CA1321422C

CA1321422C - Sampled servo code format and system for a disc drive

Info

Publication number: CA1321422C
Application number: CA000552657A
Authority: CA
Inventors: Vernon L. Knowles; Robert E. Kier
Original assignee: Hewlett Packard Co
Current assignee: HP Inc
Priority date: 1986-11-26
Filing date: 1987-11-24
Publication date: 1993-08-17
Anticipated expiration: 2010-08-17
Also published as: JPS63146281A; EP0269381A3; US4823212A; JP2593492B2; EP0269381A2

Abstract

Abstract The system and method for normalizing the ampli-tude of servo code derived signals in a disc drive, includes an improved embedded servo code format of separate magnetic dibit fields, recorded in half track steps. one magnetic dibit field is an automatic gain control field in which the magnetic head sees the same amount of magnetic dibit regardless of its position with respect to the track. A fast settling AGC loop responding to signals from the AGC field, produces a gain adjust signal while the magnetic head is still in the AGC field, which is used to normalize the amplitude of all following servo code and data code signals in that sector. The cycle of automatic gain control is repeated for each track in each sector. A sector mark following the AGC field has a magnetic dibit pattern which is fault tolerant. It is transduced in three parts to minimize error in its detection and identification and provides a timing reference for servo signals which follow. A defect dibit is recorded if the following servo gap is found to be defective during manufacture so that the next servo gap may be ignored. A
track position servo code field provides signals for track following.

Description

~32~ ~22 IMPROVED SAMPLED SERVO CODE FORMAT
AND SYSTEM:E`OR A DIS~ DRIVE

Backaround of the Invention 1. Field of the Invention This invention relates to improvements in disc memory drives and, more particularly, to improvements in system and methods and sampled servo code formats for a magnetic disc drive.

2. Description of the Prior Art Magnetic disc storaye ~ystems, called disc drives, provide large volumns of accessible memory. ~hese conventionally comprise a stack of me~ory discs mounted in axially spaced posi-tions on a common spindle to be rotated at constan~ speed. The disc~ have data recorded in concentric circular tracks on each disc surface. Corresponding tracks on the disc surfaces are cylindrically aligned. Magnetic heads on a moveable carriage, there being one head for each disc surface, are radially and circumferentially aligned, to be moved as a group to position a selected head at a s~lected track ~or reading or writing at that track locativn.
The track~ are each divided into equal radially aligned sectors, aligned from disc to di~c. ~ach sector has a servo code recording at its beginning, read by the magnetic head for use by the ~ervo in driving the carriage and head for track following and ~rack seeking operations.
The U. S. Patent to Lewis et al, 4,424,543 provides 2 ~ 3 . : : :
. :. -, . : ~ .:
, . ' ~;. , ~, , :~ .
, -"` 132~22 prerecorded s~rYo data in a section at the beginning of each sector in each track, which comprises the sequence of an erase gap, a preamble, a sector mark, a track nu~ber, a check code, and a track position code. User data follows tho~ sector at each track. The erase gap ~unction is to provide a time synchroniza-tion. The preamble provides clock synchronization. The sector mark is used as an addition~l verification of servo timing. The track num~ers ar~ us@d for track s~e~ing. The checX code is usad to check for clock hi~t.. The track position code is used for track cen~ering. The code tran~itions in the track position code are written to equ~lly ov~rlap ad~acent tra~k~ and to be offset in time. When a head is on track c~nter, the signals from transitions on one side o~ ~he track are equal to the signals ~rom the transition~ on th~ other ~ide o~ the track, which is recognized by the servo as ~ track centQred condition. The track position code i~ th~ only 5~rYO cod~ written to overlap adjacent tracks.
In providing th~ d~crib~d $QrVO code ~or~at, Lewis et al, are concerned wi~h provlsion~ ~or accura~ely transducing the servo eode, ~or d~t~cting tim~ ~hl~t errors and ~or minimizing the possibility of data overwrite into th~ embedd~d servo code, as discu3s~d ~ith re#pect to Fig. 2. Th~ disclo~ur~ conc~ntrat~s on ~r~X id~ntiricatlon an~ a clock shi~t. Nothing is said about pro~ision~ for auto~atic gain control.
The U. S. Patent ~o P~nniman, 4,S30,019 also provides prerecorde~ s~rvo da~a in a ~ec~ion at the beginning of each :
... ..

sector in each track, which comprises the sequence o~ an erase gap, an automatic gain control ~ield and A h B bursts o~ servo control infor~ation for track cenl:ering purposes. Automatic gain control i5 de~cribed as follow~;: "Inclusion of the AGC circuits (not illustrated) allows the AGC burst of tha pattern to be monitored and retained for use to ad~ust t:he gain of the circuitry used to process the head transducer output of servo information and al~o da~a to acco~nodate variations in the disk storage media. This is particularly u~;eful to provide noise i~Dmuni~y if the di~k iY to be read by a disc drive c: ther than the one on which i~ was creatQd". Nothing fur~her is said about automatic gain control, ~pecially ~GC codQ formatting, or the details of a sys~e~ ~or proces ing and using signals ~rom the AGC
f ield .

Acceptable s~rvo ~y~teDI function in a disc dxive re~uire~ c:are~ul control o~ ~ignal gain~ Although Penniman men-tions automatic gain ~:on~rol, and i'c~ u~e, a~ noted above, the thrust o~ hi~ toaching for a control for ~h~ rins~ positioning~ of a transduc~r h~ad o~ a di~c driv~a unit res~ upon the ~ tablish-ment of tiD~e~ rs~rence~ bas~d on ~hl2 ~ransition be~ween the ~rased gap and ~n AGI:: bur~t tog~th~r with a 8~rvo code decoding t~c:hnique for fine posi~ioning o~E th~ ~ran~ducer head. ~either Psnniman nor Lewi~ et al l~r~at au~omatic g2lin con~rol as a ~ac~or requiring attention.

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.

