Magnetic disc apparatus with a cosine equalizer and specific gap length head

A magnetic disc apparatus comprises a single write/read magnetic head for carrying out write/read with respect to either one of a low-density magnetic disc and a high-density magnetic disc, and a cosine equalizer supplied with a read signal from the write/read magnetic head. The write/read magnetic head has a core gap length which is between 0.60 and 1.20 times a minimum field conversion length for high-density write by a write/read signal.

BACKGROUND OF THE INVENTION 
The present invention generally relates to magnetic disc apparatuses, and 
more particularly to a magnetic disc apparatus that uses both high-density 
and low-density magnetic discs as the storage media and that can provide 
favorable write and read processing for the write and read modes, 
respectively. 
In conventional magnetic disc apparatuses that perform low-density 
read/write using floppy discs as the storage media, the magnetic layer is 
magnetized to such a deep portion that an adequate overwrite 
characteristic and read margin are obtainable. To achieve this, the core 
gap length of the write/read head is set at a maximum of 0.60 times the 
minimum field conversion length for the write/read signal current, and the 
resolution at the innermost peripheral track of the magnetic disc is set 
to have an absolute value of 70% or more. 
In magnetic disc apparatuses that perform high-density write/read and that 
have been proposed to date, the core gap length for the write/read head is 
made smaller so that the minimum field conversion length can be made 
smaller. The depth of magnetization is therefore not as deep and the 
thickness of the magnetic layer of the magnetic disc can therefore be 
reduced. 
Moreover, magnetic disc apparatuses having mode switching to perform both 
low-density write/read and high-density write/read have also been proposed 
to be compatible with both low-density and high-density magnetic discs. 
Magnetic disc apparatuses of this type have a single write/read head to 
perform write and read for both the low-density mode and the high-density 
mode. 
However, there are the following problems with this type of 
low-density/high-density compatible magnetic disc apparatus. The example 
used to explain these problems is a magnetic disc apparatus which uses a 
magnetic disc with a diameter of 90 mm (3.5 inches) and has a high-density 
write/read mode in which the recording density is 2MB and a low-density 
write/read mode in which the recording density is 1MB. 
In the 2MB mode (high-density write/read mode), there is the restriction 
that the core gap length of the head is 0.9 .mu.m or less (1.46 
.mu.m.times.0.60=0.876=0.9 .mu.m, where 1.46 .mu.m indicates the minimum 
field conversion length for the 2MB mode) so that the absolute value of 
the resolution at the innermost peripheral track of the magnetic disc has 
an absolute value of 70% or more. 
The average thickness of the magnetic layer in the write portion of a 
magnetic disc for the 2MB write/read is 1.0 .mu.m but the average 
thickness of the magnetic layer in the write portion of a magnetic disc 
for the 1MB write/read is 2.0 .mu.m which is approximately twice that of 
the magnetic disc for the 1MB write/read. In addition, the minimum field 
conversion length (min. FCL) is 1.46 .mu.m for the 2MB write and 2.91 
.mu.m for the 1MB write. 
Magnetic discs exclusively for the 2MB write/read have a thin 
(approximately 1 .mu.m) magnetic layer and so a head with the 
abovementioned gap length (0.9 .mu.m) can magnetize the magnetic layer to 
a sufficient depth during the write. However, magnetic discs exclusively 
for the 1MB write/read have a thick (approximately 2 .mu.m) magnetic layer 
and so when one of these discs is used in a low-density/high-density 
compatible magnetic disc apparatus having a head with the abovementioned 
gap length (0.9 .mu.m), it is impossible to magnetize deep into the 
magnetic layer which is approximately twice as thick. 
FIG. 1 is a sectional diagram illustrating this status, where 1 and 2 
indicate the head cores, g indicates the core gap length, 3 indicates the 
magnetic layer, and 4 indicates the base part of the disc. When the narrow 
head gap of this 2MB write/read head is used to write signals at the 
minimum field conversion length, the magnetic flux distribution curves of 
this write/read head can only magnetize down to the depth indicated by d 
in FIG. 1. Therefore, sufficient overwrite is possible for old data 
written with a conventional type of head (having a narrow gap) as shown in 
FIG. 1, but there is a poor overwrite characteristic for old data written 
with a magnetic disc apparatus exclusively for the 1MB mode (having a 
write/read head with a wide gap length of about 1.3 to 1.5 .mu.m). 
