JPH0219077A - Video camera - Google Patents

Abstract

PURPOSE:To improve the sensitivity while reducing the deterioration in the S/N by applying variable control to a gamma correction quantity consecutively in a conservative way if the illuminance is deteriorated over a lighting range able to keep an output signal level constant by gain control by an automatic gain control circuit. CONSTITUTION:A process circuit 4 consists of a gamma correction circuit 5, a white clip circuit, a black clip circuit, a blanking circuit and a setup circuit or the like. Then a detection circuit 7 and a video signal level detection circuit 8 detect an output signal level of an automatic gain control circuit 3, and a gamma correction variable control circuit 9 detects it from the detection signal level by the video signal level detection circuit 8 that the gain of the automatic gain control circuit 3 is maximized and the deterioration in the illuminance is detected and applies variable control to the gamma correction quantity of the gamma correction circuit 5 continuously. Thus, the output level if the gamma correction circuit is nearly kept constant to improve the sensitivity at low illuminance while suppressing the deterioration in the S/N to the utmost.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention maintains the output signal level from the image sensor constant even when capturing images at low illuminance, and captures images with high sensitivity regardless of changes in the illuminance of the subject. The present invention relates to a video camera equipped with an automatic gain control circuit for making it possible to perform the following steps.

[Prior Art] Figure 3 is a block diagram showing the configuration of a conventional video camera. In the figure, (1) is a lens, (2) is an image sensor, and light incident from the lens (+) marked E is shown. (3) is an automatic gain control circuit that maintains the signal level from the image sensor (2) constant regardless of changes in illuminance. (4) and (6) are process signals. A circuit and an encoder circuit convert the output signal from the automatic gain control circuit (3) into a television signal.

Next, the operation of the above 411 configuration will be explained.

The light incident on the image sensor (2) through the lens (+) is
It is converted into an electrical signal with a level proportional to the first arrival of the signal and output. Next, the electric signal is subjected to gain control in an automatic gain control circuit (3) to increase the gain during low-illuminance imaging, and is output as a signal at a constant level regardless of changes in illuminance, and then to a process circuit (4). ) and an encoder circuit (6), the signal is converted into a television signal and output.

In this way, the level of the television signal is held constant even during low illuminance, and sensitivity is maintained at a good level.

[Problem to be solved by the invention] Although conventional video cameras are configured as described above, there is a limit to the illumination range in which the output signal level can be kept constant through gain control by an automatic gain control circuit. If the gain amount of the automatic gain control circuit is increased in order to obtain sensitivity that can cope with the above-mentioned decrease in illuminance, noise components such as dark current of the image sensor will be greatly amplified as the illuminance decreases. There was a problem in that the S/N became very poor, and as the S/N deteriorated, the image quality deteriorated.

This invention was made to solve the above-mentioned problems, and it is an object of the present invention to provide a video camera that can improve sensitivity by reducing the deterioration of S/N at low illuminance. With the goal.

[Means for solving the problem] The video camera according to this video camera II+ detects a decrease in the output signal level of the automatic gain control circuit, and variably controls the gamma correction acid of the gamma correction circuit in accordance with the amount of decrease. It is characterized by what it does.

[Function] According to the present invention, during imaging at low illuminance, after the gain amount of the automatic gain control circuit reaches the maximum, the illuminance decreases, that is, according to the amount of decrease in the output signal level of the automatic gain control circuit. By continuously and modestly variable controlling the gamma correction acid, the output level of the gamma correction circuit can be kept almost constant, and the sensitivity at low illuminance can be improved while suppressing the deterioration of S/N as much as possible. can.

[Embodiment of the Invention] Hereinafter, an embodiment of the present invention will be described based on the drawings.

FIG. 1 is a block diagram showing the configuration of a video camera according to an embodiment of the present invention.
) and (6) are the same as the conventional example shown in FIG. 3, so the same reference numerals are given and the explanation thereof will be omitted.

In FIG. 1, (5) is a gamma correction circuit, and the process circuit (4) is connected to the gamma correction circuit (5), a self-cleaning circuit (not shown), a black clipping circuit, a blanking circuit, a set-up circuit, etc. It consists of

(7) and (8) are a detection circuit and a video signal level detection circuit, which detect the output signal level of the automatic gain control circuit (3). (9) is a gamma correction variable control circuit that detects that the gain amount of the automatic gain control circuit (3) has reached the maximum based on the signal level detected by the video signal level detection circuit (8) and that the illuminance has further decreased. , In accordance with the detection, the gamma correction amount of the gamma correction circuit (5) is continuously variably controlled.

