Tracking control apparatus for a rotary head, variable speed signal reproducing system

The present invention is intended for use in a signal reproducing system of the type having at least one rotary transducer for scanning traces across a movable record medium, such as a magnetic tape, to reproduce signals from previously recorded record tracks. Each transducer is supported on a displaceable support member that is responsive to control signals to displace the transducer relative to the record tracks. The record medium is movable at a speed which may be less than, equal to or greater than the speed at which the signals originally were recorded. A frequency generator generates speed representing pulses whose frequency is a function of the speed at which the record medium is moved. A cyclical, or resettable, counter cyclically counts the speed representing pulses. A level generator generates a signal level corresponding to the count then present in the counter at the time that a transducer advances to the middle portion of a scanning trace. The generated signal level is used as a control signal to correspondingly displace the transducer support member. This displacement provides general compensation for the deviation between the scanning trace of the transducer and the record track which is scanned thereby. In a preferred use of the present invention, the signal reproducing system is a VTR, and tracking compensation is provided for slow or fast modes of reproduction. In accordance with another feature, a sawtooth signal having a period equal to a head scanning period, and an amplitude determined by the speed at which the record medium (or tape) is moved is added to the generated level in order to provide substantially continuous error correction of the head scanning trace.

BACKGROUND OF THE INVENTION 
This invention relates to tracking error control apparatus for use in h 
signal reproducing system and, more particularly, to such apparatus for 
correcting tracking errors that may arise when a record medium from which 
signals are reproduced is moved relative to one or more rotary transducers 
at a speed that differs from the speed thereof at which such signals 
originally were recorded. 
In rotary head recording/reproducing systems, signals can be recorded with 
relatively high density over a relatively wide frequency spectrum in a 
series of parallel tracks on a movable record medium. Typical of such 
recording/reproducing systems is the video tape recorder (VTR) in which 
video signals are recorded in parallel, skewed record tracks along a 
moving magnetic tape. In a typical VTR, one or more heads serves as a 
recording head and also as a reproducing head; and such head or heads is 
rotated at a substantially constant speed for both recording and 
reproducing operations. If the speed at which the magnetic tape moves 
during a reproducing operation is identical to the speed at which the tape 
was moved during a recording operation, then each scanning trace of the 
reproducing head easily can be made to coincide with each previously 
recorded record track. However, such coincidence is not readily obtained 
if the tape is moved at a slower or faster speed during the reproducing 
operation. That is, if the ratio between the tape reproducing speed and 
the tape recording speed is N, then a tracking error will occur when 
N.noteq.1. 
In the aforementioned reproducing mode wherein N.noteq.1, the scanning 
traces of each reproducing head will be parallel to each other, but such 
traces will not be parallel to the record tracks on the magnetic tape. For 
example, if N&lt;1, the angle which each scanning trace makes with respect to 
the longitudinal axis of the tape is smaller than the angle which each 
record track makes with this longitudinal axis. Furthermore, when N&lt;1, a 
plurality of scanning traces may be formed across the tape, whereas only a 
single record track is recorded. Each of these traces exhibits a 
deviation, or tracking error, with respect to the particular record track 
which is being scanned. 
In order to overcome this problem, a so-called displaceable support member 
has been developed for each reproducing head. The displaceable support 
member may be an electrostriction device, such as a piezoceramic element 
known, for example, as a bi-morph leaf assembly. Typically, a bi-morph 
leaf assembly is responsive to control signals to deflect by an amount 
that is determined by such control signals. It is appreciated that if a 
head is mounted upon such a bi-morph leaf assembly, the deflection of the 
assembly will result in a displacement of the head. It has been proposed, 
heretofore, to arrange the bi-morph leaf assembly such that, as the 
reproducing head is rotated, control signals applied to the bi-morph leaf 
assembly will result in a corresponding displacement of the head in a 
direction which is transverse to the scanning trace. Consequently, even if 
the scanning trace of the head crosses, or intersects, a record track, 
compensating control signals can be applied to the bi-morph leaf assembly 
such that the head is displaced in a direction to bring it into 
coincidence with that record track. 
When N.noteq.1, the compensating drive signal which must be supplied to the 
bi-morph leaf assembly to eliminate the head tracking error generally is 
of a sawtooth waveform. If a plurality of traces are formed for each 
record track, as is the case when N&lt;1, each trace must be shifted by a 
respective, constant amount in addition to being continuously adjusted for 
coincidence with the record track. This means that, in addition to the 
sawtooth waveform which is used as a compensating drive signal for the 
bi-morph leaf assembly, a staircase waveform compensating drive signal 
also must be supplied thereto. Both the sawtooth waveform and the 
staircase waveform have parameters which are dependent upon the actual 
speed at which the tape is moved. For the proper compensation of tracking 
errors, the apparatus which is used to produce the compensating drive 
signals must, therefore, take the actual tape speed into account. 
OBJECTS OF THE INVENTION 
Therefore, it is an object of the present invention to provide improved 
apparatus for compensating head tracking errors in a signal reproducing 
system wherein the record medium from which signals are reproduced is 
movable at various different speeds. 
Another object of this invention is to provide apparatus for correcting 
head tracking errors in a signal reproducing system, such as a VTR, in 
which correction is dependent, at least in part, upon the actual speed at 
which the record medium from which the signals are reproduced is moved. 
A further object of this invention is to provide tracking error control 
apparatus for use in a signal reproducing system of the type in which one 
or more rotary heads scans a movable record medium, the heads being 
supported on respective displacement support members, in which the support 
members exhibit inherent time delays which are taken into account by the 
error correcting apparatus. 
Various other objects, advantages and features of the present invention 
will become readily apparent from the ensuing detailed description, and 
the novel features will be particularly pointed out in the appended 
claims. 
SUMMARY OF THE INVENTION 
In accordance with this invention, tracking error correcting apparatus is 
provided for use in a signal reproducing system of the type having one or 
more rotary heads which scan traces across a movable record medium, such 
as a tape, to reproduce signals which had been recorded previously in 
parallel record tracks, and wherein the speed at which the record medium 
moves during the reproducing operation is N times the speed at which the 
tape moved during the recording operation. A frequency generator generates 
speed representing pulses having a frequency representing the speed of the 
record medium. These pulses are counted by a cyclical, or resettable 
counter, and a level generator generates a level corresponding to the 
count then present in the counter at the time that a head reaches the 
middle portion of its scanning trace. The generated level is supplied as a 
compensating drive signal to the displaceable support member upon which 
the head is mounted, thereby displacing the head in a direction to 
compensate for any tracking error between its scanning trace and the 
record track being scanned thereby. In accordance with one aspect of this 
invention, a sawtooth signal is added to each generated level, and the 
summed signals are supplied to the displaceable support member. This 
sawtooth signal has a period equal to the period of a head scanning trace, 
and an amplitude which is a function of the speed of the record medium. 
Preferably, the generated level is supplied to the displaceable support 
member prior to the time that the head mounted thereon rotates into 
scanning relationship with the record medium, and the sawtooth signal is 
added to this level at the beginning of the scanning relationship. 
The cyclical, or resettable, counter is controlled in accordance with the 
speed at which the record medium is moved, so as to insure that the levels 
which are generated from the count thereof are accurate regardless of the 
speed of the record medium.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
Referring now to the drawings, wherein like reference numerals are used 
throughout, FIG. 1 is a schematic illustration of a typical recording 
medium 1 upon which parallel record tracks 2 are recorded. For the purpose 
of the present discussion, it is assumed that record medium 1 is a 
magnetic tape. However, it should be readily appreciated that the record 
medium may comprise a magnetic sheet or other conventional recording 
medium upon which signals are recorded in parallel record tracks. 
Furthermore, it is assumed that the signals which are recorded in tracks 2 
are FM video signals, such as monochrome or color video signals, including 
the usual vertical and horizontal synchronizing pulses. As an example, the 
video signals which are recorded in tracks 2 may be formed therein by a 
so-called two-head helical scan VTR. In addition to recording the video 
signals in tracks 2, the typical VTR system records control pulses P.sub.c 
in a longitudinal track 3, positioned along one edge of tape 1, and 
further records audio signals in a longitudinal track 4, the latter being 
positioned along the opposite edge of the tape. 
As is conventional, during a recording operation, tape 1 is driven at a 
servo-controlled constant speed in the direction of arrow 5, and each head 
rotatably scans a trace across the tape in the direction of arrow 6. Thus, 
tracks 2 are skewed relative to the longitudinal axis of the tape. 
Adjacent tracks are separated by a guard band, and each track contains a 
single field of FM video signals. Of course, each field of video signals 
contains horizontal synchronizing pulses P.sub.h which, preferably, are in 
alignment with each other from one track to the next. This is the 
so-called H-alignment, which is effective to reduce unwanted cross-talk 
interference between adjacent tracks. Although not shown in FIG. 1, the 
vertical synchronizing pulses are recorded at the beginning portion of 
each track 2. 
Each control pulse P.sub.c is associated with a respective track, and these 
control pulses are recorded at the frame frequency such that alternate 
tracks 2 are associated with corresponding control pulses P.sub.c. It may 
be appreciated that if a control pulse playback head is provided so as to 
reproduce the control pulses P.sub.c recorded in control pulse track 3, 
each reproduced control pulse is indicative of the particular position of 
tape 1. During a normal reproducing operation, that is, during a 
reproducing operation wherein tape 1 is moved at the same speed as during 
the recording operation, a control pulse P.sub.c is reproduced in 
predetermined phase relationship with the position of a rotary head. For 
example, the control pulse is reproduced at the same time that a rotary 
head rotates into its initial scanning relationship with respect to a 
corresponding track 2. If the predetermined phase relationship between the 
reproduced control pulse P.sub.c differs from the expected phase of the 
head, a servo system is provided to adjust the rotary phase of the head so 
as to insure the proper tracking of each record track 2 by a head. 
Furthermore, the rate at which control pulse P.sub.c is reproduced is a 
function of the speed at which tape 1 is moved. 
For the purpose of the present discussion, let it be assumed that N 
represents the ratio between the speed at which tape 1 is moved during a 
reproducing operation and the speed at which this tape was moved during a 
recording operation. Of course, if N=1, the reproducing speed is equal to 
the recording speed; and this commonly is referred to as the normal 
reproducing mode. For a slow motion mode of reproduction, N&lt;1, and for a 
fast motion mode of reproduction, N&gt;1. For a reverse mode of reproduction, 
N is negative. Finally, if N=0, the VTR is operated in its stop motion 
mode of reproduction. The present invention is directed to reproducing the 
signals which are recorded in tracks 2 for all values of N. It is 
appreciated that if N.noteq.1, then the scanning trace of each head will 
not, without any dynamic displacement thereof, coincide with a record 
track. For the case of a VTR, this tracking error between the scanning 
trace of the head and the record track results in a deterioration in the 
quality of the video picture which ultimately is reproduced. In order to 
overcome this problem, it has been proposed heretofore to support each 
head on a displaceable support member, such as an electrostriction 
element. Typical examples of electrostriction elements which can be used 
are piezo-ceramic members, such as a bi-morph leaf assembly. The advantage 
offered by such displaceable support members is that they can be driven so 
as to deflect in response to a control signal applied thereto. This 
deflection, if properly controlled, will bring the scanning trace of the 
head which is mounted on the displaceable support member into coincidence 
with the record track that is being scanned. 
