JPH02306776A - 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 applying amplitude limit to a signal of 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 by means of a limiter respectively and adding the signals. 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 two limiters 12, 13 limiting the level of each output of the main line system 20 and the flicker correction system 30 are provided. Then the gain of the image pickup output is controlled with a control signal being the sum of the outputs of the limiters 12, 13. When a dark image light is picked up and 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]

When forming the AGC gain control signal by adding the main line system that applies AGC to the image pickup output and the flicker component correction system whose level fluctuates from field to field, the amplitude of each is limited by a limiter before addition. This is a flicker removal circuit that eliminates saturation of the variable gain amplifier in the AGC loop.

[Conventional technology]

For NTSC signal video cameras, the vertical scanning frequency is 6.
Since the frequency is 0 Hz, in areas where the commercial power frequency is 50 Hz, flicker caused by flickering of fluorescent lights may appear on the screen with a cycle of 1720 seconds. In other words, since the fluorescent lamp blinks every half cycle of alternating current, flicker occurs at 20 Hz, which is the greatest common divisor of 60 Hz and 100 Hz.

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 has been made to enable field-by-field gain control for flicker removal correction to be performed without any problem without AGC becoming saturated when imaging in a relatively dark room using fluorescent lighting. With the goal.

[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 flicker whose level fluctuates for each field of the imaging output based on the AGC control signal. It is equipped with a flicker correction system 30 that detects the component, and two limiters 12.13 that limit the level of each output of the main line system 20 and flicker correction system 30, and a control signal that is the sum of the outputs of each limiter 12.13. It is characterized by controlling the gain of the imaging output.

[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 a limiter to the gain control signal of the main AGC system and the flicker correction amount and adding them together, a gain control signal that is outside the control range of the gain control amplifier will not be formed even under the worst dark image light conditions. .

〔Example〕

FIG. 1 shows a block diagram of essential parts of a flicker removal circuit according to the present invention. The input image sensor output is passed through a voltage control amplifier (VCA) 1 for gain control and then to an A/D converter 2.
The signal is converted into a digital signal and then sent 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 detected output is distributed by a switch 4 which is switched sequentially in a cycle of three fields for each field, and stored in memories (M1-3) 5-1.5-2.5-3 corresponding to each field.

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 which is connected.

The output of the adder 7 can be considered to be the average of the detection levels of 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 10b and supplied to the limiter 13 as an AGC control signal. The limiter 13 changes its input level to MI as shown in FIG.
Output is limited between N-MAX. This range is VCA
The gain of I corresponds to +6 to +24 dB.

The output of the limiter 13 is supplied to the D/A converter 15 via the adder 14, and the converted analog voltage is supplied to VCAl as an AGC control voltage. The AGC loop is configured as described above. Adder 7, comparator 9, time constant circuit 1
0b constitutes the 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 flicker components 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 10
a and the subtracter 11 constitute a flicker correction system 30.

The output of subtracter 11 is supplied to limiter 12 .

This limiter 12 converts the input into VCAl gain by ±6
limited to dB range. The output of limiter 12 is output from adder 1
In step 4, the signal is added to the main AGC control signal and is supplied from the D/A converter 15 to the VCAI as an AGC control signal that has been subjected to flicker correction.

As shown in Figure 2, the VCAI gain variable range is from 0 to
In the case of 30 dB, the AGC control signal of the main line system 20 is limited by the limiter 13 to +6 to +24 dB.

On the other hand, the output of the limiter 12 of the fluffy correction system 30 is ±6
dB. Therefore, when these are added together in the adder 14, the worst case is 0 dB (6-6) to 30 dB.
Gain control is performed within the range of B(24+6). Therefore, the gain control voltage is not applied outside the controllable range of VCAI as in the prior art, and the AGC operation for removing commercial power supply flicker is performed under any conditions.

In the configuration shown in Figure 1, the limit range of the limiter 13 is fixed (
±6 dB), but when the AGC control voltage of the main line system 20 is large, the limiting range may be further narrowed, and when the AccI]i voltage is small, the limiting range may be widened.

〔Effect of the invention〕

As described above, the present invention is configured to add the gain control signal of the AGC system and the flicker correction signal for each field via the limiter, so that the gain given to the gain control amplifier when imaging dark image light is maximized. Even if this happens, there is enough margin to perform gain correction for each field for flicker component removal without any problem, so flicker can be removed even under the worst conditions. In particular, flicker does not occur even when an image is taken under a fluorescent light in a relatively dark room.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a flicker removal circuit of an imaging apparatus to which the present invention is applied, and FIG. 2 is an input/output characteristic diagram of an AGC control amplifier in the circuit of FIG. In addition, in the symbols used in the drawings, 1...----1------VCA3.
・−−1−−・−1−−−−−−・−Detection circuit 8.9
−1−−−・−−−−1−−−Comparator 12, 1
3・−・・・・Limiter 14・−・−・−・・−
. . . Adder 20 --- Single line system 30 --- 1 --- This is a frikk 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. and two limiters that limit the level of each output of the main line system and flicker correction system, and the gain of the imaging output is controlled by a control signal obtained by adding the outputs of each limiter. Power frequency flicker removal circuit.