Combination of a magnetic record carrier and an apparatus for recording a digital information signal in a track on said record carrier

A combination is disclosed of a magnetic record carrier and an apparatus for recording a digital information signal in a track on the record carrier. The apparatus includes an input terminal for receiving the digital information signal, an encoding unit for encoding the digital information signal so as to obtain a digital channel signal suitable for recording, and a writing unit for writing the digital channel signal in the track on the record carrier. The writing unit includes at least one write head having a specific gap width. The apparatus is adapted to write the digital channel signal in the record carrier with a bit-length which is smaller than 0.25 .mu.m, and the record carrier has an oblique easy-axis angle between 30.degree. and 42.degree. with regard to the longitudinal direction of the track and in a plane perpendicular to the record carrier.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to the field of digital magnetic recording and 
reproduction and especially to recording of computer data. 
The invention relates to a combination of a magnetic record carrier and an 
apparatus for recording a digital information signal in a track on said 
record carrier, the apparatus includes 
input apparatus for receiving the digital information signal, 
encoding apparatus for encoding the digital information signal so as to 
obtain a digital channel signal suitable for recording, 
writing means for writing the digital channel signal in the track on the 
record carrier, the writing means comprising at least one write head 
having a specific gap width, the apparatus being adapted to write the 
digital channel signal in the record carrier, with a bit-length which has 
a specific value, to an apparatus and a record carrier for use in the 
combination. A combination as defined in the opening paragraph is known 
from EP-A 18,267. Those skilled in the art are directed to: J. C. 
Mallinson, "Proposal Concerning High-Density Digital Recording", IEEE 
Trans. on Magn., vol. 25, pp. 3168-3169 (1989); J. P. C. Bernards et al, 
"Vector magnetization of recording media: A new method to compensate for 
demagnetising fields", IEEE Trans. on Magn., vol 27, no. 6, pp. 4873-4875 
(1991); H. A. J. Cramer, "On the hysteresis and the recording process in 
magnetic media", Thesis, University of Twente (NL), Oct. 29, 1993; R. L. 
Wallace, "The reproduction of magnetically recorded signals", BSTJ, 30, p. 
1145 (1951). All the above citations are hereby incorporated in whole by 
reference. 
SUMMARY OF THE INVENTION 
The invention aims at providing an improved combination which enables 
higher recording densities and lower bit error rates. 
The combination in accordance with the invention is characterized in that 
the apparatus is adapted to write the digital channel signal in the record 
carrier with a bit-length which is smaller than 0.25 .mu.m and that the 
record carrier has an oblique easy-axis angle between 30.degree. and 
42.degree. with regard to the longitudinal direction of the track and in a 
plane perpendicular to the record carrier. 
The invention is based on the following recognition. The read process in 
magnetic recording (using inductive heads, or magneto-resistive heads 
which are not saturated) is linear. The write process is non-linear. 
Fortunately, for a wide range of linear recording densities (defined as 
the inverse of the recording bit-length) the write process is 
pseudo-linear. This means that the locations of magnetic transitions 
recorded in a medium using a write signal f=f.sub.1 +f.sub.2 (where 
f.sub.1 and f.sub.2 are two separate write signals) are the same as those 
found by linear superposition of the results for write signal f.sub.1 and 
f.sub.2 when they are written separately. The recording channel is thus 
linear along the time axis. The amplitude of the output signals for write 
current f=f.sub.1 +f.sub.2, however, are not simply the linear 
superposition of the amplitudes for recording using f.sub.1 and f.sub.2 
separately (thus the channel is pseudo-linear). This pseudo-linearity is 
critical because in combination with the linear read process it allows the 
development, and use, of linear equalizers in the read process (which 
correct for channel imperfections and reduce inter symbol interference in 
the detection process). As the linear recording density is increased (by 
decreasing the bit-length) the magnetic transitions start to interact with 
one another (through their demagnetizing fields) in the recording process. 
The channel is no longer pseudo-linear. The leading order impact of this 
non-linear interaction between recorded transitions is a non-linear bit 
shift of the transition location of the bit currently being written. This 
will adversely affect the system bit-error-rate since this non-linear 
shift will not be corrected by the linear read equalization. 
Other sources of non-linearities include a finite rise time of the field in 
the gap of the magnetic head or a DC content in the write current pattern, 
in combination with a system using a transformer, for example. These 
sources will however not be discussed any further. 
There are some systems solutions for non-linear bit shifts. For example 
they can be compensated for using write equalization by time shifting the 
transitions in the write current pattern appropriately. This entails some 
expense in terms of electronics. Write equalization compensation must 
operate in real time. This requires fast electronics since these circuits 
must operate at several times the bit frequency. 
In accordance with the invention, a record carrier having substantially no 
non-linear bit shift is required. With a record carrier as claimed, and 
for recording densities as defined above, such situation can be obtained, 
so that relatively higher bit densities with relatively lower bit error 
rates can be achieved. 
