<s>
In	O
computer	B-General_Concept
data	I-General_Concept
storage	I-General_Concept
,	O
partial-response	B-Device
maximum-likelihood	I-Device
(	O
PRML	B-Device
)	O
is	O
a	O
method	O
for	O
recovering	O
the	O
digital	O
data	O
from	O
the	O
weak	O
analog	O
read-back	O
signal	O
picked	O
up	O
by	O
the	O
head	O
of	O
a	O
magnetic	B-Architecture
disk	I-Architecture
drive	O
or	O
tape	O
drive	O
.	O
</s>
<s>
PRML	B-Device
was	O
introduced	O
to	O
recover	O
data	O
more	O
reliably	O
or	O
at	O
a	O
greater	O
areal-density	O
than	O
earlier	O
simpler	O
schemes	O
such	O
as	O
peak-detection	O
.	O
</s>
<s>
These	O
advances	O
are	O
important	O
because	O
most	O
of	O
the	O
digital	O
data	O
in	O
the	O
world	O
is	O
stored	O
using	O
magnetic	B-Architecture
storage	I-Architecture
on	O
hard	B-Device
disk	I-Device
or	O
tape	O
drives	O
.	O
</s>
<s>
Ampex	O
introduced	O
PRML	B-Device
in	O
a	O
tape	O
drive	O
in	O
1984	O
.	O
</s>
<s>
IBM	O
introduced	O
PRML	B-Device
in	O
a	O
disk	B-Device
drive	I-Device
in	O
1990	O
and	O
also	O
coined	O
the	O
acronym	O
PRML	B-Device
.	O
</s>
<s>
Recent	O
read/write	B-General_Concept
channels	I-General_Concept
operate	O
at	O
much	O
higher	O
data-rates	O
,	O
are	O
fully	O
adaptive	O
,	O
and	O
,	O
in	O
particular	O
,	O
include	O
the	O
ability	O
to	O
handle	O
nonlinear	O
signal	O
distortion	O
and	O
non-stationary	O
,	O
colored	O
,	O
data-dependent	O
noise	O
(	O
PDNP	B-Device
or	I-Device
NPML	I-Device
)	O
.	O
</s>
<s>
Kretzmer	O
also	O
classified	O
the	O
several	O
different	O
possible	O
responses	O
,	O
for	O
example	O
,	O
PR1	O
is	O
duobinary	O
and	O
PR4	O
is	O
the	O
response	O
used	O
in	O
the	O
classical	O
PRML	B-Device
.	O
</s>
<s>
In	O
1970	O
,	O
Kobayashi	O
and	O
Tang	O
recognized	O
the	O
value	O
of	O
PR4	O
for	O
the	O
magnetic	B-Architecture
recording	I-Architecture
channel	O
.	O
</s>
<s>
Maximum-likelihood	O
decoding	O
using	O
the	O
eponymous	O
Viterbi	B-Algorithm
algorithm	I-Algorithm
was	O
proposed	O
in	O
1967	O
by	O
Andrew	O
Viterbi	O
as	O
a	O
means	O
of	O
decoding	O
convolutional	B-Error_Name
codes	I-Error_Name
.	O
</s>
<s>
By	O
1971	O
,	O
Hisashi	O
Kobayashi	O
at	O
IBM	O
had	O
recognized	O
that	O
the	O
Viterbi	B-Algorithm
algorithm	I-Algorithm
could	O
be	O
applied	O
to	O
analog	O
channels	O
with	O
inter-symbol	O
interference	O
and	O
particularly	O
to	O
the	O
use	O
of	O
PR4	O
in	O
the	O
context	O
of	O
Magnetic	B-Architecture
Recording	I-Architecture
(	O
later	O
called	O
PRML	B-Device
)	O
.	O
</s>
<s>
(	O
The	O
wide	O
range	O
of	O
applications	O
of	O
the	O
Viterbi	B-Algorithm
algorithm	I-Algorithm
is	O
well	O
described	O
in	O
a	O
review	O
paper	O
by	O
Dave	O
Forney	O
.	O
)	O
</s>
<s>
The	O
first	O
two	O
implementations	O
were	O
in	O
Tape	O
(	O
Ampex	O
-	O
1984	O
)	O
and	O
then	O
in	O
hard	B-Device
disk	I-Device
drives	I-Device
(	O
IBM	O
-	O
1990	O
)	O
.	O
</s>
<s>
In	O
both	O
cases	O
,	O
the	O
initial	O
equalization	O
to	O
PR4	O
response	O
was	O
done	O
with	O
analog	O
circuitry	O
but	O
the	O
Viterbi	B-Algorithm
algorithm	I-Algorithm
was	O
performed	O
with	O
digital	O
logic	O
.	O
</s>
<s>
In	O
the	O
tape	O
application	O
,	O
PRML	B-Device
superseded	O
'	O
flat	O
equalization	O
 '	O
.	O
</s>
<s>
In	O
the	O
HDD	O
application	O
,	O
PRML	B-Device
superseded	O
RLL	B-Protocol
codes	I-Protocol
with	O
'	O
peak	O
detection	O
 '	O
.	O
</s>
<s>
The	O
first	O
implementation	O
of	O
PRML	B-Device
was	O
shipped	O
in	O
1984	O
in	O
the	O
Ampex	O
Digital	O
Cassette	O
Recording	O
System	O
(	O
DCRS	O
)	O
.	O
</s>
<s>
The	O
heads	O
and	O
the	O
read/write	B-General_Concept
channel	I-General_Concept
