3.2 Recording on the wire
A paper published by Oberlin Smith in an 1888 issue of Electrical World discussed the possibilities for recording sound using the property of magnetism. He envisaged a cotton thread impregnated with steel dust passing through a coil carrying a current controlled by a microphone. The variations with the sound in the strength of the current would cause corresponding magnetic fluctuations in the magnetic medium. Unfortunately he dismissed his idea because, as he said in his paper, he thought that ‘the magnetic influence would probably distribute along the wire in a most totally depraved way’.
Smith's ideas remained theoretical as he never performed any experiments. However, by the end of the nineteenth century Valdemar Poulsen, a Danish electrical engineer, had demonstrated Smith's hypothesis. Poulsen's ‘telegraphone’, shown in Figure 19, was patented in 1898. It used steel wire wrapped around a brass cylinder as the magnetic medium. At the Paris Exposition of 1900, Poulsen made a recording of Emperor Franz Josef of Austria that is the oldest magnetic recording now in existence.
The lack of appropriate technology meant that the telegraphone could not compete with the gramophone. The development of an electronic amplifier using the thermionic valve (vacuum tube) enabled the tiny magnetic fluctuations in the steel wire to be magnified to a usable level. By 1924 a German engineer, Dr Curt Stille, had developed a machine that could record sounds on a steel tape. The BBC (British Broadcasting Company) showed great interest for, at this time, they used disc recorders for pre-recording programmes and talks that were cut into acetate discs, replayed maybe twice and then discarded. So they sent two engineers to Berlin for a demonstration. They offered to buy the machine but were refused and so returned empty-handed. In 1931 Louis Blattner purchased a Stille machine, shipped it to England and renamed it the Blattnerphone, illustrated in Figure 20. It used 2-inch (50-mm) wide flat steel tape and could record for up to 20 minutes.
The BBC evaluated it but were unhappy with the signal-to-noise ratio due to a constant background hiss, caused by the physical qualities of the steel tape. Blattner eventually sold out to the Marconi Company, who in conjunction with Dr Stille further developed the recording machine. In order to provide a suitable audio bandwidth they found it was necessary to run the now one-inch-wide steel tape at a rate of 60 inches per second (152 cm per second). This meant that nearly 2 miles (3.22 km) of metal tape was required for a half-hour programme! Such was the pressure for an easy record and playback system that the BBC used steel tape recorders for a while, as demonstrated in the next activity.
Listen to the two audio tracks below. The first track contains a recording of the famous statement made by the Rt Hon. Neville Chamberlain on his return from Munich. This was recorded on a steel tape recorder in September 1938. As a comparison, the second track contains a 1932 experimental disc recording.
Notice the difference in background noise between the magnetic steel tape recording and the disc recording of 6 years earlier.
Click below (25 seconds)
Click below (1 minute 14 seconds)
Poor signal-to-noise ratio meant that steel tape was eventually discarded but one of the first home magnetic recording machines, the Webster wire recorder (described in Box 6), used thin steel wire, echoing Poulsen's idea.
Box 6: The Webster wire recorder
The Webster wire recorder was introduced in 1946 and remained popular with amateurs until the late 1950s.
The quality of this recorder was, for the time, surprisingly good – this was perhaps in part due to the fast wire speed of 30 ips (from which all the subsequent standard tape speeds – 15, 7½, 3¾ and 1 7/8 ips – were derived). Figure 21 is a photograph of the Webster wire recorder. The steel wire was 0.0036 inch in diameter and reels provided up to an hour of recording time.