CA1172357A - Method and apparatus for recording transducer positioning information - Google Patents

Method and apparatus for recording transducer positioning information

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Publication number
CA1172357A
CA1172357A CA000412259A CA412259A CA1172357A CA 1172357 A CA1172357 A CA 1172357A CA 000412259 A CA000412259 A CA 000412259A CA 412259 A CA412259 A CA 412259A CA 1172357 A CA1172357 A CA 1172357A
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CA
Canada
Prior art keywords
data
recorded
track
disc
sector
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
CA000412259A
Other languages
French (fr)
Inventor
Martyn A. Lewis
David A. Sutton
Steven K. Penniman
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DMA Systems Corp
Original Assignee
DMA Systems Corp
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Publication date
Application filed by DMA Systems Corp filed Critical DMA Systems Corp
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Publication of CA1172357A publication Critical patent/CA1172357A/en
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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
    • G11B5/58Disposition 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
    • G11B5/596Disposition 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 disks
    • G11B5/59633Servo formatting
    • G11B5/59655Sector, sample or burst servo format

Abstract

METHOD AND APPARATUS FOR RECORDING
TRANSDUCER POSITIONING INFORMATION

ABSTRACT OF THE DISCLOSURE

In a magnetic disc storage system and a magnetic disc therefor, the disc having opposed surfaces, at least one of the surfaces being coated with a magnetic material, the disc being adapted to be mounted on a spindle for rotation relative to a magnetic transducer positioned for recording data on and retrieving data from the disc, a plurality of concentric annular tracks being defined on the surface of the disc, each of the tracks being divided into a plurality of sectors, each sector having associated therewith prerecorded servo data for identification thereof, the improvement wherein the servo data for each sector comprises a unit distance track identification code recorded three times in succession, the code changing between adjacent sectors of adjacent tracks, and a clock shift check code recorded immediately following the last recorded one of the track identification codes, the same clock shift check code being recorded for each sector of every track.

Description

~L~7;~35 23 The present invention relates to a method and apparatus for 24 recording and retrieving transducer positioning information in a 25 maqnetic disc storage system and, moxe particularly, to such a 26 method and apparatus which incorporates features whicn improve
2~ system reliability in the presence of noise, media defects and _ spindle speed variations.

31 Magnetic disc storage systems are widely used to provide 32 I 1 a e volumes of relatively low-cost computer accessible memory '~ .

15'~

or storage. A typical disc storage system includes a number of discs coated with a suitable magnetic material mounted for rotation on a common spindle and a set of transducer heads carried in pairs 4 on elongated supports for insertion between adjacent discs, the heads of each pair facing in opposite directions to engage opposite ~ faces of adjacent discs. The support structure is coupled to a 7 positioner motor, the positioner motor typically including a coil mounted within a magnetic field for linear movement and oriented 9 relative to the discs to move the heads radially over ~he disc surfaces to thereby enable the headsto be positioned over any 11 annular track on the surfaces. In normal operation, the position~r 12 motor, in response to control signals from the computer, positions 13 the transducer heads radially for recording data signals on or 14 retrieving data signals from a preselected one of a set of concen-15 tric recording tracks on the discs.

17 In such a system, it is necessary to record data on a disc to 18 enable the transducer heads to locate the desired recording track.
19 Accordingly, a number of track following systems for magnetic disc 2~ drives have been developed. Most commonly, a disc surface and a 21 head have been dedicated to the recording of position information 22 for use by the track following servo system. In these systems, 23 position information is recorded continuously around the disk.

~4 Typical techniques for recording position information are disclosed in ~. S. Patent No. 3,534,344 to Santana and U. S. Patent No.
26 3,691,543 to Mueller. In bo~h of these patents, position informatio~
27 is derived from single pulse amplitudes which are time-gated from 28 recorded clock pulses. In the continuous systems for which they 29 were designed, these pulses are repeated continuously around the 30 disc and position information is continuously derived at the output 31 of a comparator.

