CA1129999A - Recording and/or reproducing apparatus for a magnetic tape - Google Patents

Recording and/or reproducing apparatus for a magnetic tape

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Publication number
CA1129999A
CA1129999A CA314,365A CA314365A CA1129999A CA 1129999 A CA1129999 A CA 1129999A CA 314365 A CA314365 A CA 314365A CA 1129999 A CA1129999 A CA 1129999A
Authority
CA
Canada
Prior art keywords
light
record carrier
magnetic head
emitting
signal
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired
Application number
CA314,365A
Other languages
French (fr)
Inventor
Edmond De Niet
Albert M.A. Rijckaert
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Koninklijke Philips NV
Original Assignee
Philips Gloeilampenfabrieken NV
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Philips Gloeilampenfabrieken NV filed Critical Philips Gloeilampenfabrieken NV
Application granted granted Critical
Publication of CA1129999A publication Critical patent/CA1129999A/en
Expired legal-status Critical Current

Links

Classifications

    • GPHYSICS
    • G11INFORMATION STORAGE
    • G11BINFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
    • G11B5/00Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
    • G11B5/48Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed
    • G11B5/54Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed with provision for moving the head into or out of its operative position or across tracks
    • G11B5/55Track change, selection or acquisition by displacement of the head
    • G11B5/5504Track change, selection or acquisition by displacement of the head across tape tracks
    • G11B5/5508Control circuits therefor
    • GPHYSICS
    • G11INFORMATION STORAGE
    • G11BINFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
    • G11B5/00Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
    • G11B5/48Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed
    • G11B5/58Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed with provision for moving the head for the purpose of maintaining alignment of the head relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following
    • G11B5/584Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed with provision for moving the head for the purpose of maintaining alignment of the head relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following for track following on tapes

Abstract

ABSTRACT
A recording and/or reproducing apparatus for a record carrier in the form of a tape, with a plura-lity of parallel longitudinal tracks. The apparatus comprises positioning means for controlling the posi-tion of the write/read head transverse to the direction of movement of the record carrier. The control signal for these positioning means is obtained with the aid of two optical systems, which each comprise a light-emitting element for emitting a radiation beam which is aimed at one edge of the record carrier and a light-receiving element for picking up the amount of light which shines past said record carrier, at least one of the elements of each optical system being rigidly connected to the write/read head. A detection device is coupled to the two light-receiving elements, which device derives the desired control signal from the amounts of light received by these two light-receiving elements.

Description

~2~a9~

The invention relates to apparatus for record-ing and/or reproducing information in a plurality of parallel longitudinal tracks on a magnetic record car-rier in the form of a tape, which apparatus comprises a magnetic head which is adapted to cooperate with the record carrier which is passed along said magnetic head, positioning means for controlling the relative position of said magnetic head with respect to the record carrier in a direction transverse to the direction of transport 10 of the record carrier and in a plane parallel to the . .
tangent plane of the record carrier with the magnetic head, and control means for the generation and applica-tion to the positioning means of a control signal which is representative of the difference between the instant-aneous and the desired position of the magnetic head.
In multichannel magnetic recording and/or re-producing apparatus with a magnetic record carrier in the form of a tape and a (multichannel) magnetic head, information being recorded and/or read in a plurality of longitudinal tracks on the record carrier9 it is ; ~ essential during information read-out that the magnetic head occupies a correct position relative to the tracks , , , '-.

