GB2026287A

GB2026287A - Method and apparatus for recording data on a carrier, in particular a magnetic carrier, irrespective of the speed of the carrier

Info

Publication number: GB2026287A
Application number: GB7915854A
Authority: GB
Original assignee: Bull SA
Current assignee: Bull SA
Priority date: 1978-06-13
Filing date: 1979-05-08
Publication date: 1980-01-30
Also published as: IT1121283B; GB2026287B; FR2428874A1; JPS54163016A; IT7923519A0; DE2923973A1; FR2428874B1

Abstract

Magneto resistors 14, 16 are positioned a distance I and L from a magnetic write lead 10 which is operated to record a "1" or "0" value of carrier 20 when a transistor is detected by the first or second magneto resistor respectively, then reducing errors due to speed fluctuations of the carrier. <IMAGE>

Description

SPECIFICATION Method and apparatus for recording data on a carrier, in particular a magnetic carrier, irrespective of the speed of the carrier The present invention relates to a method and apparatus for recording data on a carrier, in particular a magnetic carrier. It may for example be applied to the writing of coded information on magnetic tapes which can be used in data processing, or on carriers such as bank or post-office cheques, credit cards, control cards, etc.

It is known to be possible to record data on a carrier by causing in the carrier a stable change in one of its physical properties. The data so recorded is then recovered by some suitable means which enable the said change in the carrier to be detected.

Means suitable for bringing about this change in the physical properties of the carrier or for detecting such changes fall into the general category of transducers to the extent that they convert one physical magnitude into another physical magnitude, one of these generally being an electrical signal.

The physical property which is used to record an item of data may be of any kind. It may be an electrical property (a quantity of charges for example) or an optical property (refractive index, anisotropy, etc.) or again a mechanical property (surface state) etc., but in the majority of cases it is a magnetic property, based for example on the presence or absence of a magnetic induction or again on the direction of a magnetic induction.

Although the following description concentrates on methods of magnetic recording, this is done merely by way of non-limiting example and the invention relates in more general terms to any method of recording data on any kind of carrier which involves magnitudes of any kind.

In the following description, the term "signal" will be used to designate the alteration brought about in one of the physical properties of the carrier. In the case of recording based on a magnetic property of the carrier, the signals involved will thus be "magnetic signals", the recorded data thus being represented by a particular distribution of these signals.

It is for example known to be possible to record data on a magnetic carrier by applying to the carrier a magnetic field whose intensity and direction are such that the sequence of magnetic signals which are left in the carrier after the field has been applied will represent the said data translated into a certain code.

There are codes in which the data is contained in the gap between two magnetic signals. It is codes of this kind which are used in the present invention.

Their principle may be briefly summarised by taking as an example the co-calied "dual frequency" code or Aiken code.

To record binary data using this code, a magnetic field in one or other of two opposing directions is applied to a magnetic layer. The magnetic signal proper is formed by the transition between two zones having magnetic inductions of opposite senses. These transition signals are spaced apart either by a length L in cases where a 0 is recorded or by a length 1 in cases where a 1 is recorded.

Figure 1 illustrates the principle of a code of this nature in a case where 1 equals L/2. A magnetic layer C has a magnetic induction M which is directed in one or other of two opposing directions. The layer may be considered as formed by a succession of cells of the same length L = 21, each of these cells containing one item of information, either a 1 or a 0.

In any one of these cells, the induction is either always of the same sense or else reverses once. The agreed convention is that in the first case the item of data recorded is a 0 and in the second case a 1.

At the top, Figure 1 shows a sequence of items of binary data 0, 0, 1, 0. In the centre it shows the sequence of directions of the magnetic induction in four memory cells, and at the bottom it shows the variations in magnetic induction along the layer.

To record data using this code, a carrier coated with a suitable magnetic layer is passed in front of a writing transducer (or "write head") and the transducer is excited by means of a current whose direction is controlled as a function of the value of the item of data to be recorded.

At the time of reading, the reverse operation is performed. The layer is passed in front of another transducer (a read head) which is able to detect the changes in the direction of the magnetic induction in the layer and the values of the recorded items of data are derived from these.

