USRE24641E - Magnetic record testing means - Google Patents

Description

APlll 28, 1959 D. K. REYNOLDS MAGNETIC RECORD TESTING MEANS Original File'd Nov. 23, 1953 Re. 24,641 Reissued Apr. 28, 1959 Unitedl States g Patent Oice MAGNETIC RECORD TESTING MEANS Donald K. Reynolds, Seattle, Wash., assignor, by mesne assignments, to General Electric Company, New York, N.Y., a corporation of New York Original No. 2,793,344, dated May 21, 1957, Serial No. 393,869, November 23, 1953. Application for reissue January 16, 1958, Serial No. 709,429

9 Claims. (Cl. 324-34) Matter enclosed in heavy brackets appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue.

nodules, as they are called, do appear, despite these precautions on the part of manufacturers. Present day information handling machines must be accurate, since `they are being employed now in accounting systems and inventory control systems, and, indeed, even in banking systems. Therefore, great lengths are gone to, to insure their accuracy. Error checking and error correcting codes are employed, and, in many instances, there is duplication of apparatus land programming of a problem in a number of different ways, in order to check the operation of the machine.

As :a result of these requirements for accuracy storage mediums, such las magnetic tapes,-ar'e inspected thoroughly, and any defects therein uusally mean that the tape cannot be used. If defects, or nodules, are unobserved, the data is incorrectly stored at that location and is a constant source of error for the machine.

One method for checking tape which is known and used is to write a pulse pattern of a single binary digit over the entire tape. These pulses are laid down .in the plurality of tracks or channels on the tape in which normally information -s to be recorded. Then the pattern 'of pulses is read out simultaneously by means of 1a magnetic transducer head positioned over each channel. The outputs from these heads are applied to a number of gates and to a counter, which function to actuate the counter when all the heads do not simultaneously provide an output. Thus vall the imperfections on the tape which prevent a proper recording of a pulse `are counted except that more than one vnodule simultaneously present under the heads is countedl as one defect. This does not matter, since for the purpose of recording a defect in one or all of the channels at a given location, since it is customary `to record binary informationr in parallel, renders that location useless. 'I'he tape is then either -accepted 'cr rejected, based upon the nodule count and the .amount of error tolerancepermitted. If accepted with a maximum number of nodules errors are bound -to occur, but this isV cheaper than throwing the tapes away until a perfect one is found.

- vAn object of the present invention is to `provide, la system for identifying nodules in a magnetic recording medium so that errors due thereto may be avoided.

. Another object of the present invention is to provide apparatus fori-dentifying defects l in a magnetic recordingy medium, so that less perfect recording mediums may be employed and still provide error-free operation.

Still another object of the present 'invention is to provide a system for checking defects in magnetic tape which is novel and useful.

Another source of trouble in magnetic tape is known as skew.V The present system for utilizing tape is to use a plurality of tracks and simultaneously record pulses, representative of b-inary digits, in these tracks by means of magnetic heads which are aligned transversely across the tape and with each head positioned over each track. The tape is subject to a number of stresses, both from rapid starting and stopping and even in reading. Accordingly, when the tape is again passing under the magnetic heads, one section or side of the tape may be stretched more than the. other. This causes the pulses in the tracks tobe misalgned to a certain extent. Accordingly, instead of pulses being simultaneously read from the tape exactly as they were recorded on the tape, there is a delay between the reading of the digit in each of the tracks which is proportional to the stretch or skew which the tape has. Another skew difficulty results from the tape passing the aligned heads at an angle which is different in recording than in playback. This eiect is a function of the tape transport mechanical design.

An object of the present invention is to provide a system for compensating for skew.

Another object of the present invention is to provide a system for determining the amount of skew assumed by .a given tape.

The above and further objects of the invention ar'e achieved in a system wherein the outputs of the magnetic reading heads zare applied through an or gate to a l'rst variable time delay circuit to the trigger a first pulse generator. The output of the first pulse .generator is employed las va data sampling pulse. 'I'he magnetic head outputs are also applied through an and gate to a second and gate .as an inhibiting input to that second and gate. The pulse generator output is also applied to the second and gate. The output of the second .and gate is applied .to a counter and also to a recording head. Accordingly, if tape has been prepared by recording `a pattern of pulses in every track, in reading the tape this invention will count al1 the defects on the tape and will record a pulse or train of pulses, which locate this defect. With such defects being marked in this manner yalong a nodule marking channel, it is a simple matter to read the pulses on the nodule marking channel and employ them to prevent recording on the tape location where the nodules vare marked. In this manner, errors in reading from tape due to nodules are eliminated. Also, even though a tape has va large number of defects,`it can still be employed and provide substantially error-free operation.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention, itself, both as to its organization and method of operation, as well as additional objects zand advantages thereof, will be'best understood from the following description when read in connection with the accompanying drawings, in which:

