CA2159800A1 - Method and apparatus for noise reduction in magnetic media - Google Patents

Abstract

A method and apparatus is disclosed for determining the remnant noise in a magnetic medium (104) by DC saturation (102) of a region thereof and measurement (106) of the remaining DC magnetization. Conventional recording transducers may be used to determine the remnant noise. Upon determination, the remanent noise may then be compensated for in either or both of the record (110) and playback modes for all varieties of magnetic media including videotapes, cassette tapes, etc.

Description

215980~
W094/24~9PCT~S94/037 METHOD AND APPARATUS FOR
NOISE REDUCTION IN MAGNETIC MEDIA

Cross Reference to Related APPlication This application is a continuation-in-part of Serial No. 08/046,071 filed April 9, 1993.
Background and SummarY of the Invention 5The sources of noise in a readback signal from a magnetic recording medium have been investigated and identified. One of those sources includes the irregu-larities and defects in the microstructure of the magnet-ic medium itself. For many years, the noise generated 10 from this source has been thought, as with the noise generated from other identified sources, to be random and sub~ect only to statistical analysis for its determina-tion. The inventors herein have recently demonstrated that this noise component is instead deterministic, i.e.
15 is permanent and repeatable, depending entirely on the transducer-medium position and on the magnetic history of the medium. As confirmed by experiments conducted by the inventors herein, when the medium has had no signal writ-W094/24~9 215 9 8 0 ~ PCT~S94/037~

ten on it and has been recorded only with DC fields, theobserved readback signals are almost identical. The magnetic contribution to the re~h~k signal under these conditions results from spatial variations in the 5 medium's magnetization: magnetic ~o~~ins, ripple, local fluctuations of the anisotropy field and`saturization -~n~tization. These local properties,~in turn, are affected by the morpholoqy and magneti~ properties of the , ~1 individual grains which make up the domain and which do 10 not change after deposition. Hence, the noise from a nominally uniformly magnetized region measured at a fixed position on a magnetic medium is reproducible. As shown by the inventors herein, a magnetic medium may be DC
saturated and its output then measured to determine its 15 remanent state or remanent noise. The inventors have confirmed that this re~nent noise is a function of the magnetic microstructure by comparing the l- ~n~nt noise after a positive DC saturation with the re~n~nt noise after a negative DC saturation. It has been found that 20 these wave forms are virtual "mirror images" of each other thereby demonstrating a close correlation. Simi-larly, other methodologies were used to confirm that the remanent noise was determinative, repeatable, and related to the physical microstructure of the magnetic medium 25 itself. Re~^~ent noise arising from the permanent micro-structure exhibits identifiable features characteristic of that perr~nent microstructure after practically any magnetic history. See Spatial Noise phen~-en~ of Longi-t7~r7i~7 Magnetic Recording Media by Hoinville, Tn~e~k and 30 Muller, IEEE Transactions on Maqnetics, Volume 28, No. 6, November 1992, the disclosure of which is incorporated herein by reference.
The inventive t~-chnique disclosed and claimed herein relies upon the discovery that the microscopic 35 structure of the magnetic medium itself is a permanent random arrangement of microfeatures and therefore deter-~ W094/~K39 215 9 8 0 ~ PCT~S94/037~

ministic. In other words, once fabricated, the recordingmedium's physical microstructure remains fixed for all conventional recording processes. In particulate media, the position and orientation of each particle does not 5 change within the binder for any application of magnetic field, in thin film media, the microcrystalline orienta-tions and grain boundaries of the film remain stationary during the record and reproduce processes. It is the maqnetization within each of these fixed microfeatures 10 that can be rotated or modified which forms the basis of the magnetic recording process. If a region of a magnet-ic medium is saturated in one direction by a large ap-plied field, the re~-nent magnetization depends strongly on the micro-structure of the medium. This remanent 15 state is deterministic for any point on the recording surface. Each particle or grain in the medium is hun-dreds to tho~lc~s of Angstroms in dimension. Due to their small size, a small region of the magnetic surface will contain a very large number of these physical enti-20 ties. While the fabrication process normally includesefforts to align these particles, there is always some dispersion of individual orientations. The actual devia-tions will be unique to a region of the medium's surface making this orientation deterministic and making its 25 effects susceptible to elimination. As can be appreciat-ed by those of ordinary skill in the art, noise reduction enables increase in storage capacity, increase in data rates, and eases the burden on transducers, medium, and system design and fabrication.
Although this discovery has been made by the inventors herein, noise reduction t~hn;ques based on this discovery have not been implemented. As this noise component of remanent noise is deterministic, it may be reliably repeated and measured at any particular point on 35 a magnetic medium. Accordingly, the inventors have de-veloped several t~chn;ques which take advantage of this W094l~K~9 2~$ 98a ~ PCT~S94/037~