~2~ P~IN"~

on the reco:rd carrier. T.he accuracy with whi.ch said mag-netic head :i.s positi.olled relative to the tracks dictates the dis-tance to be ma:inta.ined betweeIl acljacent tracks on the record carrier and thus also deterrnines the ef`-ficiency of the information sl,orage, i.e. the amount of`
information which can be stored per unit area of` the record carrier.
The width of the trac~s and the track'distan-ces are beiDg reduced continua.l.l~ i.n order to increase the information dens:ity. As an e~ample, systems have ~ been developed in which the record carrier contains over 100 tracks with a track w:i.dth of approximately 40/um and a track distance of 10/um. Tt ~ill be evi-' dent that such systems impose very stringent` require-ments in respect of the positi.on of the magnetic head.
,, ln order to meet these stringent positional requirements servo systems of the type mentioned in the preamble have be~n developedj which serve f`or very accu-- rately controlling the positi.on of the magnetic head in a direction transverse to the direction of transport of ' the record carrier, Such a system is for example known : . from British Patent~1.260.706. The system for position-~ i.ng the magnetic head described in this Patent Applica-;; tion employs reference tracks on the record ca.rrier : 25~ which are scanned by additional magneti.c heads, the . si~nals supplied by said addi.tlonal mag-lletic heads be-. .
ing processed into a measuring signal whlch is repre-PHN~C351 Z ~ ~ 9 31.3.78 ~senl,at:ive of tl~e posit:ion of` the Irlagnetic head relative to tlle rc,~cord carrier.
The u~se of SllCil a measuring .system has some signi.fieclnt disadva1ltages. The major disadvantage is the fac-t that when SUC}l a system :is used the available i inf`ormation capacity on the record earrier eannot be utilized in. an optimull1 manner. This is first of all because of` the fact that; the reference traeks of eourse oeeupy useful informat:i.on eapaeity on the réeord earrier.
As a result of this i.t is neeessary to eompromise in res-peet of the measuring and eontrol range of the eontrol-means and the loss of`:inf`ormation capaeity on the reeord earri.er~ e3l the said referenee traeks a7-e-use the measuring range is namely dietated by the width of these traeks, so that a large measuring range is automatically atten.ded by a`substan-tial loss of information capaeity on the reeord earrier. This loss of inforrnation eapacity becomes even worse when these magnetieally seanned refer-enee traeks are used, beeause of the f`aet tha-t the po-~20 sitioning means can. t.hen be operative only when the re-eorcded inforrnation..is reproduced. As a result of this, an area along the two edges of,the reeord carrier hav-ing a width equal to o:r greater than the possible tape shift in the -trans~erse direction eannot be used as in-f`ormatlorL area~ becàuse oui.ng to the fact that duri.Mg reeordi.ng the ma.gnetie head pOsitio.1l :is IlO t; COl`ltrO1.1 ed the magneti.c head whi.eil serves f`or recc-rding in an in-11 _ P~IN 8951 31.3.78 :~ormat:iorl or refere:l1.c~ track s:i.tua-ted wi-tI-Iin said area may coll~p]etc.l.y loosc contact w:itll the record carrier.
A further dra~back of this kno~:n system is the seII.siti~ity for the type an.d irnperfections of the magnetic material. used in the record carrier. Once the i strength of the reference signals to be written in the refel-ence tracks has been adjusted, the strength of` the reference signals read during :reproduction obviously de-pends on the sensitivity of the magnetic material used i.n the record carrier. As a result of this the gain in the control loop formed by the control means and posi-tioni.ng rnean.s i.s dependent on the type of magnetic material of the record carrier, so that it is difficult to optirllise th L9 control. loop for each type of"record 1~ ~ carrler. Furtherrnore, any imperfec'tionsj such as for exarnple drop~outs, In the magneti.c layer of the-record carrier will result in failure of the positioning of the magnetic head, which may gIve rise to substantial : ! disturbances during -the i.n.formation read out.
Finally, this known system has the drawback that the bandwidth of -the transfer characteri.stic of . the control loop constituted by the control'means and '- pos:itioning rneans depends on the speed with which the record carri.er is passed along the magnetiG head, i.n parti.cular the re'là'ti~e speed between the record car-: rier and -the rnagnetic head by WhiCil the reference -tracks are soanned. The max~mulrl si.gnal fre~LIlency which Ca.ll be - ~7N 8951 ~ ~ Z ~ 31-3.7~

reacl by a magnet;ic head mai:[1.ly dcpen~ls on the speed with wh:icll t}le record carrier is moved a].ong the magnetic head. Conseq-uently, the kllown system callnot functi.on optimally at an.y arbitrary speed of thc record carrier, l in parl;i.c~lar at low speeds.
It is an object of the invention to provide a recording and/or reproducing apparatus provided wi.th control and positioning means, in which, whilst main- -tainillg an accurate positioning of the magnetic h.ead, the clisdavantages of the k.nown system are eliminated and in particular the avai]able information capacity - on the record carrier can be uti.lized in an optimum manner.
For th:;s purpose the invention is characteri~-ed in that the control means are provided with a first and a second optical system, each comprising a light-emitting element for emitting a radiation beam against one edge of -the record:carrier and a light-receiving element for receiving the amoun-t of light fromA the ra-diation beam emitted by the associated light-emitting elemen-t which shine~ past the record carrier, at least one of the said two elements of each optical systerl1 be-ing rigidly connected to the magnetic head, and the control means furthernlore being provided with a cletec--tion de-vice ~Thi.cl-l is coupled to the li.gllt-receiving - elements of the two optical systellls for generatillg -thc contro.1. s:igllal for the pos-itioning mea.ns in depelldence on the rel.at;ivc mag31.itude ol` the anlou-nts of .light re-P~-IN 8g51 ~1.3.78 ceived by -the two ligll-t-rece:iving elemellts.
In contrad:istinction -l-o the Icnown system the position o* the magnetic head :in the apparatus in ac-- cordance Wit]l the invent:ion is not related to reference tracl~s but to the two edges of the record-carrier tape.
This enables the use o~ an optical measuring system, which includes both optical systems, for measuring the posltion of said magnetic head. The use of` these optical systems for generating a measuring signal which is re-presentative of the posit:ion of the magnetic head first of all has the advantage that no information capacity on tlle record carrier need be sacrified. Moreover, as this opt:ical measuring system can also be operative during recording, this mcans that the position of the 152 infor~lation tracks is already accurately defined dur-ing recording, so that it is no longer necessary to re-serve an area along the edges of the record carrier as a safety margin, i.e. abstain f`rom using it for infor-matlon storage. This Means that owing to the use of` the ~0 opticc~l measuring system practically the entire width of the record carrier can be used for information storage.
The use of an optical measuring system which coopera-tes with the edges of the record carrier further-
2~ more results in a v-ery ~arge meclsuring range of lhe con-trol means. In fact, thiq measLIring range is equal to .