-1~2~ ~2 Th~ present invention provide~ i~nprovement over the U.S. pa~ents to Lewis et al and Penniman, in servo gain control, in one of its aspects, in the provision of a servo code format having an automatic gain control ~ield for con~rolling servo gain variations, which may vary from head to head, from track to track and from track sector to track s~ctor. Provisions for servo gain control are necessary to provide relatively unifo~m servo gain, at least in the circumstances described, if acceptable servo performance is to be achieved.
In practîcing this invention, according to this one of its aspects, a memory disc i~ divided into equal sectors. Each sector comprises a section of servo code called a servo gap which is locat~d at the beginning of the sector. The servo gap is divided into sections which comprise, proceeding from its leading edge, an automatic gain control section, a sector marX section, a gray code track number section and a track position section.
The ~our na~ed section~ in the servo gap are individually defined by magnetic dibit recordings in predeter-mined patterns. The e magnetic dibit~ are also known as magnetic transitions or magnetic ~ones. The surface O:e the magnetic d.isc is magneti7ed uniformly in one direction. The dibits are magneti~
cally poled in the opposite direction, providing a transition in the magnetic field at the leading and trailing dibit edges during scanning by a magnetic head~
Th~ magnetic dibits have a width m~asured across the ' : ;
:;
'' ; ' ~ '~

1 3 2 .L ~ 2 2 tracks approximatsly equal to one-half o~ a track. All of the magnetic dibits in th~ servo gap are the same and are recorded in half track positions on each ~ide of the cen~er line of ~ach of the tracks. They are written or recorded in half-track steps with a magnetic head having a width o~ two dibits, which corresponds to the width of a data track. Their patterns, when scanned in a direction along the tracks, det~r~ine the information which is recorded and which will be ~en~ed by a magnetic head scanninq the track. Using this recording track technique the center line of a track is defined between the confronting ends (aligned)or the ad~acent ends (A&B servo field) of the magnetic dibits in half-track positions on each side of each track.
The ~agnetic dibits on on~ ~ide of alternate trac~s are consist~ntly re~erred to a~ the A dibits and the magnetic dibits on th~ other side o~ the alternat~ track~ are consistently referred to as th~ ~ dibit~. The~e ar~ r~versed on the tracks inter~2diate to ~h~ alt~rnate ~rack~. Thi~ radially aiigns the adjac~nt A and B ~agn~tic dibi~ Tha dif~erencQ of the A and B
signals developed in a ~agne~ic head which is ~canning a track in the track po~ on ~ection of ~h~ ~ervo gap, is used by the 5ervo to position that magne~ic head a~ track center. ~he sum o~ ~he A
and B signal~ d~velop~d in a ~agnetic h~ad is used ~or servo gain compensation purpo~es, during bo~h r~ading and writing ~odes o~
disc driv@ use. Th~ guard band~ are provida~ a~ the leading and trailing edge~ of th~ 8erVo g~p to provide space for preven~ing overwri~ing o~ da~a into ~he ~ervo gap a~ ~he l~ding ~dg~ and, 1 321~L22 at the trailing edge to switch the head from a mode providing si~nals ~o the servo to a mode providing for reading or writing of data code.
The signals derived from the automatic gain control field are peak detected in a fast set~ling AGC loop and normalized a~ to amplitude as a step in ~aintaining substantially uniform servo gain. this as an i~portant fsature for reliably reading of the servo cod~ especially the sector mark which fol lows. Detection of the sector mark e~tablishe~ an exac~ kiming re~erence with the trark nu~ber and track position servo code that follow it.
The sector mark signal~ are thre~hold detected at about one-half the level of the peaX detection of the AGC signals. The sector mark pattern is designed to be fault tol0rant and is read in three parts, called space 1, spaee 2, and space 3, using bit count2rs and an algorithum providing ~or ~it count testing in each sector marX space. By thi~ expedient, positive recognition of a sector mark ic assured.
Track identification is provided using track numbers recorded in ~ray code. The ~ray code is also recorded using the half-track dibit r~cording technique. The track numbex signals are threshold det~cted an~ used in track seeking operations as a feedback siqnal to the ~ervo which i5 responding to a requested track number. This eliminate~ t~e need for seeking inner and out~r track guard bands as a referenc~ for track counting , . : , ~' '' ~ ', .
: .

purposes .
Track position servo code, also rec:orded using the hal~-track dibit racording technique, follows the track number code in the servo gap. Th~ A dibit~ ar~ recorcled on one side of each track and the ~ dibit3 ar~ record~d on th~! opposite side of each track. Due to the hal~-track recording technlqu~ the A and B
dibits swap track ~ide~ and are ra~ially aligned in adjacent half-track posi~ion ~ Th~e are formatted in two different mag-netic dibi~ patterns. ~n on~ p~ttern, the individual A and B
dibit~ al~r~ate in circ~m~r~ntlal pha~ position on opposl~e ~ides of the track~. In th~ othQr pat~ern, th~ A dibit~ are recorded in groups o~ dibit string~ or burst~ on one side of each track an~ the B ~ibits are recordsd in group~ o~ dibit strings or bursts on th~ other ~ide o~ ea~h ~rack. Th~ A an~ B di~i~ groups alternate in circum~er~ntial pha~e po~ition on oppo~it~ side~ of the ~racks. When a head i~ track cent~red over the Srack posi-tion servo code, th~ A and ~ signal a~plitude~ are equal. The A
and B signal~ ar~ procs~ed in an AGC loop coupled to a sexvo for providing servo control ~ignal~ which ~aintain servo gain substanti~lly unifo~. Th~ di~er~nc~ in the pro~essed A and B
signals i~ u~d by th~ s~rvo ~or track ~ollowing purpo~es.

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`- ~32~ ~22 Various aspects of t~e invention are as follows:
In a magnetic disc drive, the improvement comprising:
a. a rotatable magnetic memory disc haviny a plurality o~ concentric circular tracks which are each divided into sectors, each sector being radially aligned with sectors in adjacent tracks, each sector having a servo gap which is radially aligned with 5ervo gaps in adjacent tracks in that sector; and b. an automatic gain control filed in each servo gap, having magnetic dibits which have a radial length less than one-hal~ of the distance between the centers of adjacent tracks, disposed in uniformly circum-ferentially spaced positions, in radially spaced, end-to-end alignment with each other, in half-track positions on each side of the center of the tracks; whereby a magnetic head traversing the automatic gain control field, overlaps substantially the same amount o~
magnetic dibit, whether or not positioned at the center o~ a track.
In a magnetic disc drive having a magnetic memory disc provided with a plurality of concentric circular tracks which are divided into sector~, the method for magnetic dibit recording of separate magnetic dibit fields for forming a servo gap in each sector, comprising: .
a. providing a magnetic head having a width corresponding to the width of track; and b. recording with said magnetic head separate magnetic dibit fields in half-track radial steps~ placing pairs o~ said magnetic dibits in sel~cted circumferentially spaced 8a . " , .
.. . . ... . . ...

