Digital Camera Patent Abstract
A digital camera includes a CCD imager, and a CCD output signal
from the CCD imager is clamped in an analog clamp area by an analog
clamp circuit, and then, converted into a digital signal by an A/D
converter. The digital signal is thereafter applied to a clamp level
calculation circuit which calculates a clamp level of the digital
signal, whereby the digital signal can be subjected to digital clamping
by a first subtraction circuit. Digital Camera Patent Claims
What is claimed is:
1. A digital camera, comprising:
an image sensor which outputs line signals;
an analog clamper which clamps each of said line signals in an
analog clamp area of said image sensor;
a converter which converts each of analog-clamped line signals
into digital signal; and
a digital clamper which clamps said digital signal in a digital
clamp area, wherein
said analog clamp area is set such that both horizontal ends of
said analog clamp area are positioned at the outside of said digital
clamp area.
2. A digital camera as recited in claim 1, wherein said analog
clamp area covers said digital clamp area at least with respect
to a horizontal direction.
3. A digital camera as recited in claim 1, wherein said analog
clamp area is set at a different area from said digital clamp area.
4. A digital camera as recited in claim 1, further comprising a
first mode setter which sets a first photographing mode utilizing
a whole area of said image sensor, wherein said analog clamp area
and said digital clamp area are set, in said first photographing
mode, at rear edges of said line signals.
5. A digital camera as recited in claim 1, further comprising a
second mode setter which sets a second photographing mode utilizing
part of a whole area of said image sensor including one vertical
end and one horizontal end thereof, wherein said analog clamp area
and said digital clamp area are respectively set, in said second
photographing mode, at said one vertical end and said one horizontal
end.
Digital Camera Patent Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to digital cameras, and more particularly
to a digital camera adapted to process analogically and digitally
an image signal from a CCD using a mosaic-type color filter or a
stripe-type color filter, for example, so as to clamp a black level.
2. Description of the Prior Art
There is disclosed one example of a conventional digital video
camera of this kind by Japanese Provisional Patent Publication (Kokai)
H4-192677 [H04N 5/15, 5/92] laid open to public inspection dated
Jul. 10, 1992. This prior art discloses that an analog video signal
is analog-clamped and thereafter A/D-converted into a digital signal
for being subjected to digital clamping, thereby providing effective
utilization of a dynamic range during A/D conversion and keeping
constant a quantization level as a tip level of a synchronization
signal.
With such conventional techniques, however, there is a fear that
noises superposed onto the analog video signal due to analog clamping
might have an effect upon digital clamping. Nevertheless, the same
publication does not disclose a solving method therefor at all.
SUMMARY OF THE INVENTION
Therefore, a principle object of the invention is to provide a
digital camera capable of clamping a digital signal in an appropriate
manner.
According to the invention, a digital camera comprises a CCD imager
for outputting line signals, each of the line signals being clamped
in an analog clamp area of the CCD imager by an analog clamp means
and then converted into a digital signal, the digital signal being
digital-clamped in a digital clamp area by a digital clamp means,
the digital video camera characterized in that: the analog clamp
area is set such that both horizontal ends of the analog clamp area
are positioned at the outside of the digital clamp area.
The analog clamp means clamps each line signal outputted from the
CCD imager in the analog clamp area, while the digital clamp means
clamps the digital signal in the digital clamp area. Here, the analog
clamp area is set such that the both horizontal ends thereof are
positioned at the outside of the digital clamp area. That is, the
analog clamp area is set such that it covers the digital clamp area
at least with respect to a horizontal direction, or set at a different
area from the digital clamp area. By doing so, even if there occurs
noise at the both horizontal ends when each line signal is clamped
by the analog clamp means, there is no possibility that the noise
influences on the processing by the digital clamp means.
According to the present invention, the both horizontal ends of
the analog clamp area are arranged so as to be positioned at the
outside of the digital clamp area, so that it is possible to appropriately
clamp the CCD output.
The above described object, other objects, features and advantages
of this invention will be more apparent from the following description
on examples with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing a first embodiment of the invention;
FIG. 2 is an explanatory diagram showing an arrangement of a mosaic-type
color filter employed in the embodiment of FIG. 1;
FIG. 3 is an explanatory diagram showing a solid-state imaging
device unit in FIG. 1;
FIG. 4 is a timing chart showing an operation of an ordinary-speed
photographing mode (first photographing mode) in the embodiment
of FIG. 3;
FIG. 5 is an explanatory diagram showing an arrangement of a screen
for a quadruple-speed photographing mode in the embodiment of FIG.
