High energy degausser

A magnetic material degausser in which the material to be degaussed is placed within the center of a degaussing coil. A capacitive discharge system is employed for energizing the degaussing coil. The capacitor is repeatedly discharged through the coil in such a way that for at least a portion of the degaussing cycle, the energy applied to the coil decreases with each energization. The magnetic material may be rotated within the coil as the degaussing cycle progresses. In one embodiment of the present invention, the energy discharged from the capacitor first increases and then decreases during the degaussing cycle with each energization of the coil. The rotational speed of the magnetic material within the coil may be related to the frequency at which the capacitor is discharged.

BACKGROUND OF THE INVENTION 
1. Field Of The Invention 
This invention relates generally to the art of removing permanent 
magnetization from objects. 
2. Description Of The Prior Art 
In the art of making reusable magnetic recordings, usually on the 
magnetizable surface of a magnetic recording tape, it is often desirable 
to "erase" the originally recorded data so that the magnetic recording 
tape or other medium can be used properly to make a second data recording 
in pure and accurate form, independent of the previously recorded data on 
the medium. In this respect, the term "erasing" must be distinguished from 
the term "degaussing". Degaussing is a more specific term relating to the 
returning of a magnetized object, such as a magnetic recording tape or 
other medium to the totally demagnetized state or an approximation 
thereof. A system which accomplishes this purpose has become known as a 
degausser. The term "erasing" is often employed generally to describe the 
obliterating of previously magnetically recorded data from a recording 
tape by any technique. 
Two commonly employed techniques exist by which magnetically recorded data 
may be obliterated from a magnetic medium. The first technique, generally 
automatically used in purely digital, magnetically-saturated "computer" 
tapes, is simply to apply a strong, saturating, unvarying, uni-directional 
magnetic field along the recording length and direction of the tape by 
application of an "erase" magnetic head or permanent magnet. In this 
technique, the entire recorded length of a magnetic tape may simply be 
pulled across the active portion of such an "erase" head to accomplish 
obliteration of the originally recorded varying data. This procedure 
leaves all portions of the magnetic recording surface fully magnetized to 
saturation in the same direction, obliterating the previously recorded 
signal variations that comprise the recorded data. Different data may then 
be recorded on the tape by creating a magnetic field in the magnetic 
material forming the tape which is less than at saturation. This technique 
may be compared to scribbling over written material with a pencil to 
obliterate the written material and then writing something different with 
the eraser of the pencil. 
The second technique for obliterating the data on a magnetic tape actually 
accomplishes degaussing. The degaussing technique attempts to completely 
remove both all previously recorded signals and any extraneous or 
structurally generated magnetic noise pulses that might remain in the 
magnetic coating due to residual magnetization of any sort. This 
degaussing technique and a subsequent writing operation onto the tape can 
be compared to erasing written material with a pencil eraser and then 
writing again over this spot. Degaussing includes the act of removing 
signal data plus any residual magnetic noise. On the other hand, the first 
technique described above does not necessarily or commonly remove any 
residual magnetic noise. 
Degaussing is accomplished by applying to each portion of the recording 
medium surface a magnetic field which reverses direction a number of times 
and gradually decreases in absolute strength over the course of a 
degaussing cycle. The degaussing magnetic field is reduced gradually at 
each point of the tape surface from an initial level at least (usually 
more than) the level of apparent magnetic saturation through many cycles 
to a level gradually approaching zero. This leaves the magnetic recording 
surface only very slightly magnetized by the residual effects of the 
earth's magnetic field and any other residual fields applied by slight 
occasional magnetization of nearby equipment components. 
Degaussing, sometimes called AC erasure, is much more effective than 
uni-directional saturation, sometimes called DC erasure, particularly with 
analog recording tapes, since degaussing reduces false, residual 
background noise to a practical minimum so that a subsequent recording of 
data will include as little noise as possible. 
While simple DC saturation can be performed by running the tape across a 
fixed magnetic recording head supplied with a saturation strength direct 
current, degaussing can be accomplished by applying a high frequency peak 
saturation alternating current to the "erasing" head so that each portion 
of the magnetic tape experiences many alternating cycles of applied field 
which decrease gradually in intensity at a given point on the recording 
surface due to its mechanical withdrawal from the point of maximum 
magnetic action at the degaussing head active gap. 
While degaussing of magnetic tapes by transportation across a degaussing 
head is highly effective, it is time consuming and not economical in that 
it requires the use of relatively expensive magnetic tape transportation 
devices to accomplish the desired purpose. 
It is also known that full reels of magnetic tape can be degaussed, without 
unwinding and rewinding, in a "bulk" degaussing machine which applies the 
required, gradually-decreased alternating applied magnetic field to the 
entire tape roll without unwinding the tape. 
Regardless of the particular degaussing machine, the end result of 
degaussing is to make the remanent flux remaining permanently on the 
recording surfaces as small as possible in order to leave the magnetic 
recording material in as nearly a totally demagnetized condition as 
possible. This result can be accomplished only by either raising the 
temperature of the magnetized recording material to a destructively high 
level or by causing the material to traverse a very large number of 
gradually shrinking magnetic hysteresis loops by periodically reversing 
the polarity of an applied magnetic field which gradually decreases in 
intensity. 
In quality magnetic recording of analog signals, the generally accepted 
level of recorded signal and residual magnetic noise removal that must be 
accomplished by magnetic tape degaussing for both operational efficiency 
and reasonable security against unfriendly deliberate signal recovery is 
at least 90 db below the originally recorded saturation signal 
reproducible within a 10 Hz frequency bandwidth. 
