Circuit for processing image signals read out of pick-up apparatus having solid state image sensing devices adopting spatial pixel shift

A circuit for processing red, green and blue color signals read out of red, green and blue solid state image sensing devices, light receiving elements of the green solid state image sensing device are spatially shifted with respect to those of the red and blue solid state image sensing devices in a horizontal scanning direction by a half of a pitch of the arrangement of the light receiving elements, including red, green and blue channels, each having a correlation double sampling circuit, in which a color signal is sampled at feed-through and signal portions, thereof to derive two sample values and a difference between these sample values is derived as an output color signal. When the green color signal is sampled and held in the green channel, the sampling operation in the red and blue channels is inhibited by neutralizing sample values in these channels, and when the red and blue color signals are sampled and held in the red and blue channels, respectively, the sampling operation in the green channel is inhibited by neutralizing sample values. The frequency response of a brightness signal obtained by mixing the thus processed red, green and blue color signals does not become zero even if an object to be picked-up has a spatial frequency which is equal to the sampling frequency.

BACKGROUND OF THE INVENTION 
Field of the Invention and Description of the Related Art 
The present invention relates to a circuit for processing image signals 
read out of a pick-up apparatus having at least first and second solid 
state image sensing devices adopting a so-called spatial pixel shift, in 
which a number of light receiving elements of the first solid state image 
sensing device are spatially shifted with respect to light receiving 
elements of the second solid state image sensing device in a main scanning 
direction by a distance which is substantially equal to a half of a pitch 
of the arrangement of light receiving elements in the main scanning 
direction. 
In the solid state image sensing device, a number of light receiving 
elements are spatially arranged independently from each other, so that the 
spatial sampling is performed. According to the Nyquist's sampling 
theorem, a spatial frequency of an image of an object which can be 
reproduced by a single solid state image sensing device is limited to a 
frequency range up to f.sub.c /2, wherein f.sub.c is a horizontal clock 
frequency for reading signal charges stored in the solid state image 
sensing device. If a frequency range higher than f.sub.c /2 is to be 
obtained, there is produced a noise signal due to the fact that higher 
frequency components are folded back toward a lower frequency band. In a 
three-plate-type color television camera, in order to attain a higher 
resolution, a solid state image sensing device for obtaining a green color 
signal is spatially arranged with respect to the remaining two solid state 
image sensing devices for producing red and blue color signals such that 
light receiving elements of the green image sensing device are shifted 
with respect to light receiving elements of the red and blue image sensing 
devices in the horizontal scanning direction by a distant which is 
substantially equal to a half of a pitch of the arrangement of the 
elements in the horizontal scanning direction. Such a method is generally 
called a spatial pixel shift. 
FIGS. 1A to 1C show a known solid state image pick-up apparatus in which 
the above mentioned spatial pixel shift method is adopted. Light from an 
object is made incident upon a color separation optical system 2 by means 
of an objective lens 1 and is divided into red, green and blue light, 
which are then made incident upon respective solid state image sensing 
devices 3R, 3G and 3B. As illustrated in FIG. 1B, pixels of the green 
image sensing device 3G are shifted with respect to those of the red and 
blue image sensing devices 3R and 3B in the main scanning direction by a 
half of a pitch P at which the pixels are arranged in the main scanning 
direction. When such a spatial pixel shift method is utilized, the image 
of the object is spatially sampled such that the red and blue pixels are 
positioned between successive green pixels as depicted in FIG. 1C. 
Therefore, when a brightness signal is produced by mixing the red, green 
and blue color signals, the number of pixels is apparently increased, and 
thus the resolution of the brightness signal becomes higher and the false 
signal can be decreased. 
As explained above, in the spatial pixel shift method, the light receiving 
elements of the green image sensing device 3G are spatially shifted with 
respect to those of the red and blue image sensing devices 3R and 3B by 
P/2 in the main scanning direction. Therefore, prior to forming the 
brightness signal by mixing the color signals generated by these color 
image sensing devices, it is necessary to delay the green color signal 
with respect to the red and blue color signals by a time period 
corresponding to P/2, i.e. a half of a period of the signal reading clock 
in signal processors 4R, 4G and 4B. 