Figure 1 is a schematic diagram of an embodiment of the invention, and

Figure 2 is a diagram of the Wave shapes which are obtained at various portions of the circuit shown in Figure ll and which :are shown herein, in order to provide a better understanding of the operation `of this invention.

kReference is now made `to Figure 1, where for illustration of the invention it is shown being employed with a..section of magnetic tape 10 having `a plurality'of recording channels. As an illustrative embodiment, four channels are considered, three of which may be considered data recordingchannels, and the fourth the nodule marking Ichannel. The tape passes underneath the magnetic .transducer ,heads represented ...by rectangles. The lthree heads 112,14, ;16 for the data channels .aresubstantially aligned .and .thehead .18 .for the nodule .channelis displacedslig'htly .from .the .otherheads in the directionfof motion of the tape 10. The alignmentof `theheads can b e transverse, as shown,.or they may bepositionedin a skewfashionif desired. .They must be positioned, however, .so that'a recorded digit in..a binary number passes undereach one of. themsubstantially simultaneously. The tape is prepared .for testing by writing pulses in the data recording .channelsa1ong .the `.entire length of the tape. yl'hese 4pulses may all lbe .representative of vbinary ones or binary zeros. y,'1`heirnp0rtant thing is that the same plscibe written bymeans of the substantially transversely aligned data headsin the data tracks. The writing may be .donebyeither using ythesame heads used for reading and writing or'by usingseparate heads for each. The tape is .thenrunpast thefour heads shownin Figure .1. The outputstrom the . threedataheads 12, 14, .16 are applied vto.,thr.ee high-,gain.ampliiiers .22, 24, 26, each-of which arnplifies the signal picked..up and applies it to a .separatelyg'associatednetwork known as .a.slicer.3,2, 34, .-36.

vv'The sliceris acircuitfor .permitting only that portion l'o`f.a signal to passthrough which exceeds .acertain minimum level and which does not exceed :a maximum level. A suitablecircuitof this type is described and shown on pages 45 and 46 of the book Waveforms byChancefet al., ,publishedby McGrawfHillBook Company in 1949. The .output .of.each.slicer is .applied to a separate ip- llopcircuit 42, 44, 46. These flipop circuits are of the `well knowntwo-tube Eccles-Jordan type and may be .found described onpages 595 c0597 of ,the bookRadio Engineering by Terman andpublishedby McGraw-Hill Book lCompany in 1947.

`Each trigger circuit hasone input terminal designated lby "S vas the ,setterrninaL to which the application of positive, pulses causes the flip-floptohave one of its tubes lconducting andthe other tube nonconducting. .Output 14is-.taken from the .anodeof the conducting tube andap- .plied to afseparatecathode followerstage 52, 54,56. The other ,tube gridofthetrigger circuitmay beconsidered asa resetgriddesignated v.as R", input terminal,since application ,of apositivepulse .to the .other tube grid resets -the flip-flop to itsinitial .condition from which it was ,.transferredby application of a pulse to its S input. .TheinitiaLcondition maybe called a nonindicating con- -dition and .the condition to 'Whichthe ipop is driven .by thev applicationof a pulse tov its. set terminal-may be Ydesignatedas the indicating condition. Accordingly, vwhen ,.the.`jiip.-flop in each channel isdriven to the vindicating conditionby an .output .from its associated slicer, an out- .put .is .appliedto -the .associated cathode follower. Each .cathodemfollowerSL .54, .56.. applies its .output to (1) an or gate 58; (2) a first and gate 60; and (3) .adata .reading .fand. .gate 62, ,64, 66.

"Or. gatesand and gates are well known in the art, .and .asuitable type-.for each maybe foundv described in an ,articlesentitled .Diode coincidence and mixing circuits..for vdigital computers by .Tung ChangChen in the Proceedingsnf thelRE, May 1950, pagesSll to 514.

