Magnetic recording

Magnetic recording of a data continuum is effected by means of sequential impulses of recording current. The impulses occur at regular intervals providing samples of the data continuum. The impulses are of very short time duration, in that each impulse extends for only a small fraction of the time interval that is required for a point on the record medium to traverse the effective recording field of the record head. The time spacing between impulses is approximately equal to said time interval, thereby providing a magnetic recording continuum corresponding to said data continuum.

The present invention relates to magnetic recording, wherein a 
magnetization continuum is recorded on a magnetic recording medium by 
means of a transducer (record head) driven by electrical signal currents 
having an active duty cycle which is lower than that of the recorded 
magnetizations or of the analogous electrical signals reproducable from 
them. The invention provides continuous recording with only intermittent 
power consumption and low heat dissipation, an advantage which favors 
selection of low cost circuits as record head drivers. Other advantages of 
the invention will be apparent from the further description herein. 
The present invention is generally applicable to various forms of magnetic 
recording, but is described herein for purpose of illustration in its 
application to digital recording on a variety of media, e.g., magnetic 
disks, tapes, drums and cards. Although the invention is described, at 
times herein, in terms most applicable to fixed, ring head, longitudinal 
recording on magnetic tape, it is understood that it is not limited 
thereto. 
In accordance with the present invention, a magnetic record medium, such as 
a tape, is continuously driven across, or traverses, the record gap of a 
magnetic record head to record a continuum of data or intelligence 
thereon. In conventional magnetic recording, the intelligence is embodied 
in a continuum of electrical signals that are transduced continuously by 
the record head to effect a continuous magnetic recording action on the 
record tape. Pursuant to the present invention, the continuum of 
electrical signals are sampled and applied to the record head as discrete 
and discontinuous impulses of very short duration. The time duration of 
each impulse is a fraction of the time interval required for a point on 
the record medium to traverse the effective record gap of the record head 
or transducer. Also, the time spacing between successive record impulses 
is approximately equal to the aforesaid time interval of traverse. 
The present invention takes advantage of the fact that upon the application 
to the record head of a record current impulse, the value of that impulse 
is immediately recorded over the entire extent of record medium spanning 
the effective recording gap or effetive recording field of the record 
head. Each succeeding record impulse is applied to the head preferably at 
the instant that each preceding recorded increment has completed its 
traverse across the effective recording field. Thus, although the 
electrical record impulses are applied discontinuously, the magnetic 
recording is in fact substantially a continuum and will be transduced by a 
play back head into an electrical continuum corresponding to the original 
electrical signal continuum from which the record impulses are sampled. 
In order to appreciate fully the significance and advantages of the present 
invention, a summary description and analysis is provided of magnetic 
recording principles and their application in accordance with the standard 
practices of the prior art. 
MAGNETIC RECORDING PRINCIPLES AND PRIOR ART PRACTICES 
The ensuing summary description of the principles of magnetic recording and 
their application in prior art magnetic recording practices is had in 
conjunction with the accompanying FIGS. 1-11, which are described in 
sequence in the following description. 
Digital record and reproduce systems, well known and broadly applied in the 
art, provide an output electrical signal conveying binary intelligence by 
means of two signal levels, e.g., positive and negative, or by means of a 
transition sequence between levels. Numerous digital encoding methods are 
practiced and their selection depends upon factors such as application, 
need to conform to data interchange standards, bit pattern sensitivity or 
frequency spectrum restrictions of a digital system and the preference of 
the system designer. 
In digital magnetic recording systems which do not have intra-system code 
conversion, the reproduced electrical signal continuum is analogous to a 
magnetic flux continuum recorded on a medium. The output electrical signal 
level corresponds to the sense, i.e. direction, of a recorded 
magnetization, and a transition in output electrical signal level 
corresponds to a transition from one magnetization sense to another. The 
recording transducer (record head) signal current continuum producing the 
recording relates to the magnetic flux continuum in the same manner. 