An example of the tracking error which arises when N.noteq.1 is shown in 
FIGS. 2A-2F, wherein it is assumed that N=1/5. As shown in FIG. 2A, at 
this speed (i.e., wherein N=1/5), five individual scanning traces i, ii, 
iii, iv and v can be produced between adjacent record tracks 2. For the 
purpose of the present discussion, it is assumed that these scanning 
traces i-v are intended to traverse record track 2A. It is further 
assumed, for the purpose of simplification, that the commencement of trace 
i coincides with the commencement of record track 2A. It is further 
assumed that these scanning traces are formed by two rotary heads which 
may be identified as heads A and B. That is, trace i is formed by head A, 
trace ii is formed by head B, trace iii is formed by head A, trace iv is 
formed by head B, trace V is formed by head A, and so on. Of course, since 
tape 1 is moved at a constant speed (which, as assumed herein, is 
one-fifth the normal tape speed), scanning traces i-v all are parallel to 
each other and are angled to intersect record track 2A. 
If the separation between the center lines of adjacent record tracks 2 is 
assumed to be p, then the deviation d of a given trace i-v is as shown. 
When a trace deviates to the left of record track 2A, it is assumed that 
deviation d is positive and, conversely, if the trace deviates to the 
right of record track 2A, it is assumed that this deviation d is negative. 
The deviation d of trace i varies from zero at the commencement of this 
trace to a maximum of -4/5p. The deviation d of trace ii varies from +1/5p 
at the beginning of this trace to -3/5p at the end thereof. In a similar 
manner, trace iii exhibits a deviation d that varies from +2/5p to -2/5p; 
trace iv exhibits a deviation d that varies from +3/5p to -1/5p; and trace 
v exhibits a deviation d that varies from +4/5p to 0. This deviation of 
the respective traces is shown in FIG. 2B, wherein the ordinate represents 
a corresponding correction voltage E.sub.c which must be applied during 
each scanning trace to the displaceable support member upon which each 
head is mounted in order to correct for the deviation d, and the ordinate 
also represents this deviation d. 
It is seen that each trace i-v is formed during one head field period of 
time. For the purpose of the present discussion, the expression "head 
field period" means the period that each head (A or B) scans a complete 
trace across the tape. Of course, when N=1, each scanning trace coincides 
with a record track and, therefore, each head field period is equal to 
each video field period. If, during each successive head field period, a 
correction voltage having the waveform as shown generally in FIG. 2B is 
supplied to the displaceable support member upon which the head A or B is 
mounted, the deviation d of the trace formed by the head will be 
cancelled. This means that, during each head field period, the head will 
be displaced in a direction such that its scanning trace coincides with 
record trace 2A. From FIG. 2B, it is appreciated that the slope of this 
correcting voltage E.sub.c is the same for each head field period, because 
all of the traces i-v are parallel to each other. However, the DC level of 
each correcting voltage varies in a stepwise manner, of constant step 
level. 
Correction voltage E.sub.c, as shown in FIG. 2B, may be thought of as being 
constituted by a staircase component and a sawtooth component. The 
staircase component is shown as the staircase waveform E.sub.s in FIG. 2C, 
wherein the width, or duration, of each step is equal to one head field 
period, that is, the width of each step is equal to the duration of a 
scanning trace. Furthermore, since the maximum deviation of a trace from 
record track 2A is seen to be equal to 4/5p, such as the deviation d for 
traces i and v, the peak-to-peak amplitude of the staircase waveform 
E.sub.s is equal to 4/5p. The height of each step is seen to be one-fifth 
of this peak-to-peak amplitude. 
The sawtooth component which is included in correction voltage E.sub.c is 
shown in FIG. 2D as the sawtooth waveform E.sub.n. The period of each 
sawtooth wave is equal to the head field period, and the peak-to-peak 
amplitude thereof, referred to hereinafter merely as the amplitude of the 
sawtooth waveform, also is equal to 4/5p. If the displaceable support 
member upon which each head is mounted is an electrostriction element, 
such as a bi-morph leaf assembly, the staircase waveform E.sub.s and the 
superimposed sawtooth waveform E.sub.n may be staircase and sawtooth 
voltages. Thus, FIG. 2C represents the staircase waveform voltage 
component of correction voltage E.sub.c, and FIG. 2D represents the 
sawtooth waveform voltage component of the correction voltage. 
The staircase waveform voltage E.sub.s may be derived by providing a 
sawtooth voltage E.sub.k, shown by broken lines in FIG. 2C, whose 
instantaneous amplitude is sampled at the middle portion of each scanning 
trace. The period of each sawtooth voltage E.sub.k is equal to the time 
required to move tape 1 a distance equal to the separation between 
adjacent record tracks 2. Furthermore, since, as shown in FIG. 1, a 
control pulse P.sub.c is recorded in control pulse track 3 in a 
predetermined position with respect to an associated record track 2, it is 
seen that the reproduction of the control pulse may be used to 
synchronize, or commence, the sawtooth voltage E.sub.k. This is shown by 
comparing FIG. 2C with FIG. 2E. Of course, a control pulse P.sub.c is 
recorded for every other record track and, therefore, two sawtooth 
voltages E.sub.k can be produced in the period between successive control 
pulses. 
In a conventional rotary head helical-scan VTR, the drive shaft which is 
used to rotate the heads usually is provided with a sensible element, such 
as a magnetic indicia, located with predetermined angular position with 
respect to the heads. A fixed pick-up, such as a magnetic sensor, is 
disposed such that a head position pulse P.sub.g is produced whenever a 
head A or B rotates to a predetermined position on tape 1. As an example, 
this position may be the commencement of a scanning trace. FIG. 2F 
represents a train of head position pulses P.sub.g which is produced 
during the rotation of the rotary heads. Since it is assumed herein that 
N=1/5, there are five head position pulses P.sub.g produced during each 
sawtooth voltage E.sub.k. These head position pulses can be used to sample 
the sawtooth voltage E.sub.k to produce the staircase waveform voltage 
E.sub.s having five step levels during each sawtooth voltage E.sub.k. 
These head position pulses P.sub.g also may be used to trigger a sawtooth 
waveform voltage generator to produce the sawtooth waveform voltage 
E.sub.n shown in FIG. 2D. 
It may be seen that even if the commencement of scanning trace i, for 
example, does not coincide with record tracks 2A, the proper staircase 
waveform voltage E.sub.s nevertheless may be produced. If the commencement 
of a scanning trace does not coincide with the commencement of a record 
track, head position pulses P.sub.g will be shifted relative to control 
pulses P.sub.c so as not to be time coincident therewith. Nevertheless, 
sawtooth voltage E.sub.k, since it is synchronized with control pulses 
P.sub.c, will commence with the reproduction of a control pulse. The 
phase-shifted head position pulses P.sub.g, if used to sample the sawtooth 
voltage E.sub.k, will produce proper step levels which will, of course, 
account for the possibility that the commencement of a scanning trace does 
not coincide with the commencement of a record track. For example, if the 
commencement of trace i occurs to the left of the commencement of record 
track 2A, head position pulses P.sub.g will be shifted to the left of 
control pulse P.sub.c. This means that the sampling times of sawtooth 
voltage E.sub.k, attributed to the head position pulses P.sub.g, will be 
shifted to the left of the sampling times shown in FIG. 2C and, 
consequently, the staircase waveform voltage E.sub.s will appear to be 
shifted in the downward direction. This accounts for the shift of trace i, 
and the correction voltage E.sub.c formed by superimposing the staircase 
and sawtooth waveform voltages will cancel the deviation d of each shifted 
scanning trace relative to record trace 2A. 
While the waveforms shown in FIGS. 2A-2F represent the relationship between 
scanning traces of the heads A and B across tape 1 for the example wherein 
N=1/5, the waveforms shown in FIGS. 3A-3E are similar and represent the 
example wherein N=1/3. When tape 1 is moved at a speed that is one-third 
the normal tape speed, it is seen that three scanning traces i, ii, iii, 
are formed between successive record tracks 2. The maximum deviation d of 
trace i is equal to 2/3p, as opposed to the maximum deviation of 4/5p of 
trace i in FIG. 2A. The deviation d of each scanning trace, together with 
a representation of the correction voltage E.sub.c which, if applied to 
the displaceable support member upon which each head is mounted, will 
cancel this deviation, are shown in FIG. 3B. As before, this correction 
voltage E.sub.c may be produced by superimposing a staircase waveform 
voltage E.sub.s and a sawtooth waveform voltage E.sub.n. However, as shown 
in FIG. 3C, since N=1/3, three head field periods are provided in the 
interval required to advance tape 1 from one track to the next. But, since 
the heads are rotated at a constant speed irrespective of the speed at 
which tape 1 is moved, it is seen that the period of the staircase 
waveform voltage E.sub.s, shown in FIG. 3C, is less than the period of the 
staircase waveform voltage shown in FIG. 2C. Furthermore, since the 
maximum deviation d for the example shown in FIG. 3A is less than the 
maximum deviation d for the example shown in FIG. 2A, the peak-to-peak 
amplitude of the sawtooth waveform voltage E.sub.n in FIG. 3D is less than 
the peak-to-peak amplitude of the sawtooth waveform voltage shown in FIG. 
2D. 
For proper generation of the staircase waveform voltage E.sub.s in FIGS. 2C 
and 3C, the sawtooth voltage E.sub.k, which is sampled to produce the 
staircase waveform voltage, must have the same peak-to-peak amplitude in 
both examples. That is, even though the period of this sawtooth voltage 
E.sub.k may change as the tape speed N changes, its amplitude must remain 
constant. Typically, the sawtooth voltage E.sub.k is produced by charging 
a capacitor. However, if the capacitor is charged with a constant current, 
the amplitude of the sawtooth voltage E.sub.k is a function of the 
duration that this capacitor is charged. Since the period of the sawtooth 
voltage in FIG. 3C is less than the period of the sawtooth voltage in FIG. 
2C, if the same constant current is used to charge the capacitor for each 
example, it is clear that the sawtooth voltage E.sub.k in FIG. 3C will 
have a smaller amplitude than the sawtooth voltage E.sub.k in FIG. 2C. 
This means that, in order to obtain the proper amplitude for the example 
of FIG. 3C, the sawtooth voltage E.sub.k must be produced by charging the 
capacitor with a greater current. Unfortunately, this results in a very 
complicated charging circuit. Furthermore, there are significant 
limitations on the magnitude of the charging current and, therefore, it is 
difficult to obtain a sawtooth voltage E.sub.k of the same amplitude for 
all tape speeds. 
The present invention proceeds, in part, on the recognition that, as the 
speed of tape 1 is increased, the period of the reproduced control pulses 
P.sub.c is decreased, but the period of the head position pulses P.sub.g 
remains constant. This is clear from comparing FIGS. 2E, 2F with FIG. 3E. 