An .an Igate .is.a.coincidence.device lwhich provides '-anoutputonly when all.its;inputs are. present simultaneously. An or gate is a device or circuit whichprovides .an output whenanyone of its inputspare present. Thus, .the .for.`gate.58 providesanoutputwhen it,.receivesian .inputirom oneeof .thechannels from l the magnetic transducerheads `outputis applied to .an adjustable time .delay circuit v68. ,.An .adjustable time delay.circuit .Ofiatsitable type may be found described in`.Electronic "Instruments .by Greenwood et al., published by 'Mc- GrawfHilliook Companyand mas" beJfOlmd Onfpages 591 et seq.

'4 This circuit is merely -a one-shot multivibrator which provides an output pulse at a time later than the application of an input pulse by an interval which can be determined or controlled by adjusting the circuit component values.

The output of the adjustable time delay circuit 68 is applied to la'pulse `generator70. This pulse A)generator ,is wellknown. intheartvandmaybe ablocking oscillator of lthe type ,described ein .chaper .6 vof the .above noted book Waveforms. The output of the pulse generator is applied as a second input to the data and gates 62, I64, 661and v`also to'asecondand gate-72. The 'second and gate `-hasasitsother. input lthe output of the rst and gate 60, which'has'been inverted through a phase inverter 74 andfthenijapplied through Ja ,cathode follower stage 76. Thisinverted input serves to inhibit the opera- 'tion offthel-second' andgate '72,'lwhich otherwise will pass" the o utput ,of thejfirst ,pulse generator. This type. .of and gate is`known as a but-not gate, `inthat it `will pass one of its inputs but not both. This type of and L gate is foundedescribed*,byfFelker invan articlein Electrical .Engineering for .December .1952, entitled Typical block. diagram .for .fai transistor digital computer.

The rst pulse generator output is also applied toa .second time-delay .circuit l78f vvhich issubstantially-similar to the first. The outputofsthe'secondttime delay. circuit `78is applied to a-.second,.pulse generator 8.0 .which also .is similartothe.'rshbl'ocking oscillator. Its outputis vutilizedto.reset thefiipopcircuits 42, 44, :46 ,in v.each channel. .Its output, herein referredtto as .a data-shift ,pulse.has..other usesas well. .If ,the-,apparatus shown is .used .solely .for ...the ,.purpose of reading the tape, :the: iirst and second andgates V60,72'. and the apparatusfollowing them yis not -requireddata :is fread. as the outputs. of each `one .of .the data and.gates 62,64, 66. The data shift pulse .'rnay'fbe used* to transfer. the yinformation read4 from thefdata .andgates Yintoy subsequent apparatus, .to clear it .to ,receive inew data. yAlso, .-both the data .sampling gpulse and the .data.shift =pulse.may be employed as clock pulses having a given desired interval between-.them as determined-by the time delay .networks. These .clock .pulsesgmay be employed. in whatever. apparatus in which v the particularsystem vshown ...and -described .herein .are

incorporated.

The rstwandf second and .gates operate to provide pulses for marking.anodulevchannel 'on the tape. The .first and gatey onlyprovidesfan outputfwhenallthe pulsesare present and readirornthe data channels. The `second and;. gate 72 provides-aan output pulse .anytime there-isa pulseinone ofthedatachannels being. read. In l'view. of .the'roperationof the rst time delay network 68, the'fdata-'sampling pulse does not loccur Vvuntil. the. data pulse :has existed for a desired interval. Accordingly, the'lsecond "andggate'can be .inhibitedby theflrst .and :gate-.andnwill net epass the 'subsequently occurring .data lsampling-pulse if --uallfthe pulses occur. in-.the data track. :If .one .oft themdoes notjoceun due -to .thepresenceof ra 'nodule-.lor other ',impverfection, `the datasampling .pulse .passes through 'the second ?-and"gate .and then isapplied :by-means of fa; cathode' f o'llower182. to a counter'= 84 to -be counted. @Thessecond vandff'gate outputgis .also applied through another time nielay network.86-.similar.to the precedingffones .and fanother `cathode i follower 88 tto the lnodule'-ma'x'lring magnetic yrecording yhead .18. .The-mag- Lnetie recording *head records apulse int thenodule :chan- -1-n`el. `The"reason *foreits displacementis `that thetape has travelled a"--small distance fin thefinterim required for `the marking apparatus -f-to *fbe Voperated. ""Thus the nodule` marking pulse., is applied adjacent toftheI Alocation of' the'V nodule.A The number dfnodules are also counted by the jcounter. 'Thus theI endproduct is fa tape which has all its'nodule locations identified and counted. Of course, if desired, Y"othermeans' of -nodulef marking Vmay 'be'emp1oyed,==such as -an-actu`al marking, so =-.that

