Sectorized data path following servo system

A servo system for accurately positioning a transducer relative to data paths on a recording medium by detecting sync and servo signals from two parallel abutting servo tracks sharing a common longitudinal boundary within circumferentially spaced sectors of the medium. Within each sector of the recording medium, two servo waveforms of different frequencies in two abutting servo tracks are preceded by a sync reference waveform of the same frequency in both servo tracks which is used as a reference for a phase locked oscillator, connected to the transducer, generating a pair of modulation signals that are to be mixed with transducer signals from both servo waveforms. A pair of servo channels are provided for receiving the pair of modulation signals and the servo signals for deriving an imbalance error signal representing deviation of the transducer from the center line of a data path, which is the projection from the sectors of the common longitudinal boundary between servo tracks. The imbalance error signal is used to center the transducer on the data path.

BACKGROUND OF THE INVENTION 
1. Field of the Invention. 
The invention relates to servo systems, and more particularly to a servo 
system which compensates for lateral deviations of a transducer from a 
data path in a recording medium. 
2. Prior Art. 
In data recording media, it is customary to record data in formats wherein 
waveforms representing data are disposed in parallel paths. To reproduce 
the data, a transducer is used to convert the recorded waveforms into 
electrical signals. The transducer must accurately follow the center of 
the data paths in order to obtain the best reproduction of the recorded 
waveforms. Previously, servo systems have been used to maintain the 
transducer in a proper path following position, near the path center line 
as measured from side to side. A common prior art servo system relies on 
recording of servo waveforms in tracks on a separate servo medium which is 
maintained parallel to other media, usually in a stacked relationship 
having user data recorded in paths which correspond to the servo tracks. 
Typically the paths overlie one another, forming a cylinder. A transducer 
accurately positioned over a servo track is ganged with transducers over 
the data paths for very fine alignment of the latter. The paths and tracks 
are equivalent, but tracks refer to positions for servo data, while paths 
refer to position for user data. 
It has been previously recognized that there are a number of advantages in 
combining servo waveforms and user data waveforms on the same medium. This 
is especially important in small systems. For example, if only one user 
data medium is required, a separate servo medium parallel to the user 
medium would mean that one half of the total media space is dedicated to 
servo waveforms, an unacceptably high level. On the other hand, if servo 
and user data could be combined, a separate servo medium would not be 
needed. 
Momentarily disregarding the problem of where to put servo waveforms, most 
formats for representing servo data used in fine positioning of a 
transducer over a data path can be classified as: (1) phase 
discriminating, (2) limited pulse types and (3) dual frequency. In the 
first case, two servo waveforms having a known phase relationship are 
recorded on adjacent servo tracks and are simultaneously read and compared 
to control transducer position. For example, see U.S. Patent 3,427,606 to 
Black and Sordello. In the second case, positive and negative pulses are 
recorded on adjacent servo tracks with periodic polarity reversals. 
Polarity reversals in two adjacent tracks are sensed by a transducer and 
amplitudes are compared for centering the transducer. For example, see 
U.S. Pat. No. 3,534,344 to Santana and U.S. Pat. No. 3,691,543 to Mueller. 
In the third case, two waveforms representing two different frequencies 
are recorded on adjacent servo tracks with a servo signal being derived by 
separation of the frequencies and comparison of the amplitudes of the two 
signals. For example, see U.S. Pat. No.3,864,740 to Sordello and Cuda. 
One of the problems with the first format is that unwanted phase shifts due 
to speed variations of the media or recording arm head sway or anomalies 
in the electronics for reading or writing data can cause phase errors, 
creating uncertainties in the servo information. In the second format, 
media defects, such as small holes or anomalies in the media or particles 
of the media surfaces can block detection of pulses, also creating errors. 
For these reasons and others the third format has been adopted herein. 
Returning now to the problem of where to place the servo information, U.S. 
Pat. No. 3,864,741 to Schwartz, using a modification of the third format, 
the dual frequency approach, teaches that circumferentially spaced sectors 
interrupting user data paths allow servo information to be placed on the 
same medium with user data without requiring the expense of making two 
different media layers on the same disk, one for user data and one for 
servo data. However, the aforementioned patent uses two frequencies which 
must be widely spaced for unambiguous detection and equalization of the 
amplitudes of the two signals. 