If a frequency generator is coupled to the tape drive mechanism, for 
example, if a so-called frequency, or tone, wheel is mechanically coupled 
to the motor which is used to drive the usual capstan, then as the speed 
at which tape 1 is driven is increased, the frequency of the signal which 
is obtained from the frequency generator likewise is increased. Referring 
to FIGS. 4A-4C, let it be assumed that the frequency generator generates 
tape speed pulses P.sub.f whose frequency is, of course, directly related 
to the speed at which the tape is driven. Of course, the rate at which 
control pulses P.sub.c is reproduced also is directly related to the speed 
at which the tape is driven. Consequently, a constant number of tape speed 
pulses P.sub.f will be generated during the period between adjacent 
control pulses P.sub.c. In FIG. 4B, it is assumed that 32 tape speed 
pulses are generated during this period. Thus, regardless of the tape 
speed, a constant number of tape speed pulses P.sub.f always will be 
produced in the interval between adjacent control pulses P.sub.c. In the 
examples shown in FIGS. 4A and 4B, since the control pulses P.sub.c are 
reproduced at the rate of 30 Hz during a normal tape speed (N=1), and 
since 32 tape speed pulses P.sub.f are generated during the interval 
between adjacent control pulses, it is seen that the frequency of the tape 
speed pulses is 960 Hz. 
Furthermore, and as discussed above, two sawtooth voltage periods E.sub.k 
are produced during the interval between adjacent control pulses P.sub.c. 
The control pulses can be used to synchronize the sawtooth voltage 
E.sub.k, as shown in FIGS. 4A and 4C. Now, if it is assumed that 32 tape 
speed pulse P.sub.f always are generated during the interval between 
adjacent control pulses P.sub.c, then 16 tape speed pulses are generated 
during each sawtooth voltage period. In accordance with one aspect of the 
present invention, the sawtooth voltage E.sub.k is generated by counting 
the tape speed pulses P.sub.f and converting the count to an analog 
signal. When 16 tape speed pulses have been counted, the counter is reset 
so as to return the analog signal to its initial level. Furthermore, to 
insure the synchronization between the counting of tape speed pulses 
P.sub.f and the production of sawtooth voltage E.sub.k therefrom, the 
counter is forcibly reset in response to each control pulse P.sub.c. 
Since the sawtooth voltage E.sub.k is produced by converting the count of 
the tape speed pulses P.sub.f to an analog voltage, and since the same 
number of tape speed pulses are counted during the interval between 
adjacent control pulses irrespective of the speed at which the tape is 
driven, it is seen that the sawtooth voltage E.sub.k will have the same 
peak-to-peak amplitude in all instances, regardless of the particular tape 
speed. Thus, the aforenoted problem of having a constant peak-to-peak 
amplitude of the sawtooth voltage E.sub.k from which the staircase 
waveform voltage E.sub.s is produced, is overcome by the present 
invention, to be described in greater detail below. 
Referring to FIGS. 5A and 5B, the head field periods during which heads A 
and B scan alternate traces across tape 1 are shown, and the staircase 
waveform voltage E.sub.s, discussed previously with respect to FIG. 2C, is 
redrawn in FIG. 5B. This staircase waveform voltage typically is obtained 
by sampling the sawtooth voltage E.sub.k when each head A, B advances to 
about the middle portion of its scanning trace. For the purpose of the 
present explanation, the term "middle portion" means that general portion, 
not necessarily a point, which is in a central area of the scanning trace 
of the head. As shown in FIG. 5B, when head A reaches the middle portion 
of its scanning trace, sawtooth voltage E.sub.k is sampled, and the 
sampled level represents the first step of the staircase waveform voltage 
E.sub.s. Then, when head B reaches the middle portion of its scanning 
trace, sawtooth voltage E.sub.k is sampled to provide the next step of the 
staircase waveform voltage. This operation is repeated for each successive 
scanning trace of the heads, resulting in the staircase waveform voltage 
shown in FIG. 5B. Of course, the first step of this staircase waveform 
voltage represents the constant amount by which head A must be displaced 
in order to cause the beginning of its scanning trace to coincide with the 
beginning of the record track being scanned. The next step of the 
staircase waveform voltage represents the amount by which head B must be 
displaced in order to cause the beginning of its scanning trace to 
coincide with the beginning of the record track. Similarly, the remaining 
steps represent the displacement by which heads A and B must be displaced 
during successive head field periods in order to cause the beginning of 
the respective scanning traces thereof to coincide with the beginning of 
the record track being scanned. 
It is appreciated that, in order to provide proper correction for the 
deviation of each scanning trace of heads A and B relative to the record 
track being scanned, the respective steps of the staircase waveform 
voltage E.sub.s shown in FIG. 5B should be supplied to the displaceable 
support member upon which each head is mounted at least as early as the 
time that the head commences its scanning trace. However, as shown by the 
"X" locations in FIG. 5B, each step in the staircase waveform voltage is 
not produced until after the head has reached the middle portion of its 
scanning trace. As a result thereof, the step-level correcting voltage 
which must be supplied to each displaceable support member at least as 
early as the time that the head mounted on that member commences its 
scanning trace is, in fact, supplied thereto at a time that is delayed by 
one-half the head field period. As shown in FIG. 5C, each step-level thus 
is supplied to a displaceable support member at a time that is too late to 
correct the deviation of the scanning trace of the head. Thus, rather than 
supplying the first step of the staircase waveform voltage E.sub.s to the 
displaceable support member upon which head A is mounted at the beginning 
of its scanning trace, this step is supplied when the head has completed 
approximately one-half of its scanning trace. This same undesirable delay 
is present for each successive scanning trace. Thus, if the sawtooth 
voltage E.sub.k has the waveform shown in FIG. 5B, the sampling of this 
waveform each time that a head reaches the middle of its scanning trace 
will not be successful in producing step voltages which will correct for 
the deviations of the respective traces relative to the record track being 
scanned. 
As a further difficulty, a typical electrostrictive element, such as a 
bi-morph leaf assembly, exhibits an inherent time delay prior to 
responding to the control voltage applied thereto. Thus, the displaceable 
support member will not deflect immediately in response to the step-level 
which is applied thereto, even if such step levels are produced at the 
times represented in FIG. 5B. Hence, even if the staircase waveform 
voltage E.sub.s of FIG. 5B is produced, this inherent time delay of the 
displaceable support member will result in a tracking error of the head 
which is mounted thereon. 
The foregoing is overcome in accordance with an aspect of the present 
invention wherein the sawtooth voltage E.sub.k is advanced in phase, and 
then this phase-advanced sawtooth voltage is sampled when each head 
reaches the middle portion of its scanning trace. When head B reaches the 
middle portion of its scanning trace, the phase-advanced sawtooth voltage 
is sampled, and the sampled level is supplied to the displaceable support 
member upon which head A is mounted, even though head A has not yet 
commenced its scanning trace. Then, when head A reaches the middle portion 
of its scanning trace, the phase-advanced sawtooth voltage E.sub.k is 
sampled and the sampled amplitude is supplied to the displaceable support 
member upon which head B is mounted. This operation is illustrated in 
FIGS. 5D and 5E, wherein FIG. 5D represents the staircase waveform voltage 
which is supplied to the displaceable support member upon which head A is 
mounted, and FIG. 5E represents the staircase waveform voltage which is 
supplied to the displaceable support member upon which head B is mounted. 
The staircase waveform voltage E.sub.sa which is supplied to the 
displaceable support member upon which head A is mounted is produced by 
sampling phase-advanced sawtooth voltage E.sub.k only during the middle 
portion of each head field period for head B. Similarly, the staircase 
waveform voltage E.sub.sb which is supplied to the displaceable support 
member upon which head B is mounted is produced by sampling the 
phase-advanced sawtooth voltage E.sub.k only during the middle portion of 
the head field period for head A. The bold lines of the staircase waveform 
voltages E.sub.sa and E.sub.sb represent the times that heads A and B, 
respectively, scan tape 1 and, moreover, represent that the displaceable 
support members upon which heads A and B are mounted have responded fully 
to the step-level control voltages which are applied thereto. In the 
examples shown in FIGS. 5D and 5E, sawtooth voltage E.sub.k is 
phase-advanced by one head field period relative to the sawtooth voltage 
E.sub.k shown in FIG. 5B, the latter being synchronized with the 
reproduced control pulses P.sub.c. 
Thus, even though the displaceable support members exhibit inherent time 
delays to the control voltages applied thereto, if the step levels which 
are supplied to these displaceable support members are provided in advance 
of the commencement of the scanning traces of the heads mounted thereon, 
such inherent time delay is accounted for. 
Although sawtooth voltage E.sub.k, as shown in FIGS. 5D and 5E, is a 
phase-advanced by one head field period relative to the sawtooth voltage 
shown in FIG. 5B, it is recalled that the sawtooth voltage is generated by 
counting tape speed pulses P.sub.f, and then converting the digital count 
to an analog signal. Of course, the number of tape speed pulses P.sub.f 
which is produced during a head field period varies as the speed N of the 
tape varies. For example, when N=1/5, about three tape speed pulses 
P.sub.f are provided in a head field period. When N=1/3, about five tape 
speed pulses P.sub.f are provided in a head field period. Thus, in order 
to phase-advance the sawtooth voltage E.sub.k by one head field period, 
the speed at which the tape is driven must be ascertained in order to 
determine the number of tape speed pulses which correspond to a head field 
period. 
FIG. 6A is a graphical representation of the relationship between the speed 
N at which the tape is driven and the number of tape speed pulses P.sub.f 
which are generated during each head field period. As mentioned above, 
since 32 tape speed pulses are generated during the interval between 
adjacent control pulses P.sub.c, as shown in FIGS. 4A and 4B, the 
graphical representation of FIG. 6A can be normalized with reference to 16 
tape speed pulses, as shown in FIG. 6B. That is, if 16 tape speed pulses 
are used to produce the sawtooth voltage E.sub.k, then, as shown in FIG. 
6B, when N=1, one set of 16 tape speed pulses is provided during a head 
field period; when N=2, two sets (or 32) of tape speed pulses are provided 
during a heat field period; and when N=3, three sets (or 48) of tape speed 
pulses are provided during a head field period. 
In order to provide the phase-advancement of the sawtooth voltage E.sub.k, 
as shown in FIGS. 5D and 5E, the count of tape speed pulses P.sub.f at the 
time of occurrence of the control pulse P.sub.c must be delayed from a 
count of "0" by an amount which is a function of the tape speed N. FIG. 6C 
represents this delay in the count of the tape speed pulses. Stated 
otherwise, the count of the tape speed pulses P.sub.f which should be 
present at the time of occurrence of the control pulse P.sub.c is 
subtracted from a count of "16", and the amount which is subtracted is 
graphically represented in FIG. 6C. This delay in the count of the tape 
speed pulses P.sub.f results in a phase advance of the sawtooth voltage 
E.sub.k. 