An article on this subject which may be consulted is by G. E. Moore and L. J. Cote entitled "Dual Stripe Magneto-resistive Read Heads for Speed Insensitive Tape Readers" which was published in the journal "IEEE Translations on Magnetics", vol. Mag. 12, No.

6 of November 1976, may be consulted.

The major drawback of this method of recording is that it requires the speed of movement of the magnetic carrier past the write head to be absolutely constant. In effect, the reversal in the direction of the current in the head must take place at closely defined times since the reversal in the direction of the induction in the layer must appear at the end or in the middle of a cell. Fluctuations in the speed of movement of the carrier result in fluctuations in the interval separating two transitions, and this may lead to errors in recording.

The object of the present invention is precisely a method and an apparatus for recording data which do not suffer from this drawback because they are not dependent on the speed of movement of the carrier past the write head. This object is achieved in accordance with the invention, in the particular case of magnetic recording, by virtue of the fact that the reversal of the direction of the current, and thus of the magnetic induction, is brought about at the time when the previous reversal already written in the layer reaches a distance from the write head equal to either L or to 1, so that the zone situated perpendicularly in front of the write head at this moment is indeed the zone in which a fresh reversal is to be imprinted, no matter what the speed of movement of the carrier.

In a sense, the write orders are given as a function of the distance travelled by the carrier and not pre determined times with the speed of the carrier assumed to be constant.

To be more exact, the invention has as an object a method of recording data on a carrier in which the carrier is made to pass before a write transducer and a series of signals is written on the carrier, the items of data being represented by the length of the intervals separating the said signals, this method being characterised in that, in order to record a fresh item of data, there is detected the moment at which the last signal written on the carrier arrives at a distance from the write transducer equal to the interval corresponding to the said fresh item of data to be recorded and a fresh signal is written at this moment.

The signals used are preferably of a magnetic nature and are recorded on a carrier which is also magnetic. The signals may consist of a reversal in the sense of a magnetic induction.

The invention is applicable in all cases where an item of data is contained in the space between the signals written on a carrier but it finds a preferred application in cases where the dual frequency code defined above is used. In this case, the moment when the last reversal written on the carrier reaches a distance L or L/2 from the write head, depending upon whether it is desired to record a 0 or a 1, is detected and a fresh reversal is brought about on the carrier at this moment.

The invention also has an object an apparatus for recording data on a carrier which puts the method defined above into practice and which is of the kind which comprises means to pass the carrier before a write transducer suitable for writing a series of signals on the carrier, the items of data being represented by the length of the intervals separating the said signals, this apparatus characterised in that it also includes a second transducer capable of detecting the moment when the last signal written arrives at a distance from the write transducer equal to the interval corresponding to a fresh item of data to be recorded, and a circuit connected to the said second transducer which is able to bring the write trans duner into operation atthis moment.

The carrier used is preferably of a magnetic nature and the write transducer is preferably a write head which is capable of writing magnetic signals on the carrier and the second transducer a means of reading the magnetic signals. The magnetic signals may comprise reversals of magnetic induction.

In the advantageous case where the code used is a dual frequency code, the apparatus is characterised in that the detection means is capable of determining the moment when the last reversal written arrives at a distance L or L12 from the write head, depending upon whether the item of data to be recorded is a O or a 1.

Although any means capable of detecting the passage of a reversal of the magnetic induction of a magnetic layer may be suitable, the preference is for using magnetoresistors, which are highly suitable for this application. It is known that this is the name used for electrical resistors whose values depend on the magnetic field which is applied to them. Mag netoresistors of this kind are formed by thin films from a few hundred angstroms to a few microns thick which are deposited on an insulating substrate.

Such magnetoresistors may be fed by a current generator. Any variation in magnetic field then causes a variation in the voltage which appears at their terminals and this variation in voltage is very easy to measure and make use of.

