JPH02306777A - Commercial power supply frequency flicker elimination circuit in image pickup device - Google Patents

Abstract

PURPOSE:To attain the gain control for each field for flicker elimination correction without hindrance by controlling two gain variable devices of series connection corresponding respectively to a main line system applying AGC to an image pickup output and a correction system for flicker component whose level is fluctuated at each field. CONSTITUTION:A main line system 20 detecting an average level of an image pickup output to form an AGC control signal and a flicker correction system 30 detecting a flicker component whose level is fluctuated for each field of the image pickup output based on an AGC control signal as a reference and a 1st gain variable device 1a controlling the gain of an pickup output with a control signal at the main line system and a 2nd variable device 1b connected in series with the 1st gain variable device 1a whose gain is controlled with an output of the flicker correction system are provided. When the gain given to a gain control amplifier is maximized, the gain correction for each field for eliminating the flicker component is attained. Thus, the forming of a gain control signal at the outside of the control range of the gain control amplifier is avoided even under the worst condition of the dark image light.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a circuit for eliminating commercial power frequency flicker in an imaging device such as a video camera.

[Summary of the invention]

By controlling two series gain variable devices corresponding to the main line system that applies AGC to the image pickup output and the correction system for flicker components whose level fluctuates from field to field, the AGC loop can be adjusted during dark image light. This is a flicker removal circuit that eliminates flicker caused by the saturation of the gain variable device.

[Conventional technology]

For NTSC signal video cameras, the vertical scanning frequency is 6.
Since the frequency is 0Hz, in areas where the commercial power frequency is 50Hz, flickers interrupted by flashing fluorescent lights may appear on the screen every 1720 seconds. In other words, since the fluorescent lamp blinks every half cycle of alternating current, flicker occurs at the greatest common divisor of 60Hz and 100Hz, 2011z.

Conventionally, since flicker changes at a period of 3 fields (1/20 seconds), three AGC detection circuits corresponding to each field are installed in parallel, and the average detection output held by each detection circuit is calculated. The difference between the detection output and each detection output is calculated as A of the main AGC circuit (which has a time constant longer than 3 fields).
A method of adding it to the GC control voltage is used. In this method, gain control is performed such that AGC is applied using independent AGC control voltages in each section of three fields.

[Problem to be solved by the invention]

In the conventional configuration, when the AGC circuit for the main signal is operating at a large gain when the amount of incident light is low, when the AGC correction voltage is added for each field corresponding to the flicker fluctuation, the AGC control range is There was a drawback that flicker appeared on the screen.

In view of this problem, the present invention is designed to prevent AGC from saturating and to perform field-by-field gain control J for flicker removal correction without any trouble when imaging in a relatively dark room using fluorescent lighting. The purpose is to

[Means to solve the problem]

The commercial power frequency flicker removal circuit of the present invention includes a main line system 20 that detects the average level of the imaging output and forms an AGC control signal, and a main line system 20 that detects the average level of the imaging output and forms an AGC control signal, and A flicker correction system 30 that detects force components due to level fluctuations, a first variable gain device 1a that controls the gain of the imaging output using the control signal of the main line system, and a first variable gain device 1a that is connected in series with the first variable variable device. , and a second gain variable device 1b whose gain is controlled by the output of the flicker correction system.

[Effect]

In the case of an NTSC video camera, when an image is taken under a fluorescent light with a 50 Hz power supply, flickering occurs on the screen at a cycle of 3 fields (1/20 second). This flicker can be removed by correcting the gain control signal of the main AGC system with flicker components for each field. By applying the gain control signal of the main AGC system and the flicker correction component to separate series gain variable devices, a gain control signal that is outside the control range of the gain control amplifier can be formed even under the worst dark image light condition. There is no.

〔Example〕

FIG. 1 shows a block diagram of a main part of a first embodiment of a free/seven force removal circuit according to the present invention. The input imaging element output is converted into a digital signal by an A/D converter 2 via voltage control amplifiers (VCAs) la and 1b, which are variable gain devices, and then output to a processing circuit (not shown). The output of the A/D converter 2 is supplied to a detection circuit 3 that adds pixel data for one field, for example, to obtain the average level of the field screen.