As discussed hereinabove with respect to FIGS. 2D and 3D, it is seen that 
the peak-to-peak amplitude of the sawtooth waveform voltage E.sub.n is a 
function of the speed N at which tape 1 is driven. FIG. 7 is a graphical 
representation of this relationship, wherein the abscissa represents tape 
speed N and the ordinate represents the peak-to-peak amplitude of the 
sawtooth waveform voltage as a multiple of p, the center-to-center 
distance between adjacent record tracks. From the slope of the graphical 
representation in FIG. 7, it is seen that when N&lt;1, the slope of the 
sawtooth waveform voltage E.sub.n is negative, that is, the slope is 
downward to the right; whereas when N&gt;1, the slope of the sawtooth 
waveform voltage E.sub.n is positive, that is, it slopes upward to the 
right. It is recalled that, although the peak-to-peak amplitude of the 
sawtooth waveform voltage may vary as a function of the tape speed, the 
period of the sawtooth waveform voltage remains constant and equal to the 
head field. 
In accordance with another aspect of the present invention, to be 
described, in addition to providing correcting voltages by which the 
deviation of the scanning trace of the heads relative to the record track 
being scanned can be corrected during so-called special modes of 
reproduction, that is, during modes wherein N.noteq.1, tracking errors 
which may arise due to, for example, expansion or shrinkage of tape 1, 
differences in the mechanical components and tolerances from one VTR to 
another, and so on, also may be corrected. This is obtained by providing 
the closed loop correcting circuit in which the heads are vibrated, or 
dithered, from side to side across the width of the track being scanned, 
this dithering being performed at a constant frequency. The video signals 
which are reproduced by the dithering heads will exhibit an envelope 
which, when synchronously demodulated, results in an error voltage that 
represents the deviation of the average path traversed by the heads 
relative to the center of the track being scanned. This error voltage is 
fed back to the displaceable support members so as to deflect them in a 
direction whereby this deviation is cancelled. 
An embodiment of a VTR in which the present invention can be used is 
illustrated in the block diagram of FIG. 8. The VTR includes a signal 
reproducing section 10, a tape drive section 30, a head servo circuit 40, 
an operation control circuit 50, a tracking control circuit 100 and a 
closed loop correction circuit 200. Signal reproducing section 10 includes 
reproducing heads 11A and 11B which are mounted for rotation by a head 
drive shaft 41 so as to scan parallel traces across tape 1, as described 
above. These heads may be angularly displaced from each other by 
180.degree. such that when head 11A is in reproducing contact with tape 1, 
head 11B is not. 
Heads 11A and 11B are supported on displaceable support members, one 
embodiment of which is illustrated in FIGS. 10 and 11. Each displaceable 
support member is formed of an electrostriction element 21, such as a 
bi-morph leaf assembly, known to those of ordinary skill in the art. One 
end 21A of this bi-morph leaf assembly is secured to a support member 23 
by a suitable adhesive 22. As an example, electrostriction element 21 may 
be cantilever-mounted on support member 23. The other end 21B of the 
electrostriction element is adapted to support head 11A (11B) secured 
thereto by, for example, an adhesive. In addition, a damper element 24 is 
secured between the electrostriction element 21 and support member 23 so 
as to damp unwanted vibration, such as resonant vibrations, of the 
electrostriction element. As is typical, electrostriction element 21 is 
displaced in the direction of arrow 25 in response to a control voltage 
applied thereto. The direction of displacement is dependent upon the 
polarity of the control voltage, and the length, or degree, of 
displacement is determined by the magnitude of the control voltage. 
Returning now to FIG. 8, heads 11A and 11B are mounted by the displaceable 
support members shown in FIG. 10 and 11 to a rotatable guide drum (not 
shown) which, in turn, is rotated by head drive shaft 41 driven by head 
drive motor 42. Typically, tape 1 is helically wrapped about the tape 
guide drum by at least 180.degree. such that heads 11A and 11B scan 
alternate traces thereacross. Tape 1 is driven by tape drive section 30 
which includes a capstan 31 and pinch roller 32. The capstan and pinch 
roller cooperate in conventional manner such that when capstan 31 is 
driven, as by motor 33, tape 1 is moved in the direction of rotation of 
the capstan. 
A frequency generator 34 is mechanically coupled to capstan drive motor 33 
and is adapted to generate the aforementioned tape speed pulses P.sub.f. 
Consistent with the aforedescribed example, the frequency of tape speed 
pulses P.sub.f when N=1 is equal to 960 hertz. These tape speed pulses are 
supplied to a servo circuit 35 which also receives a tape speed drive 
signal from operation control circuit 50. The operation control circuit, 
although not shown in detail, may be provided with suitable 
operator-control switches and signal generators responsive to the 
selective operation of such switches to produce mode control signals. 
These mode control signals serve to determine whether the VTR is operated 
in its recording mode, playback mode, slow-motion mode, fast-motion mode, 
stop mode or reverse mode, as desired. Servo circuit 35 utilizes tape 
speed pulses P.sub.f to control the operation of capstan drive motor 33 
such that tape 1 is driven at the speed and direction determined by the 
mode which is selected by operation control circuit 50. 
Head servo circuit 40 is adapted to control head drive motor 42 in 
accordance with the phase relationship between heads 11A, 11B and the 
control pulses P.sub.c which are reproduced from tape 1. Accordingly, head 
servo circuit 40 includes a control pulse head 43 to reproduce such 
control pulses P.sub.c, a pulse generator 46 adapted to generate head 
position pulses representing the rotary position, or phase, of head 11A or 
11B and phase comparator 45 for comparing the phase of the control pulses 
P.sub.c to the phase of the head position pulses. Control pulse head 43 is 
disposed in alignment with control pulse track 3 (FIG. 1) which is 
recorded on tape 1. The control pulses P.sub.c which are reproduced by 
control pulse head 43 are supplied through an amplifier 44 to one input of 
phase comparator 45. 
Pulse generator 46 is comprised of a suitable pick-up, such as a magnetic 
sensor, fixedly disposed adjacent head drive shaft 41. A sensible element, 
such as a magnetic element, is provided on head drive shaft 41 in 
predetermined relationship with respect to head 11A. Thus, when head 11A 
rotates to a predetermined position, such as the commencement of its 
scanning trace across tape 1, the sensible element provided on head drive 
shaft 41 is sensed by the pick-up, whereby pulse generator 46 generates 
the head position pulse P.sub.ga representing the corresponding rotary 
position of head 11A. Pulse generator 46 is coupled via a wave shaping 
amplifier 47 to another input of phase comparator 45. The wave shaping 
amplifier serves to shape the head position pulses P.sub.ga. Any phase 
differential between the reproduced pulses P.sub.c and the head position 
pulses P.sub.ga is detected by phase comparator 45 which, in turn, 
supplies a phase error signal to head drive motor 42 via amplifier 48. 
This phase error signal adjusts the operation of head drive motor 42 such 
that head 11A is brought into proper scanning relationship with tape 1 at 
the time that a recorded control pulse P.sub.c is reproduced by control 
pulse head 43. Thus, head servo circuit 40 controls the rotation of heads 
11A and 11B to be in proper phase synchronism with the movement of tape 1. 
Returning to signal reproducing section 10, this section includes a 
change-over switch 13, an FM demodulator 15, a de-emphasis circuit 16 and 
a video output terminal 17. Heads 11A and 11B reproduce the FM video 
signals which are recorded in alternate record tracks on tape 1. The 
reproduced video signals from these heads are amplified by playback 
amplifiers 12A and 12B respectively, and the amplified video signals are 
supplied to respective fixed contacts of change-over switch 13. The 
change-over switch, shown schematically as a mechanical switch, includes a 
movable contact, or equivalent, which alternately couples the amplifier 
12A and then amplifier 12B to its output. More particularly, the FM video 
signal reproduced by head 11A is coupled to the output of change-over 
switch 13 when that head scans tape 1; and then change-over switch 13 is 
changed over to couple the video signal reproduced from the tape by head 
11B to the output thereof. Thus, at each head field period, the FM video 
signal S.sub.f produced by a corresponding one of heads 11A and 11B is 
supplied to the output of change-over switch 13. 
The condition of change-over switch 13 is controlled by a rectangular wave 
signal S.sub.v which alternates between high and low levels in synchronism 
with the head field periods. As an example, when rectangular wave signal 
S.sub.v is at its relatively higher level, the FM video signal S.sub.f 
reproduced by head 11A is coupled to the output of change-over switch 13; 
while when the rectangular wave signal is at its relatively lower level, 
the FM video signal reproduced by head 11B is supplied to the output of 
the change-over switch. Rectangular wave signal S.sub.v is produced by 
flip-flop circuit 78, such as a set/reset flip-flop circuit having its set 
input connected to wave shaping amplifier 47 to be set in response to each 
head position pulse P.sub.ga. The reset input of flip-flop circuit 78 is 
connected to a wave shaping amplifier 77 to be reset in response to each 
head position pulse P.sub.gb which is produced when head 11B advances to 
the beginning of its scanning trace. To this effect, another pulse 
generator 76, similar to aforementioned pulse generator 46, is provided to 
generate head position pulses P.sub. gb each time that head 11B rotates to 
the beginning of its scanning trace. Thus, during the head field period in 
which head 11A scans tape 1, flip-flop circuit 78 is in its set state, and 
rectangular wave signal S.sub.v is at its relatively higher level to 
couple the FM video signal S.sub.f reproduced by head 11A to the output of 
the change-over switch. During the next following head field interval, 
flip-flop circuit 78 is in its reset state and rectangular wave signal 
S.sub.v is at its relatively lower level to control change-over switch 13 
to couple the FM video signal reproduced by head 11B to the output 
thereof. It is appreciated that the rectangular wave signal exhibits a 
waveform that is consistent with the scanning phases of heads 11A and 11B. 
The FM video signal S.sub.f, produced during successive field intervals at 
the output of change-over switch 13, is supplied through a limiter 14 to 
FM demodulator 15. The limiter serves to eliminate unwanted amplitude 
modulations of the FM video signal. FM demodulator 15 demodulates the 
video information contained in the FM video signal. If it is assumed that 
the recorded signal is a monochrome (or black-and-white) video signal, the 
output of FM demodulator 15 is a luminous signal S.sub.y. This demodulated 
video signal then is supplied to video output terminal 17 via de-emphasis 
circuit 16; and then used by additional apparatus (not shown) to reproduce 
a video picture corresponding to the video signal. Of course, the 
reproduced video signal, provided at video output terminal 17, includes 
the usual horizontal and vertical synchronizing pulses. 
Tracking control circuit 100 corrects the tracking errors of heads 11A and 
11B during the so-called special modes of reproduction. That is, when 
N.noteq.1, the deviation of the scanning traces of heads 11A and 11B 
relative to the previously recorded track, as discussed in detail above, 
are corrected by this tracking control circuit. A more detailed 
illustration of tracking control circuit 100 is illustrated in FIG. 9. 