The features and advantages of the present invention will in any case be better apparent from the following description of embodiments, which are given by way of non-limiting example. The description refers to the accompanying drawings, in which: Figure 1 illustrates the distribution of the zones of magnetic induction on a recording tape in the case of the so-called "dual frequency" code, Figure 2 is a general diagram of the apparatus of the invention, Figure 3 shows a first embodiment of the means of the invention, in which two magnetoresistors are positioned at distances from the write head equal to L/2 and L respectively, Figure 4 illustrates the principle of operation of the apparatus in the previous Figure, Figure 5 illustrates a second embodiment of the means of the invention in which two magnetoresistors are positioned at distances from the write head equal to L and L +L/2 respectively, Figure 6 illustrates the principle of operation of the apparatus of the previous Figure, Figure7 is a diagram against time of the various signals which may be used when writing or reading, Figure 8 is a block diagram of the read circuit.

Although the invention is applicable to any method of recording data based on any of the properties of a carrier, it will be assumed in what follows, by way of illustration, that the means relied upon are of a magnetic nature. Similarly, although the invention may be applied to codes other than the so-called "dual frequency" code, this is the case which will be assumed to apply in the following description, once again purely by way of illustration. The general principles of this code have already been described with reference to Figure 1.

A block diagram of the apparatus according to the invention is shown in Figure 2. The apparatus comprises a carrier S, a means D causing the carrier to move before a first write transducer T, and before a second read transducer T2 and a circuit C for controlling the write transducer T, which receives the data to be recorded at an input E and which is connected to the read transducer T2.

The way in which this circuit operates is as follows. Transducer T2 detects the moment at which the last signal written on the carrier S by transducer T, arrives at a distance from the transducer T, equal to the interval corresponding to the item of data to be recorded, which is applied to input E. Circuit C then actuates transducer T, at this moment so that a fresh item of data to be recorded.

The following Figures refer more particularly to the case where reading and writing are performed by magnetic means.

The apparatus which is shown in Figure 3 comprises: a magnetic write head 10 which defines an air-gap 12, two magnetoresistors 14 and 16 which are deposited on a substrate 18 and which are separated from one another by a distance 1 equals L/2, the first resistor being positioned at a distance L/2 from the air gap 12. A magnetic carrier 20 on which data is recorded can pass in front of the air gap 12 and the two magnetoresistors 14 and 16.

The operation of this apparatus is illustrated in detail by Figure 4, which shows the various positions assumed by the magnetic carrier at different times.

When the carrier commences to pass under the write head, the latter is first of all excited by a constant current such that the layer 20 will be saturated.

The magnetic induction M in the layer is thus directed in a certain direction.

At time t1, the right hand end of the carrier reaches a predetermined position and a mechanical or optical means (not shown) orders a first reversal in the induction of the layer. A first transition is thus imprinted between two zones whose magnetic inductions are of opposite senses. This transition is marked 1.

The carrier continues to pass under the write head.

At time t2 transition 1 comes perpendicularly opposite the first magnetoresistor 14. The resulting change in voltage at the terminals of the magnetoresistor enables this time t2 to be detected. If a cell of length 1 is to be written on the carrier, it is at this time that it is necessary to bring about a fresh reversal in the direction of the current in the write head in order to cause a second reversal 2 in the sense of the magnetic induction.

At time t3, transition 1 comes perpendicularly opposite magnetoresistor 16 and transition 2 perpendicularly opposite magnetoresistor 14. In the case illustrated, it is assumed that a 0 is to be imprinted, which is equivalent to a cell of length L with no reversal in the sense of the induction. At time t3 the direction of the current flowing in the write head thus remains the same as it was after time t2 and the sense of the induction of the layer also remains the same as it was after time t2.

At time t4, transition 2 comes perpendicularly opposite magnetoresistor 16. This time marks the passage under air-gap 12 of the end of the cell which begins with transition 2 and whose length is L. It is therefore at this moment that it is necessary to reverse the current in the write head to produce a fresh transition 3 in the magnetic carrier. The cell of length L = 21 between transitions 2 and 3 does not contain a transition and thus does indeed represent a O.

Another 0 may be recorded by reversing the induction only at the moment to when transition 3 passes under magnetoresistor 16.

A 1 may then be recorded by reversing the induction at the time to when transition 4 reaches magnetoresistor 14. The cell of length L corresponding to this 1 ends with a transition 6, and so on.

