Solid-state image-pickup devices exhibiting faster...

Details Solid-state image-pickup devices exhibiting faster... Solid-state image-pickup devices exhibiting faster...

1998-02-05

2002-04-23

Ho, Tuan (Department: 2612)

Television

Camera, system and detail

Solid-state image sensor

Reexamination Certificate

active

06377304

ABSTRACT:

FIELD OF THE INVENTION
This invention pertains to solid-state image-pickup devices and associated drive-signal timing methods. Such solid-state image-pickup devices comprise a plurality of light-sensitive pixels arranged in a planar array and output a video signal corresponding to the image sensed by the pixels. In particular, this invention pertains to solid-state image-pickup devices that produce video signals with improved video-frame processing rates.
BACKGROUND OF THE INVENTION
Solid-state image-pickup devices are typically used in electronic camera equipment such as camcorders and digital still cameras. These devices measure light intensity at multiple discreet locations on a plane to image a scene. The devices contain an array of pixels that convert light intensity into measurable voltage signals. These voltage signals are then processed to produce a video output signal that may be stored or viewed on a video display.
Until recently, solid-state image-pickup devices provided relatively low display resolutions and generally comprised 100,000 pixels or less. More contemporary solid-state image-pickup devices have much higher resolutions, with the most recent devices employing 300,000 to 1,000,000 pixels.
While higher-resolution imaging devices are desirable, they have the drawback of requiring more signal processing to produce each frame of video output signal. This additional signal processing often results in an undesirable time-lag between the sensing and display of the image. If the imaged subject (such as a person) is moving, the time-lag may cause the displayed image (as viewed by the operator of the camera equipment) to have an inaccurate sense of what is happening in real time. This is especially a concern with digital still cameras, because if the image displayed on the viewfinder is not in real time, the camera operator will not be certain of the picture that will result when the camera shutter is actuated.
Consider a case in which a video image is displayed on the small electronic viewfinder of a camcorder or digital still camera. If the photographic subject is moving, the image displayed on the viewfinder must be updated, or “refreshed,” frequently to smoothly (and accurately) display the photographic subject. Each refresh comprises outputting a new “frame” of video information to the viewfinder. The updating frequency directly affects how smoothly and accurately the photographic subject is displayed; higher refresh rates provide for smoother displays of motion. For example, the film industry uses a refresh rate of 24 frames per second, and a television screen is typically updated at 30 frames per second. Both of these frame rates provide smooth flicker-free viewing for most people.
As the number of pixels in a solid-state image-pickup device increases, the time to refresh each frame necessarily increases. This is because the processing and display of each pixel requires a finite amount of time; the more pixels to process, the more time required. Images directly displayed from many types of conventional high-resolution solid-state image-pickup devices often exhibit prohibitively slow frame-updating speeds. It is then necessary to provide additional video processing measures, such as decreasing the number of pixels in the video signal, to provide adequate frame-updating speeds.
An additional problem inherent in the use of conventional solid-state image-pickup devices pertains to displaying video output on monitors or viewfinders that have a different pixel aspect ratio than that of the solid-state image-pickup device. For instance, a standard television has a display resolution of 755 horizontal×484 vertical pixels, which equates to an aspect ratio of 755/484 (approximately 1.6). Since the output signals from many solid-state image-pickup devices are designed to be shown on a standard television screen, these devices typically have aspect ratios close to 1.6. In contrast, many camcorder or camera viewfinders are approximately square in shape, which equates to a pixel aspect ratio of about 1.
In general, when a solid-state image-pickup device and a video display have different aspect ratios, the displayed video image will be distorted either lengthwise or width-wise. A solution to this undesirable effect is to perform additional video processing, such as adjusting the number of horizontal and/or vertical pixels in the video signal.
While additional video processing may solve the frame-updating and aspect-ratio problems, it is not the most desirable solution. Additional video processing increases the overall circuit complexity and cost, since such processing typically requires frame memory and/or subtraction circuits, etc. The additional processing steps also result in diminished frame-refresh rates.
One method for improving the screen-refresh rate is to simultaneously read-out the dark and output signals for respective horizontal lines of pixels in a continual sequential fashion. A solid-state image-pickup device that employs this technique is disclosed in Japan Kôkai Patent Document No. SHO 62-128679.
FIG. 21
schematically shows an example of an electrical circuit configuration of a solid-state image-pickup device that simultaneously reads out the dark and output signals, one horizontal line of pixels at a time.
In
FIG. 21
, multiple pixels
80
are arranged in a planar array of columns and rows. Each pixel
80
comprises a photodiode
81
to perform photoelectric conversion, a JFET (junction-type field-effect transistor)
82
to current-amplify the charge accumulated by the photodiode
81
, a MOS switch
83
to shift the charge accumulated by the photodiode
81
to the gate electrode of the JFET
82
, and a MOS switch
84
to initialize the gate potential of the JFET
82
.
The gate electrodes of the MOS switches
83
are commonly connected in each horizontal line of the pixel array. Each of these horizontal lines is individually connected to respective control pulses &PHgr;TG
1
, &PHgr;TG
2
, . . . (&PHgr;TG
1
connected to the first horizontal line, &PHgr;TG
2
connected to the second horizontal line, etc.) that are output from a vertical scanning circuit
87
.
Similarly, for a given horizontal line of pixels, the MOS switches
84
are commonly connected, with each line individually supplied with a respective control potential &PHgr;RSD
1
, &PHgr;RSD
2
, . . . , which is output from the vertical scanning circuit
87
. In addition, the gate electrodes of the MOS switches
84
are commonly connected throughout the entire pixel array and are supplied with a control pulse &PHgr;RSG.
The source electrodes of the JFETs
82
are commonly connected in each vertical column of pixels to respective vertical read-out lines
85
. A reset MOS switch
85
a
and constant current source
86
are respectively connected to each of the vertical read-out lines
85
. The gate electrodes of all the MOS switches
85
a
are commonly connected and are supplied with a control pulse &PHgr;RSTV.
The output terminal of each vertical read-out line is connected to a pair of MOS switches
88
s,
88
d.
The gate electrodes of the MOS switches
88
s
are all commonly connected and supplied with a control pulse &PHgr;Ts. The gate electrodes of the MOS switches
88
d
are also all commonly connected and supplied with a control pulse &PHgr;Td. Collectively, these MOS switches and control lines form a multiplexer circuit.
Capacitors
89
s,
89
d
are respectively connected to the output terminals of the MOS switches
88
s,
88
d.
By following each vertical read-out line
85
upward it can be seen that a respective pair of capacitors
89
s,
89
d
is provided for each vertical read-out line
85
; thus, these capacitors are marked
89
s
2
,
89
d
2
(corresponding to the first vertical read-out line),
89
s
2
,
89
d
2
(corresponding to the second vertical read-out line), etc.
Two capacitors for each vertical read-out line are provided because of the pixel-output variances associated with solid-state image-pickup devices. Each pixel in the array produces a voltage output that depends on t

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