It should be noted that J. C. Mallinson, "Proposal Concerning High-Density 
Digital Recording", IEEE Trans. on Magn., vol. 25, pp. 3168-3169 (1989), 
has predicted that for an oblique easy-angle (between 0.degree., 
longitudinal, and 90.degree., perpendicular) the non-linear bit shift will 
be zero. No indication has been given, however, how an appropriate oblique 
easy-axis angle for the record carrier should be chosen, depending on the 
bit density of the information signal to be recorded on the record 
carrier. 
It should further be noted that J. P. C. Bernards et al, in "Vector 
Magnetisation of Recording Media: A New Method to Compensate for 
Demagnetising Fields", IEEE Trans. on Magn., vol 27, No. 6, pp 4873-4875 
(1991) discloses a method to correctly measure the oblique easy axis angle 
of a magnetic medium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows the measured non-linear bit shift of a Metal Evaporated (ME) 
tape having an specific oblique easy-axis angle (as an example, 35.degree. 
from the longitudinal direction). FIG. 1a shows the behaviour of the 
non-linear bit shift (change in transition location .DELTA.x divided by 
the bit-length b) in percent as a function of the bit-length b in .mu.m. 
The bit-length is to be understood as the shortest distance between 
flux-reversals in the magnetic material of the record carrier. 
On decreasing the bit-length by 20%, from 0.25 .mu.m (a value appropriate 
for a present day standardized Digital Video Cassette system), to 0.2 
.mu.m (a value which might be of use in future, digital recording systems 
having a higher density), the bit shift increases about 25% from a value 
of approximately 17% to 21%. It should thus be concluded that in present 
day recording systems, where the bit-length is relatively large, 
non-linear bit shift has not been recognized as a problem, for the reason 
that the non-linear bit shift is low. For future recording systems, 
however, with higher bit densities, non-linear bit shift will become a 
problem, and a solution to this problem is required. In accordance with 
the invention, this solution is found in choosing an appropriate value for 
the easy-axis angle of the recording medium. 
Computer simulations using a well tested self consistent numerical model 
have been carried out to investigate the behaviour of the non-linear bit 
shift as a function of the easy-axis angle of a magnetic record carrier in 
a recording system where contact recording has been applied. Compared to 
earlier computer simulations carried out on a modelled version of a 
magnetic layer, a more refined calculation has been carried out, using 
smaller elementary units in the model and by calculating the magnetic 
field deeper within the magnetic layer. This was necessary, since the 
surface of a magnetic layer can suffer appreciable overwrite from the 
trailing edge of the recording head, resulting in a significantly 
different magnetisation field in the top layer of the magnetic medium 
compared to layers deeper in the medium. This is explained with reference 
to FIG. 1b, which shows the magnetisation in the magnetic layer of 
evaporated metal of the medium at the location of a flux reversal in the 
magnetic medium. The direction of magnetisation, given by the arrows in 
the figure, is in the direction of the easy axis angle of the medium, 
where the length of the arrows corresponds to the strength of the 
magnetisation. The magnetic field was written in the magnetic medium by 
means of a magnetic head scanning the medium from the top side. Special 
attention should be paid to the surface portions indicated by the 
reference numerals 20 and 21 in FIG. 1b. The magnetic field in these 
portions is quite different from the magnetic field more deeper in the 
medium. So, not taking into account the magnetic field deeper in the layer 
will lead to unreliable results from computer simulations. 
The improvement to the calculations further required the introduction of an 
exponential spacing loss factor that describes the influence of regions of 
the magnetic layer further from the magnetic head. This approach is well 
known in the theory of magnetic recording, see R. L. Wallace, "The 
Reproduction of Magnetically Recorded Signals" BSTJ, 30, p. 1145 (1951). 
FIG. 2 shows the behaviour of the non-linear bitshift, where the bit-length 
b of the information recorded on the record carrier equals 0.2 .mu.m and 
the write head has a gap width g of 0.25 .mu.m. The easy-axis angle for a 
substantially zero non-linear bit shift is roughly 36.degree., with a 
margin of eg. .+-.1.degree.. For a description of the self consistent 
numerical model, reference is made to the dissertation of H. A. J. Cramer, 
"On the Hysteresis and the Recording Process in Magnetic Media", Thesis, 
University of Twente (NL), Oct. 29, 1993. 
FIG. 3 shows a cross sectional view of the record carrier 1 and the write 
head 2, when recording information in the record carrier. The arrow 4 in 
the figure indicates the direction of movement of the head 2 relative to 
the record carrier 1. The cross section is along the direction of movement 
of the head relative to the record carrier and perpendicular to the record 
carrier. The head 2 has a gap width g and bits 11 of information are 
recorded with length 12 in the evaporated metal layer 13 of the record 
carrier. The writing of bits of information in the record carrier results 
in magnetization patterns in the record carrier. The angle .alpha. in FIG. 