32~2~
positions, in radial halfrtrack alignment in positions between track centers.
Xn a magnetic disc drive/ the improvement comprising:
a. a magnetic memory disc having a plurality of circular tracks thereon, each circular track having a servo gap therein disposed in rad,al alignment with servo gaps in adjacent trac}cs;
b. an automatic yain control field and a track position servo field in circumerentially spaced positions in each servo gap and respectively, radially aligned with automatic gain control fields and track position servo fi.elds in adjacent tracks; and c. pairs of half-track magnetic dibits disposed in selected circumferential positions, in radially aligned, spaced end-to-end positions disposed betw~en and spaced from adjacant track centers in both said automatic gain control field and said track position servo field.

Brie~ Description of th~ Drawinqs Figure 1 is a partial map of the surface of a memory disc.
Figure 2 is a plan view of a s~ctor of a memory disc, 8b : -~. .;

, ~ , 132~22 fragmentarily illustrating magnetically recorded tracks.
Figure 3a is an enlarg~d plan view of a ~ragment of the magnetic 20nes or dibits of one format of a sampled track posi-tion servo code d2fining a track and a typical processed, track centered, track following signal which is shown therebelow.
Figure 3b is similar to Figure 3a, illustrating a presently preferred format o~ a sampled, track position .~ervo cod~.
Figure 4 is a rectilinear enlargement of the servo gap portion of one disc sctor, illustrating the Pormatting of the servo code magnetic dibits and idealized signals derived there~rom.
Figure 5 i~ similar to Figure 4, illustrating a presently pre~erred track position servo code for~at in the servo gap (The A and ~ dibit group&);
Fi~ure 6a and 6b illustrate th~ relationship of timing signals, with th~ idealized servo code siynal~ of Figures 4 & 5, respectively;
Figure 7 i~ a block diagram of an automatic gain control circuit;
Figure 8 is a block diagram of a position error cir-cuit; and Fi~ure 9 is a ~low chart illustrating the ~teps in d~tecting the sector mark in the servo gap.

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2~22 P~c~,p,tion_o~h~y ~imen~
Figure 1 illustrates the e~sential parts of a disc drive for pres~nt purposes, co~npri~ing a disc 1, a pivotally mounted armstack or carriage 3 ~nd a ~agnetic h~ad 5 on the end of a flexure assembly 7 which i~ attach~d to tAe end of the armstack or carriage 3. A. di~c drive typic:ally includes a plurali~y of disc~ 1 which are axially spaced on a spindle and rotated in a counte:r clockwise dirQction, as view~d, at constan speed. Magn~tic hç~ad~ 5 ar,s E30si~ioned on ~ach ~ide o each disc by the carriag~ 3 and the flexurs a~se~bly 7. The carriage 3 may be part of ~ither a linear or a rotary actuator ~yste~ for moving th~ head~ across th~ di~c to ~if~e~rent track pot3itions. A
rotatably mounted ~21rriag~ hown having a pivot moun~ 9. An actuator member 11 compri~es a fixe~l, arcua~e magne~ structure 13 nd a coil 15 on one e~nd o~ th~ carriag~ 3 which is Iaagneti-cally coupled to the~ ~agnQt. In th~ E~ervo ~od~ o~ operation, for either track s~Q~k~ng or tras:k Pollowing pUrpO~B, a servo system 16 i~ conn~ct~d ~o a ~ ct~d ~nagne~ic hea~ 5 to receive signals therefrom for thQ pUrpo8~ OL'' controlling thQ actua~or member 11 to which th6~ output o~ th4~ ~ervo i~3 connected. A host computer 14 proYide~ r~ ts~ l~o ~he sQrvo ~y3te~, such as track mambers, in acc~sing in~EorDIa~ion in the di~3c drive ~or computing or data proc:e~sing ~unction~.
Th~ improv~d ~srmatting o~ ~he ~ ro code in ~h~ memory di~c is uReful in ~a~apl~3d ~nro ~yst~m~ o~ th~ type described in co-pending Canadian appl`ication serial no. 542,292 enti~led "Method And - . , .

2 ~

Apparatus For An Improved Sa~pled Servo Seek ~nd Track Follow Diqc Drive", a~signed to the assignee of this invention. As described in that application, proper formatting of the servo cod~ on the di~c, including th~ use of track numbers to identify individual tracks, obviatas the need for a dedicated servo di~c. This reduces ~yste~ complexity, in that ~witching of the ~ervo between prQsent, dedicat@d, and target h~ads i~ eli~ina~ed, a~ong other rea~on~.
The disc 1 i~ d~vided into a plurality of sectors 17 which are equal in 3iZ~. Only a f2w o~ ~he~ sectors are illustrat~d for present purpo~a3, ona b~ing detailed. Each sac-tor 17 comprise~ a s~rvo gap 19. Th~ ~rvo gaps 19 a~o~iated with individual concen~ric circular track~ 21 are radially aligned, a~ shown. The ~ction o~ ~ach track extending through a servo gap has servo cod~ record~d th~xein. The section of each track ~ollowing its ~rvo gap in a particular sector ha~ data recorded th~rein u~ful in d~a processing func~ion~ or in computer function~.
Figur~ 2 i~ a furth~r ~velop~nt o~ Figur~ rawn to an ænlarg~d scal~, illu~trating additional detail~. Only two concentric tracks are ~hown. ~nlarg~m~nt p~r~it~ ~howing additional detail3 of thQ s~rvo ~od~ in a ~ervo gap 19 as ~een in tha serYo gap on ~h~ right in Figur~ 2. Th~ ~ervo code is recorded along track sQction~ 2la ln a s~rvo gap, and comprises a trac~ positîon ~ervo cod~ ~ection ~3, a ~rac~ nu~b~r sec~ion 25, ~32:~2~
and a section 27 co~npri~ing primarily an area o~ auto~atic gain control with a sn~all guard band GB1 (not shown here) at the front. Two types of track po~;ition ~3ervo code are described in thi~ disclosure. The track position s2rVo code shown here co~prises magnetic zone~ or dibits A and B in circumferentially spaced position~ on oppo~ite ~ide~3 of the track center 1 ine, over}appin~ roughly one-hal~ o~ a track on each ~ide oî a track cen~er line 21a. The Dlagnetic dibit~ B are in circumferential positions in~er~nQdiat~ the!~ grletic dibit~ A, ~hat is, they alternate in circu~nferential pha~ position. The magnetic dibits in the saction 25 defining the trac:k nu~er~3 are al~o recorded in hal~-track position~ on 6~ach side o~ ~ traclc c0nter lin~a 21a, as ar~ the magnEatic dibit r~cording~ ln th~ au~o~tic gain control section 27. When the disc~ rotate, trac:k po~ition signals are developed in the s~l~actecq magne~ic head when ~che dibit~ of the ~rack po~ition ser~ro cod~ 23 pa~ therab~rleath~ These signals are useful in dater~ining th~ radial po~itlon o~' th~ ~agnetic head in relation ~o t~ agnQtic zon~ or d~bi1:~ ~ and B and, hence, track center.
Fi3ura 3a i~ an enlarged view o' a modified fra~nent o~ ~he track poaition 9~trV0 code 23 o~ ~igure 2, as will be d~veloped in Fig. 4. ~ ~agnQtic head 5 i~ ~hown on the left in thi~ figur~ in track c~ratl3r~d po~i~ion. Th~ ~ur'ac~ of the disc, repr~nted in th~ plan6!~ o~ the drawing, i8 0~ ona magnetic polarity. Th~ mzl~tic: zon~ or dibit~ ar~ o~. the oppo~ite magnetic polarity. DiSc rr~tion fran rignt to l~ft, as viewed, , ` , ' ' ' - :~32~22 beneath the ma~netic head 5, generate~ track position servo cod,~
voltages in tha magne~ic head, which af~er amplification and processing, are charack~rized by the tims varyi.ng track position servo code wave~orm~ A and B shown in Figure 3a. The di~ference between the A and B dibits volta~es (A-~), which is inl:egrated , indicates the radial po:~ition o~ the magnetic head with respect to the ~raGk center and is used in t~e tra ~c fol}owing mode of operation for keeping ~h~ magne~ic head track-centeredO When the differen::e b~twe~n the A and B ~ignal~ i~ z~ro, ~he Dlagnetic h~ad ~ rack-c~n~red~ Th~ u~ of th~a ~ignal~ ~ and B in track following and in track s~eking op~ration3 in a sampled se~ro type of track 3seking and tr2~clc following ~ervo ~ystam, i~ de~cribed hereina~ter. Such use i~ also dQ~crib2d in the referenced co-pending applica~ion, above.
Othar way~ in which thes~ A and B signals ar~ usPd in track seeking and ~xack following op~r~tion3 are known in the prior art. S~e thQ pa~n~ to ~wi~ Qt al and ~nnin~an, above.
Figur~ 3b illustra~e~ a presently preferred tra~k poci-tion 5e~0 cod~ ~or~at.. The A and B magnetic dibits are again recorded in halr-tr~ck po~itions, but now they are in strings or bursts coDIpri~ing, gor exa~ple, ~tring~3 oP seven dibit~ in alter-nating circu~îar~n~ial phs~3~ po3i~ion~3 on oppo~it~ sides of the track. ThQ ~ignal wav~or~ sJenera~d wh~n the magnetic h~ad 5 scan thi~3 di~ onna~ has ~he s~e appearance a~ that of Fig.
3a, bu~ ~he in~i~.ridual wav~or~ pe~ak~s, ins3~e~d o re~ulting from , . :" j;

