Data storage on magnetic tape

Magnetic tape is a ribbon of a base material such as mylar or polyester, which is coated on one side with a very thin layer of ferric oxide mixed with other compounds, and which layer acts as a magnetic material.

The surface of the tape with the magnetic material is pulled past a thin gap between the pole pieces of an electromagnet.

The electromagnet has a continually varying ac current flowing through the coil, and as a result, each element of the magnetic surface is magnetised according to the instantaineous value of the magnetic field across the gap between the poles.

As the tape is being continually pulled across the gap, the magnetic surface records the history of the varying ac current, along the tape.

If now the opposite is done, and the tape with its magnetic layer which has been magnetised as above is pulled past the same gap between the pole pieces, then a varying ac current is induced in the coil. This signal can be amplified, using a high gain amplifier, and the original signal that was used to magnetise the tape can be recovered.

The tapes used in early computer systems was 0.5 " wide, and were approximately 2,400 ft long, and were stored on reels 10.5" in diameter.

Instead of a single electromagnet recording information on the tape, early tape systems used seven electromagnets stacked on top of each other. These produced seven parallel recorded tracks along the length of the tape. Six tracks carried data for six bit character streams, and the seventh track carried parity data for each character.

Later tape systems used a head block containing nine electromagnets, producing nine tracks. These were for eight bit words, plus one for parity.

A typical tape of the above size will hold approximately 140Mbytes.

However as technology has improved, the packing density is now much higher. A modern magnetic tape within a cassette can typically store 1 to 3 Gbytes.

© 1998 Ron Turner

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