Hardware Reference
In-Depth Information
Figure 8.4. Magnetic write and read processes.
You can think of the write pattern as being a square waveform that is at a positive or negative voltage
level. When the voltage is positive, a field is generated in the head, which polarizes the magnetic
media in one direction. When the voltage changes to negative, the magnetic field induced in the media
also changes direction. Where the waveform actually transitions from positive to negative voltage, or
vice versa, the magnetic flux on the disk also changes polarity. During a read, the head senses these
flux transitions and generates a pulsed positive or negative waveform, rather than the continuously
positive or negative waveform used during the original recording. In other words, the signal when
reading is 0 volts unless the head detects a magnetic flux transition, in which case it generates a
positive or negative pulse accordingly. Pulses appear only when the head is passing over flux
transitions on the medium. By knowing the clock timing the drive uses, the controller circuitry 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 medium 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.
As you can see, hard disk drives and other storage devices read and write data by means of basic
electromagnetic principles. A drive writes data by passing electrical currents through an
electromagnet (the drive head), generating a magnetic field that is stored on the medium. The drive
reads data by passing the head back over the surface of the medium. As the head encounters changes
in the stored magnetic field, it generates a weak electrical current that indicates the presence or
absence of flux transitions in the signal as it was originally written.
Read/Write Head Designs