the alternate sensing of the A and B dibits, now results from alternate sensing of A and B dibit strings or bursts, resulting in waveform group A and waveform group B. Here, again, the difference between the A and B voltages indicates the radial position of the magnetic head with respect to track c2nter.
Processing of the A and B voltage~ for bokh formats will be described at a later point.
Figures 4 and 5 are enlarged, rectilinear developments of servo gaps according to this invention, each showing recordings of the magnetic dibit~ for 4 adjacent disc track~
which have been arbitxarily numberedr tracks three, four, five, and six. Figure 4 ~hows the track position servo code format of Fig. 3a and P'igure 5 shows the track position ~ervo code ~ormat of Fig. 3b.
The direction of disc motion in each figure is indi-cated at the top of the drawing. The radial and circumferential directions with reepect to the disc are also indicated~
Additional in~ormation with respect to the servo gap i5 evident in each figureO Proceeding from the leading edge of the servo gap which is ~t th~ le~t ~ide of each figure, as indicated, each servo gap comprises a first guard band, GB1, an automatic gain control section or field, 27, a sector mark section or field, 29, which is the same ~sr all sectors, an index bit, Il, a defect bit, D, a track number ~ection, 25, recorded in Gray code, a second index bit, I2, and the A and B track position servo code field 23. Each servo gap ends with a guard b~nd section GB2.

1~

. -~32~22 Data trarks appear on each side of the servv gaps. Eachillustrated servo gap i5 part of the sector ~hich include~ the data tracks on the right of the S2rV0 gap.
Guard ~ands, ~Bl and GB2, are write splice areas that account ~or disc rotational speed variations so that the servo code is not overwritten by the user data. The guard band, GB1, during a writa operation initiates the interruption of the writing operation at its leading edge and, thereby prevents overwriting into the AGC ~iald.
The automatic gain control fleld, 27, is de-tected (in response -to the signals from the magnetic head) by a fast settling auto~ati~ gain control loop, Fig. 7, that noxmalizes the signal amplitud~s so that ~ub3~quent ~ervo gap fields can be properly detec~ed and proc~ed. This is particularly impor~ant if a h~ad switch has just occurred which could result in a signi-ficant change in ~ignal a~plitud~. Th~ automatic gain control field, 27, is also use~ ~or automatic gain control ~or data detection circuit~.
In auto~atic gain control, the ~ignal amplitud~ is normalized. Except ~or ~agn~ic dibi~ pat~exn differences (cir-cumferential dibit ~pacing) for th~ purpose o~ identifying trac~
numbers~ ~ctor mark~ and track po~i~ion~, the au~oma~ic gain con~rol field i~ recorded at sub~tan~ially ~he ~a~e density as the Gray code and ~he ~ine s~rvo field (track position servo code). In ~ran~ducing ~he~e magnetic fi~l~s, the Gray code . . : .

~32~ ~22 field, for example, is transduced at half the voltage threshold of the threshold used for the automatic gain control field, so that detackion of the Gray code track number will change when the magnetic head is half off-track.
The sector mark, 29, also detected at half the peak voltage of the ASC field, is a special bit patt~rn that establishes an exact timing reference with the servo signzlls that ~ollow it. This eliminateæ the need for a phase locked loop that normally is used on dedicated servo disc drives or as in sampled servo drives such as used in Lewis et al~ above. The sector mark pattern (Figs. 6a and 6b~ is designed to be ~ault tolerant so that it can be correctly id~ntified even in the presence of a disc defect. It is also used to control disc rotational speed by timing the interval between s~ctor marks.
Index bits, I1 and I2, are redundant bits used to identify a ~ector called the index sector. Only one sector on each track has transistions or dibits written into bit cells, Il and I2. This is a sector zero, for example. The redundancy provides immunity to disc defects and identifies a selected sector as the initial sector on the disc from which other sector locations are determined.
The defect bit, D, is a bit cell that has a dibit written into it, if the next, or following, servo gap has been determined to be def~ctive. This bit is written into the format at the time of manufacture of the disc when the servo code is being tested. Any servo gap which happens ~o contain a disc ~32~2~

