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The Secret of Disks: How MFM Encoding Doubles Data Transfer Speeds
With the advent of the digital age, data storage technology has also advanced. Among them, the modified frequency modulation (MFM) coding technology has become one of the key technologies for disk storage due to its superior performance in data transmission. This article will explore the development of MFM coding and how it can double the speed of data storage.
MFM is a special encoding method that aims to address the shortcomings of traditional frequency modulation (FM) encoding, making it possible to significantly increase the speed of writing data on the disk.
MFM was first introduced in hard disk drives in 1970 and later in floppy drives in 1976. Improved frequency modulation technology is mainly designed for magnetic storage devices. The characteristic of MFM coding is that each input data bit has at most one polarity change. This feature not only reduces the probability of data errors, but also doubles the writing speed, so MFM disks are often called "double density" disks, while early FM disks are called "single density".
In magnetic storage, data is not stored as absolute values, but rather as changes in polarity. Changes in the magnetic field induce electric currents in nearby wires, and vice versa. This means that when the read/write head reads or writes data, it creates a pattern of magnetic polarity on the media through a series of electrical current changes, which indicates where the data is stored.
MFM is a length-limited (RLL) code that effectively limits the distance between recorded transitions during the recording process to avoid data errors due to noise.
Because of the more precise timing requirements needed for MFM, it was not economically feasible to combine the required analog and digital components on a single integrated circuit with the technology available in the late 1970s. This resulted in the need for drive vendors to design their own dedicated clock recovery circuits, a system called a data separator. The implementation of MFM encoding requires more complex hardware support for disk drives, but the cost of these devices is gradually decreasing as technology advances.
The basic rule of MFM encoding is that in a data stream, if the previous bit is zero, then when the input is zero, it will be encoded as 10; if the previous bit is one, it will be encoded as 00; and if the previous bit is one, it will be encoded as 00. It is always encoded as 01. This encoding method ensures that there are an average of 0.75 to 1 magnetic transitions per encoded data bit. Due to these limitations, MFM coding can achieve higher data density than FM coding.
The design of data separators became an art at the time, and many early controllers such as the Western Digital FD1771 series came into being.
Of course, in addition to MFM, there are also improved MFM encoding technologies (such as MMFM), which further compresses the clock bits, thereby achieving a longer maximum run length, becoming (1,4) RLL encoding. By further optimizing the coding method, MMFM performs well in specific scenarios, although standard MFM coding is still in high demand in general applications.
However, in the current data storage environment, MFM technology has gradually been replaced by more efficient data encoding methods and has become an edge technology for specific applications. However, it is still important to understand the history and use of this technology because it paved the way for later data storage technologies.
As the cornerstone of digitalization, how did MFM become a turning point that enabled us to improve data storage and transmission technology? What new technologies will make our lives more convenient in the future?
