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BACKGROUND OF THE INVENTION
The present invention is generally directed to the recording and
reproduction of signals on a magnetic medium, particularly the positioning
of a record/reproduce transducer head adjacent to a track of information
on a magnetic recording tape. More specifically, the invention is directed
to a system for dynamically positioning plural transducing heads adjacent
to tracks of video signal information in accordance with a predicted shape
for the tracks.
Information signals, for example video signals, are typically recorded on a
magnetic medium, such as magnetic tape, in discrete tracks of information.
In one type of recording system that is in widespread use for recording
video signals, the magnetic tape is disposed around the periphery of a
scanning drum and longitudinally transported relative thereto. One or more
magnetic transducing heads rotate about the circumference of the drum. The
tape follows a helical path around the drum, so that the rotating head
transcribes a path, or track, along the tape that is disposed at an angle
relative to the longitudinal direction of the tape. As the tape is
transported around the drum at a predetermined speed, successive adjacent
tracks are formed on the tape at that angle. During playback, if the tape
is transported around the scanning drum at the same speed, the rotating
transducing head will successively read the tracks in the order in which
they were recorded, under ideal conditions.
However, due to varied conditions such as stretching of the tape,
differences in the normal speed between one machine and another, etc., the
transducing head may not be precisely positioned over a recorded track. As
the location of the head moves away from the center of the track, the
quality of the reproduced signal begins to degrade.
Accordingly, in order to faithfully reproduce individual tracks of video
information, it is necessary to deflect the transducing head in a
direction that is substantially transverse to its path of movement around
the scanning drum. In other words, the transducing head must be moved in a
direction that is parallel to the axis of the drum to enable it to remain
adjacent to a particular track of recorded information. The position of
the head in this direction is sometimes referred to as its "elevation."
Various techniques have been developed to control the elevation of the head
to maintain it substantially centered over the recorded track. One popular
technique applies a continuously oscillating dither signal to a control
voltage that determines head elevation, to provide feedback information
that enables the transducing head to be precisely positioned Exemplary
servo systems which utilize a dither signal for controlling the elevation
of the transducing head are disclosed in U.S. Pat. Nos. 4,151,570,
4,163,993 and 4,485,414, among others.
Under proper tracking conditions, i.e., when the transducing head is
precisely centered over the track of recorded information that is being
reproduced, an RF information signal from the head, for example a video
signal, is at a maximum amplitude. When the head is displaced to one side
or the other of the track, the amplitude of the RF signal decreases. When
the dither signal is imposed upon the head elevation position control
signal, it causes the head to slightly oscillate to either side of the
recorded track of information, resulting in a sinusoidal envelope in the
RF waveform from the transducing head. If the average position of the head
is centered over the track of information being reproduced, this RF
envelope has a symmetrical shape. If, however, the average position of the
head is displaced from the track, the envelope will be asymmetrical with
respect to the dither signal. More particularly, the magnitude of the
reproduced RF signal will be lower when the dither oscillations are at one
extreme than when they are at the other extreme. This asymmetry in the
envelope can be detected and used to correct the position of the head.
Transducing head elevation servo systems which employ a dither signal or
similar type of signal for providing feedback information relating to the
alignment between the head and a track of recorded information utilize two
different types of information from the dither signal to control the
elevational position of the heads. In one portion of the control loop of
the servo system, real-time information that is obtained from a detected
dither signal is used to effect instantaneous position control of the
head. In another portion of the control loop of the servo system, the
feedback information obtained from the detected dither signal is averaged
over several successive scans of the recorded tracks. This information
provides an indication of the average shape and relative position of a
track, and can be used to effect dynamic or high rate error correction of
the head position. The present invention is particularly concerned with
this latter aspect of head elevation servo systems, i.e., the dynamic or
high rate error correction based upon a predicted shape of the recorded
track. Therefore, the description which appears hereinafter will be
primarily focused upon this aspect of the head elevation servo system.