The detection output is distributed by a switch 4 which is sequentially switched in a cycle of 3 fields for each field, and the memory CM1 to CM3) 5-1.5-2.corresponds to each field.
5-3.

The contents of each memory 5-1 to 5-3 are read out at the same timing and supplied to the adder 7, and are read out one by one at another timing, and are switched in the 1st to 3rd fields sequentially. The signal is supplied to the switch 6 that is connected.

The output of the adder 7 can be considered to be the average of the detection levels for three fields. The addition output is compared with the reference level ref by the comparator 9, and the error output as a result of the comparison is smoothed over a dozen field periods by the time constant circuit tab, and is supplied to the D/A converter 15 as an AGC control signal to convert the converted analog voltage. is supplied as the AGC control voltage to the first variable gain variable circuit VCAla. Due to the above, AG
A C loop is constructed. The adder 7, the comparator 9, and the time constant circuit 10b constitute a main line system 20.

On the other hand, the detection outputs read out in field order from the memories 5-1 to 5-3 are selected field by field by the switch 6, and compared with the reference ref by the comparator 8. The comparison output is smoothed in a time constant circuit 10a with a time constant of several field intervals, for example, and then in a subtracter 11,
The output of the time constant circuit 10b, which is the C control signal, is subtracted. Therefore, the subtracter 11 extracts the free/twist component of three field cycles in field sequence. Note that there are actually three systems of time constant circuits 10a corresponding to three fields, each of which holds the previous integral value and executes a new integral together with the updated value. Comparator 8, time constant circuit 1
0a and the subtracter 11 constitute a flicker correction system 30.

The output of the subtracter 11 is supplied as a gain control signal for flicker correction to VCAlb, which is a second gain variable device.

As shown in FIG. 2, the gain variable range of VCAla is, for example, 0 to 30 dB. On the other hand, VCAlb whose gain is controlled by the control voltage of the flicker correction system 30 is
As shown in the figure, the input voltage-gain characteristic is limited so that a gain of ±6 dB or more does not occur. Therefore, the gain control voltage is not applied outside the controllable range of VCAla as in the conventional case, and the AGC operation for removing commercial power supply flicker is performed under any conditions.

FIG. 4 shows a second embodiment, and the difference from FIG. 1 is that the VCA 1 b of the flicker correction system 30 is inserted after the A/D converter 2. VCAIb is a digital gain variable device, and feedforward control is performed using the gain control signal of the flicker correction system 30 to remove flicker.

The operation in Figure 4 is the same as Figure 1, and when selling a dark image, VCA
The conventional problem that la is saturated and flicker appears does not occur.

〔Effect of the invention〕

As described above, the present invention is configured to control the gain variable devices 1a and 1b in series using the gain control signal of the AGC system and the flicker correction signal for each field. Even if the applied gain is maximized, it is possible to perform gain correction for each field to remove flicker components, making it possible to remove flicker even under the worst conditions.Especially in a relatively dark room under fluorescent light. Even when taking an image, there is no frizz.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a first embodiment of a flicker removal circuit for an imaging device to which the present invention is applied; FIGS. 2 and 3 are input/output characteristic diagrams of two gain variable amplifier amplifiers in the circuit of FIG. 1; FIG. 4 is a block diagram similar to FIG. 1 showing a second embodiment. In addition, in the codes used in the drawings, la, lb -------・--V CA3-----
−−−−−−−・−・−・−Detection circuit 8, 9−−−−
---------・-Comparator 12.13----
------Limiter 14-------------1--
-----Adder 20---------Single line system 30---------Flicker correction system.

Claims (1)

[Claims] A main line system that detects the average level of the imaging output to form an AGC control signal, and a flicker correction system that detects a flicker component whose level fluctuates for each field of the imaging output based on the AGC control signal. a first gain variable device that controls the gain of the imaging output using the control signal of the main line system; and a second gain that is connected in series with the first gain variable device and whose gain is controlled by the output of the flicker correction system. A commercial power frequency flicker removal circuit in an imaging device including a variable device.