For special applications, it is sometimes desirable to reduce the residual 
magnetization of a previously recorded tape by more than 90 db below 
previous signal saturation. 
In order to degauss a tape that had previously been recorded with a 
saturated signal, it is necessary to apply a magnetizing force equal to at 
least two and preferably three times the coercive force or coercivity of 
the magnetic recording material. Most magnetic tapes in high quality use 
have a coercivity ranging from 150 to 350 oersteds. 
In more recent times, new varieties of magnetic recording tape and other 
magnetic media have been made available to the industry in which the 
coercivity has been increased substantially above the 350 oersted level. 
In the new, "high energy" magnetic recording media, the coercivity may 
approach 800 oersteds. It is not inconceivable that additional improvement 
in magnetic recording media will provide tapes with even higher coercivity 
values. 
The appearance of especially high energy recording media requires the 
provision of degaussing systems capable of applying much higher 
magnetizing forces than have been practically possible with currently 
existing commercial degaussing systems, particularly bulk degaussing 
systems. While this problem is not especially difficult to overcome in 
tape degaussing systems which pass the entire length of the tape 
sequentially across a degaussing head, developing the required higher 
degaussing energy levels in a practical sense becomes more and more 
difficult when it is desired to use the more economical method of bulk 
degaussing described above. 
The currently available best quality commercial bulk degaussing systems for 
magnetic tapes, such as, for example, the K-90 tape degausser manufactured 
by General Kinetics Incorporated, Rockville, Md., universally employ 
alternating current electromagnets to apply the required degaussing field 
from outside of the reel of tape or other package of magnetic medium to be 
degaussed. In order to achieve even the modest external field strengths 
required for ordinary, comparatively low energy magnetic tape, these 
degaussers use coils of heavy wire surrounding laminated transformer iron 
cores. These coil-core combinations generally are powered by continuous 
application of alternating current from a normal 60 or 50 Hz supply line. 
The useful portion of the magnetic field produced by iron cored coils of 
this type occurs in an air gap interrupting the iron core structure to 
allow insertion of the reel of magnetic tape. The reel of tape usually is 
inserted fully into the active air gap of such systems and either is 
caused to rotate while being slowly withdrawn geometrically from the field 
influence or is rotated at a fixed position in the air gap while the 
applied alternating electric field is gradually reduced. Either of these 
methods results in application through the sides of the tape roll of a 
gradually reducing applied alternating magnetic field required for 
degaussing. 
As a result of the electromagnetic inefficiency of the necessary air gap in 
the coilcore structures thus employed, it has been necessary to apply very 
heavy continuous alternating electric currents to the coils, with 
resultant heating of the wire due to its electrical resistance and heating 
of the core material due to hysteresis energy loss in the iron 
laminations. In addition, it is usually necessary to employ large 
electrical capacitors connected in parallel with the bulk degausser coils 
to minimize the total amount of power line current required by providing a 
degree of power factor correction. 
In a great many practical instances, the magnetic tapes to be bulk 
degaussed will be contained on reels provided with protective metal 
flanges. Since the externally applied magnetic field emanating from the 
iron core external coils must penetrate the metal flanges, eddy currents 
are induced in the flanges which cause them to react mechanically, 
producing noise, and occasionally vibrating sufficiently to damage the 
edges of the wound roll of tape. Due to eddy current losses, the metal 
reel flanges also become heated, with possible damaging results to the 
tape thereon. 
This problem has been overcome by employing a coil of wire containing no 
iron core. The coil is essentially rectangular in cross section and is 
used with a reel of tape inserted at least partially within the confines 
of the inside of the coil with the axis of the coil lying parallel to the 
real flanges. Such a degaussing system is illustrated in U.S. Pat. No. 
3,143,689 to Hall. In this patent, a storage capacitor is gradually 
charged from the rectified line current. The storage capacitor is 
periodically discharged through the degaussing coils in synchronism with 
the frequency of the line current. A second capacitor is connected in 
parallel with the degaussing coil to create a resonant circuit which rings 
after the energy in the storage capacitor has been applied to it. This 
creates a reversing magnetic field which gradually decreases in intensity 
in accordance with the time constant of the degaussing coil and the second 
capacitor. According to this patent, the storage capacitor should be 
discharged six or seven times to produce a sufficient erasure of the 
magnetic recording tape. 
U.S. Pat. Nos. 3,321,586, 2,962,560, and 2,838,720 also teach degaussing 
systems which employ capacitor discharge circuitry. 
As described above, the Hall patent relies upon the characteristics of a 
resonant circuit to control the frequency of the reversal of the magnetic 
field and the time period of the decay of the magnetic field. However, the 
characteristics of the resonant circuit are greatly affected by the nature 
of the magnetic material inserted within the coil. Thus, reels of 
different size, reels of the same size with different amounts of tape 
thereon, or reels of different material influence not only the frequency 
of field reversals, but also the time constant of the field decay. 
Furthermore, in general, in view of the losses associated with the repeated 
generation of a magnetic field within the tape and surrounding air, it is 
impractical to extend the time constant of the resonant circuit to an 
extent sufficient to perform a complete degaussing. It is for this reason 
that Hall teaches that four or five repetitions are necessary to 
completely erase the tape. 
The Hall patent also teaches the use of two orthogonally arranged sets of 
degaussing coils. Such an arrangement is necessary to uniformly erase all 
portions of the tape. This complicates not only the circuitry necessary to 
energize the coils, but also makes more difficult the problem of placing 
the tape within the orthogonal sets of coils. 
