U.S. patent application number 11/977317 was filed with the patent office on 2008-03-13 for apparatus and method for processing image signal and imaging equipment.
Invention is credited to Fumihiko Sudo.
Application Number | 20080062289 11/977317 |
Document ID | / |
Family ID | 19190790 |
Filed Date | 2008-03-13 |
United States Patent
Application |
20080062289 |
Kind Code |
A1 |
Sudo; Fumihiko |
March 13, 2008 |
Apparatus and method for processing image signal and imaging
equipment
Abstract
An image pickup device picks up an image. An image signal in
association with the image is filtered in a time axis direction
using a digital low-pass filter to generate an image signal having
a reduced dynamic resolution power, which is equivalent to an image
signal that would be obtained at a longer exposure time. The
filtered signal is then subjected to frame skipping processing to
generate an output image signal having a preferred frame period.
The resultant signal is equivalent to one obtained by a slow
shooting.
Inventors: |
Sudo; Fumihiko; (Kanagawa,
JP) |
Correspondence
Address: |
FROMMER LAWRENCE & HAUG
745 FIFTH AVENUE- 10TH FL.
NEW YORK
NY
10151
US
|
Family ID: |
19190790 |
Appl. No.: |
11/977317 |
Filed: |
October 24, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10338493 |
Jan 8, 2003 |
7304671 |
|
|
11977317 |
Oct 24, 2007 |
|
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Current U.S.
Class: |
348/241 ;
348/E5.047; 348/E5.054; 348/E5.078 |
Current CPC
Class: |
H04N 5/23241 20130101;
H04N 5/217 20130101; H04N 5/2625 20130101; H04N 5/23293
20130101 |
Class at
Publication: |
348/241 ;
348/E05.078 |
International
Class: |
H04N 5/217 20060101
H04N005/217 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 9, 2002 |
JP |
P2002-002796 |
Claims
1-14. (canceled)
15. An imaging equipment comprising: an image pickup device having
an image pickup element for forming an image signal of a dynamic
resolution power, said image pickup device having a read period
longer than a minimum frame period within which said image pickup
element operates; a digital low-pass filter configured to filter
the image signal in a time axis direction thereof to lower the
dynamic resolution power for an image based on the image signal; a
processor configured to perform frame skipping on the filtered
image signal to form an output image signal having a preferred
output frame period; and a controller configured to control (a) the
generation of said image signal by said image pickup device, (b)
the filtering characteristic of said digital low-pass filter, (c)
the reading by said image pickup element and (d) the frame skipping
of said processor; said controller controlling said image pickup
device and said processor so that, when the frame period of said
output image signal is longer than n (n is an integer) times the
minimum frame period but shorter than (n+1) times the minimum frame
period, said image pickup device sets the read period of said image
pickup device to 1/n times the output frame period and said
processor performs frame skipping after every n frames.
16. The imaging equipment according to claim 15, wherein said
digital low-pass filter includes a one-frame delay unit supplied
with a signal DV.sub.e, where DV.sub.e=(1-k) DV.sub.b+k DV.sub.f,
and DV.sub.b is the image signal from the image pickup element,
DV.sub.f is the output from said one-frame delay unit, k is a
coefficient within a range from 0-1 and said signal DV.sub.e is
supplied to said processor.
17. An imaging method comprising the steps of: forming an image
signal of a dynamic resolution power from a pickup device having a
read period longer than a minimum frame period within which an
image pickup element operates; filtering the image signal in a time
axis direction thereof to lower the dynamic resolution power for an
image based on the image signal; and performing frame skipping on
the filtered image signal to form an output image signal having a
preferred output frame period; the read period being set to 1/n
times the output frame period and frame skipping being performed
after every n frames when the frame period of said output image
signal is longer than n (n is an integer) times the minimum frame
period but shorter than (n+1) times the minimum frame period.