defec~ can be marked defectiv~ by writing the bit D into the previous servo gap. This bit informs thie servo demodulator electronics to ignore the next 5ervo gap. The data field in the sector associated with that servo gap can be processed nv.~ally.
The gray coded track numher section or field 25, is a set of magne~ic dibits that contain the track address for the track that the head is precently flying over. Thes~ are binary addresses that are encoded into a Gray code sequence so that any decoding uncertainty is limited to plus or .~inus one-half track.
In using Gray code, only one bit in the ~rack i~ntification number changes from track to track, as shown. The txack addres-ses are read and decoded even during a track seeking operation.
In fact, it is these track addre~ses that are used to provide track position feedback to control the seek operation. In prac-tice, a host computer may be employed to program ratrieval and writing of information in the dis~ drive. In this respect, the host comput~r 14 (Fig. 1) makes requests in the for~ of particu-lar track number~ which are to be acce~s~d. The present track address or number where ~he magnetic head is located provides the feedbacX signal which is used in this se~k operation~
The track position servo code, 23, also called the A
and B fine servo fields, compri~es ma~netic dibit recordings that are recorded in one-half track positions on each sid~ of the data track center line. In Figure 4, the individual B dibit recordings on one side of a track center line are circumferen-: . . ~
:, :
::
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~3~ ~2~

tially spaced in posi~ions which are intermediate the individualA dibit recording~ on the opposite ~ide of that track center lina. In ~igure 5 ~he B groups or strings of dibit recordings on one side of a track cent~rline are circumferentially spaced in positions which are intermediate the A groups or strings of dibit recordings on the oppo~ite 8id~ 0~ that track centerline. As noted above, when th~ a~plitude~ o~ the processed A and B sig-nals, developed by magneti~ h~ad transition of the track position servo code are equal, ~hat magnetic head is axactly positioned ovar the centerlin~ of ~hat parti~ular da~a tra~k. Thesa sign21s ar~ u~ed to prcvide feQdback control ~or ~he s@rvo system 16 for track following purposeq.
Th¢ u~e of the au~o~atic gain control section or field in the servo gap provide~ a basis for dev~loping an inexpensive servo sy~em. By placing the auto~atic gain control field in ~h~
fron~ of each 5ervo gap an~ by making ~he auto~atic gain control response tim~ fa t enough to s~ttla within the period o~ time in wh~ ch the automatic gain control field i~ scanned by the magnetic head, the au~oma~ic gain control voltagQ can b~ det~mined. This voltage, call~d a gain ad~ust~ant ~oltag~ her~in, is held ~ixed until th~ n~xt sarvo gap i~ rsachecl and i3 u~ed to normalize the amplitude~ o~ ~11 sign~ls ~rive~ fro~ ~ervo and data ~ode in the sector containing that ~arvo gapS
Atten~ion i~ dir~c~d ~o th~ fac~ ~ha~ th~ magn~tic dibits in all of ~h~ ~ctions or fi~ld~ wi~hin ~h~ s~rva gap are the sa~ and are recorded in hal~-track posi~ion~ on each side of 1~

` ~32~ ~22 the ihdicated track, using a magnetic head twice the width of a dibit and recording dibits in half-track steps. The result is a magnetic field pattern, in the automatic gain control field, as seen in Figs. 4 and 5, between the centers of tracks numbered 3, 4, 5 and 6, for example, in which; the magnetic dibits are radially disposed of the tracks, have a radial length less than one-half the distance between the centers of adjacent tracks, are radially positioned in pairs in spaced end-to-end relationship between and spaced from the centers of adjacent tracks, and, on opposite sides of the centers of the tracks are circumferentially spaced, to form magnetic dibit ~ields of differing formats or patterns. A magnetic head 5 in the automatic gain control field due to the uniformity and density of the dibit pattern sees the same amount of magnetic dibit whether it is track csntered, whether it is off center from a track, or whether it is involved in a track seeking operation crossing tracks as it traverses the AGC field. This solves the problem of achieving good automatic gain control when the servo is being switched from one magnetic head to another. By this half-track recording technique in which the fields on ad~acent tra~ks are written coherently, the magnetic head reads properly even when it is in a position between the tracks; thus, this arrangement also provides a proper automatic gain control function during a seek operation.
This arrangement also avoids the problem of having to settle the automatic gain control function during the performance of a head switching operation, or seek opexation, or both, at the same time that the heads are trying to settle on a particular track. This prohlem exists in systems in which the automatic gain control and the position loops are interacting loops.

.
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- ~ 3 ~ 2 ~

By formatting the automatic gain control field or section, as indicated above, the servo signal formad by summing A ~ B can be held constant or relatively constant.
The signal train depicted at the hottom of each of Figures 4 and 5 i5 idealized. Each shows the ampli.tude of the 19a . ~ . . ,, .~ , :
,: :.
: ., .

:~3~22 pattern of the ~ignals developed in the different ~ields or sections o~ ~he servo gap as i~ is scanned by the magnetic head.
The signal patterns relate to the bottom track, called track number 3. ~ track centered position of the magnetic head 5 is shown but the signal pattern~ transduced from the AGC field are valid for both on track and off-track positions. The peak ampli-tudes of the signals are relatively constant in all sections. As discussed abov~, it is desirable to detect the signal 25a, representing a track number, at hal~ the peak amplitude of the automatic gain contxol signals. By iso doing, ther~e is a confidenc~ that existing signal bit~ will be counted and that signal levels, where bits ar~ ~issing, will not be count2d. Also the track number changes as th~ magnetic head crosse~ a half-track position.
In the track position servo ~ield, the signal amplitudes are relatively con~tant and they are roughly half the peak amplitude of th~ ~ignals in the other servo fields; the reason being that ln track centered po~itio~ the magnetic head 5 overlaps one-half o~ the dibits A and B at different times~
Figures 6a and 6b relate a set of timing signals to the servo code ~ignals of ~igures 4 and 5, respecti~ely. These are rectangular wave ~ignal~ that enable selected functions in tran~-ducing the servo code. An enable signal EN(~GC) conkrols the time interval in which the AGC field may be read~ As seen in Figures 6a and 6b, a write recovery interval i5 provid~d as the he~d enters the AGC field, followed by a read interval which ~ - , . . .
-' . ~.' ':~ ....
: . ' . ', ; :