In the dynamic correction of the head position to account for the shape of
the track, the magnitude of the dither correction signal is sampled at
several locations along the length of each track being scanned. For
example, 10-15 samples might be made along the length of each track. Each
sample provides an indication of the elevational position of the head,
relative to the track, at the location of that sample. By averaging the
samples for the respective locations over several successive scans of the
tracks, information regarding the average shape of the track is obtained.
Thus, for example, if each track is generally "S" shaped, the head will be
displaced to one side of the track during an early part of its scan of
that track and displaced to the other side of the track during the latter
part of the scan, since the head generally tends to follow a linear path.
However, by sampling and storing the detected dither signal which
indicates the displacement of the head, and subsequently applying these
stored values to the head positioning control signal during subsequent
scans, the head can be dynamically positioned in accordance with the
predicted shape of the track.
Typically, the dynamic correction of the head position is carried out by
sequentially connecting storage capacitors to the detected dither signal
during successive, respective portions of the track being scanned. The
connections of the capacitors to the dither correction signal can be
controlled on a time basis, in a manner analogous to the operation of a
demultiplexer. Thus, if N storage capacitors are provided, each capacitor
could be connected to the dither correction signal for 1/N of the total
time required to scan a single track of recorded information, resulting in
N samples. These samples are then successively applied to the head
position control signal during subsequent scans of recorded tracks.
In some types of magnetic tape recorders, two or more heads are in contact
with the tape at any one time to record or reproduce plural tracks of
information simultaneously. For example, in a video tape recording machine
a field of video information can be divided over several successive
tracks. In this plural head type of arrangement, the two transducing heads
are mounted on a common deflectable arm and moved in unison during
elevational positioning. The heads can be mounted close enough to one
another to scan two physically adjacent tracks on the tape. To diminish
the likelihood of cross-talk between the two adjacent tracks, the two
heads can be offset at slight angles in opposite respective directions to
the axis of the tape. This offsetting of the heads in opposite directions
is typically referred to as "cross-azimuth". Since the two heads are
mounted on a common movable arm for elevational positioning, the dither
signal that is imposed upon the elevation control signal influences both
heads. It is possible to employ the dither feedback signal from only one
of the heads to effect elevational position correction. Since the two
heads are always moved in unison, any asymmetry in the RF envelope from
one head should be indicative of track misalignment of the other head as
well.
In order to accommodate differences in the distance between heads from one
machine to another, it would be more preferable to average the dither
envelope in the RF signals from both of the heads, and use this averaged
signal to control elevational position. When this averaging technique is
employed, however, a problem arises in connection with the sampling of the
detected dither signal for dynamic correction purposes if more than one
pair of heads is used to reproduce the recorded tracks of information. For
example, if two pairs of heads are employed each pair is alternately in
and out of contact with the tape during recording and reproduction.
Furthermore, the two heads in a pair which are in simultaneous contact
with the tape are spaced in the direction of their movement around the
drum. As a result, one head comes into contact with the tape slightly
before the other head in the pair, and the signal from the trailing head
in the direction of the scan is slightly delayed relative to the leading
head. Because of this slight delay between the signals from the two heads
in one pair and the alternating arrangement of head pairs, it is possible
that the leading head in one pair will come into contact with the tape and
begin to reproduce a track of recorded information before the trailing
head of the other pair has come out of contact with the tape. If the
dither signals from these two heads in different pairs are averaged
together, the result does not provide any meaningful information because
it pertains to two different portions of two different tracks.
Consequently, the first sample of the detected dither signal must be
disregarded.
It is possible to disregard the first sample by simply clamping it to a
ground reference potential. However, if the following samples of the
detected dither signal have a significant non-zero value, it can be
appreciated that there will be a sharp transition between the value of the
first, grounded sample and the following samples. This sharp transition
can result in a spike in the head position correction signal, which could
take some period of time to settle out of the control loop.
Accordingly, it is desirable to be able to reliably predict the value of a
missing sample of the track curve measuring system. Since the missing
sample represents the first sample in a sequence, it is most preferable to
predict the value of this missing sample on the basis of the next few
samples which can be reliably measured.