18. The imaging method according to claim 17, wherein the image
signal is filtered by a one-frame delay of a signal DVe, where
DV.sub.e=(1-k)DV.sub.b+k DV.sub.f, and DV.sub.b is the image
signal, DV.sub.f is the result of said one-frame delay, k is a
coefficient within a range from 0-1, and said signal DV.sub.e is
subjected to said frame skipping.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to an apparatus and a method for
processing an image signal, and imaging equipment.
[0003] 2. Description of Related Art
[0004] In the field of conventional movie production, film cameras
have been used. This type of film camera has a mirror 81 mounted on
a rotary disk shutter 80 off the open area of the shutter, and an
imaging lens 70 and a ground glass 82 of an optical viewfinder
aligned on the optical axis of the mirror 81 as shown in FIG. 1.
When the rotary disk shutter 80 is positioned in the open area,
photographed object images are formed on a film 83, exposing the
film 83. Then this stops displaying the photographed object images
on the optical viewfinder. When the rotary disk shutter 80 is
positioned in the non-open area, frame stepping is carried out for
the film 83, with the photographed object images reflected by the
mirror 81 and displayed on the ground glass 82 of the optical
viewfinder.
[0005] Regulation of the period of opening the shutter allows
exposure time to be regulated. Changing the frame-stepping rate
allows special effects to be obtained. For example, by replaying a
high-speed motion picture (i.e. pictures taken at a high
frame-stepping rate) at a normal speed, a high-speed motion of an
object such as a water droplet dropping on the surface of water can
be easily observed in detail. Replaying a low-speed picture
(picture taken at a low frame-stepping rate) at a normal speed can
enhance the reality of, for example, a fighting scene or a
high-speed car chase.
[0006] In the field of TV program production, filming (or
shooting), editing, and shipping of programs are in the process of
digitalization. This is also the case in the field of movie
production. That is, owing to the developments of digital
techniques for extended definition pictures and to recent
price-reduction of digital equipment, the digitalization is also
promoted in the world of movie production.
[0007] When a video camera utilizing a solid-state image pickup
device such as a charge coupled device (CCD) is used to shoot an
object, control of shutter opening period of a film camera can be
emulated by controlling exposure time using the electronic shutter
function of the solid-state image pickup device. Also by
controlling the read time required for retrieving a signal from the
solid-state image pickup device, film-feeding speed of the film
camera can be emulated. Further, since, unlike an optical
viewfinder, an electronic viewfinder displays an image based on the
signal retrieved from the solid-state image pickup device, the
image can be displayed without any break.
[0008] Exposure time of a solid-state image pickup device is
controlled using the electronic shutter function of the device as
shown in FIG. 2A. Thus, a slow shooting (or low-speed shooting) can
be obtained by prolonging the time intervals of images to be
obtained, which can be done by extending the read time required for
reading image data as shown in FIG. 2B. However, if a slow shooting
is made in this way, images displayed on the electronic viewfinder
are refreshed at a longer interval. As result, when panning the
camera to a desired object in motion such that the image of the
object is displayed at the center of the electronic viewfinder
according to the displayed image on the electronic viewfinder if
the object is fast one, the actual position of the image of the
object on the imaged picture differs from the displayed position of
the object in the viewfinder. Thus, the camera cannot correctly
follow the shooting object.
[0009] On the other hand, when exposure time is extended while
shooting a moving object, a resolution power for object in a time
axis direction (hereinafter referred to as "dynamic resolution
power") decreases in the shot picture due to an increased movement
of the object during one exposure time, making the displayed image
blurred on the electronic viewfinder.
