JP4670764B2 - Electronic camera - Google Patents

Description

  The present invention relates to an electronic camera.

Conventionally, when an excessively bright subject exists in the imaging area, tail noise is generated in the image signal after imaging. Patent Document 1 discloses a technique for estimating the tail-like abnormal level and removing tail-like noise from the image signal.
JP 2006-24985 A (Claim 3 etc.)

  An object of the present invention is to suppress the occurrence of this trailing noise in an electronic camera.

<< 1 >> The electronic camera of the present invention includes an imaging unit, a gain adjustment unit, an A / D conversion unit, and a level limiting unit.
The imaging unit photoelectrically converts the subject image to generate an analog image signal.
The gain adjusting unit adjusts the gain of the image signal according to the set value of the imaging sensitivity.
The A / D conversion unit digitizes the image signal whose gain has been adjusted by the gain adjustment unit.
The level limiting unit limits the signal level to the image signal before gain adjustment when the gain of the gain adjustment unit is larger than a predetermined threshold.
<< 2 >> Preferably, the level limiting unit narrows the amplitude limiting width of the image signal before gain adjustment as the gain of the gain adjusting unit increases.
<< 3 >> Preferably, the level limiting unit changes the reset potential of the signal line of the imaging unit to shift the DC level of the image signal on the signal line. As a result, the signal level of the image signal before gain adjustment is suppressed by the signal line and the dynamic range of the circuit.
<< 4 >> Preferably, the level limiting unit completes the circuit setting for suppressing the signal level before reading the image signal to the signal line.
<< 5 >> Also preferably, the level limiting unit detects the presence of a high-luminance subject by determining the subject. The level limiting unit predicts whether or not the image signal after gain adjustment exceeds a predetermined allowable level based on the gain of the gain adjustment unit and the presence of a high-luminance subject. When it is predicted that the level will be exceeded, the level limiter limits the signal level on the image signal before gain adjustment.

According to the present invention, the level of the image signal before gain adjustment is limited in conjunction with the high sensitivity setting of the imaging sensitivity. Usually, in a shooting situation using a high sensitivity setting, the signal level of the image signal generated by the imaging unit is generally low. Therefore, in a general shooting situation, there is almost no adverse effect on image quality due to the level restriction of the present invention.
On the other hand, when a high-luminance subject exists in the field, the signal level of the image signal generated by the imaging unit increases regardless of the high sensitivity setting. This signal location is greatly amplified in synergy with the high gain for high sensitivity setting. As a result, in the image signal processing circuit, the image signal is excessively saturated, so that charges cannot be completely removed, and tail noise is generated. In accordance with this high sensitivity setting, the present invention limits the signal level of the image signal before gain adjustment. As a result, it is possible to avoid the occurrence of trailing noise while avoiding excessive saturation during or after amplification.

<< First Embodiment >>
[Description of electronic camera configuration]
FIG. 1 is a block diagram showing an electronic camera 10 of the present embodiment.
In FIG. 1, a photographing lens 12 is attached to the electronic camera 10. In the image space of the photographic lens 12, the imaging surface of the imaging element 11 is arranged. The imaging element 11 is controlled by the imaging control unit 14. The image signal output from the image sensor 11 is temporarily stored in the memory 17 via the signal processing unit 15 (including the gain adjustment unit) and the A / D conversion unit 16.
This memory 17 is connected to a bus 18. A microprocessor 19, a recording unit 22, an image compression unit 24, a monitor display unit 30, an image processing unit 25 and the like are also connected to the bus 18. The microprocessor 19 is connected to an imaging control unit 14, a signal processing unit 15, an operation unit 19a such as a release button, and a photometry unit 19b. A recording medium 22a is detachably attached to the recording unit 22.

[Circuit example of level limiter]
FIG. 2 is a diagram illustrating a part of the peripheral circuit of the image sensor 11 described above.
A reset pulse is output from the imaging control unit 14. This reset pulse is input to the reset terminal of the image sensor 11 via the capacitor Cx.
On the other hand, the microprocessor 19 outputs the voltage Vs via the D / A converter 33 and the amplifier 34. This voltage is applied to the reset terminal of the image sensor 11 via the diode Di.

[Explanation of level limit operation linked to high sensitivity setting]
FIG. 3 is a flowchart for explaining the level limiting operation by the electronic camera 10. Hereinafter, this operation will be described along the step numbers shown in FIG.

Step S1: The microprocessor 19 determines a setting value for imaging sensitivity according to user settings, photometric values, and the like.

Step S2: The microprocessor 19 transmits the set value of the imaging sensitivity to the signal processing unit 15. The gain adjusting unit in the signal processing unit 15 determines the gain of the image signal in accordance with the set value of the imaging sensitivity.

Step S3: The microprocessor 19 determines the gain set in step S2 as a threshold (here, whether or not it is 18 dB or more), and determines the magnitude of the gain. If it is determined that the gain is large, the microprocessor 19 proceeds to step S4. If it is determined that the gain is small, the microprocessor 19 proceeds to step S5.