SUMMARY OF THE INVENTION 
The present invention overcomes these problems. The bulk degaussing system 
of the present invention employs a coil of wire having a substantially 
rectangular cross section in which the reel of tape is inserted. A 
capacitor is charged gradually and repeatedly discharged through the coil. 
A second capacitor is provided across which a control voltage develops. In 
fact, the charge across the second capacitor gradually decreases for at 
least a portion of the degaussing cycle. The voltage across the second 
capacitor is compared with the voltage across the storage capacitor. When 
the storage capacitor has charged to a level related to the control 
voltage, a switch is triggered causing the storage capacitor to discharge 
through the coil. Since the control voltage gradually decreases, the peak 
amount of energy that is stored in the storage capacitor prior to each 
discharging gradually decreases so that the strength of the magnetic field 
applied to the tape gradually decreases. 
At the same time, the tape is rotated to effectively reverse the field 
direction and ensure uniform degaussing. In fact, as the field applied to 
the tape decreases, the frequency of rotation increases to quicken the 
degaussing process. 
With some tapes, particularly relatively new tapes which have only been 
recorded on twice and are being erased for the second time, it has been 
found that starting a degaussing cycle at a lower field strength and then 
decreasing the field strength produces better results than starting at a 
higher field strength and then decreasing the field strength. After a tape 
has been erased more than two or three times, this phenomena disappears 
and the quality of erasing increases with an increase in the field 
strength applied to the tape. Since the number of times that a tape has 
previously been erased cannot always be determined, it has been discovered 
that if the applied magnetic field begins at an intermediate level, rises 
to a peak, and then decreases to zero, all tapes will be demagnetized 
efficiently. 
Finally, in order to concentrate the magnetic field in the tape, 
electrically conductive, non-magnetic blocks, e.g., made of aluminum, may 
be inserted in the coil around the tape.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
FIGS. 1 and 2 illustrate the mechanical arrangement of the present 
invention. Drawer 100 includes movable slide portion 102 which slides with 
respect to and is supported by fixed slide portion 104. Attached to 
movable slide portion 102 is drawer front 106 to which bracket 108 is 
attached. Pivotably mounted through bracket 108 is shaft 110. Fixed to 
shaft 110 for rotation therewith are pulley 112 and reel platform 114. 
Tape reel 116 may be placed on reel support 114 through shaft 110. 
Mounted on door 100 is motor 118. Pulley 120 is attached to shaft 122 of 
motor 118 for rotation with shaft 122. Drive belt 124 operatively 
interconnects pulley 120 and pulley 112. Thus, when motor 118 operates, 
tape reel 116 turns. 
Housing 126 is mounted fixed with respect to fixed slide portion 104. 
Preferably, housing 126 is made up of an electrically conductive, 
nonferromagnetic material such as aluminum. As best illustrated in FIG. 2, 
housing 126 is hollow, having a generally rectangular cross-section and 
open ends. Thus, housing 126 conforms quite closely to the outer contours 
of tape 116 except for that portion of housing 126 which must receive 
bracket 108, shaft 110 and pulley 112. As a result, the bottom portion of 
housing 126 slopes downwardly. Those portions 127 of housing 126 between 
the bottom surface, the rectangular region in which reel tape 116 may be 
inserted and the rectangular region in which pulley 112 and bracket 108 
may be inserted is filled with the electrically conducting, 
nonferromagnetic material of which the housing is made. 
Housing 126 is arranged such that when drawer 100 is closed, reel 116, 
shaft 110, pulley 112 and a portion of bracket 108 move within housing 
126. 
Wrapped around housing 126 is coil 128. In the preferred embodiment, coil 
128 is made of insulated ten gauge copper wire. Coil 128 extends along the 
length of housing 126 over the region occupied by tape reel 116 when 
drawer 100 is pushed in to its furthest extent 
Since housing 126 is conductive, the field generated by coil 128 will 
induce eddy currents therein. To prevent the eddy currents from 
counteracting the current in coil 128, housing 126 is broken along its 
length and insulator 129 is inserted. 
FIG. 3 illustrates the circuit associated with the degausser illustrated in 
FIGS. 1 and 2. Line voltage is applied to power controller 130, which, in 
turn, is electrically connected to high voltage transformer 132. The 
secondary of transformer 132 is connected to a voltage doubler including 
resistor 134, capacitor 136, diode 138, diode 140 and resistor 142. 
Resistors 134 and 142 and capacitor 136 are employed to limit the current 
drawn from the secondary coil of transformer 132. 
Resistor 142 is also connected to one terminal of main storage capacitor 
144. The other terminal of capacitor 144 is connected to a terminal of 
coil 128. The other terminal of coil 128 is connected to the secondary 
coil of transformer 132. 
Connected in parallel across capacitor 144 and coil 128 is electronic 
switch 146, which in the preferred embodiment, is a silicon controlled 
rectifier (SCR). Connected in parallel with SCR 146 is diode 148. Diode 
148 is inserted with such a polarity that current flows through diode 148 
in a direction opposite to the direction of most of the current flowing 
through SCR 146. 
When power controller 130 receives the line voltage, it provides a signal 
to control voltage generator 150 which generates a voltage level which 
starts high and gradually decreases over a degaussing cycle. When the 
control voltage reaches a predetermined low level, the degaussing cycle is 
over and control voltage generator 150 generates a signal to cause power 
controller 130 to deenergize the circuit. 
