Magtek InSpec 9000 Manuel d'utilisateur télécharger pdf (Page 61)

Appendix A. Magnetic Encoding

Notice in the figure that a coil of wire is wrapped around an iron core and that the iron core is

shaped much like a "horse shoe" magnet with the North and South Poles close together. The gap

between the North and South Poles is where the magnetic field lines are produced (when electric

current flows) and thus, it is the area of the head that touches the magnetic tape. When the

power source changes the direction of the electric current, the magnetic field lines also change

directions. This change of magnetic field direction produces a flux transition on the magnetic

tape.

To illustrate this point, refer to Figure A-4, an idealized model for magnetic tape:

Figure A-4. Magnetic Tape Model

This idealized model shows the iron oxide particles, all uniformly shaped as tiny bar magnetics

and all perfectly aligned.

Now, consider that an encode head has passed over the magnetic tape and the direction of the

electric current in the encode head coil has changed as indicated in Figure A-5.

Notice the change in the magnetic polarity on the magnetic tape where the change in encode

current occurred. This change in polarity is called a flux transition and it is what a READ head

will detect as it passes over the magnetic tape.

So, imagine a READ head passing over the magnetic tape. A READ head is basically

constructed in the same way as an encode head - “horse shoe” shaped iron with a coil of wire.

However, instead of a current being sent through the coil by a power source, an electric current

is created in the coil as the gap of the READ head passes over the flux transitions. A read back

signal is generated as illustrated in the figure above. This read back signal forms the basis for

the measurement of the position precision of the flux transitions (jitter).

This point needs to be emphasized. It is the basis for all the measurements taken by the Encoded

Card Tester: The flux transitions recorded by the encode head, on the magnetic tape, causes a

read back signal to be generated by the read head. This signal possesses amplitude whose peaks

define the position of flux transitions.

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Magtek InSpec 9000 Manuel d'utilisateur Page 61