- ~32~ ~2 ~erminates at or within the end of the AGC field. The signals IA
and IB gate the A and B signals, individually, Fig. 6a, or in groups, Fig. 6b, in sequenc~a for processing to form the signals A-B and A+B, as will be de~crib~d. More particulaxly, in Figure 6a the signals IA an~ I23 are synchronized with the inclividual A
and B voltage peak~, respectively, In Figure 6b, the signals IA
and IB are synchroni2ed with the A and B signal groups and pro-vide respQctive read intervals les3 than th~ re~pective inter-vals o~ ~ha A and 8 E~ignal group~, to avoid signal loss~s at the baginning and end of th2 individual 3ignal group~. The IA and IB
signals are gated by a sampl~d ~ervo galte signal SSGT w~len it is in the lower o~ it3 two voltage stat~ disc:harge sign~l DSCG
is provided which, in th~ high~r o~ its two voltags states, discharges individual integrating a~pli~iers to which the signals A and B are gated, aR will be ~e~ ribe~d. Circuits for producing thP timing signalR ar~ w~11 known, ~e~ l:h~ pa~ent to Lewis et al.
Provi-~ion for d~tecting a ~ec~or mark is de~cribed herein. A~ ~een in Flgur~ ~b a ~ctor mark 35 bit~a long is employed to illu~trato thi~ feature. Other bit 10ngths may be us~d. The 3~ctor mark i~ divided into thrae xpace Space 1 a~
the be~inning o thQ sec~or ~ark i~ 5 bi~ long. Cer~tral space 2 is 25 bit~ long and en~ing spac~ 3 i~ 5 bi~s long. This bit patt@rn ident~ ha~ a . ector maLrk and i~ u~ed in deter-mining that the sector mark ha~3 been found, a~a will be seç3n in the flow chart o~ Pisure 8.

- ~32~22 A fast settling AGC circuit ~or per~orming the auto-matic gain control ~unction is illustrated in ;Figure 7. Here, the electrical ou~put of the magnetic h~ad S ~raversing ~he servo gap is amplified in a prea~pli~ier 31, the outpu~t o~ which is coupled a~ on~ inpu~ to an au~omatic gain con~:rol amplifier 33.
The output of this amplifier i8 ~iltered by a filter circui~ 35, the output of which is applied to an amplifier 37. The output signals AN of the ampli~i@r 37 are peak d~tect~ by a peak detec-tor 39, controllad by the anable AGC signal 40 ~ENAGC), the output of which is diff~rentially co~pared with a re~erence AGC
signal having the de3ired amplitudx. ThQ di~fer~nce ~ignal ia inte~rated by an integrat~r 43 which prod~ce~ and ~tore~ a gain adjust signal coupled as ~2dback in the loop to the s~cond input of the AGC ampli~ier 33. This gain adju~t signal exists through out the scanning ~f th~ pr~ent ector which includes the remainder o~ ~he ~ervo gap and ths data field. Ths AGC cycl starts over again a~ th~ ~a~n~ic h~ad ~can~ the n~x~ A~C :eield in the nex~ servo qap in ~h~ pr~nc~ o~ ~h~ 3ignal 40 (EN~GC)~
Th~ gain adju3~ f~edback signal func~ion~ either to increase or decrea~ the ougpu~ of th~ ~C a~plifier 33 in a sen~e to balance th~ p2ak amplitude output o~ the peak detector with the p~ak ampli~ude~ o~ the re~erence AGC signal during the ~GC interval (ENAGC~ Thu~ in scanning the au~omatic gain control field 27 ak the front end o~ th~ s~rvo gap, an au~omatic g~in control ~unction i~ achiev~d~ When the sampled servo gating ~3~2~
signal SSGT is in the lower of its two voltage states, are coupled to a position error signal demodulator ~5, Fig.8, in the input o~ the servo system 16, Fig. 1, having its own A+B
automatic gain control ~unction. A read enable timing signal REN, Fig. 6a, in the higher of its two voltage states, following the sampled servo ~ating signal S~GT, enables a data circuit 38 as the magnetic head enters t~ data ~ield, which traMsmits data to the user 42 as requested by the host computer 14.
The position error signal ds~odulator 45 of Figur~. 7 is i~lustrated .in block diagra~ ~or~ in Figur~ is a ~art of the servo sy~tem 16 o~ Fig. 1. Th~ no~alize~ signals, AN, from the automatic gain control a~pli~ler of Figure 7 ara coupled as ~he servo signal input ~o a 8~rVo dibit current conYerter 47. This current converter is gatsd ~y the sampl~d ~ervo gating signal SSGT (s~e Figure~ 6a and 6b) ~o that only the A and B track position servo code ~lg~al~ are proce~sed. The QU~pUt 0~ this circuit 47 i8 a current I(DB) which i~ the dibit current developed only fro~ th~ ~ and ~ 3i~nal~. Thi~ current i~ coupled as an input to an AGC current divid~r 49 which ha~ a second input V(AGC) coupled ~ro~ a ~e~back loop and a re~rence inpu~ VtR), to achi~ve au~omatic gain con~rol. Its output current is de~ignated I(~C). Th~ A and ~ curr~n~ signal~ I(AGC) ~ro~ ~he A~C curren~ divider 49 are couple~ a~ input ~o a s~rvo dibit integrat~r 51 which i5 qated by th~ signal~ IA and IB (se~ ~igure ~a and 6b), synchronously wi~ the A an~ B si~nal input. This , , .. . ..
'"~ ' ' '~,' ' ~' ,, " :' ' , . . .

32:~2~
integrates ths signals from the ~ and B track po~ition ser~o field~, producing the A level and B level output signals A(LVL) and B(LVL). In practice, these signals are coupled to differen-tial and summing ampli~iers which are here represented as differential and summing circuits 53 and 55, respectively, for producing the position error signal (A-B) and the sum signal ~A~). The position error signal (A-B~ is used in the track following control loop of servo system 16 to control the actuator 11 of Figure 1, to posi~ion the head 5 on the center of a slelec-ted track. The signal (A+~ compar~d with a refer~nce signal (A&B REFERh~OE) in the circuit 57, the output of which iscoupled to an integrating amplifier 59, whic:h provide~ the ~aedback V(AGC) tothe automatic gain control curr~nt divider 49.
The servo dibit current e~onver~r 47 provides the out-put ::urrent I (DB), that i~ proportional tc~ the a~plitude of each A and B input siynal. It function~ as a half-wave rectifier. In thiR application it rocti~ only tl~e n~ga~ivç~ levels o~ ~he A
and B s~rvo signals to produc~ ~h~ curr~nt pul~es I(DE3).
Such a current coa v~rter may be impl~ent~d using a cs~nventional PNP type ~:ransi~tor, which i~ ba~e }: iased so that it is nonc:ondu::~ing or ~ antially nonconducting, and disabled by an invert~r having an outpu~ roupl~d ~o the tran~isl:or emitter, which norEally pull~ ~ha e~i~t~r voltag~ ~o a level below the basQ bias volt~ge. In t~a presenc~ o~ th~s sa~npl~d servc3 ga~ing signal SS&T, thQ invert:er cir ::ui~ permit~ th~ ter voltage to rise so tha~ the ~ransi~tor can conduct . ~hQ A and B servo f ield :, ~

