Error rate drift caused by rubbing noise in ferrite read heads

Tribology International 33 (2000) 401–408www.elsevier.com/locate/triboint

Error rate drift caused by rubbing noise in ferrite read heads

Simon Chandler*

Hewlett-Packard Ltd, Computer Peripherals Bristol, Filton Road, Stoke Gifford, Bristol BS12 6QZ, UK

Abstract

Some combinations of DDS-3 tape drive and cassette experience a rapid increase in error rate, followed by a gradual spontaneousrecovery, when a section of tape is repeatedly used. This behaviour is caused by a change in the level of rubbing noise resultingfrom changes to the surface of the read head. This paper describes an investigation into the nature of this noise, and in particularthe effect of head-tape speed on its frequency spectrum. In some cases, standing-wave vibrations in the head structure are excitedby periodic ripple features that form on the surface of the ferrite read heads. 2000 Elsevier Science Ltd. All rights reserved.

Keywords:Magnetic recording; Tape; Friction; Noise

1. Introduction

As part of an ongoing quality assurance program,Hewlett-Packard’s Digital Data Storage (DDS) helical-scan tape drives are subjected to rigorous tests. In mid-1998 some DDS-3 drives were found to experience aprogressive increase in their uncorrected error rate whensubjected to a particular test, known as “maggot”. Agradually worsening error rate of this kind is known col-loquially as “error rate drift”. Although the DDS for-mat’s error correction protects the end user from beingaffected by this degradation, HP wanted to understandthe phenomenon in order to eliminate it, and so reduceits impact on the drive’s margin to other impairments.

For error rate drift to occur two things are needed:

1. An impairment that causes the error rate to increase.2. A mechanism that systematically increases the level

of this impairment.

Error rate drift had previously been observed when HP’sDDS-2 tape drives (an earlier product) were operated inlow humidity conditions [1]. In that case the impairmentwas identified as a layer of contamination that trans-ferred from the tape to the surface of the read heads,whilst the mechanism was identified as the reduction in

* Present address: Agilent Technologies UK Ltd, White HouseRoad, Ipswich, Suffolk IP1 5PB, UK. Tel.:+44 1473 465 943; fax:+44 1473 465 438.

E-mail address:[email protected] (S. Chandler).

0301-679X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved.PII: S0301-679X(00 )00060-8

abrasivity of the tape each time it interacted with theheads. This caused the “stain” thickness to increase eachtime a section of tape was reused, resulting in a progress-ive increase in spacing loss, and an attenuation of thereadback signal amplitude. Was the error rate drift seenin the maggot test caused by this staining mechanism?

2. Initial observations

The maggot test performs a complicated sequence oftape movements, in the process of which some sectionsof the DDS-3 metal particulate (MP) tape are reusedmany times. As a first step in our investigation a greatlysimplified test was devised in which the drive wrote 768frames (|12 seconds) of random data to a section ofpreviously unused tape, and the read-after-write (RAW)error rate was measured. Using the drive’s tape counteras a guide, the tape was then rewound to the startingpoint, and the same section of tape was overwritten. Thiswrite-rewind process was repeated 200 times over thesame section of tape, producing the first region of driftshown in Fig. 1.

After 200 reuse passes the tape was moved forwardsat the normal recording speed for 5 minutes, so thatunused tape passed over the write and read heads. Thisresulted in a gradual fall in the error rate, until it returnedto its initial level. The recovery is shown in detail inFig. 2, but is simply represented by a vertical line in Fig.1 (thex-axis of which is not time, but pass number). Thegradual recovery probably results from abrasive wear of

402 S. Chandler / Tribology International 33 (2000) 401–408

Fig. 1. “Error rate drift” in the uncorrected read-after-write error rate(solid), and read error rate (open), normalised by the first measuredvalue (2.1e-3). The sequence of operations yielding this behaviour isdescribed in the text.

Fig. 2. Recovery from error rate drift by pulling unused tape for thetime shown. Points show the uncorrected read-after-write error ratenormalised by 2.1e-3. Dashed line shows error rate before recovery.Solid line showsy=7.6/3√t.

the head by the rough unused tape, and indicates thatthe underlying impairment is associated with a changeto the surface of the head, rather than the tape.

