Hardware Reference
In-Depth Information
transition cells used solely for timing take up space on the medium that could otherwise
be used for data.
Because the number of flux transitions a drive can record in a given space on a particular
medium is limited by the physical nature or density of the medium and the head techno-
logy, drive engineers have developed various ways of encoding the data by using a min-
imum number of flux reversals (taking into consideration the fact that some flux reversals
used solely for clocking are required). Signal encoding enables the system to make the
maximum use of a given drive hardware technology.
Although various encoding schemes have been tried, only a few are popular today. Over
the years, these three basic types have been the most popular:
• Frequency Modulation
• Modified Frequency Modulation
• Run Length Limited
Thefollowingsectionsexaminethesecodes,howtheywork,wheretheyareused,andany
advantages or disadvantages that apply to them. It will help to refer to
Figure 8.10
(later
in the chapter) as you read the descriptions of these encoding schemes because this figure
depicts how each of them would store an “X” on the same media.
Figure 8.10
ASCII character X write waveforms using FM, MFM, and RLL 2,7 encoding.
Frequency Modulation Encoding
One of the earliest techniques for encoding data for magnetic storage is called
Frequency
Modulation encoding
. This encoding scheme—sometimes called
Single-Density encod-
ing
—was used in the earliest floppy disk drives installed in PC systems. The original Os-