The embodiment illustrated by Figures 3 and 4 is not the only possible embodiment. Other embodiments may be conceived, given only that they enable the moments to be detected when the last transition written reaches a distance from the air gap equal to L or L/2, this last transition not necessarily being that which comes perpendicularly opposite the magnetoresistors but possibly the penultimate transition written. This is particularly the case with the embodiment illustrated in Figure Sin which the first magnetoresistor 14 is a length L away from the air gap 12, the distance between the magnetoresistors being once again equal to L12 as in the embodiment of Figure 3.

The operation of the apparatus in Figure 5 is sub stantially the same as that of the apparatus in Figure 3. It is illustrated in Figure 6, which shows the state of the carrier at four different times. It will however be noted that this embodiment calls for the first cell imprinted between times t, and t2 to be a cell of length L. Acell of length L/2 can only be written after time t2. It is this which is illustrated at t3 and t.

This is only an apparent restriction since the majority of carriers of magnetic layers in the form of cards, books, etc. have to have three long cells (i.e.

three 0's) before the data sequence proper.

The two embodiments illustrated which employ two magnetoresistors are perfectly suitable when the data density is high, that is to say when the lengths 1 and L of the cells are small. This is particularly the case with the standard lengths 1 = 165cue and L=330,uor1 =60,uandL= 120,u.

There is no special problem in producing the magnetoresistive members employed in the invention. Use may possibly be made of an additional member termed a compensating element as described in a patent application No. 13389178 entitled "Magnetic Transducer Device for Detecting Coded Magnetic Information and Method of Producing the said Device" which was filed by the present applicants on 5.4.78. Use may also be made of polarisation as described in a patent application No.

14413/78 entitled "Device for Reading Magnetic Information" which was filed by the present applicants on 12.4.78.

It is clear that if it were desired to use codes involving three cells of different lengths (rather than two) it would merely be necessary to add a third magnetoresistor to the devices illustrated.

As to the write head, this may be either of the conventional kind having pole pieces and an energising coil, or of the integrated kind having thin magnetic layers and conductive layers.

It is equally clear that the device described with its two magnetoresistors may also be used as a reading means. In this case it is merely necessary to leave the head 10 unused and to detect the changes in resistance at the terminals of the two magnetoresistors, as described in the article mentioned above.

A difficulty may arise in putting the method of the invention into practice by reason of the proximity of the magnetoresistors to the write head. In effect, these items are generally situated in the leakage field of the write head and, at the points where the magneto-resistors are situated, the value of this field may be of the same order of magnitude as that of the field coming from the magnetic layer.

This problem may be solved by operating the write head not with a continuous current but with a series of current pulses and by measuring the voltage at the terminals of the magnetoresistors only during the intervals when the writing current is zero.

Figure 7 shows the form of the principal signals used with this version of the invention. Ata is shown a writing current 1E which remains of a certain positive or negative value for the entire duration of the writing periods. Atb is the writing current i', which is used in the invention, which is formed by a series of pulses. The period separating each pair of such pulses needs to be sufficiently short to ensure that the magnetic carrier travels only a short distance during this period, so that any possible fluctuations in the induction of the carrier will be outside the limits of resolution of the reading means. Atc is shown the leakage field Bf of the write head at the points where the magnetoresistors are situated and at d is shown the magnetic field Bs due to the magnetic carrier.

In accordance with the invention, the voltage Vat the terminals of the magnetoresistors is read only during the intervals of time when the writing current 'E is zero. It is this which is shown ate.

The voltage read is then integrated, and this gives a voltage Vj which is free of the interference effect due to the leakage field of the write head, as shown atf.

A possible circuit for putting this embodiment into practice is shown in Figure 8. It comprises a generator 22 which supplies current pulses ~E to the write head 10 as a function of the items of data applied to its input E, a magnetoresistor 24 associated with a current generator 26, a logic circuit 28 which receives at its input the voltage V taken from the terminals of magnetoresistor 24 and which emits this voltage from its output only when the writing current ~E is zero (this circuit may be represented diagrammatically by a NOT gate 29 followed by an AND gate 30), and an integrating circuit formed by a capacitor 32 and a resistor 34 this circuit being followed by an amplifier 36 whose output is connected to a shaping circuit 38 which controls the actuation of generator 22.