[0010] In a solid-state image pickup device, noise component called
dark current is generated in photo-sensor for performing a
photoelectric conversion. It is known that the dark current
increases with exposure time. Hence, dark current increases when
exposure time is increased to enhance the imaging signal. As a
consequence, the margin of the dynamic range of the photo-sensor
decreases with exposure time, as shown in FIG. 3. For example, when
the object is bright, there are too many of imaging signals for
processing to obtain an image having steady reproduction of
halftones so as to cause a halation on a bright part thereof. If,
in an attempt to increase the margin of the dynamic range,
illumination is dimmed or an amount of light entering the
solid-state image pickup device is reduced by decreasing the
aperture of the object lens, a level of the dark current will not
vary but only the intensity of the image signal decreases, thereby
resulting in deterioration of the S/N ratio. Hence, a noiseless
clear image cannot be obtained in this way.
[0011] In a CCD, noise components such as dark current and smear
generated in the transmission lines for reading electric charges
generated by the photoelectric conversion in the photo-sensor,
increase with the read time required for reading signals. Hence, if
the read time is increased, the margin of the dynamic range is
further decreased according to the noise components, resulting in
further deterioration of the S/N ratio.
SUMMARY OF THE INVENTION
[0012] It is therefore an object of the invention to provide an
apparatus and a method for processing an image signal capable of
preventing the lowering of the margin of the dynamic range of the
photo-sensor even in slow shooting and capable of providing a high
quality image without deteriorating its S/N ratio.
[0013] It is another object of the invention to provide imaging
equipment capable of preventing the lowering of the margin of the
dynamic range of the photo-sensor even in slow shooting and capable
of providing a high quality image without deteriorating its S/N
ration.
[0014] An apparatus for processing an image signal in accordance
with one aspect of the invention comprises a filter for filtering
an input image signal in a time axis direction thereof, thereby
lowering the dynamic resolution power in association with the image
signal. The apparatus also comprises a frame skipping processor for
performing frame skipping processing on the input image signal thus
filtered, thereby obtaining an output image signal having a
preferred frame period. The apparatus further comprises a
controller allowing for altering a filtering coefficient of the
filter means.
[0015] A method for processing an image signal in accordance with
another aspect of the invention comprises filtering an input image
signal in a time axis direction thereof using a digital low-pass
filter, thereby lowering the dynamic resolution power in
association with the input image signal. The method also comprises
performing frame skipping processing on the input image signal thus
filtered, thereby obtaining an output image signal having a
preferred frame period.
[0016] In this arrangement, the input image signal is filtered by
means of a digital low-pass filter in time axis direction, thereby
lowering the dynamic resolution power in association with the input
image signal. The filtering coefficient of the digital low-pass
filter is varied so as to regulate the reduction of the dynamic
resolution power. The image signal having the dynamic resolution
power thus reduced is subjected to the frame skipping processing by
the frame skipping processor to obtain an output image signal
having the reduced dynamic resolution power and a preferred low
frame period.
[0017] Imaging equipment in accordance with further aspect of the
invention comprises an image pickup device for forming an image
signal. The imaging equipment also comprises a digital low-pass
filter for filtering the image signal in a time axis direction
thereof, thereby lowering the dynamic resolution power in
association with the image signal. The imaging equipment further
comprises a frame skipping processor for performing frame skipping
processing on the image signal obtained through the filtering,
thereby forming an output image signal having a preferred frame
period. The imaging equipment also comprises a controller for
controlling generation of the image signal in the image pickup
device, filtering characteristic of the digital low-pass filter,
and the frame skipping processing in the frame skipping processor.
The imaging equipment still further comprises a switch for
selecting either the image signal generated by the image pickup
device or the image filtered signal by the digital low-pass filter
to output the selected image signal to an image monitor such as an
image viewfinder.
[0018] In this arrangement, the image signal generated by the image
pickup device is filtered by means of a digital low-pass filter in
a time axis direction, thereby lowering the dynamic resolution
power in association with the image signal. The filtering
coefficient of the digital low-pass filter is varied so as to
regulate the reduction of the dynamic resolution power. The image
signal having the dynamic resolution power thus reduced is
subjected to the frame skipping processing to obtain an output
image signal having the reduced dynamic resolution power and a
preferred low frame period. In addition, either the image signal
generated by the image pickup device or the filtered image signal
is selected as the output image signal to be displayed on an image
monitor such as an image viewfinder.
