Tracking control using signals reproduced by erase head

There is disclosed an information signal reproducing apparatus for reproducing information signals from a magnetic record bearing medium on which information signals are recorded with pilot signals of four different frequencies recurrently superimposed thereon. The apparatus comprises magnetic heads which are arranged to reproduce said information signals and said pilot signals by tracing said magnetic record bearing medium and an erasing head which is arranged to trace said magnetic record bearing medium in equal tracing direction to that of said magnetic heads and which is able to erase at least the information signals recorded on said magnetic record bearing medium and to reproduce said pilot signals. At the time of controlling a tracing position of said magnetic heads relative to said magnetic record bearing medium, four local pilot signals having the same frequencies as those of said pilot signals, respectively, are recurrently generated in a predetermined rotation. The state of said pilot signals reproduced by said magnetic heads is monitored, using said four local pilot signals and the tracing position on said medium of said magnetic heads is controlled, depending upon the state of said pilot signals. The level of a specific pilot signal from among said pilot signals reproduced by said erasing head is detected and the generating sequence of the local pilot signals is changed over, depending upon a result of such detention. According to such construction it is possible to effect the tracking control with high precision and high response speed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an information signal reproducing apparatus and 
more particularly to an apparatus for reproducing information signals from 
a recording medium on which pilot signals of different frequencies are 
recurrently superimposed in rotation on the information signals. 
2. Description of the Related Art 
In the field of high density magnetic recording which typically includes 
the video tape recorder (hereinafter referred to as VTR), it has recently 
come to be popularly practiced to have pilot signals of four different 
frequencies superimposed on information signals during recording and to 
carry out tracking control by using the pilot signals reproduced during 
reproduction of the information signals. The conventional signal 
processing operation which has been performed to obtain signals for 
tracking control is as briefly described below: 
FIG. 1 of the accompanying drawings shows the tracks of recording 
magnetization formed on a magnetic record bearing medium by a VTR which is 
arranged to perform tracking control in the four-frequency method. FIG. 2 
is a diagram showing the essential parts of a reproducing circuit arranged 
to obtain a tracking error signal. In FIG. 1, an arrow 2 indicates the 
travelling direction of a magnetic tape 1. Magnetic heads A and B which 
have a given azimuth angle are arranged to form recording tracks A1, B1, 
A2, B2 and so on. Another arrow 3 indicates the direction in which these 
heads perform a scanning action. In each of recording tracks 4, one of 
pilot signals of four different frequencies f1 to f4 is recorded together 
with the video signal for every field, that is, in each of the tracks one 
after another. 
The sequence of rotation in which these pilot signals are recorded is as 
shown in FIG. 1. For example, the pilot signal of frequency f1 which is 
102.5 KHz.apprxeq.6.5 fH is superimposed on the video signal in the track 
A1; the pilot signal of frequency f1 which is 118.9 KHz.apprxeq.7.5 fH in 
the track B1; the pilot signal of frequency f3 which is 165.2 
KHz.apprxeq.10.5 fH in the track A2; and the pilot signal of frequency f4 
which is 148.7 KHz.apprxeq.9.5 fH in the track B2 (fH: the frequency of 
horizontal synchronizing signal). 
The frequency difference between the pilot signals recorded in adjacent 
recording tracks is either fH or 3 fH as shown in FIG. 1. When the head is 
scanning the recording tracks Ai (wherein "i" represents 1, 2, 3 and so 
on), the frequency difference of the pilot signal of the scanning track 
(the main scanning track) is always fH from that of the track on the 
right-hand side as viewed on FIG. 1 and is always 3 fH from that of the 
left-hand track. Further, when the head is scanning the recording tracks 
Bi ("i": 1, 2, 3, . . . ), the frequency difference of the pilot signal of 
the scanning track is always 3 fH from that of the right-hand track and is 
always fH from that of the left-hand track. 