It is common practice to use record current levels sufficient to produce 
near saturation remanence in the medium; higher current levels are used if 
there is a need to reliably record new data over prerecorded data 
(overwrite) without benefit of an erase cycle. 
FIG. 1 illustrates a record current waveform with the corresponding 
reproduce system voltage waveform and the corresponding remanent 
magnetization pattern. The magnetization pattern is represented by a 
simple planar (parallel to plane of medium) vector model. This 
illustration is typical of a non-return-to-zero-level (NRZ-L) digital 
system. The planar vector model disregards any normal (perpendicular) 
components of magnetization which exist in regions of flux transitions in 
the medium. 
FIG. 2 illustrates an elementary ring record head which converts record 
signal current from a head driver (generator) to a fringing magnetic 
recording field which penetrates into the magnetic layer of a recording 
medium. 
Record head structures of a type widely applied in the current art are 
usually more complex than that depicted in FIG. 2, but the essential 
principles are the same; signal current through the core windings produces 
magnetic flux within the material of the head core, some of which emanates 
from the region of a non-magnetic gap to form the recording field. 
A more detailed illustration of the magnetic field in the region of a 
record head gap is given in FIG. 3. 
As a medium is being recorded the instantaneous magnetization of each of 
its particles or domains is determined by the magnetic susceptibility of 
the particle and the intensity and direction of the recording field at the 
particle site. The susceptibility of a particle may vary with its 
orientation. The particles of typical commercial recording media are 
capable of being switched to a new state of magnetization within 3 
nanoseconds of a change in the recording field. 
During recording, the magnetization of a particle changes as a function of 
field direction and intensity attributable to position change or record 
signal change while the particle moves through the entire recording gap 
region; but, the final determinant of remanent magnetization is the 
direction and intensity of the field near the trailing edge of the 
recording gap. Therefore, the recording zone of a head is generally 
considered to exist along contours emanating from the trailing edge in 
which the field intensity approximately equals the coercivity of the 
medium. FIG. 4 illustrates such a recording zone. The contours shown 
depict contours of constant magnetic field intensity as opposed to filed 
lines. The numerals 1, 0.9, 0.8, etc. indicate field strength relative to 
the strength in the center of the deep gap field. The gap length of a 
record head is usually designed to produce a desired record zone 
penetration depth into a medium. Gap length values ranging from one to two 
times the minimum wavelength to be recorded are typical. 
Another factor affecting remanence is demagnetization. Demagnetization 
occurs when fields emanating from a magnetization return to oppose the 
sense of adjacent magnetizations. If the geometry of a magnetized region 
is such that its field generally emanates and returns externally, the 
demagnetization factor will be low; if its field generally returns 
internally through the region of the magnetization, the demagnetization 
factor will be high. 
FIG. 5 illustrates the demagnetization effect for a thin, flat sheet whose 
length and width dimensions are very large with respect to thickness. When 
most of the field returns externally, as in FIG. 5a, the demagnetization 
factor is nearly zero and remanence is high; when most of the field 
returns internally, as in FIG. 5b, the demagnetization factor is nearly 
unity and remanence is low. 
Similarly, for a continuum of recorded planar magnetizations, long compared 
to their depth (magnetic layer thickness), the demagnetization factor is 
low. As the recorded wavelength of such planar magnetizations decreases, 
the demagnetization factor increases, i.e., for purely planar 
magnetizations, remanence decreases with recorded wavelength. 
If a long array of normal magnetizations of like sense is recorded, the 
situation is similar to that depicted in FIG. 5b, and the demagnetization 
factor is high. However, if the normal magnetizations alternate in sense 
over closely spaced intervals, as occurs in short wavelength recording, 
then their fields are mutually supportive and the demagnetization is low. 
For purely normal components of magnetization, remanence increases as the 
recorded wavelength decreases. 
