Digital Camera Patent Abstract
A digital camera apparatus has an analog emphasis circuitry arranged
such that when an output from a charge-coupled device (CCD) forming
the imager and cooperating with a complementary color, checkered
pattern filter is processed in a digital fashion, an analog carrier
chrominance signal component is emphasized before being converted
to a digital signal, thus making it possible to reduce adverse influence
caused by quantization noise. Digital Camera Patent Claims
WHAT IS CLAIMED IS
1. A digital camera apparatus comprising:
a color filter assembly in which one primary color filter element
and three complementary color filter elements are arranged in a
plurality of rows and columns in a checkered pattern;
a solid-state imager element having light receiving picture elements
arranged in a two-dimensional array, wherein said primary and complementary
color filter elements and said light receiving picture elements
are opposed to each other, and from which an analog luminance signal
component and an analog carrier chrominance signal component are
produced upon horizontally scanning two rows of said light receiving
picture elements simultaneously;
an emphasizing circuit for emphasizing said carrier chrominance
signal component; and
processing means for digitally processing said analog luminance
signal component and said emphasized carrier chrominance signal
component for producing a digital luminance signal and a digital
chrominance signal.
2. The apparatus according to claim 1, wherein said carrier chrominance
signal component processing circuit includes a color matrix circuit.
3. The apparatus according to claim 1, wherein said emphasizing
circuit emphasizes the level of a frequency component of said carrier
chrominance signal component near a repetitive frequency of said
color filter assembly.
4. The apparatus according to claim 1, wherein a horizontal scanning
output from said light receiving picture elements is supplied through
an automatic gain control circuit and a sample and hold circuit
to an analog-to-digital converter, and said emphasizing circuit
is connected between and automatic gain control circuit and said
sample and hold circuit.
5. The apparatus according to claim 4, wherein an output from said
analog-to-digital converter is separated into a luminance signal
component and a carrier chrominance signal component by a filter
circuit.
6. The apparatus according to claim 4, wherein a repetitive frequency
of said color filter assembly is selected to be one-half the value
of a sampling frequency of said sample and hold circuit.
Digital Camera Patent Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a color solid-state
imager and, more particularly, is directed to a camera that digitally
processes the image signal.
2. Description of the Background
A color solid-state imager is known in which a four-color filter
made-up of the three complementary colors, cyan (Cy), magenta (M),
yellow (Ye), plus green (G) arranged in a checkered pattern is combined
with a solid state imager element. An example of such solid-state
imager is found in Japanese Patent Laid-Open Gazette No. 59-161989.
FIG. 1 shows a color filter 10 in which cyan and yellow filter
elements 1c and 1y are alternately arrayed on an n.sup.th row, and
magenta and green filter elements 1m and 1g are alternately arrayed
on the (n+1) row. The rows then alternate between lines of cyan
and yellow filter elements and rows of magenta and green filter
elements. The pattern arrangement of the cyan and yellow filter
elements in the n.sup.th, (n+2), and (n+4) rows are the same, however,
the pattern arrangements of the magenta and green filter elements
in the (n+1), (n+3), and (n+5) rows are opposite in phase, that
is, they are offset by one filter element every row.
In FIG. 2, a color solid-state imager system is comprised of an
analog signal system, shown generally at 20, and a digital signal
processing system, shown generally at 30. In the analog signal system
20, a solid-state imager element 21 is provided in an opposing relation
to the color filter 10 that is formed as shown in FIG. 1. This solid-state
imager element 21 includes light receiving picture elements arranged
in a two-dimensional manner, that is, in an X-Y matrix form, and
transfer elements for transferring signal charges of respective
picture elements, for example, when imager element is a charge-coupled
device (CCD). An output signal from solid-state imager element 21
is supplied through a correlative double sampling circuit 22 to
an automatic gain control amplifier (AGC) 23. An output from the
AGC amplifier 23 is sampled and held by a sample and hold circuit
24, and an output from sample and hold circuit 24 is supplied to
an analog-to-digital (A/D) converter 25, in which it is converted
into a digital signal of, for example, 10 bits.