Next, the operation of the above configuration will be explained.

When imaging in an illuminance range where the gain amount of the automatic gain control circuit (3) does not reach its maximum, it operates in the same way as in the conventional example.
Maintains good sensitivity in low light conditions. At this time, (γ) in the complementary IE of the gamma correction circuit (5) in the process circuit (4) is
γ=1/2.2#0.45, which reproduces faithful images and ensures good image quality.

Next, when the gain amount of the automatic gain control circuit (3) reaches the maximum and the illuminance further decreases, the output signal level of the automatic gain control circuit (3) decreases in proportion to the illuminance.

The detection circuit (7) and the video signal level detection circuit (8) detect this, and the gamma correction variable control circuit (9)
The gamma correction amount of the gamma correction circuit (5) is continuously controlled to be modest. That is, the gamma value is increased. As a result, the compression rate of the signal that has been compressed by gamma correction is continuously reduced, and the output signal level of the gamma correction circuit (5) is kept approximately constant. As shown in Fig. 2 (11), the S/N ratio at this time is reduced by the amount of gamma correction and the increase in the peak part of the IE at the human input signal level. The S/
It is possible to reduce N. In other words, when increasing the gain star of the automatic gain control circuit (a), the amplitude of the entire signal is increased due to the influence of the noise component, and the S/
In the visual evaluation of the N state, the poor S/N ratio, especially on the black level side, becomes noticeable, or in the case of controlling the gamma correction amount, it is possible to suppress the deterioration of the S/N ratio to a small extent.

According to the following, the sensitivity at low illuminance of the television signal converted by passing the output signal of the gamma correction IF circuit (5) through the process circuit (4) and the encoder circuit (6) is determined by the deterioration of S/N. It is possible to recover in a small number of situations.

Note that FIG. 2(A) shows a gamma correction characteristic diagram based on trifold line approximation, which is generally used in video cameras, etc., and FIG. 2(8) shows gamma correction (far Gamma compensation 1 [2 characteristic diagrams are shown below. In both countries, the same human power level (at)
When compared, the output level (b2) in FIG. 2 (R) is higher than the output level (bl) in FIG. 2 (8). Also, when comparing both countries at the same output level (b2), the input level (a2) in Figure 2 (A) is the same as that in Figure 2 (n).
is larger than the input level (al) of .

This means that in order to obtain the same output level at low illuminance between a conventional video camera and a video camera according to the present invention, the gain edge of the automatic gain control circuit (3) must be set larger than that of the conventional video camera. Therefore, as mentioned above, in conventional video cameras, the amplitude of the entire signal is expanded and the S/N is deteriorated.

In addition, in the embodiment L, the automatic gain control circuit (3)
When the gain end is maximum and the output signal level is low -
If there was, the gamma correction amount of the gamma correction circuit (5) could be variably controlled, or the detection circuit (7) could be configured to detect the illuminance level using, for example, a light receiving element. The gamma correction amount of the gamma correction circuit (5) may be variably controlled depending on the detection level. In this case, an illuminance level detection circuit is used in place of the video signal level detection circuit (8).

[Effects of the Invention] As described above, according to the present invention, the output signal level can be kept constant through gain control by the automatic gain control circuit! 4.
If the illuminance decreases beyond the brightness range, the gamma correction amount can be continuously reduced to 1 and the sensitivity at low illuminances can be improved while minimizing S/N deterioration. It has the effect of making you feel better.

[Brief explanation of the drawing]

FIG. 1 is a solo diagram showing the configuration of a video camera according to an embodiment of the present invention, FIG. Block diagram showing. (1)...Lens, (2)...Image sensor, (3)...
... automatic gain control circuit, (4) ... process circuit, (
5)...Gamma correction circuit, (6)...Encoder circuit, (8)...Video signal (j level detection circuit, (9)...
...Gamma correction variable control circuit. Note that the same reference numerals in 1'A indicate the same or corresponding parts.

Claims (1)

[Claims] (1) A lens, an image sensor that converts the light incident from this lens into an electrical signal, an automatic gain control circuit that maintains a constant signal level from this image sensor, and an output signal of this automatic gain control circuit that converts the output signal to a television. a signal level detection circuit that detects the output signal level of the automatic gain control circuit when the gain amount of the automatic gain control circuit reaches a maximum in the video camera equipped with a process circuit and an encoder circuit that converts the signal into a signal; A video camera comprising: a variable gamma correction control circuit that variably controls the amount of gamma correction of the gamma correction circuit in accordance with the amount of decrease in the output signal level by the signal level detection circuit.