Hardware Reference
In-Depth Information
cuitry can determine whether a pulse (and therefore a flux transition) falls within a given
transition cell time period.
The electrical pulse currents generated in the head while it is passing over the storage me-
dium in read mode are weak and can contain significant noise. Sensitive electronics in
the drive and controller assembly amplify the signal above the noise level and decode the
train of weak pulse currents back into binary data that is (theoretically) identical to the
data originally recorded.
Asyoucansee,harddiskdrivesandotherstoragedevicesreadandwritedatabymeansof
basicelectromagneticprinciples.Adrivewritesdatabypassingelectricalcurrentsthrough
an electromagnet (the drive head), generating a magnetic field that is stored on the medi-
um. The drive reads data by passing the head back over the surface of the medium. As the
headencounterschangesinthestoredmagneticfield,itgeneratesaweakelectricalcurrent
that indicates the presence or absence of flux transitions in the signal as it was originally
written.
Read/Write Head Designs
As disk drive technology has evolved, so has the design of the read/write head. The earli-
est heads were simple iron cores with coil windings (electromagnets). By today's standar-
ds, the original head designs were enormous in physical size and operated at low record-
ing densities. Over the years, head designs have evolved from the first simple ferrite core
designs into the several types and technologies available today. This section discusses the
various types of heads found in PC hard disk drives, including the applications and relat-
ive strengths and weaknesses of each.
Several types of heads have been used in hard disk drives over the years:
• Ferrite
• Metal-In-Gap (MIG)
• Thin-film (TF)
• Magneto-resistive (MR)
• Giant magneto-resistive (GMR)
• Perpendicular magnetic recording (PMR)
Note
By the end of 2005, hard drives based on PMR were being used in devices such as portable
music players and laptop PCs. Desktop PC hard drives based on the technology became
available in 2006.
