Magnetic recording system with peak shift compensation

A method and system provide for the compensation of peak shift of digital data recorded on D.C. premagnetized (erased) magnetic media. D.C. erasure results in the entire magnetic medium being aligned (polarized) in a single direction. The subsequent magnetic recording of data on such premagnetized medium normally results in a peak shift whenever the subsequently recorded signal traverses in the same direction of polarization as the single polarized direction of the D.C. erased medium. Such peak shift is compensated for in the present invention by utilization of a compensation circuit which, upon the receipt of an encoding signal which would cause data to be recorded which would traverse (be polarized) in the same direction as the premagnetized medium, is activated to provide a modified encoding signal to vary the placement of the magnetic boundaries of the recorded signal to compensate for its anticipated peak shift. The compensating circuit of the present invention is intermediate the source of the data encoding signal and the magnetic write head transducer. The compensating circuit of the present invention may either delay the return transition of the magnetically recorded signal, or cause the switching transition to be written earlier.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a data recording and decoding system, and 
more particularly to a method and system for improving the reliability of 
digital recording by correcting peak shift data errors caused by the use 
of D.C. premagnetized (erased) magnetic medium. The present invention is 
particularly useful in connection with the writing of digital data on D.C. 
premagnetized (erased) magnetic medium, and with the accurate reproduction 
and decoding of such recorded data by magnetic read/write systems. More 
particularly it relates to a system for modifying the timing of a write 
current in order to avoid or eliminate peak shift due to magnetic 
recording on D.C. premagnetized magnetic medium. It relates mainly to high 
density recording systems, but improved performance will also be 
experienced in lower density recording systems. 
2. Description of The Prior Art 
Modern data processing systems include a variety of means for recording or 
writing digital data on a variety of recording medium. The present 
invention is described in the context generally of magnetic medium such as 
flexible magnetic tape; however, the present invention is applicable to 
any form of recording of digital data having predictable characteristics 
on any magnetic recording medium. The words "recording" and "writing" are 
used interchangeably herein to designate the recording of magnetic data 
signals on any form of magnetic recording storage medium. 
It is desirable in such systems to maximize the reliability of data writing 
and reproduction, while at the same time maximizing the data throughput, 
all with a minimum of data errors. Such maximization is achieved in 
present day magnetic recording systems by increasing both the storage 
(writing) and reproduction (reading) speeds, and by increasing the data 
density (bits per unit area) on the magnetic medium. As the data recording 
density is increased, various undesirable effects are known to occur which 
cause data errors as a result of the interaction of the magnetic domains 
which comprise the adjacent data bits on the magnetic medium. Such 
interaction effects the density at which data can be reliably written and 
read. Various data encoding techniques have been developed for reducing 
these effects, including run length limited coding, group code recording, 
and others; however, in any encoding scheme, the above mentioned 
undesirable interaction effects occur at some given data density. One such 
undesirable effect is called "peak shift," and it most often occurs as a 
result of pulse crowding of the data bits on the magnetic recording 
medium. Peak shift is characterized by a shifting of the data transition 
locations from their proper (expected or timed) location. Peak shift will 
often result in a data error. This is due to the fact that in such 
recording systems individual data bits are recorded during a specific bit 
cell time in such a manner that a change of magnetic flux, or a magnetic 
flux peak at the discrete locations within the data bit cell or at its 
boundaries is read as being indicative of the recorded data. Such recorded 
data is written and then read on the magnetic medium as, for example, a 
logical "1" or a logical "0". Such flux transitions may be either a 
reversal of polarity or a change from one level of magnetization or flux 
to another. As used herein, a "flux reversal" is defined as that point 
which exhibits the maximum free space surface flux density normal to the 
surface of the magnetic medium, and is used interchangeably with the term 
"transition". In NRZ encoding, for example, such a transition occurs 
whenever a logical "1" is to be recorded. In MFM encoding, whenever a 
transition occurs at a boundary it is read as a logical "0," while a 
transition at the center of a data bit cell represents a logical "1". 
Also, as used herein, a "data bit cell" is defined as that time period 
during which one data representative flux transition should properly 
occur. 