Since all the pilot signals (of frequencies) f1 to f4 have relatively low 
frequency values, the head can reproduce, as cross-talk, the pilot signals 
from tracks located adjacent to a track mainly scanned or traced. For 
example, when the head is mainly scanning the track A2, the pilot signal 
thus detected (or reproduced) is a composite signal consisting of the 
frequency components f4, f2 and f3 representing these pilot signals. In 
case that the head is in a so-called on-track state with the center of the 
tracing locus of the head precisely coinciding with the center line of the 
main scanning track, the pilot signals f2 and f4 of the two adjacent 
tracks are at the same level. The reproduced level of the pilot signal f4 
becomes higher than that of the pilot signal f2 when the head slightly 
deviates from the center line of the track A2 toward the track B2. The 
reproduced level of the pilot signal f2 becomes higher than that of the 
pilot signal f4 when the head slightly deviates toward the track B1. 
Therefore, the direction and extent of deviation of the head from the main 
scanning track are obtainable by separately taking out difference signals 
fH and 3 fH representing differences between the pilot signal of the main 
scanning track and the pilot signals of the two adjacent tracks 
respectively and by comparing the levels of the difference signals thus 
obtained with each other. 
FIG. 2 shows in a block diagram a circuit of the above-stated 
four-frequency method. A terminal 5 is arranged to receive a reproduced 
signal in which the pilot signals are superimposed on the video signal. A 
low-pass filter (hereinafter referred to as LPF) 6 is arranged to separate 
the pilot signal component from the reproduced signal. A local pilot 
signal generating circuit 7 is arranged to generate local pilot signals. A 
multiplier 8 is arranged to perform a multiplying operation on the pilot 
signal component and the local pilot signal generated by the generating 
circuit 7. The local pilot signal is of the same frequency as that of the 
pilot signal recorded on the main scanning track. As mentioned with 
reference to FIG. 1 in the foregoing, assuming that the main scanning 
track is the track A2, the output of the LPF 6 includes frequency 
components f2, f4 and f3. Meanwhile, the local pilot signal is of the 
frequency f3. Therefore, the multiplier 8 produces a signal of a frequency 
representing the sum of and a difference between the frequency f3 and the 
frequencies f2, f4 and f3. A band-pass filter (hereinafter referred to as 
BPF) 9 is arranged to take out only a signal of fH from the sum and 
difference signal while another BPF 10 is arranged to take out only a 
signal of 3 fH from the sum and differrence signal. The outputs of the 
BPF's 9 and 10 are detected and rectified by detection circuits 11 and 12 
respectively. Following that, the signal components fH and 3 fH thus 
obtained are supplied to a level comparison circuit 13. The circuit 13 
then produces a signal corresponding to a level difference between these 
signal components. More specifically, in case where the reproduced level 
of the signal of the frequency fH is higher than that of the signal of the 
frequency 3fH, the output of the circuit 13 is of a positive potential 
corresponding to the level difference. In the event of a case opposite to 
that, the output of the circuit 13 is of a negative potential. The signal 
produced from the level comparison circuit 13 can be used as a tracking 
error signal as it contains information on the extent and direction of the 
deviation of head from the track. 
As apparent from the foregoing description given with reference to FIG. 1, 
the deviating direction of the head from the tracks Ai is opposite to the 
deviating direction from the tracks Bi. Therefore, a switching circuit 16 
is arranged to selectively produce the output of the level comparison 
circuit 13 either directly or through an inverting amplifier 14 according 
to a head change-over signal 15. 
While tracking control can be adequately accomplished for reproduction of a 
video signal under a normal condition, let us consider now a tracking 
signal processing operation under a transient condition. With the main 
scanning track assumed to be the track A2 (see FIG. 1) in the beginning of 
reproduction and the local pilot signal component used for the 
multiplication process assumed to be of the frequency f1, the detection 
circuits 11 and 12 respectively produces the frequency components fH and 3 
fH in the manner similar to the tracking control under the normal 
condition. In this instance, however, the tracking control is performed in 
such a manner as to cause the reproducing head to further deviate from the 
track A2 because the polarity obtained in this instance is opposite to the 
polarity obtained under the normal condition. Then, the pilot signal 
recorded in the main scanning track and the local pilot signal eventually 
come to coincide with each other. 