Table 1 is presented as FIG. 6A, and it qualitatively summarizes 
demagnetization effects for long and short wavelength planar and normal 
magnetizations. 
As a region of a medium moves through an active recording zone, the shape 
of that zone (FIG. 4) produces both planar and normal magnetization 
components, instant by instant; these components are then subject to 
modification by demagnetization effects. 
FIG. 6B shows a record current transition and a model of the resulting 
magnetization pattern before and after demagnetization. The long planar 
component array is essentially unchanged by demagnetization. However, the 
normal components are diminished by demagnetization with the exception of 
those located in the region corresponding to the transition. These 
transition region normal components are less affected by demagnetization 
because of a supporting field from the planar continuum which they 
terminate. 
In general, the longitudinal recording of a digital data signal continuum 
produces a magnetization pattern of normal components marking transition 
regions which are supported (flux linked) by planar components. The longer 
the recorded half-wavelength, i.e., the longer the recording signal 
remains at one polarity, the longer will be the continuum of planar 
components, the deeper will be its penetration into the medium, and the 
longer will be the region of possible interaction between normal 
components terminating adjacent, opposite sense, half-wavelengths. Such 
interaction is a cause of pulse crowding which contributes to pattern 
sensitivity. FIG. 7 illustrates a simplified planar and normal component 
model associated with a record current waveform having various single 
polarity time intervals. 
Compensating pattern sensitivity by means of advancing or delaying record 
current transitions is known in the art. Some effects contributing to 
pattern sensitivity can be minimized by means of thin magnetic medium 
coatings, record heads having short gap lengths to limit fringing flux 
penetration and to improve the record field gradient, and by means of 
reduced record current to limit flux penetration and to improve the field 
gradient. The latter means, of course, compromises overwrite performance. 
When a recording current changes polarity rapidly, as in the recording of a 
square wave, a subsequent cycle partial erasure occurs. FIG. 8 illustrates 
this effect. As the polarity of the record current changes, previously 
recorded magnetizations leave the record zone to be affected by a weaker 
field of opposite sense, i.e. partially erased. Thus, output levels are 
reduced for signals having closely spaced reversals of polarity. 
The more abruptly the field strength decreases with distance from the 
trailing edge of the record gap, i.e. the higher the recording field 
gradient, the less will be the effects of subsequent cycle erasure. In 
general, increasing record current, which may be done to assure adequate 
overwrite levels in a digital system, decreases the recording field 
gradient and increases the effects of subsequent cycle erasure. 
Opposite sense normal magnetizations in close proximity may have a mutual 
flux circuit of such low reluctance as to preclude their flux contribution 
being sampled by a reproduce head circuit, as is illustrated in FIG. 9. 
This proximity mutual flux "loss" can reduce signal output and shift the 
point at which a transition is sensed in the medium by the reproduce 
circuit. 
FIG. 10 illustrates a typical digital data record current waveform. Current 
is maintained in one direction or the other through a record head winding. 
Heat energy dissipated by a head and head driver is a function of the 
shaded area of the waveform shown. The sustained heating effect of the 
relatively high currents required in some digital systems precludes use of 
low power rated and inexpensive components in head driver circuits, 
particularly for multi-track systems. 
The remanent flux produced by sustained currents through the record head 
can be modulated at any time a medium is being recorded by variation in 
head to medium contact, thereby producing noise. This noise modulation is 
caused by random irregularities in surface quality of the medium. 
Some digital recording systems utilize a read-after-write protocol and, to 
accomodate it, very closely spaced write (record) and read (reproduce) 
head gaps, e.g., 0.150 inch. In such systems, crossfeed, the transformer 
coupling of write head energy to read circuits, is a major design 
consideration. Crossfeed can interfere with the data being reproduced from 
the recording medium. Transformer coupling of write energy generally 
increases with frequency, however, frequencies higher than the fundamental 
frequency of the highest data rate are of less concern because the 
reproduce head circuit can include low pass filter elements to reject 
them. 
