Television – Image signal processing circuitry specific to television – Gray scale transformation
Reexamination Certificate
1999-08-17
2003-04-01
Miller, John (Department: 2614)
Television
Image signal processing circuitry specific to television
Gray scale transformation
C348S679000, C348S683000, C348S222100, C348S229100, C348S255000, C455S136000, C455S138000, C455S240100, C455S245100
Reexamination Certificate
active
06542202
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a video signal processing apparatus and, more particularly, to a video signal processing apparatus improving image quality when illuminance is low.
2. Description of the Background Art
When an image is picked up by a video terminal having a camera device using an image pickup device such as a CCD (Charge Coupled Device) under low illuminance condition, for example indoors or at night outside, video images picked up by the camera device may be extremely dark or suffer from much noise.
The picked up video images become extremely dark as sensitivity of the image pickup device such as the CCD employed in the camera device is low. As semiconductors have been developed to be smaller and smaller recently, the CCD has also been reduced in size, and hence the number of signal electrons handled by one pixel has been reduced. Therefore, the output level of the CCD for an image picked up under dark condition becomes small, resulting in low sensitivity.
The cause of extremely conspicuous noise is that shot noise generated at the time of photoelectric conversion becomes innegligible, as the number of signal electrons per pixel has been reduced.
The following methods have been conventionally known as effective in improving sensitivity.
a) To enlarge aperture ratio of iris.
b) To set shutter speed slower and to make exposure time longer.
c) To improve signal level by signal processing such as AGC (Auto Gain Control).
d) To improve signal level by signal processing through frame addition method.
As to the method a), that is, to increase aperture ratio of an iris, the larger the aperture ratio, the larger lens must be mounted. When an image is picked up under bright condition, a camera sensor would be saturated when the aperture ratio is large. Therefore, there is a limit in increasing the aperture ratio.
When automatic iris mechanism is mounted on the lens, the lens body will be increased in size, and the cost is also increased. Therefore, for a video terminal of which cost or power consumption is of priority, a fixed iris having relatively small aperture ratio must be used.
As to the method b), most of the image pickup devices such as the CCD allows free adjustment of shutter speed, by variable electronic shutter control. When an NTSC (National Television System Committee) video signal is to be generated, there is a limit of shutter speed in accordance with NTSC standard, which requires video signals of thirty (30) frames per second. Therefore, most image pickup devices allow control of shutter speed to the minimum (longest exposure time) of {fraction (1/30)} sec.
A specific example of the prior art utilizing the method c) is shown in FIG.
15
. Referring to
FIG. 15
, an AGC circuit includes a GCA (Gain Control Amplifier) circuit
1
connected to an output of D/A (Digital to Analog) converter
4
, which will be described later, an A/D (Analog to Digital) converter
2
connected to an output of GCA circuit
1
, a signal level detecting circuit
47
connected to an output of A/D converter
2
, and a D/A converter
4
connected to an output of signal level detecting circuit
47
.
A video signal output from an image pickup device is first input to GCA circuit
1
, as an input signal to AGC circuit. GCA circuit
1
amplifies the input video signal, in accordance with a gain control signal G
1
′, which will be described later, obtained from D/A converter
4
.
The amplified video signal is input to A/D converter
2
and converted to a digital video signal. The video signal which has been converted to the digital video signal is output as an output video signal of AGC circuit.
The output video signal is at the same time supplied to signal level detecting circuit
47
. Signal level detecting circuit
47
detects the level of the present video signal, from the output video signal.
The result of level detection is input to D/A converter
4
as a gain control coefficient K
1
′. D/A converter
4
converts gain control coefficient K
1
′ to an analog control signal, and provides the gain control signal G
1
′.
The output gain control signal G
1
′ is fed back to GCA circuit
1
, and automatic gain control takes place.
Referring to
FIG. 16
, the change in gain control coefficient K
1
′ output from signal level detecting circuit
47
when illuminance gradually changes from high (blight) to low (dark) will be described.
When the illuminance is high, the signal level is high, and therefore a small gain control coefficient K
1
′=min is output to provide low gain.
As the illuminance gradually lowers, the signal level becomes higher, and therefore gain control coefficient gradually increases to K
1
′=max to attain higher gain.
When the luminance attains still lower (darker) after gain control coefficient K
1
′ attains to K
1
′=max, the maximum value of gain control coefficient K
1
′=max is maintained. Therefore, the signal level becomes lower.
As to the method d), that is, frame addition method, Japanese Patent Laying-Open No. 5-344417 entitled “High Sensitivity Camera” proposes a video signal processing apparatus which improves output level of the image pickup device under low illuminance.
Referring to
FIG. 17
, a frame addition circuit constituting a part of a conventional video signal processing apparatus includes an A/D converter
2
, an image memory
5
connected to an output of A/D converter
2
, and an adder
6
connected to image memory
5
and to an output of A/D converter
2
.
The video signal output from the image pickup device is first input to A/D converter
2
. A/D converter
2
converts the input video signal to a digital video signal, and outputs the digital video signal to image memory
5
and adder
6
. Image memory
5
has a storage capacity of one frame, and operates as a one frame delay circuit. The video signal delayed by one frame and the present video signal not delayed, output from A/D converter
2
are input to adder
6
and added to each other. The added video signal is output as an output video signal of the frame addition circuit.
The AGC circuit implementing the method c) shown in
FIG. 15
is capable of amplifying a video signal of a very low level picked up with low illuminance always to a constant level, by automatic gain control in accordance with the present video signal level. Therefore, the AGC circuit has been used in many video terminals.
When a video signal of a very low level picked up with low illuminance is amplified with high gain, however, shot noise is also amplified, which means that the noise component is increased, degrading S/N (signal
oise) ratio.
Further, the frame addition circuit implementing the method d) shown in
FIG. 17
provides video signals of twice the amplitude level, as a video signal delayed by one frame and a present video signal not delayed are added.
Further, the noise component is a shot noise with low frame correlation. Therefore, when frames with noise components are added, in most cases the addition is between a pixel with noise component and another pixel without noise component. Therefore, the original video signal is doubled by addition, while noise component is not added but averaged in the added two frames. Therefore, noise component level can be suppressed to some extent.
As still images are added and averaged, the signal to noise ratio S/N of each image can be improved in the following manner.
Generally, the S/N ratio of an optical system is represented by the following equation.
S
N
=
signal
(
P
,
M
)
quantum
noise
(
P
,
M
)
+
thermal
noise
+
system
noise
Namely, the signal and the quantum noise are increased in accordance with the input photoelectric power P and the sensor magnification M. On the other hand, thermal noise is not dependent thereon.
Accordingly, thermal noise is negligible when an image is picked up using a sensor having high magnification with suf
Oda Mamoru
Takeda Mitsuhiko
Birch & Stewart Kolasch & Birch, LLP
Miller John
Natnael Paulos
Sharp Kabushiki Kaisha
LandOfFree
Video signal processing apparatus improving signal level by... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Video signal processing apparatus improving signal level by..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Video signal processing apparatus improving signal level by... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3003980