Control voltage generator 150 provides its control voltage as one input to 
comparator 152. Comparator 152 compares the control voltage with the 
voltage across the main storage capacitor 144. When capacitor 144 has been 
charged to a particular level determined by the control voltage, 
comparator 152 generates a signal for trigger circuit 154 which causes SCR 
146 to become conductive, discharging capacitor 144 through coil 128. 
Every time comparator 152 produces a favorable comparison, a signal is fed 
back from the output of comparator 152 on line 156 to cause control 
voltage generator 150 to decrease the control voltage by a predetermined 
amount. In the preferred embodiment, the control voltage is decreased by a 
fixed percentage. 
The output of comparator 152 is also applied to motor speed controller 158. 
Motor speed controller 158 controls the operation of motor 118. 
In operation, power is applied to power controller 130. This immediately 
causes control voltage generator 150 to generate a high control voltage. 
Then, when a cycle start switch is closed, power controller 130 applies 
power to transformer 132. This power is transformed by transformer 132 
into a higher voltage and is employed to charge capacitor 144. Eventually, 
the charge on capacitor 144 becomes sufficiently great to cause comparator 
152 to produce a favorable comparison. As a result, trigger circuit 154 
triggers SCR 146 to cause current to flow from capacitor 144 through SCR 
146 and coil 128. This causes a field to be produced in housing 126. 
Eventually, the charge in capacitor 144 will become depleted. At this 
point, the energy stored in coil 128 will cause capacitor 144 to become 
charged with an opposite polarity. 
Thus, as illustrated in FIG. 4, as SCR 146 begins to conduct, current 
through both SCR 146 and coil 128 increases until the energy stored in 
capacitor 144 has been dissipated. FIG. 5 illustrates the voltage across 
capacitor 144. Note that it begins at a relatively high level and 
decreases to zero. Then, the energy stored in coil 128 causes the current 
to continue to flow in the same direction through capacitor 144 and SCR 
146, although in a diminishing amount. This causes a voltage to develop a 
cross capacitor 144 of reverse polarity as illustrated in FIG. 5. At some 
point, the current through coil 128 and SCR 146 reaches zero. At this 
time, SCR 146 stops conducting. However, a voltage across capacitor 144 
exists having a polarity opposite to the polarity of the voltage across 
capacitor 144 just prior to triggering. This voltage causes a current flow 
through coil 128 and diode 148. As can be seen in FIG. 4, this current 
gradually increases and then decreases. As illustrated in FIG. 5, at some 
point, the energy stored in capacitor 144 reaches zero. Nevertheless, 
current continues to flow through coil 128 and diode 148 as a result of 
the energy stored in coil 128. This causes a voltage to develop across 
capacitor 144 having a polarity similar to the polarity of the voltage 
across capacitor 144 just prior to triggering. Eventually, all of the 
energy stored in coil 128 is exhausted, and the current flow stops. 
Current flowing through resistor 142 then gradually continues to increase 
the charge across capacitor 144 until it reaches a level comparable to the 
control voltage so that trigger circuit 154 causes SCR 146 to become 
conductive again. 
Signals produced by comparator 152 are fed back on line 156 to control 
voltage generator 150. In the preferred embodiment, this causes the 
control voltage to decrease by a fixed percentage. 
During the degaussing cycle, motor speed controller 158 causes motor 118 to 
turn reel 116. As a result of this mechanical turning, all portions of the 
tape experience magnetic fields of reversing polarity so that degaussing 
can occur. When current is flowing through diode 148, a magnetic field of 
opposite polarity is generated. However, the strength of this field is 
generally not sufficient as compared to the strength of the field created 
when current flows through SCR 146 to effectively accomplish degaussing. 
Therefore, the rotation of reel 116 improves the quality of degaussing and 
its uniformity. 
As the control voltage decreases, it obviously takes less and less time to 
charge capacitor 144 to a level comparable with the control signal. 
Therefore, SCR 146 becomes triggered more and more frequently. Therefore, 
in order to approximately maintain the same number of SCR 146 triggerings 
per rotation of tape reel 116, it is necessary to cause tape reel 116 to 
turn more rapidly. Accordingly, the signals from comparator 152 are 
applied to motor speed control 158 so that the speed of motor 118 
increases as the frequency of the signals from comparator 152 increases. 
FIGS. 6 and 7 are a detailed circuit diagram of an embodiment of the 
present invention similar to that described above with respect to FIG. 3 
with some additional features. In FIGURE 6, line power is applied to high 
voltage transformer 132 through main switches 160, fuse 162 and relay 
contacts 164. Power is also directed to low voltage transformer 166 
through fuse 168. The secondary coil transformer 166 is connected to 
rectifier 169, including capacitor 170 which provides some smoothing AC 
waveform. The output of rectifier 169 is provided to regulator 172 which 
produces a constant 12.5 volts. This voltage is applied to contact 171 of 
power controller 135. Connected in parallel with relay contact 166 is 
pushbutton start switch 173. Relay contact 171 and pushbutton start switch 
173 are connected to interlock switch 175 which closes only when drawer 
100 is closed. The other terminal of interlock switch 175 is connected to 
coil 177 of a relay. It is coil 177 that operates contacts 164, 171 and 
174. Contacts 164 and 171 are normally opened and close when coil 177 is 
energized, while contact 174 is normally closed and is opened when coil 
177 is energized. Transistor 176 controls the current flowing through coil 
177. Diode 178 is provided to shunt any voltage spikes produced by coil 
177 to the 12.5 volt source when transistor 176 shuts off. The conductance 
of transistor 176 is controlled by operational amplifier 180 which has an 
inverting input supplied with a reference voltage V.sub.REF2 and a 
noninverting input supplied with the control voltage from control voltage 
generator 150. 