[0019] The concluding portion of this specification particularly
points out and directly claims the subject matter of the present
invention. However those skill in the art will best understand both
the organization and method of operation of the invention, together
with further advantages and objects thereof, by reading the
remaining portions of the specification in view of the accompanying
drawing(s) wherein like reference characters refer to like
elements.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a diagram showing operations of a conventional
film camera.
[0021] FIGS. 2A and 2B illustrate a case in which time of
retrieving signals is elongated to emulate slow shooting.
[0022] FIG. 3 is a diagram showing the margins of the dynamic range
of a photo-sensor for short and long exposure times,
[0023] FIG. 4 is a block diagram representation of a video camera
in accordance with the invention.
[0024] FIG. 5 shows a block diagram representation of a
speed-dependent processing section.
[0025] FIG. 6 is a graph showing step response characteristic of
the digital low-pass filter.
[0026] FIG. 7 is a graph showing a frequency characteristic of the
digital low-pass filter.
[0027] FIGS. 8A-8D show operations of an image pickup device
operating at a low shooting speed.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
[0028] The invention will now be described in detail by way of
example with reference to the accompanying drawings. FIG. 4 shows
an arrangement of a video camera. Incident beam of light enters
through an imaging lens 10 into an imaging pickup section 11 and
then an image of an object is formed on the imaging surface of the
imaging pickup section 11. The image pickup section 11 is formed of
a solid-state image pickup device, e.g. CCD. The image pickup
section 11 retrieves, based on a drive signal RC received from a
drive section 52 as described later, a signal associated with a
picture of the object obtained through photoelectric conversion of
the image of the object. The section 11 generates RGB signals Spa,
for example, on the imaged picture, and supplies the resultant RGB
signals to a pre-processing section 12.
[0029] The pre-processing section 12 executes
noise-component-removing processing (e.g. correlated double
sampling) on the image signal Spa to remove therefrom noise
components, and supplies the resultant signal Spa that is removed
of noises as an image signal Spb to an A/D conversion section 13.
The A/D conversion section 13 converts the image signal Spb into a
digital image signal DVa and supplies it to a feedback clamp
section 14. The feedback clamp section 14 controls A/D conversion
so that an image signal DVa having a stable black level within a
predetermined range can be obtained with the difference between the
black level signal of a blanking period and a reference signal
being detected and supplied to the A/D conversion section 13. A
corrective processing section 15 executes corrections to the image
signal Dva such as a shading correction and compensation for the
defects of the image pickup device. The image signal DVa corrected
in the corrective processing section 15 is supplied as an image
signal DVb to a speed-dependent processing section 20.
[0030] FIG. 5 shows a configuration of the speed-dependent
processing section 20. A subtracter 211 and an adder 213 of a
digital low-pass filter 21 and a terminal "a" of a switch 23
receive the image signal DVb from the corrective processing section
15. The subtracter 211 also receives an image signal DVf from a
one-frame delay circuit 214, which will be described later, and
subtracts the image signal DVb from the image signal DVf. The
resultant subtraction signal DVc obtained by the subtracter 211 is
supplied to a multiplier 212.
[0031] The multiplier 212 receives a filtering coefficient k from a
controller 50 for regulating the filter characteristic of the
digital low-pass filter to obtain the same result as could be
obtained by the regulation of exposure time. The subtraction signal
DVc is multiplied by the filtering coefficient k. The multiplier
212 then obtains and supplies a multiplication signal DVd to the
adder 213.
[0032] The adder 213 adds the image signal DVb supplied from the
corrective processing section 15 and the multiplication signal DVd
supplied from the multiplier 212 to generate an image signal DVe.