Most prior art peak shift problems have been due to and inherent in the 
coding scheme and the resulting transition between two or more 
sequentially occurring bit cells. For example, in high density recording, 
and in particular when no data transition (polarization reversal) is 
present for two or more sequential bit cells, the point in time on the 
magnetic medium at which the next following transition peak occurs is 
found to shift from its proper (expected) place. This causes the width of 
the bit cell to vary, with the result that normal decoding circuits may 
decode (read) erroneous data due to loss of synchronization of incoming 
data, or due to the decoding of a transition (polarization reversal) 
occuring in an improper (adjacent) bit cell. When the pulses are close 
together the trailing edge of a previous pulse, or the leading edge of a 
succeeding pulse may extend past the bit cell of the pulse peak under 
consideration. When this happens the time of occurrance at the peak will 
be shifted toward either the preceding or succeeding pulse depending on 
which pulse's edge is overlapping the peak. Descriptions and drawings of 
this peak shift phenomenon are set forth in U.S. Pat. No. 3,623,041 
(MacDougall) and 3,537,084 (Behr). 
Additionally, since the art has advanced to higher recording densities it 
has become a common practice to not magnetically saturate the magnetic 
medium as deeply as was the common practice in lower density recording 
systems. However, this lack of saturation of the medium has presented an 
undesirable effect during the writing of new data over old magnetically 
recorded data. This is due to the fact that recording at lower 
frequencies, but at relatively high magnetic saturations may penetrate the 
medium more deeply than subsequent overwriting at higher frequencies, but 
with less magnetic saturation. In order to avoid difficulties due to 
unerased data which might remain after such overwriting, it is now common 
practice to magnetically erase the magnetic recording medium before 
recording (overwriting) on it. In the prior art, both A.C. and D.C. 
magnetic erase techniques have been used for erasure. The A.C. method of 
erasure is commonly used, but is relatively more costly in terms of the 
erase head and the circuit for the erase head which are required. However, 
despite its higher cost, A.C. erasure has the advantage of not producing 
premagnetized magnetic media which may introduce additional errors into 
the subsequently recorded data. By comparison, D.C. premagnetization 
(erasure) of magnetic medium requires a less expensive erase head and 
circuit, but has a tendency to introduce yet another kind of peak shift 
error into the data recorded and then read from such D.C. premagnetized 
medium. These errors in D.C. premagnetized (erased) magnetic medium are 
seen as a peak shift which occurs when signals are subsequently recorded 
on the medium, which recorded signals traverse (are polarized) in the same 
direction as the polarization of the D.C. premagnetized magnetic media. 
Various approaches have been taken in the prior art in an effort to avoid 
or compensate for peak shift in magnetic recording; however, such prior 
art approaches have been primarily directed to the correction of errors 
caused by sequencing, rather than to errors inherent in the character of 
the polarization of the magnetic recording medium, at the time it is 
written, for example due to D.C. erasure. 
One class of solutions to peak shift problems caused by sequencing has 
entailed compensating the signal at the time the data is written or 
encoded, e.g. when it is known that a particular peak will be shifted in a 
particular direction, by writing or encoding the data earlier or later in 
an effort to compensate for the shift which is expected to occur. This 
solution was at first treated as unsatisfactory since the pulse adjacent 
the pulse being compensated will also cause peak shift in the opposite 
direction. Thus, for a time it was taught by the prior art that using 
techniques of writing earlier or later were of little value as a means to 
avoid peak shift, and that in fact such techniques would cause other 
problems. More recently, techniques for timing adjustment have been found 
which do not cause opposite peak shift, but they have been quite complex. 
In any event, no prior art technique is known for adjusting data encoding 
to compensate for peak shift error due to the use of D.C. premagnetized 
medium. 
U.S. Pat. No. 3,503,059 (Ambrico) discloses the most commonly used method 
of correcting pulse shift errors due to sequencing. Ambrico teaches the 
use of minor distortions (step write compensation) in the magnetic flux 
after each major transition so that upon read-back the peaks will occur at 
the proper time. U.S. Pat. No. 3,573,770 (Norris) employs the same 
technique, but different means to avoid peak shifting. U.S. Pat. No. 
3,623,041 (MacDougall) uses a different approach which is quite successful 
as well. MacDougall provides a new system of encoding which has fewer 
signal transitions. Fewer transitions means fewer pulses, and therefore, 
less pulse crowding for similar data rates or intensities. U.S. Pat. No. 
3,537,084 (Behr) employs a technique in which writing is not modified, but 
in which the read back is compensated. 