Referring to FIG. 9 in detail, tracking control circuit 100 includes a 
counter 101, a digital-to-analog (D/A) converter 102, sample-and-hold 
circuits 103A, 103B and a sampling pulse generator comprised of monostable 
multivibrators 106A, 107A and monostable multi-vibrators 106B, 107B. 
Counter 101 is a cyclical, or resettable counter which is adapted to count 
the tape speed pulses P.sub.f generated by frequency generator 34 (FIG. 
8). These tape speed pulses are supplied through a wave shaping amplifier 
108 to the count input of counter 101. In one embodiment, counter 101 is 
an UP/DOWN counter whose counting direction is determined by a suitable 
count control signal produced by operation control circuit 50. For 
example, when N is positive, that is, when tape is driven in the forward 
direction, operation control circuit 50 supplies a count UP signal to 
counter 101. Conversely, when N is negative, that is, when tape 1 is 
driven in the reverse direction, operation control circuit 50 supplies a 
count DOWN signal to counter 101. The counter may comprise a hexadecimal 
counter, whereby it counts to a count of 16 and then resets itself to 
resume the counting of the tape speed pulses. 
The count produced by cyclical or resettable counter 101 is supplied as a 
digital count to D/A converter 102. The D/A converter is adapted to 
convert this count to a corresponding analog signal. Thus, as the count of 
counter 101 is incremented, D/A converter 102 produces a sawtooth voltage 
E.sub.k of positive slope. If the count of counter 101 is decremented, the 
D/A converter produces a sawtooth voltage of negative slope. The sawtooth 
voltage E.sub.k is supplied to sample-and-hold circuits 103A and 103B 
which are adapted to sample the sawtooth voltage at a time that 
corresponds to the middle portion of a scanning trace. The sampling pulses 
which are supplied to, for example, sample-and-hold circuit 103A are 
produced substantially when head 11B reaches the middle portion of its 
scanning trace and, conversely, the sampling pulses which are supplied to 
sample-and-hold circuit 103B are produced when head 11B reaches the middle 
portion of its scanning trace. 
The aforementioned sampling pulses, designated sampling pulses P.sub.a and 
P.sub.b, are derived from the aforementioned rectangular wave signal 
S.sub.v. For convenience, pulse generators 46 and 76, together with wave 
shaping amplifiers 47 and 77 and flip-flop circuit 78, described 
previously with respect to FIG. 8, are shown in FIG. 9. Let it be assumed 
that the rectangular wave signal is at its relatively higher level during 
the period that head 11A scans tape 1, and that this rectangular wave 
signal is at its relatively lower level during the period that head 11B 
scans the tape. Monostable multivibrator 106A is coupled to flip-flop 
circuit 78 to receive the rectangular wave signal S.sub.v and is adapted 
to be triggered in response to the trailing edge or negative transition, 
in the rectangular wave signal. The time constant of monostable 
multivibrator 106A is equal to approximately one-half of a head field 
interval. At the completion of that time constant, monostable 
multivibrator 106A, which had been triggered to its quasi-stable state, 
returns to its stable state, and this transition in the output thereof 
triggers monostable multivibrator 107A to produce sampling pulse P.sub.a. 
Monostable multivibrator 106B is similar to monostable multivibrator 106A, 
but is triggered to its quasi-stable state and responds to the leading 
edge, or positive transition, of rectangular wave signal S.sub.v. The time 
constant of monostable multivibrator 106B is approximately equal to 
one-half of a head field interval, at which time it returns to its stable 
state. This transition in the output of monostable multivibrator 106B 
triggers monostable multivibrator 107B to produce the sampling pulse 
P.sub.b. 
The waveforms which are produced when sampling pulses P.sub.a and P.sub.b 
are generated are shown in FIGS. 12A-12E. FIG. 12A represents the head 
field periods during which heads 11A and 11B scan alternate traces across 
tape 1. As shown in FIG. 12B, the rectangular wave signal S.sub.v is at 
its relatively higher level during the head field period in which head 11A 
scans tape 1, and the rectangular wave signal is at its relatively lower 
level during the head field period in which head 11B scans the tape. 
Monostable multivibrator 106A is triggered to its quasi-stable state and 
responds to the negative transition in rectangular wave signal S.sub.v, 
and at the conclusion of the time constant of this monostable 
multivibrator, monostable multivibrator 107A is triggered to generate 
sampling pulse P.sub.a, as shown in FIG. 12C. Conversely, monostable 
multivibrator 106B is triggered to its quasi-stable state and responds to 
the positive transition in the rectangular wave signal S.sub.v, and when 
this monostable multivibrator returns to its stable state, monostable 
multivibrator 107B is triggered to generate sampling pulse P.sub.b, as 
shown in FIG. 12D. Sampling pulses P.sub.a are supplied to sample-and-hold 
circuit 103A; and sampling pulses P.sub.b are supplied to sample-and-hold 
circuit 103B. In addition, sampling pulses P.sub.a and P.sub.b are 
supplied through an OR circuit 165 to produce sampling pulses P.sub.m 
(FIG. 12E) for a purpose soon to be described. It is seen that sampling 
pulses P.sub.m are generated at the head field rate. 
It is appreciated that D/A converter 102 generates the sawtooth voltage 
E.sub.k which has been described previously with respect to FIGS. 5D and 
5E. It is seen from FIG. 12C that sampling pulses P.sub.a are generated 
each time that head 11B reaches the middle portion of its scanning trace, 
that is, at about one-half of a head field period prior to the time that 
head 11A commences its scanning trace. Thus, the sampling of the sawtooth 
voltage E.sub.k in sample-and-hold circuit 103A by sampling pulses P.sub.a 
results in the staircase waveform voltage E.sub.sa shown in FIG. 5D. In a 
similar manner, the sampling of the sawtooth voltage E.sub.k in 
sample-and-hold circuit 103B by sampling pulses P.sub.d results in the 
staircase waveform voltage E.sub.sb shown in FIG. 5E. The staircase 
waveform voltage E.sub.sa is supplied as one component of the correction 
voltage E.sub.ca to electrostriction element 21A upon which head 11A is 
mounted. This component is supplied via an adder circuit 104A and an 
amplifier 105A. The purpose of adder circuit 104A will be described below. 
Similarly, the staircase waveform voltage component E.sub.sb of correction 
voltage E.sub.cd is supplied to the electrostriction element 21B, upon 
which head 11B is mounted, via adder circuit 104B and amplifier 105B. 
Although the circuitry which has been shown and described for producing the 
staircase waveform voltages E.sub.sa and E.sub.sb includes a 
digital-to-analog converter for converting the count of counter 101 prior 
to the sampling of the converted sawtooth voltage E.sub.k, it is 
appreciated that, if desired, the count of counter 101 first may be 
sampled by sampling pulses P.sub.a and P.sub.b, and then each sampled 
digital count may be converted by a respective digital-to-analog converter 
to produce the staircase waveform voltages E.sub.sa and E.sub.sb. 
It is seen that, regardless of the speed at which tape 1 is moved, counter 
101 will be incremented to produce the same counts during the interval 
between adjacent control pulses P.sub.c. Hence, the peak-to-peak amplitude 
of sawtooth voltage E.sub.k, as produced by D/A converter 102, will be 
constant regardless of the tape speed. Of course, the period of each 
sawtooth waveform is dependent upon the frequency at which tape speed 
pulses P.sub.f are produced, and thus dependent upon the speed N of tape 
1. 
Tracking control circuit 100 also includes a counter control circuit 110 
which is adapted to advance the phase of sawtooth voltage E.sub.k by one 
head field period, relative to the time of occurrence of the reproduced 
control pulse P.sub.c, as discussed hereinabove with respect to FIGS. 
5A-5E. Of course, the number of tape speed pulses P.sub.f which are 
generated during one head field period is a function of the speed N of 
tape 1. For the slow-motion mode of reproduction, that is, when N&lt;1, the 
slower the tape speed, the smaller the number of tape speed pulses which 
are produced in one head field period. If, as shown in FIG. 5D, the count 
of counter 1 should be reset to an initial count of, for example, 0 at one 
head field period prior to the occurrence of a control pulse P.sub.c, then 
the count of counter 101 should be equal to the number of tape speed 
pulses which are generated during one head field period at the time of 
occurrence of the control pulse P.sub.c. Counter control circuit 110 is 
adapted to reset counter 101 at the appropriate time so as to effectively 
phase-advance the sawtooth voltage E.sub.k by one head field period, 
regardless of the speed N of tape 1. 
Counter control circuit 110 includes a frequency discriminator 111, an 
analog-to-digital (A/D) converter 120 and a counter reset circuit 
including a programmable counter 115, a flip-flop circuit 116 and a 
differentiating circuit 117. Frequency discriminator 111 is a so-called 
pulse count type of discriminator comprised of, for example, a monostable 
multi-vibrator and an integrator. The frequency discriminator is adapted 
to produce a speed voltage E.sub.f having a level, such as a DC level, 
corresponding to the frequency of tape speed pulses P.sub.f. Thus, speed 
voltage E.sub.f represents the actual speed of tape 1. A/D converter 120 
is adapted to convert this speed voltage E.sub.f to a corresponding 
digital signal. To this effect, A/D converter 120 includes a voltage 
comparator 121, an oscillator 122, a counter 123, a D/A converter 124, and 
a latch circuit 126. Oscillator 122 is adapted to supply timing pulses of 
predetermined frequency to counter 123. The counter is, for example, a 
count-to-64 counter and counts the timing pulses supplied thereto by 
oscillator 122 until a count of 64 is obtained. At that count, counter 123 
resets itself to resume the counting of the timing pulses. The count of 
counter 123 is supplied to D/A converter 124 and also to latch circuit 
126. The D/A converter converts the changing count of counter 123 to a 
corresponding analog voltage. This analog voltage is supplied to one input 
of voltage comparator 121, the voltage comparator including another input 
supplied with speed voltage E.sub.f. When the voltages provided at both 
inputs of the voltage comparator are equal, an output pulse is generated, 
this output pulse being supplied to latch circuit 126 and, through a delay 
circuit 125, to the reset input of counter 123. When a latch pulse is 
received by latch circuit 126, the count then supplied thereto by counter 
123 is latched therein. 
The manner in which A/D converter 120 operates will best be understood by 
reference to the waveform diagram shown in FIGS. 14A-14E. FIG. 14A 
represents the timing pulses supplied to counter 123 by oscillator 122. As 
the count of counter 123 is incremented, the digital count thereof is 
converted to an analog voltage E.sub.a by D/A converter 124. This analog 
voltage E.sub.a exhibits a staircase waveform, as shown in FIG. 14B. 
Voltage comparator 121 compares the step-wise increasing analog voltage 
E.sub.a to the constant level speed voltage E.sub.f, as shown in FIG. 14C. 
When a voltage comparison is obtained, that is, when the level of the 
analog voltage E.sub.a is equal to the speed voltage E.sub.f, latch pulse 
P.sub.e is produced by the comparator. This latch pulse, shown in FIG. 