The aforementioned U.S. Pat. No. 3,864,740 to Sordello and Cuda teaches the 
advantage of using two closely spaced servo frequencies, as opposed to the 
previously mentioned widely spaced frequencies, for correcting transducer 
position. In review, certain advantages accrue in the use of such closely 
spaced servo frequencies because (1) the recording characteristics of the 
transducer change with frequency and therefore the magnitude of the signal 
from the recorded tracks varies with frequency; (2) the flying height of 
the transducer varies and as a result the two signals are attenuated at 
rates varying with frequency; (3) the readback characteristics of the 
read/write head are different for the two frequencies; (4) the magnetic 
characteristics of the recording medium change with frequency; (5) the 
electronic characteristics of the circuit may change with frequency; and 
(6) changes in the relative speed between the recording medium and the 
transducer can alter the frequency of the readout signal sufficiently to 
detune the electronic filters which are frequency dependent. 
Sordello and Cuda relied upon a modulation technique to reduce their two 
closely spaced servo frequencies to two low frequencies suitable for 
controlling an actuator. The prior art Sordello and Cuda modulation 
technique used a phase locked oscillator to generate modulating signals to 
be combined with the closely spaced servo frequencies. However, a new 
problem now arises. 
The phase locked oscillator requires a synchronizing reference frequency. 
Of course, such a synchronizing reference can be provided by a separate 
source such as a clock track or even by a separate glass disk or timing 
gear rotating with the medium and having marks which initiate timing 
pulses at a desired rate. However, these sync reference frequency sources 
introduce additional tolerances, components and space requirements which 
are to be avoided where only a single or a few stacked recording media are 
to be used. 
SUMMARY OF THE INVENTION 
An object of the invention is to devise a servo system having a modulated, 
two frequency, narrow band, position correcting signal for fine tracking 
of a transducer over a user data path in a recording medium wherein servo 
signals are derived from such medium containing the user data. 
A further object of the invention is to use a phase-locked oscillator for 
generating modulation frequencies in such a system while having 
synchronizing reference signal for the oscillator derived from the same 
medium from which the servo signals are derived. 
Still another object is to derive all frequencies in such a servo system, 
including data writing frequencies, from a single phase-locked oscillator. 
The above objects have been attained in a servo system wherein data path 
recording areas of a recording medium are interrupted by a plurality of 
spaced servo sectors containing regions with sync and servo waveforms. 
Each sector has parallel tracks called servo tracks for recording sync and 
servo waveforms in each track wherein a common longitudinal boundary of 
two parallel adjacent servo tracks is a projection of the center line of a 
data path. Since servo tracks are recorded on a recording medium before 
data paths are established the common boundary between servo tracks 
defines the center line of a data path and thereby establishes the path. 
Thus, a transducer directly over the boundary will be in the center of the 
data path after traversing a sector. 
The transducer, which converts disk waveforms into electrical signals, is 
connected to a phase-locked oscillator. The oscillator provides multiple 
output frequencies for the servo system, including modulation frequencies 
for mixing with signals derived from two abutting servo tracks, the 
resultant combinations of which are two frequencies within a narrow 
bandwidth to be used to correct transducer position to be exactly over the 
center line of a data track. 
A sync reference frequency for the phase-locked oscillator is picked up 
from the sync reference waveforms in the servo tracks. The sync reference 
and servo waveforms in each sector are divided into two regions with sync 
reference waveforms all of a single frequency recorded in two abutting 
servo tracks in one region of the sector preceding and in linear alignment 
with servo waveforms in another region of the sector. A transducer 
traversing a sector will first encounter sync reference waveforms followed 
by servo waveforms. If the phase-locked oscillator has drifted in 
frequency or phase, circuits are provided for making a correction. The 
corrected oscillator frequency is divided down yielding two modulation 
frequencies for mixing with two servo frequencies from the two recorded 
servo waveforms. Mixing occurs in a pair of servo channels wherein sum and 
difference resultant frequencies are separated by rectification and filter 
circuits and then combined to produce an amplitude imbalance error signal 
representing deviation of the transducer from the center line of a data 
path. The imbalance error signal is fed to an actuator for centering the 
transducer on a data path. 