The second region of drift in Fig. 1 starts on unusedtape. During the first pass over this, data was writtenand the RAW error rate was measured. In all subsequentpasses, however, no writing took place, but the data wasread and the read error rate (RER) was measured. Thisproduced almost exactly the same drift behaviour as seenin the first drift (RAW), indicating that the drift is domi-nated by an impairment to the reading process.

On completing the second drift, the error rate wasagain lowered by pulling unused tape for 5 minutes. The

third region of drift was then produced in an identicalway to the first region.

Fig. 3 plots the increase in error rate (known as “driftfactor”) of the A-channel against that of the B-channelfor 60 drives that had previously failed the maggot test.The weak correlation suggests that the process thatcauses drift affects both the A-channel and B-channel,but that the level of response depends on some propertythat is slightly different for the two channels — probablysomething associated with the heads themselves.

The similarity of the three drift regions shows thatthe drifting behaviourwas reproducible within a singleexperiment. Moreover, it was observed that as moredrift-recovery cycles were performed with a particulardrive and cassette, consecutive drifts became increas-ingly similar. Repeating the experiment with the samedrive and cassette but on a different day would producea significantly different amount of drift. An even greaterdifference would be seen if the cassette was changed —even to another from the same batch. With differentdrive–tape combinations the extent of drift varied from1× (i.e., no drift) to 30× when performing the 200-pass test.

The highly variable nature of the drifting phenomenonwas consistent with experience from the manufacturingline, where drift had not been seen in the maggot testprior to mid-1998. A detailed investigation revealed thatthe occurrence of drift was strongly affected by the tapebatch, with a smaller dependence on the manufacturingdate of the drive, and an interaction effect between thedrive and tape. Consequently, only some “bad” combi-nations of drive and tape batches produced problematiclevels of drift. This suggests that the cause of error ratedrift may be associated with an uncontrolled variationin material properties of both the tapes and the heads.Unfortunately we were unable to identify any change in

Fig. 3. Drift factors of A-channel vs. B-channel of 60 drives that hadpreviously failed the “maggot” test. Points show the median of threemeasurements, whilst error bars show the standard error.

403S. Chandler / Tribology International 33 (2000) 401–408

either the tape (e.g., abrasivity, roughness) or the readheads that could be associated with the onset of drift inthe maggot test.

Experiments were carried out to measure the effect oftape tension on the extent of error rate drift. Theseshowed that lowering the tension reduces drift, and caneven make the error rate improve as the tape is reused,probably because the tape surface is worn smoother andthe head-tape spacing decreases [2]. Increasing the ten-sion appeared to make the drift worse; however, detailedexperiments revealed that as the tape tension is increasedthere is a gradual transition to dominance by a second,larger, and probably unrelated type of drift, caused bydamage to the tape (Appendix A).

The information presented so far is consistent with thepreviously encountered staining-based form of error ratedrift; however, several observations were puzzling.

1. Significant drift in the maggot test had been seen atmodest levels of relative humidity (RH|40%), andspot checks on whether drift became worse at lowhumidity (10%) in the simple 200-pass experimentindicated that there was no effect. This was contraryto the drift caused by staining, which increased rap-idly as the humidity was reduced below 30% [1].

2. An Atomic Force Microscope (AFM) was used toexamine the heads from a drive that had just experi-enced large error rate drift in the 200-pass test (Fig.4). The ferrite tape bearing surface was not coveredwith the 10–30 nm thick patches of contaminationthat are known to build-up during the stain-baseddrifting process. Although stain was present on theglass, this is recessed with respect to the ferrite and,it is believed, does not contribute to spacing loss [1].

3. Measurements were made of the change that occursin the signal-to-noise ratio (SNR) of a DDS-3 tape

Fig. 4. Topography of a single crystal MnZn ferrite read head froma Hewlett-Packard DDS-3 tape drive. Measured with a Digital Instru-ments Dimension 3000 Atomic Force Microscope (tapping mode). Theposition of the gap is indicated by the dotted line. Tape makes contactwith the entire region shown in this image.

drive as the error rate drift took place. Unlike similarexperiments with stain-based drift, there was verylittle attenuation of the playback signal amplitude,with the calculated channel frequency response indi-cating that no significant spacing loss was occurring.In fact the degradation in SNR, and hence error rate,could be totally attributed to a gradual increase inincoherent noise.

