Recording sound

In 1877, Thomas Edison was experimenting with a way to repeat Morse-coded telegraph messages using a waxed paper tape on which the message was written by a stylus. He noticed that if he pulled an already inscribed tape past the stylus it produced a note, and reasoned that he should be able to use the same kind of system to record notes of his choice and play them at will.

The “phonograph” that resulted had a stylus that followed a spiral groove on a tinfoil-covered drum. Sound vibrated a diaphragm, which vibrated a stylus, which made indentations in the groove. Running the stylus through the indented groove reversed the process, causing the diaphragm to vibrate, reproducing the word “Halloo,” in Edison’s first successful attempt.

A needle following a groove became the basis of a new industry. Edison’s cylinders led the way for years, but disks, introduced by Emile Berliner in1888, eventually supplanted the cylinders because they could be more easily mass-produced.

Originally phonographs were entirely mechanical, with amplification being provided by a large megaphone-like horn. Later, electronic amplification and loudspeakers came along, along with vinyl (until the Second World War, records were made of shellac), the long-playing record, and stereophonic sound. But the base technology was the same as Edison’s tinfoil-covered drum.

In 1898, though, Danish inventor Valdemar Poulsen patented a device called the Telegraphone that recorded sound on a long steel wire, without stylus or groove: the first magnetic recorder.

An electric current flowing in a coil generates a magnetic field. A magnetic field moving near a wire generates an electric current in the wire. A magnetic recorder turns sound vibrations into a varying electric current, which is used to create a varying magnetic field, which can be recorded by passing something through it that can be magnetized, like steel. If you then pass that magnetized steel by a wire, the varying magnetic field sets up a varying electric current, which can be used to create vibrations in a diaphragm, reproducing the original sound.

The Telegraphone worked, but its sound quality was poor. Some people experimented with half-inch-wide steel tape, but it was heavy and expensive. Magnetic recording didn’t really catch on until the Germans came up with a lightweight tape made of a cellulose acetate film (today plastics used) covered with particles of iron oxide — rust.

Improvements in magnetic recording over the years have brought us to today’s era of high-quality cassettes. But sometimes you need a bigger tape — in the Kramer IMAX Theatre, for example.

In ordinary 35-millimetre motion pictures, the soundtrack is optically encoded on the film, as a track that varies from dark to light. Light shines through this track onto a photoelectric cell, generating a varying electrical current which is turned back into sound.

However, IMAX films boast six channels of surround sound, with a supplementary “sub-bass” channel for those nice low frequencies that shake the theatre in Rolling Stones At the Max. Optically coding that information would take far too much space on the film. Instead, it is recorded on 35-millimetre magnetic tape, which runs on a separate unit electronically synchronized with the film.

Some IMAX theatres now have digital sound — which brings us nicely to today’s recording medium of choice, the compact disk.

The CD seems something like a high-tech version of a vinyl record, replacing the stylus with a laser beam and the groove with tiny reflective pits. The big difference is that vinyl discs are “analog” recordings, while a laser disc is a “digital” recording.

In analog recordings, the pattern of rapidly varying air pressure that we call sound is imposed directly on the storage medium, as an undulating groove on a vinyl record, or as a changing magnetic pattern on a tape. Ina digital recording, sound is recorded as a string of “on” and “off” signals. (On a compact disk, the signal is “on” if the laser beam is reflected by a pit, and “off” if it isn’t.)

You can arrange 16 on-off signals in 65,536 different ways. Each of these combinations, to the computer, represents a particular sound waveform. During digital recording, the sound is sampled 44,000 times per second, and the computer assigns each sample whichever of the 65,536 values is closest to the sound at that instant. In playback, another computer reads those coded sequences (while checking for and correcting errors) and turns them into an electrical signal.

It is possible to record digital information on magnetic tape, and even on vinyl, but the combination of digital recording, resistance to wear and even aesthetic beauty the CD offers has made it the format to be reckoned with. In fact, the vinyl record is all but dead.

Thanks to the CD, we’re leaving future generations perfectly preserved records of artists like Vanilla Ice.

One only hopes future generations are forgiving.

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