~ 3~ii7 1 In such a continuous system, each track crossing can be detected by the trac]c followiny circuitry no matter how fast the
3 head carriage might be moving. For this reason, track identi-~ fication information can be derived by simply decrementing a track difference counter until the difference is Pqual to zero, 6 meaning that the transducer head has arrived at the desired track.

~ For a variety of practical reasons, it is desirable to place 9 track position information on the same surface as the data infor-mation and to eliminate the use of a dedicated surface and head for track position informaticn. One reason is that misregis-12 tration of the disc center due to disc interchange or temperature 13 variations can be accommodated since the head is moved directly 1~ to the track of interest. Another reason is that the physical alignment of the heads in a disc drive is not as critical as it 16 is where there are multiple heads which must be aligned on 17 multiple surfaces. As a result, no field adjustments are gen-18 erally re~uired. Another obvious reason is that an entire disc 19 surface need not be dedicated to track position information.

21 As a result, most recently developed systems have em~loyed 22 embedded servo information ~i.e., prerecorded identification 23 information, on the same surface used for recording data, for 24 use by the head tracking servo system). In the most practical form of embedded servo system, each track is divided into a 26 plurality of sectors and the track identification and fine 27 position information is recorded at the beginning of each data 28 sector. This information is then read by the same head that 29 reads and writes data on the disc. Previous embedded servo systems are exemplified by U. S. Patent No. 4,208,679 to 31 Hertrich,U. S. Patent No. 4,163,265 to Van Herk et al, 32 U. S. Patent No. 4,149,201 to Card, U. S. Patent No. 3,812~533 :1.17;~357 1 ¦ to Kimura, V. S. Patent No. 3, 185,972 to Sipple and British ¦ Patent Application No. 2017364 to Droux.
3 l
4 ¦ Several problems arise from the use of embedded servo
5 ¦ information~ The data/servo head is capable of writing over
6 ¦ and therefore destroying the servo information and this must be
7 ¦ prevented. During a high speed search for a given track and
8 ¦ sector, the head may cross several tracks between sectors of
9 ¦ embedded servo infonnation. Since servo information is recorded lO ¦ only once per sector in a short burst, the effect of a defect 11 ¦ in the disc or a noise burst is much more severe.

13 l 14 ¦ According to the present invention, there is provided a 15 ¦ method and apparatus for recording and retrieving embedded servo 16 ¦ information which incorporates a variety of ~eatures which 17 ¦ improve system reliability in the presence of noise, media 18 ¦ defects and spindle speed variations. The present method and 19 ¦ apparatus is capable of an extremely high degree of accuracy, 20 ¦ even in the presence of high head carriage speeds. The present 21 ¦ system has a significantly reduced susceptibility to noise bursts 22 ¦ and to defects in a disc surface. The present system virtually 23 ¦ eliminates the possibility of the data/servo head writing over 24 ¦ and therefore destroying the servo information.

26 ¦ Briefly, the present invention achieves the above by 27 ¦ recording a unique pattern of servo data at the beginning of 28 ¦ each data sector. Specifically, the embedded servo data includes 29 ¦ a gap which is fully DC erased and which is used as an initial 30 ¦ time sync for the servo information recovery system. A multiple-31 ¦ bit burst of "zeros"is recorded following the erase gap in 32 order t provlde a clock synch onization signal for a data ~ 35~7 l ¦ separator. This burst of zeros is followed by a special sector 2 ¦ mark code c~nsisting of a multiple-bit ~urst of "ones" which is 3 ¦ used as an additional verification of embedded servo timing 4 ¦ information from which the next multiple bits are expected to be 5 ¦ track identification data and a check code. A track identifi-6 ¦ cation code in Gray code format is repeated three times in 7 ¦ succession. This code changes between adjacent sectors of 8 ¦ adjacent tracks and provides tolerance for media defects.
9 ¦ Following the three track identification codes, a special clock lO ¦ shift check code which is the same for all sectors and all tracks ll ¦ is recorded as a means of verifying that a noise pulse or media 12 ¦ defect has not caused a time shift error in the previous bits of 13 ¦ track identification data. Two bursts of high density transi-14 ¦ tions are recorded on the disc following the check code for the 15 ¦ purpose of providing fine position information. These bursts are 16 ¦ written so as to e~ually overlap adjacent tracks and to be offset 17 ¦ in time so that they may be readily separated by the servo 18 information recovery system. These bursts are recorded so ihat l9 ¦ when a head is on track, the amplitude of the signal from one 20 ¦ burst is equal to the amplitude of the signal from the other burst ~1 l ~2 l 23 ¦ It is therefore an object of the present invention to solve 24 ¦ the problems associated with the unreliability of detected servo 2~ ¦ information as a result of noise, media defects and spindle speed 26 ¦ variations. It is a feature of the present invention to solve 27 ¦ these problems by recording servo data in a format such that a 28 ¦ unit distance track identification code is recorded a plurality of 29 ¦ times in succession followed by an unvarying clock shift check ~d~

