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Gap 3 follows the data field and provides extra time in the sector to allow for spindle speed variations, write
oscillator tolerance and to mask the write to read recovery time. For example a track could be formatted with the
spindle running faster than nominal the thus occupy less disk space. The track may then be write updated with the
spindle running slow and require more disk space for the same number of data bytes.
Gap 4 follows the last Gap 3 on the track and provides extra time in the track to allow for spindle speed and
write oscillator tolerances. Gap 4 must be long enough to prevent the format operation from overflowing the track
capacity when the write oscillator is running slow and the spindle is rotating
30/0
faster than normal.
3.6.3
WRITE PRE-COMPENSATION
Recording data at high bit densities creates a phenomenon known as intersymbol interference. This occurs when
two bits must be recorded closer to each other than to neighboring bits. When these two bits are read back, inter-
symbol interference causes the bits to appear shifted away from each other. This is the major cause of bit shift in
data recording systems. Other factors such as head/media resolution, speed variations, write current level and ran-
dom circuit noise contribute to bit shift to a lesser degree. When bit shift is great enough, read errors occur.
Bit shift is density related and therefore, more apparent on the shorter innermost cylinders. There are two
methods that the Q2080 user may employ to reduce bit shift: reduce write current, and write pre-compensate. Both
methods should be used when writing above cylinder 586. The Q2080 provides an input that when driven will reduce
write current by approximately 10%. Write pre-compensation is a technique whereby the bit patterns that cause the
greatest inter-symbol interference are detected and those bits that are predicted to be read early or late are corre-
spondingly written late or early. A practical amount of pre-compensation for the Q2080 is 12 nanoseconds for both
early and late written bits.
Table 3-1 shows those bit patterns that require pre-compensation, the shift direction expected, the precompensa-
tion direction and the type (clock or data) pulse to write for MFM encoding.
TABLE 3-1
WRITE PRE-COMPENSATION
Data Pattern
Expected Shift Direction
Pre-compensate
Type of Pulse
1
2
3
4
For Data in Cell 3
Direction
To Write
0
0
0
0
None
On Time
Clock
0
0
0
1
Late
Early
Clock
0
0
1
0
None
On Time
Data
0
0
1
1
Early
Late
Data
0
1
0
0
None
No Clock or Data Written
0
1
0
1
None
No Clock or Data Written
0
1
1
0
Late
Early
Data
0
1
1
1
None
On Time
Data
1
0
0
0
Early
Late
Clock
1
0
0
1
None
On Time
Clock
1
0
1
0
None
On Time
Date
1
0
1
1
Early
Late
Data
1
1
0
0
None
No Clock or Data Written
1
1
0
1
None
No Clock or Data Written
1
1
1
0
Late
Early
Data
1
1
1
1
None
On Time
Data
Data is encoded in the sequence 1,2,3,4 and written from Cell 3. Clock pulses are written positioned at the
beginning of a cell time and data pulses are written in the center of a cell time.
REV A (01/84)
3-4
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