1;
FIG. 6 is a timing chart showing an operation of a quadruple-speed
photographing mode (second photographing mode) in an embodiment
of FIG. 2;
FIG. 7 is a block diagram showing a rearrangement circuit for the
embodiment of FIG. 1;
FIG. 8 is a timing chart showing an operation of writing-in of
a memory for the embodiment of FIG. 7;
FIG. 9 is a timing chart showing an operation of reading-out of
a memory for the embodiment of FIG. 7;
FIG. 10 is a block diagram showing an interpolation calculating
circuit for the embodiment of FIG. 1;
FIG. 11 is a diagram showing an operation of an interpolation process
in the interpolation calculating circuit for an embodiment of FIG.
10;
FIG. 12 is a block diagram showing a signal processing circuit
in the embodiment of FIG. 1;
FIG. 13 is an explanatory diagram showing a first analog clamp
area and a second analog clamp area for performing analog clamping
in an embodiment of FIG. 9;
FIG. 14 is an explanatory diagram showing a first analog clamp
area and a second analog clamp area for performing analog clamping
in an embodiment of FIG. 12;
FIG. 15(a) is a waveform showing an output of an AGC circuit,
FIG. 15(b) is a waveform showing a horizontal pulse H1, and
FIG. 15(c) is a waveform showing an analog clamp circuit output;
FIG. 16 is a block diagram showing one example of a clamp-level
calculating circuit in the embodiment of FIG. 12;
FIG. 17 is a timing chart showing the operation of the clamp-level
calculating circuit of FIG. 16 in a first predetermined period;
FIG. 18 is a timing chart showing the operation of the clamp-level
calculating circuit of FIG. 16 in a second predetermined period;
FIG. 19 is a block diagram showing a portion of a timing generator
defining the first predetermined period and the second predetermined
period in the embodiment of FIG. 12;
FIG. 20 is a timing chart showing a first digital clamp area and
a second digital clamp area;
FIG. 21 is a waveform diagram showing a comparative example in
a case of adding an offset value without subtraction in the embodiment
of FIG. 12;
FIG. 22 is a waveform diagram showing an operation of the embodiment
of FIG. 12;
FIG. 23 is a block diagram showing in detail the clamp-level calculating
circuit in a case of using CCD imager with a stripe-type color filter
in the embodiment of FIG. 12; and
FIG. 24 is an explanatory diagram showing an arrangement of a mosaic-type
color filter;
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A digital video camera 10 of a first embodiment shown in FIG. 1
includes a solid-state imaging device unit or CCD imager 12, and
this solid-state imaging device unit 12 includes as shown in FIG.
3 the number of M of photodiodes 14 for photoelectric conversion
and vertical transfer CCDs 16. The vertical transfer CCDs 16 are
driven by a vertical driving circuit 18, while a horizontal transfer
CCDs 20a and 20b are driven by a horizontal driving circuit 22.
The horizontal transfer CCDs 20a and 20b are arranged in parallel
therewith a drain 24, which drain 24 is utilized for sweeping off
unwanted electric charges during high-speed photographing. That
is, the electric charges, which have delivered through sweep-off
control gates 26 from the horizontal transfer CCDs 20a and 20b,
are swept off out of the drain 24.
The solid-state imaging device unit or CCD imager 12 is attached
with a primary-color filter shown in FIG. 2, so that signals of
the same number of lines as the number of pixels in a vertical direction,
i.e. 480 are read out of the horizontal transfer CCDs 20a and 20b.
That is, this solid-state imaging device unit 12 is of dual-channel
structure that is intended to improve on a resolution by once reading
out data of all pixels in one field, without mixing electric charges
between vertical two pixels.
In an ordinary-speed photographing mode, the solid-state imaging
unit or CCD imager 12 operates in accordance with a field storage
mode shown in FIG. 4. That is, the electric charges stored on photodiodes
14 are read once per one field to the vertical transfer CCDs 16.
Thereafter, two vertical transfer pulses are successively supplied
to the vertical transfer CCDs 16, and electric charges of two lines
are simultaneously transferred once per 1H respectively to the horizontal
transfer CCDs 20a and 20b. The horizontal transfer CCDs 20a and
20b each output electric charges of one line per 1H depending upon
a horizontal transfer clock. That is, the horizontal CCDs 20a outputs
an output of an odd line, 1, 3, 5 . . . , as a first channel output
therefrom, while the horizontal CCDs 20b outputs an output of an
even line, 2, 4, 6 . . . , as a second channel output therefrom.
This enables outputting, from the solid-state imaging device unit
12, of electric charges for all the pixels included in an effective
pixel area and a shield area shown in FIG. 12.
