Audio Systems. Technology Development

At the start of the twentieth century, mechanical recording and reproduction of sound by cylinder or disk phonographs was dominant (Edison’s cylinder, Edison’s Diamond Disk, Emile Berliner’s gramophone, and the Victor Talking Machine Company’s Victrola). Playback quality was limited by the lack of amplification methods to increase the volume of the recorded performance above the background noise of the mechanical record.

Thus inexperienced performers were rarely used. Electronic recording methods with microphones and amplifiers were developed after World War II, and valve amplifiers and the transistor, which created more efficient signal amplification, caused a revolution in audio playback.

Throughout the twentieth century, turntables or decks for playing disk records were normally kept horizontal, so the downward pressure of the stylus would be constant across the disk’s surface. Eldridge Johnson, founder of the Victor Talking Machine Company in 1901, devised a spring motor to rotate the turntable at an even speed in 1896, and battery motors were also used prior to the electrical drives developed in 1925.

Although disk records were mastered on lathe-type mechanisms that moved the cutter in a straight line across the disk, records were usually played with a pivoted ‘‘pickup arm’’ that made a curved rather than straight track across the record, introducing distortion as well as wearing the record. Careful geometric design and motors that moved the rear end of the arm reduced tracking error and improved audio quality.

Early playback was acoustomechanical, as was sound recording. Following the development of electronic sound recording, the first fully electrical playback device, the Brunswick ‘‘Panatrope’’ in 1926, used magnetic playback pickups (cartridges) to convert the motion of the stylus in the groove to an electrical signal. Brunswick’s player also incorporated an electronic amplifier using the new vacuum tubes, and the electrical signal could be played through a loudspeaker.

The earliest material for disk records was called ‘‘shellac.’’ Actually a mixture of materials, shellac was an organic binding material for which no synthetic substitute was found. Before the first electronic version, sounds were reproduced and amplified mechanically. The acoustic energy was emitted by a ‘‘soundbox’’ and fed into a conical horn providing a suitable acoustical load. Ultimately, this energy came from the rotation of the disk itself.

Therefore the disks had to resist wear; and to this day, shellac has proven to be one of the longest lasting sound storage media. Shellac was used until about 1955 in the U.S., 1959 in the U.K, and even later in developing countries. The major drawback to shellac is that it is very prone to breakage.

During World War II the Allied forces made effective use of music for maintaining the morale of the troops, and music had to be distributed without risk of breakage. Shellac was gradually replaced with ‘‘vinyl’’ (polyvinyl chloride), which was flexible and had much less ‘‘grain,’’ meaning lower background noise. Until then, heavy steel needles, or pickups, were used for playing disks, typically with downward pressures of 100-150 grams.

When long-playing disks (LPs) were introduced after the war, the same vinyl was used. It was much more expensive than shellac, but the longer playing time offset this. Pressures from 10 to 20 grams became normal, with reduced stylus tip size. Vinyl disks also featured extended frequency range, and the extended range of good pickups also reduced wear.

Also, because of vinyl’s inherent flexibility, turntable designers were forced to introduce a pickup with a carefully crafted playback stylus made of jewel such as sapphire to resist wear. A number of mathematical studies showed how to optimize various dimensions of quality sound to achieve faithful reproduction, low distortion, and simultaneously low damage to disks. Vinyl records were still used by enthusiasts at the end of the twentieth century.

The faithful capturing of sound performed under ‘‘normal’’ conditions was led by radio broadcasting in the early 1920s. The technology of the electronic amplifier aided both the recording and the reproduction of sound. The first electronic amplifiers were based on thermionic valves, or vacuum tubes. Since vacuum tube technology was familiar to radio engineers, the principal developments in both radio and audio work tended to be described in books and magazines for the radio market. The demobilization of armed forces after World War II and expansion of the consumer market seems to have led directly to pure audio research and literature.

From 1950 onward, the pursuit of innovative, if not always faithful, sound reproduction was fed by specialist manufacturers, dealers, exhibitions, and advertising claiming ‘‘high fidelity,’’ or hi-fi, playback. A disk deck tended to form the centerpiece of a practical domestic hi-fi system. Tape decks could also used for playing reels of magnetic tape, with cassette decks added for analog audiocassettes after 1970. A tuner, primarily for receiving analog frequency-modulated (FM) radio broadcasts, was a normal component.

The actual amplification took place in two or three separate units, depending on whether monophonic or stereophonic reproduction was intended. The preamplifier brought the various sources of audio to similar electronic voltages, allowed the bass and treble ranges to be controlled for individual preference and, in the case of the disk deck, applied equalization. This compensated for deliberate changes in frequency response to improve the capacity of a mechanical disk record, called a recording characteristic, internationally standardized in 1955.

In vacuum tube days, the main amplifier (or amplifiers for stereo) tended to be a heavy, delicate, and hot unit, so it was usually placed on the floor out of the way. It had enough output power to feed a separate loudspeaker enclosure, or two for stereo. Hi-fi loudspeakers might require between 5 and 50 watts apiece. Most 1950s designs were no more efficient than those at the end of the century, the electroacoustic efficiency being only about 1 to 3 percent. Quality transistorized amplifiers became dominant toward the end of the 1960s.

Each loudspeaker cabinet often contained several electrical-to-acoustical transducers covering different parts of the frequency range, with the audio being suitably divided by a ‘‘crossover unit.’’ Compared with anything else in the high-fidelity system, loudspeakers had the most radical effects upon the reproduced sound quality. They were relatively cheap and simple for enthusiasts to assemble, and many did.

As the actual hardware became cheaper, with greater appeal to nonspecialists, the hi-fi craze became democratized. No longer was it confined to the wealthy or dedicated enthusiast. By the end of the twentieth century, everyone could enjoy high fidelity sound very comparable to leading technology of the 1950s.