The image signal DVe thus obtained in the adder 213 is supplied to
the one-frame delay circuit 214, a frame-skipping circuit 22, and
the terminal "b" of the switch 23.
[0033] The one-frame delay circuit 214 delays the image signal DVe
by 1 frame before it supplies the signal as the image signal DVf to
the subtracter 211. The frame-skipping circuit 22 executes frame
skipping processing based on a skipping control signal CTa received
from the controller 50 to generate an image signal DVg having a
preferred frame number (defined by the ratio of frame period/frame
rate), which signal is supplied to a main line signal processing
section 31 as shown in FIG. 4. The movable terminal "c" of the
switch 23 is connected to a monitoring signal processing section 41
shown in FIG. 4. The switch 23 is adapted to select either the
image signal DVb or the image signal DVe based on a control signal
CTb from the controller 50 and supplies the selected signal as the
image signal DVh to the monitoring signal processing section
41.
[0034] The maim line signal processing section 31 executes on the
image signal DVg the signal processing such as gamma (.gamma.)
processing for a video output and outline compensation processing
as well as Knee correction. The image signal DVg done with the
above signal processing is supplied as an image signal DVj to a
main line output processing section 32.
[0035] The main line output processing section 32 converts the
image signal DVj to a signal CMout suitable for the device
connected to the video camera before it is output to the device.
For example, when the device complies with component signals and/or
composite signals, the image signal DVj is converted into the
signal CMout suitable for this device. When the image signal is to
be transferred via a serial digital interface that complies with a
format known as SMPTE292M, the output processing section 32 casts
the image signal DVj into the signal CMout suitable for
SMPTE292M-format and outputs it.
[0036] The monitoring signal processing section 41 also executes on
the image signal DVh the signal processing such as gamma (.gamma.)
processing and/or outline compensation processing as well as Knee
correction in accordance with an image display device implemented
for monitoring the imaged picture. For example, depending on
whether the image display device is constituted of a CRT or an LCD,
the monitoring signal processing section 41 is switched for any one
of Gamma correction characteristic, Knee correction characteristic,
and outline compensation characteristic. A monitoring output
processing section 42 receives the image signal DVh thus processed
from the monitoring signal processing section 41 as an image signal
DVk.
[0037] The monitoring output processing section 42 converts the
received image signal DVk into a signal MTout adequate for a given
image display device of image monitor. For example, if the image
display device is an analog device, the image signal DVk is
converted into an analog signal that is output as the signal
MTout.
[0038] On the other hand, the controller 50 is connected to an
operation section 51. As a user of a video camera operates the
operation section 51, the section 51 supplies an operation signal
PS to the controller 50. The controller 50 generates various
control signals CT based on the operation signal PS to control the
associated devices so that the video camera can operate as intended
by the user. When, for example, an operation is made on the
operation section 51 to select an imaging speed and/or a shutter
time (shutter opening time), the controller 50 supplies to the
speed-dependent processing section 20 an appropriate filtering
coefficient k in accordance with the selected shutter time, causing
the speed-dependent processing section 20 to generate an image
signal having a dynamic resolution power lowered in accord with the
selected shutter time. The controller 50 generates a control signal
TC for setting a frame period on reading a signal (hereinafter
referred to as "signal read frame period") for the image pickup
section 11 in accord with the selected imaging speed and supplies
the signal to the drive section 52. The controller 50 also
generates control signals CTa and CTb in accord with the selected
imaging speed, which are supplied to the speed-dependent processing
section 20. The drive section 52 generates the drive signal RC
based on the control signal TC and supplies it to the image pickup
section 11 in which a signal is read in a set frame period. The
speed-dependent processing section 20 performs the frame skipping
processing based on the control signal CTa and selects the image
signal based on the control signal CTb. The main line signal output
processing section 32 outputs the image signal corresponding to the
selected imaging speed as well as the monitoring signal output
processing section 42 outputs the image signal in accordance with
user's operation.