On the contrary, when a quadruple-speed switch 28 in FIG. 1 is
manipulated, i.e. in a case of quadruple-speed photographing, the
output from the 1/4 region on upper left in the ordinary-photographing
CCD output denoted by internal hatching as shown in FIG. 5(A) is
taken out for utilization. This area is, if explained by FIG. 13,
a lower left 1/4 in the region of the effective pixel area plus
the shield area. This is because the image given in FIG. 5(A) is
to be reflected upside down by the solid-state imaging device unit.
Detailing further on the quadruple-speed photographing mode (second
photographing mode), an electric-charge read-out pulse in this mode
is supplied once per 1/4 field to the vertical transfer CCDs 16
as shown in FIG. 6. Further the vertical transfer CCDs 16 are driven
by vertical transfer pulses outputted 2 in number every 1/2H and
in number of 240 during a horizontal blanking period every 1/4 field.
Accordingly, the electric charges of two lines are transferred every
1/2H to the horizontal transfer CCDs 20a and 20b. That is, since
the horizontal transfer CCDs 20a and 20b are driven by the same
horizontal transfer clock as the aforesaid ordinary photographing
mode, while the electric charges for the first half of one line
have been transferred but the electric charges for the latter half
thereof still remain left, the electric charges for the next line
are transferred to come from the vertical transfer CCDs 16.
On the other hand, at that time a sweep-off pulse is supplied to
thereby open the sweep-off control gate 26. Accordingly, the electric
charges for the latter half of the one line are swept off from the
drain 26 through the sweep-off control gate 26. That is, in the
quadruple-speed photographing mode, the electric charges only for
the first half on each line are outputted from the horizontal transfer
CCDs 20a and 20b, while the electric charges for the latter half
on each line are swept off from the drain 24. Specifically, the
first half signals on odd lines 1 to 239 are outputted from the
horizontal transfer CCD 20a, and the first half signals on even
lines 2 to 240 are outputted from the horizontal transfer CCD 20b.
Incidentally, the electric charges for pixels involved in the first
four lines, amongst the electric charges outputted out of the vertical
transfer CCDs 16, are the electric charges being in the shield area.
Accordingly, in the quadruple-speed photographing mode, one screen
is constituted by eight images which are horizontally compressed
to 1/2 in image at the upper left 1/4 portion of the CCD screen.
Here, the images on the left and right are images to be respectively
formed by the odd line and the even line on the same time base.
In this manner, the solid-state imaging device unit 12 operates
on the ordinary photographing mode when the quadruple-speed switch
28 is not manipulated, and operates on the quadruple-speed mode
when the quadruple-speed switch 28 is manipulated. In either case,
the camera signal from the solid-state imaging device unit or CCD
imager 12 is delivered by each channel to the signal processing
circuits 30a and 30b where it is subjected to processing such as
AGC, analog clamping, A/D conversion, digital clamping, etc. Therefore,
color camera signals are outputted from the signal processing circuits
30a and 30b.
The color camera signal from the signal processing circuit 30a
is delivered as it is to a contact 32a of a switch 132, and to a
contact 132b of the switch 132 through a rearrangement circuit 34a.
The color camera signal from the signal processing circuit 30b is
delivered as it is to a contact 232a of a switch 232, and to a contact
232b of a switch 232 through a reordering circuit 34b. The switches
132 and 232 are in interlocking to a quadruple-speed switch 28,
so that they are connected to the contacts 132a and 232a when the
quadruple-speed switch 28 is off and connected to the contacts 132b
and 232b when the quadruple-speed switch 28 is on. Accordingly,
from the switches 132 and 232 are outputted the color camera signals
from the signal processing circuits 30a and 30b when the quadruple-speed
switch 28 is off, and the color camera signals from the reordering
circuits 30a and 30b are outputted therefrom when the quadruple-speed
switch 28 is on.
The reordering circuit 34a, as shown in FIG. 7, includes eight
memories al, a2, b1, b2, c1, c2, d1, and d2, and a memory control
circuit 36. The respective memories al, a2, b1, b2, c1, c2, d1,
and d2 are a memory for 1/8 screen to store signals for eight regions
A1, A2, B1, B2, C1, C2, D1 and D2 shown in FIG. 5. That is, during
writing-in of the memories a1, a2, b1, b2, c1, c2, d1 and d2, the
memories a1 and a2 are alternately written with signals every 1/2H
for a first 1/4 field, and the memories b1 and b2 are alternately
written with signals every 1/2H for a next 1/4 field. In this manner,
all the memories a1, a2, b1, b2, c1, c2, d1 and d2 are written with
color camera signals.