BRIEF STATEMENT OF THE INVENTION
In accordance with the present invention, the elevational position of a
magnetic transducing head is detected relative to a track of recorded
information being scanned by the head. This detection is carried out at
several locations along the length of the track, and a value is stored for
each of these locations. Based upon the stored information, an estimation
is made of a position value for the first and/or last location along the
length of the track. This estimation is determined from a weighted
extrapolation of a few samples adjacent the location whose value is being
estimated. Preferably, the estimated first sample is equal to the
algebraic sum of the adjacent second sample and the difference between the
second and third samples, or a percentage of this difference. If desired,
the value for the last sample can also be estimated in a similar manner
using the two previously measured samples.
The elevational position of the head is dynamically controlled in
accordance with the actual and estimated head position values. The
weighting can be such that each value which is used to determine an
estimated value is given equal consideration. More preferably, however,
the actual value which is closest in position to the estimated value,
e.g., the value for the second sampled location, is given a greater weight
than subsequent measured values. By giving the closest value a greater
weight than subsequent values, the estimated value is less likely to be
erroneously influenced by sharp curves in the shape of the recorded
tracks.
Further features of the invention, as well as the advantages offered
thereby, are explained in greater detail hereinafter with specific
reference to preferred embodiments illustrated in the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a partial schematic and partial block diagram view of a portion
of a magnetic tape reproduction system of the type to which the present
invention is applicable;
FIG. 2 is a schematic plan view of the arrangement of a magnetic recording
tape on a scanning drum;
FIG. 3 is partial block and partial schematic diagram of a track curvature
filter which includes the estimation feature of the present invention;
FIG. 4 is a timing diagram of the signals generated during the operation of
the system depicted in FIGS. 1 and 3;
FIGS. 5A and 5B are graphic illustrations of two examples of the estimation
of the first sample in accordance with the invention; and
FIGS. 6A and 6B are illustrations of the track error correction signals
that are respectively obtained without and with sample estimation.
DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS
In the following description of a preferred embodiment of the invention,
reference is made to the recording and reproduction of video signals using
dual heads and a helical scan format to record/reproduce tracks of video
information as well as a dither signal to detect the position of the
heads. Although the principles which underlie the present invention are
particularly suited for use in this type of recording and reproduction
system, it will be appreciated by those familiar with the relevant
technology that the applicability of the invention is not limited thereto.
Rather, this particular example is chosen to facilitate an understanding
of the invention and an appreciation of the advantages offered thereby.
An exemplary magnetic tape recording and reproducing system of the type to
which the present invention is applicable is schematically illustrated in
FIGS. 1 and 2. FIG. 1 contains a perspective illustration of the head
scanning mechanism along with a block diagram of the head elevation
control system, and FIG. 2 is a top view of the head scanning system.
Referring to the figures, the recording and reproducing system includes a
scanning drum 10 about which a magnetic recording tape 12 is partially
wrapped. Typically, the drum 10 is comprised of an upper drum and a lower
drum. The lower drum remains stationary while the upper drum is rotated.
In the embodiment illustrated in FIGS. 1 and 2, the rotation of the drum
is in a clockwise direction. As best shown in FIG. 2, the tape 12 is
guided around a pair of guide rollers 14, 16 and the drum 10 so that it is
in contact with approximately 180.degree. of the surface of the drum. The
tape is longitudinally moved by a suitable transport mechanism, for
example a capstan (not shown), so that it traverses the surface of the
drum. In the embodiment illustrated in the figures, the direction of tape
movement across the surface of the drum is counterclockwise, i.e.,
opposite to the direction of drum movement.
Two pairs of magnetic playback transducing heads, labelled 1, 2, 3, 4, are
disposed at the circumferential surface of the upper rotating drum. One
pair of heads 1, 2 is disposed 180.degree. from the other pair 3, 4.
Typically, one pair of heads is in contact with the portion of the tape 12
disposed around the periphery of the drum 10, while the other pair is out
of contact with the tape. As the drum rotates, the pairs alternately come
in contact with the tape. One head in each pair, e.g., heads 1 and 3, are
associated with one channel of information, Channel A, and the other head
in each pair, i.e., heads 2 and 4, are associated with Channel B
information.