Connected to high voltage transformer 132 are resistors 134 and 142, 
capacitors 136 and 144, diodes 138, 140 and 148 and SCR 146. The only 
difference between FIG. 3 and FIG. 6 is that in FIG. 6, main storage 
capacitor 144 is actually made up of three capacitors connected in 
parallel in order to increase the amount of energy that may be stored 
therein at a given voltage. Connected in series with SCR 146 is inductor 
182. When SCR 146 begins to conduct, inductor 182 slows the rate at which 
current begins to flow through SCR 146, thus reducing the possibility of 
destroying SCR 146. Connected in parallel with SCR 146 is a network 
consisting of resistors 184, 186 and 188, capacitor 190 and diodes 192 and 
194. This network slows the rate at which the voltage across SCR 146 
increases when SCR 146 turns off. Again, this network helps prevent 
erroneous triggering of and possible damage to SCR 146. 
Control voltage generator 150 includes diode 196 and resistor 198. 
Connected between resistor 198 and ground is control voltage capacitor 
200. Before coil 177 is energized, contact 174 is closed, causing current 
to flow from regulator 172 through diode 196 and resistor 198 to charge 
control voltage capacitor 200 to a high level. The voltage on capacitor 
200 passes through buffer 202 to comparator 204 of comparator circuit 152. 
The voltage level on main storage capacitor 144 is applied to the 
inverting input of comparator 204. The output of comparator 204 can have 
one of two states. When the difference between the value of the input 
signals on its noninverting and inverting inputs is positive, the output 
of comparator 204 floats. When the difference is negative, the output of 
comparator 204 is connected to ground. 
Also included in comparator circuit 152 is comparator 206. The noninverting 
input of comparator 206 is supplied with a reference voltage V.sub.REF3. 
The inverting input of comparator 206 is also connected to main storage 
capacitor 144. Comparator 206 operates in the same manner as comparator 
204 in that its output either floats or is grounded. 
Trigger circuit 154 receives the output of comparators 204 and 206. Trigger 
circuit 154 generates a signal to turn on SCR 146 in response to a low 
signal from either comparator 204 or comparator 206. 
Operational amplifier 208 acts as a monostable multivibrator. When the 
output of either comparator 204 or 206 becomes low, it causes the output 
of operational amplifier 208 to become low. This low signal is fed back 
through resistor 210 to hold the input to trigger circuit 154 low 
independent of comparators 204 and 206. Since the output of operational 
amplifier 208 is low, capacitor 212 slowly discharges through diode 214. 
Eventually, the inverting input of operational amplifier 208 becomes lower 
than the noninverting input so that the output of operational amplifier 
208 floats. 
The output of comparator circuit 152 is provided to an input of monostable 
multivibrator or one shot 216 of control voltage generator 150. The output 
of one shot 216 is normally low, but becomes high for a fixed period of 
time in response to an input signal. The output of one shot 216 is 
provided to the inverting input of operational amplifier 218. The 
noninverting input of operational amplifier 218 is supplied with a 
reference voltage V.sub.REF4 which is between the high and low levels 
outputted by one shot 216. Operational amplifier 218 is similar to 
comparators 204 and 206 in that its output either floats or is connected 
to ground. Normally, the output of one shot 216 is less than reference 
voltage V.sub.REF4 so that the output of operational amplifier 218 floats. 
Therefore, control voltage capacitor 200 is not discharged. However, when 
a signal is applied to the input of one shot 216, its output becomes 
higher than reference voltage V.sub.REF4. As a result, the output of 
operational amplifier 218 is grounded so that capacitor 200 discharges 
through resistor 220. Thus, the control voltage applied to comparator 204 
decreases. 
Motor control circuit 158 is also responsive to the output of comparator 
circuit 152 in that the output is applied to monostable multivibrator or 
one shot 222. The output of one shot 222 is integrated by means of 
resistors 224 and 226 and capacitor 228. The output of this integrator is 
applied to buffer 230 whose output is applied to differential amplifier 
232. Capacitor 234 provides further smoothing of the signal. The output of 
differential amplifier 232 is applied to transistor 236 which drives motor 
118. 
Resistor 238 and diode 240 connect a terminal of contacts 274 with the 
inverting input of operational amplifier 232. 
In operation, main power switches 160 are closed, causing regulator 172 to 
produce 12.5 volts. Since relay coil 177 is deenergized, contacts 174 are 
closed so that the 12.5 volts from regulator 172 passes through diode 196 
and resistor 198 to charge control voltage capacitor 200. The energy which 
passes through contacts 174 also passes through resistor 238 and diode 240 
to be applied to the inverting input of operational amplifier 232 to 
insure that motor 118 remains off. 
Then, a reel of tape is placed on spindle 110 and drawer 100 is closed, 
closing interlock switch 175. Start switch 173 may then be pressed which 
provides a voltage at one terminal of relay coil 177. The voltage that has 
charged capacitor 200 passes through buffer 220 and is applied to 
operational amplifier 180 to cause its output to become high so that 
transistor 176 is energized. As a result current flows through relay coil 
177, closing contacts 164 and 171 and opening contacts 174. As a result, 
capacitors 144 begin to charge. At the same time, the closing of contacts 
171 guarantees that current will continue to flow through relay coil 177. 