~JJN ~9~1 31.3.78
3 ~2~!9 the ~:idth o:~ the record carrier, so th.a-t a h.igh rclia-bility :in respect of -the positioning of` the magnetic hcad is ob-tained.
Furthernlore, this optical measuring system is ¦ independent of the -type o:f magnetic material of the re-cord carrier, so that the control loop, which is con-stituted by the contro:L means and the positioning means~
can be designed in an .opti.mum manner independently of the type of record carrier. Morcover, the control sys-tern is insensi-tive to dropo~lts and the like in the mag-netic layer of said record carrier Finally, the fre-q~ency response of this optical measuring sys-tem is in-dependent of` the speed with which. the record carrier is . passed alo-ng the magnetic head, so that an optimum opera-tion of the control 1QP is also ens~lred at an arbitrary low speed.
Thre operation of the system in accordance with the invention is based on the recognition that when the magnetic head is correctly posi-tioned, the record car-rier either masks~exactly half the radiation ~eam ernit-ted by each of the light-emitting elements or masks half of each of the light-receiving elements. The light-receiv-ing elements of each o:t` the optical systems then recei-ve equal amounts of light. If there is a relative d:Lsplace-ment between the recorrl carr:i.er and magneti.c head i.n a directi.on transverse to the d:irect:ioll ol` transpo:~t ol the record carri..er, the two light rece:i.ving elements re-Pf-~ g~51 31.3.7 ~ ~ 2 ~ ~ ~9 ceiv~ unequal amoun-ts of`~ight. The diI`L`erence bet~ee the a.rnounts of l.i.gll-t recei.ved by these two .Light-re-ce.i.~lng elenleIlts is then representative of the diff`er-ence betl~een th.e :instantaneolls and the desired position ~of the magnetic head and may therefore be used as con-trol sigllal for the positionong means. For this, it SUffiCeS to generate an electric signal ~ith the aid of a detection device, whic]-l signal is dependent on the relative mag:n.:itude of the amounts of ligllt receiv-ed by the two light~receiving elements.
There are several possibil:ities of realising the optical system and the associated detection device.
II1 accordance with a first embodiment, each of the two optical systems comprises a light-transducer f`or con-vertlng the amount of ligllt received by the light-re-ceiving element of the relevan-t optical system into an electric output signal and the detection device is pro-vided with an adder circui.t for adding the output sig~
' nals of the two l.ight transd-ucers to each other with opposite polarity and supplying the sum signal as the - control signal. Furthermore, this embodiment is pre-f`erably characterized in that tlle light-emitting e:Le-ments of the two optical systems a.re coupled t;o a mo~
dulati.on device for the in~phase variation of ~the in-tensit;y oI` the radiati.on heatns em:itted by the two light-em;t-ti:ng elemen-t.; wi.tl-l a rnodul.ation frequency and tha-t;
the de-tection device for obta:ining t:he control signal 31.3.7~

comprises a synchrono-ls detector, to whi.ch. the sulrl sig-nal I`rom the aclder c:ircl~it ancl a signal derive~ f`rom the modulat:ion devi.ce, having a frequency equal to the modulation frequency, are applied~ r`his embodiment has I the aclvantage that a.c. amplifiers can be used in thr detection device. Suitably, this embodiment is charac-ter:ized in that the detection device comprises a band-pass filter wlth a passband around the modulation fre- -quen.cy, v:La which band-pass f~ ter the sum signal is applied to the syllchronous detector. This ensures a satisfact~ry elirnination o:f the inf`luence of stray light in a simple manner. In these embodiments of the apparatus in accordance with the invention the light-emitting elernent of each of` the two optical systems may consist of a light guide~which is optically coupled to a cornmon llght source. Thus it is ensured that after a one-ti.me adjustment the twc, light-emit-ting elements emit a radiation b~am wlth the correct relative intens.ity rati.o irrespective of variations of the intensi.ty of the luminous energy produced by the ligh~ source.
An other preferred embodimellt of the apparatus i accordance with th.e invent:ion is characterized in that the light~emit-ting elements of the t-wo optical systerl~s are coupled to a modulatioll device for vary-ing the in~en.sits~ of the racliation beams ern:i.ttecl by the two ligl)t-emitt:i.ngr elements wi-th a modll1.ation :fre-P~lN 895 1 3 1 . 3 . 78 .: .
quency in phasQ oppos:i.t:ion ancl that the detection de-vi.ce :is aclapted to supply a surn signa:L ~hich represents t~.le SUIll 0:~ the amounts of lig:ht received by the two :l.ight-:receivin~,r elemeIlts, whilst sa:icl detection device furthermore cornpr:ises a synchronous de tecto:r, to which t:his electric surn s:ignal and a sign~l derived from the modulation device, having a frequency equal -to the mo-dulation frequency, are appl.led. In this emhodiment of the apparatus in accorcl.ance wlth the invention the light-emitting elements of the two optical systems consequelltly emit a radia tion Ijeam whi.ch varies in phase opposition. The desired control signal can then be obtained by summa.tlon of the amounts of l:ight re-ce:ived by the two light-receiving elemen-ts and by syn-chronous detection. Sald sum~nat~ on can be obtained by convertingr the amoun-ts of ~ight received by the two light-receivingr elements lnto electric signals with the aid of light transduoers and~ add~ng~these to each `: o-ther. In a preferred embodiment this summation is ~: ~: 20 performed optically, namely by the use o:E a light ~:: :
guide as light-reseiving element for each of the two optical systems ancl by optically coupling these to a common light transdu¢er.
s in the~se two l.ast-mentioned prefe:l-red em-2~5 ~odl nents the light--emi t ting elerrlents agaiIl emit vary-ing ligrllt bearlls5 i.t i s .possible again to effectively elim .nate the ef *ect of stray li.ght by the inclus:ion :~ ~, , ' 31.3.7~

