Stereophonic Sound System

Component systems are made up of individual components (parts) that are purchased separately and connected by the consumer. People can combine different models and types of components to suit their individual needs, budgets, and tastes. In general, component systems provide the best sound quality.

Rack systems consist of units that a manufacturer has wired together and assembled into a frame or cabinet. In the past, such systems were called consoles. Rack systems have the same kinds of audio parts as component systems, but the parts cannot be separated. Some preassembled systems are small enough to fit on a table or desk. These are known as tabletop, or desktop, systems. Rack systems, as well as smaller preassembled systems, cost less to produce than component systems but generally have poorer sound quality.

Portable systems are lightweight preassembled systems that can be carried easily while in use. They are powered by batteries. Some portable systems, popularly called boom boxes, have small speakers. Others are small enough to fit into a pocket. Such units are used with headphones.

Home theater systems combine high-quality sound with picture playback. In such systems, various components are connected to a large-screen television set. The sound and picture come from a videocassette recorder (VCR) or a videodisc player.

Program sources include tuners, compact disc players, tape decks, and phonographs.

A tuner receives signals broadcast from radio stations and converts them into electric signals. The user adjusts the tuner to receive a radio signal of only a certain frequency (vibrations per second). Each radio station broadcasts its program signals at a specific frequency. In many stereo systems, the tuner is combined with the amplifier into a single unit called a receiver.

A compact disc (CD) player produces sound that has been recorded on a small, round disc in digital (numerical) code. The disc is made of hard plastic with a reflective metal coating. As the player spins the disc, a laser beam shines on the disc. The beam reflects off the disc as pulses of light. The CD player uses the pulses, which correspond to the code on the disc, to create a signal.

CD players are an excellent program source because they produce virtually no background noise. In addition, playback does not damage the disc. As a result, CD's last longer than tapes and records. With a CD player, the user can quickly access any part of the recording. Most models can be programmed so that tracks (recordings) can be played in a certain order. A device called a CD changer accepts multiple discs for extended playback.

A tape deck, also called a cassette deck, records and reproduces sounds on magnetic tape. During recording, electromagnetic heads convert electric signals into varying magnetic patterns on the tape. During playback, heads translate the magnetic patterns into electric signals. Decks typically have electronic noise reduction systems that reduce the faint hissing noise made by the tape during playback.

A phonograph, also called a record player, reproduces sound from a plastic disc called a record. An analog (likeness) of the original sound waves is stored as jagged waves within a spiral groove on the record. The record is placed on a circular turntable. As the turntable spins the record, a needle called a stylus rides along the groove. The waves in the groove cause the stylus to vibrate. The phonograph converts the vibrations into electric signals.

An amplifier strengthens the electric signals that it receives from the program source. Most home stereo systems use amplifiers with at least 50 watts of power per channel. Stereo systems require at least two amplifier channels. Amplifiers used in portable systems produce only a few watts because of the limitations of battery power and size. In general, amplifiers with greater power provide better sound because distortion is lessened. However, speakers are designed to use only a certain range of power, so the amplifier in a system must be coordinated with the speakers. Too much power from an amplifier can damage speakers.

Amplifiers or receivers serve as the control center of stereo systems. These components may have switches to adjust loudness, tone, and the balance of stereophonic sound between the channels. Tone controls alter the strength of sound signals within limited ranges of frequencies. There are three basic types of tone controls--bass, treble, and midrange. Bass controls cut or boost low-frequency signals, which produce low-pitched sounds. Treble controls control high-frequency signals, which produce high-pitched sounds.

An amplifier may include an equalizer to improve sound quality. An equalizer operates similarly to tone controls but can cut or boost more specific ranges of frequencies, thus enabling more precise tone control. An equalizer can correct acoustic problems caused by the speakers or by the shape of the room in which the speakers are kept.

A speaker receives electric signals from the amplifier and converts them into vibrations, which create sound waves. Stereophonic systems require at least two speakers for reproducing sound--one for each channel of recorded sound. Because it is difficult for one speaker element to reproduce the entire range of frequencies, high-quality units contain three speakers, one each for bass, midrange, and treble.

Speakers in component systems are mounted in wooden or plastic cabinets. The size and shape of the cabinet affects the tone quality of the sound made by the speakers. The location of speakers in a room also affects the quality of the sound they produce.

Headphones, like speakers, change electric signals into sound waves. Headphones, also called earphones, typically have two cushioned earpieces connected by a band. The device fits over the head and ears for private listening. Each earpiece carries one sound channel.

Until the mid-1980's, phonographs were the most common program source. Since that time, however, many people have replaced their records with tapes or compact discs.

Contributor: Ken C. Pohlmann, M.S.E.E., Prof. of Music Engineering, Univ. of Miami.

See also Compact Disc; Phonograph; Radio; Speaker; Tape Recorder.

Pohlmann, Ken C. The Compact Disc Handbook. 2nd ed. A-R Editions, 1992.

White, Glenn D. The Audio Dictionary. 2nd ed. Univ. of Wash. Pr., 1991

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