Handbook for Sound Engineers

Magnetic Recording and Playback 1083

magnetic field. The rear layer of the sandwich shown in

Fig. 28-53 has a fixed or pinned magnetic field that

serves as a reference. The filler of the sandwich is a

magnetoresistive (MR) material chosen for a large

change in resistance per change in magnetic flux. The

front outer layer is a magnetic probe that actually

samples the magnetic flux of the bits on the disk. As the

magnetic polarity of the data bits reverses, the angle of

the magnetic field in the outer layer spins back and

forth. Part of this field bridges through the center layer

to the pinned rear layer, causing the resistance of the

MR layer to change. The resulting output signal has a

much better SNR than an equivalent read head.

Since the GMR effect does not work in reverse to

generate a varying magnetic field when driven by an

electrical signal, we still need a coiled conductor for

writing the data onto the disk. The solution is a

composite head that has both a GMR read element and a

coil for writing. The entire head, including the GMR

read element and the coil for writing, can be fabricated

together using thin film techniques. A single thin film

wafer may contain up to 20,000 heads.

Most digital recording schemes drive the record head

hard enough to saturate the medium in one polarity or

the other. If the head is tracking exactly over any prior

data, the old data will be completely overwritten. Unfor-

tunately, the tolerances of the head tracking system may

cause slight alignment errors that leave a bit of the old

signal unerased.

One method to remove the residue is to use a

straddle erase technique that resembles the outriggers

on a Hawaiian canoe. Two thin erase cores straddle the

desired track and trim off any of the prior signal that

wasn’t covered by the new recording.

A newer technique is to write wide and read narrow.

Just as we discussed for analog recording, we can write

a track that is wider than the read core. The extra width

of the recorded track allows for a small tracking error.

This technique is easily implemented with GMR heads

since these heads have separate read and write elements.

The read element is fabricated slightly narrower than

the write element to create the desired overlap.

28.5.6 Magnetic Disks

A 2500 foot roll of 2 inch recording tape has enough

surface to carpet a large living room. A 60 minute

DTRS tape would only cover half of a couch. In

comparison, a multigigabyte hard disk in a digital audio

workstation uses a few 3½ inch (89 mm) diameter

magnetic disks with a working area about the size of

your footprint. Although the basic technology of all of

these products is similar, the precision required in their

manufacturing increases rapidly as the size shrinks and

the density increases.

28.5.6.1 Floppy Disks

Floppy disks were close cousins to magnetic tape.

Although the disks were cut from large rolls much like

the jumbo rolls from which magnetic tape is slit, the

coatings parameters were very different.

The diskette, which we now call the floppy disk,

was developed around 1970 as a read-only device. The

contents of the prerecorded diskette were loaded into a

Figure 28-52. Thin film digital tape head.

Figure 28-53. Giant magnetoresistive head.

I bias I bias Pinned layer(Exchange layer

not shown)

Exchange layer

Pinned layer

Conducting spacer

Contact

Sensing layer

I bias

Sensing layer

Conducting

spacer

Contacts

x

x

x