Vertical deflection circuit for a camera tube in a television camera

A vertical deflection circuit is used in a camera tube in a color television camera in which the camera tube is provided with an optical filter having an optically black part extending in parallel with a beam scanning direction of the camera tube. A vertical deflection circuit comprises a circuit for generating a saw-tooth voltage for vertical deflection, a vertical deflection coil for the camera tube, and a circuit for supplying the saw-tooth voltage thus generated to one end of the vertical deflection coil in DC coupling. Due to temperature fluctuation, a vertical deflection width fluctuates with respect to a position corresponding to a black part of the filter, as a reference.

BACKGROUND OF THE INVENTION 
The present invention relates generally to vertical deflection circuits for 
camera tubes in television cameras, and more particularly to a circuit for 
carrying out vertical deflection so as to carry out DC restoration of 
black level of a video signal irrespective of amplitude fluctuations of a 
vertical deflection voltage. 
In general, a dark current exists in the camera tube of a color television 
camera. This dark current is not continually constant but fluctuates or 
becomes irregular due to temperature variation. When there is a 
fluctuation in this dark current, the white balance is destroyed. 
Particularly in a frequency separation system in which an optical stripe 
filter is used, the green color signal transmitted at low frequency is 
directly influenced by the dark current. 
Heretofore, in color television cameras of the single-tube or two-tube type 
of a color multiplex system in which vidicon camera tubes are used as 
camera tubes for chrominance signals, optical filters provided with 
optically black parts vertically at the end portions in the horizontal 
scanning direction of the effective picture have been used. A video signal 
obtained from a camera tube by using an optical filter of this character 
has a black level portion produced by the optically black part in the 
trailing edge of the horizontal beam blanking of each horizontal scanning 
period. Heretofore, correction for black level fluctuation due to dark 
current fluctuation has been carried out by clamping this black level 
portion, whereby the DC restoration has been carried out. 
However, when there is a flaw or damage in the optically black part of the 
optical filter, or when there is a flaw in the photoconductive film or 
nesa film of the camera tube corresponding to this black part, a signal of 
high level in pulse form is generated in the signal part corresponding to 
the optically black part in the image pickup signal. In the above 
mentioned known system, however, when the unwanted pulse signal due to a 
flaw is generated, clamping occurs with this unwanted signal as a 
reference, and accurate black level clamping cannot be carried out. 
Furthermore, the dark current level, in general, is not uniform over the 
entire photoconductive surface of the camera tube in the horizontal 
scanning direction but is higher at the two end parts than the central 
part. (This level distribution will hereinafter be referred to as "dark 
current shading".) By the above mentioned known system, the effect of this 
dark current shading could not be reduced. 
Another example of a color television camera is that wherein use is made of 
an optical filter comprising a color stripe filter provided on the upper 
or lower portion thereof laterally with an optically black part. In this 
color television camera, the DC restoration is generally carried out by 
clamping a black level output signal part in the output signal of the 
camera tube by a clamping pulse. 
When there exists a variation in environmental temperature, the amplitude 
of the saw-tooth voltage to be applied to a vertical deflection coil 
undergoes change, whereby the vertical deflection width thus changes. 
A vertical deflection circuit known heretofore has been arranged so that a 
saw-tooth voltage for vertical deflection is supplied by way of a 
capacitor to a vertical deflection coil. In this known circuit, the 
average level of the saw-tooth voltage is adapted to correspond to the 
center of vertical deflection in the camera tube, and deflection is 
carried out upward and downward with respect to the above described 
center. Accordingly, as will be described in conjunction with drawings 
hereinafter, when the amplitude of the saw-tooth voltage fluctuates due to 
causes such a temperature fluctuation, there occurs fluctuation in a 
position of the black level output signal part corresponding to the black 
part of the optical filter, among the output signal of the camera tube. 
The clamping pulse generated from an electrical circuitry for clamping the 
output black level signal for DC restoration has a timing for clamping the 
black level in a case where the saw-tooth voltage has a normal amplitude 
thereby carrying out vertical deflection normally, and accordingly the 
output black level signal is positioned at the normal position. 
Heretofore, position of the output black level signal undergoes 
fluctuation due to causes such a temperature fluctuation, as described 
above. For this reason, there give rise to difficulties that clamping 
cannot be carried out appropriately, which results in improper DC 
restoration of black level. 
For getting rid of the above described effect of dark current shading and 
for causing the black level signal not to appear in the final video 
signal, it is required to narrow the width of black part of the optical 
filter, thus shortening a period of time when the output black level 
signal exists. However, this arrangement involves difficulty that the 
accurate DC restoration of the black level cannot carried out under even 
slightest position fluctuation, because the period of time when the output 
black level signal exists becomes short. 