31 An advantage to be derived is a system capable of an extremely 3~

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1 high degree of accuracy, Another advantage is a system having a ¦

2 significantly reduced susceptibil:ity to media defects. A still 3 further advantage is the virtual ellmination of the possibility 4 of a data/servo head writing over and therefore destroying servo information, Another advantage is a system which,detects spindle 6 speed variations, Still another advantage is a system which detects 7 time shift errors, Still another advantage is the elimination of 8 precise timing accuracy for the obtaining of position data. Still 9 another advantage is an improvement in signal-to-noise ratio.
11 Still other objects, features and attendant advantages of the lZ present invention will become apparent to those skilled in the art 13 from a reading of the following detailed description of the preferre 14 embodiment constructed~in accordance therewith, taken in conjunction 15 with the accompanying drawings wherein like numerals designate like 16 ~arts in the several figures and wherein:
1~

19 Figure 1 shows diagrammatically the information recorded on disc surface together with a timing diagram showing the normal 21 iming of the various signals; and 22 Figure 2 is a block diagram of a preferred implementation of 23 servo information recovery system.

26 Referring now to Figure 1, there is diagrammatically shown the 27 nformation recorded on the surface of a disc (not shown? adapted 28 or use in a magnetic disc recording system ~not shown). Figure 1 29 hows adjacent portions of adjacent txacks 10, specifically the 3i ortions of tracks 10 at the beginning of a sector where the head 32 ~ -6-'Z357 1 positioning data is recorded ~or use by a servo system ~or derivi~
2 position information. From an inspection of Figure 1, it is seen 3 that the embedded servo data for each data sector includes an 4 erase gap 11, i.e. a band at the beginning of each servo sector which is fully DC erased. In the recording codes generally used 6 in magnetic disc xecording systems, "ones" and "zeros" are both 7 identified by the existence o:E recorded transitions so that a 8 fully erased area is distinct:Ly identifiable. Erase gap 11 is 9 used for initial time synchronization for the servo in~ormation recovery system, to be described more fully hereinafter. Follow-11 ing erase gap 11 are two bands 12 and 13 of prerecorded sector 12 identification data. Band 12 is referred to herein as a preamble 13 and preferably consists of a thirteen-bit burst of "zeros"
14 recorded i~nmediately following erase gap 11 in order to provide a clock synchronizing signal Eor a data separator, to be described 16 more fully hereinafter. Immediately following the preamble in 17 band 12 is a sector mark code in band 13, preferably a three-bit burst of "ones" used as an additional verification of embedded 19 servo timing information to indicate that the next bits should be track identification data and a check code.

2~ In magnetic disc drives using embedded servo techniques, the 23 requirement exists for knowledge of which track the head is 24 currently positioned over. To accommodate this requirement, track boundary crossings are typically counted. In continuously 26 recorded servo implementations, counting boundaries during high 27 speed head moves is no problem since information is available 28 continuously. ~owever, in an embedded servo system where track 29 boundary inEormation is available only on an intermittent basis, as here, it is possible to skip track boundaries. Hence, there 31 arises a need for track identification. This identification could 32 ~ go to the treme of recordlng a~ a~601ute track address ~or i~.~'7'~35~

~ every track on the disc. A more practical implementation is to 2 record repeated bands ~herein the tracks are identified ~ithin 3 the band, i.e. groups of sixteen tracks per band where the 4 tracks within the band are numbered 0-15. This requires only ~ four bits of data ~or track identification.