As illustrated in FIG. 2, the drum can also include two pairs of recording
heads 5, 6, 7, 8. In a manner similar to the playback heads, the two pairs
of recording heads are disposed on diagonally opposite sides of the drum,
and displaced 90.degree. around the circumferential surface of the drum
from the playback heads. The tape 12 is disposed along a path that forms a
helix around the surface of the drum 10. Thus, as the upper drum rotates,
the individual heads transcribe tracks which are oriented at an acute
angle relative to the longitudinal direction of the tape.
Assuming the axis of the drum 10 is vertically oriented, all of the heads
normally travel about a horizontal path. To improve packing density on the
tape, one head in each pair, e.g., 1, 3, 5, 7 is oriented at an angle of
about 15.degree. relative to the horizontal plane. The other head, 2, 4,
6, 8 in each pair is oriented at the same angle but in the opposite
direction, to provide a cross azimuth relationship. With this arrangement,
the tracks transcribed by the respective heads in each pair can be located
directly adjacent one another, without interference between them during
playback.
Each of the two heads in a pair is mounted on a common deflectable arm 18.
The ends of these arms on which the heads are mounted can be moved in a
direction that is transverse to the path transcribed by the rotating
heads, i.e., they can be deflected in a generally vertical direction.
Preferably, the vertical movement of the arms to determine the elevational
positions of the heads is effected by means of a voice coil (not shown)
upon which each arm is mounted. The positioning of the arm is controlled
in response to a voltage signal applied to the voice coil.
The circuit for generating this control voltage signal is illustrated in
block diagram form in FIG. 1. To enable the elevational position of the
heads relative to a scanned track to be determined, a low frequency
oscillating signal, for example a 1 KHz sinusoidal signal, is produced by
a dither generator 20 and applied to a summing junction 22. At the summing
junction 22 the dither signal is combined with other head position
correction signals (described in greater detail hereinafter) and fed as an
input signal to an amplifier 24. The output signal from the amplifier
comprises the control voltage that is applied to the voice coils for the
deflectable arms on which the heads are mounted.
As explained in detail in the previously mentioned patents, the dither
signal causes a sinusoidal amplitude envelope to be imposed upon the RF
output signal from the playback heads 1-4. The RF output signals are fed
to associated preamplifiers 26 and then selectively connected to a pair of
equalizers 28 for the respective channels. The selective connection of the
amplified RF signals to the equalizers is carried out by means of a pair
of switches 30, 31 whose positions are controlled by respective head
switch signals in accordance with the rotational position of the scanning
drum 10. During one half of a revolution of the drum, the switches are in
the positions illustrated in FIG. 1, to connect the equalizers to the two
heads which are in operative contact with the tape. When these two heads
come out of contact with the tape and the other two heads come into
contact with the tape, the positions of the respective switches 30, 31 are
changed, so that the equalizers are always connected to the two heads
which are in operative relationship with the tape at any time. The output
signals from the equalizers are applied to suitable signal processing
circuitry which decodes the signals to determine the information stored on
the tape.
The output signals from the equalizers are also applied to a pair of RF
envelope detectors 32. These detectors filter out the high frequency RF
information signal and produce output signals indicative of the shape of
the RF envelope which is imposed on the output signals from the heads as a
result of the applied dither signal. The two envelope signals from the
respective RF envelope detectors 32 are averaged together in a suitable
averaging circuit 4, and the resulting signal is applied as one input to a
synchronous detector 36. The synchronous detector correlates the averaged
envelope signal with the dither signal from the generator 20, and produces
a detected dither signal.
The detected dither signal from the synchronous detector 36 provides an
indication of the elevational position of the heads relative to the track
being scanned. This signal is applied to a ramp generator and dither
correction circuit 38, which generates a position compensation signal to
correct any misalignment between the heads and the tracks being scanned.