:in tJle clet;ection dev;.ce of a band-pass :[`ilter wi-th a pa.ss-band around the modula-t:iorl frecluency.
The invel~tion will be described in more de-tail with referellce to the dra~;i.ng, in which Figs. 1, 2 ancl 3 show three different embo-di.ments of the apparatus in accordance with the inven-tion.
Fig. 4 shows a circuit: for driv:ing the light emitting diodes eMployed in the embod:iment of Fig. 3, 10 and Fig. 5 shows the varia-tion of the drive cur-.
rents for these diodes.
Fig. 1 in a perspective view shows a holder 1 to whicll a magnetic head 2 is secured, which in fact comprises a multitude of magnetic heads K. Alon~ this magnetic head 2 a magnetic tape 3 is passed in a direc-tion V, on which magnetic tape information can be re-. corded or read in a number of tracks corresponding to . , ` the number of heads K by cooperation with the mag~etic .
: 20 head 2 In order to ensure that the magnetic heads K
.
. are always aligned relative to the relevant tracks on the magnetic tape 3, positioning n-~eans are provi.ded, comprising a drive means 4, which is capable o.~ mov-~; 25 ing the holder 1 iD a directioIl R. This dri.ve means 1 receives a control si.gnal from control means ~h:i.ch : -detect the po~.tion of` the holder 1 re.1.ative to the PHN ~ ~ ,5 1 31-3.7 2 ~ ~ 9 rnagnet:i.c tape 3.
These control means compri,se a :first optical ~ystem 5, consisting of a ligllt-errlitt:ing element 6 and a light-receiving element 7, and a second optical sys-teln 8, comprising 1 .I.igrht-ernitting element 9 and a light-i receiving elemen-t 10. The two light-emitting elements 6 and 9 may for example cons:is-t of light-emitting diodes `' (L~D~s)~ which are.energized fIom a power supply source 11. These elements G and 9 are arranged 90 that they ea.ch shine on one edge S1 and S2 of the magnetic tape.
The ligh.t-receiving elements 7 and 10 may f`or example consist'of` photodiodes which are moullted on th,e holder 1 at a centre-to-centre disLa.nce f`ronl each other'which, viewed iIl the direction R, is equal to the width B o.f the magnetic tape 3. ' The radiation beams ernitted by the light-emitting elements 6 and 9 towards -the respective light-recei,vin:g elemen~ts~7 and 10 are part],y masked by tlle , magneti.c tape 3. When~ the holder 'I is in the correct position relative to the magnetic tape 3, i.e. when ' the magnetic head. 2 exac'tly coincides with the magne- :
1 : : tic tape 3, the portions of` the radiation beam emitted by the eléments 6 and 9 which radiate past the magnetic tape 3, ar.d;are incldent on the photodiodes 7 and 'lO
are equal., so that thc outpllt signals of the photo-diodes 7 and 10 a-re also eq~lalr l~hen l:he ho].der 'I moves . .
: in the d:i.rectioll R~ the magnetic tape will masl; OIle of :

;' 13 P~IN 8CJ~1 31.3.7g the rad:iatio:n beams I`or a greater par-t alld the other radiatio.tl beam for a .smaller part, so that the output signals of` the photodetectors 7 and 10 dif'fer fxom each o -tller .
This data i5 used for deriving a suitable -! control signal l'or the dri.ve means l~. In the present .-'' embodirnent, in which :Lt is assumed that the two light-ernitting elementvs 6 a:nd 9 emit a continuous radiation ' beam, a detection d.evice 12 has been included for this purpose, which determines the difference between the output signals'of the two photodiodes 7 and 10 and which .:
for this purpose may for example comprise a differential amplifier. The. difference signalis then applied to the drive means v].a a servo ampli,fier 13 and thus corrects the pos,ition of the ho'lde.r '1~ ~.e. of the magnetic head 2~ relative to the magneti,c tape 3. ' '`
The construction of the dri-ve means 4 is ir-relevant for the prinolple of the invsntion. An e~a.mplc ' of this is a magnetic system I.n which the hol-der 1 is rigidly secured to a mag7let coil 1 which'is disposed in a magnetic field, which coll is energized by the con,trol signal~. As a second example i-t is possible to sec~re th,e holder 1 to thecllassis via piezoelectric bencLing elernents, whi,ch ars energized by the control 2~ 5ig~al.
' In the pr,esent embodimeIlt, the linear -measuring rangs of the measur:ing systsm, con9titu-tecl . - -- 1L~ .