7 In recording this type of data, it is preferable to use a 8 unit distance code, commonly referred to as a Gray code. These 9 codes were developed so that only one bit of the code changes as a boundary between tracks is crossed. Use of a Gray code 11 limits ambiguity to a head position uncertainty of - 1/2 track.

13 According to the present invention, sector mark band 13 is 14 followed by a band 14 containing a follr-bit track identification code in Gray code format which is repeated three times in 16 succession (a total of twelve bits). Recording the Gray code 17 track identification information three times provides for more 18 than mere redundancy. More specifically, one of the purposes and 19 objects of the present invention is to record data in a way that 2~ the servo information recovery system is insensitive to media 21 defects. Such media defects typically occur in the form of 22 pinholes on the surface of the disc where magnetic signals are 23 not recorded. If a track identification code is recorded only ~4 once and a pinhole obliterates one or more of the bits of data, it would be impossible to read a correct code. If a track ~6 identification code is recorded twice and a pinhole obliterates 27 one or more of the bits of data in one of the codes, again it 2~ would be impossible to determine which code is the correct one.
29 On the other hand, by recording the track identification code three times, a two-out-of-three vote can be implemented in a 31 microprocessor such that if two out of the three track iden~i-32 fication cocles agree, it may be assumed that valid track ~ 3~7 1 identification information has been detected.

3 Even if the microprocessor determines that two out of the 4 three or all three track identification codes agree, it still cannot ~e stated with assurance that valid track identification 6 information has been detected. The reason for this is that the 7 loss of one or more data bits could cause a time shift in the data 8 so that all data bits are shifted by one or more bits. If this 9 should occur, each of the three track identification codes could be the same, but each could be wrong. In order to eliminate time shift errors, band 14 is followed by a band 15 having recorded 12 therein a special clock shift check code as a means of verifying 13 that a noise pulse or media defect has not caused a time shift 14 error in the previous twelve bits of track identification data.
16 As shown in Figure 1, the check code selected consists of 17 four bits, specifically "0010". This exact same check code 18 appears at the end of the track identification data in every 19 sector of every track 10. The servo information recovery system looks to see whether this bit pattern ollows the track iden-~1 tification data and, if it exists, it is now assumed that the 22 data just read is correct. From an inspection of the check code, 23 it will be apparent that any time shift error will cause a 24 change in the location of the "1" so as to invalidate the track identification data.

27 The fact that the exact same check code appears in every 28 sector of every track 1~ is significant~ Conventional check 29 codes would be ineffective in a magnetic disc recording system.
~0 Conventional check codes are generated by performing a mathe-31 matical algorithm on recorded data and forming data bits which ~2 relate to the recorded data. In data retrieval, the mathematical 1~ 235~

algorithm is perEormed on the recovered data and the derived 2 ¦ check code is compared with the recorded check code. However, 3 ¦ in a magnetic disc storage system where a head is posit'ioned 4 ¦ between adjacent tracks, recording data in a Gray code format 5 ¦ will limit head position ambiguity, but there is no way to insure 6 ¦ that the retrievea check code will relate to the retrievea track 7 ¦ identification data if the check code is different for each track.
I This leads to the present use of the exact same check code in 9 ¦ every sector of every track.
10 1 ¦ According to the preferred embodiment of the present 12 ¦ invention, band 15 is followed by bands 16 and 17 in which there 13 ¦ is recorded high density transitions, generally designated "A"
14 ¦ and "B", respectively, for the purpose of ~roviding fine position 15 ¦ information. Bands 16 and 17 are offset in time and are offset 16 ¦ laterally relative to each other and relative to tracks 10 so 17 ¦ that each band equally overlaps adjacent tracks 10. It will be 18 ¦ apparent that when a transducer head, to be described more fully 19 ¦ hereinafter, is in position 18 (Figure 1), where it is exactly 20 ¦ aligned with a track 10, the amplitude of the signal received by 21 ¦ such head from band 16 will be equal to~ the amplitude of the ¦ signal received from band 17. This information can be used by 23 ¦ the servo information recovery system to indicate that the head ~ ¦ is "on track". On the other hand, when the head is in position 25 ¦ 19 between adjacent tracks 10, the head will receive a signal 26 ¦ only from band 17. In intermediate positions, the inequality 27 ¦ - of the A and B signals from bands 16 and 17 may be used to 28 ¦ indicate the location of the head relative to any one of 29 ¦ tracks 10.