5. visual identification may be made. The tape may then be spliced, with the imperfect portion cut out. Otherwise, with a magnetic nodule marking, itis well within the skill of one versedin the art to require that the nodule channel be read in advance of writing on the tape. The output from the nodule channel can then be used to prevent writing until the imperfect portion'of the tape has passedby. Another and also a preferred embodiment of the invention is to place the nodule marking head slightly in advance of the nodule along the direction of motionl pulses have been recorded in the three channels, but the` third channel has a nodule, and, therefore, no pulse is recorded transversely with the two pulses in the othertwo data channels. The magnetic flux in the transducer head during playback is shown as the second curve 13 of Figure 2. The output voltage .from the transducer head will have the waveform 15 shown in the third curve. The output of the slicer will be the rectangular pulses 17 shown in the fourth curve, and the-ilip-op will provide the pulses 19 as shown in` thesixth curve. The data sampling pulse shown b y the seventh curve 21 occurs during the interval of,the existence of the outputs from the flip-Hops. Accordingly, itfis during -this interval that the data.and \gateswill. provide an output. The first and gatewill not provide l'an' output in view of the fact that.

all of its inputs are notfpr'esen't.i-"Accordingly, the second "and gate will pass a pulse from the firs-t pulse generator which serves to provide av marking pulse transversely aligned on the tape with" the defective portion of the data channels. The reset pulse shown by the last curve 23 occurs at a short time after the data sampling pulse and serves to reset the flip-Hops so they may be in condition for indicating the next signals coming under the magnetic transducer heads.

If the signals read by the magnetic transducer heads do not occur simultaneously as a result either of the skew lof the tape or of the misalignment of the recording heads (not shown), then the first an gate will not provide an output and the indication is of the same type (noninhibiting) as if a nodule had occurred. By increasing lthe delay of the rst variable time delay network, the data s-ampling pulse can be further delayed until Ian output is provided from the first and gate. Thus there is available a means of compensating for tape skew. There is also available a means for determining the extent of the tape skew since the amount of delay required in order to permit the first and gate to operate and inhibit the second and gate can be readily measured. When this is considered together with the rate of tape travel, the amount of tape skew can be easily computed.

Although the system is here shown and described as one wherein the location of the nodules are identified by a pulse in the nodule channel, this also can be performed by marking a pulse everywhere along the nodule channel except where a nodule occurs in the data channels. Thus the presence of the nodule is identified as the absence of a pulse. This can be accomplished by omitting the inverter in Figure l and changing the second and gate to be the same type as the first and gate.

It will be appreciated that the present invention is employed where recording of closely spaced pulses on tape is desired, but not so closely spaced as to smear together so that the pulses cannot be treated as discrete units. Furthermore, if the test recording is made with large gaps between test pulses, the large gaps will remain untested and the system will provide' accurate results only if the laying down of pulses is subsequently performed in the same regions as the ones in which the original test pulses were laid down. The usual defects typically eect pulses over about 25 to 50 miles of tape and With the present recording systems between ve and ten pulses are usually packed in that space.

The lower limit settingof the slicer 32 can be employed, to a certain extent, to increase or decrease the number of nodules or tape imperfections marked by the subject invention. The nodules on a tape are characterized not only by losing percent of the signal recorded thereover but also by causing a considerable reduction in amplitude of the recorded signal. Therefore, a determination should be made of the minimum amplitude signal to be tolerated on readout and the slicing level should be adjusted accordingly.

Accordingly, 'there has been described and shown above a system for identifying, counting, and marking on tape the location of nodules so that tapes which have heretofore been deemed not usable for accurate recording and reproducingare now made usable. Also, compensation for tape skew and the measurement thereof is now afforded by means of the above described system. The defect marking system herein obviously may also be employed with magnetic drums and other magnetic recording mediums wherein recording is made in the manner,

described above.

I claim:

1. Apparatus for testing a magnetic recording medium employed in a system wherein said recording medium has a'plurality of parallel data tracks, and a separate magnetic transducer head is positioned over each of said tracks, said apparatus comprising means to record substantially transversely aligned pulses in all said data tracks, a means for each channel to generate a signal indicative of one of said aligned pulses being read by the transducer for that channel, means to generate a data sampling pulse after a desired interval responsive to output from. anyy one of said means to generate a signal, means responsive to a coincident output from all said means to generate a signal for each channel to generate a coincidence indicating pulse, and means responsive to a data sampling pulse and to the absence of a coincidence indicating pulse to mark said magnetic recording medium at the location of the pulses which fail to generate a coincidence indicating pulse.

l 2. Apparatus as recited in claim l wherein said means to generate a data sampling pulse after a desired interval includes an adjustable time delay network tol permit determination of the extent of skew 0f said magnetic record ing medium.