In the digital signal processing system 30, the digital signal
from A/D converter 25 is supplied to a low-pass filter 31 and to
a bandpass filter 32. An output from low-pass filter 31 is supplied
to a luminance signal processing circuit 33, in which it undergoes
necessary signal processing, such as, aperture correction, gamma
correction, and the like. On the other hand, an output of bandpass
filter 32 is supplied to a matrix circuit 34, in which it is processed
to provide three primary color signals, red, green, and blue (R,
G, and B). The red (R), green (G), and blue (B) primary color signals
are supplied to a color signal processing circuit 35, in which they
undergo necessary signal processing, such as white balance adjustment,
gamma correction, encoding, hue correction, and the like. In the
conventional color solid-state imager, since the signal processing
is performed in a digital fashion as described above, the color
solid-state imager can be made compact in size and requires relatively
low power.
The output from luminance signal processing circuit 33 and the
output from color signal processing circuit 35 are supplied to digital-to-analog
(D/A) converters 36 and 37, respectively, wherein they are converted
into an analog luminance signal Y and an analog color signal C,
respectively.
In the operation of the color solid-state imager described above,
the light reflected from an object is spatially modulated by each
of the filter elements of color filter 10 and each modulated light
beam is photoelectrically converted by each respective light receiving
element (not shown) of solid state imager element 21 and then sampled.
In the sampling operation, a sampling frequency fsp is selected
to be four times as high as the color subcarrier frequency fsc,
for example, fsp may equal 14.3 MHz. Further, a repetitive frequency
of the color filter 10 is selected to be about 1/2 of the sampling
frequency fsp.
In the odd-numbered field, two rows of the light receiving picture
elements corresponding to the n.sup.th row and the (n+1) row; the
(n+2) row and the (n+3) row; and so on, of the color filter 10 are
horizontally scanned at the same time to sequentially generate the
photoelectric-converted outputs at every two columns. That is, the
output signals are calculated at every four picture elements of
two rows and two columns. In the even-numbered field, two rows of
the light receiving elements corresponding to rows (n+1) and (n+2);
the (n+3) and N+4) rows; and so on of the color filter 10 are horizontally
scanned at the same time to sequentially generate the photoelectrically
converted outputs at every two columns.
The light receiving picture elements corresponding to the cyan
and magenta filter elements 1c and 1m of n.sup.th row and (n+1)
row derive signal components (B+G) and (R+B), respectively, and
the light receiving picture elements corresponding to the yellow
and green filter elements 1y and 1g derive signal components (R+G)
and G, respectively, as shown in FIGS. 3A and 3B.
Further, the light receiving elements corresponding to the cyan
and green filter elements 1c and 1g of the (n+2) row and the (n+3)
row, respectively, of color filter 10 derive signal components (B+G)
and G and the light receiving picture elements corresponding to
the yellow and magenta filter elements 1y and 1m color filter 10
derive signal components (R+G) and (R+B), respectively, as shown
in FIGS. 3C and 3D.
In the horizontal scanning of the odd-numbered and even-numbered
fields, a total sum Ss of the output signals generated at every
4 picture elements of 2 rows and 2 columns is expressed as: Ss=(2R+3G+2B).
Thereafter, the signal is processed by low-pass filter 31 and the
resultant signal is a luminance signal that is uniform and compensates
any lens distortion.
If a sum of the output signals of each of the columns is calculated
at every four picture elements of two rows and two columns on the
n.sup.th row and the (n+1) row in the odd-numbered field and a difference
between the sum signals of each of the columns is calculated, then
a differential signal Sdb is expressed as:
Similarly, if a sum of output signals of each of the columns is
calculated at every four picture elements of two rows and two columns
on the (n+2) row and the (n+3) row and a difference between the
sum signals of each of the columns is calculated, then a differential
signal Sdr is expressed as:
Further, the same differential signals Sdb and Sdr are obtained
on the (n+1) row, the (n+2) row, the (n+3) row and (n+4) row of
the even-numbered field.
The differential signals, Sbd and Sdr, are subcarrier chrominance
signal components that are formed on the basis of the spatial modulation
performed by the respective filter elements of color filter 10.
The repetitive frequencies thereof become one half of the sampling
frequency fsp, that is, twice as high as the color subcarrier frequency
fsc, for example, fsp may equal 7.16 MHz.
Nevertheless, in the previously proposed color solid-state imager
the level of the analog carrier chrominance signal component delivered
from the AGC amplifier 23 is decreased, for example, to 1/4 to 1/2
of the level of the luminance signal component, so that when the
analog signal is digitally converted by the A/D converter, even
according to the linear quantization process, the accuracy of the
carrier chrominance signal is about 8 to 9 bits for the standard
input level equivalent, which is, for example, 10 bits.