The phase-locked oscillator is used as a source, not only of the modulation 
signals, but also of the data write clocking frequency. Electrical 
circuits are provided for the gating of the oscillator signals to the 
modulators only after the phase-locked oscillator frequency is set by the 
sync waveform, while the previously mentioned filter circuits in the two 
servo channels provide narrow bandwidth output for derivation of the 
imbalance signal. Thus, both time and frequency domain discrimination 
enhance the system signal to noise ratio, where system noise would include 
noise derived from servo channels. 
One of the chief advantages of the present invention is that a two 
frequency, narrow bandwith servo system can be implemented with sector 
contained servo tracks at a small cost in terms of the data surface of a 
typical recording medium. This is particularly useful in single or dual 
disk recording systems, as well as in larger systems. 
Another advantage is that multiple frequencies used in the above mentioned 
servo system can be produced in a single phase-locked oscillator whose 
reference frequency is based on sync reference waveforms contained in the 
servo sector.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
With reference to FIG. 1, a recording medium 11 is shown. The servo system 
of the present invention is particularly useful for magnetic disk 
recording media, but nothing herein restricts the servo system of the 
present invention to magnetic media or to circular disks. The invention is 
applicable in general to linear data path arrays, whether circular or 
straight. However, for purposes of this discription concentric, circular 
data paths will be described. 
One aspect of the present invention involves an improvement in data paths 
achieved by providing servo sectors for a recording medium which will keep 
a transducer over the center of a data path. Another aspect involves 
electronics which use the sectors and information therein to perform 
operations which achieve the desired result. 
In order to understand the reason for providing the servo sectors of the 
present invention, and the construction thereof, it is necessary to 
understand the appearance of a recording medium having user data thereon 
between sectors. 
Recording medium 11, occupying the space between inner and outer guard band 
edges 12 and 14, respectively, has a recording area for plurality of 
parallel concentric circular data paths which are closely spaced. The 
paths have a radius originating at the axis of disk rotation 26. It is 
common to position hundreds of parallel data paths for user data on a disk 
at spacings of a few thousandths of an inch, with the path width being 
approximately equal to the spacing between paths. The path width is its 
lateral or radial dimension while the orthogonal dimension or direction is 
termed the longitudinal dimension or direction. The same terms apply to 
the servo tracks. 
Recording medium 11 is shown in plan form to have a data path 13 whose 
dimensions are greatly magnified for purposes of illustration. A 
transducer 15 is shown to be positioned in the center of data path 13. The 
lateral position of transducer 15 is adjusted by means of an actuator, not 
shown, which provides coarse adjustment for accessing a specified data 
path as well as fine adjustment within a data path so that the transducer 
15 may be positioned in the center of the data path 13. The present 
invention deals with fine lateral positioning of a transducer over a data 
path. 
Data path 13, like other data paths parallel to it, is interrupted by a 
plurality of servo sectors, only one of which is shown in FIG. 1, defined 
between radial lines 23,25. Within this sector are parallel, servo tracks, 
such as the odd servo track 17 and the even servo track 19, both of said 
servo tracks having circumferential edges which are arcs of circles having 
a common center 26. These servo tracks are of equal lateral dimension and 
abut in a common longitudinal boundary 20. Since servo tracks in all servo 
sectors are established by recording waveforms on the medium prior to 
recording user data in data paths, the projection 21 of common 
longitudinal boundary 20 outside of the servo sector, in both directions, 
defines the center of a data path 13. By exact positioning of transducer 
15 over the boundary 20, the transducer will automatically be over the 
center of data path 13. 
For purposes of defining data path center lines (DP), one pair of odd and 
even (O and E) servo tracks are required. For example, DP1 is defined by 
the boundary of O1, E1; DP2 is defined by the boundary of E1, O2; DP3 is 
defined by the boundary of O2, E2. It will be realized that n+1 laterally 
abutting servo tracks define n data path centers. Each of the servo tracks 
within a sector has two regions divided by dashed line 27. A first region 
of each servo track, to the left of dashed line 27 is known as the header 
portion 16 and contains a waveform reproduced in two laterally abutting 
tracks known as the sync reference. The sync reference waveform recorded 
in abutting odd and even servo tracks is identical in amplitude, phase and 
frequency. To the right of dashed line 27 is the tail region 18 of each 
servo track. The tail region of the odd and even servo tracks contain 
recorded servo waveforms recorded at a frequency f1 in the odd servo track 
and at a frequency of f2 in the even servo track. The tail region 18 of 
each servo track terminates at radial line 25 whereupon the data path 13 
continues in longitudinal geometric alignment with the previously 
interrupted portion of the same data path. Similarly, the header region of 
an odd or even servo track in one servo sector will be in longitudinal 
geometric alignment with the tail region of an odd or even servo track in 
a preceding servo sector. Header regions of servo tracks of different 
radii are circumferentially aligned, i.e. are defined between the same 
radial lines, and so are the tail regions. This is referred to as regional 
alignment of header and tail regions, i.e. the longitudinal boundaries of 
sectors subtend an equal angle with respect to the center of rotation of a 
disk. 