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1 ¦ Following bands 16 and 17 appear bands 20 having no data 2 ¦ recorded therein for receipt of data from the user of the 3 ¦ system. Each band 20 will be followed by a band 11 and the 4 ¦ pattern will repeat.

6 ¦ Referring now to Figure 2, there is shown a preferred 7 ¦ implementation of a servo information recovery ~ystem, generally 8 ¦ designated 30. System 30 is adapted for use in a magnetic 9 ¦ disc storage system of the type described hereinbefore. Such 10 ¦ a system incorporates one or more transducer heads 31 carried
11 ¦ at the end of an elongated support~to thereby enable such head
12 ¦ to be positioned over any annular trac]c on the surface of a
13 ¦ disc. For a fuller discussion of a magnetic disc storage
14 ¦ system, reference should be had to copending U. S. patent
15 ¦ application Serial No. filed
16 ¦ , entitled "Head Loading and Retraction
17 ¦ Apparatus for Magnetic Disc Storage Systems" and assigned to
18 ¦ DMA Systems Corporation, the assignee of the present application.
19 l
20 ¦ Head 31 is flying over the surface of the rotating magnetic
21 ¦ disc producing read back signals in response to the prerecorded
22 ¦ servo data shown in Figure 1. The signal from head 31 is
23 ¦ amplified by an amplifier 32, the output of which is conducted ~4 ¦ to additional amplifiers 33 and 34 and a differentiator and 25 ¦ crossover detector 35. Amplifiers 32~34 simply increase the ,26 ¦ amplitude of the signal from head 31 in order to provide an 27 ¦ ade~uate signal lev~l for the various recovery electronic 28 ¦ functions. The analog signal from amplifier 32 is differen-29 ¦ tiated and crossover detected by circuit 35 to provide a digital version of the recorded analog information. Accordingly, .

i~ 3~

1 the output of circuit 35 is a logic level signal with pulses 2 occurring at each plus or minus peak of the original wave~orm 3 from magnetic head 31. Such a differentiator and crossover 4 detector is well known to those skilled in the art. ~-6 The output of circuit 35 is then applied to a data q separator 36 which is capable of decoding the train of pulses 8 from circuit 35 into "ones" and "zeros" according to the 9 recording code used when the servo data format was recorded.
The exact construction and function of data separator 36 will, 11 therefore, depend upon the particular recording data format 1~ used. Data separator 36 requires a preamble of all "zeros" in 13 order to distinguish the clock transitions of the recorded 14 data code and to synchronize its circuit function for reliable data decoding. Thus, data separator 36 utilizes the burst 16 of zeros recorded in preamble 12. In any event, circuits 17 capable of decoding a train of pulses into "ones" and "zeros"
18 in accordance with a particular recording code are well known 9 to those skilled in the art.
The output of data separator 36 on a line 37 consists of a 22 serial string of data which is fed to the input of a sixteen-bit 23 shift register 3~. In addition, data separator 36 generates a
24 clock signal that is applied to the clock input of shift register 38, to be used as a shift clock, and to a sixteen-clock counter 26 40. Shift register 38 is a conventional shift register and 27 counter 40 is a conventional digital counter.