During reading out, as shown in FIG. 9, the memories a1, b1, a2
and b2 are repeatedly read out in order responsive to a read enable
signal for a first 1/2 field period (first half field), and the
memories c1, d1, c2 and d2 are repeatedly read out in order for
a next 1/2 field period (latter half field). Incidentally, the reordering
circuit 34b is similarly configured to the reordering circuit 34a,
and duplicated descriptions will be omitted.
In this manner, the color camera signals shown in FIG. 5(C) are
outputted from the reordering circuit 34a and 34b. The color camera
signal outputted from the switches 132 and 232 is inputted to an
interpolation processing circuit 100. The interpolation processing
circuit 100 is configured as in FIG. 10. That is, a first channel
output inputted through the switch 132 is supplied directly or through
a 1H delay circuit 116 respectively as D0 and D2 to a selecting
circuit 118, and a second channel output inputted through the switch
232 is supplied directly or through a 1H delay circuit 117 respectively
as D1 and D3 to the selecting circuit 118. The 1H delay circuits
116 and 117 are memories which can store an inputted CCD output
for a 1H time period, with a result that a CCD output with 1H delay
is obtained by passing a signal through the same circuit. Incidentally,
write-in or read-out of a signals to the 1H delay circuit is executed
in synchronism with the horizontal transfer in the horizontal transfer
CCD.
The selecting circuit 118 selects digital signals of 3 lines out
of digital signals of adjacent 4 lines depending upon whether a
field under processing is of an odd field or an even field. For
the odd field, D0 to D3 are respectively outputted as L0 to L2,
whereas for the odd field D0 to D2 are respectively outputted as
L0 to 2.
The outputs L0 to L2 of the selecting circuit 118 are inputted
directly to an interpolation calculating circuit 133 and to a delay
circuit 130. Further, the outputs of the delay circuit 130 are inputted
to a delay circuit 131. Here, both the delay circuits 130 and 131
have a delay time for one pixel, and the outputs of the delay circuits
130 and 131 are inputted to the interpolation calculating circuit
133. Accordingly, the interpolation calculating circuit 133 is inputted
with signals for successive 3 pixels of adjacent 3 lines, i.e. signals
for 9 pixels are simultaneously inputted to the interpolation calculating
circuit 133.
Since the color filter in this embodiment has filters of three
primary colors arranged in a mosaic form wherein either only one
of R, G and B can be obtained from any of pixels, the signals for
the other two colors are interpolated with the surrounding pixels
by the interpolation calculating circuit 133. In this case, the
relations between the arrangement of pixels on the solid-state imaging
unit 12 and pixels to be selected are represented as in FIG. 11.
As was stated hereinbefore, during in an odd field, the line signals
D1 to D3 are selected so that a pixel pattern at an odd number is
represented as FIG. 11(b). Also, a pixel pattern at an even number
becomes as shown in FIG. 11(c). Meanwhile, during in an even field,
the line signals D0 to D2 are selected so that a pixel pattern at
an odd number is represented as FIG. 11(d), whereas a pixel pattern
at an even number is represented as shown in FIG. 11(e). Incidentally,
FIG. 11(a) demonstrates a typical arrangement of part of pixels
on the solid-state imaging unit 12.
As clear from FIG. 10, if determinations are made based on various
timing signals whether the field under process is either of an odd
field or an even field, and whether the pixel under process is either
at an odd number or an even number, then a pixel pattern is determined
as any of FIGS. 11(b) to (e). Accordingly, the interpolation calculating
circuit 132, for example in the case of FIG. 11(b), obtains a G
signal from the central pixel and outputs the G signal as it is,
obtains R signals from upper and lower pixels in the centered vertical
column and taking an average on these two pixel signals to output
as R signals, and obtains B signals from the left and right two
pixels in the centered horizontal line and taking an average on
these two pixel signals to output as B signals. Also, in the case
of (c), R and G signals are available by taking average on the same
color signals for adjacent four pixels.
In this manner, the signals for the pixels under process lacking
two colors are prepared and interpolated by the peripheral same-colored
pixel signal, thereby outputting R, G, and B signals for pixels.