In the lower portion of FIG. 1, the data path 13 and the servo tracks 17, 
19 have been enlarged to show the waveforms which exist in the odd and 
even servo tracks. The term "waveforms" refers to oscillations recorded on 
a recording medium, such as a disk, while the term "frequency" refers to a 
number of oscillations per second present in an electrical signal which 
may be recorded as a waveform or derived therefrom, or used in the 
operation of circuits. To the left of radial 23, data has been written on 
data path 13 at data recording frequencies F, 1.33F, and 2F. After passing 
radial line 23, the header region 16 of the odd 17 and even 19 servo 
tracks can be seen to contain a sinusoidal waveform which is to become the 
sync or reference frequency. This frequency is recorded at frequency 4F 
and extends completely across the longitudinal dimension of header region 
16. The two sync reference waveforms in each servo tract are in phase so 
that upon pickup by a transducer the waveforms will be mutually 
reinforced, yielding the same reference signal independent of radial 
transducer position. 
Upon crossing dashed line 27 which separates the header region 16 from the 
tail region 18, the odd servo track 17 has a first servo waveform thereon 
written at a frequency f1, while the even servo track 19 has a second 
servo frequency f2 written thereon. Both of the waveforms f1 and f2 extend 
completely across the longitudinal dimension of tail region 18 of each 
servo sector from the dashed line 27 to radial line 25. To the right of 
radial line 25 the data path 13 again continues in geometric alignment 
with the previously interrupted data path 13. The servo waveforms f1 and 
f2 are sinusoidal continuations of the sync waveform, but at different 
frequencies. The frequencies of f1 and f2 are selected so that they are 
closely spaced, i.e. the difference between them is relatively low, but 
are easy to separate by the techniques described in U.S. Pat. No. 
3,864,740. By closely spacing the frequencies, both magnetic recording and 
electronic frequency dependent errors are greatly reduced. Furthermore, 
the frequencies may be in the data read channel where data is not 
separated from servo signals near the front end of the system. For 
example, a frequency of 2.42 MHz has been used for f1 and 2.58 MHz has 
been used for f2. 
Less than sixty-four periods of the sync reference frequency are needed, 
although this number is not critical. In a disk system thirty-two servo 
sections, circumferentially spaced at equal distances, each containing 
sync and servo regions, as described, are preferred. Note that 1F, 1.33F, 
and 2F are the data code recording frequencies for digital recording. The 
servo frequency duration is approximately 50 microseconds so that the time 
for a transducer to traverse a sector is approximately 60 microseconds. 
These times are exemplary and one skilled in the art could readily select 
other appropriate times. 
While only one odd and one abutting even servo tracks have been shown in 
FIG. 1, similar odd and even servo tracks have circumferential boundaries 
forming arcs in regional alignment extending from the inner guard band 
edge 12 of the disk all the way to the outer guard band edge 14, in each 
case odd and even servo tracks abutting so that boundary therebetween 
extended outwardly from a sector defines the center line of a data path. 
Each data path is interrupted by abutting odd and even servo tracks in 
servo sectors for fine adjustment of a transducer over the data path. Each 
pair of odd and even servo tracks bears the same waveforms as the odd and 
even servo tracks 17, 19. 
Both the sync reference waveforms and the servo waveforms are recorded in 
the servo tracks in the servo sectors before any data is recorded on data 
paths. In other words, each of the sectors on a disk is formed or 
"formatted" prior to the time that data is to be recorded. The 
circumferential boundaries of servo tracks form concentric arcs 
interrupted by the radial line which defines each sector and forms 
boundaries for the area wherein user data is to be recorded. User data 
paths are subsequently defined on the disk such that the center line of 
each user data path corresponds to the projection of a common boundary 
between abutting odd and even servo tracks. Each user data path extends 
from a radial line which defines a boundary of one sector to a radial line 
which defines a boundary of an adjacent sector. 