Eventually, sufficient energy has been stored in capacitors 144 so that the 
voltage on the inverting input of comparator 206 is greater than 
V.sub.REF3. As a result, the output of comparator 206 becomes low, 
triggering SCR 146. The output of comparator 206 is also fed back to one 
shot 216 which produces a timed pulse which causes the voltage across 
capacitor 200 to decrease by a fixed percentage. At the same time, the 
output of comparator 206 also triggers one shot 222 to produce a signal 
which places a small charge on capacitor 228. As a result, motor 118 
begins to turn slowly. 
After SCR 146 and diode 148 stop conducting, energy from transformer 132 
again begins to charge capacitors 144. Eventually, this causes a favorable 
comparison in comparator 206, triggering SCR 146 again. This process 
continues, and as it does, the control voltage across capacitor 200 
gradually reduces and the charge on capacitor 228 gradually increases so 
that the motor increases in speed. 
Eventually, the charge on capacitor 200 becomes sufficiently small so that 
the voltage applied to the noninverting input of comparator 204 is less 
than V.sub.REF3. When this occurs, the next time capacitors 144 are 
charged to a sufficiently high level, the output of comparator 204 becomes 
low, triggering SCR 146 and one shots 116 and 222. From this point in the 
cycle on, each time one shot 216 is triggered, the charge on control 
voltage capacitor 200 decreases, so that the amount of time necessary to 
charge capacitors 144 to the appropriate level decreases. At the same 
time, the triggering of one shot 222 charges capacitor 228 so that the 
speed of motor 118 increases. Thus, as the cycle continues, the rate of 
discharging of capacitors 144 increases as the speed of rotation of the 
reel of tape increases. This is to keep approximately constant the number 
of discharges per tape revolution. 
Eventually, the charge on control voltage capacitor 200 is so low that 
voltage level V.sub.REF2 becomes greater than the output of buffer 202. As 
a result, the output of comparator 180 becomes low, turning off transistor 
176. As a result, current stops flowing in coil 177, opening contacts 164 
and 171 and closing contact 174. With contacts 174 closed, the voltage 
applied to the inverting input of operational amplifier 232 insures that 
motor 118 is no longer driven. This completes one degaussing cycle. 
Intuition would lead one to believe that the higher the voltage across 
capacitors 144 prior to the initial discharging in each degaussing cycle, 
the better the degaussing will be. In fact, this is generally the rule. 
However, it has been found that at certain times, when a degaussing cycle 
is started with a lower energy on capacitors 144, more effective 
degaussing occurs. It appears that this phenomenon occurs with tapes that 
have been written on twice and are about to be erased for the second time. 
As the tapes are erased more than two times, this phenomenon disappears. A 
problem arises in that new tapes are often mixed in with old tapes so that 
it is impossible to tell whether a new tape or an old tape is being 
degaussed. Accordingly, it is desirable to design a circuit which 
efficiently erases all tapes, no matter how many times they have been 
written on. 
FIG. 8 in conjunction with FIG. 7 illustrate a circuit for accomplishing 
this efficient erasure. Thus FIG. 7 is employed to illustrate a portion of 
the circuitry in two embodiments. Instead of beginning degaussing at a 
high energy plateau which gradually decreases, the circuit in FIGS. 7 and 
8 begin degaussing at an intermediate energy level which builds to a 
plateau and then drops to a very low value. The circuit in FIGS. 7 and 8 
is very similar to the circuit in FIGS. 6 and 7. In fact, the circuits are 
identical except for the control voltage circuit. Accordingly, similar 
elements are designated similarly and will not be described in detail 
again, except for the manner in which they interreact with the new control 
voltage generator. 
In FIG. 8, control voltage generator 250 includes comparator 252 which has 
a noninverting input connected to the output of buffer 202 (representing 
the control voltage stored on capacitor 200) and an inverting input 
connected to a reference voltage value V.sub.REF5. Comparator 252 is of 
the type similar to comparators 204 and 206 in that its output either 
floats or is connected to ground. Resistors 254 and 256 form a voltage 
divider connected to the output of comparator 252 which provides an input 
to the inverting terminal of operational amplifier 258. The noninverting 
input of operational amplifier 258 receives a voltage from contact 174 
through resistor 260 and diode 262. Connected between the output of 
operational amplifier 258 and its noninverting input is resistor 264. 
Operational amplifier 258 is of the same type as comparator 252 in that 
its output is either floating or connected to ground. 
The values of resistors 254, 256, 260 and 264 are chosen such that 
operational amplifier 258 operates as a bistable multivibrator or a 
flip-flop. Initially, the output of flip-flop 258 is high as set by the 
voltage from contact 274 which passes through resistor 260 and diode 262. 
Feedback through resistor 264 maintains the output of flip-flop 258 high 
even after contact 274 is opened. At some point in the operation of the 
circuitry as will be described in detail below, the output of comparator 
252 becomes high so that 12 volts are applied to the inverting input of 
flip-flop 258. This causes the output of flip-flop 258 to become low, and 
this low value is fed back through resistor 264 to the noninverting input 
of flip-flop 258. Since the inverting input of flip-flop 258 cannot go 
below a certain level determined by resistors 254 and 256, the output of 
flip-flop 258 remains low until reset by the closing of contact 274. 
The output of flip-flop 258 is employed to control the voltages at the 
nodes of a voltage divider network made up of resistors 266, 268 and 270 
connected in series between 12.5 volts in ground. Ramp-up driver 272 and 
ramp-down driver 274 each have a noninverting input connected between 
resistors 266 and 268. The inverting inputs of drivers 272 and 274 are 
respectively connected to the Q and Q outputs of monostable multivibrator 
or one shot 276. In response to an input signal, the Q output of one shot 
276 becomes high for a fixed period of time and the Q output becomes low 
for the same time. After that period of time, the Q output becomes low and 
the Q output becomes high. Operational amplifiers 272 and 274 are both of 
the same type as comparator 252 in that their output either floats or is 
grounded. 
