We shall consider the principles of recording sound on tape and on film, and its reproduction. In the world of electronics where music and sound systems play very crucial roles in our social life, there is the need for the production of high quality music and sound equipments that would meet the demands of evolving technology. Therefore, we shall take a look at the working principles and technology involved in recording sound on tape or a film.
The diagram below illustrates the principle of tape recording in which flexible tape is used. It is coated with a fine uniform layer of a special form of ferric oxide which can be magnetized. The backing is a smooth plastic base tape. When recording, the tape moves at a constant speed past the narrow gap between the poles of a ring of soft iron which has a coil round it. The coil carries the audio-frequency (af) current due to the sound recorded. On one half of a cycle, that part of the same cycle, the next piece of tape in the gap is magnetized in the opposite direction, as shown. The rate at which pairs of such magnets is produced is equal to the frequency of the af current. The strength of the magnets is a measure of the magnetizing current and hence of the intensity of the sound recorded.
In ‘playback’, the magnetized tape is now run at exactly the same speed at the same or another ring, the playback head. As the small magnets pass the gap between the poles, the flux in the iron changes. An induced emf is thus obtained of the same frequency and strength as that due to the original tape recording. This is amplified and passed to the loudspeaker, which reproduces the sound.
While recording sound, in order to obtain high-quality sound reproduction from tape, the output from a special high frequency bias oscillator is applied to the recording head in addition to the recording signal. This ensures that the magnets formed on the tape have strengths which are proportional to the recording signal. If the bias oscillator was not used, severe distortion due to non-linearity would occur. The bias oscillator can also be used to erase the recording on the tape. This is done by applying its output to a coil in a special erase head. The erase head is similar to the record head but has a larger gap, so that the tape is in the magnetic field for a longer period. The bias signal takes the magnetic material on the tape through many thousands of hysteresis loops. These become progressively smaller until the magnetism disappears.
One method of recording sound on film, in the form of a variable area sound track, is illustrated in the diagram below. A triangular aperture or mask T is brightly lit by a high-wattage lamp. After passing through T the light is reflected by a mirror M, and the rays are brought to a focus on to a slit S by a lens L1. By rotating the mirror slightly, as shown, the image T’ of T, produced by the slit, can be moved up and down. This varies the length of slit illuminated. The light passing through the slit is collected by another lens system L2 and focused on a trip of moving unexposed film. The mirror M is mounted on the moving system of a galvanometer, so that it moves at the same frequency as the audio frequency currents passed through the galvanometer coil. The area of the film exposed thus varies as the audio-frequency signal. Hence when the film is developed, a permanent sound recording or sound track is the film is developed, a permanent sound recording or sound track is obtained.
The diagram below illustrates the principle of reproducing the sound track and passes through to a photo-electric cell. This contains a light-sensitive metal surface such as caesium, which then emits a number of electrons proportional to the light intensity. A current therefore flows in a resistor R. the sound track is coupled to the film, and as it moves, an audio-frequency current flows in R. the pd developed is amplified and passed to the loudspeaker.
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