fact for producing uncorrupted pre-recorded signals which may be played back by any playback device but which, when played back, have already been compensated for the rema-nent noise co,..ponent. In other words, a ~gnetic record-5 ing may be recorded at the factory with a signal whichhas been first compensated for re~-ne~t nolse such that as the signal is played back later the~p1ayback signal or read signal has the remanent noise co~ponent virtually eliminated. As the remanent noise c ~ponent may very 10 well be the most significant factor in noise emanating from pre-recorded magnetic media, this noise reduction terhn~que may very well provide a dramatic reduction in noise with no required modification to the ~l ~n~ous number of playback r~chi ~e~ presently in the public's 15 hands. This would include playback m~ch~n~c for the entert~inment industry, etc. In a first erho~?nt of the invention, the remanent noise is first determined and the recording device compensates the original signal for the ~2 ~n~nt noise before writing the comp~cated signal 20 on the magnetic medium. These steps may be readily achieved with cullventlo~l recording transducers, as explained herein. Consequently, very little, if any, modification to existing recording equipment need be made to achieve these noise compensated recordings.
A second methodology will also create uncorrupted pre-recorded signals on magnetic medium. With this meth-od, the signal is first written on the magnetic medium, the written signal is then read from the ~gnetic medium, this read signal is then compared with the original sig-30 nal. The differences therebetween are deter~i~e~ to be noise, the greatest component of which is deterministic medium noise. The original signal is compensated to eliminate this noise before being recorded back at the same location on the magnetic medium. Thus, after the 35 compensated signal has been recorded onto the magnetic medium, any other readback or playback ~chine would then ~ W094/2~9 215 9 8 ~ ~ PCT~S94/03722 produce a signal which has been compensated for re~n~nt noise.
In still another embodiment of the present inven-tion, the inventors have developed a methodology for 5 compensating a signal read from a magnetic medium for remanent noise in real time. This methodology permits a playback device to be manufactured and sold which can play back pre-recorded m-gnetic medium which has not itself been compensated prior to recording, and produce a 10 signal which is compensated on readback. With this meth-od, the signal is first read from the magnetic medium, the re~nent noise is determined for said magnetic medi-um, such as by saturating the magnetic medium and re~ng the remanent noise directly therefrom, and the signals 15 are then compared to eliminate the noise from the origi-nal corrupted signal prior to use. As determining the 1 2 -n~nt noise, as envisioned by the inventors, involves de~ ying the original recorded signal when the medium is saturated, another step to the method may well include 20 re-lec~lding either the original signal or its compensat-ed counterpart. Thus, with this methodology, a playback device may take a pre-recorded magnetic medium whose signal has not been compensated, and transform it into a magnetic medium with a compensated signal recorded there-25 on such that further playbacks of the same ~gn~tic medi-um would possibly not require compensation. With this methodology, if implemented in one alternative embo~im~nt thereof, a user with a suitable playback machine may very well transform his entire collection of recorded media 30 from non-compensated to compensated magnetic media. In other words, one may readily convert a collection of analog cassette tapes having original non-compensated signals thereon to a collection of analog cassette tapes having compensated signals recorded thereon which may 35 then be played back by any playback device and produce WO9412K~9 2 15 9 8 ~ ~ PCT~S94/037~ ~

what should be an enh~n~ signal because of the noise reduction.
In essence, the present invention is elegantly simple and adapted for implementation by conventional re-5 cording transducers as are commonly found and used invirtually every read or read/write device presently uti-lized by the public at large. Such examples include cassette players, magneto-optic disc players, and VCRs.
In its simplest implementation, a ~o~ventional recording 10 transducer need merely DC saturate a specified portion of a magnetic medium, and then "read" or "play back" the L ~ -n~nt noise which remains. This L~ ~n~nt noise, which is an analog signal, may then be used to compensate an original signal, such as a musical program, dramatic 15 reading, etc.
While the principal advantages and features of the invention have been described above, and a nl h~ of examples given, a greater underst~n~ng of the invention may be att~l n~ by referring to the drawings and the 20 description of the preferred embodiment which follow.
Brief Description of the Drawinqs Figure 1 is a magnified representative depiction of the microscopic structure of a region of magnetic medium;
Figure 2 is a magnified depiction of several tracks of a magnetic medium having microscopic structure representatively shown thereon;
Figure 3 depicts three conventional recording transducers and a magnetic medium traveling thereunder;
Figure 4 is a perspective view of a magneto-optic disc player with a magneto-optic disc in its tray;
Figure 5 is a cassette player depicting a cassette tape for play therein;
Figure 6 is a perspective view of a VCR with a 35 tape ready for insertion;