Basically, the ramp generator and dither correction circuit 38 provides
instantaneous correction of the elevational position of the heads in
response to the detected dither signal. Although not illustrated in FIG.
1, the ramp generation and dither correction circuit receives other input
signals in addition to the detected dither signal that are indicative of
the particular mode of operation of the recording and reproduction system.
For example, these other input signals indicate whether the system is in a
normal playback mode or a special effects mode, such as fast play or
freeze frame. In response to these signals, the ramp generation and dither
correction circuit 38 generates suitable control voltages to position the
head in accordance with the anticipated location of the track for the mode
of operation.
In addition, the detected dither signal is applied to a track curvature
correction circuit 40. The track curvature correction circuit essentially
averages the detected dither signal over several successive scans of
recorded tracks to determine the general shape and position of the tracks.
For example, if the tape is stretched during playback the tracks could
have a curved shape rather than being perfectly linear. Also, if the speed
of the tape differs slightly during reproduction than the speed at which
it was recorded, or the rotational speed of the scanner on a playback
machine is different from the rotational speed of the scanner for the
machine on which the tape was recorded, the angle of the tracks might be
slightly offset. The track curvature correction circuit 40 determines the
average shape and orientation of the tracks on the basis of the detected
dither signal, and produces an output signal to control the elevational
position of the heads in accordance with the predicted shape and
orientation of the track being scanned. The output signals from the dither
correction circuit 38 and the track curvature correction circuit 40 are
summed with the dither signal at the junction 22, and applied to the
amplifier 24 to properly position the transducing heads during playback.
An example of a track curvature correction circuit is illustrated in
greater detail in FIG. 3. The detected dither signal from the synchronous
detector 36 is applied to the common terminals of a pair of
multiplexers/demultiplexers 42, 44. These multiplexers/demultiplexers are
responsive to a 4-bit address signal from a counter 46 to connect the
detected dither signal at a common input/output terminal Q to one of a
plurality of selectable terminals I.sub.n. In the embodiment illustrated
in FIG. 3, twelve samples of the detected dither signal are taken during
each scan of a track. In the implementation shown in FIG. 3, one of the
multiplexers/demultiplexers 42 has eight selectable terminals which are
employed, and the other multiplexer/demultiplexer 44 also has eight
selectable terminals, but only four are used. Seven of the eight
selectable terminals I.sub.1 -I.sub.7 of the first
multiplexer/demultiplexer 42 are respectively connected to storage
capacitors C2-C8. Similarly, the four selectable terminals I.sub.0
-I.sub.3 of the multiplexer/demultiplexer 44 that are utilized are
connected to storage capacitors C9-C12.
The counter 46 functions to count an input clock signal having the same
frequency as the dither signal. In fact, the clock signal can be a square
wave version of the dither signal. In operation, the counter 46 is reset
at the beginning of each scan of a recorded track on the tape. The signal
which is used to reset the counter can be the same signal that controls
the switch 30 to indicate when the respective pairs of heads go into and
out of contact with the tape. Each transition in the head switch signal
causes the counter 46 to be reset. After being reset, the counter 46
counts the pulses in the dither clock signal to produce the 4-bit address
signal that is applied to the multiplexers/demultiplexers 42 and 44. The
most significant bit on an output line 48 determines which of the two
multiplexers/demultiplexers 42 or 44 is to be operative at any time, and
the other three bits indicate which one of the eight selectable terminals
I.sub.n in the operative multiplexer/demultiplexer is to be connected to
its common terminal Q. As a result, the detected dither signal is
sequentially applied to the storage capacitors C2-C12 during successive
respective cycles of the dither signal. Thus, it will be appreciated that
the number of samples that are taken per track will be a function of the
frequency of the dither signal and the length of the recorded track.