The new evidence suggested that the impairment under-lying error rate drift in the maggot test was not stain,but was a source of noise that somehow increased as thetape was reused. The next section describes experimentsthat were carried out to characterise this noise.

3. Measurement of rubbing noise during drift

A spectrum analyser1 was used to monitor changes inthe output of the read head’s pre-amplifier as error ratedrift took place. On each pass over the media the RAWerror rate measurement was used to monitor changes inthe output of the read head’s pre-amplifier as error ratedrift took place. On each pass over the media the RAWerror rate measurement2 was immediately followed by20 seconds of blank tape. Spectra were measured fromthe data region and from the blank tape, and saved tothe host computer that controlled the drive and instru-mentation.

When no tape was present the output was solely fromelectronics noise. However, when blank (i.e.,demagnetised) MP tape runs in contact over a ferrite readhead, stress generated by friction creates, throughinverse-magnetostriction, a change in the head’s internalmagnetisation. In turn, this generates a noise voltage inthe head’s windings, and hence a noise in the pre-ampli-fier output [3]. This so-called “rubbing noise” can beclearly seen in Fig. 5 (top two curves), as the increasethat occurs between the electronics noise, and the outputwith previously unused tape (pass 1). To clarify the ter-minology used in this paper, “blank-tape noise” is thetotal noise measured when pulling a.c. erased tape overthe heads, and consists of rubbing noise plus elec-tronic noise.

When the read heads first pass over previously unusedtape the rubbing noise of some drives only contains a+2 dB background that extends from DC to about 17MHz; however, most spectra also have several strong,sharp peaks at frequencies between 6.2 MHz and 9.5MHz, and a few smaller broad peaks at higher fre-quencies.

1 HP 3585B or HP 4395 spectrum analyzer, RBW=30 kHz, VBW=1kHz, external gated trigger set to capture the central 3.0 ms of thedata track.

2 768 frames of DDS-3 format, random data.


Fig. 5. Change in the blank-tape noise spectrum during 101 passesover the same section of tape. The curves are offset for clarity. Elec-tronic noise spectra (EN) are shown for the first and last curves. DriveL509753, cassette 125M92N8M1, 45% RH, 21°C.

Strong peaks in rubbing noise spectra are usually asso-ciated with distinct standing wave vibrations that are setup in the head, at frequencies determined by the dimen-sions and acoustic propagation velocities in the headmaterial [4]. Magnetostriction in the ferrite converts theoscillating change in dimensions into a changing flux,and so produces an induced noise voltage peak at thefrequency of the standing wave. The read heads in aDDS-3 tape drive are composed of two non-identical,asymmetric halves, that are joined together to form thefront and back gap. In addition, sections of the ferriteare removed to accommodate the windings, and otherparts are thinned down to define the gap width andincrease the magnetic reluctance of the front gap. Thiscomplicated structure means that many vibrationalmodes are supported, and many peaks occur in the rub-bing noise spectrum. Attempts are often made to identifya head’s resonant modes from the observed spectralpeaks [5], but this can only be done with confidencewhen using simplified gapless test heads [4]. Conse-

quently, we have not attempted to make the identifi-cation.

The reproducibility of rubbing noise spectra was fairlygood, with different measurements (made 8 monthsapart) of the same drive yielding peaks at identical peakpositions and with similar magnitude. In contrast, rub-bing noise spectra from different heads — even twoheads from the same drive — have peaks at differentfrequencies (compare Fig. 5 and Fig. 8). This is probablycaused by slight differences in their geometry resultingfrom manufacturing variation (e.g., tolerance in dimen-sions, different amounts of in-fill glass used) and differ-ences in wear affecting their gap depth.

As the tape is reused there is no change in the fre-quencies at which peaks occur; however, the relative sizeof the peaks does change — with a large increase in theheight of initially small peaks (and the background level)above 9 MHz.