Another prior art technique is described by U.S. Pat. No. 3,879,342 (Patel) 
in which a means of compensation for peak shift present in three frequency 
coding is introduced. A pulse shift circuit advances or delays the writing 
pulses; however, it is a complicated system in which three separate clock 
signals are required. In U.S. Pat. No. 3,483,539 (Poumakis) high density 
self-clocking information storage along a magnetic track is taught in 
which it is necessary to determine whether each successive pulse occurs 
after a predetermined short interval following the preceding pulse, or 
after a predetermined long interval following the preceding pulse. 
Poumakis then distinguishes between long and short intervals and 
repositions each pulse which occurs less than a minimum short interval 
following a preceding pulse so that each pulse always occurs after such a 
minimum interval. 
In U.S. Pat. No. 4,000,513 (Precourt) peak shift due to pulse crowding of 
data recorded on a magnetic medium is reduced by preemphasizing the 
recorded data time pattern in order to compensate for peak shift of the 
magnetic pattern recorded on the magnetic medium. Preemphasis of recorded 
peak shift errors is accomplished either by delaying or advancing the time 
when a particular peak shift data transition caused by the encoding data 
pattern will occur. This complex system causes the data to either be 
delayed or advanced before it is written, thereby compensating for peak 
shift error which would otherwise be present in the recorded data. 
Again it is noted that no known peak shift compensation system has been 
divised which simply and inexpensively adjusts the timing of the encoding 
signal used with D.C. premagnetized medium. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a method and 
system for the compensation of peak shift of digital data recorded on D.C. 
premagnetized (erased) magnetic media. Where magnetic medium is D.C. 
erased substantially all of the magnetic domains on the entire magnetic 
medium are aligned (polarized) in a single direction. The subsequent 
magnetic recording of data on such D.C. premagnetized medium normally 
results in a peak shift whenever the subsequently recorded magnetic data 
traverses (is polarized) in the same direction as the premagnetized 
direction of the D.C. erased medium. Such peak shift is compensated for in 
the present invention by utilization of a simple adjusting circuit which, 
upon the receipt of an encoding signal which will cause data to be 
recorded which will traverse (be polarized) in the same direction as the 
premagnetized medium, is activated to provide a modified encoding signal 
which will vary the placement of the magnetic boundaries of the recorded 
signal to compensate for its anticipated peak shift. The compensating 
circuit, of the present invention may either delay the return transition 
of the recording signal, or cause the switching transition to be written 
earlier. 
Another and additional object of the present invention is to provide a very 
simple, yet significantly effective improvement over any similar systems 
and methods of the prior art.

DETAILED DESCRIPTION OF THE INVENTION 
Referring first to FIG. 1, a prior art high density recording scheme on 
D.C. premagnetized magnetic recording medium 12, in this case in the form 
of magnetic tape, is schematically shown in cross-section. In order to 
illustrate and explain the problem of the prior art and the solution of 
the present invention the cross-section of magnetic medium 12 has been 
greatly enlarged and arbitrarily divided to illustrate six imaginary 
magnetic levels, L1 through L6, supported by non-magnetic substrate 14. In 
use level L1 is most closely adjacent any magnetic transducer, e.g. write, 
read or erase head, none of which are shown in FIG. 1, (but see FIG. 3). 
The length of media 12 is divided to illustrate a number of data bit or 
magnetic cells designated T1 through T4 and the transition boundaries 
B1-2, B2-3 and B3-4 therebetween. 
Now referring to D.C. premagnetized media 12 at levels L4 through L6, it is 
seen that all of the magnetic domains are polarized in the same direction 
from south (S) to north (N) as represented by the arrows at levels L4 
through L6. Indeed, the entire D.C. premagnetized media 12 was so 
polarized after D.C. erasure and prior to being written upon magnetically. 
