Sound is stored for playback through the process of sound recording. Recording devices capture sound waves from the air and convert them into electrical signals or digital data, which are then stored on media such as records, compact discs (CDs), and computer hard drives. Playback equipment reverses the recording process to reproduce the original sound. Sound recording is used in the music, movie, television, and radio industries.

History of Recording Technology

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The first device to record and reproduce sound was the phonograph, which was invented by Thomas Alva Edison in 1877. The first phonographs used a vibrating stylus, or needle, to cut pits representing sound waves into tinfoil-wrapped cylinders. The sound was reproduced when a second stylus traced over the pits. Emile Berliner’s Gramophone, invented in 1887, replaced the cylinder with a flat disc and made many improvements in Edison’s invention possible—for example, easier mass production and enhanced fidelity. Lee De Forest patented the amplifying, three-element electron tube, called the Audion, in 1907, and electric phonographs using amplifiers with one or more tubes appeared in the 1920s.

The standard format for playback on early phonographs was the 78-rpm (revolutions-per-minute) record; it had a playing time of about 41/2 minutes. In the late 1940s manufacturers introduced 331/3-rpm records, which provided up to 30 minutes of playing time per side. These long-playing records, or LPs, made 78-rpm discs nearly obsolete. Smaller 45-rpm discs, playing up to eight minutes per side, soon followed.

Around the same time, magnetic tape recorders came into widespread use. Magnetic tape consists of a plastic backing coated with tiny particles of magnetic powder. After a microphone converts sound waves to electrical signals, the tape recorder magnetizes the powder in a way that preserves a record of the signals. The most popular tape-recording format was the cassette. By 1982 sales of recorded music on cassettes had overtaken those of LPs in the United States.

The recording devices discussed thus far use analog technology—that is, they produce and store a physical representation (an “analog”) of sound waves. Most modern recording, however, is done digitally. Digital recording analyzes an analog signal and samples, or measures, it thousands of times a second. Each of these samples is assigned a value, and that value is converted into binary number data (numbers containing only 0s and 1s). Binary numbers are easily stored, retrieved, and edited on computers and playback devices. Digital audio recordings are free of the noise and distortion typical of analog recordings, thus offering a superior experience for the listener.

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The first popular digital audio format was the CD, which eclipsed the LP in popularity in the late 1980s. In addition to offering improved sound, CDs could store up to 80 minutes of recorded music on a side, while LPs offered only 60 or 70 minutes on two sides. In the late 1980s digital audio tape (DAT) recorders became available for sound recording and reproduction. Although DAT was meant to replace the cassette, the format never won wide acceptance.

Courtesy of Apple

The next milestone in digital audio was the introduction of digital audio files. The most popular type is the MP3, but various formats exist. Digital audio files provide near-CD quality sound at vastly reduced file sizes. The size of CD audio files is reduced by discarding certain sounds based on assumptions of what the ear is least likely to miss. The smaller size made it feasible to download music files from the Internet and then listen to them on a computer or transfer them to a portable digital music player, such as Apple Inc.’s iPod. During the first decade of the 21st century digital downloads surpassed CDs in terms of units sold.

The Recording Industry

The recording industry plays an integral part in the music and motion picture branches of the entertainment industry. The techniques used to record sound in each medium are vastly different. Motion picture sound, for example, must be synchronized with the visual performances so it is transferred to film, rather than to discs or digital audio files.

Music Recording

The two basic approaches to recording music are documentary and studio recording. Documentary-style recording is most often used to record classical music and live concerts of popular music. Most rock musicians prefer to record their music in studios, where they can experiment freely, and other pop performers may rely upon electronic gimmicks to alter their sound.

In documentary-style recording, recording engineers set up portable equipment at the recording site. They may employ one or more microphones with or without a microphone mixer. With only two microphones positioned at 90- to 120-degree angles to the performers, engineers can record an entire orchestra.

Sometimes recording engineers separate the microphones with a screen made of special acoustical insulation in order to approximate the binaural (two-eared) experience of human hearing. Sound personnel may mount a microphone on a tall stand to pick up the airborne sound waves and the wall-conducted sound waves. The opposite effect is obtained by placing a microphone on a low tripod. The objective is to permit only one sound path to the microphone; otherwise, echoes tend to cloud the original signal.

In studio recording, the central components are tracks—individual channels that record each microphone or instrument independently. That way, a singer might record on one track one night, the guitarist on another the next night, and the percussionist on still another the third night. The recording engineer can play back the individual tracks at whatever volume is desired for each. The engineer can also re-equalize (change the ratio of high sounds to low and middle sounds) one track at a time or create a wholly new track by mixing two of the original tracks. In fact, multitrack recording and mixing allow all kinds of enhancement after the original recording dates.

Studio recording involves the use of several pieces of equipment: a special, acoustically enhanced recording environment (the studio itself); a microphone for each voice and acoustic musical instrument; line preamplifiers for electronic instruments; a multitrack mixing console that permits the engineer to adjust the volume of each input separately; and a multitrack recorder to store the output signal from the console. This recorder might be digital or analog. Most digital recording is computer-based and is done with multitrack hardware and software that can have as many tracks as the computer’s memory allows.

Recording for Motion Pictures

When sound was first added to motion pictures in the 1920s, the sound-on-disc process was used. This meant that the sound was recorded onto a record separately from the movie. The record player then regulated the speed of the movie projector so that the sound and picture were synchronous.

In the 1930s sound-on-film cameras were developed. These cameras record a magnetic or optical track running parallel to the picture area. Editing such film is difficult, however, and the film’s sound quality is inferior. Inventors solved these problems with a technique called double-system sound. Although the sound tape recorder and film camera are separate pieces of equipment, an electronic synchronization signal assures correspondence between the two during filming. Laboratory personnel copy the tape to magnetic film with the same proportions as the picture film.

During editing, picture and sound films are locked side-by-side into an editing console. When an editor cuts one film, the others must be cut at the same place. If synchronization is lost, it can be regained by advancing one film or another. Back at the laboratory, the picture and sound are married to create a single film with a synchronous sound track. This film, called the release print, is what viewers see and hear on television or in the theater. It is equipped with either an optical or, more likely, a magnetic sound track.

Optical sound tracks consist of one or more narrow stripes at the edge of the film. Viewers cannot see these tracks on the screen because they are outside the projected area of the film. There are two basic types of optical sound tracks—variable density and variable area. The stripe on a variable-density sound track uses a series of darker or fainter parallel lines to represent sound waves. The stripe on a variable-area sound track has a clear center of varying width edged with black. The width differences represent the sound waves. In both types, the sound is reproduced by passing a beam of light through the track. The pattern on the track constantly alters the amount of light falling on a photoelectric pickup, which is connected to a speaker that produces the sound.

The two main digital playback formats for film are DTS and Dolby Digital (AC-3). DTS audio is encoded on a separate CD-ROM that is played along with the film. To synchronize the audio with the action on screen, a time code is optically printed on the edge of the film. The sensor on the projector reads the time code and uses it to keep the audio in time with the picture when playing back the CD-ROM. The AC-3 format optically encodes the digital audio data in small squares in between the physical sprocket holes on the film. The sensor on the projector reads the digital data and recompiles it as a steady stream of bits (binary digits), which is then decoded in synchronization with the picture for playback.

The optical and both digital sound track formats can all be imprinted on the same film. This allows a theater to choose which playback format is appropriate for its particular system.