Commodore Amiga A500 Technical Reference Manual page 170

TWO INDEX TIMEOUT
Selectable Precomp
MFM Decode
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 t o
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 t o 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 AI,
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
t o
l 0
MHz.