However, as shown in FIG. 1 magnetic medium 12 has already been written 
upon by a magnetic write head transducer, not shown, operating under the 
driving force of writing current 16 in MFM (modified frequency modulation) 
form to write a series of "0"s so that D.C. premagnetized media 12 is 
modified with one magnetic polarity and then with the opposite polarity so 
that the polarity is reversed at each data bit cell boundary to represent 
the information ("0"s) recorded. However, as previously noted, in high 
density recording, magnetic medium 12 is not completely saturated, but is 
only magnetized to a given depth, say through L3, and even then the 
saturation or magnetization varies from L1 to L2 to L3, and so on, due to 
the effect of the existing D.C. premagnetization already present in 
magnetic media 12 and also due to other known magnetic phenomenon. The 
outermost portions of the magnetic medium which have been effected by 
writing flux magnetization are designated D1 through D4. It is thus seen 
that the magnetic polarity of domains D1 through D4 which are recorded in 
magnetic medium 12 vary at boundaries B1-2 and B3-4 from those which were 
intended to be written, and thus provide composite read-out signal 22 
which is subject to peak shift for recorded signals at boundaries B1-2 and 
B3-4. 
Referring again to FIG. 1, when encoding signal 15 causes write current 16 
to be generated and applied to D.C. premagnetized magnetic medium 12 
through a magnetic transducer to generate magnetic flux, magnetic polarity 
enhancement or reversal is caused to take place substantially within each 
bit cell boundary. At bit cell T1, which is subject to magnetic 
polarization in the same direction as the polarity of D.C. premagnetized 
magnetic medium 12, there is an additive magnetic effect, with the result 
that switching transition 18 in bit cell T2 is effected by a carry-over of 
magnetic polarization from L1 and L2 of bit cell T1. On read out boundary 
B1-2 between cells T1 and T2 is read as the composite sum of the polarity 
of the magnetic flux of levels, L1, L2 and L3, and to some extent L4-L6 
(not shown as an element of the read-out signal), the result is that the 
composite magnetic flux 22 peaks to the right of transition boundary B1-2. 
This is out of timing sequence, and is thus a peak shift problem. In a 
similar manner a peak shift problem occurs at boundary B3-4. When write 
current 16 is in a direction which will cause polarization flux to be 
recorded opposite to the D.C. premagnetized polarity of magnetic medium 
12, as in bit cell T2, the composite sum of the magnetic flux 22 tends to 
cancel, and thus does not result in a peak shift, for example at boundary 
B2-3. 
Having now defined the problem, it is seen that peak shift due to D.C. 
premagnetization is predictable for a given recording system using D.C. 
premagnetized magnetic medium. Referring to FIG. 2, as a solution to the 
problem of peak shift due to D.C. premagnetized medium, the present 
invention modifies the timing of write current 16 to displace the 
resulting recorded magnetic flux (and boundaries) D1, D2, D3 and D4 in 
such a manner that it compensates for the anticipated peak shift, and so 
that the modified composite read-out flux peak 22M is correctly timed. 
Referring again to FIG. 2, in which like portions are given the same 
numbers and in which similar portions are given related numbers as in FIG. 
1, an application of the present invention is shown in which, writing 
current 16M has been modified, in this case by delaying the signal or the 
portion of the signal which will cause the writing of magnetic data which 
will traverse (be polarized) in the same direction as the polarization of 
the D.C. premagnetized magnetic medium 12. This results in a composite 
read-out signal 22M in which the magnetic flux peaks at B1-2 and B3-4 now 
occur at the correct time relationship, and in which there is therefore no 
peak shift problem. It should be noted that although writing current 16M 
is modified by the present invention in response to a newly generated 
encoding signal 17, encoding signal 15 is the same as that of the prior 
art as shown in FIG. 1. 
A magnetic recording system including the present invention is set forth 
schematically and diagrammatically at FIG. 3. As illustrated, magnetic 
medium 12 is shown to be subjected to D.C. premagnetization by D.C. erase 
head transducer 32 driven by D.C. amplifier 34 and its enabling function, 
as is well known in the art. This D.C. premagnetization is most 
practically carried out on-line, as illustrated. However, D.C. 
premagnetization of the magnetic medium may also be carried out remotely 
and separately from the system shown. Any form of D.C. magnet, whether 
permanent or electromagnetic may be used to perform this D.C. 
premagnetization function so long as it saturates (erases) magnetic medium 
12 to a greater depth than it has been or will be written or read. 