~ W094/2K~9 215 9 8 0 ~ PCT~S94/037~

Figure 7 is a schematic diagram of the write-read-write embodiment of the invention; and Figure 8 is a block diagram of the electronics shown in Figure 7.
5 Detailed DescriPtion of the Preferred Embo~;~?nt As shown in Figure 1, a region of magnetic medium 20 is built up with a plurality of microcrystalline structures 22 in a random pattern. This mi~lo~y~alline structure 22 is comprised of particles or grains varying 10 from hundreds to thousands of Angstroms in diameter. The view of Figure 1 is greatly enlarged and magnified in order to depict this physical ph~nomena. As shown in Figure 2, this microcrystalline structure extends throughout the magnetic medium even though the magnetic 15 medium 24 shown in Figure 2 may be itself comprised of tracks 26, 28, 30 as well known in the art.
Referring now to Figure 3, a plurality of conven-tional recording transducers 32, 34, 36 are shown mounted in a transducer transport 37 with a traveling magnetic 20 medium 38 controllably driven past recording transducers 32, 34, 36 all as is well known in the art. Recording transducers 32-36 are all ~onnected to electronic cir-cuitry 40, as well known in the art, to control and read their input and output and further process signals for 25 playback or other use. Although only three transducers 32, 34, 36 are being shown in Figure 3, it will be well undel~ood to those of ordinary skill in the art that a plurality of recording transducers of any number may just as easily be provided and, as taught herein, may be re-30 quired in order to effect the purposes of the presentinvention. In implementing the present invention, the recording transducers 32-36 as shown in Figure 3 may be considered as part of a device which is used to create pre-recorded ~gnetic medium with re~nent noise compen-35 sated recordings. Alternately, the device shown in Fig-ure 3 may be considered as a playback unit of either a W09412K~9 2 1~ 9 8 a ~ PCT~S94103722 specialized playback device with means for creating a remanent noise ~ L~ sated signal from a non-~o,..~ensated pre-recorded signal, or a st~n~rd playback device which may be used to play back a r~nent noise compensated 5 magnetic medium. All of these functions are achieved with conventional recording transducers and therefore are readily implemented using existing and àvailable t~chnol-ogy.
A r~n~nt noise compensated signal may be pre-lO recorded onto a magnetic medium by utilizing the follow-ing method. The remanent noise of the magnetic medium may first be detel ~ ne~ by DC saturating the medium and then r~A~1~g the remanent noise with a conventional re-cording transducer. This would take transducer 32 to l5 saturate the medium and transducer 34 to read the rema-nent noise. The original signal would then be compensat-ed, using coLl~elltional compensation circuits as is well known in the art to modify the original signal such that it may then be le~oLded by recording transducer 36. In 20 this -nn~r, using this method and device as shown in Figure 3, a pre-_ ,Qncated recording, pre-compensated for r~-~n~nt noise, may be created on magnetic medium 38.
While there is a fixed and close spacing between trans-ducers 32-36, the remanent noise is itself capable of 25 being used for in~e~g the transducers 32-36 to thereby ensure that the ~omrencated signal is recorded by trans-ducer 36 for the remanent noise which in fact appears at that point on the magnetic medium for which said compen-sation has been made. This is because, as explained 30 earlier herein, while the remanent noise is random, it is unigue to any particular point on the magnetic medium and thus can be used to identify such point for benchm~rk purposes. While this is the perferred embo~me~t, it should be understood that the r~m~n~nt noise is always 35 there, whether the medium has been recorded over or not.
Therefore, it is not strictly necessary that the speci-~ W094l~39 2 ~ 5 9 8 ~ ~ PCT~S94/03722 fied portion of medium cont~;n;ng the r~n~nt noise beDC saturated, or DC saturated in the same polarity in order to obtain the re-~n~nt noise.
In a variation of the first embodiment hereof, 5 still another methodology may be used to create a pre-recorded ~gnetic medium having a signal recorded thereon which is r~nent noise compensated. This second embodi-ment involves the steps of first writing the original signal on the magnetic medium, such as for example by 10 transducer 32 in Figure 3, reading the recorded signal from said magnetic medium such as by transducer 34, com-paring the read signal with the original signal to deter-mine the differences therebetween, compensating the orig-inal signal, and then writing the compensated signal with 15 transducer 36. Using this methodology, as with the first embodiment of the present invention, magnetic medium 38 would thus receive a recorded signal which has been com-pensated for the r~--nent noise inherent in the magnetic medium 38. These compensated recordings may then be 20 played back by any conventional playback device and pro-duce a signal which is noise compensated. This is impor-tant as with this implementation of this embodiment, uncorrupted copies or noise compensated copies of pre-recorded signals may be produced and made available for 25 play back by the large number of playback devices already in the public's hands. This could very well be imple-mented for improving the pre-recorded playback of musical and dramatic programs on magneto-optic discs, cassette tapes (analog and digital), and VCR video tapes.
The inventors have developed a generalized model with an algorithm for implementing the write-read-write embodiment of the present invention. This generalized model is explained in Exhibit A. As noted therein, and referring to Figure 3 thereof, this generalized model 35 compensates for additive medium noise and explains a design approach for implementing this embodiment with a W094/2~9 21~ 9 8 ~ ~ PCT~S94/03722 silicon tap delay line. As shown in Figure 3 of Exhibit A, the signal sl(t) is processed by a write head, repre-sented by h(t) onto a magnetic medium. As it is written, the signal is corrupted by two kinds of medium noise, 5 non-repeatable medium noise n1(t) and rep~eatable additive medium noise nd(t). This corrupted signal is then read by a read head which processes it as rep~esented by a func-tion g(t). In order to determine th-ë error function portion of the signal, the signal sl(t) is processed as 10 represented by a function b(t) which is the equivalent of a write and read function, and then subtracted from the output of the read head. Additionally, an electronics noise signal wl(t) is added to represent the electronics noise. The result is an error function e(t) which is 15 representative of the total noise introduced in the sig-nal sl(t) by a write and read function. Next, the error function e(t) is then processed by a filter function c(t) which is the inverse of the noise expected to be added by a later write and read function. Finally, the output of 20 the filter function c(t) is subtracted from the data signal (t) and a write head processes this signal with the function h(t) to record it onto a magnetic medium where it again suffers corruption through the two kinds of ~gn~tic medium noise, repeatable additive medium 25 noise nd(t) and also non-repeatable medium noise n2(t).
At this point in the model, a signal desired to be re-corded, s(t) has been recorded in a precompensated manner as the function c(t) subtracts out the effects of the write function, a later expected read function and the 30 expected repeatable additive medium noise nd(t). As the magnetic medium is then read by a read head and the sig-nal processed with the function g(t), a signal output y(t) is produced which is clearly compensated for.
Computer simulations have been run on this method-35 ology utilizing the mathematical solutions for the vari-ous components of the system. As indicated at page 9 of ~ ~094t~4~9 215 g 8 0 ~ PCT~S94/03722 Exhibit A as well as Figures 5-7 of Exhibit A, the precompensation model disclosed therein with the write-read-write scheme yields a reduction in noise power on average. The amount of the reduction depends on the 5 ratio between the repeatable noise power and all other noise. A distribution of noise powers is shown in Exhib-it Figure 5 for 1,000 runs demonstrating the significant-ly decreased noise levels expected for recording with signal precompensation. Exhibit Figures 6 and 7 further 0 ~Q -_ strate the improved signal wave form which is also achieved.
Further explanation of this write-read-write embodiment of the present invention is found in Figures 7 and 8. As shown in Figure 7, a first write head 102 15 writes the signal sl(t) on the medium 104. The recorded signal is then read by read head 106 which produces an output yl(t) to an electronics circuit 108, as explained in Figure 8. The electronics 108 produces a compensated data signal which is then written by write head 110 back 20 on the magnetic medium 104. The write head 110 thus writes a precompensated version of the data signal s(t) which, after being read by another read head (not shown) produces an output of data signal s(t) which has been compensated for additive repeatable magnetic noise.
As shown in Figure 8, the electronics 108 includes an adder 112 which subtracts an output signal d(t) from ideal ch~nnpl 114 having a function b(t) which processes the diagnostic signal sl(t) as equivalent to a write and read function. Diagnostic signal generator 115 processes 30 the data signal s(t) to produce the diagnostic signal sl(t). For example, sl(t) could be a DC saturation sig-nal. The output of adder 112 produces an error signal e(t) which is then compensated by compensation filter 116 through a signal transformation function c(t). As ex-35 plained above, the compensation filter function c(t) isthe inverse of the noise expected to be added by a write W094l~K~9 215 9 8 ~ a PCT~S94/03722 and read function. A second adder 118 subtracts the output of compensation filter 116 from the data signal 8 ( t) to produce a signal which corresponds to a pre- -compensated data signal for writing onto the magnetic 5 medium by write head 110. As noted above, the general-ized model and algorithm for each of the ~unctions in-cluded in Figures 7 and 8 may be readily determined by one of ordinary skill in the art from the equations given in Exhibit A.
lOIn still another implementation of the noise compensation methodology of the present invention, a playback device may be manufactured and sold which is capable of producing a noise compensated signal from recordings on magnetic media which have not been noise 15 compensated. In this e~hoAirent of the present inven-tion, the signal is first read, such as by recording transducer 32 in Figure 3, the re~n~nt noise is then de~e i n~.~ such as by saturating the magnetic medium with a signal from transducer 34 and re~Ai ng the re~ne~t 20 noise with transducer 36, and then the original signal would be compensated with said remanent noise prior to playback or other processing. Although not specifically shown, a fourth transducer may be provided to re-record either the original signal or the compensated signal back 25 on the magnetic medium 38 for subsequent playback. With this device and method, conventional recordings on mag-netic media may be compensated for re-~nent noise prior to playback. Also, perhaps while being played, magnetic media may be transformed from ~ omp~ated to noise 30 comr~n~ted recordings. Thus, with this implementation, a device may be made and sold for use with the vast in-ventory of pre-recorded magnetic media presently in the public's hands.
In still another implementation of the present 35 invention, the uni~ue remanent noise pattern may be used as a benchmark to locate a transducer at a particular ~ W094/24~9 PCT~S94/037~