I'IIN ~51 31.3.78 ~2~9!9 by the two optical systems corresponds. to -the dirnens.ion of` the two p}~otodiodos 7 and 10 :in the ~lirection R. In this embodilnellt it has been ass~med thal; the two li~ht-emitting elements ~ and 9 ecLch emi.t a comparatively wide radiation beam and that the two photodiodes 7 and 10 are partly covered by the record carrier 3, i.e. each f'or one half in the case o~ a correct positioning of the ho].der 1. ~`he limits of this li.near measuring range~
within which the output signal of -the detecti.on devi.ce 12 varies linearly with a movelnent of the holder 1 in the directi.on R, ocour when one of the photodiodes 7 and 10 is f`ully covered by the rnag.netic tape 3. How-: . ever~ dlSO outside said l.:i.near measuring range the pre-' sent control s~stem operates correctly~ because iIl the case of a larger relative displacement between holder .~ . . .
~ 1 and ma~eti.c tape 3 one of the photodiodes remains ~, -fully covered, so tllat~a maxlmum control signal for the dri.ve means l~ remain~s available. The control Sys-' : tem becomes inoperative only if the relative displace-20 : ment between holder i and magnetic tape 3 îs, SUCil that -the two photodiodes~ 7:and ! are not at.all covered by the magnetic tape 3j which corresponds to a maxirl1um permlssi.ble displacernent in both directions equal to the width~B.~or the magnet.i~o tape 3. ln practice the-:: :2~ . -- measuri.ng a.n.d control range o:~ the system shownin ~ l~ig. 1 :i3 thereI`ore enti.rely deterlniIled by the bearn .:' ' ~
width of the radiatioIl beams produced by l,he li~ht :

'15 ;: : ~ , ~ ~ , .
.: ' ~ . . , : .

31.3.78 ~L2~9 en1itting e:Lelllent-3 6 and 9, which can be optimized by the choice of the lig~1lt-em:i.ttiIlg elemen-ts 6 and 9 ahd, as the case may be, by lenses which are coupled there-to. In this respect tlle use o* lenses is useful in or-der to ensure that each of the emitted radiation beams, i as far as possible, is a parallel beam with a most ho-mogeneous light distribu-tion.
If light-emi.tting elements with a smaller beam wi.d-th of the~emitted radiation beam are to be used, the light-enlitting elements 6 and 9 can be made to rnove along with the holder 1, i.e. by also moving the drive means ln the direction R. For the light-emitting elements 6 and 9 li.ght guides (fibers) may ,, then for e~ample be used, which results in the embo-dimen.t as schematicaI.ly shown in cross section in Fig.
2, Corresponding elements in th~s Figure bear the same reference numerals~ as~ln Flg.~
The oonstrUotlon~o~ the~llolder 1 with mag-netic head (not~shown~aIld the phot~diodes 7 and 10 20 ~ is identlcal to that in~accordance with Fig. 1. II7 contrQdistinotlon~to tIle embodiment o* Fig. 1 the ght-ein-itt~lng~elemeDt~s~6 ancl 9~ are no~l aooon7rnodated in a second~hold~er 15~whloh is rlgidly couple-d to the holder 1` and which~can thus also be rnoved in the direc-25~ tlon R by tho drlve means l~. These light--emitting ele-mel7ts 6 and 9 110W consist of l:ight guides (fibres) ~ ~ ~7~ ends are fixed in the holder 15. Tlle other end :; ~ ,: :

-~16 31,3,78 o~ each o:f tlLese l:igllt guides may be connected to an ind:ividual light source, but because the two light guides in -the present embod:iment always emit light the same time, it is po,ssible to employ a common light source 16 for the two ligrh.t guides 9 which is I driven by a power supply source 11, The control signal for the dr:ive n~eans 4 can be obtained in a manner identical to that shown in :Fig, 1 with the aid of the . detecti.on.device 12 and the servo-ampli.~ier 13. -Owing to the rigid coupling be-tween the holder 15 and the holcler 1 it is achieved that the beam width of the radiation beams emitted by the light-emitting elements 6 and 9 no longer limits the measur-ing and con-tro] range of the system, so that in this embodlrrlent the control range corrésponds to twice the ; width B of the recorcl carrler, In the embodlment of Fig. 2 a modulation de-vice 20 is incIuded between the light-emitting diode 16 and the power supply source 11, which device re-ceives a modulation slgnal~ from an oscillator 19, This rnodulat,ion devioe 20 serves to ensure that the light-emitting elements 6 and 9 do not emit a continuous : radiation beam bu-t a periodically varying radiation ~ béam. The form of this periodic variation is not es-.
2~ senti.al., A9 a.n exarnple~, the light~emit;-tLng diode l6 -. may ~e energize~ w:ith a direct current ~:i.th a sinusoidal .. ,............................... i alternating current superimposed on i.t, which resu1.ts : . . .

