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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic recording and reproducing
apparatus such as VTR adapted to detect the occurrence of clogging of a
head.
2. Description of the Prior Art
When video heads get clogged, efforts to perform recording on a magnetic
tape may result in imperfect recording quality or become impossible. If
such an imperfect magnetic tape on which recording has been performed is
used for reproduction, even with a normal VTR having video heads that are
not suffering from clogging, the reproduced image may frequently include
noises and it is even possible that no images at all will be reproduced.
Conversely, even if a magnetic tape which has been properly recorded is
reproduced by means of a video head which is clogged, many noises may
similarly be generated and on occasion no image will be reproduced.
The majority of the causes of clogging are attributed to the fact that the
magnetic powder generated by a magnetic tape may stick to the gaps defined
by the video heads, inhibiting formation of a normal magnetic circuit.
This is inevitable with a VTR or the like.
Such clogging may be easily removed by cleaning the tip of a video head
with a cleaning liquid or having a cleaning tape run along said tip.
It is to be understood, however, that the clogging may be easily observed
on the monitor screen during reproduction, but clogging is difficult to
detect during recording. When it is desired to detect the presence of
clogging during recording, recording has to be suspended and the
reproduction mode tested in order to detect any clogging.
In particular, video tape recorders used for surveillance for long periods
of time (hereafter referred to as "time-lapse VTR") according to the prior
art are assumed to continue recording in an unmanned mode all day long, or
for months on end in some cases. Thus it is quite unlikely to be possible
to reproduce a magnetic record to confirm whether proper recording has
definitely been effected. Consequently, it often happens that the subject
matter to be recorded is not properly recorded, which is a critical
problem with this kind of VTR when so-called "clogging" occurs as a result
of adherence to video heads of magnetic powders or the like from the
surface of video tapes. This makes it impossible to properly record and
reproduce records. As countermeasures directed at solving this problem,
not only has it been the practice for users to frequently check the
condition of reproduced images and/or to have a specialist periodically
check for the presence of "clogging", but there has also been a practice
of attaching a head cleaning mechanism as shown in FIG. 1 in order to
clean the video heads regardless of whether "clogging" actually occurs or
not.
This cleaning mechanism according to the prior art will now be explained by
referring to FIG. 1. The numeral 1 in FIG. 1 designates a rotary drum,
numeral 10 a tape guide, numeral 2 a rotary roller for cleaning, numeral
11 a support arm for the rotary roller, numeral 12 a spring for pressing,
numeral 13 a cam for moving the support arm and numeral 14 a pin for
moving the support arm 11.
Operation of the above cleaning mechanism will next be explained by
referring to FIGS. 2 through 4. When the tape guide 10 commences movement
in the direction indicated by an arrow d.sub.1 in FIG. 2 for the purpose
of recording or reproduction, the cam 13 is caused to rotate synchronously
in the direction indicated by an arrow d.sub.2 and the rotary drum 1 is
also caused to start rotating. As shown in FIG. 3, the roller 2 is urged
against the rotary drum 1 by the biasing spring 12 while the tape guide 10
is moving so that the video head may be cleaned by the roller. At the
position where the tape guide 10 completes this movement, the pin 14 will
move the support arm 11 against the force of the biasing spring 12, as
shown in FIG. 4, whereby the roller 2 is moved away from the rotary drum
1.
In the magnetic recording and reproduction apparatus of prior art, since
the video head cleaning mechanism is so constructed that the roller is
urged against the head at the time of loading or unloading a tape in the
manner described above, and that the head will be cleaned at the time of
and with the same frequency as loading regardless of whether the heads are
clogged or not, there have been certain problems in that recording cannot
always be performed as soon as is necessary, and in that cleaning is
performed so often that the heads may be damaged.
SUMMARY OF THE INVENTION
The present invention has been proposed with a view to eliminating such
problems as those pointed out above and a first object of the present
invention is to provide a magnetic recording and reproducing apparatus
having a head cleaning mechanism that allows the heads to be cleaned only
when they become clogged so that the heads will not become damaged due to
excessive cleaning.
It is a second object of the present invention to provide an apparatus for
detecting clogging of video heads in an intermittent magnetic recording
apparatus, which is capable of automatically and properly detecting any
clogging of the heads during recording without affecting the recording
operation at all.
