When a piece of magnetic material is moved past a magnetic field, usually created by an electromagnet, it becomes magnetised. Similarly, when the magnetised material is moved past an unenergised coil, it induces a voltage across the coil. This is Faraday's law of electromagnetic induction, which relates the voltage induced to the magnetic field strength. A number of parameters are used to characterise magnetic materials:
(a) Coercivity H c is the measurement the level of difficulty to magnetise the material. For storage media, a high coercivity is desired in order that information stored will be preserved in the presence of stray magnetic fields that may be present. High coercivity also implies that a strong magnetic field is needed to record information onto it. Magnets with high coercivity are called hard magnets.
(b) Remanence B r is the amount of magnetisation that remains after the magnetic field is removed. Soft iron used for electromagnets are chosen to have low remanence so that it will respond efficiently to the applied electromagnetic field.
(c) Magnetic domains are small regions in the magnetic media which may be magnetised independently of adjacent regions, so that adjacent domains can have opposite polarities. The size or granularity of these domains have an important bearing on the density of information that can be stored.
(d) Flux reversal occurs when a change in polarity is encountered while moving from one domain to the next. The storage density of the media is measured by the flux reversal per inch (frpi) or the flux change per inch (fcpi).
The main requirements of a magnetic recording system are: the write head, the magnetic medium, and the read head. (The write head could be the same as the read head and usually has been the case for disk drives.) The write head is driven by a current source that carries the information to be stored. The write head radiates flux, which changes the state of magnetization of the magnetic medium immediately under the head. Actually, since the head is moving with respect to the magnetic medium, any point on the magnetic medium retains the state of magnetization corresponding to the last flux it experienced from the write head as the head moves away from that point.
On a rigid disk, the disk moves in a circular motion under the head. Information is stored on the disk in concentric tracks, the width of a track roughly being governed by the size of the write head. The density of recording per sq inch (known as areal density) is the product of the number of tracks per inch (tpi) and the linear density of information along a track measured in bits per inch (bpi).
The current into the write head induces a magnetization pattern on the track immediately below the write head. When a track is to be read, a read head is positioned over the track. Then, the magnetization pattern formed on that track radiates flux that is sensed, by the read head. The read head produces a voltage that corresponds to the magnetization on the track being read. There are primarily two types of read head: inductive heads which contain coils of very fine wire and which produce a voltage proportional to the time derivative of the flux that passes through its coils, and magneto-resistive (MR) heads which produce a voltage directly proportional to the flux sensed by the head. MR heads produce larger read voltages than inductive heads, but have a limited dynamic range for linear operation.
The recording/playback head is made of easily magnetised ferrite material and has a small air-gap at the point where it comes into contact with the recording tape. When energised, the coil winding on the structure is used to create a strong and concentrated magnetic field on the recording media as it moves along with a velocity v. During the playback mode, this coil detects the induced voltages.
The recording media in this case is a powdered ferric oxide compound which is magnetisable and has high remanence. This layer of magnetic material in the unmagnetised state may be conceived as made up of dipoles, tiny magnets with N-S poles randomly positioned. Under the influence of the external magnetic field, these dipoles will align their N-S poles in line with the applied field thus becoming magnetised. Upon removal of the applied field some of these dipoles remain aligned. By either increasing the rate v the media is moved across the head, or by decreasing the granularity of the magnetic material, (i.e. making the tiny magnets smaller), we can record faster changes in the applied magnetic field, that is, the frequency response is increased. For digital data, the density of the stored information increase with decrease in the granularity of the magnetic media.
With a weak field, only a small number of the dipoles retain their alignment. As the field gets stronger, more and more of them will remain aligned, that is, the stored magnetic field increases. A saturation level is reached when increases in the applied field does not result in a corresponding increase in the stored magnetic field. Digital recording generally operate in the saturation region.
Digital recording systems may be considered as communications channels. There is an input signal and an output signal which is a transformed and noisy version of the input. The source data to be recorded or saved is prepared by the CPU, which also provides the storage device with information concerning the address of the storage locations. Compression and other source data preprocessing takes place before the data passes into the channel encoder which adds error-correcting bits and converts the data stream into a form suitable for recording. This signal is passed through the equalisation filters, and amplified as the write current for the recording head, creating the pattern of magnetic fluxes reversals on the storage medium.
To recover the stored data at the output, a reverse sequence takes place. Starting at the storage medium, the flux reversals are sensed by the magnetic head. The pulses are demodulated, equalised, decoded and finally presented at the output as the read data. In digital recording, the magnetic medium is saturated and flux reversals are used to represent the digital information. The figure below shows the relationship between the write signal, flux changes and subsequent induced current in the write and read modes.
During the write process, a write current is passed through the coil. Since the current needs a finite time to build up and the media is moving under the head, the result is a magnetic transition with a finite rise-time. Disks and tapes employ the longitudinal recording method illustrated above in which the flux lines of the magnetic field are oriented in the direction of the motion of the media. Although higher recording densities are attained using the vertical or perpendicular recording method in which the flux lines are perpendicular to the surface of the media this method is more difficult and correspondingly more expensive to implement. As shown above, the direction of the write current determines the polarity of the magnetisation of the recording medium. When the current reverses, it creates a flux reversal.
During the read process, the magnetisation on the recording surface is detected by the head and some of the magnetic flux is diverted through the coil, producing an induced voltage which is proportional to the rate of change of flux. These detected current pulses are hard to distinguish from noise pulses found in magnetic media and techniques are required to properly encode and decode the data for magnetic recording purposes. Under ideal conditions, the peak of the read pulse indicates the position of the flux transition.
There are a number of methods for encoding digital data when recording onto magnetic material, some examples are:
 | Non Return to Zero (NRZ) |
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Modified Frequency Modulation (MFM) |
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Run Length Limited (RLL) |
There are many ways of encoding the data as a function of flux reversals. The simplest would be to represent each 1 with a pulse leaving the signal low for each 0. This is the return-to-zero (RZ) code as the level always drop back to the zero state. If the level for the 1 is held high for the whole bit period, we have the special case of the non-return-to-zero (NRZ). The Non-return to zero (NRZ) technique of recording is the simplest and most efficient, but it is not self-clocking, and we cannot obtain the bit cell information from it. Flux reversals occur only at mid-cells (or cell boundaries in some implementation).
Modified frequency modulation (MFM) refines data encoding to reduce the number of pulses written on the disk. Using MFM, a logic ONE is always encoded as no pulse followed by a pulse. A logic ZERO, when preceded by a logic ONE, is encoded as two no pulses. A logic ZERO, when preceded by another logic ZERO, is encoded as a pulse followed by no pulse. Using MFM, the byte 11000101 would be encoded NPNPNNPNPNNPNNNP for a total of six pulses or flux reversals on the disk. Compare this with the 12 pulses required to store the same data using FM. MFM is currently used with all floppy drives, most large disk memory sets, and many fixed disk systems.
The run length limited encoding schemes take data encoding to a new level. Usually the RLL specification will be followed by two numbers such as 1, 7 or 2, 7. These numbers represent the minimum and maximum run of O bits between two 1s. The most common RLL scheme is RLL 2,7. RLL 2,7 is a complex encoding scheme that groups bits together and uses a table to encode the data in these groups. for example, 1100 is encoded as NNNNPNNN, 1101 is NNPNNPNN, and 111 is NNNPNN. RLL increases the density and transfer rate of data by 50 percent.