Revolutions in sound recording
Revolutions in sound recording

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Revolutions in sound recording

3.3 Magnetic tape recorders

Experiments showed that the use of paper tape coated with iron oxide particles significantly improved the signal-to-noise ratio and enabled a lower tape speed to be used. A plastic-based version of this magnetic tape, developed by the German company BASF, led to the development of a commercial tape recorder with audio characteristics that could nearly match those of the gramophone record, but not at an economical price. Secret work on tape recorders was undertaken by the Germans throughout the Second World War. This was revealed when Allied forces captured the Radio Luxemburg studios in 1944 and discovered machines capable of outperforming discs in both sound quality and playing time. In America the Minnesota Mining Manufacturing Company (3M Co.) further refined the tape (their experience with adhesive tapes proved advantageous) while the Ampex Corporation developed a machine with a frequency response of 30 to 15 000 Hz at a tape speed of 7½ ips (19 cm/s) and a signal-to-noise ratio of 50 dB, certainly equalling, if not bettering, the characteristics of discs at that time. The tape speeds used for different recording characteristics are discussed in Box 7.

Box 7: Tape speed

The audio bandwidth of a tape recorder is determined to an extent by the selection of the tape speed, i.e. the rate at which the tape is drawn across the record and play heads, shown in Figure 22. The wavelength of the audio signal recorded onto the magnetic tape is proportional to the tape speed. As the tape speed is increased, a greater proportion of the tape is used to store the audio signal, allowing higher frequencies to be retained on the tape. Because high tape speeds are less economical on tape usage, tape recorders had speed controls to allow users to select the tape speed to suit the audio quality required, as detailed in Table 3.

Figure 22: The tape path on a tape recorder

Table 3: Tape speed vs bandwidth

Tape speed Bandwidth Use
38 cm/s (15 ips) 20 Hz–20 kHz Studio recording
19 cm/s (7½ ips) 30 Hz–15 kHz High-quality home recording
9.5 cm/s (3¾ ips) 40 Hz–13 kHz General domestic music and speech
4.8 cm/s (1 7/8 ips) 50 Hz–6 kHz Recording speech (dictation)

Soon tape recorders were in use by the American radio networks for pre-recording their broadcasts, the entertainer Bing Crosby being one of the greatest proponents of the technology. Recording companies were also quick to embrace the benefits of tape – especially the ease with which mistakes could be edited and retakes inserted. Also the ability to record for longer periods (30 minutes or more) meant less need for recording sessions to be split into short takes. Early domestic recorders were used primarily for playing stereo recordings, but they were costly in terms of both the hardware and the media: a pre-recorded stereo tape cost five times that of the equivalent mono LP disc. The sales of pre-recorded tape plummeted once stereo LPs became available in 1958. From that point on, domestic tape recorders (similar to the one illustrated in Figure 23) were used mainly by enthusiasts for home recording.

Figure 23: An enthusiast's home tape recorder

Activity 21

By referring to the information given earlier, construct a table that compares the frequency response and playing time of the newly developed magnetic tape with that of 78 rpm discs of the same period (1945).

Answer

Table 4: Comparison of 78 rpm disc and magnetic tape media

Characteristic 78 rpm disc Magnetic tape
Frequency response 30 Hz–8 kHz 30 Hz–15 kHz
Playing time 5 minutes per side 30 minutes (minimum)

Tape could offer twice the bandwidth and six times the playing time.

The frequency response of magnetic tape was between 30 Hz and 15 kHz, and the playing time was up to 30 minutes. The frequency response of a 78 rpm record was 30 Hz to 8 kHz. The playing time was up to 5 minutes for each side of a 12-inch (30-cm) disc.

TA212_3

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