Accordingly, a manner for correcting electrically the amplitude of the 
saw-tooth voltage so that it does not fluctuates irrespective of 
temperature fluctuation may be conceivable. However, a circuit arrangement 
according to this manner will be extremely complex, whereby the television 
camera becomes expensive and bulky. Thus, the above manner is not suitable 
for potable type television cameras which are preferred to be inexpensive 
and of small size. 
SUMMARY OF THE INVENTION 
Accordingly, it is a general object of the present invention to provide a 
novel and useful vertical deflection circuit for a camera tube in a 
television camera in which the above described difficulties encountered in 
the prior art have been overcome. 
Another and specific object of the invention is to provide a vertical 
deflection circuit for a camera tube in a television camera which is 
adapted to supply saw-tooth voltage from a saw-tooth voltage generation 
circuit to a vertical deflection coil in DC coupling, without passing 
through a capacitor. According to the circuit of the present invention, 
vertical deflection is carried out with respect to, as a reference, the 
black part of the optical filter. In this connection, even if the vertical 
deflection width fluctuates due to the temperature fluctuation, the output 
black level signal derived in correspondance with the black part of the 
optical filter among the output signal of the camera tube will be always 
located at an appropriate position. Accordingly, the DC restoration of the 
black level part can be always accurately and positively accomplished. 
Other objects and further features of the invention will be apparent from 
the following detailed description when read in conjunction with the 
accompanying drawings.

DETAILED DESCRIPTION 
One example of an optical filter used in a color television camera in which 
the vertical deflection circuit according to the present invention can be 
used is shown in FIG. 1. This optical filter 10 comprises a glass plate 11 
provided thereon with an optical color stripe filter 12 for color 
multiplexing and an optically black part 13 extending laterally. Light 
from an object to be image-picked up is passed through this optical filter 
and projected onto the photoconductive surface of a camera tube. As a 
result of beam scanning of this photoconductive surface by the camera 
tube, a camera tube output signal (video signal) as indicated in FIG. 2(C) 
through FIG. 2 (H) is led out through the signal electrodes of the camera 
tube. In FIG. 2(A), the interval 1V indicates one vertical scanning period 
between vertical beam blankings 15 and 15. 
One embodiment of a vertical deflection circuit according to the present 
invention is shown in FIG. 4. A vertical driving pulse indicated in FIG. 
3(A) is supplied to an input terminal 20, and is then applied through a 
capacitor C1 to the base of an NPN transistor Q1. The transistor Q1 has an 
emitter which is grounded and a collector which is connected to a terminal 
22 of the power source +Vcc through a resistor R2. A resistor R1 is 
connected between the base of the transistor Q1 and the ground. Moreover, 
a capacitor C2 is connected between the collector of the transistor Q1 and 
the earth. 
The transistor Q1 carries out switching operation in response to the 
driving pulse applied to the base thereof. During period of time when the 
transistor Q1 assumes its OFF state, a current flows through the resistor 
R2 to the capacitor C2 whereby the capacitor C2 is charged. While, during 
a period of time when the transistor Q1 assumes its ON state, the charges 
stored in the capacitor C2 are discharged through the transistor Q1. 
Accordingly, at a junction point between the collector of the transistor 
Q1 and the capacitor C2, is obtained a saw-tooth voltage indicated in FIG. 
3(B). 
In the known vertical deflection circuit, the saw-tooth voltage generated 
in the well-known saw-tooth voltage generation circuit as described above 
has been fed through the coupling capacitor to a vertical deflection coil. 
The average level of the saw-tooth voltage has been set so as to 
correspond to a center of the vertical deflection width, whereby the 
vertical deflection is carried out upward or downward with respect to the 
center thus set. Accordingly, when the vertical deflection is being 
carried out with an appropriate deflection width A1, as indicated in FIG. 
1, there occurs no problem. However, the deflection width undergoes charge 
due to temperature fluctuation, as referred to above. For example, the 
vertical deflection is carried out over a width of A2, when the deflection 
width is smaller than the normal width A1, and over a width of A3, when 
the deflection width is larger than the normal width A1. 
Accordingly, output signal of the camera tube of the television camera 
provided with the above described known circuit has a waveform indicated 
in FIG. 2(C) when the deflection width is A1, a waveform indicated in FIG. 
2(D) for the deflection width A2, and a waveform indicated in FIG. 2(E) 
for the deflection width A3. In FIGS. 2(C), 2(D), and 2(E), reference 
numerals 17a, 17b, and 17c denote respectively the output black level 
signal parts obtained in responsive correspondence with the black part 13 
of the optical filter 10, and reference numerals 18a, 18b, and 18c denote 
respectively the video signal parts obtained in responsive correspondence 
with the stripe filter 12. As apparent from each drawing, in response to 
variation of the deflection width, the video signal period width undergoes 
variation, and futhermore the position of the output black level signal 
fluctuates. 
