Circuit used in a camcorder for monitoring the recorded state of a tape during recording by simultaneous playing back of the tape

A circuit used in a camcorder for monitoring the recorded state of a tape during recording by simultaneous playing back of the tape comprising first and second record/playback heads mounted on a head and drum assembly for performing a record and playback of the tape, a second amplifier for amplifying the signals of the first and second heads received via switches to produce chrominance record signals, third and fourth playback only heads mounted on the head and drum assembly between the first and second heads with a given interval for exclusively performing the simultaneous playing back, first and third amplifiers connected with the third and fourth heads via switches for amplifying the video signals picked up by the third and fourth heads, third and fourth switches for respectively connecting the signals of the third and fourth heads to the first and third amplifiers, or the output signals of the first and second heads selected by first and second switches to a second amplifier, and a head switching means for selectively switching the output signals of the first and third amplifiers.

FIELD OF THE INVENTION 
The present invention concerns a circuit used in a camcorder for monitoring 
the recorded state of a tape during recording by simultaneous playing back 
of the tape in order to prevent possible recording errors. 
TECHNICAL BACKGROUND 
In a conventional camcorder, the recorded state of a tape may be checked 
out only by playing back of the tape. Namely, the recorded state of a tape 
may not possibly be checked out during recording. 
Referring to FIG. 1, the image signals originating from an object are 
transmitted through lens 20 to charge coupled devices (not shown) to 
convert the image signals into electrical signals that are processed by an 
image processor 22 to produce the image signals of EE mode through 
audio/video output terminals. In addition, the image signals of EE mode 
are monitored by an electronic viewfinder 23. 
In such a conventional camcorder, it is impossible to check out during 
recording the noises that may be recorded on the tape together with the 
desired signals, caused by foreign matters attached on the record head or 
others, because the electronic viewfinder only provides the visual images 
directly taken from the object. Namely, it is impossible to check out the 
recorded state of the tape during recording. In order to check out the 
noises recorded on the tape, the recording mode of the camcorder is 
temporarily stopped, and the reviewing mode is performed. However, this 
may cause the failure of an important record timing, and does not resolve 
the record troubles caused by foreign matters attached on the record head 
after the reviewing of the tape. 
As shown in FIGS. 2 and 3, when the tape wound around the head drum with an 
envelope angle of 180.degree.+.theta..degree. runs from left to right with 
magnets sensing positions to produce head switching signals, the record 
head Ha and the playback head Hb alternately and sequentially scan the 
recorded track from the first track "a" and the 14th track "n" to play 
back the recorded image signals. Of course, the playback of the recorded 
image signals is possibly performed only after rewinding the recorded 
tape. 
Thus, the conventional camcorder does not have means for checking out the 
recorded state of the tape without stopping the recording operation, so 
that it is impossible to immediately prevent recording errors occurring 
during the recording operation. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide means for checking out 
the recorded state of a tape without stopping the recording operation of a 
camcorder. 
It is another object of the present invention to provide means for 
immediately preventing the recording errors occurring during the recording 
operation of a camcorder caused by the foreign matters attached on the 
head, etc. 
According to the present invention, a camcorder for monitoring the recorded 
state of a tape during recording by simultaneous playing back of the tape 
comprises first and second record/playback heads for performing a record 
and playback of the tape, third and fourth heads arranged between the 
first and second heads with a given interval for exclusively performing 
the simultaneous playing back, first and second magnet means for 
generating the switching signals of the third and fourth heads, whereby 
playing back the tape occurs by the third and fourth heads simultaneously 
with the recording. 
The present invention will now be described more specifically with 
reference to the drawings attached only by way of example.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 4, a circuit used in a camcorder for monitoring the 
recorded state of the tape during recording by simultaneous playing back 
of the tape comprises first and second record/playback heads H1 and H2 
mounted on a head and drum assembly for performing a record and playback 
of the tape, and a third and fourth playback only heads H1 and H2 mounted 
on the head and drum assembly between the first and second heads with an 
interval of 90.degree. for exclusively performing the simultaneous playing 
back. 
A first and second switches SW1 and SW2 is to connect the signals of the 
first and second heads to a first, second and third amplifiers 1, 2 and 3. 
The image signals picked up by the third and fourth heads H3 and H4 are 
delivered through third and fourth switches SW3 and SW4 to the first and 
third amplifiers 1 and 3. A head switching means 4 selectively switches 
the output signals of the first and third amplifiers 1 and 3 so as to 
produce playback high frequency signals. 