FIG. 11 shows a square wave which might represent the highest data rate 
record current waveform of a digital record/reproduce system. The relative 
amplitudes of the fundamental frequency, the third harmonic, and the fifth 
harmonic comprising part of the frequency spectrum of this waveform are 
also shown. The harmonics can, of course, be filtered by the system; but 
the fundamental frequency which has a higher peak value than the recording 
waveform can crossfeed to the reproduce circuit. Design elements of the 
read/write head assembly and its shielding are dictated by crossfeed 
performance versus cost considerations.

DESCRIPTION OF THE INVENTION 
In accordance with the present invention, a magnetic flux continuum 
analogous to a digital information signal continuum is recorded as a 
sequence of end-to-end juxtaposed magnetized increments. The sequence of 
magnetized increments is recorded along a track of a recording medium as 
the medium moves at constant speed relative to a record transducer (head). 
Each magnetized increment comprising the 14 magnetic flux continuum is 
produced by an instantaneous magnetic recording field. This instantaneous 
recording field results from the record head being driven by a current 
impulse. The current impulse is of short duration compared to the time 
required for a point on the track to traverse the length of the recording 
field which it produces. The length of each magnetized increment so 
produced includes the length along the track penetrated by the recording 
field; for a ring head, this length is the entire gap leading edge to gap 
trailing edge penetrating field length which includes all contours of 
sufficient intensity to produce a recording effect. The length of each 
magnetized increment also includes the relatively small distance traversed 
by a point on the track during application of the current impulse. The 
timing of the sequence of current impulses which produce the sequence of 
magnetic increments is such that the end of one magnetized increment on 
the medium is made to approximately coincide with the beginning of the 
next magnetized increment, in sequence, along the track. Of course, the 
timing is a function of the speed of the record medium relative to the 
record head. 
Thus, a magnetic flux continuum is produced by a sequence of current 
impulses having a low active duty cycle. 
In one embodiment of the present invention, the aforesaid magnetized 
increment length (determined, in part, by the record head gap length) is 
made to coincide with the length of an NRZ-L bit cell recorded on magnetic 
tape. Thus, it can be appreciated that a sequence of bits can be recorded 
on tape by means of uniformly time-separated, short duration, current 
impulses of appropriate sense, one current impulse corresponding to one 
recorded bit; and that the magnetization pattern thus produced will be 
similar to that produced by systems which apply current of one sense or 
the other to the record head continuously (continuous current systems); 
and that the heating effect (average power consumption) of such current 
impulses in head driver electronic components can be low, and that media 
surface modulation noise can be statistically limited during recording by 
the short duration of the active current impulse. 
It can also be appreciated that the sequential current impulse waveforms 
are not characterized by instantaneous reversals of polarity, as are 
continuous current system waveforms. Therefore, there are minimal 
subsequent cycle erasure effects and proximity mutual flux loss effects. 
In consequence, peak record currents can be made relatively high to 
achieve good overwrite performance without causing a significant loss of 
output for high transition rate signals and without causing at least one 
transition shifting mechanism. 
An additional factor to be appreciated is that the sequential current 
impulse waveform spectrum includes a fundamental frequency of lower peak 
value than the impulse itself, and use of sequential current impulse 
recording can result in low crossfeed interference by the fundamental 
frequency component. 
Therefore, the objects of the present invention are: 
To provide for sequential impulse magnetic recording of information on a 
magnetic medium in a manner which can be compatible with the reproduce 
techniques of continuous current recording systems, which systems are 
widely known and applied in the art. 
To provide for magnetically recording information in a manner which 
requires less energy and dissipates less heat than do continuous recording 
systems. 
To provide for magnetically recording information in a manner less affected 
by media surface modulation noise. 