PHN 8~51 31.3.7~
9~9 in radiat.i.on beams wh:ich in addition to a cont:Lnuous luminous inte:llsity exhibit a sinusoidally varying li.g'ht intensity. In~stead of a sinusoidal variation it is also possible to vary the luminous intensity in ac-cordance with a squarewave~ In tho last~e.ntioned case it is even possible to operate the light-emitting diode internllttently by dispensing wlth the d.c. component, i.e~ to turn lt on and off perlodi.cally. In'order to derlve a suitable control signal'for the drive means
4 from the surll signal supplied by the det~ction de-vice 12, it is necessary to apply said sum signal to a synchronous detector 18 t~hlch also receives the control ; signal for the modulation devlce 20, ~.. e. the signal ,, : wlth a frequency equal to the modulation frequency, : :
from the oscll'lator~l9. The output s~ignal of this ~, ~ : s~rnchronous detector 18~is then agaln representatlve : of the msgnltude~and ths dlreotion of the positional , error,of the hol~er 1.: ~ ~
; The uss~of a varylng lntenslty of the radia-~ tion beams first~:of all has the advantage that'only a.o.; ampli~flsrs nsed~be ussd ln the control loop.
far more:lmportan.t advantage is ob-tained in respect o~ ths slllnlnatlon:of~the in~luence of stray light on the position measurement. It will be evident 'that stray 25~ ght whlch~is~lncident~on~the two photodlodes 7 and 10 ~: , , ;
: in wle~qual amo~lnt~ provides a contrib~;ti,on to the surn signal,of the detection devi.ce l2and thus lmpalrs the :

~ 18 ..

.~,,,., ~ .

31.3.7g ~l2~9~9 pos.ition measurelllellt. Generally, this stray light will g:ive rise to a d~c. component in said sum signal but in addition an a.c. component, *or example in the case of artificial light. By selecting the modula-tion frequency ¦ ~or the variation o* the radiation beam su*ficiently highl for example 100 kHz and by including a band-pass ~ilter 1ll with a pass-band around said modulation I`re-quency in the co'nnection between the detection device 12 a.nd the synchronous detector 1~ the contributi OIl in the sum signal of the detection device caused by stray light is blocked, so that the ef*ect o* stray light on the measurement is *ully eliminated~
Fig. 3 shows an embodiment o* the apparatus ''' in accordance with the invention employing two indi-1~ vidual light-emitting diodes 6 and 9. By means o* a mo-dulation device 20, included between these light-emit-ting diodes 6 an.d 9, and the power supply source 11 and the oscillator 19, it is again:ensured that the light-em.it-ting dlodes 6 and 9~emit a radiation beam which ': 20 vari.es wi,th the modulation *req-uency. However, in con-tra-distinction. to the embodi.ment o* Fig. 2, in which the ~: var.iati,olls of the two radiation beams are :in phase, the .

two li,ght-emi-tting diodes 6 and 9 are now driven in phase OppOsltioll-, so t.hat th.e variations o~ the radiat;on beams are also in phase opposi.ti.on.
- ln the holcLer 1 the two light-r.eceivillg ele-ments 7 and 10 are, agai.n arranged opposite the li.g1rt:-.
,.: I 9 ~.

,, .

PHN g~51 3 1 . 3 , 78 ~s emittiIlg d:iodes 6 and 9, which light-receivi~g elements are con3tituted by two :Light gui.des (fibres) in the pre-sent embodiment. These two light guides are coupled to a common photodiode 17 whose output signal consequent-ly represents the sum of the amounts of light received ¦ by the two li.ght-guides. When the magnetic tape 3 is situated halfway between the ends of the two light gu.ides 7 and 10, these two light guides receive unequal amounts of light at substanti.ally any instant, but the sum of these a~ounts of light is constant, which means that the output signal of the photodiode 17 is then con-.
stant. If the magnetic tape has shifted relative to this desired position, the photodiode 17 supplies an output signal with an a.c. oomponent of a f-requency equal to ~ ~ the modulation frequency (for example 100 kHz), whose arrlplitude lS a measure of the magnitude of the posi-tional error between the magnetlc head and the mag~leti.c tape~ whilst the~;phase~relatlve to the modulation signal l for the rnodulation device 20 indicates the di~ection of the positional error.~The contr~ol slgnQl f`or the drive means 4 oan therefore be obtained by applylng the out-put signal of said photodiode~17 to a syncllronous detec-tor 18, whi.oh-also reoei~es~the modulation signal SUp-plied by the oscillator 19. In~order to eliminate the ;25~ infLuence oI` s-tray l.l.ght; a band-pass filter 1~ ith : ~ ~
~ a passband arou~d the modulation frequency (100 kll~) ..... .
can again be incl.uded between the photodiode 17 and : ~ ' .. , .. . ~ . . - . .
:

