All magnetic storage devices read and write data by using electromagnetism. This basic principle of physics states that as an electric current flows through a conductor (wire), a magnetic field is generated around the conductor. Electrons actually flow from negative to positive, although we normally think of current flowing in the other direction. The read/write heads in a magnetic storage device are U-shaped pieces of conductive material, with the ends of the U situated directly above (or next to) the surface of the actual data storage medium. The U-shaped head is wrapped with coils or windings of conductive wire, through which an electric current can flow.When the drive logic passes a current through these coils, it generates a magnetic field in the drive head. Reversing the polarity of the electric current causes the polarity of the generated field to change also. In essence, the heads are electromagnets whose voltage can be switched in polarity very quickly.
The disk or tape that constitutes the actual storage medium consists of some form of substrate material (such as Mylar for floppy disks or aluminum or glass for hard disks) on which a layer of magnetizable material has been deposited. This material usually is a form of iron oxide with various other
elements added. Each of the individual magnetic particles on the storage medium has its own magnetic field. When the medium is blank, the polarities of those magnetic fields are normally in a state of random disarray. Because the fields of the individual particles point in random directions, each tiny magnetic field is canceled out by one that points in the opposite direction; the cumulative effect of this is a surface with no observable field polarity.
The drive logic or controller takes the data to be stored and encodes it as a series of flux reversals over a period of time, according to the pattern dictated by the encoding method it uses.
Where the waveform actually transitions from positive to negative voltages,
or vice versa, the magnetic flux on the disk also changes polarity. 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.
When the PC produces the electrical signal, it sends voltages to the head causing the magnetic mediums to impart flux reversals on the disk. A drive head creates flux reversals on the medium to record data. For each data bit (or bits) that a drive writes, it creates a pattern of positive-to-negative and negative-to-positive flux reversals on the medium in specific areas known as bit cells. As the surface of the medium moves under the drive head, the head can generate what is called a magnetic flux of a given polarity 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 or flux in the head gap. This flux reversal in the head causes the polarity of the magnetized particles on the disk medium to reverse.
When the heads slide across the magnetic domains and gathers the enduced flux reversals it converts that information through elecrtonical voltages allowing the computer to read the storage. 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. During a read operation, each flux reversal the drive detects
generates a positive or negative pulse that the device uses to reconstruct the original binary data.
To optimize the placement of flux transitions during magnetic storage, the drive passes the raw digital input data through a device called an endec. During a read operation, the endec reverses the process and decodes the pulse train back into the original binary data. What engineers and designers came up with, essentially, is a design in which the bits of information are decoded not only from the presence or absence of flux reversals, but from the timing between them. The more accurately they can time the reversals, the
more information that can be encoded (and subsequently decoded) from that timing information. Timing is what defines a particular bit cell—that is, the time window within which the drive is either writing or reading a transition. If the timing is off, a given voltage transition might be recognized at the wrong time as being in a different cell, which would throw the conversion or encoding off, resulting in bits being missed, added, or misinterpreted. To ensure that the timing is precise, the transmitting and receiving devices must be in perfect synchronization.
This encoding scheme—sometimes called Single-Density encoding—was used in the earliest floppy disk drives installed in PC systems. FM encoding is no longer used.
This encoding was devised to reduce the number of flux reversals used in
the original FM encoding scheme and, therefore, to pack more data onto the disk. MFM encoding minimizes the use of clock transitions, leaving more room for the data. It records clock transitions only when a stored 0 bit is preceded by another 0 bit; in all other cases, a clock transition is not required. Because MFM minimizes the use of clock transitions, it can double the clock frequency used by FM encoding, enabling it to store twice as many data bits in the same number of flux transitions. This means a drive using MFM encoding reads and writes data at twice the speed of FM, even though the drive sees the flux reversals arriving at the same frequency as in FM.
This is the most popular encoding scheme for hard disks. It packs up to twice the information on a given disk than MFM does and three times as much information as FM. In RLL encoding, the drive combines groups of bits into a unit to generate specific patterns of flux reversals. By combining the clock and data signals in these patterns, the clock rate can be further increased while maintaining the same basic distance between the flux transitions on the storage medium.