14D, is supplied to latch circuit 126 which responds thereto to latch the 
count then present in counter 123. It is appreciated that this count is 
the digital representation of the analog speed voltage E.sub.f. At a 
delayed time determined by delay circuit 125, for example, at the next 
following timing pulse, a delay pulse P.sub.d is supplied to counter 123 
to reset the latter to its initial count, as shown in FIG. 14C. In this 
manner, latch circuit 126 stores a digital representation, in the form of 
a digital count, of the actual speed of tape 1. 
As one example thereof, the frequency of the timing pulses generated by 
oscillator 122 may be on the order of about 119 KHz. 
FIG. 13A is a graphical representation of the speed N at which tape 1 is 
driven and the level of speed voltage E.sub.f. It is seen that, as the 
tape speed is increased, the level of the speed voltage is reduced. FIG. 
13B is a graphical representation of the relationship between tape speed N 
and a digital count representation of speed voltage E.sub.f. When N=0, the 
digital count corresponding to speed voltage E.sub.f is equal to "48". 
When N=1, the digital count corresponding to the tape speed is equal to 
"32". When N=2, the digital count corresponding to the tape speed is equal 
to "16". It may be appreciated that, in a typical binary counter, the four 
least significant bits in the count corresponding to tape speed E.sub.f 
will be the same for changing tape speed when 0&lt;N&lt;1, and also for 1&lt;N&lt;2, 
and also for 2&lt;N&lt;3, and so on. Thus, rather than store the entire count 
obtained by counter 123 at the time that latch pulse P.sub.e is generated, 
it is sufficient to store that portion of the count represented by the 
four least significant bits. Thus, the four least significant bits of 
counter 123 are supplied to latch circuit 126, and the binary values of 
these bits are stored in the latch circuit in response to latch pulse 
P.sub.e. The contents of latch circuit 126 are represented in FIG. 13C for 
different tape speeds N. 
Programmable counter 115 is supplied with the count stored in latch circuit 
126 and is further supplied with the tape speed pulses P.sub.f. The 
programmable counter is adapted to count the tape speed pulses until the 
programmed count, corresponding to the count supplied thereto by latch 
circuit 126, is reached and at that time, programmable counter 115 
generates a carry pulse P.sub.u and, moveover, is reset to an initial 
count to resume counting the tape speed pulses. Thus, it is recognized 
that programmable counter 115 functions as a programmable frequency 
divider to divide the frequency of the tape speed pulses P.sub.f by a 
dividing ratio determined by the count supplied thereto by latch circuit 
126. Since this dividing ratio is a function of the actual speed at which 
tape 1 is driven, it is appreciated that the time of occurrence of carry 
pulse P.sub.u also is a function of the tape speed. Programmable counter 
115 additionally includes a reset input connected to amplifier 44 for 
receiving control pulses P.sub.c which function to reset the programmable 
counter to its initial count. Thus, the time of occurrence of control 
pulse P.sub.c may be considered to be a reference time, and the time of 
occurrence of carry pulse P.sub.u following the occurrence of control 
pulse P.sub.c is a function of the tape speed. 
The carry pulse P.sub.u generated by programmable counter 115 is supplied 
to the set input of flip-flop circuit 116. The reset input of this 
flip-flop circuit is supplied with control pulses P.sub.c. Thus, flip-flop 
circuit 116 is adapted to be set and reset by carry pulse P.sub.u and 
control pulse P.sub.c, respectively. As a result of this operation, 
flip-flop circuit 116 generates an output signal S.sub.i whose positive 
transitions are differentiated by differentiating circuit 117 to generate 
a reset pulse P.sub.i. 
Let it be assumed that the count stored in latch circuit 126 and supplied 
to programmable counter 115 establishes a frequency dividing ratio of "X". 
This means that after "X" tape speed pulse P.sub.f are counted, the 
programmable counter generates the carry pulse P.sub.u. Furthermore, since 
the programmable counter always is reset by the control pulse P.sub.c, it 
is seen, from FIGS. 15A and 15B, that carry pulse P.sub.u is generated at 
a time following the control pulse P.sub.c that is determined by the 
dividing ratio "X". This dividing ratio is, of course, a function of the 
tape speed. The carry pulse P.sub.u sets flip-flop circuit 116 and the 
control circuit P.sub.c resets this flip-flop circuit, resulting in the 
output signal S.sub.i, shown in FIG. 15C. Each positive transition in the 
output signal S.sub.i is differentiated and supplied as a reset pulse 
P.sub.i to counter 101. Thus, the count of counter 101 is reset to its 
initial count in response to each reset pulse P.sub.i. This results in 
phase-advancing the sawtooth voltage E.sub.k derived from the count of 
counter 101 by one head field period relative to the control pulse 
P.sub.c, as shown by the solid sawtooth voltage in FIG. 15E. The broken 
sawtooth voltage is that which would be produced if counter 101 is reset 
in response to control pulses P.sub.c. 
It is seen from FIGS. 15A, 15B and 15E, that the resetting of counter 101 
actually is delayed from control pulse P.sub.c by an amount determined by 
the dividing ration "X" of programmable counter 115. This delay is 
sufficient to result in an effective phase-advance of the sawtooth 
voltage. Furthermore, since the time of occurrence of the reset pulse 
P.sub.i is a function of the actual speed at which tape 1 is driven, the 
resetting of counter 101 likewise is controlled in accordance with this 
tape speed. Hence, the effective phase-advance of the sawtooth voltage 
will be equal to one head field period, even though the number of tape 
speed pulses P.sub.f which is generated during a head field period varies 
as a function of the tape speed. This all is taken into account by the 
operation of counter 123, latch circuit 126, programmable counter 115 and 
flip-flop circuit 116. The delay between control pulse P.sub.c and reset 
pulse P.sub.i corresponds to the count delay shown in FIG. 6C, this delay 
being a function of tape speed N. 
As an example, as mentioned above, if N=1/5, then about three tape speed 
pulses P.sub.f are generated in one head field interval. At this tape 
speed (N=1/5), the digital count corresponding thereto, as stored in latch 
circuit 126 may set programmable counter 115 with a dividing ratio equal 
to 13. Consequently, after being reset by a control pulse P.sub.c, 
programmable counter 115 counts 13 tape speed pulses P.sub.f and then 
generates the carry pulse P.sub.u. Hence, the carry pulse is generated at 
a delayed time corresponding to 13 tape speed pulses. At this time, 
differentiating circuit 117 generates the reset pulse P.sub.i to reset the 
count of counter 101. As shown in FIG. 15E, counter 101 is reset at a time 
corresponding to 13 tape speed pulses following the occurrence of control 
pulse P.sub.c. This means that the sawtooth voltage E.sub.k derived from 
the counter 101 is phase-advanced, relative to the control pulse P.sub.c, 
by an amount equal to about three tape speed pulses. In the present 
example wherein N=1/5, this is equal to one head field period. 
The foregoing has described the manner in which the level steps of the 
staircase waveform voltage component E.sub.sa, E.sub.sb of correction 
voltage E.sub.ca, E.sub.cb is produced. It is recalled that this 
correction voltage also includes a sawtooth waveform voltage component 
E.sub.n. More particularly, and as now will be described, the correction 
voltage E.sub.ca which is supplied to displaceable support member 21A 
includes a sawtooth waveform voltage component E.sub.na, and correction 
voltage E.sub.cb which is supplied to displaceable support member 21B is 
provided with a sawtooth waveform voltage component E.sub.nb. These 
sawtooth waveform voltage components are generated by sawtooth waveform 
circuit 150. This sawtooth waveform circuit includes an OR circuit 151, a 
sawtooth generator 152, a sample-and-hold circuit 153 and a change-over 
switch 154. OR circuit 151 is connected to receive head position pulses 
P.sub.ga and P.sub.gb, produced by pulse generators 46 and 76, 
respectively. Head position pulses P.sub.ga and P.sub.gb are produced at 
the head frame rate, and OR circuit 151 combines these position pulses to 
produce head position pulses P.sub.g at the head field rate. These head 
position pulses are supplied to sawtooth generator 152 to trigger the 
sawtooth generator to produce the sawtooth waveform voltage E.sub.n, 
described previously with respect to FIGS. 2D and 3D. The output of 
sawtooth generator 152 is connected through a difference amplifier 161, 
described in greater detail below, to sample-and-hold circuit 153 and, 
additionally, directly to one set of inputs of change-over switch 154. 
Sample-and-hold circuit 153 is connected to OR circuit 151 to receive head 
position pulses P.sub.g as sampling pulses. It may be appreciated that the 
sample-and-hold circuit is adapted to sample the amplitude of the sawtooth 
waveform voltage E.sub.n supplied thereto by sawtooth generator 152 at the 
beginning of each scanning trace. The sampled amplitude of the sawtooth 
waveform voltage is supplied to another set of inputs of change-over 
switch 154. The change-over switch is shown schematically as a mechanical 
switch having a pair of movable contacts, each movable contact being 
selectively engageable with one or another of a respective pair of fixed 
contacts. The switching condition of change-over switch 154 is determined 
by the rectangular wave signal S.sub.v which is supplied thereto as a 
switching signal. One movable contact of the change-over switch is 
connected to an input of adder circuit 104A, and the other movable contact 
of the change-over switch is connected to an input of adder circuit 104B. 
Sawtooth waveform circuit 150 also is coupled to a level correcting circuit 
160, the latter being comprised of difference amplifier 161, a 
sample-and-hold circuit 162 and a difference amplifier 163. The purpose of 
level correcting circuit 160 is to adjust the DC, or average, level of the 
sawtooth waveform voltage E.sub.n produced by sawtooth generator 152 as a 
function of the detected deviation between the scanning path of each head 
11A and 11B and the record track which is scanned thereby. As will be 
described below, the signal reproducing apparatus shown in FIG. 8 includes 
a closed loop correction circuit 200 which serves to detect when the 
average path traversed by a head deviates from the center-line of the 
record track being scanned. Although the tracking control circuit 
described herein is adapted to correct for tracking errors between the 
scanning traces of the heads and the record track being scanned, other 
tracking errors may arise due to, for example, tape shrinkage, tape 
stretching, or different characteristics of the VTR which is used for a 
reproducing operation than that which is used for a recording operation. 
As will be described, such tracking errors are corrected by closed loop 
correction circuit 200. This closed loop correction circuit is adapted to 
produce an error correcting voltage, and this error correcting voltage is 
supplied as a reference voltage E.sub.r to one input of difference 
amplifier 163. For the purpose of simplification, this reference voltage 
E.sub.r is shown as being produced by a reference voltage source 164. 
Difference amplifier 161 includes a positive (+) input connected to 
sawtooth generator 152 to receive the sawtooth waveform voltage E.sub.n 
therefrom, and a negative input (-) connected to receive the output of 
difference amplifier 163. As mentioned above, difference amplifier 163 is 
supplied with the reference voltage E.sub.r at its negative (-) input and 
has its positive (+) input connected to the output of sample-and-hold 
circuit 162. This sample-and-hold circuit is adapted to sample the 
sawtooth waveform voltage E.sub.n at the midpoint of each period. To this 
effect, sample-and-hold circuit 162 is supplied with sampling pulses 
P.sub.m of the head field rate, these sampling pulses being produced by 
OR-circuit 165, as described above, It is recalled, from FIG. 12E, that 
sampling pulses P.sub.m are produced at the middle portion of each trace 
of heads 11A and 11B. 
