In the year 1898, the magnetic recording technique was discovered by Poulsen. In magnetic recording techniques, the property of certain magnetic material, to retain magnetism, when once they are magnetized is used. The magnetic material used is gamma ferric oxide (Fe2O3). This material is coated on a plastic base and tape will be manufactured. The Tape Recorder Working Principle is explained below.
Magnetic Recording Principle:
The given figure shows the Tape Recorder Working Principle or tape recording method. The magnetic tape will be moved at a constant speed under the influence of a concentrated magnetic field created by the recording head. The recording head will be supplied with input signal frequency current. The recording head is nothing but an electromagnet, which has a very fine gap. In this fine gap a strong alternating magnetic field will be created, when the input signals are supplied to the recording head.
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| The principle of Magnetic Recording and Reproduction |
The magnetic tape is moved past gently pressed against the fine gap of the recording head. so the tape will be magnetized in accordance with the magnitude of the magnetic field created at the air gap. Thus we have length of magnetized tape that has been magnetized to different densities depending on the flux variations at the air gap.
At the time of reproduction, the tape on which the magnetic recording is done will be run past over a head called the play back head. This is done at the same speed with which recording was done. As the tape is magnetized its field variations will induce voltages accordingly in the play back head. This voltage is amplified in an amplifier, which integrates the signal to provide flat frequency characteristics.
Hence it is clear that, different signals can be recorded on magnetic tapes and it can be reproduced. For good quality recording and reproduction, the following conditions are to be satisfied
1. The speed of tape must be the same at the time of recording and reproduction.
2. The recording and reproducing head gap must be very small to create a strong magnetic field at the time of recording and to receive maximum induced voltage at the time of reproduction.
The recorded information can be removed and fresh recording can be made on the same tape when desired. This is one of the, best advantages of tape recording technique. This process of removing the recording is called Erasing. Erasing can be done by demagnetizing the recorded tape.
Tapes Used for Recording :
The tapes used for recording are Coated tapes. The tapes are manufactured by applying a coating of a magnetic oxide and binding agents over a thin film. This is known as the base. The base material for magnetic tapes is obtained from Cellulose Acetate (C.A.), Polyvinyl Chloride (P.V.C.), or Polyester (Mylor).
The tapes made on Cellulose Acetate are weak and are cheap in cost. Tapes made on P.V.C. base are costly and are stronger than those made on C.A. The strongest tapes are made on Mylar base and are the costliest.
Magnetic Recording tapes manufacturing process :
The steps in manufacturing the tape are :
Preparation of the base
The base film is made as a broad film by suitable process.
The thickness of the base is : 0.381 mm for standard tapes
0.0254 mm for long play tapes
0.0127 mm double play tapes It will be more thinner for triple play tapes.
Coating of the base with magnetic material
Finely divided particles of Ferric Oxide generally needle shaped Fe2O3, a wetting agent, a binder and a lubricant are all thoroughly mixed, in a ball mill. Using a special process the base will be coated with this mixture. Chromium dioxide (CrO) can also be used as the magnetic material in the manufacture of tapes.
The advantage of chromium dioxide as the material is that it does not suffer from self demagnetization, as is the case with Ferric Oxide. However chromium dioxide tapes require stronger fields at the head for recording. Also chrome tapes need more stronger bias.
High quality tapes are made using iron alloy that has high coercivity.
It is a generally well known fact that, for making a high quality tape the following factors should be satisfied: The ferric oxide particles should be of uniform size, dispersed in the mixture and a uniform coating is to be made. Strict quality control measures are taken to get high quality coating on the base.
After coating the tapes will be dried and polished. The tape so manufactured will be cut to desired width of 6.35 mm for standard tape and 3.81 mm for cassettes. Magnetic tapes with 12.7 mm, 25.4 mm and 50.8 mm are available.
After slitting to the desired width, tape will be spooled on spools or spooled on to cassettes. The spools or cassettes will be stored for later use.
Tags:
Data Recorders