2~5 ~8~

position in a magnetic medium. For example, for editing purposes, and as previously explained above, the conven-tional recording transducers 32-36 as shown in Figure 3 could be readily used to determine the r~nP-nt noise at 5 a particular position on the magnetic medium 38. This could then be used to reposition the transducers 32-36 at the start or finish of an edit, or otherwise to precisely position a conventional recording transducer with respect to the magnetic medium. This application would provide 10 significant advantages in dubbing, etc. which is commonly used for taking rough cuts of many kinds of programs and editing them for final production. For that matter, editing is used in a large number of applications too numerous to mention herein. In each of these applica-15 tions, it is desired to accurately and reliably reposi-tion a recording transducer to ensure the continuity of the signal and ~loylam through the discontinuity created by the editing process. As the inventors' methodology provides a convenient and simple way to most accurately 20 determine the exact position on a magnetic medium, and to find that exact position, the present invention provides a unique and novel way to position a recording transducer for editing.
As shown in Figure 4, a magneto-optic disc player 25 64 has a ~gneto-optic disc 66 in its tray 68 ready for play. As explained herein, a magneto-optic disc player 64 may play back re~nent noise compensated magneto-optic discs 66. Furthermore, although not presently ~o~er-cially available for home use, ~gneto-optic disc players 30 64 may soon be available which are capable of recording onto magneto-optic discs 66. In such event, all of the embodiments of the present invention may be implemented such that magneto-optic discs 66 may be noise compensated when played back, even though its original signal was not 35 recorded in a noise compensated format, and CD player 64 W094/24639 ~ PCT~S94/03722 used to re-record a noise compensated signal back onto magneto-optic disc 66.
Similarly, a cassette player 72 as shown in Figure 5 has a cassette 70 being inserted therein for play.
5 This magnetic medium is also susceptible to implementa-tion of the inventors' methodologies to ~nh~nC~- the re-cord and/or playback of cassette-70 in remanent noise compencated format.
A last example of an implementation of the in-lO ventors' methodologies is shown in Figure 6 and includesa VCR 74 with a video tape cassette 76 being inserted therein. As the video tape cassette 76 is a magnetic medium, it is also susceptible to the noise compensation methodologies disclosed and claimed herein.
There are various changes and modifications which may be made to the invention as would be apparent to those skilled in the art. However, these changes or modifications are included in the teaching of the disclo-sure, and it is int~n~ that the invention be limited 20 only by the scope of the claims appended hereto.

wo 94/24639 1 5 2 1 5 9 8 0 ~ PCTIUS94/037Z2 Magnetic Recording System Design to Reduce Medium Noise Through Signal Precompensation .
Joseph A. O'Sullivan - Dakshi Agrawal Donald G. Porter ~onald 5. Iru~eck ~arcel ~ ~uller Dep~ln.ent of FlPc~ n~ineering Washington University St. Louis, MO 63130 ABSTRACT