[0039] Next, operations of the imaging equipment will be described
below. In the speed-dependent processing section 20 as shown in
FIG. 5, the image signal DVb and the image signal DVe obtained by
filtering the image signal DVb in a time axis direction using a
digital low-pass filter 21 have a relationship given by Equation
(1) below. The amplitude characteristic of the digital low-pass
filter 21 is given by Equation (2) below, where a normalization is
made such that the amplitude at .omega.=0 equals 1. DVe .function.
[ nT ] = { DVe .function. [ ( n - 1 ) .times. T ] - DVb .function.
[ nT ] } .times. k + DVb .function. [ nT ] = ( 1 - k ) .times. DVb
.function. [ nT ] + k .times. .times. DVe .function. [ ( n - 1 )
.times. T ] ] ( 1 ) 1 - k 1 + k 2 - 2 .times. k .times. .times. cos
.times. .times. .omega. .times. .times. T .times. 1 + k 2 - 2
.times. k .times. .times. cos .times. .times. .omega. .times.
.times. T ( 2 ) ##EQU1## where the angular frequency .omega. is
related to the frame period T of the image signal DVb by
.omega.=.pi./T, k is the filtering coefficient, and n is a positive
integer.
[0040] The filter characteristic of the digital low-pass filter 21
of the speed-dependent processing section 20 varies with k. The
step response of the filter can be obtained for varying k in the
range from 0 to 1. The response becomes less steep for larger k, as
shown in FIG. 6. The frequency characteristic of a digital low-pass
filter 21 is shown in FIG. 7, from which it is seen that the pass
bandwidth decreases with the filtering coefficient k. That is, as
the filtering coefficient k increases, the residual image due to
the image signal DVe increases and high-frequency components are
lost, which manifests that the image is similar to that obtained by
incrementing exposure time to lower the dynamic resolution of the
image. In other words, by filtering the image signal DVb with a
digital low-pass filter 21 having a controlled filtering
characteristic, an image signal of a picture similar to that
obtained by controlling the exposure time (i.e. controlling the
shutter opening time) can be obtained. In short, it is possible to
provide emulated control of shutter opening time by changing the
filtering coefficient k of the digital low-pass filter 21.
[0041] To do this, filtering coefficient k of the digital low-pass
filter 21 may be altered to obtain an emulated image signal DVe for
a slow shooting by prolonging the exposure time to reduce the
dynamic resolution.
[0042] Further, the frame-skipping circuit 22 executes frame
skipping processing on the image signal DVe in accordance with the
imaging speed. In the frame skipping processing performed in the
frame-skipping circuit 22, the number of frames to be skipped may
be regulated to obtain an image signal having a preferred frame
period. Thus, the frame-skipping circuit 22 may emulate variable
frame number function of the film camera.
[0043] As a result of the frame skipping by the frame-skipping
circuit 22 with a regulated number of skipped frame, it is possible
to provide an image signal DVg of prescribed frame period that
results in a picture similar to the one obtained by reducing the
dynamic resolution with a prolonged exposure time.
[0044] It will be understood that if the movable terminal "c" of
the switch 23 is coupled to the terminal "a", the image signal DVb
is supplied to the image display device. Then, an image having a
full dynamic resolution and complete frames can be displayed on the
image display device. This enables slow shooting of an object to
correctly follow the object. Further, by supplying the image signal
DVe to the image display device by coupling the movable terminal
"c" to the terminal "b", an image can be obtained on the image
display device to confirm how much the dynamic resolution is
lowered.
[0045] For example, if exposure and reading of the image signal are
carried out by the solid-state image pickup device of the image
pickup section 11 at a frame period of Fa, as shown in FIGS. 8A-8B,
and if three frames per four frames are skipped by the
frame-skipping circuit 22 of the speed-dependent processing section
20, as shown in FIG. 8C, then the image signal DVg (to be supplied
to the signal processing section 31) will have a frame period of
Fa/4 and a reduced dynamic resolution that would be obtained by
prolonging the exposure time in accord with an appropriate
filtering coefficient k.