Let it be assumed that the average level of the sawtooth waveform voltage 
E.sub.n is greater than the reference voltage E.sub.r. This average level 
is sampled by sample-and-hold circuit 162 and compared in difference 
amplifier 163 to the reference voltage E.sub.r. Since it has been assumed 
that the average level of the sawtooth waveform voltage exceeds the 
reference voltage, difference amplifier 163 supplies the difference 
therebetween to the negative input (-) of difference amplifier 161. Hence, 
the average level of the sawtooth waveform voltage E.sub.n is reduced 
accordingly so as to be equal to the reference voltage. If, on the other 
hand, the average level of the sawtooth waveform voltage had been less 
than the reference voltage E.sub.r, difference amplifier 163 supplies a 
negative voltage level to the negative (-) input of difference amplifier 
161, resulting in an increase in the average level of the sawtooth 
waveform voltage. Thus, level correcting circuit 160 functions to insure 
that the average level of the sawtooth waveform voltage E.sub.n is equal 
to the reference voltage E.sub.r. As will be described below, this 
reference voltage E.sub. r is equal to the error correcting voltage which 
is needed to make the average path of heads 11A and 11B coincide with the 
tracks being scanned thereby. 
Referring to FIGS. 16A-16H, the manner in which sawtooth waveform circuit 
150 operates now will be described. Let it be assumed that the head 
position pulses P.sub.g appear as shown in FIG. 16A. In the following 
explanation, reference is made to the left-hand portion of the drawing 
figures wherein N&lt;1. As will be described below with respect to FIG. 18, 
each head position pulse P.sub.g triggers sawtooth generator 152 to 
generate the sawtooth waveform voltage E.sub.n. Since it is assumed that 
N&lt;1, the slope of the sawtooth waveform voltage is seen to be negative. 
After passing through difference amplifier 161 so as to have its average 
level corrected, the sawtooth waveform voltage E.sub.n whose average level 
is equal to the reference voltage E.sub.r, as shown in FIG. 16D, is 
sampled in sample-and-hold circuit 153 by head position pulses P.sub.g. It 
is appreciated that these head position pulses occur at the beginning, or 
maximum amplitude, of the sawtooth waveform voltage. Thus, sample-and-hold 
circuit 153 supplies the peak of the sawtooth amplitude voltage E.sub.p, 
shown in FIG. 16E, to one set of inputs of change-over switch 154. 
The rectangular wave signal S.sub.v, shown again in FIG. 16F, is assumed to 
operate change-over switch 154 such that, when the rectangular wave signal 
is at its relatively higher level, the change-over switch supplies the 
level-corrected sawtooth waveform voltage E.sub.n to adder circuit 104A, 
and also supplies the sampled sawtooth amplitude E.sub.p to adder circuit 
104B. Conversely, when the rectangular wave signal S.sub.v is at its 
relatively lower level, change-over switch 154 supplies the sampled 
sawtooth amplitude E.sub.p to adder circuit 104A, and also supplies the 
level-corrected sawtooth waveform voltage E.sub.n to adder circuit 104B. 
The voltage which is supplied by change-over switch 154 to adder circuit 
104A is the sawtooth waveform voltage E.sub.na, shown in FIG. 16G, and the 
voltage which is supplied to adder circuit 104B by the change-over switch 
is the sawtooth waveform voltage E.sub.nb, shown in FIG. 16H. It may be 
recognized that, when head 11A scans tape 1, the sawtooth waveform voltage 
E.sub.na is supplied to adder circuit 104A. Similarly, when head 11B scans 
tape 1, the sawtooth waveform voltage E.sub.nb is supplied to adder 
circuit 104B. Consequently, during the scanning trace of each head, a 
sawtooth waveform voltage component of the correction voltage E.sub.c is 
supplied to the displaceable support member upon which that head is 
mounted. 
In the foregoing description, it has been assumed that N&lt;1. However, if 
N&gt;1, then the sawtooth waveform voltage E.sub.n exhibits a positive slope, 
as shown in the right-hand portion of FIGS. 16B, 16D and 16E. Also, since 
the amplitude of the sawtooth waveform voltage at the time of occurrence 
of each head position pulse P.sub.g is at its minimum level, the sampled 
sawtooth amplitude produced by sample-and-hold circuit 153 appears as the 
lower DC level E.sub.p, shown in the right-hand portion of FIG. 16E. It is 
recognized that the sawtooth waveform voltage E.sub.na, supplied to adder 
circuit 104A when head 11A scans tape 1, is as shown in the right-hand 
portion of FIG. 16G; and the sawtooth waveform voltage E.sub.nb supplied 
to adder circuit 104B when head 11B scans tape 1 is as shown in the 
right-portion of FIG. 16H. 
A waveform representation of the manner in which correction voltages 
E.sub.ca and E.sub.cb are produced by summing the respective staircase 
waveform voltages and sawtooth waveform voltages is depicted in FIGS. 
17A-17F. FIG. 17A represents the staircase waveform voltage E.sub.sa which 
is produced by sample-and-hold circuit 103A. This staircase waveform 
voltage is recognized to be identical to the staircase waveform voltage 
described above with respect to FIG. 5D. Similarly, FIG. 17B illustrates 
the staircase waveform voltage E.sub.sb produced by sample-and-hold 
circuit 103B. This staircase waveform voltage is recognized to be 
identical to the staircase waveform voltage shown in FIG. 5E and discussed 
above. This, it is recalled that each step-level of, for example, the 
staircase waveform voltage E.sub.sa is produced by sampling the sawtooth 
voltage E.sub.k produced by D/A converter 102 each time that head 11B 
reaches the middle portion of its scanning trace. That is, each step-level 
is produced at approximately one-half of a head field period prior to the 
time that head 11A commences its scanning trace. Similarly, each 
step-level in the staircase waveform voltage E.sub.sb is produced one-half 
of a head field period prior to the time that head 11B commences its 
scanning trace. The bold lines in FIGS. 17A and 17B represent the actual 
scanning times of heads 11A and 11B, respectively. 
FIG. 17C illustrates the sawtooth waveform voltage E.sub.na supplied to 
adder circuit 104A by change-over switch 154, and described previously 
with respect to FIG. 16G. Similarly, FIG. 17D illustrates the sawtooth 
waveform voltage E.sub.nb, discussed previously with respect to FIG. 16H, 
which is supplied to adder circuit 104B by change-over switch 154. Adder 
circuit 104A sums the staircase waveform voltage E.sub.sa and the sawtooth 
waveform voltage E.sub.na to produce the error correction voltage 
E.sub.ca, shown in FIG. 17E. The staircase waveform voltage E.sub.sa is 
superimposed onto FIG. 17E, shown by the broken lines. The bold lines in 
FIG. 17E represent the correction voltage E.sub.ca that is applied to 
displaceable support member 21A at the time that head 11A scans tape 1. It 
is appreciated that the DC level, or average level, of the sawtooth 
waveform component in this correction voltage changes as the step-level of 
the staircase waveform component changes. The correction voltage E.sub.ca, 
represented by the broad lines of FIG. 17E, is seen to correspond to the 
correction voltage E.sub.c shown in FIG. 2B. Thus, this correction voltage 
E.sub.ca corrects the deviation d of each scanning trace formed by head 
11A relative to the record track which then is being scanned. 
Similarly, the error correction voltage E.sub.cb which is produced by adder 
circuit 104B is obtained by summing the staircase waveform voltage 
E.sub.sb and the sawtooth waveform voltage E.sub.nb. In FIG. 17F, the 
staircase waveform voltage E.sub.sb is superimposed, as shown by the 
broken lines. The bold lines of the error correction voltage of FIG. 17F 
represent the error correction voltage which is supplied to displaceable 
support member 21B at the time that head 11B scans tape 1. It is 
appreciated that the average, or DC level of the sawtooth waveform 
component in this error correction voltage changes as the step-level of 
the staircase waveform component changes. Error correction voltage 
E.sub.cb is seen to be the same as the error correction voltage E.sub.c 
shown in FIG. 2B. Hence, the deviation of each scanning trace formed by 
head 11B relative to the record track which is scanned by this head 
(wherein N.noteq.1) is corrected. 
A preferred embodiment of sawtooth generator 152 is shown schematically in 
FIG. 18. The sawtooth generator includes a capacitor C.sub.1, a 
charge/discharge transistor Q.sub.1, a reference transistor Q.sub.2 and a 
switching transistor Q.sub.3. Transistors Q.sub.1 and Q.sub.2 both are 
connected in emitter-follower configuration, and the emitter electrode of 
transistor Q.sub.1 is connected via a resistor R.sub.1 to capacitor 
C.sub.1. The emitter electrode of transistor Q.sub.2 is connected to 
capacitor C.sub.1 via the collector-emitter circuit of transistor Q.sub.3. 
The base electrode of transistor Q.sub.1 is connected to be supplied with 
a voltage E.sub.j which, as will be described, has a magnitude that 
represents both the speed and direction of movement of tape 1. The base 
electrode of transistor Q.sub.2 is supplied with a reference voltage 
E.sub.1, this reference voltage being equal to the magnitude of voltage 
E.sub.j when N=1. 
The voltage E.sub.j is produced by a change-over switch 156 having one 
fixed input thereof connected to receive the speed voltage E.sub.f 
produced by frequency discriminator 111, described above with respect to 
FIG. 9. The other fixed input of change-over switch 156 is connected to 
receive an inverted version E.sub.f of the speed voltage, this inverted 
version being produced by an inverting circuit 157. Change-over switch 156 
is schematically illustrated as a mechanical switch whose movable contact 
is controlled by operation control circuit 50 to receive the inverted 
version E.sub.f of the speed voltage during forward tape movement (i.e., 
N&gt;0), and to receive speed voltage E.sub.f during reverse tape movement 
(i.e., N&lt;0). 
The operation of sawtooth generator 152, shown in FIG. 18, now will be 
described with reference to the graphical representation of FIG. 19. Speed 
voltage E.sub.f, produced by frequency discriminator 111, is related to 
the speed N of tape 1 as represented by the broken line. It is recalled 
that this graphical representation of the speed voltage has been discussed 
above with respect to FIG. 13A. The inverted version E.sub.f of the speed 
voltage, as produced by inverting circuit 157, is represented by the chain 
line in FIG. 19. It is appreciated that, when N=0, speed voltage E.sub.f, 
and the inverted version E.sub.f thereof, are of equal magnitude. However, 
as N increases, the magnitude of the speed voltage E.sub.f decreases; 
whereas the magnitude of the inverted version E.sub.f thereof increases. 
During forward tape movement, operation control circuit 50 controls 
change-over switch 156 to supply the inverted version E.sub.f of the speed 
voltage to the base electrode of transistor Q.sub.1. During reverse tape 
movement, the operation control circuit controls the change-over switch to 
supply the speed voltage E.sub.f to transistor Q.sub.1. Hence, the voltage 
E.sub.j which is supplied to the base electrode of transistor Q.sub.1 by 
change-over switch 156 is represented by the solid line in FIG. 19. 