ran	O
at	O
the	O
(	O
then	O
)	O
remarkably	O
high	O
data-rate	O
of	O
117	O
Mbits/s	O
.	O
</s>
<s>
The	O
PRML	B-Device
electronics	O
were	O
implemented	O
with	O
four	O
4-bit	O
,	O
Plessey	O
analog-to-digital	O
converters	O
(	O
A/D	O
)	O
and	O
.	O
</s>
<s>
The	O
PRML	B-Device
channel	O
outperformed	O
a	O
competing	O
implementation	O
based	O
on	O
"	O
Null-Zone	O
Detection	O
"	O
.	O
</s>
<s>
A	O
prototype	O
PRML	B-Device
channel	O
was	O
implemented	O
earlier	O
at	O
20	O
Mbit/s	O
on	O
a	O
prototype	O
8-inch	O
HDD	O
,	O
but	O
Ampex	O
exited	O
the	O
HDD	O
business	O
in	O
1985	O
.	O
</s>
<s>
Petersen	O
was	O
granted	O
a	O
patent	O
on	O
the	O
PRML	B-Device
channel	O
but	O
it	O
was	O
never	O
leveraged	O
by	O
Ampex	O
.	O
</s>
<s>
In	O
1990	O
,	O
IBM	O
shipped	O
the	O
first	O
PRML	B-Device
channel	O
in	O
an	O
HDD	O
in	O
the	O
IBM	O
0681	O
It	O
was	O
full-height	O
5¼-inch	O
form-factor	O
with	O
up	O
to	O
12	O
of	O
130mm	O
disks	O
and	O
had	O
a	O
maximum	O
capacity	O
of	O
857MB	O
.	O
</s>
<s>
The	O
PRML	B-Device
channel	O
for	O
the	O
IBM	O
0681	O
was	O
developed	O
in	O
IBM	O
Rochester	O
lab	O
.	O
</s>
<s>
in	O
Switzerland	B-Protocol
.	O
</s>
<s>
The	O
IBM	O
0681	O
read/write	B-General_Concept
channel	I-General_Concept
ran	O
at	O
a	O
data-rate	O
of	O
24Mbits/s	O
but	O
was	O
more	O
highly	O
integrated	O
with	O
the	O
entire	O
channel	O
contained	O
in	O
a	O
single	O
68-pin	O
PLCC	B-Algorithm
integrated	O
circuit	O
operating	O
off	O
a	O
5	O
volt	O
supply	O
.	O
</s>
<s>
The	O
presence	O
of	O
nonlinear	O
transition-shift	O
(	O
NLTS	O
)	O
distortion	O
on	O
NRZ	B-Protocol
recording	O
at	O
high	O
density	O
and/or	O
high	O
data-rate	O
was	O
recognized	O
in	O
1979	O
.	O
</s>
<s>
and	O
was	O
first	O
to	O
implement	O
Write	B-Device
precompensation	I-Device
for	O
PRML	B-Device
NRZ	B-Protocol
recording	O
.	O
</s>
<s>
Precompensation	O
is	O
viewed	O
as	O
a	O
necessity	O
for	O
a	O
PRML	B-Device
system	O
and	O
is	O
important	O
enough	O
to	O
appear	O
in	O
the	O
BIOS	B-Operating_System
HDD	O
setup	O
although	O
it	O
is	O
now	O
handled	O
automatically	O
by	O
the	O
HDD	O
.	O
</s>
<s>
The	O
target	O
( +1	O
,	O
+1	O
,	O
-1	O
,	O
-1	O
)	O
or	O
(	O
1-D	O
)	O
(	O
1+D	O
)	O
^2	O
is	O
called	O
Extended	O
PRML	B-Device
(	O
or	O
EPRML	O
)	O
.	O
</s>
<s>
PRML	B-Device
)	O
is	O
followed	O
by	O
a	O
post-processor	O
which	O
examines	O
the	O
residual	O
waveform	O
error	O
and	O
looks	O
for	O
the	O
occurrence	O
of	O
likely	O
bit	O
pattern	O
errors	O
.	O
</s>
<s>
Such	O
detectors	O
are	O
referred	O
to	O
as	O
Pattern-Dependent	O
Noise-Prediction	O
(	O
PDNP	O
)	O
detectors	O
or	O
noise-predictive	B-Device
maximum-likelihood	I-Device
detectors	I-Device
(	O
NPML	B-Device
)	O
.	O
</s>
<s>
Although	O
the	O
PRML	B-Device
acronym	O
is	O
still	O
occasionally	O
used	O
,	O
advanced	O
detectors	O
are	O
more	O
complex	O
PRML	B-Device
operate	O
at	O
higher	O
data-rates	O
.	O
</s>
<s>
(	O
TDMR	B-Architecture
uses	O
a	O
2-input	O
,	O
1-output	O
equalizer	O
.	O
)	O
</s>
<s>
The	O
detector	O
uses	O
the	O
PDNP/NPML	O
approach	O
but	O
the	O
hard-decision	O
Viterbi	B-Algorithm
algorithm	I-Algorithm
is	O
replaced	O
with	O
a	O
detector	O
providing	O
soft-outputs	O
(	O
additional	O
information	O
about	O
the	O
reliability	O
of	O
each	O
bit	O
)	O
.	O
</s>
<s>
Such	O
detectors	O
using	O
a	O
soft	O
Viterbi	B-Algorithm
algorithm	I-Algorithm
or	O
BCJR	B-Error_Name
algorithm	I-Error_Name
are	O
essential	O
in	O
iteratively	O
decoding	O
the	O
low-density	B-Error_Name
parity-check	I-Error_Name
code	I-Error_Name
used	O
in	O
modern	O
HDDs	O
.	O
</s>