A simple metric that can be used to quantify the totalrubbing noise is the area contained between the curvesof electronics noise and blank-tape noise, between DCand 20 MHz. This metric increases as the tape is reused,and is strongly correlated with the degradation thatoccurs in error rate (Fig. 6).

Whilst investigating this correlation, extended testswere performed in which more than 200 passes tookplace. Surprisingly, the error rate and noise metric werealways found to gradually recover, and after many hun-dreds of passes over the same section of tape, they returnto the initial value seen with new unused media (Fig. 7).The rate at which this spontaneous “self-recovery”occurs, and the position (i.e., pass number) of themaximum value of error rate, varies by a large amountfrom drive-to-drive, although they appear to be veryreproducible characteristics of the drive under test. Dur-ing self-recovery the error rate and noise metric

Fig. 6. Correlation between error rate and rubbing noise metric(dimensionless). Points are labelled with the pass number in which themeasurements were made. Data were taken from the same experimentas Fig. 5.


Fig. 7. Spontaneous gradual recovery in noise metric after 100passes. In other drives the recovery may be much more gradual, andmay start after 500 or more passes. Solid circles represent the A-chan-nel. Open circles represent the B-channel.

remained correlated in the same way as during the pre-ceding drift. Fig. 7 also illustrates how the absolute noisemetric of the two channels is always very similar in size.

4. Effect of head-tape speed on rubbing noise

To confirm that peaks in the rubbing noise spectra aredue to standing modes determined by the head’sgeometry, measurements were made to check that thepeak positions were invariant when the head-tape speedwas changed. Two approaches were used:

1. A section of previously unused tape was reused 101times at a chosen “drifting speed”, usually nominal,to develop a drifted state with large rubbing noise.The drum speed was then varied from 0.5× to 1.5×nominal, and at each new speed a spectrum was meas-ured on the fifth pass over the well-used tape.

2. The speed was set to the desired value and a new101-pass reuse script was started on unused tape. Aspectrum was measured on pass 101, then the speedwas changed to a new value and the system was resetto the undrifted state by pulling unused tape. This wasrepeated for speeds from 0.6× to 1.4× nominal.

In both cases the order of speed settings was randomised,and the measurements were repeated to check for repro-ducibility. The same behaviour was observed with bothmethods, although method 1 gave slightly less scatterdue to there being a smaller change to the systembetween each measurement. Unless otherwise stated, allresults presented in this paper are from method 1.

As the head–tape speed is increased, the frequenciesat which peaks (i.e., local maxima) occur remains

unchanged, as expected; however, the relative amplitudeof the peaks and the shape of the background do change,so that the overall spectral shape changes (Fig. 8). Thisis different from the results of Johnson and Huijer [4],where the height of all peaks increased to the sameextent so that the spectrum grew but retained the sameshape. In fact, this latter result can obtained if the drumspeed is changed when the drive is in an undrifted state,having just pulled unused tape for 5 minutes.

Inspection of Fig. 8 shows that as the speed isincreased the location of the largest peak (globalmaximum) moves to higher frequencies. This change,shown more clearly in Fig. 9, occurs in a very uniformway until the preferred location of the highest peakmoves above the roll-off frequency in the channel’stransfer function (|20 MHz), after which any noise gen-erated in the head will not be detected by the spectrumanalyser. For the same reason, the total rubbing noisemetric does not increase monotonically with increasingspeed, but reaches a maximum at about 1× (i.e., nominal)

Fig. 8. Blank-tape noise spectra for head–tape speeds from 0.6× to1.5× nominal speed (6.01 ms21). The curves are offset for clarity, andlabelled with the head–tape speed. Drive L509834, cassette125M92N8M1, 40% RH, 22°C.


Fig. 9. Illustrating the change in position and size of peaks in a rub-bing noise spectrum as the head–tape speed is changed. Points identifythe position of peaks, the greylevel shows the size of the peak(darker=larger), and the line shows the position of the largest peak.The linear region (0.6–1.3×) corresponds to a wavelength of 433±4 nm.

speed, after which it decreases as the speed is furtherincreased.