The Q output of one shot 276 is connected to ramp-up driver 272 through a 
voltage divider network including resistors 278 and 280, which in the 
preferred embodiment, are of equal value. This permits the inverting input 
of driver 272 to assume the values of either six volts or 12 volts. The Q 
output of one shot 276 is connected to the inverting input of driver 274 
through a voltage divider network including resistors 282 and 284. In the 
preferred embodiment, resistors 282 and 284 are selected so as to cause 
the inverting input of driver 274 to assume either a zero volt or a six 
volts value. Resistors 266, 268 and 270 have values chosen in the 
preferred embodiment so that the voltage between resistors 266 and 268 can 
vary with the output of flip-flop 258 between either three volts or nine 
volts. 
Resistor 286 and diode 288 connect a terminal of contacts 174 with the 
inverting input of operational amplifier 274. 
In operation, switches 160 are closed so that regulator 172 produces 12.5 
volts. Since, at this point, relay coil 177 is not energized, relay 
contact 174 is closed and relay contacts 164 and 171 are open. As a 
result, the 12.5 volts flows through contact 174, resistor 260 and diode 
262 to set flip-flop 258 so that its output is high. As long as the output 
of flip-flop 258 is high, the voltage on capacitor 200 can only ramp 
upward once start switch 173 is closed. At the same time, 12.5 volts flows 
through contact 174, resistor 286 and diode 288 to cause the inverting 
input of ramp-down driver 274 to become high. This results in the output 
of driver 274 to be connected to ground. The 12.5 volts from contact 274 
also passes through diode 296 and 298 through resistor 220 and then to 
ground within driver 274. This causes a voltage to appear across control 
voltage capacitor 200 which represents an intermediate value. 
Once switch 173, is momentarily closed and the drawer is pushed in, closing 
switch 175, coil 177 is energized opening contact 174 and closing contacts 
164 and 171. Since contact 174 is opened, the inverting input of driver 
274 becomes low so that the output of driver 274 floats. Therefore, the 
intermediate voltage stored on capacitor 200 remains. 
At this time, a voltage begins to develop across capacitors 144. When 
comparator 204 determines that this voltage is greater than the reference 
voltage across control voltage capacitor 200, comparator 204 causes SCR 
146 to fire. The output of comparator 204 also triggers one shot 276 so 
that its Q output becomes high and its Q output becomes low. Since the 
output of flip-flop 258 is high, the voltage in the preferred embodiment 
between resistors 266 and 268 is at nine volts. The triggering of one shot 
276 causes the inverting input of driver 274 to change from zero volts to 
six volts so that the output of driver 274 does not change. On the other 
hand, the Q output of one shot 276 causes the voltage to the inverting 
terminal of ramp-up driver 272 to change from 12 volts to 6 volts. This 
causes the output of ramp-up driver 272 to float so that current may flow 
from resistor 290 through diode 292 to charge capacitor 200. As soon as 
one shot 276 returns to its normal state, the output of ramp-up driver 272 
becomes grounded so that current flowing through resistor 290 passes to 
ground, instead of through diode 292. 
In this manner, capacitors 144 are periodically discharged. Each time 
discharge occurs, one shot 276 is actuated, causing the output of ramp-up 
driver 272 to float so that the charge across capacitor 200 increases. 
As this process continues, eventually the output of buffer 202 becomes 
higher than reference value V.sub.REF3 so that comparator 206 changes 
state before comparator 204. As a result, for this period, the amount of 
energy discharged from capacitors 144 remains at a constant plateau level. 
Nevertheless, the voltage across capacitor 200, as reflected by the output 
of operational amplifier 202, continues to increase. Eventually, this 
voltage becomes higher than reference voltage V.sub.REF5 so that the 
output of comparator 252 becomes high. This places a voltage on the 
inverting input of flip-flop 258 greater than the voltage on the 
noninverting input so that the output of flip-flop 258 becomes low. 
Feedback through resistor 264 insures that the noninverting input of 
flip-flop 258 remains lower than the voltage at its inverting input. 
Therefore, the voltage of the point between resistors 268 and 270 is held 
at ground so that the voltage between resistors 266 and 268 is held at 
three volts in the preferred embodiment. 
The next time comparator circuit 152 produces an output signal, and one 
shot 276 is triggered, the Q output of one shot 276 causes the inverting 
input of ramp-up driver 272 to change from 12 volts to six volts. Thus, 
the output of driver 272 does not change. At the same time, the Q output 
of one shot 276 causes the inverting input of ramp-down driver 274 to 
change from zero volts to six volts. As a result, the output of ramp-down 
driver 274 changes from floating to being grounded. Therefore, energy 
stored in capacitor 200 can flow through resistor 220. As a result, the 
voltage across capacitor 200 decreases. Thus, with each triggering of SCR 
146, the voltage across capacitor 200 decreases by a fixed percentage. 
Eventually, the voltage on the output of buffer 220 drops below reference 
voltage V.sub.REF3 so that comparator 204 changes state before comparator 
206 when the voltage across capacitors 144 reaches a sufficiently high 
level. Thereafter, the voltage across capacitor 144 prior to each 
triggering of SCR 146 gradually decreases until the cycle ends. 