If the value of resistor 34 is called Rc, the value of capacitor 32 C, and the mean value of magnetoresistor 24 Rj, the components mentioned are selected in such a way that the time constant RCC will be long compared with the time constant RIC, thus maintaining the integrated voltage substantially constant between two measurement pulses.

Claims

1. Method of recording data on a carrier in which the carrier can pass before a write transducer and a series of signals is written on the carrier, the items of data being represented by the length of the intervals separating the said signals, characterised in that in order to record a fresh item of data, there is detected the moment at which the last signal written on the carrier arrives at a distance from the write transducer equal to the interval corresponding to the said fresh item of data, and in that a fresh signal is written on the carrier at this moment.

2. Method according to claim 1, characterised in that a magnetic carrier is used and in that magnetic signals are written on this carrier by means of a magnetic write head.

3. Method according to claim 2, in which the data to be recorded may be of two values, 0 and 1, and in which each magnetic signal written consists in a reversal in the sense of the magnetic induction of the carrier, a 0 item of data corresponding to the absence of a reversal of induction in an interval of length L, two consecutive reversals then being separated by a length L, and a 1 item of data corresponding to the presence of a reversal of induction in the said interval of length L, two consecutive reversals then being separated by a length L/2, characterised in that there is detected the moment when the last reversal written on the carrier arrives at a distance L or L/2 from the write head, depending upon whether a 0 or a 1 is to be recorded, and in that a fresh reversal is written on the carrier at this moment.

4. Method according to claim 3, characterised in that, after passing before of the write head, the carrier passes before two magnetoresistors spaced from one another by a length L/2, one of the two magnetoresistors being separated from the write head by a length L, and in that the variations in the resistance of these magnetoresistors are detected.

5. Method according to claim 4, characterised in that the write head is fed with a series of current pulses, and in that the variations in the resistance of the magnetoresistors are measured during the intervals of time separating the said pulses.

6. Apparatus for recording data on a carrier which puts into practice the method according to the claim 1, of the kind which comprises means for passing the carrier before a write transducer capable of writing a series of signals on the carrier, the data being represented by the length of the intervals separating the said signals, characterised in that it also includes a second transducer capable of detecting the moment at which the last signal written arrives at a distance from the write transducer equal to the interval corresponding to a fresh item of data to be recorded, and a circuit connected to the said second transducer which is capable of operating the write transducer at this moment.

7. Apparatus according to claim 6, characterised in that the carrier is magnetic, the write transducer is a magnetic write head capable of writing magnetic signals on the carrier, and the second transducer is a means of reading magnetic signals.

8. Apparatus according to claim 7, in which the data to be recorded may assume two values, 0 and 1, and in which each magnetic signal written consists of a reversal in the sense of the magnetic induction of the carrier, a 0 item of data corresponding to the absence of a reversal of induction in an interval L, two consecutive reversals then being separated by a length L, and a 1 item of data corresponding to the presence of a reversal of induction in an interval of length L, two consecutive reversals then being separated by a length L/2, characterised in that the second transducer is capable of determining the moment at which the last reversal written arrives at a distance L or L12 from the write head depending upon whether the item of data to be recorded is a 0 or a 1.

9. Apparatus according to claim 8, characterised in that the said second transducer has two magnetoresistors before which the carrier passes, these magnetoresistors being separated from one another by a distance L/2, one of the two magnetoresistors being a length L away from the write head, and in that the two magnetoresistors are connected to a circuit for detecting variations in their resistance.

10. Apparatus according to claim 9, characterised in that it includes a generator for generating current pulses which is connected to the write head and, for each magnetoresistor, a logic circuit having two inputs of which one is connected to the said generator and the other to the said magnetoresistor, and an integrating circuit arranged at the output of the logic circuit.

11. A method of recording data on a carrier substantially as hereinbefore described with reference to the accompanying drawings.

12. Apparatus for recording data on a carrier substantially as hereinbefore described with reference to the accompanying drawings.