FIG. 2 illustrates in block form the electronics for using information in 
the servo sectors. With reference to FIG. 2, a coil 31 is shown which is 
part of transducer 15 of FIG. 1. The coil 31 may have a grounded center 
tap 33 and serves to convert waveforms on the recording medium into 
electrical signals. Coil 31 is connected to a variable gain amplifier with 
gain controlled by an AGC circuit 37 and the output of summing amplifier 
38. The AGC circuit tends to maintain the envelope of the output signal 
from variable gain amplifier 35 at a constant level, compensating for 
signal variations or low and high signal levels from various causes. The 
AGC circuit is supplied with a reference voltage from an external fixed 
supply, such as a battery or other d.c. source. The AGC circuit is 
selected to have a wideband characteristic for attenuation, but a narrow 
band characteristic for amplification. The AGC gain is initially maximum, 
but lowers its gain as corrective feedback from the servo channel signals 
48, and 58, combined in summing amplifier 38, an operational amplifier and 
the AGC reference act on the initial output. 
The output of variable gain amplifier 35, which may alternatively be a wide 
band amplifier, includes user data signals and signals from the servo 
tracks. Analog gate 39 passes user data signals to a data signal channel 
for further read processing. It is not necessary to have a user data 
channel separate from servo channels, but at some point, user data signals 
are removed from the servo information. One place to conveniently remove 
user data is immediately after the variable gain amplifier 35. The signals 
from the servo tracks are transmitted through a band pass filter 36 to a 
pair of servo channels indicated by the dashed lines 41, 51. Band pass 
filter 36 has its center frequency at approximately 2.5 MHz with 12 
decibel or more rolloff per octive on both sides of the center frequency. 
Band pass filter 36 is needed to eliminate any harmonic content of the 
servo signals at the output of variable gain amplifier 35. Servo channel 
41 includes a modulator 43 which may be a balanced modulator, followed by 
a second band pass filter 45. Band pass filter 45 is tuned to have the 
center of its pass band at a frequency which is the difference of the 
recorded servo waveform frequencies f1 minus f2. Filter 45 is connected to 
a demodulating peak detector 47 which converts the maximum amplitude of 
the signal received therein into a voltage signal representative of head 
position which is applied to the sample and hold circuit 49 which retains 
the servo positional information between servo sector times. 
Similarly, in servo channel 51, the modulator 53 receives signals from the 
servo tracks from bandpass amplifier 36. Modulator 53 is connected to a 
band pass filter 55 tuned to have the center of its pass band at a 
frequency which is the difference of the recorded servo waveform 
frequencies f2 minus f1. Band pass filter 55 is connected to demodulator 
peak detector 57 which converts input signals into corresponding voltage 
signals representative of head position which are then transmitted to the 
sample and hold circuit 59 which functions the same as sample and hold 
circuit 49. The analog outputs of servo channels 41, 51 are head 
positional signals which are taken along lines 48, 58 and transmitted to a 
difference amplifier 61. The output of difference amplifier 61 represents 
an imbalance error signal derived by comparing the two head positional 
signals in difference amplifier 61, an operational amplifier. The 
generation of the imbalance error signal has previously been explained in 
U.S. Pat. No. 3,864,740 to Sordello and Cuda but will be briefly reviewed 
herein. 
Each of the servo channels 41, 51 contains a corresponding modulator 43, 
53. To each of the modulators is applied a modulation signal. For example, 
modulation signal frequency f1 is applied on line 42 from a phase-locked 
oscillator 71. Modulation frequency f1 modulates the input to modulator 43 
producing sum and difference frequencies. Since modulation frequency f1 is 
selected to be equal to the frequency of servo signal f1 the frequency 
terms which appear out of the modulator are 2f1, (f1-f2), (f1+f2) and 
(f1-f1). Filter 45 removes all but the (f1-f2) term. Servo channel 41 
processes positional information carried by recorded servo signal f2. 
Similarly, in servo channel 51, the phase-locked oscillator applies a 
modulation frequency f2 along line 52 to modulator 53. Modulation 
frequency f2 is selected to be equal to the frequency of servo signal f2. 