Now a method of effecting the above delay will be explained for a known 
correlation double sampling method which has been widely used to remove 
reset noise and amp-noise from an output signal generated by CCD (Charge 
Coupled Device) which has been commonly utilized as the solid state image 
sensing device FIG. 2 shows the construction of the correlation double 
sampling circuit An input color signal read out of a solid state image 
sensing device is parallelly supplied to first and second sample and hold 
circuits 5 and 6. Output signals of the first and second sample and hold 
circuits 5 and 6 are then supplied to a differential amplifier 7. 
FIG. 3A depicts the input color signal. To the first sample and hold 
circuit 5 is supplied a first sampling signal SH-1 shown in FIG. 3B and 
the input color signal is sampled at a feed-through portion thereof. To 
the second sample and hold circuit 6 is supplied a second sampling signal 
SH-2 illustrated in FIG. 3C and the input color signal is sampled at a 
signal portion thereof. In this manner, the input color signal is sampled 
and held by the first and second sampling signals SH-1 and SH-2 at 
suitable timings and then a difference between these sample values is 
derived by the differential amplifier 7. In this manner, it is possible to 
derive the output color signal having high S/N without being influenced by 
reset noise and amp-noise. 
FIG. 4 shows the construction of the known correlation double sampling 
circuit. The green color signal read out of the green image sensing device 
11G is amplified by a buffer amplifier 12G and is then parallelly supplied 
to first and second switches 13G and 14G. The first switch 13G is operated 
by the first sampling signal SH-1 shown in FIG. 3B and the green color 
signal generated from the green image sensing device 11G is sampled at the 
feed-through portion. The sampled signal is held in a first hold circuit 
15G. The second switch 14G is driven by the second sampling signal SH-2 
illustrated in FIG. 3C and the green color signal is sampled at the signal 
portion to derive a sample value which is stored in a second hold circuit 
16G. The sample value held in the first hold circuit 15G is transferred 
via buffer amplifier 17G and third switch 18G to a third hold circuit 19G, 
and is further supplied via a buffer amplifier 20G to one input of a 
differential amplifier 21G. The sample value stored in the second hold 
circuit 16G is supplied via a buffer amplifier 22G to the other input of 
the differential amplifier 21G. The second and third switches 14G and 18G 
are driven by the second sampling signal SH-2. Therefore, the green color 
signal read out of the green image sensing device 11G is sampled at the 
signal portion to derive a sample value and the thus derived sample value 
is stored in the second hold circuit 16G. At the same time, the sample 
value at the feed-through portion is stored in the third hold circuit 19G. 
In this manner, the sample values representing the signal levels of the 
green color signal sampled at the feed-through portion and signal portion 
are simultaneously supplied to the differential amplifier 21G, and thus 
the differential amplifier produces an output green signal. 
The red and blue color signals read out of the red and blue image sensing 
devices are processed in the entirely same manner, so that here only the 
processing circuit for the red color signal is shown in FIG. 4. In the red 
color signal processing circuit, portions similar to those of the green 
color signal processing circuit are denoted by the same reference numerals 
with R instead of G. As explained above, the light receiving elements of 
the red image sensing device 11R are spatially shifted with respect to 
those of the green image sensing device 11G in the main scanning direction 
by P/2, so that sampling timings for the red color signal have to be 
changed with respect to those for the green color signal. To this end, 
there are arranged switches 23R, 24R, hold circuits 25R, 26R and buffer 
amplifiers 27R, 28R. The switches 23R and 24R are driven by a third 
sampling signal SH-3 shown in FIG. 3D. The third sampling signal SH-3 is 
shifted with respect to the second sampling signal SH-2 by a half of the 
period of the clock pulses for reading the solid state image sensing 
devices. Therefore, the sample values stored in the hold circuits 16R and 
19R are transferred into the hold circuits 25R and 26R, respectively at 
the timing of the third sampling signal SH-3, and are then supplied to the 
differential amplifier 21R by means of the buffer amplifiers 28R and 27R, 
respectively. In this manner, from the differential amplifier 21R there is 
derived an output red color signal which has been delayed with respect to 
the green color signal by a half of the clock period. The thus processed 
color signals are then supplied to succeeding stages and there are 
produced the brightness signal and color difference signals in a usual 
manner. 