Much of the noise in m~ne~ic rccording systems is due to intrinsic pLopellies of the m~ net;c mcdium itself. Much of this noise is repeatable in that iden~ic~ veformc.
recorded in the same place on thc mcdium (with an erasure in b~,L~ ) have highly corre-latcd noise. This effec~ is eYrloit~l in the present paper through the design of systems to csl;,.~ and subsequently correct the tistortion of ~e recorded waveÇorm due to m~-linm noise, The appr~ach may be applicable to othcr storagc ch~nnelc whose noise is ...~,.1;...., dcpcndcnt such as m~n~o-t -op~ic media A method for opt~mally re~lcing rcpeatable addi-tive medium noisc is ~lu~osed. ~ t;ons of this system have been ~un and the results arc prornisin~-1. Introduction Most models of ma~n~tic recording ch~nn~lc use a traditional commllnir~ n thcorymodel as shown in figure 1. Thc ch~nnÇl is thc magnetic m~inm. The medium rnay cûrIupt the signal, and if modeled as additivc noisc, avcrage sta~stics of this medium noise may be c,~ i and used in system deslgn to improvc the p~,fu~ ancc of the system. Other sources of noisc inc~udc receiver noisc and head noise. Even ~Yhen these latter two noise sources are consi~.red, the ~ . noise l~mits the system ~ rOl... t~ Thc authors have dcveloped models for m~En~tjC media [1,2] as have other_ 13,4,5,6,7]. These models account for thc fact that m~ n noise arises from the miclosco~ic pLo~-,.Lies ûf the record-ing m~ m These ~op~.lies are l~t,~ .Il;l~ic1;c once the "lediulll is lllall~Çactured4 While it is infeasible to have a cu~lplP microscopic scan of the 1ll~ l-l for use in signal design, it may be possible to n~e~su~e local ~.e~l;t.~ eatures on-line and to use those features when ~1~.cignin~ ~he recorded si~nal. This ~ y accoun~s not only for the avcrage e~fects of the mçtlillm, but also for l~cal effects, The ~lu~osed ~ tc~ is to make on-line n.ea~ulc~ r~t~ of the m~diuln noise and then to use tbesc mea~u.~ c.lls in signal dcsign. Possible ways to accolll~lish this ~lldtC~,~ are ~ ccllcsed below. They may be cl~c~ified as "w~ite-read-write" recording strategies. First a diagnostic signal is written on the m~iom, second the res~lting m~neti7~tion pattcrn is read, and ~ird an inform~tion carrying signal is recorded. The design of the second written signal dcpends on thc modd for the l~lcdiul~ noise.

~ n n PcTruss4/03722 W0 94/24639 2 1 5 g ~ U 1;~

0.1. C-~
lCc-lc e~l' r' Ko~r~rior~
I ~^eo~e~ I I Deco~r I

l!!neo~C E-/';
Mo~vl~ion ~ o~ul~;c ~ncode~ I l D-,so~e ,o,-.o.o. ~ ,o,c,l,o L Geler ~o~ ¦ ¦ Vet-clor I Eql~ E~R--Id ~C O ~

Figure 1. Traditional model for a magnet~c record;ng syste~.
This figure 1s from the Introductlon to the Special Issue on Coding for Storage Dev~ces, IEE~ Transactions on Informat;on ~heory, vol. 37, May 1991, p. 709.

WO 94/24639 ~ 7 21 5 9 8 0 (1 PCT/US94/03722 There is no generally accepted model for the medium noise. There is some evidence th~t the medium noisc has a l~rge mllltiplir~ive portion. This does not adequately account for medium noise when the medium is tlem~en~tizcd, however, Some initial s;m--lations using our medium model in~ e grcatcr v?-iability of the written magnetiZ~tion in the demagnctized case lhan whcn ~ strong unidirec~ion~l magnetic field has been applied. This scems to be consistent with llleasU~ S from m~e~C media. It should bc noted that the noise in the de~n~g~eti7~d ca~e is correlated with the previous signal wdtten; however, presently we are not including prcviously writlen signals when modeling the mag.-f l;7~.~.orl rrsllhin~ ~om wri~i~g a new signal. While our m~~ model has been useful for co~ uLing the capacity of m~etic media, it has not yet bcen used for ~l~ci~nin~ magn~iC recording systcms, Presen(ly our mcdium model is being used to analyze medium noise in received volt~ge wav~forms ~8].
The approach herc is based on simplirled models for the cffects of lllc.iiuLIl noise. As we gain expcrience wilh ~his approach and as more sophisticate~ models beeom~ avaiiable, we will use them 2. Write-Read-WritePre~ n: AdditiveNoiseCase ~ e begin the analysis for this sche~e with a line~ approximation to the recording pro-cess. l~ecognizing that the process is inherenlly n~nl;nf~r, we anticipate that the approach presenled here must bc improved by using a more accurate nonlinear model for Ihe r~cording system. Due to Ihe specd of clcctronics, dle a~l-oach p~posed holds the possibili~ of being implemented in silicon, Shown in E~gures 2 is a block fli~r~m descdption of the wri~e-read-write recording pro-ce~s Figure 2 shows thrce heads ~ying over a m~gnetir m~iuTn The first records a diag-nostic signal sl(t). The second reads ~e res~lltitlg m~neti7~tio~ on the mP~ m Third, a signal that has been computed by thc ckct~onics and includes ~e dcsircd signal and Cu~
sation for the ~,r"l;~.." noise is wdtten onto the ~ .3i~ . When the info,...~tion is read at a later time, the signal-to-noise ratio will be ci~nific~ntly higher, resllltin~ in better recovery of ~c desired signal, The crucial design of the elecl~n~cs block is baced an a modd for the m~nn~.r in which the .. eA;.. noise is lll~r~ d. The ~liccllscion in this paper is based on an additive model for the l~ ll noise.
Shown in ~igure 3 is an appr~ fe linear model of a rn~ tic leeoldil~g system ~n this system, all blocks are a~surn~d to be linear and time-inYariant, all random processes are ~CSurn~ to be wide-sense st~tion~ A ~li~nsstir signal sl(t) is wntten using the w~itc head (h(t)), mt~Ail)m noise is addcd (nd(t) + nl(t)), the signal is read (g(t)) introAl~cing electronics noise (wl (t)). Thcrc may be an eqn~li7P.r in the system and this is i~lcolpo~aled into h(t) or g(t) as a~?plu~liate. The desired t~h~nncl response is d(t). We assume here tha~b(t) = (g ~ h)(t) (where * ~n~ s convolution)~ Denoting the output of g(t) plus wl(t) by yl(t), the crror signal e(t) = y~ (b * sl)(t) equals the part of the received voltage wave-f~rm due to systcm noise. The noise has a f~pCatal~ cc,nlæc>l~cnt, n"(t), due to the m~ lm, and t~o unrepeatable co~ ~n~n~c~ n1(t) due to the ~ ,., and wl(t) due to the dect~n~cs.
Thc goal is to co..,~n~te for the repeatable cG~ onellt. To do this, e(t) is filtered, sub-tracted from s(t), the infol~,lation-bearing signa~ to be recûrded, then recorded with the writc head again. Physic~lly, a signal is wdtten on the medium then read; the desired signal, d(~), computed elc.,l~u-~i~lly, is subtracted from this signal producing the error signal e(t); then the new signal (s(t) - (c * e)(f is wdtten at the same place on ~e medium.