To provide for magnetically recording information in a manner which can 
simultaneously achieve less interference from overwritten data, higher 
output of high transition rate data signals, and fewer transition shifting 
mechanisms than can simultaneously be achieved by continuous current 
recording systems. 
To provide for magnetically recording digital information in a manner which 
can produce less fundamental frequency crossfeed interference energy than 
is produced by continuous current recording systems. 
Other objects and advantages of the invention will be apparent to those 
skilled in the art from the foregoing general description of the 
invention, and from the following description of one embodiment of the 
invention. This embodiment is presented only as illustrative of the 
invention in order to facilitate a complete understanding thereof by those 
skilled in the art, and to facilitate their making and using the 
invention. This embodiment represents the best mode contemplated at this 
time for practicing the invention, although it is obvious that other modes 
are possible and might indeed ultimately prove more practical. 
The following description is had in conjunction primarily with FIGS. 12-17 
of the accompanying drawings, wherein like reference characters refer to 
like or corresponding parts, and wherein: 
FIG. 12 is a block diagram of a recording system for practicing the present 
invention, with associated schematic waveform diagrams for explaining the 
operation; 
FIGS. 13 and 14 are time, waveform and magnetic vector charts showing the 
details of impulse recording in accordance with the present invention; 
FIG. 15 is a schematic diagram of the record circuit used in the present 
embodiment of the invention; 
FIG. 16 depicts the magnetic vector components wherein the present 
invention is practiced with a small overlap in recording increments; and 
FIG. 17 depicts the magnetic vector components wherein the present 
invention is practiced with perpendicular field recording. 
In one single channel (track) embodiment of the present invention, digital 
information is recorded on magnetic tape by means of sequential recording 
current impulses. The magnetization patterns created on magnetic tape by 
this embodiment are similar to the patterns created by continuous current 
recording systems in all characteristics essential to their being 
reproduceable as electrical signals by reproduce systems of types known in 
the art and widely used in conjunction with continuous current recorders. 
FIG. 12 shows a functional block diagram of the present embodiment of the 
invention, and of a reproduce system. Waveforms associated with the NRZ-L 
code used, are also shown. 
A data signal conveying each bit (level) to be recorded and a clock signal 
defining each bit period (cell) are applied to their respective record 
logic circuit inputs 101 and 102. The record logic circuit 103 processes 
the clock and data to produce dual polarity voltage impulses. The voltage 
impulses are converted to current impulses of appropriate amplitude and 
sense by a head driver 104 having an input level control. The head driver, 
in turn, drives the record head 105. The record head creates an 
instantaneous magnetic recording field and a magnetically recorded 
increment in the tape 106, for each intermittent current impulse. The 
length of each increment thus recorded is related to the record head gap 
length. The spacing of these recorded increments is defined by the timing 
of current impulses and the speed of the magnetic tape, which speed is 
well regulated by means known in the art. 
For this embodiment, design parameters such as record head gap length, 
current impulse timing, and tape speed are chosen to produce closely 
spaced recorded increments on tape, each of which corresponds to a bit 
cell. A tape thus produced can then be reproduced by a system functionally 
equivalent to that shown in FIG. 12 and described herein, in general 
terms, for benefit of an example. 
The reproduce head 107 generates a voltage proportional to the rate of 
change of flux sensed across its gap as the tape moves over it at a 
uniform speed. The reproduce head signal is amplified by a preamplifier 
108, then equalized at 109 to compensate for the non-constant amplitude 
versus frequency (data level change rate) transfer characteristic of the 
rate-responsive reproduce head. Phase equalization to compensate for 
pattern sensitivity or for phase erros introduced by amplitude 
equalization may also be employed. The equalized reproduce signal is then 
processed by the reproduce logic circuit 110 which detects and shapes the 
output data signal, synthesizes a stable clock signal, and accurately 
synchronizes the data signal to the clock signal, at output terminals 111 
and 112. 
A dimensioned timing diagram and vector magnetization model for the 
recording of a 1-0 bit sequence by the subject emobidment is shown in FIG. 