With conventional low-density/high-density compatible magnetic disc 
apparatus, this write/read head has a narrow core gap length set to 0.6 to 
0.9 .mu.m for use in either the low-density or high-density mode. This 
causes no problem for write to 2MB magnetic discs, but there is an 
insufficient depth of magnetization with respect to the thick magnetic 
layer for write with respect to 1MB magnetic discs and this creates the 
problem of a poor overwrite characteristic. 
If the core gap length of the magnetic head is widened so that a sufficient 
depth of magnetization is obtained with respect to the 1MB magnetic discs, 
the resolution for high-density (2MB) write will drop. If this resolution 
drops to 60% or less, then there will be much interference between the 
waveforms of adjacent bits so that the peak shift indicated in FIG. 2 
occurs to hinder favorable write and read processing. 
FIG. 2 shows the read waveform with respect to input data that is written 
at a high density. In FIG. 2, (A) represents the data pattern, (B) 
represents the write data and (C) represents the write current. For (C), 
the period for which the write current is applied (indicated by "X" in the 
figure) corresponds to the minimum field conversion length. In FIG. 2, (D) 
indicates an isolated read waveform with respect to each magnetic 
conversion when the resolution is 100%, and (E) indicates an isolated read 
waveform with respect to each magnetic conversion when the resolution is 
60%. FIG. 2 shows that the peak shift indicated by .DELTA.I occurs due to 
interference between the waveforms of adjacent bits when the resolution 
becomes about 60%. 
Simply setting the core gap length to enable low-density write therefore 
means that it is impossible to avoid lowering the read margin for data 
written at high-density. Accomplishing these mutually incompatible 
objectives has therefore involved compromising the range for which 
suitable characteristics could be obtained. 
SUMMARY OF THE INVENTION 
Accordingly, it is a general object of the present invention to provide a 
novel and useful magnetic disc apparatus in which the problems described 
above are eliminated. 
Another object of the present invention is to provide a magnetic disc 
apparatus that can achieve favorable write/read characteristics for both 
the low-density and high-density read and write modes. 
Still another object of the present invention is to provide a magnetic disc 
apparatus that uses one write/read head to perform write and read with 
respect to both low-density and high-density magnetic discs and in which 
the core gap length of the write/read head is between 0.60 and 1.20 times 
the minimum field conversion length for the write and read signals, and 
having a cosine equalizer provided in a read circuit to which the read 
signals from the write/read head are supplied. 
According to the present invention, the core gap length of the write/read 
head is set to a value which permits sufficient magnetization when writing 
to the thick magnetic layer of low-density magnetic discs to therefore 
prevent lowering of the read margin. Also, providing the cosine equalizer 
enables compensation for the peak shift caused by having the core gap 
length which is larger than the normal core gap length. The write and read 
characteristics are therefore improved for both the low-density and 
high-density magnetic discs. 
Unlike the conventional low-density/high-density compatible magnetic disc 
apparatus in which the characteristics had to be compromised to satisfy 
the mutually contradictory problems of overwrite, lowering of the read 
margin and the generation of peak shift, the present invention solves all 
of these problems and enables favorable write/read characteristics to be 
obtained for both low-density and high-density magnetic discs. The effect 
of the present invention can be particularly anticipated with respect to 
compact magnetic disc apparatus using magnetic discs with a diameter of 90 
mm (3.5 inches).

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of a magnetic disc apparatus according to the present 
invention will be described with reference to FIGS. 3 through 5. 
In FIG. 3, a gap length g0 of a write/read head 5 comprising head cores 6 
and 7 is between 0.60 and 1.20 times the minimum field conversion length. 
This is a first feature of the present invention. The gap length g is set 
at a minimum of 0.60 times the minimum field conversion length in the 
conventional low-density/high-density compatible magnetic disc apparatus, 
but the gap length g.sub.0 of the present invention is set to be wider 
than the gap length g. 
A magnetic disc 8 for low-density write/read is comprised of a magnetic 
layer 9 and a disc base 10 and when the write/read head 5 with the gap 
length g.sub.0 performs write with respect to such a magnetic disc 8, it 
is possible to magnetize to a deep portion of the magnetic layer 9 
indicated by d.sub.0. It is therefore possible to obtain a favorable 
overwrite characteristic and read margin for the low-density mode. This is 
to say that the first feature of the present invention functions 
effectively in the low-density mode and that the write/read 
characteristics for this mode are improved. 