31.3.78 :~2~9 sa:i.d synchrono-ls detector 18.
Instead of the two light guides 7 and 10 it is obviously also possible to accommodate two separate photo-diodes in the holder 1 in the present embodiment, ¦ and to sum the output signals of these photodiodes ! electrically. Furthermore, it will be evident that in respect of the location. of the light~emitting elements 6 and 9 and the~light-receivi.ng elements 7 and 10 rela-.
tive to each other~ there are rnany variants, which not 10 only appl:ies in respec-t of the embodiment shown in Fig. 3 but also in respect of the embocliments in ac-cordance with Figs. 1 and 2. For example it is readily possible to repl.ace the light-receiving ele~lents 7 and 10 in the holder 1 by one or both light-emitting ele-ments 6 and g. The two elements which are not placed on the holder 1 may then optlonally be rigidly coupled to the drive means 4, i.e. so-that they move along with the holder 1.
It will be ev~.dent that for the accuracy of : 20 the systems shown it is essential that the two opti-cal channels of the m~asuring system are, as far as possi.ble, identical. ~ deviati.on between these two optical channels can be compensated for by adjustment of the dr:ive of the li.ght-emitti.ng elements or of the de-tection device.
Fig. ~I shows a circui.t fo:r dr:i~i.ng the light~
emi-tting diodes 6 and 9 used in t.he elllbodinlellt o~ Fig. 3 Pl:IN 8951 31.3.7~
~2'~9 by means o~ which said adjustment is possible. This cir-cuit includes the transistors Tl and T2, whose emitters are i.ntercollnected via a resistor R and are also con-nected to -the negative terminal of the supply voltage I source v~a resistors R3 and RL~. The light-emitting diodes 6 and 9 are connected to the collectors of these tran-i sistors T1 and T2 and receive control currents I1 and I2 from these transis-tors.
To the base of transis-tor T1 a control signal is applied which consists o~ a d.c. component and an a.c. component. The d.c. component is derived from a reference vo]tage Vx via a voltage divider R6. The a.c.
~- component is derlved from ~the modulation signal of the oscillator 19 (Fig. 3) applied to input terminal K via the isolati.ng~capaoitors C1 and~C2 and the voltage di-vider~R5, By adjus~tlng the voltage dividers ~5 and R6 the ratio~between~the amp~litude of the a.e. component and the value~of the d~.c.~ component~can be adjusted, 'the amplitude o~f the a.e.~oomponent~being~ selected to be smaller than the d.c. component.
The~bas~e~or transls~tor T2 recelves a cont-rol slgnal from an operational~amplifier ~. The emitter of trarislstor T1~is oonnecLed to the non-in~rertillg input (+) of~said operational amplifier A via a lo~-pass fil-~25 ~ ter F3, whi]st the eml-t~ter of transistor T2 is eon~leet~
ed to thG inverting input (-) of said operational am-plif1er A ~_a a Low-pass fllter ~2. Thus it is achieved ~ ~ ' ' P~IN 8g51 31,3.7~
:~2~9 that the direct voltages on the emitters of the tran-sistors T1 and T2 are necessarily equal and thus al50 the direct currents through these transistors T1 and T2. Th:is means that the drive currents I1 and I2 for the light-emitting diodes 6 and 9 contain equal d.c. components I =, as is indicated in Fig~ 5.
Thus, one of the most important conditions is created to ensure that the light-emitting diodes ernit equal amounts of light. The amount of light emitted by a light-emitting dibde substantia]ly depends on two factors, i.e. the temperature and the drive current.
; Since the two light-emitting diodes receive the same direct current the:ir d-Lssipations are equal. By ensur-ing that the thermal resistance between the two diodes is small, for example by including them in a common cooling body~ it is ensured that the two diodes have the same temperature with a high accuracy.
The aJoc component of the drive current I1 is directly derived from the modulation signal applied ZO to the terminal K. The a.o. component of che drive cur-rent I2 is deterrnlned by connecting the tapping point of the resistor R1 to the inverting input (-) of the operational amplifier A via a high-pass filter F3.
Tllus it is achieved that the alternating voltage on the emi-tter of transistor T2 is exactly in phase op-pos:itivn to the alterllating voltage on the emitter of transistor T1, the ratio of the anlplitlldes of these t~o .

.. ... . .

PHN 8g~1 31.3,7g ~2~9 al-ternatirlg voltages being determined by the adjustment of the tapping of the resistor ~1~ Thus both the pha~se relationship of the a.c. components of the drive cur-rer3ts I1 and I2 and their amplitude r~tio are accura-tely defined.
The circuit can be adjus-ted simply when the control loop is interrupted before the record carrier 3 is inserted. ~n that case the co~trol signal for the drive means should be ~ero. If this is not the case ow-ing to an inequality in the optical paths, said control signal need merely be reduced to zero by changing the tapping point of` the resistor R1, i.e. by changing the ratio of the a.c. components of the drive currents for the light-emitting diodes.
Obviously~the-light output of the light-emitting diodes can be~f`urther stabilised inter alia by means of negative~feedbaok. This can be effected fully electrically~but, if desired also partly op-tically~, for exatnple by tapping a part of each of the 20~ ; radiation beams with thé aid~of light guides, summing ~ :
these~and convertlng~thern into an electric feedback signaI~for;-the~control circult via a synchronous de-tector.
As has~been lndicated for the embodirllent 25~ described with ~eference to ~ig. 1 the Ina~etic head ~.
2 may comprlse a rnu:Ltitude of adjacently disposèd heads K~ which each cooperate Witll a separate track ::
~ - 2L~ -: : , . :.