13. The timing of the reproduce waveform for that sequence is also shown. 
The data rate of this embodiment is 250,000 bits per second (BPS) recorded 
(and reproduced) at a tape speed of 71/2 inches per second (IPS) to 
produce a bit packing density on tape of 33,333 bits per inch (BPI). For 
the NRZ-L code used, 33,333 is also the maximum number of flux changes per 
inch (FCI), i.e. the maximum number of sense reversed, adjacent increments 
(half-cycles) recorded per inch of tape. Each bit cell corresponds to a 
time interval of 4 microseconds and a recorded length of tape of 30 
microinches. The upper frequency of the record/reproduce channel pass band 
need only be 125,000 Hertz for 250,000 BPS at the Nyquist rate of 2 bits 
per Hertz. 
The clock period is 4 microseconds to provide a positive-going voltage 
transition at the beginning of each bit cell as shown. The 50 percent duty 
cycle clock waveform also provides a negative-going transition at the 
center of each bit cell. Each negative-going transition of the clock is 
used to trigger a recording current impulse. The duration of each current 
impulse is 400 nanoseconds and its sense is defined by the data level of 
the cell to which it corresponds. 
In the vector magnetization model of FIG. 13, vectors marked "1" represent 
components recorded at the leading edge of a current impulse; those marked 
"t" represent components recorded 400 nanoseconds later at the trailing 
edge of the current impulse. The length, L.sub.A, is the distance traveled 
by the tape during an active current impulse (3 microinches). The shaded 
area of FIG. 13 represents the effective recording field penetration into 
the tape, and the length L.sub.E, the length of that penetration. The 
length of each recorded increment, L.sub.I, is 30 microinches, the sum of 
a field penetration length (L.sub.E) of 27 microinches plus the 3 
microinches traveled during its recording current impulse. The length of 
the record head gap, L.sub.G, producing the desired penetration length is 
approximately 20 microinches. 
FIG. 14 illustrates waveforms and a planar/normal vector magnetization 
model associated with the subject embodiment and a random bit sequence. 
The long sequence of identical bits, e.g., 0--0--0, are recorded as 
identical increments having adjacent terminations of opposite sense 
components. These components, shown circled in FIG. 14, have energy 
stable, proximity, mutual fields which cannot be sensed by a reproduce 
circuit. Therefore, the effective component pattern is similar to one 
produced by continuous current recording. 
FIG. 15 is schematic diagram of the record circuit used for the subject 
embodiment. Dual polarity, positive logic is used, i.e. a positive voltage 
signifies a logic "1", a negative voltage signifies a logic "0". U1 and U2 
are non-inverting buffers for the clock and data signal respectively. The 
clock buffer U1 drives the inverting trigger input of a monostable 
multivibrator, U3. U3 generates a positive pulse at its Q output for each 
negative-going clock transition. R1 and C1 are timing components which 
determine the duration of each positive pulse (400 nanoseconds). The Q 
output of U3 is applied to the control input C of a bilateral switch, U4. 
The output of U2 is connected to the data input I of U4. When the control 
input of U4 is negative, it is in a high impedance state and its output at 
0 is held to ground potential (0 volts) by R2. When the control input is 
positive, during the 400 nanosecond pulses, the output of U4 is of the 
same polarity as the data signal. The dual polarity voltage impulses thus 
derived are divided by level control potentiometer, R2. The R2 signal is 
connected to a transconductance head driver consisting of Q1 through Q4 
and R3 through R7. The head driver converts voltage impulses to current 
impulses and presents a high source impedance to the record head, L1. The 
driver, acting as a current source, provides a small L/R time constant in 
conjunction with record head inductance and results in a broad range of 
inductances possible in a record head designed to be driven by short 
duration current impulses. When the input to the bases of Q1 and Q2 is 
grounded (the quiescent state), Q1 through Q4 are not conducting and no 
current is supplied to the head. When the input is positive, Q2 and Q4 are 
not conducting, but Q1 does conduct. The current of Q1 is determined by 
the value of R5 and the input voltage. Q1 current causes a voltage drop 
across Q1 collector resistor, R3. The R3 voltage and the value of R6 
determine the collector current of Q3. The collector of Q3 drives the 
record head during positive sense impulses. When the input to the bases of 
Q1 and Q2 is negative, Q1 and Q3 are not conducting, while Q2 conducts 
current in an amount determined by the input voltage and the value of R5. 