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The design problem may be stated as minimi7in~ the distortion in the w~veio~ y(t) that is eventually read due to the r~ le component of m~ noisc. From this ~ie~-point, C(t) may be ~P.ci~ 3 from lhe re~luccd system shown in Figurc 4. In that figure, the information-beanng signal s(~ d thc t1i~gnos~ic signal sl(t) have been rcmoYcd. Let ~e distortion ~e measurcd in temls of noise power. Then Ihe goal is to de~ign c(t) to miniml~e ~e ~ignal power in y~(t), the lloiSC co~ .t of the output, ~or any real systcm dcsign, there will be a co~ctr~;nt class, C, for Ihe c(t) so thal they are re~ll7~ble. A typical Co~ ;nt is that it be the output of a tr~nsversal filter Yvith a fixed number of taps. The problcm state-ment l.e;~ cs miC E[l(g * nd)(t) - ~g * h * c $ e)(t)l2]~ (1) c~ .
For many co~ ail ts, the problem reduoes to solving ~e ~ell-known normal equations.
To minimi~e (1), SlJ~5tihlt~ .
e(t) = yl (t)--sl (t) = wl (t) + (g $ (nl ~ n,~))(t) . (2) puting the exl~ect~d value, the problem sli~t~ nt (2) ~uccs to miC ~(c * ~ ~ q)~O) - 2(~* p)(O)~, (33 c~
where for any signal f(t), f(~) - f(~
q(t) = (~ * b * (~,~" + g * g $ (R~ + ~n~)))(t)~ (4) p(~) = (g ~ b* h~)(t), (5) ~o(t) = (g * h)(t), and ~"",,.(t), R""(t), and R~(t) are the aul~cu~ance functions for wi(t), nj(t), and n,2(t), L~s~lively (~ = 1 or 2).
The unconstrained solution to (3), if it cxists is ~u(~) = P(a~)lQ(a~), (6) wher~ P(~) and Q(a7~ are the Founer tra~sforms of p(t) and g(t), ~ yecLively~ and the sub-script ~c in cu(t) ~ sçnts the fact that this is an un-nn~ra~nod optimum solution. This may not ex~st bec~usc Q(~) may haYc ze~os. Also, evcn if this solution cxists, it may not be real-istically re~ hle- Usually some type of co..~ J~t is added so that the solution far c(t) exists and is well-behavcd (easily realizable). In the following section, the con~traint of ~e solution bcing impl~ nt~ble by a tapped dday line (and thus easily re~li7able IlSing stan-dard VLSI design) is added.
Noticc that if all noise were additive and (~allssj~n, and the lln~ rained so]utioll (6) is used, then the limit~ ~ in ca~aci(y is dct~ ....;..~ by ~c change in ~e power spec-trum of ~e cl~nnel As opposed to using a sl~dal~ writing srh~ , ~e col~ cnt due to the repeatable ~"r~ noise is ~t~nll~t~ s ~ r~ tioll increases as the energy of the repe~t:lble c~ ponent increases rdative to thc cnergy of dle ul~ ~a able component~ That is. if the power Sp~llu~l due to the noise in an unco~ t~d system is Sy(~7) = S,(~) + S"~a~), wherc Sr(a)) is thc rcpeatable component, then ~e power s~ecL~ Ulll of ~e colll~cnsated system is Sy~(~) = 5~ )) + S~(Q') s,(~;+ S"~Q') Wo 94/24639 2 1 S 9 8 0 0 22 PCT/US94/03722 ~

Note that S"(~) = IG~)12S""(~) + S""~ ) and S,~a7) = IG(~)12Sdd(a~; here S,~"", 5"", and Sdd are the Pourier t,ansÇo.llls of RWW, Rn~ and R~ es~iv~ly. The capac-ity of thc Gaussi~n channel is thcn i~ erl; ~e increase t~ s on the spectral shapes (~at is, sys~em factor~ such as ~e g(t), h(t3, and the noise levds) and detailed analysis of the standard water-filling c~Lpaci~y formula t9- p. 267~. In our f~ J ;...~,t~1 setup, depcnding on the m~ 1m testcd, we have found dlat thc rcpeatable co~ oncnt is one half to n~ne tenths the total noise power. If a~l of ~c repe~hl~ noise is adequatcly modeled as addi-tive noise, equation (7) implics a potential increasc in signal t~ noise ratio of l .2 to 7.2 dB.