29 An erase gap detector 41 responsive to the output of amplifier 34 is the circuit that starts the entire process 31 of servo data recovery. That is, at the end of erase gap lli 32 which is detected by an interval wherein no signal is received 3 :~7;2~

1 ¦ from head 31, the onset of preamble data in band 12 causes 2 ¦ erase gap detector 41 to generate a logic level "one" on a 3 ¦ line 42 indicating that there is "signal present". This signal 4 ¦ on line 42 is applied together with a "timer enable" signal 5 ¦ on a line 43 from a timer 44 t:o the input of an AND gate 45.
6 ¦ When both inputs to AND gate 45 are true, an "erase gap enable"
~ ¦ signal is generated on a line 46 which is used to activate a 8 ¦ sector mark decoder 47. Sector mark decoder 47 is connected g ¦ to the output of shift register 38 so as to monitor the first 10 ¦ three bits of data. Accordingly, sector mark decoder 47 11 ¦ outputs a "start" pulse on a line 48 on the occurrence of the 12 ¦ sector mark code "111" provided "erase gap enable" line 46 is ~3 ¦ true. The start pulse on line 48 is applied to counter 40.
14 ¦ Erase gap detector 41 may be implemented by combining a 15 ¦ conventional peak detector with a threshold circuit in order 16 ¦ to detect the presence of a signal from head 31.
17 l 18 ¦ Sector mark decoder 47 may be a simple three-input AND
19 ¦ gate to detect the presence of three "ones" in shift register 38.
20 l 21 ¦ System 30 has now been conditioned to receive and analyze 22 ¦ the next sixteen bits of data from magnetic head 31. It shou~d 23 ¦ be noted that in order for this to occur, erase gap detector 41 2~ ¦ must sense the erase gap, timer 44 must enable gate 45, and 2~ ¦ sector mark decoder 47 must detect the sector mark code recorded 26 ¦ in band 13.

28 ¦ The start pulse on line 43 initiates a counting of the next 29 ¦ sixteen clock pulses from data separator 36 by counter ~0, at the 30 ¦ end of which an "end 16 clock count" line 49 goes true. The 32 ~ signal on line 49 is applied as one input to a three input AND

Z3~i~J
1 gate 50. AND gate 50 also receives an input on line 43 from 2 timer 44 and a signal from a clock check decoder 51. Clock 3 check decoder 51 may be a circuit similar to sector mark decoder 4 47 and is coupled to the output of shift register 38 so as to ~ monitor the most recently recorded four bits of data. Since the 6 track identification data on band 14 is received first, the most 7 recently recorded ~our bits should be the data in check code 8 band 15. When these four bits are "0010", the output of decoder ~ 51 will be true. Thus, if the three inputs to AND gate 50 are simultaneously true, gate 50 will generate a true "load Gray code" signal on a line 52 connected to a microprocessor 53.
12 Microprocessor 53 ls also coupled to the output of shift 13 register 3a.

It should be noted here that at the end of sixteen clock 16 pulses, the three successively recorded track iden~ification 17 bits in Gray code format and the clock check code should be 18 present in register 38. As mentioned previously, the clock 19 check oode is used to insure that no noise or media defects have caused a clock pulse failure which may mean that every bit of 21 the sixteen bits could be mispositioned in register 38 by one 22 or more bits. In such a case, even a majority vote of three 23 Gray codes could be in error. The bit pattern selected for the 24 clock check code in band 15 is absolutely reliable for protection 26 against a -2 bit shift error so that the presence of a "load 26 Gray code" signal on line 52 is a highly reliable indication of 27 the v~al;idity of the Gray code data. ~ccordingly, a command on 2~ line 52 is used to instruct microprocessor 53 to read the 29 twelve bits of track identification data and to determine 31 a majori clecision on the track just rea~ by magnetic heaù 31.

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1 If two out of the three Gray code signals are consistent, 2 microprocessor 53 outputs track information on a line 54 3 which is used by the servo control circuitry (not shown).

