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Tuesday, 29 April 2014

Magnetic Recording and Reproduction of Sound

Magnetic Recording and Reproduction of Sound: This is widely used system for recording audio signals. The basic principle involved in magnetic recording is the phenomenon of electro-magnetic induction. The audio and video tapes are coated with a magnetic material. The signal (audio or video) currents induce magnetism in the tape. The track is in the form of magnetic path on the tape.
For audio recording, the audio signal is given to the microphone which converts it into electrical pulses. This electrical signal is amplified and the amplified current is sent through the audio head coil. If magnetises the head. The magnetic strength of the audio head goes on changing in accordance with intensity of the audio signal being produced at the microphone.

The magnetized audio head is moved on the audio tape. The audio tape is covered with a layer of ferromagnetic material like ferric oxide. As a result tape is also magnetized; the strength goes on varying according to the signal. By magnetic induction, “magnetic track” of sound is formed on the tape which contains audio signal.

The voltage obtained at the output of amplifier should not have any distortion otherwise during replay; the same will come into play.

The core of the head is made of soft alloy, hence it does not have any residual magnetism, therefore it will not give any distortion. However, “demagnetisers” may be used to demagnetize the head immediately after it has done its job.

Magnetic Recording Methods:

For magnetic recording, generally the following methods are used.
1. Longitudinal Magnetic Recording
2. Transversal Magnetic Recording
3. Perpendicular Magnetic Recording

1. Longitudinal Magnetic Recording: In this method, the tape is moved longitudinally to the recording head. The North and South, both poles are kept on same side of the tape.
The method is simple and gives satisfactory results. This is commercial and a very popular method of recording.

2. Transversal Magnetic Recording: In this recording, the poles of the record head are kept transversal to the tape. In this case the record head needs special construction and also it is a complicated method. Results are also not so satisfactory as in the first case, hence it is not used at commercial level.

3.Perpendicular Magnetic Recording: In this method, the tape moves normal (at perpendicular) to the poles of the record head – one pole on each side.

Reproduction or Replaying: For reproduction of the audio signal “play head” is moved on the tape. This activates the play head. The recorded magnetic path (sound track) induces currents in the play head coil (by electromagnetic induction).These induced currents are amplified and given to speaker which by transducer action converts the currents into the original sound. 

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