The color camera signal outputted from the interpolation calculating
circuit 132 is delivered to a recording circuit (not shown), as
well as to a contact 38a of a switch 38 and to a contact 38b of
the switch 38 by way of a zooming-up circuit 40. The magnifying
circuit 40, in the quadruple-speed photographing mode, magnifies
twice the color camera signal from the switch 32 in respective horizontal
and vertical directions. The switch 38 interlocks to the quadruple-speed
switch 28, similarly to the aforesaid switch 32. Consequently, when
the quadruple-speed switch 28 is off, the color camera signal from
the switch 32 is delivered as it is to a viewfinder (not shown),
while, when the quadruple switch 28 is on, the viewfinder is delivered
with a color camera signal magnified by the magnifying circuit 40
to be outputted from the switch 38.
So far explained were the ordinary photographing mode, i.e. the
first photographing mode, and the quadruple-speed photographing
mode, i.e. the second photographing mode, of the digital video camera
10. A clamping process in the signal processing circuit 30a will
be explained hereinbelow. Incidentally, the clamping process in
the signal processing circuit 30a also similar to the signal processing
circuit 30b, and duplicated explanations will be omitted.
The signal processing circuit 30a shown in FIG. 12 of this embodiment
includes an automatic gain control (AGC) circuit 42 for receiving
an output signal, which has been removed of reset noise by a correlated
double sampling circuit (not shown), from the solid-state imaging
device unit or CCD imager 12. This AGC circuit 42 processes an output
signal from the solid-state imaging device unit 12 to an optimal
amplitude in dependence upon an AGC voltage from the AGC-voltage
generating circuit (not shown) for generation of an AGC voltage
based on a gain control signal delivered as for example a PWM signal
from a CPU 44.
The output signal adjusted in amplitude by the AGC circuit 42 is
clamped by the analog clamp circuit 43. More specifically, a switch
43a included in the analog clamp circuit 43 is turned on corresponding
to, in a shield area shown in FIG. 13, a first analog clamp area
set at a rear edge of each line or a second analog clamp area set
at a lower edge to an effective pixel area. That is, in the ordinary-speed
photographing mode the switch 43a is turned on by gate signals of
a vertical pulse V1 and a horizontal pulse H1 corresponding to the
first analog clamp area, whereas in the quadruple-speed photographing
the switch 43a is turned on by gate pulses of a vertical pulse V1
and a horizontal pulse H2 corresponding to the second analog clamp
area. This causes the output signal of the AGC circuit 42 to be
clamped at a reference voltage V2, and the clamped signal is inputted
to an A/D converter 46 through a buffer amplifier 43a.
The A/D converter 43 is given of a reference voltage V1 by which
a minimal value level of the A/D converter is prescribed. Consequently,
the A/D converter 46 quantizes an input signal with the reference
voltage V2 as reference, creating a digital signal. Incidentally,
the reference voltages V1 and V2 are obtained by voltage-dividing
a power supply voltage Vcc with resistances R1 to R3, resulting
in the reference voltage V1>V2.
In this manner, the digital signal is adjusted in black level by
clamping the output signal from the AGC circuit 42 by the reference
voltage V2 and quantize it with the reference voltage V1 as a minimum
value. The adjustment like this is due to the following reason.
That is, although the digital signal from the A/D converter 46 is
clamped at a level shown in FIG. 21(A) and FIG. 22(A), if the black
level of the digital signal outputted from the A/D converter 46
rises excessively high or falls excessively low, there occurs saturation
in white level for the digital signal or saturation in part of random-noise
components superimposed by the black level, with the result that
the dynamic range is reduced small or "black floating"
becomes impossible to be prevented.
Also, the first analog clamp area and the second analog clamp area
are set such that their respective horizontal ends are respectively
positioned at first and second clamp areas so that analog clamping
will not have adverse effects on digital clamping. That is, in the
ordinary-speed photographing mode, the turning on/off of the switch
43a at both horizontal ends of the first analog clamp area causes
superposition of noise on the output signal from the AGC circuit
42, as shown in FIG. 15. However, as will be understood from FIG.
13 and FIG. 14, since the first analog clamp area is set broader
than the first digital clamp area, there is no possibility that
this noise badly influences upon the later processes. Also, in the
quadruple-speed photographing mode, a second analog clamp area and
a second digital clamp area are set such that they are not overlapped
with so that, in also this mode, the noise caused during analog
clamping will not badly affect the digital clamping.
The digital signal is delivered to an adding circuit 48 and a clamp-level
calculating circuit 50. In the adding circuit 48, an offset value
given from an CPU 44 is added to the digital signal outputted from
the A/D converter 46. The offset value is preferably set in accordance
with the magnitude of a gain to be set in the AGC circuit 42. For
example, where the gain of the AGC circuit 42 is great the offset
value is rendered large, and where reverse the offset value is rendered
small.