In addition to being successively applied to the storage capacitors during
respective segments of the track being scanned, the detected dither signal
is also applied to the non-inverting input terminal of a differential
amplifier 50. In operation, when the common terminal Q of one of the
multiplexers/demultiplexers 42 or 44 is connected to one of its selectable
terminals, the storage capacitor connected to that terminal discharges
through a resistor 52 connected between the non-inverting input terminal
of the amplifier 50 and ground. Thus, the detected dither signal is
averaged with the voltage stored in the capacitor under consideration, to
provide a dynamic correction signal based upon previously detected
positions of the head relative to the track for the particular portion
under consideration. This dynamic correction signal appears at the output
terminal of the amplifier 50 and is applied to the summing junction 22.
FIG. 4 illustrates a timing diagram for signals which are produced during
the operation of the circuits shown in FIGS. 1 and 3. A pair of head
switch signals provide an indication of the particular heads which are in
operative contact with the tape at any time. One signal provides an
indication as to which of the two leading heads in each of the pairs,
i.e., head 1 or head 3, is in contact with the tape, and another signal
indicates which of the two trailing heads, 2 or 4, is in contact with the
tape. The two heads in each pair are displaced from one another in the
direction of the scan, i.e., in the horizontal direction. As a result, the
leading head in each pair will come into contact with the tape a short
period of time prior to the trailing head of the pair. For example, the
length of this time period might be in the neighborhood of 180
microseconds. During this time, the two head switch signals will indicate
that the leading head of one pair and the trailing head of the other pair
are in contact with the tape. This period of time is referred to as the
head switch overlap, and is identified in FIG. 4.
The RF envelope signals which are obtained from the heads in operative
contact with the tape are respectively illustrated in FIG. 4 immediately
below the head switch signals. These two RF envelope signals are averaged
together and a detected dither signal is obtained from the synchronous
detector 36, as shown. During each cycle, the amplitude of the detected
dither signal is stored in a respective one of the storage capacitors
C2-C12.
Due to the averaging of the two detected envelope signals from the
respective heads in contact with the tape, the sample of the detected
dither signal which is obtained during the period of the head switch
overlap does not provide any useful information. More particularly, during
the period of the overlap the detected dither signal is an average of the
output signals from two heads in two different pairs. The signal from the
leading head, for example head 1, relates to the beginning of a scan of a
new track. However, the signal from the trailing head at this time, for
example head 4, pertains to the end of the previously scanned track. Since
these two signals are averaged together, the result does not provide
useful information with respect to either of the two tracks. As a result,
the sample which is obtained during the period of the head switch overlap
must be disregarded. For this reason, the first selectable terminal of the
multiplexer/demultiplexer 42 is not connected to a storage capacitor.
It is possible to disregard the first sample of the detected dither signal
by simply shunting it to ground or some other reference voltage. However,
this approach can produce undesirable results if the next few samples are
not close to the reference voltage. For example, as shown in the example
of FIG. 4, a sharp transition can exist for the grounded voltage
representative of the first sample and the stored value for the second
sample. Since the stored samples are A.C. coupled to the elevation error
of the head, a zero voltage value for the first sample can result in a
large step function from the average value of the track curvature error.
In other words, a sharp spike would be produced in the tracking error
correction signal that is applied to the voice coil for the deflectable
arms 18, from which the arms would not be able to immediately recover.
Accordingly, it is desirable to provide an estimated value for the first
sample which is based upon the adjacently measured values of the track
curvature error. In accordance with the present invention, such an
estimated value is generated by means of a weighted extrapolation of the
second and third values.
Referring to FIG. 3, the storage capacitors C2 and C3 store the sampled
track error values for the second and third sampled segments of the track,
as represented in FIG. 4. These two capacitors are respectively connected
to the second and third selectable terminals I.sub.1 and I.sub.2 of the
multiplexer/demultiplexer 42. In addition, the capacitor C2 is directly
connected to the non-inverting input terminal of a differential amplifier
54. The storage capacitor C3 is connected to the inverting input terminal
of the amplifier through a series resistor 56. A feedback resistor 58 is
connected between the output terminal of the amplifier 54 and the
inverting input terminal of the amplifier. This output terminal of the
amplifier is also connected to the first selectable terminal I.sub.0 of
the multiplexer/demultiplexer 42.
In operation, the output signal V.sub.1 fr | | |