The magnitude of the voltage E.sub.j when N=1 is shown in FIG. 19 to be 
equal to E.sub.1. This voltage E.sub.1 is supplied as a reference voltage 
to the base electrode of transistor Q.sub.2, such as by a voltage divider 
circuit. If the base-emitter losses of transistors Q.sub.1 and Q.sub.2 are 
ignored, it is appreciated that the emitter voltages of these transistors 
are equal to E.sub.j and E.sub.1, respectively. 
At the beginning of each head field period, that is, at the beginning of 
each scanning trace, OR circuit 151 supplies a head position pulse P.sub.g 
to the base electrode of transistor Q.sub.3. These head position pulses 
have been discussed previously with respect to FIG. 16A. Transistor 
Q.sub.3 is rendered conductive in response to each head position pulse 
P.sub.g such that its collector-emitter path promptly charges capacitor 
C.sub.1 with the reference voltage E.sub.1 provided at the emitter 
electrode of transistor Q.sub.2. Transistor Q.sub.3 thereafter is returned 
to its non-conductive state, awaiting the next occurrence of a head 
position pulse P.sub.g. While this transistor is non-conductive, the 
voltage across capacitor C.sub.1 changes toward the voltage E.sub.j 
provided at the emitter electrode of transistor Q.sub.1. For example, if 
the tape is moved in its slow motion mode (N&lt;1), then the voltage E.sub.j 
at the emitter electrode of transistor Q.sub.1 is less than the voltage 
E.sub.1 to which capacitor C.sub.1 has been charged. Consequently, this 
charged voltage now discharges through resistor R.sub.1. If the voltage 
across capacitor C.sub.1 is used as the sawtooth waveform voltage E.sub.n, 
it is appreciated that this sawtooth waveform exhibits a negative slope 
when N&lt;1. Conversely, when the tape is moved in its fast motion mode 
(N&gt;1), the voltage E.sub.j at the emitter electrode of transistor Q.sub.1 
is greater than the reference voltage E.sub.1 to which capacitor C.sub.1 
has been charged. Thus, the capacitor now is charged through resistor 
R.sub.1 ; whereby the sawtooth waveform of this voltage exhibits a 
positive slope. 
Although capacitor C.sub.1 always is charged to the level E.sub.1 at the 
beginning of each scanning trace, the level to which it gradually is 
charged or discharged is a function of the voltage E.sub.j which, in turn, 
is determined by the actual speed at which the tape is driven. That is, 
for the slow motion mode, capacitor C.sub.1 is discharged from its initial 
level of E.sub.1 to a level which is a function of the voltage E.sub.j. 
Conversely, for the fast motion mode, capacitor C.sub.1 is charged from 
its initial level of E.sub.1 to a higher level that is a function of the 
voltage E.sub.j. Thus, it is seen that the peak-to-peak amplitude of the 
sawtooth waveform voltage E.sub.n produced by capacitor C.sub.1 is related 
to the tape speed in the manner graphically represented by FIG. 7. 
Returning now to FIG. 9, it is seen that, in addition to tracking control 
circuit 100 which supplies correction voltages E.sub.ca and E.sub.cb to 
displaceable support members 21A and 21B, a closed loop correction circuit 
200 also is provided to supply tracking error voltages to the displaceable 
support members. This closed loop correction circuit is shown in greater 
detail in FIG. 8. Closed loop correction circuit 200 is comprised of an 
oscillator 201, an envelope detecting circuit including a tuned amplifier 
202, a sample-and-hold circuit 203 and a tuned amplifier 205, a 
synchronous detector 206 and sample-and-hold circuits 208A and 208B. 
Oscillator 201 is adapted to generate an oscillating signal whose frequency 
is on the order of about 450 Hz. This oscillating signal is supplied to 
displaceable support members 21A and 21B via adder circuits 104A and 104B, 
respectively, for the purpose of vibrating the displaceable support 
members, resulting in the dithering of heads 11A and 11B from side-to-side 
across a record track being scanned thereby. Referring to FIG. 20, it is 
seen that, as the head 11A (or 11B) vibrates, or dithers, relative to 
track 2, the center of the head oscillates relative to the center line of 
the track with a peak-to-peak amplitude of 2.DELTA.w, wherein .DELTA.w is 
on the order of about 10% of the width T.sub.w of the track. As a result 
of this dithering of the head, the envelope of the FM video signal 
reproduced thereby exhibits an amplitude modulation component. Of course, 
this amplitude modulation is removed from the reproduced video signal by 
limiter 14 included in signal reproducing section 10. 
Tuned amplifier 202 is connected to receive the reproduced FM video signal 
S.sub.f having the amplitude modulated envelope. The tuned amplifier is 
tuned to the FM frequency corresponding to the frequency modulated 
horizontal synchronizing pulse. Thus, tuned amplifier 202 particularly 
amplifies the FM horizontal synchronizing pulse S.sub.t whose envelope is 
amplitude modulated because of the dithering of the heads. This amplified 
FM horizontal synchronizing pulse S.sub.t is supplied to sample-and-hold 
circuit 203. A synchronizing separator circuit 204 is connected to video 
output terminal 17 of signal reproducing section 10 and is adapted to 
separate the horizontal synchronizing pulses P.sub.h from the demodulated 
video signal. These separated horizontal synchronizing pulses P.sub.h are 
supplied as sampling pulses to sample-and-hold circuit 203. Thus, the 
sample-and-hold circuit samples the FM horizontal synchronizing pulses 
S.sub.t to produce the sampled horizontal synchronizing pulse S.sub.h, 
which appears as a burst of frequency corresponding to the FM frequency 
which had been selected to modulate the originally recorded horizontal 
synchronizing pulse. The samples of the FM horizontal synchronizing pulse 
S.sub.h exhibit an envelope which is amplitude modulated in accordance 
with the dithering of the heads. This amplitude modulation of the envelope 
of the sampled FM horizontal synchronizing pulses is, of course, 
determined by the frequency of the oscillating signal generated by 
oscillator 201 and supplied via adder circuits 104A and 104B to 
displaceable support members 21A and 21B. 
The samples of the FM horizontal synchronizing pulses produced by 
sample-and-hold circuit 203 are supplied to synchronous detector 206 by 
tuned amplifier 205, the latter being tuned to the dithering frequency of 
450Hz. Thus, the tuned amplifier supplies the envelope of the samples of 
the FM horizontal synchronizing pulses to the synchronous detector. In 
addition to this envelope, synchronous detector 206 receives the 
oscillating signal generated by oscillator 201, which is supplied thereto 
by a phase shift circuit 207. This oscillating signal is used to 
synchronously detect the envelope of the samples of the FM horizontal 
synchronizing pulses S.sub.h so as to produce a tracking error voltage 
E.sub.w. The level of this tracking error voltage represents the deviation 
of the average path traversed by head 11A (or 11B) from the center line of 
track 2, shown in FIG. 20. 
The tracking error voltage E.sub.w, produced by synchronous detector 206, 
and representative of the tracking error of heads 11A and 11B, is supplied 
to sample-and-hold circuits 208A and 208B. The rectangular wave signal 
S.sub.v is supplied as a sampling signal to sample-and-hold circuit 208A; 
and the inverted version S.sub.v of this rectangular wave signal is 
supplied as a sampling signal to sample-and-hold circuit 208B. From FIG. 
12B, it is seen that sample-and-hold circuit 208A samples the tracking 
error voltage E.sub.w during each head field period that head 11A scans 
tape 1, so as to supply the tracking error voltage E.sub.wa associated 
with head 11A to displaceable support member 21A. Similarly, 
sample-and-hold circuit 208B samples the tracking error voltge E.sub.w 
during each head field period that head 11B scans tape 1 so as to supply 
the tracking error voltage E.sub.wb associated with head 11B to 
displaceable support member 21B. 
If the average path traversed by each of heads 11A and 11B is represented 
as x, then the tracking error voltage E.sub.w which is supplied to 
sample-and-hold circuits 208A and 208B is a function of this average path 
in accordance with the graphical representation shown in FIG. 21. This 
tracking voltage is supplied as a correcting voltage to the respective 
displaceable support members so as to bring the average path traversed by 
each head into coincidence with the center line of each track being 
scanned. 
As mentioned above with respect to level correcting circuit 160, the 
reference voltage E.sub.r supplied to difference amplifier 163 corresponds 
to the tracking error voltage E.sub.w. 
It now should be apparent that the present invention serves to control the 
scanning traces of each head such that, regardless of the speed at which 
the tape is driven the previously recorded record tracks are scanned 
accurately. The correction voltage E.sub.c which is supplied to the 
displaceable support members upon which the respective heads are mounted 
is a function of the speed at which the tape is driven. Moreover, a closed 
loop correction circuit also is provided to make sure that each head 
correctly scans a record track despite changes that might take place in 
the physical characteristics of the tape or the operating parameters of 
the VTR' which are used. 
In the foregoing description, it has been assumed that the video signals 
recorded on tape 1 are monochrome (or black-and-white) video signals. It 
should be readily appreciated that the recorded video signals may consist 
of composite color video signals, such as an FM color video signal or a 
color video signal in which the chrominance component is 
frequency-converted to a relatively lower band and the luminance component 
is frequency modulated to a relatively higher band. If the recorded video 
signal is a color video signal, then the dithering frequency of the heads, 
that is, the frequency of the oscillating signal generated by oscillator 
201 of closed loop correction circuit 200, should be equal to an odd 
multiple of one-half the field frequency. With this dithering frequency, 
even though the amplitude of the reproduced video signals changes as a 
function of the dithering of the heads, resulting in an irregular density 
of the color saturation, this irregularity is cancelled out by reason of 
the fact that the fields are interleaved. Hence, the irregular density is 
not perceived by a viewer. 
Furthermore, in recording the video signals, heads 11A and 11B may exhibit 
different azimuth angles such that the alternate record tracks that are 
recorded thereby are recorded with different azimuth angles. For proper 
reproduction, the reproducing heads should exhibit the same azimuth angles 
as those heads which were used to record the respective tracks. 
While the present invention has been particularly shown and described with 
reference to a preferred embodiment, it should be readily apparent to 
those of ordinary skill in the art that various changes and modifications 
in form and details may be made without departing from the spirit and 
scope of the invention. For example, frequency generator 34 may be 
constituted by a separate longitudinal track on tape 1 having clock 
signals recorded therein. These clock signals are reproduced as tape speed 
pulses P.sub.f having a frequency which is directly related to the speed 
at which the tape is driven. Furthermore, although the recording medium 
with which the present invention is used has been described herein as a 
magnetic tape, it should be appreciated that any other movable recording 
medium containing signals which are recorded in parallel tracks can be 
used. For example, the recording medium may be a magnetic sheet or an 
optical recording medium. It is intended that the appended claims be 
interpreted as including the foregoing as well as other such changes and 
modifications.