Step S4: Here, the signal level of the image signal before gain adjustment is limited in conjunction with the determination of the high gain setting (high sensitivity setting).
For example, this signal level restriction is implemented by the following circuit operation. First, the microprocessor 19 changes the data value given to the D / A converter 33 and increases the voltage Vs output from the amplifier 34 (see FIG. 2).
During the period when the reset pulse is at a low level, the diode Di is turned on by this voltage Vs, and charges are accumulated in the capacitor Cx. Along with the charge accumulation of the capacitor Cx, a voltage of (Vs−Vd) appears at the reset terminal of the signal line, where the forward voltage of the diode Di is Vd.
In this state, the reset pulse rises by the voltage width Vp and changes to the high level. Then, the voltage at the reset terminal increases by an amount corresponding to the holding voltage of the capacitor Cx. As a result, the diode Di is reverse-biased and electrically disconnected, and the voltage at the reset terminal rises to (Vp + Vs−Vd).
Thus, in conjunction with the high gain setting (high sensitivity setting) of the electronic camera 10, the applied voltage of the reset terminal (gate voltage of the reset gate) is set higher. As a result, in the image sensor 11, the reset potential of the signal line is shifted to the saturation level side as compared with the low gain setting (low sensitivity setting) (see FIG. 4). Due to the shift of the reset potential, the DC level of the image signal passing on the signal line approaches the saturation level side. As a result, the level difference between the DC level and the saturation level of the image signal is narrowed, and the amplitude limit width of the image signal is set to be narrower.
In this way, the circuit setting for suppressing the signal level of the image signal is completed each time the signal line is reset prior to reading the image signal.
After the setting operation described above, the microprocessor 19 shifts the operation to step S6.

Step S5: On the other hand, in the case of the low gain setting (low sensitivity setting), the microprocessor 19 reduces the voltage Vs output from the amplifier 34. As a result, the reset potential of the signal line is separated from the saturation level side, and the amplitude limit width of the image signal can be ensured as wide as possible (see FIG. 4).
After the setting operation described above, the microprocessor 19 shifts the operation to step S6.

Step S6: Upon receiving a release operation from the user, the microprocessor 19 instructs the imaging control unit 14 to start imaging. The imaging control unit 14 performs exposure control of the imaging device 11 with exposure settings (charge accumulation time, aperture value, etc.) based on the imaging sensitivity setting values. After completing the exposure, the imaging control unit 14 sequentially reads image signals from the imaging element 11.
At this time, the signal level of the image signal is limited according to the preset amplitude limit width by passing through the signal line in the image sensor 11 (see FIG. 4).
The gain adjustment unit in the signal processing unit 15 amplifies the image signal output from the image sensor 11 with a gain corresponding to the set value of the imaging sensitivity. The A / D converter 16 digitizes the gain-adjusted image signal and stores it in the memory 17. The image data stored in the memory 17 is subjected to data processing by the image processing unit 25, the image compression unit 24, and the like, and then saved in a file on the recording medium 22a via the recording unit 22.

[Effects of First Embodiment]
In the first embodiment, in conjunction with the high sensitivity setting of the electronic camera 10, level restriction is applied to the image signal before gain adjustment. By this level restriction, it is possible to suppress the signal level of the image signal from the high-brightness subject and prevent an excessive saturation state caused by high gain amplification at the time of setting high sensitivity. As a result, it becomes possible to eliminate the tail noise caused by excessive circuit saturation.

  In a general high-sensitivity shooting situation, the signal level of the image signal generated by the image sensor 11 is generally small, and the signal level is not limited. For this reason, image quality degradation due to signal level limitations can be almost ignored.

  In the first embodiment, in conjunction with the high sensitivity setting, the reset potential of the signal line in the image sensor 11 is shifted, and the DC level of the image signal passing through the signal line is shifted to the saturation level side. By this operation, the level difference between the DC level and the saturation level of the image signal is narrowed, and the amplitude limit width of the image signal can be narrowed. As a result, the signal level of the image signal before gain adjustment can be suppressed without separately adding a dedicated circuit for level limitation.

  Furthermore, in the first embodiment, the circuit setting for suppressing the signal level is completed prior to reading the image signal. Therefore, the signal level of the image signal before gain adjustment can be more reliably suppressed.

<< Second Embodiment >>
FIG. 5 is a flowchart for explaining the operation of the electronic camera in the second embodiment. Since the configuration of this electronic camera is the same as the configuration of the first embodiment (FIGS. 1 and 2), duplicate description is omitted. The operation of the second embodiment will be described below along the step numbers shown in FIG.

Steps S11 to S12: Same operations as steps S1 to S2 of the first embodiment

Step S13: The microprocessor 19 acquires the divided photometry result of the imaging area from the photometry unit 19b. The microprocessor 19 determines whether or not there is a subject (high brightness subject) that shows an excessive brightness value compared to the average brightness in the imaging area based on the divided photometry result.
Here, by pre-determining whether or not the gain-adjusted image signal causes excessive circuit saturation from the conditions of (divided photometric brightness value, imaging sensitivity, exposure setting), It is preferable to determine the presence or absence.