However, with the levels of the components fH and 3 fH becoming about the 
same while the reproducing head is justly tracing the track A2, when the 
reproducing head is shifted either in the direction of arrow 2 or in the 
direction opposite thereto, a considerably long period of time is also 
required before the pilot signal of the main scanning track and the local 
pilot signal come to coincide with each other. Therefore, the conventional 
circuit arrangement has presented a problem that, during the above-stated 
long period of time, the quality of the reproduced picture tends to 
degrade. Further, in accordance with the conventional method described, 
the level difference between the pilot signals which are for example f2 
and f4 and recorded in the two adjacent tracks B1 and B2 with the track A2 
assumed to be mainly scanned or traced, is arranged to be detected in 
obtaining the tracking error signal. However, since the pilot signals f2 
and f4 are reproduced as cross-talk and thus do not have any dynamic 
range, they are poor in the S/N ratio. Thus, it has been impossible to 
carry out tracking control with a sufficient degree of accuracy. 
SUMMARY OF THE INVENTION 
It is a general object of this invention to solve the problems mentioned in 
the foregoing with simple arrangement. 
It is a specific object of this invention to provide an information signal 
reproducing apparatus which is capable of performing tracking control with 
quick retraction time without necessitating complex arrangement. 
It is another specific object of this invention to provide an information 
signal reproducing apparatus which is capable of performing tracking 
control with an adequate degree of accuracy with simple arrangement. 
Under this object, an information signal reproducing apparatus arranged 
according to this invention as an embodiment thereof to reproduce 
information signals which are recorded in recording tracks formed on a 
magnetic record bearing medium in the direction of crossing the travelling 
direction of the medium at a given pitch with pilot signals of different 
frequencies recurrently superimposed in rotation on the information 
signals comprises: magnetic heads arranged to reproduce at least the 
information signals by tracing the recording tracks on the medium in the 
track forming direction; means for causing the medium tracing action of 
the heads; an erasing head arranged to erase the information signals 
recorded on the medium by tracing the recording tracks in the same 
direction as the magnetic heads and to be capable of reproducing the pilot 
signals; and tracking control means arranged to control the tracing 
position on the medium of the magnetic heads by controlling the medium 
tracing action causing means at least according to the pilot signals 
reproduced by the erasing head. 
It is a further object of this invention to provide an information signal 
reproducing apparatus which is capable of promptly bringing tracking 
control from a transient state into a normal state. 
It is a still further object of this invention to provide an information 
signal reproducing apparatus which is capable of obtaining a 
satisfactorily reproduced signal even at the beginning of a reproducing 
operation. 
Under this object, an information signal reproducing apparatus arranged as 
another embodiment of this invention to reproduce information signals 
which are recorded on a magnetic record bearing medium with pilot signals 
of different frequencies recurrently superimposed in rotation on the 
information signals comprises: magnetic heads arranged to reproduce the 
information signals and the pilot signals by tracing the magnetic record 
bearing medium; means for causing the heads to trace the medium; an 
erasing head arranged to erase the information signals recorded on the 
medium by tracing the medium in the same direction as the magnetic heads 
and to be capable of reproducing the pilot signals; tracking control means 
arranged to control the tracing position on the medium of the magnetic 
heads by controlling, according to the state of the pilot signals 
reproduced by the magnetic heads, the means for causing the heads to trace 
the medium; level detecting means for detecting the level of a specific 
pilot signal from among the pilot signals reproduced by the erasing head; 
and switching means for switching the controlling state of the tracking 
control means from one state over to another according to the level of the 
specific pilot signal detected by the level detecting means. 