3 shows the easy-axis angle of the magnetic medium. More specifically, the 
preferred direction of magnetization is the direction of the arrow 6. A 
`one` bit could result in a magnetization given by the arrow 8, whereas a 
`zero` bit could result in a magnetization given by the arrow 10. 
Note that the bit shift in FIG. 2 is a very sensitive function of the 
easy-axis angle. Varying the easy-axis angle around the optimum value of 
36.degree. leads to a rapid change in the bit shift. There exists, 
therefore, a very well defined optimum value, for particular ME tape 
properties (the value of the optimum easy-axis angle will be different for 
an ME tape with different properties). This optimum value is also function 
of the system geometry. For example Table I shows the calculated impact of 
varying the write head gap width g, while holding the field over the head 
gap H.sub.g =440 kA/m (which maximizes the output signal at a read 
wavelength of 1 .mu.m), and the easy-axis angle .alpha.=36.degree. fixed. 
Changing g by .+-.20% has the same impact as changing the easy-axis angle 
by .+-.2.degree. around the optimum point. The point is that for a given 
system and ME tape, there exists an optimal easy-axis angle where the 
non-linear bit shift is zero. 
TABLE I 
______________________________________ 
The calculated bit shift as a function of the 
write head gap g for an easy-axis angle 
.alpha. of 36.degree., a bit-length b of 0.2 .mu.m, 
and the field over the head gap H.sub.g of 440 kA/m. 
Write Gap g (.mu.m) 
Calculated Bit shift (%) 
______________________________________ 
0.2 -2.3 
0.25 +0.05 
0.3 +4.1 
______________________________________ 
One could conclude from the table that for a gap width of 0.2 .mu.m, the 
non-linear bit shift would become quite large. By using a record carrier 
with a slightly different oblique easy-axis angle, the non-linear bit 
shift will again be substantially zero. Such change in the value of the 
oblique easy axis angle can be small, eg. within one degree, as the 
dependency of the non-linear bit shift from the oblique easy axis angle is 
rather strong, see FIG. 2. 
Investigations have resulted in the knowledge that in future systems which 
will have smaller bit-lengths than presently used, which use an 
arbitrarily chosen media (ME or Metal Particle MP), will experience 
significant increases in non-linear bit shift. In accordance with the 
present invention, a solution has been found for future systems (with a 
given bit-length b, gap width g, and the standard contact recording based 
configuration using in magnetic recording systems) to use such (ME) tapes 
that substantially no non-linear bit shift occurs. Appropriate record 
carrier magnetic materials for ME tapes are Co--O and Co--Ni--O based 
materials. 
FIG. 4 shows an embodiment of the recording apparatus. The apparatus has an 
input terminal 40 for receiving the information signal to be recorded. The 
input terminal 40 is coupled to an input of a formatter unit 42, an output 
of which is coupled to an input of a write unit 44. The formatter unit 42 
encodes the information signal such that the encoded signal is suitable 
for recording in the magnetic record carrier. The formatter unit 42 may 
thus include a channel encoder, well known in the art, for channel 
encoding the information signal. The write unit 44 comprises at least one 
write head 45, for writing the channel signal in a track on the record 
carrier 46. The write head 45 has a gap width g which is smaller than 0.25 
.mu.m, more specifically, smaller than 0.20 .mu.m. The apparatus 47 is 
adapted to write the channel signal in the record carrier such that the 
bit-length of the signal written in the record carrier is smaller than 
0.25 .mu.m. This can be realized by a specific processing speed in the 
formatter unit and by choosing a specific speed of the head 45 relative to 
the record carrier 46 during recording. 
The easy-axis angle of the record carrier is between 30.degree. and 
42.degree.. More specifically, the easy-axis angle lies between 33.degree. 
and 39.degree.. A preferred value for the easy-axis angle is substantially 
36.degree.. 
The present invention allows a simple modification in medium property, more 
specifically, the easy-axis angle of the medium, to move the non-linearity 
threshold to higher linear bit densities. 
In the foregoing, the description has remained silent about the possibility 
that the write head may have a write gap positioned at a specific azimuth 
angle compared to the direction of movement of the write head relative to 
the record carrier. In such a situation, the gap width is defined as the 
width of the gap viewed in the said direction of movement of the head and 
the bit-length is defined as the shortest distance between flux reversals 
on the record carrier, viewed in the longitudinal direction of the track. 
Whilst the present invention has been described with respect to preferred 
embodiments thereof, it is to be understood that these are not limitative 
examples. Thus various modifications may become apparent to those skilled 
in the art, without departing from the scope of the invention, as defined 
in the appended claims. As a consequence, the record carrier may be in the 
form of a longitudinal record carrier (tape) or in disk form (hard or 
floppy disks). Further, the recording process may be implemented using 
contact recording, however, also a non-contact recording process is 
possible. 
A recording apparatus in accordance with the invention may now be devoid of 
any write equalization means for time shifting the transitions in the 
write current.