The first object is achieved by a magnetic recording and reproducing
apparatus wherein reproduction signals are constantly detected even during
recording, the absolute value of the detected signals are watched, and
when the absolute value becomes lower due to "clogging", the head cleaning
mechanism is caused to operate so as to effect cleaning of the heads.
The second object is achieved by an apparatus for detecting clogging of
video heads wherein a recording/reproducing command circuit is adapted to
put the video head in the recording mode while intermittent recording is
being conducted by means of the intermittent magnetic recording apparatus
and to switch the video head into the reproducing mode during periods when
intermittent recording is not being performed upon generation of head
operation switching signals, a reproduction envelope comparison circuit is
adapted to compare an envelope of reproduction signals during the
reproducing mode with a predetermined envelope clogging value, and a
clogging output circuit is adapted to detect any clogging of the video
head in response to the output from the comparison circuit and to generate
clogging signals.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a tape movement route of a magnetic recording and
reproducing apparatus according to a prior art;
FIGS. 2 through 4 are explanatory views showing operation of the apparatus
shown in FIG. 1;
FIG. 5 illustrates a tape movement route of a magnetic recording and
reproducing apparatus according to an embodiment of the present invention;
FIG. 6 is a perspective view of the essential part of the route shown in
FIG. 5;
FIG. 7 is a partial sectional view showing assembly of the rotary roller
2A, roller supporting part 3 and shaft 4 shown in FIG. 6;
FIG. 8 is a block diagram showing the constitution of an apparatus for
detecting clogging of video heads according to a second embodiment of the
present invention;
FIG. 9 is a waveform diagram showing operation timing; and
FIG. 10 is a graph showing the relationship between a reproduction envelope
and the reproduction S/N ratio.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
An embodiment of the present invention will now be explained by referring
to FIGS. 5 through 7, which show the constitution of an assembly for
cleaning clogging provided in a tape movement route.
In the drawings, numeral 1 designates a rotary drum, numeral 2A shows a
rotary roller having fibrous cloth for cleaning wrapped around the outer
circumference, and numeral 3 designates a roller support section which is
movable in a bearing part 7 by means of a shaft 4 press-fitted into the
support section. Numeral 6 designates a solenoid which is energized or
de-energized to move the roller supporting section 3 forwardly or
backwardly through a transmitting plate 5, a biasing spring 8 and a
release spring 9.
Operation of the embodiment shown in FIG. 5 will next be explained.
A time-lapse VTR or the like is normally provided with two sets of heads
for normal recording and reproducing and one or two sets of heads for
special reproduction. According to the embodiment, two recording heads are
used for recording and simultaneously reproducing signals at the portion
of the magnetic tape just recorded by use of a head which is not used for
recording, so that the absolute value of the signals is continuously read
out. Since recording continues over long periods, magnetic powder or the
like removed from the tape by the head due to abrasion may stick to the
head and cause "clogging", whereupon the absolute value of the output of
the reproduction signals will decrease. When the absolute value is
detected to be below a predetermined value by a comparison circuit (not
shown in FIGS. 5 through 7), the solenoid 6 will be energized for a fixed
period of time, for example 2-5 seconds. This energization of the solenoid
6 will cause the rotary roller 2A to be urged against the rotary head part
of the rotary drum 1 through the plate 5 and springs 8 and 9 for the same
period of time as that during which the solenoid is energized and the head
is thus cleaned by the operation of the roller 2A. By detecting the
reproduction signals again, if the value of the reproduction signals is
lower than a predetermined value, the above-described cleaning operation
will be repeated until the value of the signals will become more than the
predetermined value, thus "clogging" condition may be eliminated.
It is to be understood that the above-described embodiment relates to a
time-lapse VTR, but similar effect may be obtained in application to a
business type VTR. It is also to be understood that according to the
above-described embodiment, although a solenoid 6 has been applied to the
driving part of the head cleaning mechanism, similar effect may be
obtained if such a mechanism as to receive electrical signals and operate
in response thereto will be used in place of the solenoid.
A second embodiment of the present invention which is adapted for use as an
intermittent magnetic recording apparatus will now be explained by
referring to FIGS. 8 through 10.
FIG. 8 is a block diagram showing the constitution of an apparatus for
detecting clogging of video heads according to an embodiment of the
present invention. In FIG. 8, numeral 100 designates the intermittent
magnetic recording apparatus, numeral 200 designates a recording and
reproduction command circuit, numeral 300 designates a reproduction
envelope comparison circuit, numeral 400 designates a clogging output
circuit and numeral 500 designates an output terminal for clogging
signals.