3. Tapped Delay Lin~ h~ c~.tation As m~n~iQn~1 in the previous se~tion, the actual impl~-mP~t~t~oo would be dirre~nl from (6). Some realizAbility cor,.~ r~ must be illlposcd. One natural conctraint (but not the only possible one~ other constl~lts ha~ve been concid~red and thc following derivation can easily bc modificd to ~ccount for other conc~r~intc) is that the imrleIn~.nt~fjon be Kali~ed by a tapped dclay line. Tapped tclay lincs (finile ;mr~1se response filters wi~h a finite l~uuibcr of coefficients) may be easily built using VLSI technology.
Thc col .clr~int class is the set of c(t) such that c(t)= ~, c[n]~(t-nT~, (8) where ~ is a Dirac delta function, and 1- is ~c timc interval bct~cell the taps. Let for each integer n, let - p~n]--p(n~) and qln] = q(nr) . (g) The solution for the optimal tap weights is obtaincd by s~ ;t~ g thc form (8) for c(t) into (3), then taking thc dcr~vatives with respcct to the t~p weights c~nl. This results in a set of 2N + 1 cquations in Ihe 2N I 1 u~ own tap wcights c~n]. The equaLions are ~4tn - k]c[~l = p[n], for - ~ ~ n 5 N. (lO) If the values of c{n~ and p~n3 a~e put into ~rectors c and p, and the values of q[n] are put into thc matrix Q such that the n, k entry of Q is q[n - k], then equatlon (lO) may be re~rrittcn compactly as Qcsp~ (ll) This cg.uation may be solved directly to give cOp, = Q-lp . (12) r.~ ~ively, ~ y p~o~,.1ies of q, c, and p may bc exploited to ~e~uce ~e co~ a-tional comrl~ity of the matrLlc inYe~ u~d ~ (12). These ~ll~-e~ p~S may be s~a~ed co...~ r ~ ;.J = q[-n] and p[n] =- pl-nl; ~ese imply that ~c solution for c is o~d S~ C~iC so c[n~ = - cl-nl-~ wo 94/24639 215 9 8 0 0 PCTIUS94/03722 4. Slmulations In this hrst simulation we made many simplifying assump80ns. Wc assume that thcimpulse responsc function h(t) of the write head is S(t) and the unit stcp response function o~
the read head is a I,orcntz pulse We also sssume that head mech~ m is precise enough to let us write at a predecided position on the medium.
We assume Ihat all noise sourscs are while and G~lls~i~n. Additive repc~t~ble media noisc, head noise and clc~LIunic nois~ are ~ccl~m.~l to have total noisc po~er distributed among themsclves in the ratio of 8~1:1. Total noise power is 10% of thc the signal power The step rcsponsc Or the rcad head is givcn by s(t) = 1 ~i 1 ~ ~2 ~TS, Here TS12 is the half width.of the Lorentz pulse. Differçn~iA~ing this with respect to t we get the impulse response function g(t) of the read hcad _~ 8t g(t)=
2~2 1 ~ _ I
~SJ
Taking the Fourier transform of g(r), we get G(f) - jaf exp(-~lfl) where a = - ~r2TS and b = ~cTS, Let the sampling ratc bc fs(~ KIT). Then the encrgy in the f~qucI~cies greater than fS/2 gets aliased. Thus the total aIiased energy is Ea(fs) = 2 IG~f )12df ~ v exp~-X) 1 + X ~ 2 where v - ~(2T~) ~nd X = fSb = rrSK, K being the nurnbcr of times (fs = Kl~) a bit is bdng samplcd. From the graph of E(~) versus X, it can be concluded that for X = 12 lcss than 0~1% encrgy gets aliased.
In our ~imll1a~ion we choose S = l/~r, which represents a fairly high linear density and we choose K = 10, at which less than 0. 3% of the energy of g(~) and less than 4% of the energy of a square wave get aliased, Thc percentage of the energy of the square wave that gets aliased is high for K = lO . hou~cver it does not matter in this ~imlllation since by exploiting linearity o~ the system, we can compute noise powers indeperl-lçntly of the sig~al.
Note that g[n], the disc~,Li~d vcrsion of g(f) is nonzero even for large Inl. However it is WO 94/24639 21 S 9 8 0 ~ PCT/US94/03722 ~

monotonically decreasing in magnitude. For our ,cim~ ion we have tr lnr~ed g[n] suitably in such a manner ~at thc trllnrate~ g[n~;-8 < n < 8; has more than 99% of the energy of ~e ~ri~in~t nont~nc~ted g~n~. Tap weights in lhe filter c[n] were comrnt~d ~cs~ming-165nS 16.
~ im~ on results show that under above stated as~ ?tions write-rcad-write schcme yields 4. ~4 dB reduction in noise power on thc aver~ge. Figure S compares the noise powers bcfore and after signal ~.~co...~ s~;on ob~ined in IOOO ~u~s. Figuses 6 and ~ show typical read wavcro.,.,s obtained by writing lhe signa~ out and with col~ c~tion.

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Figute 5: Comparision between read noise bc~ore ond ater sign~l precompcnsion U~ or~ r~ . . ~, , E~
~.
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o ~o o ~, 100 --Figurc 6: Rcad signal beforc signal p~_v...pension ~'`~ r, t t ~

.J~o ~r;o ~0 0 ~C ~00 1~0 Figure 7: Read 8ign~1 ~fter ~i~nal pr~compension , 10 WO 94/24639 215 ~8~- Q~ PCT/US94/03722 ~

5. ~oncl~lcions ~ ecently, ~e~iulents have demonstrated that medium noise h~ a repeatable compo-nent [10~. In this ,cullln~a~y we have outlined a strategy ~or preconl~cl-c~ing for local mcdium effects th~t are additive through the use of a write-~ead-write re~ording protocol.
We ha~e a~so explorcd the use of more soph;c~ir~ted models for ~e effects of mediun~ noise, includiog mllltiE~lir~tive effectc. ~iti~ studies show that for multiplicative noise, thc rela-tively simple analysis presentcd abovc is not adequate Whilc estirnatcs of thc effects of the mnltir1lr ~jve and additive compone~ts may be estim~t~A, their op~im~1 use in sign~l com-pensation is c~ mplirflt~ :
References 1. J. A. O'Sullivan, D. G. Porter, R. S. Indeck, and 1~1. W. Muller, "Physically BasedInforrnation Sci~-nre of Ma~tic Recording I. L~fo,~llalion Capacity of a Medium ~qodel," ~EEE Trans. ~agnetics, vol. 29, 1993.
2. J. A. O'Sullivan, D. G. Portcr, R S. Indeck, and ~. W. MulleL "Recording Medium Propcr~ies and Capaci~y Bounds," J. Applicd P}~ysics, 1993, 3. M. Mansuripur and R. Giles, "~im~ rjon of the Ma~n~i7~ion-Reversal Dynamics on tllc Connection M~ in~ " Computcrs fn Phrysics, vol. 5, pp. 291-302, l99l.
4. R. Giles and M. M~ncunrur, "P~s~i~le Sources of Co~ ,ivil~ in lhin Films of Amor-phous R~re ~arth Tr~ncition Metal Alloys," Complltcrs u2 P~ysics, voI. 5, pp. 204-219, 1991.
5. H. N. Bertram and J. Zhu, and ~ Ehrenreich and D. lbrnbull, eds., in Solid State Physic.s, A~ad~rnic Press, New York, 1 g92.

