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Theory of Operation
2-13
The HP Format uses a double-density encoding scheme known as Modified Modified Fre-
quency Modulation (MMFM). The rules for MMFM coding are as follows:
•A 0 bit cell has no data transition.
•A 1 bit cell has a data transition.
•A 0 bit cell has a clock transition if there is no transition in the preceding bit cell.
•A 1 bit cell never has a clock transition.
For the same recording density (flux transitions per inch), there are twice as many data
transitions In MMFM coding as in FM coding.
Precompensation. Transitions which are written close together tend to appear shifted
apart when they are read back. This effect is known as bit shift. Due to its large bit cell, the
IBM Format is not affected by bit shift. However, because of the smaller bit cell used in the
HP Format, bit shift is noticeable. To compensate for bit shift, certain MMFM transitions are
written closer together. This action is called precompensation. The actual precompensation
used on a transition is dependent upon the pattern being recorded.
Address Marks. There are four types of HP address marks. All are unique in that they
include a bit cell with an extra clock transition. The HP address mark byte is not included in
CRC generation. Address mark is abbreviated AM.
Name
Data Pattern
Clock Pattern
Where Found
!DAM
!70
!OE
ID Field
Defective Track AM
!FO
!OE
ID Field
Data AM
!50
!OE
Data Field
ECC Data AM
!DO
!OE
Data Field
At the command set level, a clear D bit indicates an ID AM in the ID field, and a set D bit
indicates a defective track AM in the ID field.
There are four types of IBM Format address marks. All are unique In that they include bit
cells which contain no clock transition. The IBM address mark byte is included in CRC
generation.
Name
Data Pattern
Clock Pattern
Where Found
Index AM
!FC
!07
At Index Hole
!DAM
!FE
!C7
lD Field
Data AM
!FB
!C7
Data Field
Deleted Data AM
!F8
!C7
Data Field
!
=
Hexl!ldecima.l
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