The intermittent magnetic recording apparatus 100 is constituted in the
following manner. Specifically, referring to FIG. 8, numeral 115
designates a rotary drum having video heads 116A and 116B mounted on the
circumference thereof. Numeral 124 designates a drum motor which is
controlled by a drum controller 125 to synchronize with the vertical
synchronizing signals spaced from the image signals in respect of the
frequency and phase. Numeral 120 designates a capstan shaft which is
driven by a capstan motor 121 to run a magnetic tape 119. Numeral 122 is a
capstan controller which is adapted to intermittently drive the capstan
motor 121 in accordance with the intermittent driving command signals
output from a intermittent driving command circuit 123 and controls
movement of the magnetic tape 119 at a predetermined travelling pitch.
Numeral 111 designates an input terminal for the image signals, numeral 112
designates a processing circuit for the recording signals, numeral 113
designates a recording head amplifier, numerals 117A and 117B designate a
reproduction head amplifier, numeral 114A and 114B designate head
change-over switches adapted to change over the heads between the
recording side (hereinafter referred to as REC) and the reproducing side
(hereinafter referred to as PB), wherein the image signals input from the
input terminal 111 are converted in the recording signal processing
circuit (112) to FM signals which are necessary for magnetic recording and
are then supplied to the video heads 116A, 116B through the recording head
amplifier (113) and the video head change-over switches 114A, 114B.
Numeral 118 designates a reproducing change-over switch which is adapted to
receive commands from the drum controller 125 and to be synchronized so
that the video heads 116A, 116B track the magnetic tape 119.
The operation of the apparatus with the constitution described above will
next be explained.
The capstan motor 121 is designed to be intermittently driven in accordance
with a speed pattern of a trapezoidal wave such as that shown in FIG.
9(b), i.e., acceleration-constant speed-deceleration, and the speed
pattern differs depending on different control modes of the intermittent
drive.
The timing chart in FIG. 9(c) shows the supply of signal current for
recording signals to the video heads 116A, 116B. It is to be understood
that there are two types of timing; the timing immediately before and
after the rotation speed of the capstan motor 121 has reached its maximum,
and the timing immediately after the capstan motor 121 has finished its
intermittent movement and stopped. In the case of the former, since it is
not necessary to provide any significant head step at the video heads
116A, 116B, two-hour mode recording which is a normal type of continuous
recording may be conducted with these video heads 116A, 116B, in which
case, however, it is necessary for the speed pattern of the capstan motor
121 to be constant.
In the case of the latter type of timing, i.e. still recording, the head
step of the video heads 116A, 116B has to conform to the specified width
of the recording track, such as 58 .mu.m in the case of two-hour mode
recording with the NTSC standard of the VHS system, in order to allow
intermittent recording to be performed for one frame. Consequently, since
two-hour mode recording which is the normal type of continuous recording
has a considerable head step which does not permit intermittent recording
to be performed, it is necessary to provide another set of video heads at
the rotary drum for exclusive use during two-hour mode recording. However,
in this case the speed pattern of the capstan motor 121 may be controlled
roughly.
The intermittent recording current supplied to the video heads 116A, 116B
is classified as being of two kinds, i.e., the frame recording type in
which intermittent supply of the current is for a period equivalent to one
frame of the image signal or for one rotation of the rotary drum 115, and
the field recording type in which the intermittent current supply is for
one field of the image signal or one-half rotation of the rotary drum 115.
The above operation of an intermittent magnetic recording apparatus 100 is
similar to prior arts, and the head change-over switch 114A, 114B has been
fixed at the REC side in the prior arts.
It is to be noted here that, according to the constitution shown in FIG. 8,
in the course of intermittent recording, intermittent drive command
signals such as those shown in FIG. 9(a) are supplied to the
recording/reproducing command circuit 200 from the intermittent drive
command circuit 123. When the intermittent command signals (a) are
supplied in this way to the recording/reproducing command circuit 200, the
recording command signals shown in FIG. 9(d) are applied to the recording
head amplifier 113 which is thus caused to start operating. These
recording command signals (d) are applied at a certain time (t.sub.1)
before the recording signals shown in FIG. 9(c) so that they avoid the
transitionary state of the recording head amplifier during start-up.