In the above mentioned known signal processing system, the frequency 
response of the brightness signal is decreased in proportion to the 
increase in the spatial frequency of the object, and when the spatial 
frequency becomes equal to the clock frequency for the solid state image 
sensing devices, the frequency response becomes zero. This will be further 
explained in detail with reference to FIGS. 5A to 5F. FIG. 5A illustrates 
the object having a regular repetition of bright portions and dark 
portions, and FIGS. 5B and 5C show the second and third sampling signals 
SH-2 and SH-3. When such an object is picked-up and the read out signal is 
processed by the known correlation double sampling circuit, each of the 
green and red signals lasts for a time interval between successive 
sampling points as shown in FIGS. 5D and 5E. Then, the frequency response 
of the brightness signal obtained by mixing the green, red and blue color 
signals with each other at a predetermined ratio becomes zero as shown in 
FIG. 5F. 
SUMMARY OF THE INVENTION 
The present invention has for its object to provide a novel and useful 
circuit for processing image signals read out of the pick-up apparatus 
including a plurality of solid state image sensing devices arranged by 
using the spatial pixel shift method, in which the frequency response does 
not become zero even when the spatial frequency of the object becomes 
comparable to the read out clock frequency, so that the maximum resolution 
of the brightness signal can be increased. 
According to the invention, a circuit for processing image signals read out 
of a pick-up apparatus which includes at least first and second solid 
state image sensing devices, each having a number of light receiving 
elements arranged in matrix, the light receiving elements of the first 
solid state image sensing device being spatially shifted with respect to 
those of the second solid state image sensing device in a main scanning 
direction by a distance substantially equal to a half of a pitch of the 
arrangement of the light receiving elements in the main scanning 
direction, comprises: 
a first sampling and holding means for sampling a first image signal read 
out of the first solid state image sensing device to derive sample values 
and holding the thus derived sample values of the first signal; 
a second sampling means for sampling a second image signal read out of the 
second solid state image sensing device to derive sample values and 
holding the thus derived sample values of the second signal; and 
inhibiting means for inhibiting the operation of said first sampling means 
for a time period from a spatial sampling point at which said second image 
signal read out of said second solid state image sensing device is sampled 
by said second sampling means to a spatial sampling point at which said 
first image signal is sampled by said first sampling means, and for 
inhibiting the operation of said second sampling means for a time period 
from a spatial sampling point at which said first image signal is sampled 
by said first sampling means to a spatial sampling point at which said 
second image signal is sampled by said second sampling means. 
According to the present invention, for a time period during which one of 
the first and second image signals is sampled, the sampling of the other 
image signal is inhibited, and thus when the brightness signal is derived 
by mixing the thus sampled signals with each other, the brightness signal 
reproduces the spatial frequency of the object faithfully and the 
resolution of the brightness signal becomes very high. 
It should be noted that the present invention is most suitably applied to a 
color television camera having red, green and blue solid state image 
sensing devices, but the invention can be equally applied to a 
monochromatic television camera having at least two solid state image 
sensing devices

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 6 is a circuit diagram showing a first embodiment of the signal 
processing circuit according to the invention. A green color signal read 
out of a green solid state image sensing device 41G is amplified by a 
buffer amplifier 42G and is then supplied to first and second switches 43G 
and 44G. The first switch 43G is driven by the first sampling signal SH-1 
shown in FIG. 3B, and the green color signal read out of the solid state 
image sensing device 41G is sampled at the feed-through portion and a 
sample value thus obtained is stored in a first hold circuit 45G. The 
second switch 44G is operated by the second sampling signal SH-2 
illustrated in FIG. 3C and a sample value is stored in a second hold 
circuit 46G. The sample value stored in the first hold circuit 45G is 
amplified by a buffer amplifier 47G and is then transferred via a third 
switch 48G to a third hold circuit 49G. The sample value held in the third 
hold circuit 49G is supplied via a buffer amplifier 50G to one input of a 
differential amplifier 51G. The sample value stored in the second hold 
circuit 46G is supplied via a buffer amplifier 52G to the other input of 
the differential amplifier 51G. The second and third switches 44G and 48G 
are driven by the second sampling signal SH-2 such that the green color 
signal read out of the solid state image sensing device 41G is sampled at 
the signal portion thereof and a sample value thus obtained is stored in 
the second hold circuit 46G. At the same time, the sample value obtained 
at the feed-through portion and stored in the first hold circuit 45G is 
transferred into the third hold circuit 49G and is stored therein. In this 
manner, to the inputs of the differential amplifier 51G are simultaneously 
supplied the first and second sample values obtained at the feed-through 
portion and signal portion of the input green color signal, respectively. 