Q2 current then causes a voltage drop across R4 which, in conjunction with 
the value of R7, determines the collector current of Q4. The collector of 
Q4 drives the record head during negative sense impulses. 
Thus, the data signal and clock signal are processed to provide 400 
nanosecond current impulses of dual sense for recording. 
In summary, the subject embodiment records bits as 30 microinch magnetized 
increments on tape: the length of each increment is largely determined by 
head design, not tape motion; and the field energy of each increment is 
derived from a current impulse having a tape motion related, 
half-wavelength of only 3 microinches, one tenth of the recorded increment 
length. Expressed in terms of frequency, the 400 nanosecond recording 
current impulse relates to the half-wave period of 1.25 megaHertz signal, 
a frequency ten times higher than the pass band required for the subject 
embodiment data channel. It is significant that, in comparison to the heat 
dissipated by elements (e.g. transistor junctions) of comparable 
continuous current head drivers, the heat dissipated by the impulse 
current head driver of this embodiment is reduced by 90 percent. A further 
90 percent reduction in heat could easily be achieved by reducing the 400 
nanosecond current impulses (10 percent duty cycle) to 40 nanoseconds (1 
percent duty cycle), an entirely practical value considering that less 
than 3 nanoseconds is required to switch the particles of the magnetic 
tape and that any active record current duty cycle including current 
impulses of at least 3 nanoseconds will, in theory, be sufficient. As a 
practical matter, the minimum active record current duty cycle acceptable 
for a given application will be determined by the minimum pulse width 
handling capacity of components selected for other considerations such as 
cost. At the other extreme, determining the maximum duty cycle acceptable 
for a given application requires a more complex analysis of the effects of 
increased duty on the various benefits expected. In practice, it has been 
found that substantial benefits of sequential current impulse recording 
over continuous current recording are obtained by using an active record 
current duty cycle of 50 percent or less. The vector magnetization 
resulting from the subject embodiment is effectively the same as that from 
its continuous current counterpart; but, the magnetizations are produced 
without the need to continuously record and thereby overwrite record head 
leading edge components; in fact, it is inefficient to do so. 
In the magnetic tape recorder embodiment described herein, the shortest 
recorded half-wavelength was determined by the data rate and the tape 
speed, both of which were constant. However, in typical magnetic disk 
applications, data rate and angular velocity are constant; track 
(cylinder) speed varies with circumference as does bit packing density and 
recorded half-wavelength. If, in a disk embodiment of the present 
invention, design parameters are chosen to yield a recording increment 
length corresponding to the minimum half-wavelength to be recorded on the 
outer track, then these recording increments will overlap on inner tracks. 
Impulse recording with overlapped increments is modeled in FIG. 16. The 
bold vectors are shown to have overwritten the "over-length" components 
(shown dashed) of the previously recorded half-wavelength. This increment 
overlap is similar to overwriting which occurs in continuous current 
recording systems, except that some benefits of the current impulse 
recording technique are retained, which benefits are generally associated 
with the nature of the recording waveform. 