The behaviour shown in Figs. 8 and 9 suggests thatas the head–tape speed is increased a speed-relatedvibration increases in frequency, exciting both the back-ground and nearby modal frequencies in the head. Theline in Fig. 9 shows how an increasing excitation fre-quency causes the position of the largest peak to jumpfrom one pre-existing modal frequency to another. Thisline is quite linear, and can be extrapolated to interceptthe frequency axis close to the origin. This suggests thatthe excitation has a characteristic wavelength (l),given by

l5f p RPMnom

60 gradient(1)

where f is the diameter of the drum (30 mm), andRPMnom is the nominal drum speed (3826.5 RPM). Fit-ting a straight line (y=mx) to this data gives a gradient of13.87±0.14 MHz, and hence a wavelength of 433±4 nm.3

The wavelength calculated from the observed speeddependence is a very reproducible characteristic of ahead. For a chosen head, the same wavelength wasobtained using either of the two methods describedabove. Moreover, in one experiment using method 1, tendifferent drifting speeds from 0.6× to 1.4× gave wave-lengths that were consistent (at a high level of confi-dence,c2=10.1, degrees of freedom=9) with a singlevalue of 467±4 nm. This indicates that the characteristicwavelength of a head is independent of the drum speed,and hence head–tape speed.

Figs. 8 and 9 show a very clear linear dependence ondrum speed, which was a repeatable characteristic of this

3 This error estimate is based on adjusting the chi-squared value toequal the number of degrees of freedom.

drive when in the drifted state. Other drives, however,showed a much weaker dependence on drum speed, anda linear region could not be clearly discerned even whenthey had just undergone considerable error rate drift.

5. Detailed AFM measurements

An Atomic Force Microscope was used to examinethe ferrite read heads of DDS-3 tape drives in each ofthe following conditions:

1. Immediately after the error rate had been reduced bypulling new tape for five minutes (as in Fig. 2).

2. When the drive was in a drifted state (due to increasedrubbing noise) after reusing a section of tape 200times.

3. After a section of tape had been used many times(|1000) and the error rate had substantially recover-ed.

After pulling unused tape over the heads, the only fea-tures present on the ferrite were the many, relativelydeep scratches due to abrasive ploughing by hard par-ticles in the tape coating.

In the drifted state the surface of the ferrite alsoappeared featureless (Fig. 4) until it was examined inmore detail. This revealed a subtle “ripple” pattern inboth the topography and phase images. Ripples wereonly ever seen in the head–tape contact region on theferrite — never on the glass. They had a height rangeof less than 3 nm and an RMS roughness of 0.4 nm.Analysis of the ripples using 2-D Fourier transformsshowed that they were always aligned at±10° normalto the direction of tape movement, and always had awavelength that closely matched that calculated from thespeed dependence of the head’s rubbing noise. As anexample, the ripples shown in Fig. 10 have a wavelengthof 426±80 nm, whilst the speed dependence yielded awavelength of 422±4 nm. The actual wavelength (bothof the ripples and derived from the speed dependence)appeared to be a characteristic of individual heads, sincethe two read heads on one drum could have significantlydifferent wavelengths. In addition, the spacing of theripples could sometimes be different on some parts ofthe head, as shown in Fig. 10.

The clarity of the ripple features appears to be linkedwith the clarity of the linear region in the speed depen-dence measurements (Fig. 9). With some drives, evenwhen they had just undergone considerable error ratedrift, ripples were difficult to see in the AFM images,and the position of the highest peak in their rubbingnoise spectrum showed only a weak dependence ondrum speed, with no clearly discernible linear region.

Ripples could also be seen on the ferrite by using an


Fig. 10. Phase image of the same head shown in Fig. 4. Note thatthe “ripples” are more closely spaced in the top-left corner. The direc-tion of tape movement is indicated by the approximately horizontalscratches on the surface of the head. Tape makes contact with theentire region shown in this image.

optical microscope with differential interference contrast(DIC) filters.

Other authors have reported seeing ripples on mag-netic recording heads when used with MP tape [6,7], butthe mechanism by which these were formed was notexplained.