Concurrently, the head change-over switches 114A, 114B are changed to the
REC side by the head change-over signals output from the
recording/reproducing command circuit 200.
If the intermittent magnetic recording apparatus 100 is of the frame
recording type, a recording current equivalent to one frame of the image
signals is simultaneously supplied to the video heads 116A, 116B and an
image signal equivalent to one frame is recorded on the magnetic tape 119.
On the other hand, if the recording apparatus is of the field recording
type, the recording current are supplied simultaneously to the video heads
116A, 116B for a period equivalent to one field during which the magnetic
tape 119 is traced by the designated one of the video heads 116A, 116B,
and the image signals equivalent to one field will thus be recorded on the
magnetic tape 119.
When the intermittent recording explained above is over, the reproduction
command signals as shown in FIG. 9(e) are supplied immediately to the
reproduction head amplifiers 117A, 118B from the recording/reproducing
command circuit 20 and the recording head amplifier 113 is shut off, so
that the head change-over switches 114A, 114B are changed over to the PB
side.
The above operation during reproduction is regarded as an intermittent
recording operation if viewed from the perspective of the entire
intermittent magnetic recording apparatus 100. Namely, while intermittent
recording is not being executed, only the video heads 116A, 116B, the head
change-over switches 114A, 114B and the reproduction head amplifiers 117A,
118B are forced to the side of reproducing during the intervals between
respective intermittent recording operations.
The signals from the video heads 116A, 116B are amplified by the
reproduction head amplifiers 117A, 118B and are selectively switched by
the reproduction change-over switch 118 to provide the reproduction
signals shown in FIG. 9(f) which are then fed to the reproduction envelope
comparison circuit 300 in the next step. The timing for the switching of
the reproduction change-over switch 118 is controlled by the drum
controller 125 and synchronized with the timing of the video heads 116A,
116B which are tracing the magnetic tape 119.
The relationship between the wave form of the envelope of the reproduction
signals (f) (hereinafter referred to as "reproduction envelope") which are
FM signals and the type of recording conducted by the intermittent
magnetic recording apparatus 100, which is either of the moving record
type or the still record type, and also either of the frame record type or
the field record type, will now be explained.
Firstly, in the case of the moving record type of recording, since the
reproduction track angle is smaller than the recording track angle, the
relevant reproduction envelope will present a triangular wave form which
is ramping as shown in FIG. 9(f).
In the case of the still record type of recording, since the recording
track angle and the reproduction track angle are the same, the
reproduction envelope presents a simple flat wave form.
Further, in the case of both the frame record and moving record types of
recording, since the magnetic tape 119 is caused to run forward a little,
the reproduction envelope of the recording track which has been recorded
earlier at either of the video heads 116A, 116B becomes proportionally
smaller than that of the recording track which has been recorded later.
The fraction of the earlier reproduction envelope that is proportionally
smaller is inversely proportional to the running speed. With regard to the
still record type of recording in which the recording tape 119 is stopped
during recording, since the respective video heads 116A, 116B will trace
the same track during both recording and reproduction, a satisfactory
reproduction envelope can be provided.
In the case of the field record type too, since the video head which has
been used for recording may also be used for tracing a track at the time
of reproduction, a satisfactory reproduction envelope can be attained.
As explained, the reproduction envelopes may differ depending on the type
of recording. In any of the types of recording, a sufficient amount of the
reproduction envelope to be used for the detection of clogging at the
video heads may be applied to the reproduction envelope comparison circuit
300. It is to be noted, however, that, as explained above, in the cases of
the frame record and moving record types of recording, as far as the video
head which has recorded earlier is concerned, when the speed of the tape
is increased above a certain level at the time of recording, the
reproduction envelope may drop and thus may not be distinguishable from
clogging.
The reproduction envelope which has been applied to the reproduction
envelope comparison circuit 300 is compared to a predetermined value of
envelope clogging and, depending on the result of such comparison,
comparison signals such as those shown in FIG. 9(g) may be provided by the
circuit 300.
The reproduction envelope which occurs when the video heads 116A, 116B
become clogged will next be explained. Since the amplified gains of the
reproduction head amplifiers 117A, 118B are normally designed to be
approximately 200 dB, the output of the amplifiers will be approximately
200 mV if the reproduction signals detected by the video heads 116A, 116B
are supposed to be 200 .mu.V.