The benefit of compatibility with reproduce systems of continuous current 
recorders is obviously retained regardless of the degree of overlap (or 
increase in active record current duty cycle caused by overlap) for 
continuous current recording could be regarded as an infinite sequence of 
overlaps. The benefits of reduced modulation noise and reduced heat 
dissipation are inversely related to the active duty cycle of the 
sequential current impulse recording waveform. The benefits of improved 
overwrite performance without loss of output for high transition rate 
signals depends on maintaining a recording medium motion related distance 
separating the occurrence of opposite sense record current impulses, which 
distance is at least equal to the length of the record zone located at the 
trailing edge of the record head gap (refer to FIG. 8.) Of course, the 
record zone length increases with peak record current but the condition of 
separation of opposite sense recording impulses is generally met so long 
as the combination of the impulse duration and the overlap provides an 
inactive time interval between impulses which allows this distance to 
traverse the head. The benefit of reduced crossfeed interference depends 
on the degree of reduction of the fundamental frequency energy for the 
impulse current waveform compared to that of a continuous recording 
current waveform. In summary, the overall benefits of sequential current 
impluse recording are retained substantially for moderately overlapped 
increments. This fact permits application of reasonable gap length 
tolerances in the manufacture of current impulse record heads for all 
applications. Operation with overlapped recording increments also permits 
use of the current impulse recording method with codes having possible 
transitions at sites other than those defined by integer multiples of the 
minimum half-wavelength increment. 
In an embodiment of the present invention which utilizes perpendicular 
field recording, the length of the effective field region (L.sub.E) is 
defined by head pole geometry rather than record head gap length. Purely 
normal magnetization components of one sense are recorded in each 
increment length as shown in FIG. 17. Each recorded increment is produced 
by a single, low duty cycle, current impulse of appropriate sense in a 
manner similar to that described herein for the longitudinal recording 
embodiment. 
Restating the invention in light of all the foregoing description and 
analysis, in reference to the embodiment utilizing the linear traverse of 
a magnetic record head relative to the surface of a magnetic record 
medium, the following terms are defined: 
L.sub.E is the length along said line of traverse of the effective 
recording field of the record head (see FIG. 13); 
L.sub.A is the length of said linear traverse during the time of 
application of an active record current impulse to said record head (see 
FIG. 13); 
L.sub.I is the length of a recorded increment resulting from the 
application of a record current impulse to said record head (see FIG. 13); 
S.sub.T is the speed of said traverse; 
T.sub.A is the period of an active recording current impulse (see FIG. 13); 
and 
T.sub.R is the period between initiation of successive recording current 
impulses (see FIG. 13). 
In accordance with the present invention the relationships among the terms 
defined above are expressed by the following equations: 
1. L.sub.A =S.sub.T .times.T.sub.A ; 
2. L.sub.I =L.sub.E +L.sub.A ; and 
3. L.sub.A &lt;L.sub.E 
In the referenced embodiment of the invention, T.sub.A /T.sub.R is the 
active record current duty cycle, it may conveniently equal about 1/4, or 
1/10, or can be made as small as is practical for the minimum pulse width 
capacity of the components selected for the recording circuit. Of course, 
the recording current impulse must be of sufficient duration to accomplish 
switching of the magnetic particles. Also, ideally, L.sub.E =S.sub.T 
(T.sub.R -T.sub.A) and T.sub.R =L.sub.I /S.sub.T as this would mean that 
the effective length of the recording field (L.sub.E, determined by record 
head design and magnitude of record current) exactly corresponded to the 
length of traverse of the magnetic medium during the inactive time 
interval between successive, active recording current impulses; in 
consequence, a continuous magnetic recording without overlap or separation 
of successive increments of recording would be provided. However, the 
advantages of the present invention are still obtained with some measure 
of overlap or separation of the successive increments of recording. 
The invention may be embodied in other specific forms without departing 
from the spirit or essential characteristics thereof. The present 
embodiments are therefore to be considered in all respects as illustrative 
and not restrictive, the scope of the invention being indicated by the 
appended claims rather than by the foregoing description, and all changes 
which come within the meaning and range of equivalency of the claims are 
therefore intended to be embraced therein.