The output of modulator 53 contains the signals 2f2, (f2-f1), (f2+f1) and 
(f2-f2) which may deviate slightly from ideal values because of variations 
in RPM of disk or from tolerances in the recording or readback process. 
However, the actual frequencies should approximate the ideal with a 
deviation approximately equal to two percent. Similarly (f1-f1) or (f2-f2) 
should be zero frequency, but in practice may be a slowly varying DC or a 
low frequency signal. Servo channel 51 processes positional information 
carried by recorded servo signal f1. 
The two remaining signals, (f1-f2) in channel 41 and (f2-f1) in channel 51 
contain signals which after demodulation by peak detection, are balanced 
against each other in the difference amplifier 61, as previously 
described. Any remaining error out of difference amplifier 61 represents a 
positive or negative signal which after being combined with positional 
information in servo compensator 66 is fed to a power amplifier 62 which 
has sufficient output power to drive actuator 63 which has a voice coil 
for adjusting the position of transducer 15, suspended a slight distance 
above recording medium 11. 
The phase-locked oscillator which generates modulation signals f1 and f2 
does so in response to a sync reference signal which is derived from the 
header region 16 of each servo track. The sync reference signal is a 
reference for the phase-locked oscillator which controls electronically 
generated modulation frequencies f1 and f2. This may be more clearly 
understood with reference to FIG. 3 which illustrates the phase-locked 
oscillator in block form. 
In FIG. 3, the sync reference signal, at a frequency of 4F or 6.46 MHz, is 
applied to a pulse shaper 73 which squares the input signals at a given 
amplitude for treatment of the sync reference signal by digital circuits. 
A frequency divider 75, which is a flip-flop scaler, divides the pulse 
frequency in half for convenience. This signal is then transmitted to a 
phase detector 77 which compares the phase of the pulses received from 
frequency divider 75 with the phase of pulses received from voltage 
controlled oscillator 81 through the frequency divider 83. The phase 
detector compares leading edges of input pulses. When phase differences 
occur, a corrective signal is transmitted to compensating network 79 which 
translates the phase-locked oscillator corrective signals from phase 
detector 77 into voltages which are applied to the voltage controlled 
oscillator (VCO) 81 thereby controlling its frequency. Under zero 
phase-locked oscillator corrective conditions the two input signals to the 
phase detector 77 are of the same phase and of the same frequency. 
VCO 81 is tuned to a given frequency 77.41 MHz, which is an integer 
multiple of the modulation frequencies and also the data write clocking 
frequency. VCO 81 produces square waves which, as previously mentioned, 
are divided by frequency divider 83 for transmission to the phase detector 
77. The output from VCO 81 is also transmitted to frequency dividers 85, 
87 and 89. A first frequency divider 85 divides the VCO output by an 
integer. The second frequency divider 87 divides the VCO frequency by a 
different integer, while the third frequency divider 89 divides the VCO 
output by a third integer. For example, in this embodiment a VCO frequency 
of 77.41 MHz may be used to derive a first modulation frequency f1 of 2.42 
MHz by dividing by 32 while a second modulation frequency f2 of 2.58 MHz 
may be derived by dividing by 30. A data write clocking frequency, 
f.sub.write of 12.9 MHz may be derived by dividing by 6. The derived 
modulation frequencies are corrected when an enabling signal is 
transmitted along the PLO enable line 91. The enable signal occurs a time 
after a sync reference waveform has been received by the transducer. 
Phase detector 77, compensation network 79, VCO 81 and frequency divider 83 
form a phase-locked loop, or phase-locked oscillator, the heart of which 
is VCO 81, which together with the first and second frequency dividers 85 
and 87 form an oscillator means for generating the two modulation signals 
in response to the sync reference signal. 
FIG. 4 schematically shows the details of phase detector 77 of FIG. 3. The 
circuit includes a pair of identical J-K flip-flops 82, 84. Flip-flop 82 
has a clock terminal connected to receive the sync reference signal, which 
has been shaped by pulse shaper 73 and divided by two by frequency divider 
75 in FIG. 2. Flip-flop 84 has a clock terminal which is connected to 
receive the divided VCO output from frequency divider 83 in FIG. 3. 
The Q output of flip-flop 84 is connected to the J input of flip-flop 82; 
the Q output of flip-flop 82 is connected to the J input of flip-flop 84. 
