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To most users, the hard disk drive is the most important, yet most mysterious, part of a computer system. A hard disk drive is a sealed unit that holds the data in a system. When the hard disk fails, the consequences usually are very serious. To maintain, service, and expand a PC system properly, you must fully understand the hard disk unit.
Magnetic media essentially is an analog storage medium. The data that we store on it, however, is digital information — that is, ones and zeros. When digital information is applied to a magnetic recording head, the head creates magnetic domains on the disk media with specific polarities. When a positive current is applied to the write head, the magnetic domains are polarized in one direction; when negative voltage is applied, the magnetic domains are polarized in the opposite direction. When the digital waveform that is recorded switches from a positive to a negative voltage, the polarity of the magnetic domains is reversed.
During a readback, the head actually generates no voltage signal when it encounters a group of magnetic domains with the same polarity, but it generates a voltage pulse every time it detects a switch in polarity. Each flux reversal generates a voltage pulse in the read head; it is these pulses that the drive detects when reading data. A read head does not generate the same waveform that was written; instead, it generates a series of pulses, each pulse appearing where a magnetic flux transition has occurred.
To optimize the placement of pulses during magnetic storage, the raw digital input data is passed through a device called an encoder/decoder (endec), which converts the raw binary information to a waveform that is more concerned with the optimum placement of the flux transitions (pulses). During a read operation, the endec reverses the process and decodes the pulse train back into the original binary data.
Because the number of flux transitions that can be recorded on a particular medium is limited by the disk media and head technology, disk drive engineers have been trying various ways of encoding the data into a minimum number of flux reversals, taking into consideration the fact that some flux reversals, used solely for proper timing, are required. This method permits maximum use of a given drive hardware technology.
Although various encoding schemes have been tried, only a few are popular today. Over the years, the following three basic types have been the most popular:
The following sections examines these codes, discusses how they work, where they have been used, and any advantages or disadvantages that apply to them.
One of the earliest techniques for encoding data for magnetic storage is called Frequency Modulation (FM) encoding. This encoding scheme, sometimes called Single Density encoding, was used in the earliest floppy disk drives that were installed in PC systems. The original Osborne portable computer, for example, used these Single Density floppy drives, which stored about 80KB of data on a single disk. Although it was popular until the late 1970s, FM encoding no longer is used today.
Modified Frequency Modulation (MFM) 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. In MFM encoding, the use of the clock transition cells is minimized, leaving more room for the data. Clock transitions are recorded only if a stored 0 bit is preceded by another 0 bit. In all other cases, a clock transition is not required. Because the use of the clock transitions has been minimized, the actual clock frequency can be doubled from FM encoding, resulting in twice as many data bits being stored in the same number of flux transitions as in FM.
Because it is twice as efficient as FM encoding, MFM encoding also has been called Double Density recording. MFM is used in virtually all PC floppy drives today and was used in nearly all PC hard disks for a number of years. Today, most hard disks use Run Length Limited encoding, which provides even greater efficiency than MFM.
Because MFM encoding places twice as many data bits in the same number of flux reversals as FM, the clock speed of the data is doubled so that the drive actually sees the same number of total flux reversals as with FM. This means that data is read and written at twice the speed in MFM encoding, even though the drive sees the flux reversals arriving at the same frequency as in FM. This method allows existing drive technology to store twice the data and deliver it twice as fast.
The only caveat is that MFM encoding requires improved disk controller and drive circuitry because the timing of the flux reversals must be more precise than in FM. As it turned out, these improvements were not difficult to achieve, and MFM encoding became the most popular encoding scheme for many years.
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