HP 9845B/C CE Service Manual

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 a bit cell has no data transition.
• A 1 bit cell has a data transition.
• A a 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
eRe generation. Address mark is abbreviated AM.
Name
Data Pattern Clock Pattern
Where Found
lOAM !70 !OE
10 Field
Defective Track
AM !FO
!OE
10 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 eRe
generation.
Name
Index AM
IDAM
Data
AM
Deleted Data AM
Data Pattern
!FC
!FE
!FB
!F8
Clock Pattern
!D7
!C7
!C7
!C7
Where Found
At Index Hole
10 Field
Data Field
Data Field

HP 9845B/C CE Specifications

General

Brand	HP
Model	9845B/C CE
Category	Storage
Language	English