Meanwhile, the clamplevel calculating circuit 50, to be brief,
calculates an average value (referred to as "OB level")
of an optical black (OB) portion of each line in a first digital
clamp area set at a rear edge of each line or a second digital clamp
area set at a front edge of each line, in a shading area shown in
FIG. 14, to calculates a clamp level based thereon. Specifically
the clamp level calculating circuit 50 is shown in FIG. 16.
Referring to FIG. 16, the clamp level calculating circuit 50 includes
a selecting circuit 52 for receiving a digital signal (input data)
of for example 8 bits from the A/D converter 46. This selecting
circuit 52 selectively outputs one times the input data ((1) or
two times thereof ((2) to deliver it to an adding circuit 54. The
adding circuit 54 is delivered with data selected by a selecting
circuits 58, 60 through an AND gate 56. The AND gate 56 is controlled
by a signal VSET shown in FIG. 17 and FIG. 18. The signal VSET is
a signal that becomes a low level only at respective odd and even
one pixels at which the first digital clamp area and the second
digital clamp area begins.
Also, the selecting circuits 58 and 60 are controlled by a signal
HSET shown in FIG. 17 and FIG. 18, to selectively output 1/2 times
(1/2) or one times (1) a data delivered. This signal HSET, as shown
in FIG. 17 or FIG. 18, is a signal that becomes a low level at odd-and-even
two pixels at a beginning end for the first digital clamp area or
the second digital clamp area on each line. The selecting circuit
62 is controlled by a signal ODD/EVEN, which selects to output an
output of the selecting circuit 58 when the signal ODD/EVEN is at
a high level, and an output of the selecting circuit 60 when at
a low level. This signal ODD/EVEN, as shown in FIG. 17 or FIG. 18,
goes to a high level for an odd pixel of the CCD and to a low level
for an even pixel.
Further, the output of the adding circuit 54 is delivered to an
adding register 262. This adding register 262 includes an odd register
64 and an even register 66, so that an odd pixel data of the output
of the adding circuit 54 is delivered to the odd register 64 and
an even pixel data is delivered to the even register 66. The outputs
from the odd register 64 and the even register 66 are respectively
delivered to the aforesaid two selecting circuits 58 and 60, as
well as to an odd register 70 and an even register 72 included by
a clampvalue register 68. The adding register 262 latches the output
from the adding circuit 54, while the clamp-value register 68 latches
the output from the adding register 262. Note that the outputs from
the odd register 64 and the even register 66 of the adding register
262 are respectively rendered 1/16 to be latched to the odd register
70 and the even register 72 of the clamp-value register 68.
The odd register 64 of the adding register 262 receives a signal
CLKODD as a latch signal, whereas the even register 66 receives
CLKEVEN as a latch signal. As shown in FIG. 17 and FIG. 18, the
signal CLKODD is a clock that is outputted on every odd pixel, and
the signal CLKEVEN a clock outputted on every even pixel. The odd
register 70 and the even register 72 of the clamp-value register
68 are delivered with a signal CLKHD as a common latch timing signal.
The signal CLKHD is a signal that goes to a high level only at one
pixel at the right end of each line shown in FIG. 14.
The outputs of the odd register 70 and the even register 72 of
the clamp-value register 68 are delivered to a selecting circuit
74. The selecting circuit 74 is controlled by the signal ODD/EVEN
similarly to the aforesaid selective circuit 62, and select to output
an output of the odd register 70 when the signal ODD/EVEN is at
the high level and an output of the even register 72 when at the
low level.
Each of timing signals shown in FIG. 17 and FIG. 18 is created
by a timing generator 76 shown in FIG. 1. That is, the timing generator
76 includes as part thereof a front-edge clamp timing circuit 78
and a rear-edge clamp timing circuit 80 sown in FIG. 19. The front-edge
clamp timing circuit 78 and the rear-edge clamp timing circuit 80
both receive a master clock CLK and a horizontal synchronizing signal
HD to prescribe a second digital clamp area (second predetermined
time period) and a first digital clamp area (first predetermined
time period).
More specifically, the front-edge clamp timing circuit 78 is configured
by an appropriate gate circuit and a ROM decoder, to output a timing
signal shown in FIG. 18 only for a time (second predetermined time
period) for odd two pixels and even two pixels of each line signal
in the second digital clamp area, based on the master clock CLK
and the horizontal synchronizing signal HD.
The rear-edge clamp timing circuit 80 is similarly configured by
an appropriate gate circuit and a ROM decoder, to output a timing
signal shown in FIG. 17 only for a time by odd two pixels and even
eight pixels of each line signal in the first digital clamp area,
based on the master clock CLK and the horizontal synchronizing signal
HD.