In FIG. 6, the switches and hold circuits are shown separately, but in 
practice, they are formed by IC to constitute sample and hold circuits. In 
FIG. 6, respective sample and hold circuits are denoted by broken line 
blocks. 
In the present embodiment, input sides of the second and third hold 
circuits 46G and 49G can be selectively short-circuited by means of a 
switch 60G. When the switch 60G is closed, the signal charges stored in 
the second and third hold circuits 46G and 49G can be neutralized. That is 
to say, the switch 60G is driven by the third sampling signal SH-3 shown 
in FIG. 3D such that the signal charges stored in the hold circuits 46G 
and 49G are neutralized for a time period during which the third sampling 
signal SH-3 assumes a higher level, so that the output signal of the 
differential amplifier 51G becomes zero for time period from the raising 
edge of the third sampling signal SH-3 to the raising edge of the second 
sampling signal SH-2. 
Also in the present invention, the red and blue color signals are processed 
in the entirely same manner, so that only the signal processing circuit 
for the red color signal read out of a red image sensing device 41R is 
shown in FIG. 6. In the red channel, portions similar to those of the 
green channel are denoted by the same reference numerals with R instead of 
G. The light receiving elements of the green image sensing device 41G and 
those of the red image sensing device 41R are shifted with respect to each 
other by P/2 in order to perform the spatial pixel shift method, and 
therefore it is necessary to change the sampling timings for the red color 
signal with respect to those for the green color signal. To this end, 
there are provided fourth and fifth switches 53R and 54R, fourth and fifth 
hold circuits 55R and 56R and fourth and fifth buffer amplifiers 57R and 
58R. The fourth and fifth switches 53R and 54R are driven by the third 
sampling signal SH-3 which is shifted with respect to the second sampling 
signal SH-2 by a time which is equal to a half of a period of the clock 
pulse for reading the image signal out of the solid state image sensing 
device. Therefore, the signal charges stored in the second and third hold 
circuits 46R and 49R are transferred into the fourth and fifth hold 
circuits 55R and 56R at the timing of the third sampling signal SH-3 and 
are stored therein. Then the signal charges stored in the fourth and fifth 
hold circuits 55R and 56R are supplied via the fourth and fifth buffer 
amplifiers 57R and 58R, respectively to the differential amplifier 51R 
which derives a difference therebetween to produce an output red color 
signal. In this manner, from the differential amplifier 51R there is 
derived the output red color signal which is shifted with respect to the 
green signal by the time equal to a half of the reading out clock period. 
It should be noted that this time corresponds to a half the pitch P of the 
arrangement of the light receiving elements in the main scanning 
direction. 
In the present embodiment, between the input sides of the fourth and fifth 
hold circuits 55R and 56R is arranged a sixth switch 60R and this switch 
is driven by the second sampling signal SH-2. That is to say, the signal 
charges stored in the fourth and fifth hold circuits 55R and 56R are 
neutralized when the sixth switch 60R is closed and this condition is 
continued until the third sampling signal SH-3 is supplied. Therefore, the 
output of the differential amplifier 51R is kept zero for a time period 
from the raising edge of the second sampling signal SH-2 to the raising 
edge of the third sampling signal SH-3. In this manner, according to the 
present invention, when the sampling and holding operation is performed in 
the green channel, the holding operation is not conducted in the red and 
blue channels, and when the sampling and holding operation is carried out 
in the red and blue channels, the holding operation is inhibited in the 
green channel. 