Both of the K inputs to the flip-flops are grounded. Both of the Q outputs 
are used to close switches 76, 78 which are solid state switches, 
preferably transistors, operated by signals supplied by the Q outputs. The 
current sourcing function of each transistor is indicated by the current 
sources 72, 74 connected to each switch. 
Since the arrangement of flip-flops 82, 84 is such that only one of the 
flip-flops at a time can have a high level output, only one of the 
switches 76,78 can be closed at one time. This event occurs when there is 
a phase or frequency difference between the divided VCO and the divided 
sync reference signal. When such a phase or frequency difference occurs, 
the Q1 output of flip-flop 82 will produce a pulse whose duration is 
proportional to the phase difference if the divided VCO signal is lagging 
the divided sync reference signal. This is known as a "speed-up signal", 
which is used to close switch 76. When switch 76 is closed, the charge on 
capacitor 80 increases thereby increasing the holding voltage which is fed 
to compensation network 79. 
On the other hand, when the phase of the divided VCO signal lags the 
divided sync reference signal, the Q2 output of flip-flop 84 produces a 
pulse which is proportional to the phase difference and this pulse is used 
to close switch 78, drawing a charge from capicitor 80 until the switch is 
again opened at the end of the Q2 output pulse. 
The capacitor 80, in combination with the switches 76, 78 form a sample and 
hold circuit for adjusting the holding voltage which controls the 
frequency of VCO 81 in FIG. 2. The capacitor 80 helps the VCO maintain the 
desired frequency. However, a slight amount of leakage from the capacitor 
and associated circuitry may cause this holding voltage to change slightly 
between sectors. Furthermore, slight variations in the magnetic or 
electronic assemblies associated with the recording or playback systems 
used in connection with the data medium may also cause slight variations 
in the holding voltage. For that reason the holding voltage may have to be 
corrected at each servo sector. 
In a sampled data system wherein sync reference waveforms are interlaced in 
sectors between data waveforms, the sample and hold circuit of the present 
invention provides a means for intermittent adjustment of the reference 
voltage for a voltage controlled oscillator of a phase locked loop. 
FIG. 5 illustrates the manner in which the signals discussed with reference 
to FIG. 4 operate to correct the holding voltage for the voltage 
controlled oscillator. The top two signals in FIG. 5 illustrate the sync 
reference signal in comparison to a divided VCO signal. At first, the 
divided VCO signal is shown to be lagging the divided sync reference 
signal, then is shown to be even with it and lastly is shown to be leading 
it. In response thereto, the Q1 output of flip-flop 82 initially produces 
a pulse which is proportional to the phase difference between the sync 
reference signal and the divided VCO signal. 
As the phase difference diminishes and becomes zero, the duration of the 
pulses Q1 becomes shorter until the two signals divided sync reference and 
divided VCO occur at the same time whereupon the Q1 pulse vanishes and 
remains absent as the divided VCO phase leads the sync reference signal. 
However, the Q2 output of flip-flop 84, while initially absent when the 
divided VCO signal was behind the divided sync reference signal yields a 
positive pulse when the divided VCO signal leads the divided sync 
reference signal. The Q1 and Q2 outputs are used to control the switches 
76, 78 which apply charge to and remove charge from capacitor 80 in FIG. 
4. 
The holding voltage, V.sub.HOLD, is corrected in the manner shown by the 
bottom plot in FIG. 5 wherein the holding voltage is made more positive 
during the positive pulses of the Q1 output and remains at the higher 
positive levels until made negative by the Q2 output pulse which drains 
charge from the capacitor 80. Once the divided sync reference signal 
establishes the correct holding voltage during a divided sync reference 
burst, capacitor 80 maintains holding voltage level until correction by 
the next burst. The holding voltage of capacitor 80 keeps the voltage 
controlled oscillator of the phase locked loop at a relatively constant 
frequency except for small errors which are adjusted as previously 
described at the next servo sector. The continuously running oscillator 
provides a data write clocking frequency which may be used at any time, in 
addition to the modulation frequencies f1 and f2 which are used in the 
servo channels. 