Step S14: If there is no high-luminance subject in the imaging area, the microprocessor 19 proceeds to step S16. On the other hand, if there is a high-luminance subject in the imaging area, the microprocessor 19 proceeds to step S15.

Step S15: The microprocessor 19 narrows the amplitude limit width of the image signal before gain adjustment as the gain of the gain adjustment unit increases so that the image signal does not exceed an excessive circuit saturation level after gain adjustment. For example, the amplitude limit width can be varied by increasing / decreasing the voltage Vs output from the amplifier 34 shown in FIG.
After this operation, the microprocessor 19 proceeds to step S17.

Steps S16 to S17: The same operations as Steps S5 to S6 of the first embodiment.

[Effects of Second Embodiment, etc.]
In the second embodiment, the same effect as in the first embodiment can be obtained.
Furthermore, in the second embodiment, the amplitude limit width of the image signal before gain adjustment is narrowed stepwise or continuously as the gain of the gain adjustment unit increases. Therefore, the image sensitivity setting closer to the low sensitivity does not limit the signal level of the image signal more than necessary, and the tone reproducibility and color reproducibility of the high luminance region of the image signal can be improved.

  In the second embodiment, whether or not the image signal after gain adjustment exceeds a predetermined allowable level based on the gain (imaging sensitivity setting) of the gain adjustment unit and the presence of a high-luminance subject. Predict. If it is predicted that the allowable level is exceeded, the signal level of the image signal before gain adjustment is limited. Therefore, when there is no high brightness subject in the imaging area, the signal level of the image signal is not limited. As a result, the gradation reproducibility and color reproducibility of the high luminance region of the image signal can be further improved.

<< Additional items of embodiment >>
In the above-described embodiment, the signal line of the image signal is suppressed by shifting the reset potential of the signal line in the image sensor 11. However, the present invention is not limited to this. In conjunction with the imaging sensitivity setting, a limit circuit or a level compression circuit for limiting the signal level may be added to the image signal before gain adjustment.

  In the above-described embodiment, the signal level saturation side is limited with respect to the entire image. However, the present invention is not limited to this. For example, an image area where a high-brightness subject exists may be determined, and the signal level may be suppressed only in that image area.

  In the second embodiment, a high-luminance subject is detected and the signal level of the image signal before gain adjustment is limited. However, the present invention is not limited to this. In general, tail noise is conspicuous in a pattern in which a medium to low luminance region exists adjacent to a high luminance subject. For this reason, when a high-brightness and high-contrast subject is detected, the signal level of the image signal before gain adjustment may be limited.

  As described above, the present invention is a technique that can be used for an electronic camera or the like.

1 is a block diagram showing an electronic camera 10. FIG. 3 is a diagram illustrating a part of a peripheral circuit of the image sensor 11. FIG. 5 is a flowchart for explaining the level limiting operation by the electronic camera 10. It is a figure explaining the image signal on a signal line. It is a flowchart explaining operation | movement of the electronic camera in 2nd Embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Electronic camera, 11 ... Imaging device, 12 ... Imaging lens, 14 ... Imaging control part, 15 ... Signal processing part, 16 ... A / D conversion part, 17 ... Memory, 18 ... Bus, 19 ... Microprocessor, 19a ... Operation unit, 19b ... metering unit, 22 ... recording unit, 24 ... image compression unit, 25 ... image processing unit, 30 ... monitor display unit, 33 ... D / A conversion unit, 34 ... amplifier

Claims (5)

An imaging unit that photoelectrically converts a subject image to generate an analog image signal;
A gain adjusting unit that adjusts the gain of the image signal in accordance with a setting value of imaging sensitivity;
An electronic camera comprising: an A / D conversion unit that digitizes the image signal gain-adjusted by the gain adjustment unit;
An electronic camera comprising: a level limiting unit that limits a signal level with respect to the image signal before the gain adjustment when the gain of the gain adjustment unit is larger than a predetermined threshold value. The electronic camera according to claim 1,
The level limiter is
The electronic camera characterized in that the larger the gain of the gain adjustment unit, the narrower the amplitude limit width of the image signal before gain adjustment. The electronic camera according to claim 1 or 2,
The level limiter is
The signal potential of the image signal before the gain adjustment is limited by changing the reset potential of the signal line of the imaging unit and shifting the DC level of the image signal on the signal line. And an electronic camera. The electronic camera according to claim 3.
The level limiter is
An electronic camera comprising: completing circuit setting for limiting the signal level before reading the image signal to the signal line. The electronic camera according to any one of claims 1 to 4,
The level limiter is
Detects the presence of a high brightness subject by determining the subject,
Before performing the gain adjustment when the image signal after the gain adjustment is predicted to exceed a predetermined allowable level from the gain of the gain adjustment unit and the presence of the high-luminance subject. A signal level is limited with respect to the image signal.

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