FIG. 10 is a graph showing the relationship between the S/N ratio of the
image signals which are demodulated to the base bands and the reproduction
envelope which has decreased due to clogging. As can be seen from this
graph, a good video head suffering from no clogging maintains a value of
more than 200 mvpp. When clogging occurs and the reproduction envelope
falls to about 100 mvpp, a relatively fair picture quality can still be
maintained. If the reproduction envelope falls to less than 100 mvpp,
however, the picture quality will rapidly deteriorate due to the clogging.
It will be understood from the foregoing explanation that clogging may be
suitably detected by setting the value of envelope clogging (m) at a value
lower than one half of the reproduction envelope of a clean video head.
As the reproduction signals comprise FM signals of 3-5 MHz band, one
example of the constitution of the reproduction envelope comparison
circuit 300 is designed to have a band amplifier adapted to amplify
signals in the band that are more than 20 dB and an AM detector using
diodes as means of detection, whereby the reproduction signals may be
amplified and AM-detected thereafter. In this way the reproduction
envelope can be provided. The circuit 300 also includes a comparator using
an envelope clogging value (m') as a reference for comparison. The
comparator may provide comparison signals as shown in FIG. 9(g) as outputs
thereof. It is necessary to multiply the envelope clogging value (m) shown
in FIG. 10 by the gain value of the band amplifier in order to obtain the
reference envelope clogging value (m') of the comparator.
Another example of the constitution of the reproduction envelope comparison
circuit 300 is so designed that the reproduction FM signals are directly
passed through a comparator when the FM signals input to the circuit 300
have a value above the envelope clogging value (m) which is set as a
comparison reference value and these signals output from the comparator
are directed through the low pass filter comprising resistor(s) and
capacitor(s), whereby comparison signals are provided as shown in FIG.
9(g).
Thus, the comparison signals (g) provided by the reproduction envelope
comparison circuit 300 are applied to the clogging output circuit 400 in
the next stage. The role of the clogging output circuit 400 is as follows.
Specifically, the reproduction envelopes have wave forms of a triangular
configuration or somewhat uneven flat wave forms, so that constant
comparison outputs cannot necessarily be provided for the entire period of
a field. At the final part of one field, if any comparison outputs are
obtained in such a field, it is then concluded that there has been no
clogging in this field. On the other hand, if no comparison outputs have
been provided in the entire period of one field, it is concluded that
there has been some clogging in the field. Further, in order to eliminate
malfunction of the circuit due to noises and to positively detect the
presence of any clogging, a circuit designed to determine clogging only if
clogging is detected successively in several fields may be incorporated
after or within the circuit 400.
The constitution of the clogging output circuit 400 as explained above may
be readily provided in combination with flip-flops.
The clogging signals shown in FIG. 9(h) which are the final outputs may be
output from the clogging output circuit 400 and appear at the clogging
signal output terminal 500 as explained above.
As seen from FIG. 9, while the intermittent recording is stopped, the video
heads 116A, 116B are changed to the reproduction mode in order to detect
whether there is any clogging or not. When the intermittent drive command
signal is applied in the next cycle as denoted by FIG. 9(a), the video
heads 116A, 116B are changed again to the recording mode so that
intermittent recording may be executed. Subsequently, similar cycles are
repeated whereby intermittent recording and detection of clogging may be
repeated alternately.
It is to be noted that according to the second embodiment described above,
the video heads 116A, 116B comprise a so-called two-head system wherein
the video head which has been used for intermittent recording will be
changed to the reproduction mode in order to determine whether there is
any clogging or not. It is also possible to provide a so-called three- or
four-head system comprising a composite video head wherein the video head
for exclusive of reproduction use is used for clogging detection, so that
clogging may be checked during both recording and reproduction.
Incidentally. It is to be noted that any clogging detected may be
automatically cleaned by a suitable method like that employed in the first
embodiment.
As explained above, according to the present invention, when "clogging"
occurs at the heads due to any change in environmental conditions or
certain problems with the magnetic tape, the head and/or its driving
system, the clogging is automatically and immediately detected. The head
may thus be cleaned rapidly as soon the clogging is detected, so that
stable image recording may be accomplished even if recording is continued
for long periods of time in a time-lapse VTR or the like.
While preferred embodiments have been described, variations thereof will
occur to those skilled in the art within the scope of the present
invention concepts which are delineated by the following claims.
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