,. - ~:

~1 3.53~ ~2 voltages are AC coupled into the base o~ the ~ransistor. The negative excursions cause the transistor to conduct to produce the current I(D~
An AGC current divider such as the circuit 49, may include a differential pair of PN~ transisl:ors that divide the incoming current ~(DB), and deliver some fraction oE it as the output current I(AGC). Th~ amount of the input current, I(DB) that is delivered as I~GC) is controlled by the difference between the feedback voltage V(AGC~, and a re~erence voltaye V(R). V(R) is established at a nominal value compatible wi~h the syst2m.
The servo dibit integrater may comprise a PNP transis-tor pair, in which the signal I(AGC) is coupled to both emitters which are also clamped at a predetermined volkage. The collec-tors are individually capacitor coupled to ground. Fixed biases on the bases o~ these transistors bias them to cut off, or sub-stantially to cut o~. The signals IA and IB are coupled, respectively, to the transistor bases and switch the transistors synchronously with th~ individual ~ and B voltages o~ Figur~ 6a, or with the A and B voltaye groups o~ Figure 6b~ The A level and B lev~l vQltages, A~LVL) and B(~VL), ar~ output buffered ~rom the respective collector circuits of the transistors. The discharge signal ~SC& controls individual switches in the collector cir-cui~s ahead o~ the capacitors, to couple the capacitors to ground, so that the capacitors may b~ discharged just prior to ., : . ~ ~ ; , , 2 ~
the time of the magnetic head transistion of the track position servo code.
The automatic gain control amplifier together with the sum and difference circuits 55 and 57 sums the A(LVL) and B(LVL) inputs to rreate the servo gain fac~or signal (A+B), which is compared to an A+B re~erence signal in the input circuit 57 to the integrater amplifier 59. The circuit 57 represents th~
positive and negative inputs to the integrater ampli~ier. The output voltage V(AGC) adjusts itself to whatever value is necessary so that th A(LVL) and B(LVL~ signal sum will exactly match the desired reference level.
The di~erPncing circuit 53 represents a conventional amplifier which generates a di~ference signal corresponding to A(LVL) - B(LVL~ Thus when th~se two input signals are equal, the output voltage may be zero~ or may be some predetermined voltage indicative o~ a head position at track center.
Circuits of the type discussed above are neither illustrated herein nor ~escribed in greater detail since their details are conven~ional and are no~ necessary to an unders~anding of this invention. Additionally such details have not been claimed~ These circuits, however, are pxesently implemented as described.
Sector mark detection is essential to system timing.
Figure 6b illustrates the division o~ the sector mark into the 3 space~ described above, comprising in thi~ example, a 1st space S bits long, a 2nd space 25 bits long and a 3rd space 5 bits ,.
. : :

. .: ~:
, ~ :.:
:

L321l~22 long. The way in which this sector mar]c i~ detected is illustrated in the flow chart of Figur~ 9.
The sector m~rk detectiorl cycle beg:ins while the mag-netic head is still in the automatic gain cont].ol ~ield. It is initiated by a sector mark search window signall 60 (SMSW), Fig.
6a, in the highar of i~ two voltage sta~e~. The search now begins for the 5 bit sector mark gap, space 1 1 at the beginning of the ~;ector mar~, which includes 4 zero bl~s and a 1 bit, 00001. The 1 bit re~ul~s ~ro~n dibit signal 62. Spac~ 1 of ~he sec~or mark begins at ~h~ erld of ~hQ AGC ~ield, Fig.~ 6k. A bit time and tim2 interval counter 6 3, Fig. 9, provid2s bit time counting and interval ti~ing. ~pace 1 o~ the sector mark has been found if 00001 is countsd in th6~ space 1 tila~ intertlal, space 2 of the sector marX haE~ been ~ound i~ 24 zero~ and a 1 are counted in ~he time pace ~ ic scann~d by th~ magnetic head. Bit count 1 is due to th~ dibit ~ nal 64. Space 3 ha~ b~en found if f ive zero bit c:oun~-~, ûOûO0, ar~ counted during ~canning .
Referring to ~lg. g, th~ search ~or th~ ~ector mark is initiated at thQ l3tart block 66 by the ~ector mark search window siynal ~0. During the timing in~rval for space 1, moni~ored by the s~arch timeout dQci3ion func~ion 68, ths ~ignals from the AGC
field are shifted in, shirt function 65. A change in ~ignal pattern fro~ o 0 indicat6a~ thQ b~ginning o~ Bp~lCe~ 1 which is confirm~d by th~ z~ro~ wh~ch ~ollow. E~it ~ zero~ zlra counted, 00001 indicating spac:~3 1 ha~ ~een ~ound, i~ counted within the ~ ., 3.3~22 space 1 time intert~al rnonitored by the search timeout dacision function 68. This i5 th~ ideal case. A ~it count of 00000 is also acceptable.
In the presence of either of the space 1 bit counts, deci~ion functis:n 67 inil:iates the ~hift function 69.
The next 25 bits are now counted. ~herea:~ter a correct bit ~unt for space 2 enables thQ shift ~unction 73 and thi3 count for the five bit counts for ~pace 3 i~ started. Otherwise the sector mark search is terminat~d by d~ci~ion ~unction 71 for ~pac~ 2. Decision function 70 r~ponding to the fail~lr~ to detect space 2 defers the sector ~ark ~earch until th~ next sector.
I~ the flv~ bit courlt~ for ~p~c~ 3 are all zeros, deci~ion ~unction 75 providQ~ an indica~ion to dQcision function 77 which determine~ that tim~ synchronization has been established. Signal procças~in~, in this c~rcu~ns~ance, in the remainder of th2 8~3~0 g~p continu~3. Otherwi~e the sector ~ark search i~ d~2rred until ~he n~xt ~ctor.
Other s~c~sr ~ark dibit configuratiorl~ may be employed.
Th~t which ha~ be~n illu~trated and d~cri~ed has been implemen-ted and i9 fault tol~ran~: and i~ readily iden1:ifiable among other dibit~ patt~rns, e~pecially th~ high derl~ity dibi~ pat~ern of the AGC f iald which prac~de~ it irl th~ ser~o ga p .
Thi~ imE~roved m~s~ory ~i~ç ~:rvo code ~ormak, sys~em and method i~or nomlalizing ~ign~l amplitude~, with r~pec:~ to its overall organization an~l with ra~pec:~ ~o the ~pecii~ic recording ~8 .
- . .
: .. . .