FIGS. 7A to 7F are schematic views for explaining the function of the 
signal processing circuit shown in FIG. 6. FIGS. 7A shows the bright and 
dark pattern of the object, and FIGS. 7B and 7C illustrate the second and 
third sampling signals SH-2 and SH-3. FIG. 7D depicts the output signal of 
the differential amplifier 51G in the green channel and is generated at 
the raising edge of the second sampling signal SH-2 and becomes zero at 
the raising edge of the third sampling signal SH-3. The red color signal 
derived from the red channel is raised at the raising edge of the third 
sampling signal SH-3 and becomes zero at the raising edge of the second 
sampling signal SH-2 as depicted in FIG. 7E. Therefore, the brightness 
signal obtained by mixing the green, red and blue color signals becomes as 
shown in FIG. 7F and can reproduce the pattern of the object faithfully. 
FIG. 8 is a graph showing the frequency response of the signal processing 
circuit according to the invention and that of the known circuit. In the 
known circuit, the frequency response is abruptly decreased in accordance 
with the increase of the frequency and becomes zero at the clock frequency 
fC as illustrated by a broken curve A. However, in the circuit according 
to the invention, the frequency response is not decreased abruptly and 
does not become zero even at the clock frequency fC as shown by a solid 
curve B. Therefore, according to the invention, it is possible to improve 
the maximum resolution of the brightness signal to a great extent. 
FIG. 9 is a circuit diagram showing a second embodiment of the signal 
processing circuit according to the invention. In the first embodiment 
explained above, the correlation double sampling is performed by using the 
first to third sampling signals SH-1 to SH-3. In the present embodiment, 
the correlation double sampling is carried out by using a delay circuit to 
remove noise from the output signal generated by the solid state image 
sensing device The green color signal is supplied to a delay circuit 65 
having a delay time which is equal to a time difference between the first 
and second sampling signals SH-1 and SH-2, and the delayed signal and 
non-delayed signals are supplied to sample and hold circuits 66 and 67, 
respectively, which are driven by the second sampling signal SH-2. Output 
signals from these sample and hold circuits 66 and 67 are supplied to a 
differential amplifier 69. Further the input sides of the sample and hold 
circuits 66 and 67 are selectively short-circuited by means of a switch 68 
which is driven by the third sampling signal SH-3. When the switch 68 is 
closed by the third sampling signal SH-3, the green signal is held to zero 
for a time period during which the red and blue color signals are sampled 
and held. By selectively closing switches in the red and blue channels 
corresponding to the switch 68 in the green channel in accordance with the 
second sampling signal SH-2, the red and blue color signals are held zero 
for a time period during which the green color signal is sampled and held. 
FIG. 10 is a circuit diagram showing a third embodiment of the the signal 
processing circuit according to the invention. Also in this embodiment, 
portions similar to those illustrated in FIG. 6 are denoted by the same 
reference numerals used in FIG. 6. In the first embodiment depicted in 
FIG. 6, the red color signal is delayed with respect to the green and blue 
color signals by the period corresponding to a half of the read-out clock 
period by means of the sample hold circuit 54R, 55R, 56R, 58R, 60R and the 
third sampling signal SH-3. In the present embodiment, an analog delay 
line 71 is provided at the output side of the differential amplifier 51R, 
said delay line having a delay time r equal to a half of the read-out 
clock period. In the third embodiment, the delay is not performed by the 
switching, so that it is possible to obtain the color signal with low 
noise. Moreover, since the correlation double sampling circuit can be 
utilized commonly for all the color channels, the cost of the signal 
processing circuit can be reduced. 