The above and further objects and features of this invention will become 
apparent from the following detailed description of preferred embodiments 
thereof taken in connection with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The following is description of embodiments in which this invention is 
applied to VTR's: FIG. 3 shows in outline a VTR arranged as an embodiment 
of this invention. FIG. 4 shows the allocation of heads employed in the 
same VTR. FIG. 5 shows the state of the recording medium tracing action of 
the same VTR performed during a reproducing operation. As shown in FIG. 4, 
recording/reproducing heads 21 and 22 are mounted on a rotary cylinder or 
drum 20 with a phase difference of 180 degrees between them. A rotary 
erasing head 24 is mounted also on the cylinder 20 with a phase difference 
of 90 degrees from each of the heads 21 and 22. The erasing head 24 has a 
width measuring twice as wide as the width of each recording track. An 
arrow 23 indicates the rotating direction of these heads. The rotary 
erasing head 24 must precede the recording/reproducing heads 21 and 22 in 
tracing the recording tracks. For example, when the head 21 is at a 
position as shown in FIG. 5, the head 24 is located at a position as shown 
in FIG. 5. Then, assuming that the reproducing head 21 is mainly scanning 
a track A1, the erasing head 24 begins to trace when the scanning or 
tracing action of the head 21 comes to the latter half of the tracing 
action on the track A1 and the tracing action of the head 24 on the track 
A1 comes to an end after the reproducing head 22 begins to mainly scan or 
trace another track B1. During this process, the pilot signal of the 
frequency f1 is not reproduced via the rotary erasing head 24. Whereas, in 
case that the main scanning track of the head 21 is the track A2 and that 
of the head 22 is another track B2, the erasing head 24 produces an output 
which contains a frequency component f1 to a great degree. 
In view of this, this specific embodiment is arranged to determine the 
reproducing operation to be abnormal in the event that the frequency 
component f1 is reproduced from the rotary erasing head 24 while a 
recording track having the pilot signal frequency f1 or f2 is mainly 
scanned or traced by the reproducing head. 
The following describes the operation of the various parts of FIG. 3 with 
reference to the timing chart of FIG. 6: In FIG. 3, the reference numerals 
used in FIGS. 2 and 4 are also used in indicating the corresponding parts. 
The recording/reproducing heads 21 and 22 are connected to a video signal 
processing circuit 32. A video signal coming via a terminal 33i is 
recorded. A reproduced video signal is produced via another terminal 33o. 
A rotation detector 40 is arranged to detect the rotation of a rotary drum 
(or cylinder) 20 and to produce a rectangular wave signal HSW one period 
of which represents one turn of the drum. The signal HSW is arranged to 
control a head switch 31. The output of the head switch 31 is subjected to 
a tracking signal processing operation which is performed at an LPF 6 and 
ensuing circuit elements. As a result, a tracking control signal is 
obtained from a differential amplifier 13. In this instance, the tracking 
control signal from the amplifier 13 is supplied directly to a capstan 
control circuit 36 without passing through an inverting amplifier. The 
output of the differential amplifier 13 is thus arranged to be used for 
tracking control only when the reproducing head is mainly scanning a 
recording track having the pilot signal frequency f1 or f3 according to 
the above-stated signal HSW. As for the local pilot signal, the local 
pilot signal of frequency f1 or f3 is employed and is selectively supplied 
via a switch 43 to a multiplying circuit 8. 
A frequency divider 42 is arranged to frequency divide by 1/2 the signal 
HSW using the output of an edge detecting circuit 51. The connecting 
position of the switch 43 is shifted according to the output of this 
frequency divider 42. Referring to FIG. 6, when the pilot signal recorded 
in the main scanning track is of frequency f1 or f2, the local pilot 
signal f1 is used. The local pilot signal f3 is used when the pilot signal 
f3 or f4 is recorded in the main scanning track. 
The switch 43 supplies the multiplying circuit 8 with the output of an f1 
oscillator 34 when the output of the 1/2 frequency divider 42 is at a high 
level and with the output of an f3 oscillator 35 when the output of the 
1/2 frequency divider 42 is at a low level. Such being the arrangement, if 
the output of the rotary erasing head 24 includes an f1 frequency 
component to an excessive degree when the output level of the 1/2 
frequency divider is high, the reproducing operation is determined to be 
abnormal. 