Further, when a dynamic range that is three times as wide as the
standard input level is maintained by the gamma correction circuit,
the high-level region is compressed, so that with the standard input
level the accuracy of the carrier chrominance signal component is
decreased to about 6 to 7 bits.
As a result, fine gradation of the color signal cannot be obtained
and a so-called false contour occurs in the reproduced picture,
in which there are step changes in the brightness of certain colors
in place of the desired gradual change in brightness. Furthermore,
a satisfactory signal-to-noise (S/N) ratio cannot be obtained due
to the quantization noise produced by the analog-to-digital conversion.
OBJECTS AND SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide
an improved digital camera apparatus having analog emphasis circuitry
in which the aforenoted shortcomings and disadvantages of previously
proposed systems can be eliminated.
It is another object of the present invention to provide a digital
camera apparatus having analog emphasis circuitry, in which the
quality of the reproduced picture can be prevented from deteriorating
due to quantization noise.
It is a further object of the present invention to provide a digital
camera apparatus having analog emphasis circuitry, in which the
occurrence of a so-called false contour can be reduced.
According to an aspect of the present invention, a digital camera
apparatus having analog emphasis circuitry includes a color filter
in which one primary color filter element and three complementary
color filter elements are arranged in a checkered pattern, and a
solid-state imager in which light receiving picture elements are
arranged in a two-dimensional manner, wherein the filter elements
and the light receiving picture elements are opposed to each other.
The imager produces an analog luminance signal component and an
analog carrier chrominance signal component, each of which results
from horizontally scanning two rows of the light receiving picture
elements simultaneously. These signal components are processed by
a processing circuit in a digital fashion. This digital camera apparatus
is further characterized by an emphasizing circuit for emphasizing
the carrier chrominance signal component.
The above and other objects, features, and advantages of the present
invention will become apparent from the following detailed description
of a preferred embodiment when read in con]unction with the accompanying
drawings, in which like reference numerals are used to identify
the same or similar parts in the several views.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view illustrating an arrangement of color filter
elements used in a prior-art solid-state imager;
FIG. 2 is a schematic in block diagram form of a prior-art color
solid-state imager;
FIG. 3A to 3D are diagrammatic representations showing respective
color signal components useful in explaining the operation of the
color solid-state imager of FIG. 2;
FIG. 4 is a schematic in block diagram form of a digital camera
apparatus having analog emphasis circuitry according to an embodiment
of the present invention; and
FIG. 5 is a graphical representation of the characteristics of
a main portion of the embodiment of FIG. 4.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
An embodiment of a digital camera apparatus having analog emphasis
circuitry according to the present invention is shown in FIG. 4
in which like parts corresponding to those of FIG. 2 are identified
with the same reference numerals and are not described again in
detail.
Referring to FIG. 4, in an analog signal system, shown generally
at 20E, an emphasizing circuit 26 is connected between AGC amplifier
23 and sample and hold circuit 24 and various constants of the matrix
circuit 34 are varied accordingly, specifically, the gain constants
are varied for de-emphasis. The remainder of the circuit arrangement
of FIG. 4 is the same as that of FIG. 2.
The emphasizing circuit 26 in this embodiment increases, or emphasizes,
the level of the analog carrier chrominance signal by, for example,
6 dB near a frequency corresponding to one half the sampling frequency
fsp, that is, near a frequency that is twice as high as the color
subcarrier frequency fsc on the basis of the low frequency and the
sampling frequency, as shown by the frequency characteristic response
curve of FIG. 5.
Thus, the accuracy of the digital carrier chrominance signal component
delivered from A/D converter 25 is increased by, for example, 1
bit, whereby fine gradation of color difference signals can be maintained,
the influence of quantization noise can be reduced, and the occurrence
of false contour as described hereinbefore can be reduced.
As set out above in detail according to the present invention,
because the emphasizing circuit emphasizes the analog carrier chrominance
signal component that results from simultaneously horizontally scanning
the light receiving elements of two adjacent rows of the solid-state
imager opposing the complementary color checkered pattern filter,
it is possible to provide a digital camera in which the quality
of the resultant pictures will not be deteriorated due to the quantization
of the color signal.
Having described a preferred embodiment of the invention with reference
to the accompanying drawings, it is to be understood that the invention
is not limited to that precise embodiment and that various changes
and modifications thereof could be effected by one skilled in the
art without departing from the spirit or scope of the novel concepts
of the invention as defined in the appended claims.
|