In the embodiments illustrated in FIGS. 3 and 6-8, the voltage polarity 
across capacitors 144 reverses. Thus, as illustrated in FIG. 5, the 
voltage is initially positive, then swings to a negative value before 
returning to a positive value. As a result, it is impossible to employ 
inexpensive electrolytic capacitors as capacitors 144. This possible 
problem is eliminated with the circuitry illustrated in FIG. 9. Note that 
the circuit in FIG. 9 is just the circuit for charging and discharging the 
main storage capacitor. Circuitry for triggering the SCR may be as in the 
previous Figures. 
In FIG. 9, high voltage transformer 132 is again connected to a voltage 
doubler including resistor 134, capacitor 136 and diodes 138 and 140. Main 
storage capacitor 290 is connected between the cathode of diode 140 and 
the anode of diode 138. Thus, the voltage doubler generates a voltage 
which gradually charges capacitor 290. Connected in parallel with 
capacitor 290 is a series circuit consisting of coil 128 and SCR 146. 
Connected parallel to coil 128 is diode 292 which has a polarity such that 
diode 292 does not initially conduct when current flows through coil 128. 
In operation, as soon as SCR 146 is triggered, energy flows from capacitor 
290 through coil 128 and SCR 146. Eventually, the energy in capacitor 290 
is depleted so that the voltage across capacitor 290 is zero. At this 
point, SCR 146 stops conducting. Nevertheless, energy is stored in coil 
128 which causes voltage to flow from coil 128 through diode 292. This 
flow continues until the energy is absorbed as losses in the system. 
The advantage of this arrangement, as described above, is that the voltage 
on capacitor 290 never reverses polarity. Therefore, capacitor 290 may be 
an electrolytic capacitor. The drawback is that capacitor 290 is not 
recharged in the last portion of a coil excitation cycle. As illustrated 
in FIG. 5 with respect to the embodiments illustrated in FIGS. 3 and 6-8, 
although the voltage across capacitor 144 becomes negative, the voltage 
eventually returns to a positive value as energy stored in coil 128 is 
reapplied to capacitor 144. Therefore, for the next degaussing cycle, it 
is only necessary to raise the voltage a relatively small amount. However, 
with the embodiment illustrated in FIG. 9, the voltage across capacitor 
290 is reduced to zero. Therefore, it is necessary to charge the voltage 
all the way up to the desired value. This necessarily extends the time 
period of a degaussing cycle. 
In the mechanical arrangement illustrated in FIGS. 1 and 2, the bottom 
portion of housing 126 must be deformed downwardly in order to provide for 
pulley 112, bracket 108 and shaft 110. This must necessarily decrease the 
strength of the field within the housing in relation to the current 
flowing through coil 128 and affect the uniformity of the field. FIG. 10 
illustrates a mechanical embodiment of the present invention in which the 
housing may have a perfectly rectangular cross section. In FIG. 10, drawer 
300 includes slide sections 302, 304 and 306. Slide portion 306 is fixed. 
Intermediate slide portion 304 telescopes within outer slide section 306 
and inner slide section 302 telescopes within intermediate slide section 
304. In this embodiment, coil housing section 308 is fixed to outer slide 
section 306 by means of coil supports 310. Housing portion 312 is attached 
to inner slide section 302 by means of coil supports 314. Bearing housing 
316 and motor 318 are mounted to intermediate slide section 304. Shaft 320 
is rotatably mounted in bearing housing 316. Reel support 322 forms a 
platform on which tape reel 116 may be positioned. Attached to shaft 320 
is pulley 324. Pulley 326 is attached to shaft 328 of motor 318. Belt 330 
operatively connects pulleys 324 and 326. Coil 332 is wrapped around 
housing sections 308 and 312. 
In this embodiment, drawer 300 is pulled all the way out, separating the 
three slide sections. Tape reel 116 is then mounted on platform 322. As 
drawer 300 is closed, tape reel 116 moves within housing portion 308 and 
housing portion 312 encloses the other half of reel 116. When drawer 300 
is pushed all the way in, tape reel 116 is completely enclosed within 
housing portions 308 and 312. Space must be left at the bottom of housing 
portions 308 and 312 to permit shaft 320 to rotate freely. 
Obviously, housing 126 in the first mechanical embodiment or housing 
portions 308 and 312 in the second mechanical embodiment must be made 
sufficiently large to hold the largest reel which may be desired to be 
degaussed. However, smaller reels may also be degaussed. In this 
situtation, portions within the housing may remain empty. FIG. 11 
illustrates a technique for concentrating the magnetic field in the area 
occupied by the tape. Thus, blocks 334 may be inserted at the edges of the 
housing. Blocks 334 are electrically conductive and nonferromagnetic. In 
the preferred embodiment, they are made of aluminum. Blocks 334 help 
concentrate the magnetic field within that portion of the housing occupied 
by the tape. 
Although only a few exemplary embodiments of this invention have been 
described in detail above, those skilled in the art will readily 
appreciate that many modifications are possible in the exemplary 
embodiments without materially departing from the novel teachings and 
advantages of this invention. For example, the embodiments described above 
employ a feedback triggering system for the SCR in which the voltage 
across the main storage capacitors is monitored and compared with a 
changing reference voltage. Instead, those skilled in the art will readily 
appreciate that the SCR may be triggered by a timer which produces trigger 
signals with increasing frequency without the use of feedback. Similarly, 
although the exemplary embodiments of this invention have all related to 
the degaussing of magnetic tape, the present invention can be employed to 
degauss any magnetized material. 
Accordingly, all such modifications are intended to be included within the 
scope of this invention as defined in the following claims.