A switch 45 is arranged to make selection between recording and 
reproduction. The position of the switch 45 is on the side of a terminal R 
thereof when recording is performed by the heads 21 and 22 and is on the 
side of another terminal P when reproduction is performed. In recording, 
an erasing signal produced from an erasing signal generator 46 is supplied 
to the head 24. In response to this signal, the head 24 performs an 
erasing action. In the case of reproduction, the f1 frequency component of 
a reproduced signal produced via the terminal P of the switch 45 is 
separated by a band-pass filter (BPF) 47. The output of the BPF 47 is 
subjected to a detection process at a detection circuit 50. The detection 
output thus obtained is compared with a reference voltage (Vref) 48 at a 
comparator 49. When the output of the detection circuit 50 exceeds the 
reference voltage Vref, the output level of the comparator 49 becomes 
high. In this instance, if the output level of the 1/2 frequency divider 
42 is high, there arises an abnormal condition as mentioned above. Then 
the level of the output of an AND gate 44 becomes high to trigger a 
one-shot multivibrator 52 to obtain a clock input of the 1/2 frequency 
divider 42. Accordingly, the output of the 1/2 frequency divider 42 is 
inverted and the switch 43 comes to produce the output f3 of the f3 
oscillator 35. Since the reproducing head is tracing at that moment a 
track adjacent to a track in which the pilot signal f3 is superimposed on 
the video signal, tracking control is quickly brought or pulled back into 
a normal condition. In accordance with the arrangement of this embodiment 
described, the tracking control can be thus promptly brought into a normal 
condition even during the transient period after commencement of 
reproduction. 
Further, in the embodiment described, the tracking control operation is 
determined to be abnormal when the rotary erasing head reproduces a large 
amount of the frequency component f1 while the main scanning track 
includes the pilot signal (of frequency) f1 or f2. However, this 
arrangement may be changed as desired by changing the allocation of the 
reproducing head and the rotary erasing head. Further, the tracking 
control may be accomplished according to the conventional method of 
generating the local pilot signals in rotation of frequencies f1, f2, f3 
and f4. In that instance, the tracking control can be promptly brought 
into a normal condition by shifting the local pilot signal frequency in 
such a manner as f1.fwdarw.f3, f2.fwdarw.f4, f3.fwdarw.f1 and 
f4.fwdarw.f2. 
The VTR arranged according to this invention as described above is capable 
of bringing its operation into a normal condition even in the transient 
period of the tracking control. Therefore, the VTR is capable of giving an 
adequately reproduced signal even in the beginning of a reproducing 
operation. 
FIG. 7 shows in outline the arrangement of a VTR as another embodiment of 
this invention. FIGS. 8 and 9 show the allocation of heads of the same 
VTR. FIG. 10 shows the tracing action of the heads performed on a record 
bearing medium during a reproducing operation. Referring to FIGS. 8 and 9, 
recording/reproducing heads 53 and 54 are mounted on a rotary drum 55 with 
a phase difference of 180 degrees between them. An erasing head 56 is 
mounted also on the drum 55 with a phase difference of 90 degrees from the 
heads 53 and 54. Further, assuming that the track pitch is Tp, the erasing 
head 56 is positioned on the rotary drum with its lower end shifted about 
2.5 Tp. As a result, during recording and reproduction, the tracing 
position of the erasing head deviates to an extent corresponding to three 
tracks as shown in FIG. 10. 
Next, the principle of bringing or pulling the tracking control into a 
normal condition according to the arrangement of this embodiment is as 
follows: While the recording/reproducing heads are tracing the tracks in 
which the pilot signals f1 and f2 are superimposed on the video signal, 
the rotary erasing head 56 traces two tracks in which the pilot signals f3 
and f4 are superimposed on the video signal as shown in FIG. 10. Further, 
conversely, the erasing head 56 simultaneously traces two tracks in which 
the pilot signals f1 and f2 are superimposed while the heads 53 and 54 are 
simultaneously tracing the tracks in which the pilot signals f3 and f4 are 
superimposed. FIG. 11 shows this mode of operation. 
Assuming that the head 53 is in the tracing position as shown in FIG. 10, 
it is apparent that a tracking error signal can be obtained by comparing 
the frequency components f3 and f4 included in the output of the rotary 
erasing head 56. Further, while the heads 53 and 54 are tracing the tracks 
in which the pilot signals f3 and f4 are superimposed, a tracking error 
signal is also obtainable by comparing the frequency components f1 and f2 
produced from the erasing head. However, arrangement to detect and process 
all of these frequency components necessitate a large circuit arrangement. 
To avoid it, therefore, the local pilot signal is arranged to be 
superimposed on the output of the erasing head 56 as shown in FIG. 11. By 
virtue of this arrangement, the frequency components f1 and f3 can be 
extracted as a 3 fH component and the frequency components f2 and f4 as a 
2 fH component. 
