The main competitor of the µPD765 FDC chip on the market was the WD179x FDC chip family. The primary difference between 765 and 179x controllers is that the 765 only does standard track formats (preamble, marks and data fields), while the 179x will write anything you tell it in the write track (formatting a track) mode.
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=== FM Encoding ===
FM encoding, or Frequency Modulation, was one of the earliest methods used to store data on floppy disks by creating changes in the magnetic field, known as flux transitions, to represent binary data.
In FM encoding, each bit of data is divided into two distinct parts: a clock signal and a data signal. The clock signal is always present to indicate the timing, while the data signal only appears when the bit is a 1, and remains absent when the bit is a 0.
This constant clock pulse ensures that the floppy disk controller (FDC) always knows where each bit begins and ends. Essentially, for every bit of information written to the disk, there’s always one clock signal and, if the data is a 1, a data signal as well.
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=== MFM Encoding ===
MFM, or Modified Frequency Modulation, was developed as a more efficient alternative to FM encoding. The core idea behind MFM is to reduce the number of magnetic transitions needed to represent data, thereby allowing more information to be stored in the same amount of space.
In MFM, clock pulses are not always present. Instead, a clock pulse only occurs between two consecutive 0s. If a 1 appears, it doesn’t need a clock pulse to mark it; instead, a transition is placed in the middle of the bit cell to represent the 1.
This method dramatically reduces the number of transitions compared to FM. Additionally, when a 0 follows a 1, no transition is needed, further minimizing the number of magnetic changes.
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