3. Apparatus for testing a magnetic recording medium employed in a system wherein said recording medium has a plurality of parallel data tracks, and a separate magnetic transducer head is positioned over each of said tracks, said apparatus comprising, means to record substantially transversely aligned pulses in all said data tracks, a fliplop circuit for each channel having a non-indicating and an indicating state, means to couple each head to an associated flip-flop to drive it to an indicating state upon reading one of said pulses, means to generate a data sampling pulse after a desired interval responsive to any one of said flip-flops being driven to `an indicating state, means responsive to a coincidence of all said flip-flop circuits being in said indicating state to generate a coincidence indicating pulse, means responsive to a data sampling pulse and the absence of a coincidence indicating pulse to mark said recording medium at the location of the pulses which fail to provide a coincidence indicating pulse, and means responsive to a data sampling pulse to reset said flip-flops after a predetermined interval to their nonindicatng state.

4. Apparatus as recited in claim 3 wherein said means to generate a data sampling pulse after a desired interval emploi/'edi ufa system' "hereinfsidireeording medium; hasond-'time Ydelay' network; measftvceuple jsaidflfrst-andf gate output to said second and gate, means t'c'rni'arkA 7. Apparatusgfordetelingsand# indicating the presence determined llocations 'inf each*ofifsa'idarays,l said arrays' being disposed longitudinally along said medium,v said medium including an index-'tradk'disposdlongitudinally endl, umefdely network? a said second pulse generator toTgit'wte"apulse'responsive'1 thereto, and'm'eansf-'t'o lapply' sid-iscond'pulse generator' for detecting a1:- d'efect" inll'anyi otteo: said; predetermined loctitiorts` of said one array. and for"1 producing a'- firstV outpub signalA whenever a' defect isdet'ectedl thereby,v means'VA resp'onsivefftoany one ofIsaid-bits'of'saidmne array for providnga second-output signal;A and-^m`eans` responsivel tothe simultaneous occurrence'of said first and second' output signalv orproducing` a third outputy signal to control the recording' of an index marker atv a`4 position along said `index track which lcorresponds to the longitudinali position of said-one array;

8. Apparatus "for producing:asubstantially simultaneous deliverywof a plurality of signals-corresponding respectively tolthe' binary digits stored i representative form ina transverse recording position on afrecording medium provided witha-plurality of substantially parallel information'storagetracks, said mediumfbeingemployed in'a system wherein each ofanv array of transducer heads is positionedoppositevr a= respeclive'-on'efof saidtracks Vand wherein relative` motion-*isfprovidedlfbetween'said heads andfsaid medium nadireetion substantially parallel to said tracks,` comprising-af plurality of bistableI devices each' having twof stable states and'operable 'upon 'receipt` of the output signals from a respective one of saidV transducer heads` to assume thev stable state.*correspondingA to the stored binary digit? passingtthe-co'rresponding head lat that instanhar delay'signalgenerator v'coupledfto all of said bistable dvice's'and 'responsive to the first of said bistable devices`- to 'assumef a'- predetermined stable state when ra` transverse recording" positionV on saidu recording mediuml re'aohessad varray off heads 4for"generating"y a sampling signalifafter a^predetermned delay, andVr means coupled 10J-said delayfsignalgerteratnr andtoeach of said bistable devices and'responsivet'.sidsatplingl signall for deliver`v ing-:substantially simultaneously'apluality of signals each` representing .the-statefof' afrespective'one of said Ibistable devices whenfsaidf samplingfsignal' occurs.U

9.' Apparatus as"'in"claimf 8 further including. means coupledto all of said'bistable devices and responsive` to said sampling vsignal for Y resetting all --ofisad' f bistable' d'e` vices to acommo'n predeteminedstable stateafter said sampling-signal is' generated."

References vCited-l in the` `ile of vthis patent or the voriginal patent UNITED :STATI- ".Sy PATENTSy 2,528,2950'1' WiegandH Oct'. 3'1`, 1950 2,528,682 Bianey` Nov.` 7', 1950 2,628,346 Buridiart Feb. 10, 153.53 2,817,829" Lubki'n" Dec. 24, 1957

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