Principles of Magnetic Storage

Define Electromagnetism Flux FM encoding MFM encoding RLL

What is electromagnetism?

Electromagnetism Basic principle of physics states that as an electric current flows through a conductor, a magnetic field is generated around the conductor. This magnetic field can then exert an influence on magnetic material in the field

Electromagnetism When the direction of the flow of current is reversed, the magnetic field’s polarity also is reversed.

How is electromagnetism important to the hard drive? All magnetic storage devices read and write data by using electromagnetism. When the direction of the flow of electric current is reversed, the magnetic field’s polarity also is reversed

An electric motor uses electromagnetism to exert pushing and pulling forces on magnets attached to a rotating shaft.

Flux

A magnetic field that has a specific direction

Flux As the surface of a medium moves across the drive head, the head can create a magnetic flux over a specific region of the medium. When the flow of electric current through the coils in the head is reversed, so is the magnetic field polarity of the flux being created on the medium to reverse.

What does the flux have to do with the encoding method? Everything. The particular pattern of flux reversals within the specific area of the medium are used to store a given data bit (or bits) is called the encoding method.

Summary Hard disk drives & 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.

Data Encoding Schemes FM (Frequency Modulation) MFM (Modified Frequency Modulation) RLL (Run Length Limited)

What is data encoding? The conversion of digital information for use on an analog medium.

Flux reversals form the boundaries between areas of positive and negative polarity that the drive controller uses to encode the digital data onto the analog medium

During the read operation, each flux reversal that the drive detects generates a positive or negative pulse that the device uses to reconstruct the original (digital) binary data.

The drive passes raw digital data through a device called an encoder/decoder (endec), which converts the data into a waveform that is more conductive to the flux transitions (pulses).

During the read operation, the endec reverses the process and converts the information back to digital data.

Types of data encoding FM (Frequency Modulation) No longer used Used in the earliest floppy drives MFM (Modified Frequency Modulation) Devised to reduce the number of flux reversals used in the original FM encoding scheme and pack more data on the disk.

MFM Twice as efficient as FM encoding Used in hard drives for several years, but has now been phased out and replaced RLL (Run Length Limited) TODAY Most popular encoding scheme for hard disks Packs 50% more information on a given disk