ABSTRACT
A steady increase in recording densities and data rates of magnetic hard drives during last 15 years are mostly due to advances in recording materials, read/write heads, and mechanical designs. The role of signal processing and coding has been to make the best use of the capacity and speed potentials offered by these advances. As the recording technology matures, the read channel is becoming more and more advanced, reaching the point where it uses equally or even more complicated signal processing, coding
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To record data on a surface of a disk, the modulated signal current, typically bipolar, is passed through the electromagnet coils thus generating a fringing magnetic field. The fringing magnetic field creates a rémanent magnetization on the ferromagnetic surface, i.e., the ferromagnetic surface becomes permanently magnetic. The magnetic domains in the surface act like tiny magnets themselves and create their own fringing magnetic field above the ferromagnetic surface. The data are recorded in concentric tracks as a sequence of small magnetic domains with two senses of magnetization depending on a sign of writing current. In this, so-called saturation recording, the amplitude of two writing current signal levels are chosen sufficiently large so as to magnetize to saturation the magnetic medium in one of two directions. In this way, the nonlinear hysteresis effect does not affect domains recorded over previously recorded ones.