In addition, there are provided first and second magnets MG1 and MG2 
arranged with a given interval from the first and second heads H1 and H2, 
as shown in FIG. 9. The magnets are to generate the head switching signals 
that are amplified by fifth and sixth amplifiers 5 and 6. A first and 
second waveform shaping circuits 7 and 8 are to shape the output signals 
of the fifth and sixth amplifiers 5 and 6. The first and second 
mono-stable oscillators 9 and 10 are to convert the output signals of the 
waveform shaping circuits 7 and 8 into pulses delayed by a given time. A 
flip-flop circuit 13 is to convert the output signals of the mono-stable 
oscillators 9 and 10 to head switching pulses and to produce the playback 
high frequency signals through a seventh switching means SW7 receiving the 
outputs of the first and second record/playback heads H1 and H2. The 
mono-stable oscillators 9 and 10 respectively have first and second delay 
circuits 11 and 12. 
In operation, the first and second magnets MG1 and MG2 produce the head 
switching delay signals for delaying the head switching signals. Further, 
the first and second magnets MG1 and MG2 produce the switching signals for 
indicating the positions of the third and fourth heads H3 and H4 relative 
to the first and second record/playback only heads H1 and H2. In this 
case, the third and fourth heads H3 and H4 are respectively mounted in 
angular positions of +.pi./2 (90.degree.) and -.pi./2 (-90.degree.) 
between the first and second heads H1 and H2. The head drum revolution 
phase signals from the first and second magnets MG1 and MG2 are 
respectively amplified by the fifth and sixth amplifiers 5 and 6, whose 
outputs are respectively shaped by the first and second waveform shaping 
circuits 7 and 8. 
The output signals of the fifth and sixth amplifiers 5 and 6 and the first 
and second waveform shaping circuits 7 and 8 are as shown in FIGS. 7B, 7C, 
7D and 7E. 
The revolution phase signals of the first and second MG1 and MG2 shaped by 
the first and second waveform shaping circuits 7 and 8 are converted into 
pulses delayed by t1 selected by the first and second delay circuits 11 
and 12 connected with the first and second mono-stable oscillators 9 and 
10, as shown in FIGS. 7F and 7G. 
The signals as shown by FIGS. 7H and 7K produce the playback high frequency 
signals through the seventh switching means SW7 receiving the output 
signals of the first and second heads H1 and H2. 
The pulses delayed by t1 generates head switching pulse signals as shown in 
FIG. 7H used in the flip-flop circuit 13 in playback, switching the output 
signals of the first and third amplifiers 1 and 3 scanned by the first and 
second record/playback heads H1 and H2. 
In order to perform the playing back operation simultaneously with the 
recording operation, the delay time constants connected to the first and 
second mono-stable oscillators 9 and 10 are switched delayed to 8.33 msec 
(t2) as shown in FIGS. 7I and 7J of the control timing for switching the 
signals of the first and third amplifiers 1 and 3 that is the output of 
the flip-flop circuit 13, so that there is obtained a switching operation 
delayed by 90.degree. (.pi./2) as shown by FIG. 7K. 
Thus, as shown in FIG. 8, the A and B record tracks are sequentially played 
back by the third and fourth heads H3 and H4 during the first and second 
record/playback heads H1 and H2 sequentially performing recording 
operation. 
When the first head H1 has performed the recordation by 1/2 "A" track (or 
1/2 odd field), the fourth head H4 plays back the "A" track on the 
electronic viewfinder with the delay of 1/2 field time (1/60 sec.). 
Meanwhile, when the second head H2 has performed the recordation by 1/2 "B" 
track (or 1/2 even field), the third head H3 plays back the "B" track on 
the electronic viewfinder with the delay of 1/2 field time (1/60 sec.). 
The delay time constants connected with the first and second mono-stable 
oscillators 9 and 10 are switched by the fifth and sixth switch means SW5 
and SW6 of the delay circuits 11 and 12, so as to delay by 8.33 msec the 
timing for controlling the switching signals of the first and third 
amplifiers 1 and 3 outputted from the flip-flop circuit 13, thus 
performing switching operation delayed by 90.degree.. In this case, the 
delay times t1 and t2 according to the delay time constants are determined 
by the resistors R1-R4 and capacitors C1 and C2 of the first and second 
delay circuits 11 and 12. 
Namely, the time constants for the normal playing back of the recorded tape 
are determined by the resistor R1 and capacitor C1 of the first delay 
circuit 11 and the resistor R3 and capacitor C2 of the second delay 
circuit 12 to adjust the switching times of the heads H1 and H2, while the 
time constants for the simultaneous playing back during the recording are 
determined by the resistor R2 and capacitor C1 of the first delay circuit 
11 and the resistor R4 and capacitor C2 of the second delay circuit 12 to 
adjust the switching times of the third and fourth heads H3 and H4. 
As stated above, the inventive circuit provides means for monitoring the 
recorded state of the tape without stopping the recording operation, so as 
to immediately correct recording errors occurring during the recording. 
Although the invention has been described in conjunction with specific 
embodiments, it is evident that many alternatives and variations will be 
apparent to those skilled in the art in light of the foregoing 
description. Accordingly, the invention is intended to embrace all of the 
alternatives and variations that fall within the spirit and scope of the 
appended claims.