A second features of the present invention is the provision of a cosine 
equalizer in a read circuit of the magnetic disc apparatus. FIG. 4 shows 
the basic construction of a cosine equalizer 11 which comprises a 
differential amplifier 12, a variable resistor 13 for gain adjustment, a 
delay line 14 with a delay time .tau., and a matching resistor 14. FIG. 5 
shows the waveforms obtained at positions A through D in FIG. 4 when a 
signal having the waveform indicated by (A) in FIG. 5 arrives at an input 
terminal. By the appropriate selection of the delay time .tau., it is 
possible for the cosine equalizer 11 to reduce the amount of peak shift 
included in the arriving read signal. This is to say that the cosine 
equalizer 11 can be used as a circuit to compensate for the 
characteristics resulting from the peak shift. Details of the use of a 
cosine equalizer to reduce the peak shift may be found in "Read Waveform 
Correction in Magnetic Recording", Technical Report NK-6759 of Nippon 
Tsushin Gijutsu Kabushiki Kaisha published October 1984. 
The following will consider a magnetic disc apparatus having a high-density 
magnetic disc for write/read and its write/read characteristics in the 
high-density mode. The resolution will lower and increased interference 
between the waveforms of adjacent bits may generate the peak shift if the 
aforementioned gap length g.sub.0 is made larger. However, by providing 
the cosine equalizer 11 in the read circuit makes it possible to prevent 
the peak shift and the lowering of the resolution that are problems in the 
high-density mode. This is to say that the second feature of the present 
invention functions effectively in the high-density mode and that the 
write/read characteristics are improved for this mode. Accordingly, the 
magnetic disc apparatus of the present invention can provide favorable 
write/read characteristics in both the low-density and high-density modes. 
It must be mentioned that the abovementioned first feature and second 
features are not independent of each other, but are closely related. In 
the low-density mode for example, if the gap length g.sub.0 is made more 
than 1.2 times the minimum field conversion length for high-density write, 
then the overwrite characteristic and the read margin will improve since 
magnetization made deep into the magnetic layer 9. However, if this 
write/read head is used in the high-density mode, then this will mean that 
the cosine equalizer 11 will not be able to provide sufficient 
compensation for the peak shift and the write/read characteristics in the 
high-density mode will consequently deteriorate. On the other hand, if the 
gap length g.sub.0 is made smaller so as to improve the write/read 
characteristics in the high-density mode, then the deterioration of the 
low-density characteristics will be as already described. 
Because of this, according to experiments conducted by the present 
inventors, the gap length g.sub.0 set within a range of 0.60 to 1.20 times 
the minimum field conversion length will provide the full benefit of the 
cosine equalizer 11 and allow a sufficient depth of magnetization to be 
obtained for the low-density mode. It is therefore possible to obtain 
favorable write/read characteristics for both the low-density and 
high-density modes. 
The following Table gives a comparison of the characteristics of the 
present invention and the characteristics of the conventional compact 
magnetic disc apparatus for use with both the 2MB and 1MB magnetic discs. 
TABLE 
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Conventional 
This 
Characteristic 
device invention 
Notes 
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Overwrite 
-18 to -24 dB 
-26 to -32 dB 
The smaller the 
characteristic overwrite character- 
istic the better. 
Resolution 
approx. 72% 
approx. 63% 
Ratio of read output 
(for 2 MB) voltages with respect 
to input signals 1 F 
and 2 F for innermost 
track. The larger the 
better. 
Time margin 
500 nS min. 
500 nS min. 
The larger the 
(for 2 MB) better. 
Compatible 
60% min. 70% min. 
Ratio of read output 
(For 1 MB) voltages for 
read/write with a 
dedicated 1 MB appa- 
ratus, and output 
voltages for read 
with a dedicated 1 MB 
apparatus when a disk 
has signal 2 F written 
to the innermost 
track in the 1 MB 
mode by a 2 MB/1 MB 
compatible apparatus. 
The larger the ratio 
the better. 
R/W head gap 
0.75 .mu.m 
0.95 .mu.m 
width 
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Further, the present invention is not limited to these embodiments, but 
various variations and modifications may be made without departing from 
the scope of the present invention.