The output signals of the front-edge timing circuit 78 and the
rear-edge timing circuit 80 are switched over by a switch 81 interlocking
to the quadruple-speed switch 28 for being outputted. Consequently,
the clamp level calculating circuit 50 operates responsive to the
timing signals outputted from the front-edge clamp timing circuit
78 or the rear-edge clamp timing circuit 80 of the timing generator
76.
Here, the operation of the clamp level calculating circuit 50 in
the first predetermined period will be explained. First, an OB level
of 8 pixels is calculated as to odd pixels. For this, the selecting
circuit 52 is selected at "(2" at timing of a line to
be clamped. Accordingly, the input data is shifted up by one bit
with the result that a data two times the input is outputted from
the selecting circuit 52. At this time, the signal VSET as an control
input to the AND gate 56 becomes a low level as shown in FIG. 17
so that the output of the adding circuit 54 is of a data two times
a first odd pixel data outputted from the selecting circuit 52.
This two-times data is inputted to each of registers 64 and 66 of
the adding register 262, to be latched to the odd register 64 depending
on a signal CLKODD shown in FIG. 17. This data latched to the odd
register 64 is delivered to the selecting circuit 58. The selecting
circuit 58 outputs to the selecting circuit 62 the data one-time
multiplied of the input data in accordance with the low level of
the signal HSET. The selecting circuit 62 selects an upper side
input when the signal ODD/EVEN is at a high level, and selects an
lower side input when it is at a low level, with the result that
the one-times of odd pixel data from the selecting circuit 58 is
delivered from the selecting circuit 62 to the AND gate 56. That
is, the data inputted to the AND gate 56 is a data two times the
first odd pixel.
When a next odd pixel data is inputted, a two-times data is again
delivered from the selecting circuit 52 to the adding circuit 54.
At this time, since the signal VSET is at a high level, a data two
times the first odd pixel is delivered through the AND gate 56 to
the adding circuit 54. Consequently, the adding circuit 54 add the
data two times the first odd pixel and the data two times the second
odd pixel, and the adding result is latched to the odd register
64 of the adding register 262. The operation like this is thereafter
repeated up to an 8th odd pixel, and the odd register 64 of the
adding register 262 is resultingly latched with a data of 16 times
the input data.
Responsive to the signal CLKHD turning a high level at the right
end (termination) on each line on the screen, the data, which is
latched to the odd register 64 of the adding register 262, is rendered
1/16 or shifted down by 4 bits to be latched to the odd register
70 of the clamp-value register 68. Accordingly, a selecting circuit
74 outputs an OB level data of odd 8 pixels of a first line in the
first digital clamp area (FIG. 14) latched to the odd register 70
responsive to the high level of the ODD/EVEN signal.
The subsequent lines, that is a second line and the subsequent,
the selecting circuit 52 selects "(1", and the selecting
circuits 58 and 60 select "1/2" only for the first one
pixel and "1" for the subsequent 7 pixels. Accordingly,
a 16-times data is outputted from the adding register 62 and rendered
"1/16", resulting in outputting of an OB level of 8 pixels
on each line from the selecting circuit 74. That is, a recursive
filter is configured by a circuit passing from the adding circuit
54 through the adding register 262, the selecting circuits 58, 60
and 62, and the AND gate 56.
Note that, although in this embodiment the weighting coefficient
(k) of the recursive filter is set at "1/2", this coefficient
can be set arbitrary within a range of 0<k<1. That is, by
setting the coefficient (k) the clamp level of the first line remains
in the OB level value as it is, and the clamp level for the line
2 and the subsequent becomes a weighted mean of coefficient (k)
for the clamp level of the previous line and a newly obtained OB
level. Therefore, even if there is noise contained in the OB portion
of the line 2 and the subsequent, the influence thereof is relieved
to 1/k times.
Incidentally, although the operation of determining the OB level
and calculating the clamp level was explained solely on odd pixels,
as for even pixels the operation is similar to the case of odd pixels
except that the low level of the signal ODD/EVEN causes the selecting
circuits 62 and 74 to select an under side input. In any case, the
circuit of FIG. 17 calculates on each odd or even pixel.
Then, the operation of the clamp level calculating circuit 50 for
the second predetermined period is similar to the operation of the
first predetermined period, and duplicated explanations will be
omitted. Note that the OB level of two pixels is calculated for
the second predetermined period.