Write head transducer 42 is located adjacent medium 12 in a manner such 
that when it is enabled it magnetizes medium 12 in one direction or in 
another (usually opposite) direction in accordance with the timing and 
direction of the write current 16M which it receives from its enabling 
circuitry and controls. Typically, data originates at or is transmitted 
through data encoding circuit 44, then through compensating circuit 46 of 
the present invention, and thence in parallel through amplifier 48 and 
inverter 50- amplifier 52 to write head 42. The initial timing of the 
encoding signal 15 is controlled by encoding circuit 44 and may include 
art known timing adjustments and variations of the current to compensate 
for peak shift expected due to certain data patterns and due to close 
packing of magnetic bits, of the kind discussed above in the Prior Art 
section of this specification. Encoding signal 15 from encoding circuit 44 
is then subjected to further modification by novel compensating circuit 46 
of the present invention to produce write current 16M, as explained in 
greater detail hereinafter. As a matter of reference it is noted that 
encoding signal 15 in both FIGS. 1 and 2 has been represented as a simple 
square wave, while in practice it will normally be a more complex 
function. 
To complete the explanation of the utility of the present invention, the 
read-out of magnetic medium 12, which has been D.C. premagnetized and then 
written upon by magnetic write head 42 under the control of write current 
16M from compensating circuit 46 is now detailed. Read-out of medium 12 is 
accomplished by magnetic read head transducer 62 using standard circuitry 
and controls, including, for example amplifier 64, equalizer 66, 
differentiator 68 and data recovery circuit 70. In the practice of the 
present invention the signals 22M and data thus read from medium 12 have 
their pulse peaks in the proper timing sequence, and do not experience 
peak shift problems or require any special read-out circuitry or 
adjustments. Thus D.C. premagnetized magnetic medium 12 recorded in 
accordance with the method and using the system of the present invention 
can be read accurately and without peak shift errors of the type normally 
caused by D.C. premagnetization of medium 12 by, for example, D.C. erase 
head 32. 
Forms and implementations of D.C. compensation circuit 46 are further 
detailed in FIGS. 4 and 5. The preferred embodiment of D.C. compensation 
circuit 46 is set forth in FIG. 4. It includes inverter 80, resistor 82, 
capacitor 84 and additional inverter 86. Resistor 82 and capacitor 84 are 
in series with inverter 80 which is in series with encoded output signal 
15 from data encoding circuit 44. Resistor 82 and capacitor 84 are in 
parallel with one another. Inverter 80 is an open collector having low 
impedance which causes capacitor 84 to discharge rapidly. Inverter 80, by 
its nature also causes a 180 degree phase change in signal 15. Capacitor 
84 is selected for activation, and thus delay of the signal, only when the 
direction of the resulting write current to head 42 will cause the 
production of a magnetic record bit which will traverse or be polarized in 
the same direction as the polarity of D.C. premagnetized magnetic medium 
12. Inverter 86 causes a second 180 degree phase change to the now 
compensated write current, and thus effectively returns the signal to its 
original phase, but in its modified timing form as modified encoding 
signal 17. A compensated write current 16M is subsequently produced by, 
for example, the action of amplifier 48 and inverter 50-amplifier 52. 
Another preferred embodiment of D.C. compensation circuit 46 is set forth 
in FIG. 5. It includes a delay line 92 and a "D" type flip-flop 94, with 
delay line 92 being in series between data encoder 44 and flip-flop 94, 
but with data from encoder 44 also being shunted directly in series to 
flip-flop 94. In operation, this circuit is set for activation to delay 
the signal from encoding circuit 44 only when the direction of the 
resulting write current 16M to head 42 will cause the production of a 
magnetic record bit which will traverse or be polarized in the same 
direction as the polarity of D.C. premagnetized magnetic medium 12. 
The output signal 16M to transducer 42 from the circuits of either FIGS. 4 
and 5 is substantially as illustrated in FIG. 2. It is again noted that 
encoding signal 15 in both FIGS. 1 and 2 are substantially identical, with 
a modified encoding signal 17 being produced as a result of the present 
invention. 
While the embodiments of the present invention have been discussed and 
illustrated as providing a return transition delay, the problem identified 
by the present invention can be solved with equal facility by the 
expedient of providing a form of compensation circuit 46 which writes the 
switching transition early, and thus in that way avoids the writing of a 
polarized signal which would otherwise cause peak shift in D.C. 
premagnetized medium 12. 
Additional embodiments of the present invention will be apparent to those 
skilled in the art. It is therefore intended that the scope of the 
invention be limited only by the appended claims and the prior art and not 
by the preferred embodiments described herein. Accordingly, reference 
should be made to the following claims in determining the full scope of 
the present invention.