An AFM was also used to examine the heads in adrive that had used tape many times, and whose errorrate had substantially recovered. Unfortunately theresults have been inconclusive. We currently believe thatthe ripples are less clear than in the drifted state, but thatthey are still present.

6. Discussion

The underlying impairment that causes the error rateto increase is clearly rubbing noise, caused by frictionbetween the tape and the head. This has been directlymeasured, and explains the sensitivity of this phenom-enon to factors that affect interaction between the tapeand the head, such as the tape tension.

The speed dependence of the rubbing noise spectrumcan be attributed to the ripples that form on the surfaceof the read head. As asperities in the tape move over thehead they will interact with successive ripples with aperiod determined by the head–tape speed and the ripplespacing (i.e., wavelength). Frictional stress between thetape and head will, therefore, be periodic, and a largerubbing noise signal will be generated when this exci-tation frequency coincides with modal frequencies in thehead. Well defined ripples on the head will result in aclearly defined linear dependence of the rubbing noisespectrum on the head–tape speed, but this behaviour willbe less clear with weaker ripples.

In the absence of ripples, stress between the tape andthe head will be random and continuous, so resonantmodes in the head will not be excited, and the overalllevel of rubbing noise may be lower. However, error ratedrift still takes place, so the ripples exacerbate the prob-lem but do not cause it.

The nature of the ripples, and the origin of theircharacteristic wavelength, is still not clear. Is the wave-length an intrinsic property of the head’s material (e.g.,a periodic variation in composition or defect density), oris it an imposed feature, created by a mechanism that isexternal to the head (e.g., due to a mechanical resonancein the tape path). Several observations lead us to preferthe former explanation. Firstly, an external mechanismwould have to scale linearly with the drum speed inorder to make the resulting ripples be speed-invariant.Secondly, to explain the observation of varying ripplespacing over the surface of one head, an external mech-anism would have to affect different parts of a head indifferent ways. Similarly, the external mechanism wouldhave to affect a drum in a way that could lead to differentwavelengths on its two read heads. Clearly, more workneeds to be done to properly identify the ripples, themechanism by which they form, and the way in whichthey interact with the tape.

An interesting question that remains to be answered,is “what is the systematic change that makes the rubbingnoise worse as the tape is reused?” One hypothesis, sug-gested by the published work of Kawamata et al. [5],was simply that the reduced abrasivity of used tapeincreases the level of rubbing noise. However, thisexplanation is based on a change only to the tape, so isunable to account for thegradual reduction in error rate(and rubbing noise metric) that occurs when unused tapeis pulled across the heads. This latter observation sug-gests strongly that when the tape is reused, the system-atic change that takes place occurs to the head, not thetape. In addition, a simple reduction in abrasivity isunable to explain the gradual self-recovery that takesplace when the tape is used many times.

We do know that the rubbing noise will only increaseif stress to the head is increased. This statement is prob-ably a key to explaining the observed behaviour. Reusingthe tape must change the head in a way that initiallyincreases the stress, resulting in an increased error rate.If the tape continues to be reused then a very gradualreduction occurs in the rubbing noise, so the stress mustbe slowly reducing; however, it is not clear whether thischange is associated with the head or the tape. Pullingunused tape across the head gradually changes the headin a way that reduces the stress.

One possible explanation for the change that takesplace is that as the tape is reused and its abrasivity androughness are reduced [1], the wear mechanism at thehead–tape interface changes. Initially, abrasive plough-ing by large particles of head cleaning agent (HCA) in


the new unused tape creates a scratched, rough surfaceon the head. The rough tape and the rough head have alow area of contact, so interaction stresses are small, andthe rubbing noise and error rate are low. As the tape isreused, larger asperities will be removed or deformed sothat the tape surface becomes smoother. The aggressiveploughing is replaced by a gentler polishing action, andthe head surface also becomes smoother. The area ofcontact between the tape and the head will increase,increasing the tangential friction stress and hence therubbing noise and error rate. In this regime of gentlewear the ripple features may form, exacerbating the rub-bing noise, as described above. We currently do not havea convincing hypothesis for the final change that takesplace, causing the gradual self-recovery in rubbing noiseand error rate. It may be related to a further reductionin roughness and abrasivity of the tape, or possibly to achange in the conditions of lubrication at the headtape interface.

