MPI 40 Series - Data Recovery; Bit Shift; 3.7.2.2 Write Precompensation; Data Separation

3. 7 .2 Data Recovery
Data recovery refers to the retrieving of data from the flexible
diskette. The following paragraphs describe the problems asso-
ciated with data recovery and methods to ensure data reliability.
3.7.2.1 Bit Shift
Bit shift refers to the displacement of a bit, as detected by
the drive, from its nominal position. The causes of bit shift
are manifold -- R/W head resolution, media resolution, diskette
speed variation, signal-to-noise ratio of read head output.
3.7.2.2 Write Precompensation
Certain data patterns cause more bit shift than other patterns.
This bit shift is predictable and can thus be partially compen-
sated for. For example, if it is known that a bit will be
shifted by 500 ns when it is read back, then the bit can be
deliberately written 200 ns early. This would give a bit shift
of about 350 ns when it is read back. This method of reducing
bit shift is called write precompensation.
Bit shift is greater on the inner tracks of the diskette than
on the outer tracks, making write precompensation necessary only
on the inner tracks. Write precomp of 250 to 300 ns should be
used on tracks 44 through 77. If due to controller limitations,
write precomp must be used on all tracks, then 125 to 150 ns
should be used.
3. 7 .2.3 Data Separation
Data separation refers to the separating of the composite data
coming from the drive into separate clock and separate data
bits.
Normally in a read operation raw read data or encoded data is
provided to the controller and data separation or decoding is
done by the controller. But as an option for FM code, separated
data and clock are provided along with .the raw read data. For
MFM encoded data, decoding should be done by the controller
using a suitable phase lock loop circuit or else maximum read
channel margin ( and hence the specified soft error rate )
cannot be achieved. See Figure 3-9 for FM encoded data separ-
ation. The optional data separator properly separates data and
clock bits through the soft-sectored IBM standard format and
address mark areas.
In MFM recording, data bits and clock bits are subject to the
same amount of bit shift. A PLL separator with a 50% data
window and a 50% clock window should be used.
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