Developments in magnetic sound recording technology, 1898-1948
The next major development in came in 1898 when Danish inventor Valdemar Poulsen patented the telegraphone (telephonograph), the first device to record sounds magnetically. For his first experiment Poulsen moved a recording apparatus along a piece of steel wire stretched across his laboratory. By running beside it and talking into its microphone he recorded onto the wire the varying magnetic fields produced by the sounds.
His original machine consisted of a microphone, a grooved cylinder around which steel wire was wound and an electromagnet. The two poles of the electromagnet rotated around the cylinder thus magnetising the wire by amounts corresponding to the strength of the voice-induced currents. To replay the message the microphone was switched for a telephone speaker and the electromagnet was rotated once again.
Poulsen demonstrated the machine at the Paris Exhibition in 1901 by recording and playing back the voice of Austrian Emperor Franz Joseph. Admittedly, his 'telephone-recording' machine failed to gain commercial popularity but, importantly, Poulsen had proven the principle of magnetic sound recording.
Lee De Forest contributed the triode vacuum tube that amplified electrical impulses making it possible to reproduce sound at much greater volume.
The next revolution in magnetic recording, the invention of magnetic tape, took place in Germany in 1928. This new medium consisted of a plastic tape coated with a ferromagnetic iron oxide powder that when exposed to a magnetic field is permanently magnetised. Consequently, anything recorded was remembered for playback at any time, and sounds could be erased and then the tape re-record onto. Magnetic tape recorders were invented in the early 1930s and in 1936, tape manufacturer BASF made the recording of a symphony during a performance by the visiting London Philharmonic Orchestra.
The previous year, AEG exhibited a tape to tape recorder called the Magnetophon, which soon became the standard device used by German radio stations. Wire recorders continued to be used outside of Germany until after the war when the technology was 'liberated' and brought to the United States where it soon became the industry standard as well. The wire recorder, however, had been the first to make recording sound at home possible, and so attributed to it, as well as the newly invented television and radio, was the start of the popular love affair with gadgets.
Among the first notable uses of the magnetophon was the recording of 26 radio shows for delayed broadcast by Bing Crosby in 1947/48.
The mechanics of all tape recorders are very simple. The magnetic tape is drawn by a drive shaft, called a capstan, from a supply reel on the left to a take-up reel on the right. On its way, the tape passes over an electromagnetic coil, or head, consisting of an iron core wrapped in wire. There are two heads, each recording one channel of a stereo program.
During recording, the electric signal is fed to a transducer and converted it into a magnetic flux that magnetises the oxide particles on the tape. The playback head reads the magnetic field of the tape and converts it back into electrical signals of varying voltage. These are boosted in an amplifier and converted into sound by a loudspeaker. The erase head generates a current that arranges the tape's magnetic particles in a neutral position, thereby removing the previous sounds.
Much progress had been made in the quality of sound recording, however the major problem of deterioration of reproduction quality over time still remained. This was overcome by the development of digital technology.