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Hard Disk Data Encoding and Decoding
Digital information is a stream of ones and zeros. Hard disks store information in the form of magnetic pulses. In order for the PC's data to be stored on the hard disk, therefore, it must be converted to magnetic information. When it is read from the disk, it must be converted back to digital information. This work is done by the integrated controller built into the hard drive, in combination with sense and amplification circuits that are used to interpret the weak signals read from the platters themselves.
Magnetic information on the disk consists of a stream of (very, very small) magnetic fields. As you know, a magnet has two poles, north and south, and magnetic energy (called flux) flows from the north pole to the south pole. Information is stored on the hard disk by encoding information into a series of magnetic fields. This is done by placing the magnetic fields in one of two polarities: either so the north pole arrives before the south pole as the disk spins (N-S), or so the south pole arrives before the north (S-N). This is discussed in detail where the read/write heads are described.
Although it is conceptually simple to match "0 and 1" digital information to "N-S and S-N" magnetic fields, the reality is much more complex: a 1-to-1 correspondence is not possible, and special techniques must be employed to ensure that the data is written and read correctly. This section discusses the technical issues involved in encoding and decoding hard disk data.