The "load Gray code" signal on line 52 is ~lso used to 6 start timer 44, which has several functions. Timer 44 is a very 7 accurate device, preferably including a precise crystal oscillator, 8 so that time decodes can be accurate to within 0.01~. The first 9 function of timer 44 is to bring true the "timer enable" signal 0 on line 43 at a time just before the next expected "signal 11 present" occurrence on line 42 and to set it false again just 12 after the next expected clock check decode signal from decoder 13 51. This time accurate "timer enable" gate provides protection 14 against the possibility that the speed of the spindle driving the disc is out of speed tolerance, an occurrence which could 16 endanger the embedded servo data. Writing of data on the disc 17 is allowed by the write control circuitry (not shown) only after 8 a proper occurrence of a "load Gray code" signal on line 52 in 1~ the embedded servo field preceding the data field to be written.
This assures that the track following servo information is 21 acceptable and that the spindle speed control system is func-22 tioning properly so as to prevent the head 31 from writing over 23 and therefore destroying the embedded servo information.

Figur~ 1 also shows the normal timing of the various signals.
~6 That is, the "timer enable" pulse on line ~3 from timer 44 is 27 shown at 55. It is seen that this signal goes true just prior to 28 band 12 and goes false again just after band 15. The distance 29 between the time when the "timer enable" pulse goes true and the "signal present" signal on line 42 goes true, as shown at 56, is 32 the degre l~ spee~ tolerance permitted by system 30. The o~tput ~ 35i~

1 ¦of sector mar~ decoder 47 on line 48 is shown in Figure 1 at 57, 2 ¦the "end 16 clock count" slgnal on line 49 from counter ~0 is shown 3 ¦at 58, the output of clock check decoder 51 is shown at 59, and the 4 ¦"load Gray code" signal on line 52 from gate 50 is shown at ~0.

6 ¦ Timer 44 also provides time gate pulses for sampling the high q ¦density data bursts from bands 16 and 17. Thus, timer 44 provides 8 ¦accurately timed gate signals on lines 61 and 62 ~shown at 63 and 9 ¦64, respectively, in Figure 1~ which are applied to A and B sample 10 ¦and hold circuits 65 and ~6, respectively. Circuits 65 and 66 11 ¦receive the output of amplifier 33. The outputs of circuits 65 12 ¦and 66 are applied to a third A-B sample and hold circuit 67 which 13 ¦receives a timing signal from timer 44 over a line 68.
14 l 15 ¦ Circuits 65 and 66 are conventional analog sample and hold 16 circuits which, when activated, sample the analog level of an 17 input signal and hold it for further use. As can be seen from an 18 inspection of the timing diagrams of Figure 1, timer 44 activates 19 circuit 65 at a time when the A position data is expected from 20 band 16 and activates circuit 66 at a time when the s position data 21 is expected from band 17. The A and B data from bands 16 and 17 ~2 as sampled by circuits 65 and 66 is applied to circuit 67 which 23 forms the difference (A-B) therebetween. This difference signal 24 is a "position error signall' which is applied via a line 69 to the servo control circuitry to correct the location of magnetic head 31 ~ and to hold it "on track".

28 In the event of noise at the output of magnetic head 31 during 29 the servo data time, the absence of a "load Gray code" signal on line 52 will inhibit the issuance of new l'track information" on :~7Z3S7 line 54 and an updated q'position error signal" on line 69 so that bad data is not sent to the servo control circuitry, thus preserving 3 the last good (noise-free) sample of information. If two consecu-4 tive "load Gray code" signals on line 52 are missed by timer 44, a "fault" signal is sent over a line 70 to microprocessor 53 and 6 appropriate system action is taken. A "clear" signal is issued ~ by microprocessor 53 and applied to timer 44 over a line 71 to 8 enable timer 44 to accomplish a restart of servo information recover 9 system 30.

11 In summary, in embedded servo applications where position 12 information is available only intermittently at the beginning 13 of each data sector, it is extremely important to acquire position 14 information in a timely manner. It is also imperative that this 15 position information be free from noise effects from all sources.
1~ It is particularly important that minor defects in the recording 17 media itself not affect the integrity of the position information.

