent to a loudspeaker, which has a diaphragm housed in a magnetic field in much the same way as a microphone`s diaphragm is housed. The electrical signal creates a disturbance in the magnetic field. These resultant variations in the magnetic field cause the diaphragm to move. As the diaphragm moves, it pushes out and pulls in, creating changes in air pressure to recreate the sound that was originally recorded.
Sound recording and reproduction form theSound recording and reproduction form the foundation of many industries, including entertainment, communications, and multimedia businesses. Recording and reproduction of sound allow people to play their favorite music, whether it was recorded yesterday or many years ago. Radio networks rely on sound recording and reproduction for storing news and other types of programming. Television and motion pictures combine images with music, speech, and sound effects to provide the viewer with an enriched experience. Computer programs, multimedia software, and video games also use sound to make programs more engaging. "See also "Broadcasting, Radio and Television.
Sound is stored in one of two basic waysSound is stored in one of two basic ways, called formats. The earlier format, which is still in use today, stores sound in a form that resembles the original sound wave and is known as analog recording. This format is called analog because the form of the sound wave, called the waveform, in the recording is analogous to the original waveform. In other words, the recorded waveform is a copy of the original waveform. A waveform has two qualities: frequency and amplitude. Frequency corresponds to the number of times per second that the air pressure changes, and amplitude registers the amount of each change. An analog signal varies in frequency and amplitude in exactly the same way that the original waveform does.
The newer format for storing sound is digitalThe newer format for storing sound is digital recording, which represents a sound wave as a series of numbers. The numbers are defined in the binary, or base-2, numbering system, using only the numbers 0 and 1. These numbers can be represented in many ways. Computer disk drives (including those found in MP3 players) use the position of magnetic particles to define binary numbers. Compact discs use laser light reflected from smooth areas and interrupted by small pits on the disc to indicate 0s and 1s. A number of digital formats exist, and they determine the quality of the recorded sound, and the number of channels of sound playback.
The term "format" is also used to describeThe term "format" is also used to describe the number of channels, or streams of sound, that are used to record and play back sound. Two of the most common formats are monaural, or mono, and stereophonic, or stereo ( "see "Stereophonic Sound). Early recordings were made in mono, using a single channel to record and play back sound. Broadcasts on AM radio are also in mono. Stereo recording, introduced in the 1960s, uses two channels for sound. Stereo has long been the accepted format of the music industry and for FM radio broadcast. Surround sound became increasing popular during the first years of the 2000s, as the motion picture industry settled on the 5.1 surround-sound format and the music industry recognized the need to align themselves with the consumer systems being widely purchased for home theater use. Surround sound is also used for some video games and high-definition (HD) television broadcasts.
In stereo recordingIn stereo recording, each channel has a different form of the sound. The signals are sent to separate loudspeakers during playback. When played, the sounds from these two separate channels combine in the air and give the illusion of direction that the human ear naturally perceives when hearing sound. A different percentage of each sound might appear at each loudspeaker, giving each sound source its own location between the two loudspeakers. The spatial qualities afforded by stereo recordings are absent from monaural recordings.
Quadraphonic recordingsQuadraphonic recordings, which were popular in the 1970s, use four channels. Each channel is different, and each of four loudspeakers is placed in a separate corner of a room. This approach expands on stereo and gives the listener a sense of sound coming from all directions. Quadraphonic recording is the failed predecessor of today`s surround-sound systems, used with home theaters.
The 5.1 surround-sound playback formatThe 5.1 surround-sound playback format uses five primary loudspeakers surrounding the listener`s location. These five speakers are similar or the same in quality. Two speakers are located front left and right just as in stereo playback, one speaker is between those front two speakers, and two speakers are located behind the listener left and right. The spacing and loudness balance of the five speakers must be calibrated correctly to hear the sound as intended. A sixth speaker or subwoofer (the .1) handles the very low frequencies of all five channels. This extra speaker allows the system to be less expensive since the other five speakers do not need to be designed to reproduce the lowest frequency area. Mostly for logistical and musical reasons, music recordings make little use of the subwoofer. However, the low frequencies provided by the subwoofer are often very important for motion picture soundtracks or video games, especially for reproducing sound effects.
Analog Recording Systems
Analog recording is done in one of threeAnalog recording is done in one of three ways, depending on the storage medium used. Tape recording uses magnetic tape to store signals. Phonograph recording mechanically engraves sound waves onto vinyl records. Optical recording, used for motion-picture soundtracks, works by changing the intensity of light aimed at a photoelectric cell.
In analog tape recordingIn analog tape recording, electrical signals from a microphone are transformed into magnetic signals. These signals are encoded onto a thin plastic ribbon of recording tape. Recording tape is coated with tiny magnetic particles. Chromium dioxide and ferric oxide are two magnetic materials commonly used. A chemical binder coats the particles to the tape, and a back coating prevents the magnetic charge from traveling from one layer of tape to the next.
Tape is wound onto reelsTape is wound onto reels, which can vary in diameter and size. Professional reel-to-reel tape, which is 6.2 mm (0.25 in) wide, is wound on large metal or plastic reels. Reel-to-reel tapes must be loaded onto a reel-to-reel tape recorder by hand. Cassette tape is only 3.81 mm (0.15 in) wide and is completely self-enclosed for convenience. Regardless of size, all magnetic tape is drawn from a supply reel on the left side of the recorder to a take-up reel on the right. A drive shaft, called a capstan, rolls against a pinch roller and pulls the tape along. Various guides and rollers are used to mechanically regulate the speed and tension of the tape, since any variations in speed or tension will affect sound quality.
As the tape is drawn from the supply reelAs the tape is drawn from the supply reel to the take-up reel, it passes over a series of three magnetic coils called heads. The erase head is activated only while recording. It generates a current that places the tape`s magnetic particles in a neutral position in order to remove any previous sounds. The record head transforms the electrical signal coming into the recorder into a magnetic flux and thus applies the original electrical signal onto the tape. The sound wave is now physically present on the analog tape. The playback head reads the magnetic field on the tape and converts this field back to electric energy.
Unwanted noiseUnwanted noise, such as hiss, is a frequent problem with recording on tape. To combat this problem, sound engineers developed noise reduction systems that help reduce unwanted sounds. Many different systems exist, such as the Dolby System, which is used to reduce hiss on musical recordings and motion-picture soundtracks. Most noise occurs around the weakest sounds on a tape recording. Noise reduction systems work by boosting weak signals during recording. When the tape is played, the boosted signals are reduced to their normal levels. This reduction to normal levels also minimizes any noise that might have been present.
Today cassette tapes have largely beenToday cassette tapes have largely been eliminated from common use, replaced by inexpensive compressed digital formats. Professional quality analog tape recording has returned to use because of the unique sound qualities it can add to music recordings, qualities that some artists, producers, and engineers have found attractive and desirable.
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