6. J. Zhu and H. N. Bertr~m, "Miclo~ tic Sludies of Melallic Thin Films," Journal of Appli~d Physics, vol. 63, pp. 3248-3253, Ig88 7. J. Zhu and H. ~. Berlram, "M~ A~ion Reversal in CoCr Thin Films,-' Journal of Applied Physics, vol. 66, pp. 1291-1307, 1989.

8. J. A. O'Sullivan, D. G. Portes R. S. Indeck, and M. W. Muller, "Physically 33ased Information Srj~ce of ~A~ ic Recording II. Physical Models for Me~ Noise,"
submttted to ~EEE Trans. Magnetics, 1994.

9. R. Blahut, Principles and Pracficc of Information ~heory, Addison ~esley, ~rlin~, MA, 198~.

10. J. R. ~Ioinville, R. S. IndecJ~, and M. W. Muller, "Spatial Noise Phenomena of Longibl-dinal M~n~tic Recording Media," ~EEE ~rans. Magnet~'cs, vol. MAG-28, pp.
3398-3406, 1992.

Claims (15)

What Is Claimed Is:

1. A method for writing a noise compensated signal onto a magnetic medium with a device having a convention-al recording transducer, said method comprising the steps of:
determining the remanent noise of said magnetic medium as represented by an analog electrical sig-nal;
compensating said signal for said remanent noise; and writing said compensated signal on said magnet-ic medium.

2. The method of Claim 1 wherein the step of determining includes the steps of:
saturating said magnetic medium; and reading said remanent noise from said saturated magnetic medium.

3. The method of Claim 2 wherein said device in-cludes three aligned recording transducers and wherein the step of saturating is implemented with one of said recording transducers, the step of reading is implemented with another of said recording transducers, and the step of writing is implemented with the third of said record-ing transducers.

4. The method of Claim 3 further comprising the step of:
indexing said recording transducers with said remanent noise to thereby write said compensated signal at the point on said magnetic medium where said remanent noise has been compensated for.

5. The method of Claim 1 further comprising the steps of:
first writing said signal onto said magnetic medium; and reading said signal from said magnetic medium.

6. The method of Claim 5 wherein the step of determining the remanent noise includes the step of com-paring said read signal with the signal as first written.

7. A method for creating a magnetic medium with a signal written thereon which when read, has already been compensated for the remanent noise in said magnetic medi-um, the method comprising the steps of:
writing said signal on said magnetic medium;
reading said signal from said magnetic medium;
compensating said signal for the differences between it and said read signal, said differences being indicative at least in part of the remanent noise of said magnetic medium; and writing said compensated signal onto said mag-netic medium and at the same location thereon as originally written.

8. A method for compensating a signal read from a magnetic medium for the remanent noise of said medium, said method comprising the steps of:
reading said signal from said magnetic medium;
determining the remanent noise of said magnetic medium; and compensating said read signal for said remanent noise.

9. The method of Claim 8 wherein the step of determining further comprises the steps of:
saturating said magnetic medium; and reading said saturated magnetic medium to thereby determining its remanent noise.

10. The method of Claim 9 wherein the step of determining the remanent noise is performed closely after the step of reading said signal so that said signal may be compensated in a relatively short time delay.

11. A method for compensating a signal read from a magnetic medium for the remanent noise of said magnetic medium in real time, said method comprising the steps of:

reading said signal from said magnetic medium with a first recording transducer;
saturating said magnetic medium with a second recording transducer, said second recording trans-ducer being aligned with and closely spaced behind said first recording transducer to thereby satu-rate said magnetic medium after only a short time delay from reading;
reading said saturated magnetic medium with a third recording transducer to thereby continuously determine the remanent noise thereof, said third recording transducer being aligned with and close-ly spaced behind said first recording transducer to thereby determine said remanent noise after only a short time delay from saturating; and continuously compensating said read signal with said remanent noise signal as both said signals are generated to thereby continuously produce a compensated signal in real time.

12. A method for determining a benchmark in a magnetic medium, said method comprising the steps of:
saturating a portion of said magnetic medium;
and reading said saturated portion of said magnetic medium to thereby determine its remanent noise, said remanent noise being unique to said portion and therefore a benchmark identifying said por-tion.

13. A magnetic medium having a remanent noise compensated signal recorded thereon made by implementing a method comprising the steps of:
determining the remanent noise of said magnetic medium as represented by an analog electrical sig-nal;
compensating said signal for said remanent noise; and writing said compensated signal on said magnet-ic medium.

14. A magnetic medium having a remanent noise compensated signal recorded thereon made by implementing a method comprising the steps of:
writing said signal on said magnetic medium;
reading said signal from said magnetic medium;
compensating said signal for the differences between it and said read signal, said differences being indicative at least in part of the remanent noise of said magnetic medium; and writing said compensated signal onto said mag-netic medium and at the same location thereon as originally written.

15. A device for compensating a signal read from a magnetic medium for the remanent noise of said magnetic medium, said device including:
means for reading said signal from said magnet-ic medium with a first recording transducer;
means for saturating said magnetic medium with a second recording transducer, said second record-ing transducer being aligned with and closely spaced behind said first recording transducer to thereby saturate said magnetic medium after only a short time delay from reading;
means for reading said saturated magnetic medi-um with a third recording transducer to thereby continuously determine the remanent noise thereof, said third recording transducer being aligned with and closely spaced behind said first recording transducer to thereby determine said remanent noise after only a short time delay from saturat-ing; and means for continuously compensating said read signal with said remanent noise signal as both said signals are generated to thereby continuously produce a compensated signal in real time.