Commodore Amiga A500 Technical Reference Manual page 167

Selectable Precomp
MFM Decode
TWO INDEX TIMEOUT
This function insures accurate control over the number of attempts
to find a header (i.e., it is not "mislead" by counting false address
marks).
MFM ENCODE
The DJC converts all parallel data to serial and then to MFM. This
function is followed by Precomp, if selected.
In Precomp, a "string" of pulses is analyzed to determine if they are
arranged in the unique manner that could cause them to crowd once
written on the disk. It also determines which way the crowding
would distort the pulses when read. The write pulse stream is then
shifted, early or late, to compensate for the crowding conditions.
which normally occur on the innermost tracks of the drive.
Under the processor's control, the DJC precomps the disk MFM data
by using external inductive delays. Precomp is selectable and is de-
signed to shift the MFM data early or late by 12 nanoseconds to im-
prove read margins.
The use of this feature should be performed in conjunction with the
particular drive manufacturer's specification.
Data received from a disk drive is MFM, a self-clocking serial data
stream which contains a phase locked loop, lock detect, missing
clock detect and the data seperator.
When the DJC asserts Read Gate, the 8465 data seperator will at-
tempt to lock its phase locked loop on the read data. If this does not
occur within 4.8 usec, the DJC will turn off Read Gate, causing the
8465 to be placed into the low track rate for increased stability.
The MFM data is now decoded into NRZ data and clock for the DJC.
The 8465 decodes a missing clock bit and a hexidecimal A1, FD or
an A1, F8 in the sync field. This data indicates the start of a valid
header or data field. Receiving any other data causes the DJC to
abort the read. Another read would be tried after resyncing the
8465 to 10 Mhz.
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