[0046] If the movable terminal "c" of the switch 23 is coupled to
the terminal "a" to select the image signal DVb; then the image
display device of the image monitor will show an image having a
frame period of Fa and a full dynamic resolution, as shown in FIG.
8D. Moreover, If the movable terminal "c" of the switch 23 is
coupled to the terminal "b" to select the image signal DVe, then an
image having a frame period Fa and based on the image signal DVe
supplied to the main signal processing section 31 can be obtained,
from which one may estimate how much the dynamic resolution thereof
is lowered.
[0047] If a slow shooting is not used, the filtering coefficient k
is set to 0, and then no frame skipping will be performed in the
frame-skipping circuit 22. In this instance, the image signal DVb
is the same as the image signal DVe, so that the image signal DVb
is supplied to the signal processing section 31 and the monitoring
signal processing section 41 without performing the frame skipping
processing thereon. Thus, the signal CMout having the maximum frame
rate can be output from the video camera. It is also possible to
display on the image display device an image having full dynamic
resolution. Since the electronic shutter function can be used to
shorten exposure time by controlling the read of the signal from
the solid-state image pickup device, a high-speed shooting is also
possible.
[0048] It has been described above that the signal CMout of a video
camera is produced to have a preferred frame rate by altering the
number of skipped frames in the frame skipping circuit 22. However,
it is also possible to generate a signal having a preferred frame
rate by altering not only the number of skipped frames but also the
data read frame period of the image pickup section 11 by the
control signal TC.
[0049] In this case, an appropriate number of the frame to be
skipped and an appropriate data read frame period of the image
pickup section 11 are selected such that the data read frame period
of the image pickup section 11 does not exceed a predetermined
integral multiple of the minimum frame period that can be attained
by the image pickup device. For example, if the frame period To of
a signal CMout (referred to as "output frame period") does not
exceed twice the minimum frame period Tmin that can be attained by
the image pickup device (i.e. Tmin<To .ltoreq.2Tmin), no frame
skipping is performed and only the signal read frame period Ti is
altered to obtain a signal having a preferred frame rate. If the
output frame period To exceeds 2Tmin but not 3Tmin (i.e.
2Tmin<To .ltoreq.3Tmin), then signal read frame period Ti is
reduced to one half of it, so that frame skipping is performed for
every two frames. When the frame period To exceeds 3Tmin but not
4Tmin (3Tmin<To .ltoreq.4Tmin), the signal read frame period Ti
is reduced to 1/3 of it, so that frame skipping is performed every
three frames. In this manner, when the output frame period To
exceeds n times of the minimum frame period Tmin but not (n+1)
times (nTmin<To (n+1)Tmin), then the signal read frame period Ti
is reduced to 1/n of it, so that frame skipping is performed for
every n frames. Thus, the signal read frame period Ti is always
shortened.
[0050] Since the signal read frame period Ti does not exceed a
predetermined period if the output frame period To is made
arbitrarily longer, the frame period To can be made longer and the
influences of dark current and smear generated in the image pickup
section 11 can be limited. Thus, deterioration of the S/N ratio and
the loss of the margin of dynamic range can be suppressed
accordingly. The number of frames to be skipped can be decreased by
making the signal read frame period longer. In a case where the
movable terminal "c" of the switch 23 is coupled to the terminal
"a" to select the image signal DVb to monitor the imaged picture on
the electronic viewfinder, lowering of the dynamic resolution can
be suppressed since the frame period of the image signal DVb will
remain within the predetermined period.
[0051] While the foregoing specification has described preferred
embodiment(s) of the present invention, one skilled in the art may
make many modifications to the preferred embodiment without
departing from the invention in its broader aspects. The appended
claims therefore are intended to cover all such modifications as
fall within the true scope and spirit of the invention.
* * * * *