Accordingly, the clamp pulses 16 (FIG. 2(B)) for clamping the output black 
level signal thereby carrying out DC restoration of the black level are 
generated so as to be located at the positions on the time axis 
corresponding to the positions of the output black level signal 17a on the 
time axis. The signal 17a is obtained when the deflection width is the 
normal width A1. 
Accordingly, the circuit known heretofore has been involved with 
difficulties that, while the output black level signal 17a at the normal 
position is clamped positively by the clamping pulse 16 thereby rendered 
DC restoration, the black level signals 17b and 17c which are present at 
positions deviated from the normal position of the signal 17a are not 
clamped normally, whereby the DC restoration is not carried out normally. 
The circuit of the present invention has overcome the above described 
difficulties encountered in the above circuits. In a circuit of the 
present invention, the saw-tooth voltage generated as described 
hereinbefore is applied to the non-inverting input terminal of an 
operational amplifier 22. The resulting output voltage of the operational 
amplifier 22 is directly applied to one terminal a of a vertical 
deflection coil 23, without passing through any capacitor. The other 
terminal b of the vertical deflection coil 23 is connected to the 
inverting input terminal of the operational amplifier 22. 
Here, the saw-tooth voltage which has been generated, as indicated in FIG. 
3(B), in the saw-tooth voltage is now considered. In the above saw-tooth 
voltage, the voltage at points t1, t2, . . . of the termination of 
discharging (or at the initiating points of charging) is always constant, 
and the voltage (or the wave height value) at the charging termination 
undergoes fluctuation due to causes such a temperature fluctuation. 
Accordingly, it can be considered that the voltage at the points t1, t2, . 
. . of the saw-tooth voltage has been originally restored of DC. 
According to the circuit of the present invention, when the deflection 
width fluctuation arises due to causes such a temperature fluctuation, the 
vertical deflection is carried out in a range of a width A4 for the 
deflection width smaller than the normal width A1, and in another range of 
a width A5 for the deflection width larger than the normal width A1. 
Specifically, even if the deflection width changes, the one end of the 
deflection width is kept to be held at the position corresponding to the 
black part 13 of the optical filter 10. The deflection width changes only 
accompanied with shift of the position of the other end of the deflection 
width. 
Accordingly, output signals of the camera tube in the television camera 
provided with the circuit of the present invention have a waveform as 
indicated in FIG. 2(F) for the deflection width A1 (which waveform is the 
same as that in FIG. 2(C)), and a waveform as indicated in FIG. 2(G) for 
the deflection width A4, and a still another waveform as indicated in FIG. 
2(H) for the deflection width A5. In FIGS. 2(F), 2(G), and 2(H), the 
reference numerals 17a denote the black level signal part obtained in 
responsive correspondence with the black part 13 of the optical filter 10, 
and reference numerals 18a, 18d, and 18e respectively denote the video 
signal parts obtained in responsive correspondence with the stripe filter 
12. 
As apparent from each drawing, whereas the video signal period width 
changes in response to fluctuation of the deflection width, the position 
of the black level signal 17a is always kept constant and is located at 
normal position corresponding to the clamping pulse 16. 
Therefore, according to the circuit of the present invention, even if there 
arises fluctuation in the vertical deflection width, the black level of 
the output signal of the camera tube will be accurately clamped all the 
time, whereby the DC restoration is always carried out accurately. 
Referring to FIG. 4, a reference DC voltage source 24 is connected through 
a resistor R3 to the terminal b of the vertical deflection coil 23. This 
reference DC voltage source 24 is adjusted and set so that a reference DC 
voltage thus adjusted is supplied to vertical deflection circuit 23, 
whereby the one end of the above described deflection width corresponds to 
the black part 13 of the optical filter 10. 
FIG. 5 shows another embodiment of the vertical deflection circuit of the 
present invention. Parts in FIG. 5 which correspond to parts in FIG. 4 are 
designated by like reference numerals. Detailed description of such parts 
will not be repeated. 
In general, the waveform of the saw-tooth voltage generated by the 
saw-tooth voltage generation circuit indicated in FIG. 4 is curved due to 
a charging and discharging characteristic of the capacitor, and has a poor 
linearity. The present embodiment is adapted to improve the linearity of 
the waveform of the saw-tooth voltage. 
A diode D is connected and interposed between the power source terminal 21 
and the resistor R2. A capacitor C3 is connected between a junction point 
of the diode D and the resistor R2, and the inversion input terminal of 
the operational amplifier 22 and the terminal b of the vertical deflection 
coil 23. The diode D and the capacitor C3 constitutes a bootstrap circuit. 
By this bootstrap circuit, a curve of an inclined part of the waveform of 
the saw-tooth voltage (especially, a curved part at the top of the 
waveform indicated in FIG. 3(B)) is corrected to be linear, whereby 
linearity of the saw-tooth voltage waveform is improved. 
Further, this invention is not limited to these embodiments but various 
variations and modifications may be made without departing from the scope 
of the invention.