In this manner, the clamp level is outputted from the clamp level
calculating circuit 50 to be inputted to a first subtraction circuit
82 shown in FIG. 12. Accordingly, the first subtracting circuit
82 subtracts the clamp level calculated by the clamp level calculating
circuit 50 from the data added with the offset value by the adding
circuit 50, and the digital signal from the A/D converter 46 is
digitally clamped.
The output of the first subtraction circuit 82 is zero-clipped
(a negative numeral value is forced to a zero-level) zero-clip circuit
84 including for example such as an OR gate, and then delivered
through a digital low-pass filter 86 to a second subtraction circuit
88. The subtraction input to the second subtraction circuit 88 is
given of the same offset value as that delivered from the CPU 44
to the adding circuit 48.
That is, explanations will be made in detail on a case of this
embodiment that the offset value is added to or subtracted from
by the adding circuit 48 and the second subtracting circuit 88,
and a case that no offset value is used, with reference to FIG.
21 and FIG. 22. FIG. 21(A) and FIG. 22(A) each shows a CCD output
signal (digital signal) for one line outputted from the A/D converter
46 where photographing a subject in black on the left and white
on the right. The actual signal assumes a waveform as shown in FIG.
21(A) or FIG. 22(A) because it contains random noises. At this time,
the clamp level is at a level denoted by the dotted line, at which
level clamping is done by the first subtraction circuit 82.
Where no offset value is added, the output of the zero-clip circuit
84 is a signal as in FIG. 21(B). This signal, if passes through
the low-pass filter 80, has a waveform as in FIG. 21(C), wherein
a portion of the signal to be in black is not zero and hence there
occurs a phenomenon called "black floating".
However, if the offset value is added by the adding circuit 48
conforming to this embodiment, the output signal of the zero-clip
circuit 84 has a waveform shown in FIG. 22(B). If this is processed
by the low-pass filter 80, the waveform becomes as shown in FIG.
22(C). Thereafter, by subtracting the offset value by the second
subtraction circuit 88, the optically-black portion in the signal
becomes nearly zero, providing an image signal free from "black
floating".
As explained hereinbefore, the offset value is varied in accord
with the gain by the AC circuit 42. That is, when the gain is large,
noise is also large, so that the offset value is taken great to
prevent against black floating. When the gain is small, the noise
also small, and accordingly the offset value is taken small to broaden
the dynamic range for the signal.
Incidentally, in the above both embodiments explanations were made
for the case that the CCD imager has the mosaic-type color filter
with primary and complementary colors. However, this invention is
also applicable to a case of processing an output signal from a
CCD with a stripe-type color filter.
That is, a clamp level calculating circuit shown in FIG. 23 includes,
similarly to the clamp level calculating circuit 50, a selecting
circuit 52, an adding circuit 54, an AND gate 56, an adding register
262, a selecting circuits 58', 59 and 60, a selecting circuit 52',
a clamp value register 68, and a selecting circuit 74'. The selecting
circuits 52' and 74' are controlled by a signal (color-separation
pulse) SELAB that becomes to a high level on every pixel of a stripe-type
color filter (not shown). If the signal SELAB is in the high level
for a first pixel, the selecting circuits 52' and 74' select an
upper side input. If the signal SELAB is in the high level at timing
of a second pixel, the selecting circuits 52' and 74' select a middle
input. And, when the signal SELAB is in the high level at timing
of a third pixel, the selecting circuits 52' and 74' select a lower
side input. Other operations than the above can be easily understood
from FIG. 11 mentioned before, and repetitive detailed explanations
will be omitted here. In any case, the circuit of FIG. 16 calculates
clamp levels every three pixel on each pixel.
The invention is also applicable to the case of processing output
signals from a CCD with a complementary-color mosaic color filter
shown in FIG. 24(a). In this case, electric charges for vertical
two pixels are mixed in an odd field as shown in FIG. 24(b) or in
an even field as shown in FIG. 24(c), so that the number of vertical
pixels of a solid-state imaging unit 12 is "480". However
the number of lines to be actually outputted from the solid-state
imaging unit in each field is "240". Here, the technique
of creating video signals from the image signals as shown in FIG.
24(c) and FIG. 24(d) is disclosed in detail by Japanese Provisional
Patent Publication (Kokai) No. H6-46431.
In addition, in the above described embodiments, the present invention
is applied to the digital video camera having the quadruple-speed
photographing mode; however, the inventive concept for setting the
analog clamp area and the digital clamp area can be also applied
to a digital still camera having nothing to do with the high-speed
photographing mode.
Although the present invention has been described and illustrated
in detail, it is clearly understood that the same is by way of illustration
and example only and is not to be taken by way of limitation, the
spirit and scope of the present invention being limited only by
the terms of the appended claims. |