In this instance, if the local pilot signal is applied in a reverse manner, 
the 2 fH and 3 fH components would be generated in smaller quantities. 
This indicates an abnormal condition. In that event, therefore, the local 
pilot signals are switched over between signal frequencies f2 and f4. By 
this arrangement, the length of time required for pulling the tracking 
control into a normal condition can be shortened. The operation of each of 
the parts shown in FIG. 7 is as follows: 
The signal reproduced by the recording/reproducing heads 53 and 54 is 
processed by a video signal processing circuit 57 into a signal form 
suited for television and is produced from a terminal 58o. Meanwhile, a 
video signal which comes via a terminal 58i is processed by the circuit 57 
into a signal form suited for recording and is recorded by the heads 53 
and 54 on a record bearing medium. 
In recording, the output of an erasing signal generating circuit 62 is 
supplied as an erasing signal to the erasing head 56. In reproduction, the 
pilot signal component is separated by an LPF 59 from the output signal of 
the head 56 and is supplied to a multiplying circuit 60 to be subjected to 
a multiplying operation with the local pilot signal f2 or f4. A BPF 63 
separates a 2 fH component from the output of the multiplying circuit 60. 
Another BPF 64 separates a 3 fH component from the output of the circuit 
60. The outputs of these BPF's are respectively subjected to detection 
processes performed at detection circuits 65 and 66. The outputs of these 
detection circuits 65 and 66 are supplied to comparators 67 and 68 to be 
compared with a reference voltage 69. The outputs of the comparators 67 
and 68 are supplied to a logical circuit 70. The reference voltage 69 (or 
Vref) is set at a value which is slightly lower than the level of the 
pilot signals to be reproduced from one Tp portion of the record bearing 
medium. 
The heads are considered to be in an on-track state when output levels of 
both the comparators 67 and 68 are high. In that case, a capstan control 
circuit 71 is left in a state of merely performing speed control. In the 
event that the output of the comparator 67 is at a high level while that 
of the other comparator 68 is at a low level, the tracing position of the 
head is deviating in the direction reverse to the direction of arrow 2 of 
FIG. 10. In that event, the phase of a capstan is adjusted in such a way 
as to lower the travelling speed of the tape (or the record bearing 
medium). In this instance, the logical circuit 70 produces an output C as 
shown in FIG. 7. 
When the output of the comparator 68 is at a high level while that of the 
comparator 67 is at a low level, the logical circuit 70 produces an output 
B. This signal causes the phase of the capstan adjusted to increase the 
travelling speed of the tape. 
When the outputs of both the comparators 67 and 68 are at low levels, the 
logical circuit 70 produces an output A which indicates that the tracking 
control is in a transient state. The output A triggers a one-shot 
multivibrator 78 and a clock pulse signal is supplied via an OR gate 77 to 
a 1/2 frequency divider 79. 
Under a normal condition, a rectangular wave signal which has one period 
thereof corresponding to one turn of a rotary drum 55 and is produced from 
a rotation detector 74 is supplied to an edge detecting circuit 76. The 
output of the circuit 76 is used as the above-stated clock pulse signal to 
be supplied to the 1/2 frequency divider 79. Following this, a switch 75 
selectively supplies the local pilot signal f2 or f4 to the multiplying 
circuit 60. Accordingly, when the output A is produced from the logical 
circuit 70, the local pilot signal is switched over between the pilot 
signal frequencies f2 and f4, so that the tracking control can be 
instantly pulled into a normal condition. 
In the embodiment described, the capstan control may be modified to be 
carried out by comparing the outputs of the detection circuits 65 and 66 
with each other. Further, in the event that the erasing head 56 is 
arranged in a position which is further away from the 
recording/reproducing heads 53 and 54 by one track pitch (Tp) distance, 
the arrangement of the BPF's 63 and 64 must be changed to separate an fH 
component and a 2 fH component respectively. 
As described in the foregoing, the VTR according to this invention is 
capable of promptly bringing (or pulling) the tracking control into a 
normal condition even in the event of a transient state, so that an 
adequately reproduced signal can be obtained without delay.