19 It can therefore be seen that according to the present inven-20 tion there is provided a method and apparatus for recording embedded 21 servo information which incorporates a variety of features which 22 improve system reliability in the presence of noise, media defects, 23 and spindle speed variations. The present method and apparatus is 24 -apable of an extremely high degree of accuracy, even in the
25 presence of high head carriage speeds. The present system has a 2~ significantly reduced susceptibility to noise bursts and to defects 27 in the~disc surface. Furthermore, the present system virtually 28 liminates the possibility of the data/servo head writing over and 29 jhe ~!fore destroying the servo information.

:~ :1 7~31L9~

1 ¦ While the invention has been described with respect to the ¦preferred physical embodiment constructed i.n accordance therewith, ¦it will be apparent to those skilled in the art that various 4 ¦modifications and improvements may be made without departing from ¦ the scope and spirit of the invention. Accordingly, it is to be 6 ¦understood that the invention is not to be limited by the specific 7 ¦illustrative embodiment, but only by the scope of the appended 1- c1aims.

2~ I .
26 I ;

281 '

Claims (9)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-
1. In a disc having opposed surfaces, at least one of said surfaces being coated with a magnetic material, said disc being adapted to be mounted on a spindle for rotation relative to a magnetic transducer positioned for recording data on and reading data from said surface, a plurality of concentric annular tracks being defined on said surface of said disc, each of said tracks being divided into a plurality of sectors, each such sector having associated therewith prerecorded data for identification thereof, the improvement wherein said prerecorded identification data comprises:
a track identification code recorded a plurality of times in succession, said code changing between adjacent sectors in adjacent tracks; and a clock shift check code recorded immediately following the last recorded one of said track identification codes, the same clock shift check code being recorded for each sector of every track.
2. In a disc according to claim 1, the improvement wherein said track identification code is a unit distance code.
3. In a disc according to claim 1, the improvement wherein said track identification code is recorded in a Gray code format.
4. In a disc according to claim 1, 2 or 3, the improvement wherein said track identification code is recorded three times.
5. In a disc according to claim 1 or 2, the improvement wherein said clock shift check code consists of a plurality of bits of data wherein said bits of data are the same for each sector of every track.
6. In a disc according to claim 1 the improvement wherein said prerecorded identification data for each sector further comprises:
a pair of bursts of high density transitions recorded following each check code and further recorded so as to equally overlap adjacent tracks and being offset in time for the purpose of providing fine position information.
7. In a disc according to claim 6, the improvement wherein said bursts of high density transitions are recorded so that when said magnetic transducer is aligned with a track, the amplitude of the signal received by said transducer from the bursts of each pair are equal.
8. In a disc according to claim 6, the improvement wherein said prerecorded identification data for each sector further comprises:
a fully DC erased gap for providing initial time synchroni-zation; and a predetermined pattern of data bits recorded following each DC erased gap for providing clock synchronization, said predeter-mined pattern being recorded for each sector and for each track, said track identification code following said predetermined pattern of data bits.
9. In a disc according to claim 1 or 2, the improvement wherein said prerecorded identification data for each sector further comprises:
a fully DC erased gap for providing initial time synchroni-zation; and a predetermined pattern of data bits recorded following each DC erased gap for providing clock synchronization, said predeter-mined pattern being recorded for each sector and for each track, said track identification code following said predetermined pattern of data bits.
CA000412259A 1981-11-16 1982-09-27 Method and apparatus for recording transducer positioning information Expired CA1172357A (en)

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US321,747 1981-11-16
US06/321,747 US4424543A (en) 1981-11-16 1981-11-16 Method and apparatus for recording transducer positioning information

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JP (1) JPS58501644A (en)
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GB2123997B (en) 1985-07-17
US4424543A (en) 1984-01-03
EP0093716A4 (en) 1985-09-16
JPS58501644A (en) 1983-09-29
GB8317986D0 (en) 1983-08-03
GB2123997A (en) 1984-02-08
BR8207979A (en) 1983-10-04
EP0093716A1 (en) 1983-11-16
DE3278728D1 (en) 1988-08-04
EP0093716B1 (en) 1988-06-29
WO1983001858A1 (en) 1983-05-26

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