7. Conclusion

Some combinations of DDS-3 tape drive and cassetteexperienced a rapid increase in error rate, followed bya gradual spontaneous recovery, when a section of tapewas repeatedly used. The increased error rate was shownto be caused by an increase in rubbing noise, resultingfrom changes to the surface of the read head as the tapebecame smoother and less abrasive. Peaks in the rubbingnoise spectrum occur at fixed frequencies that do notchange as the tape is reused, or as the head–tape speedis increased. This suggests that the peaks are associatedwith standing-wave vibrations that are set up in the fer-rite read head. In some cases, periodic ripple featuresthat form on the surface of the read head excite thesevibrational modes. The exact nature of these ripples, andthe mechanism that causes the self-recovery are stillunknown.

Acknowledgements

The work described in this paper was carried out asone small part of a large investigation into the cause oferror rate drift. The author would like to thank his col-leagues at Hewlett-Packard for their role in the work,particularly: Tim Troup, Andy Goddard, Philip Turnerand Jonathan Lord for many valuable technical dis-cussions, Rafel Jibry and Mike Robson for performingthe SNR breakdown measurements, and Chris Calderand Dan Brain for carrying out long and often tediousexperiments. We would all like to acknowledge the workdone by engineers at Mitsumi with whom we collabor-ated during the investigation.

Appendix A. Error rate drift with increased tapetension

At an early stage in our investigation we observed thatlowering the tension of the tape could reduce error ratedrift. This suggested that the drifting process could be“accelerated” (both speeded-up and enlarged) by raisingthe tension. Indeed, with the tension set to 1.7× the nom-inal value, drift of up to 200× could occur in only 40passes.

Unfortunately, after considerable work had been com-pleted, we realised that the increased tension had createda completely different form of error rate drift! Attemptsto speed-up our work by using raised tension had side-tracked the investigation for many weeks, and ultimatelydelayed the work.

As the tape tension is increased there is a gradual tran-sition to dominance by this second, unrelated type ofdrift. The impairment associated with the high-tensiondrift is a physical damage to the tape that has a similareffect on both the reading and the writing processes.Recovery to the error rate occurs immediately the headsmove onto unused tape. The damage is localised on thetape, and occurs to a greater extent near the top of thetrack. This suggests it may be caused by contact betweenthe drum and the tape.

Attempts that were made to measure the effect of rela-tive humidity on the drift were done with the tape underhigh tension, so were dominated by the tape-damagedrift. Due to lack of time the humidity dependenceexperiment was not repeated at the nominal tape tension.

References

[1] Chandler S, Tucker B, Turner P, Heard P. Case study: An investi-gation into the cause of “error rate drift”. Tribology International1998;31(8):443–8.

[2] Tan S, Baugh E, Talke FE. Measurement of non-elastic asperitycompliance of magnetic tapes. IEEE Trans Magnetics1998;34(4):1726–8.

[3] Okita K, Horikawa JI, Hoshi T, Saji H. Rubbing noise of singlecrystal ferrite heads. J Magnetism and Magnetic Materials1986;53(4):374–8.

[4] Johnson MT, Huijer E. Rubbing noise generated by acousticvibrations in ferrite test heads. IEEE Trans Magnetics1988;24(6):2398–400.

[5] Kawamata T, Mizoh Y, Ushigome H, Hagiwara H. Abrasivityeffect of magnetic recording tape on rubbing noise. Transactionsof the Institute of Electronics and Communication Engineers ofJapan 1984;J67C(1):62–9.

[6] Katori K, Ohmori H, Shoji M, Hayashi K. Wear and corrosionresistance of Fe-N based micrograin soft magnetic thin films. IEEETrans Mag 1994;30(2):331–6.

[7] Harrison MJK, Sullivan JL, Theunissen GSAM. Pole tip recessionin sandwich heads incorporating a FeTaN soft magnetic track. Tri-bology International 1998;31(9):491–500.

Error rate drift caused by rubbing noise in ferrite read heads

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