FIG. 11 is a circuit diagram illustrating a fourth embodiment of the signal 
processing circuit according to the present invention. Also in this 
embodiment, portions similar to those illustrated in FIG. 6 are denoted by 
the same reference numerals in FIG. 6. Between the outputs of the sample 
and hold circuits and the differential amplifiers 51G and 51R are provided 
switches 72G and 72R, respectively and these switches are driven by fourth 
and fifth sampling signals SH-4 and SH-5, respectively having the duty 
cycle of 50%. FIGS. 12A to 12H are signal waveforms for explaining the 
operation of the circuit shown in FIG. 11. As illustrated in FIGS. 12D and 
12E, the fourth and fifth sampling signals SH-4 and SH-5 have the duty 
cycle of 50% and are in opposite phases. As depicted in FIG. 11, the fifth 
sampling signal SH-5 may be derived by passing the fourth sampling signal 
SH-4 through an inverter circuit 73. The switches 72R and 72G are driven 
by the fourth and fifth sampling signals SH-4 and SH-5, respectively. 
It is also possible to drive the switches provided at the output sides of 
the sample and hold circuits by the second or third sampling signal SH-2 
or SH-3, but in this case the resolution might be decreased to a large 
extent, because the duty cycle of these sampling signals is not equal to 
50%. 
FIG. 13 is a circuit diagram showing a fifth embodiment of the signal 
processing circuit according to the invention. In the present embodiment, 
portions similar to those shown in the first embodiment illustrated in 
FIG. 6 are represented by the same reference numerals used in FIG. 6. The 
neutralization of the signal charges are performed by short-circuiting the 
switches 44G and 48G by closing the switch 60G by the third sampling 
signal SH-3 and by short-circuiting the switches 53R and 54R by closing 
the switch 60R by the second sampling signal SH-2. In the present 
embodiment, the output sides of the switches 44G and 48G are connected via 
switches 74G and 75G to the same potential point such as a ground 
potential point and the output sides of the switches 53R and 54R are 
connected via switches 76R and 77R to the ground potential point. The 
switches 74G and 75G are driven by the third sampling signal SH-3 and the 
switches 76R and 77R are driven by the second sampling signal SH-2. In 
this manner, the circuit shown in FIG. 13 can operate in the same manner 
as that of the first embodiment illustrated in FIG. 6. 
The present invention is not limited to the embodiments explained above, 
but many modifications and alternations may be conceived by those skilled 
in the art within the scope of the invention. For instance, in the above 
embodiments, the three primary color signals generated from the 
three-plate-type color television camera having the green, red and blue 
solid state image sensing devices are processed. According to the 
invention, it is also possible to process two brightness signals generated 
from a two-plate-type monochromatic television camera or six color signals 
produced by a six-plate-type color television camera having two green 
solid state image sensing devices, two red solid state image sensing 
devices and two blue solid state image sensing devices. 
As explained above in detail, in the signal processing circuit according to 
the present invention, the output image signals read out of the solid 
state image sensing devices in which the spatial pixel shift method is 
adopted are processed such that when the output signal of a certain solid 
state image sensing device is sampled and held, the sampled and held 
signal of the other solid state image sensing device is forcedly made 
zero. Therefore, it is possible to obtain the very high frequency response 
even if the object having a high spatial frequency is picked-up, so that 
the maximum resolution can be increased to a greater extent. In the first, 
third and fifth embodiments in which the correlation double sampling is 
performed, the sampled and held value can be made zero by utilizing the 
sampling signal which is inherently required to effect the correlation 
double sampling. Therefore, it is no more necessary to prepare a separate 
sampling signal, and thus the circuit construction can be made simple and 
can be easily installed in the existent television camera. Further, it is 
possible to manage easily the pulse phase which is intimately related to 
the resolution of the pick-up apparatus, so that the merit of the present 
invention can be obtained most effectively. 
Since the signal charges stored in the sample and hold circuits are 
neutralized by closing the switch for a short time period, the resolution 
of the pick-up apparatus is predominantly determined by the phase of the 
neutralizing pulse and is not affected by its duty cycle. Moreover, the 
non-inverted input and inverted input of the differential amplifier are 
short-circuited by the phase of the neutralizing pulse, the same phase 
rejection ratio of the differential amplifier can be made high and the 
output signal is hardly affected by the DC voltage variation due to 
surrounding noise and temperature variation of the circuit elements. 
Further the switching noise due to the on and off operation of the 
neutralizing switch can be removed at a very high ratio.