:., . .: , .
-. :
.

~.~2~

of the magne~ic dibits of the servo code in the s~rvo gap, provide~ several advantages.
The sampled ser~o cod~ format in the 5e~0 gap provides the signals necessary for the system to mainta.in disc speed, to saek, to track follow, and to perform head switches. No dedi-cated ser~ro surface or other encoders are required.
~ h~ embedded sampled ~ervo and i~s utilization eliminates problem~ associated with mechanical and thermal head offse~s. Therefore, the data head is more closely cerltered over the data track.
Any amount oi~ radial head mi~align~an~ can b~ tolerated by u~ing the Gray codQd track addre3~es becau~3e during a head switching operation, the ~earch i~ m~d~ ~or a particular track number at which track centering tak~s plac:~.
Significant a~ount~ of circumferentialhead misalignm~slt, called ~lc~w, can b~ ~olerat~d due to timing recovery from ~he~ or mark pa~rn. Thus, th~re i~ no need to try to d~ermin~ a corrQction n~c~sEIary for a particular circum-~erential head ~k~w du~ ~o ~i~alig;r~ent, du~ ~o ~ilting o~ th~.
carriag3 or du~ ~o till~ o~ the di~c spindle, or all o~ these.
The u~a o~ the dei~ac:t bit in ths ser~o gap eliminates th~ ne~d for a flawle 8 di~c ~;urface. The presenc~ of the dei~ect kit, when sensed by the ~agale~ic head resul~s in ~kipping O:e the next sector.
In so~e disc: drive~ re i~3 a ne!ed to move ~h~ heads . .

~32.1l422 to the inner diameter or the outer diameter guard bands to obtain synchronization lock with the disc signals. This is not neces-sary with the present servo gap formatting. The sector mark pattern provides suoh a function.
In some disc drives, there's a requirement to recalibrate the system in order to ~ind track zero. Such a recalibration ope~ation i~ not necessary with the present arrange~ent since the track addresses are now read directly.
The servo signal is never lo~t due to signal amplitude fluctuations when switching fro~ one head to another because the AGC field provides a signal that is used to reestabl.ish the correct signal amplitude before processing the remainder of the servo gap.

, .: ::

. . .

~ ~,

Claims

1. In a magnetic disc drive, the improvement comprising:
a. a rotatable magnetic memory disc having a plurality of concentric circular tracks which are each divided into sectors, each sector being radially aligned with sectors in adjacent tracks, each sector having a servo gap which is radially aligned with servo gaps in adjacent tracks in that sector; and b. an automatic gain control filed in each servo gap, having magnetic dibits which have a radial length less than one-half of the distance between the centers of adjacent tracks, disposed in uniformly circum-ferentially spaced positions, in radially spaced, end-to-end alignment with each other, in half-track positions on each side of the center of the tracks; whereby a magnetic head traversing the automatic gain control field, overlaps substantially the same amount of magnetic dibit, whether or not positioned at the center of a track.

2. In a magnetic disc drive having a magnetic memory disc provided with a plurality of concentric circular tracks which are divided into sectors, the method for magnetic dibit recording of separate magnetic dibit fields for forming a servo gap in each sector, comprising:
a. providing a magnetic head having a width corresponding to the width of track; and b. recording with said magnetic head separate magnetic dibit fields in half-track radial steps, placing pairs of said magnetic dibits in selected circumferentially spaced positions, in radial half-track alignment in positions between track centers.

3. The method according to Claim 2, comprising:
a. recording one of said separate magnetic dibit fields as an automatic gain control field at the beginning of said servo gap; and b. recording a second of said separate magnetic dibit fields as a sector mark of identical dibit patterns in each track adjacent to and following said automatic gain control field.

4. The method of Claim 3, in which:
a. the magnetic dibits of said automatic gain control field are uniformly circumferentially spaced.

5. In a magnetic disc drive, the improvement comprising:
a. a magnetic memory disc having a plurality of circular tracks which are divided into sectors, each sector being radially aligned with sectors in adjacent tracks, each sector having a servo gap which is radially aligned with servo gaps in adjacent tracks in that sector;
b. a plurality of magnetic dibit fields of differing formats defining sectors within each servo gap;
c. each magnetic dibit field comprising magnetic dibits which are radially disposed of said tracks, which magnetic dibits have a radial length less than one-half the distance between the centers of adjacent tracks, which magnetic dibits are radially positioned in pairs in spaced end-to-end relationship between said spaced from the centers of adjacent tracks, and which pairs of magnetic dibits on opposite sides of the centers of said tracks are selectively circumferentially spaced to form said magnetic dibit fields of differing formats.

6. A magnetic memory disc according to Claim 5, in which:
a. one of said magnetic dibit fields is an automatic gain control field of a format in which circumferential spacing of the pairs of magnetic dibits is uniform and the pairs of magnetic dibits on one side of the center of each track are radially aligned with pairs of magnetic dibits on the opposite side of the center each track.

7. A magnetic memory disc according to Claim 5, in which:
a. one of said magnetic dibit fields is a track position servo field of a format in which circumferential spacing of the pairs of magnetic dibits is uniform and the pairs of magnetic dibits on one side of the center of a track are circumferentially positioned substantially midway between adjacent pairs of magnetic dibits on the opposite side of the center of that track.

8. A magnetic memory disc according to Claim 5, in which one of said magnetic dibit fields is a track position servo field of a format in which said pairs of said magnetic dibit are uniformly circumferentially spaced in groups on opposite sides of the center of a track, one group being circumferentially spaced from the group on the opposite side of that track.

9. In a magnetic disc drive, the improvement comprising:
a. a magnetic memory disc having a plurality of circular tracks thereon, each circular track having a servo gap therein disposed in radial alignment with servo gaps in adjacent tracks;
b. an automatic gain control field and a track position servo field in circumferentially spaced positions in each servo gap and respectively, radially aligned with automatic gain control fields and track position servo fields in adjacent tracks; and c. pairs of half-track magnetic dibits disposed in selected circumferential positions, in radially aligned, spaced end-to-end positions disposed between and spaced from adjacent track centers in both said automatic gain control field and said track position servo field.