P~IN 8951 31.3 78 ~Z~

0~ the record carrier. In -this case it is possible to record a multitude o~ di-~Ierent inrormation signals on the record carrier at the same time. However, this ar-rangement is equally suitable Yor recording one infor-mation signal only. The available storage capacity of the record carrier can then simply be u-tilized in an optimum manner by applying the in~ormation signal to a dif~erent (adjacent) head K when the end of the re-cord carrier is reached and simultaneously reversing the direction o~ movelnent of the record carrier, so that adjacen-t tracks on the record carrier can be re-corded consecutively. Obviously, it is also possible to use a single magnetic head in this en-lbodiment, which head when the end of the record carrier is reached is subjected to a displacemen-t equal to the desired track dlstance between the tracks on the record carrier transverse to the direction o~ movement.

:

~ ~ ' : :

_ 2~ -~

': . : ' ''. ':

Claims (7)

31.3.78 THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS
1. Apparatus for recording and/or reproducing information in a plurality of parallel tracks on a mag-netic record carrier in the form of a tape, which appa-ratus comprises a magnetic head which is adapted to co-operate with the record carrier which is passed along this magnetic head, positioning means for controlling the relative position of this magnetic head with res-pect to the record carrier in a direction transverse to the direction of transport of the record carrier and in a plane parallel to the tangent plane of the record carrier with the magnetic head, and control means for the generation and application to the positioning means of a control signal which is representative of the dif-ference between the instantaneous and the desired po-sition, of the magnetic head, characterized in that the control means are provided with a first and a second optical system each comprising a light-emitting ele-ment for emitting a radiation beam against an indi-vidual edge of the record carrier and a light-receiv-ing element for receiving the amount of light from the radiation beam emitted by the associated light-emitting element which shines past the record carrier, at least one of the said two elements of each optical system be-ing rigidly connected to the magnetic head, and the con-31.3.78 trol means furthermore being provided with a detection device which is coupled to the light-receiving elements of the two optical systems for generating the control signal for the positioning means in dependence on the relative magnitude of the amounts of light received by the two light-receiving elements.
2. Apparatus as claimed in Claim 1, characteriz-ed in that each of the two optical systems comprises a light transducer for coverting the amount of light received by the light-receiving element of the rele-vant optical system into an electric output signal, and that the detection device is provided with an adder circuit for adding the output signals of the two light transducers to each other with opposite polarity.
3. Apparatus as claimed in Claim 2, charac-terized in that the light-emitting elements of the two optical systems are coupled to a modulation device for the in-phase variation of the intensity of the radia-tion beams emitted by the two light emitting elements with a modulation frequency and that the detection de-vice for deriving the control signal comprises a syn-chronous detector, to which the sum signal from the adder circuit and a signal derived from the modulation device, having a frequency equal to the modulation fre-quency, are applied.
4. Apparatus as claimed in Claim 2 or 3, char-adcterized in that the light-emitting element of each of the two optical systems consists of a light guide and that these two light guides are optically coupled to a common light source.
5. Apparatus as claimed in Claim 1, character-ized in that the light-emitting elements of the two optical systems are coupled to a modulation device for varying the intensity of the radiation beams emitted by the two light-emitting elements with a modulation frequency in phase opposition, and that the detection device is adapted to supply an electric sum signal which represents the sum of the amounts of light received by the two light-receiving elements, whilst said detection device furthermore comprises a synchronous detector, to which this electric sum signal and a signal derived from the modulation device, having a frequency equal to the modulation frequency, are applied.
6. Apparatus as claimed in Claim 5, character ized in that the light-receiving element of each of the two optical systems consists of a light guide, and that the detection device comprises a light transducer with which the two light guides are optically coupled and at whose output the electric sum signal is available.
7. Apparatus as claimed in Claim 3, 5 or 6, char-acterized in that the detection device comprises a band-pass filter having a pass-band round the modulation frequency, via which band-pass filter the electric sum signal is applied to the synchronous filter.
CA314,365A 1977-11-11 1978-10-26 Recording and/or reproducing apparatus for a magnetic tape Expired CA1129999A (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
NL7712418 1977-11-11
NL7712418A NL7712418A (en) 1977-11-11 1977-11-11 RECORDING AND / OR DISPLAY DEVICE FOR A BAND-SHAPED MAGNETIC RECORDING CARRIER EQUIPPED WITH A MAGNETIC HEAD POSITIONING CONTROL SYSTEM.

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CA1129999A true CA1129999A (en) 1982-08-17

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AT (1) AT358842B (en)
AU (1) AU521060B2 (en)
BE (1) BE871892A (en)
CA (1) CA1129999A (en)
CH (1) CH637782A5 (en)
DE (1) DE2848047A1 (en)
ES (1) ES474925A1 (en)
FR (1) FR2408893A1 (en)
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IT7829602A0 (en) 1978-11-08
NL7712418A (en) 1979-05-15
FR2408893A1 (en) 1979-06-08
ATA807078A (en) 1980-02-15
ES474925A1 (en) 1979-03-16
CH637782A5 (en) 1983-08-15
GB2008290A (en) 1979-05-31
BE871892A (en) 1979-05-09
DE2848047A1 (en) 1979-05-17
US4176381A (en) 1979-11-27
GB2008290B (en) 1982-03-24
AT358842B (en) 1980-10-10
JPS5476219A (en) 1979-06-18
AU521060B2 (en) 1982-03-11
IT1101037B (en) 1985-09-28
AU4143378A (en) 1979-05-17

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