FIG. 6 is a schematic of a time gating circuit for showing the manner in 
which the PLO enable and demod enable signals are derived. Signals from 
the data transducer are fed from pulse shaper 73 along line 100 to 
monostable multivibrator, single-shot 101 which is interrogating for data 
at a rate corresponding to frequency 4F, which is the sync reference 
frequency. This is fast enough so that transitions of data pulses at 
frequencies 1F, 1.33F, and 2F will not be detected. This is shown 
graphically in FIG. 7 wherein the interrogating pulses of single-shot 101 
are labeled S/S while the data frequencies 1F, 1.33F and 2F are shown 
above this. The top waveform in FIG. 7 shows square wave sync reference 
pulses at the frequency 4F. The interrogation by multivibrator 101 takes 
place on the transition from low to high as indicated by the arrows 
labeled "interrogate for clear." 
In FIG. 6 it will be seen that the output of single-shot 101 is transmitted 
to D flip-flop 103 as a clock for the flip-flop. Input data is clocked 
through flip-flop 103 and is used to clear shift register 105 when the 
input to flip-flop 103 is high during the interrogation interval. However, 
this occurs only at the data frequencies 1F, 1.33F, and 2F. The sync 
frequency 4F is such that the D input to flip-flop 103 is low at the time 
of an interrogation and thus the shift register 105 is not reset. 
Shift register 105 counts eight cycles of sync reference frequency 4F and 
then transmits a pulse to flip-flop 107 and to flip-flop 109. The eight 
counted cycles of the sync reference frequency thoroughly insures proper 
identification of the sync frequency. The output of flip-flop 109 is the 
PLO enable signal which resets frequency divider 83 in FIG. 3. By putting 
the first eight sync reference pulses through shift register 105 before 
generating the the PLO enable signal, the first eight sync reference 
pulses are effectively ignored. The advantage in doing this is that it 
eliminates the need to compensate for a slight bit pattern shift in the 
position of the first sync reference pulse which may occur in magnetic 
recording media due to the fact that another sync pulse does not precede 
the first pulse. 
The PLO enable signal which is derived from the flip-flop 109 is used in 
FIG. 3 to reset frequency divider 83, whose output is compared to the 
divided sync reference signal in phase detector 77. The advantage in 
resetting frequency divider 83 to zero is that at PLO enable time, the 
phase detector 77 is unable to determine whether the sync reference pulse 
is leading or lagging, with respect to phase, the divided VCO pulse 
received from frequency divider 83. By resetting frequency divider 83 to 
zero with the PLO enable pulse, the phase detector 77 now has set both the 
sync reference signal and the divided VCO signal initially in phase so 
that the phase detector will not make the mistake of correcting in the 
wrong direction and thereby waste time in completing its generation of a 
true corrective voltage for the VCO. This savings of lock-on time 
represents a corresponding savings in area of servo information dedicated 
to the sync reference signal. The output of flip-flop 107 is fed to 
counters 111, 113, which are clocked by the divided VCO signal for 
counting further eight bit sequences of frequency 4F. After 20 counts of 
divided VCO signal, the nand gate 115 is activated thereby clearing 
flip-flop 109 and halting the PLO enable signal. By this time, the 
phase-locked oscillator has locked onto the sync reference signal, 
whereupon the enabling signal is no longer needed and the frequency of VCO 
81 is adjusted as previously described. After 32 counts the flip-flop 117 
is activated for enabling the servo channels as previously described. 
After 144 counts flip-flop 117 is cleared through a count summation in 
nand gate 118 and similarly flip-flop 107 is cleared since a sufficient 
amount of time is elasped for passage of the servo sector past the 
transducer. A positional error signal has been derived and the electronics 
must be reset for the next servo sector. 
Flip-flop 109 and shift register 105 may be regarded as a first time gating 
circuit which enables the phase-locked oscillator frequency correction to 
assure production of accurate modulation frequencies by the frequency 
dividers associated with the oscillator at a time interval after 8 periods 
of sync reference frequency 4F and lasting for an interval equal to 40 
periods of frequency 4F. 
The flip-flop 117 together with counters 111, 113 and shift register 105 
may be regarded to be a second time gating circuit which enables the servo 
channels after 32 counts of the counter 113 and lasting for 144 counts of 
the same counter. In FIG. 2, the first and second time gating circuits of 
FIG. 6 are represented by time gates 40. 
These time gating circuits, together with signal processing provided in 
each servo channel, provide an enhanced signal to noise ratio for the 
sectorized data path following servo system of the present invention.