JPS63119388A - Automatic gain control circuit - Google Patents

Abstract

PURPOSE:To make the screen bright by adopting two-stage of operating ranges for an AGC circuit so as to place the priority to the S/N when the brightness of the screen is not so deficient and increasing the gain if the screen becomes darker to give insufficient observation. CONSTITUTION:When an input video signal level is over a level A, since the level of the output of a 1st variable gain amplifier circuit 1 is always over J, an output of a level comparator circuit 8 opens the switch circuit 5, a 2nd variable gain amplifier circuit 2 does not amplify the signal and the output of the 1st variable gain amplifier circuit 1 is outputted as it is to an output terminal OUT. Thus, when the illuminance of an object is high, the output level keeps a high level. When the input video signal level is within levels C-D, the switch circuit 5 is closed and the output of the 2nd variable gain amplifier circuit 2 is fixed to the level J by the gain control signal GC2 and when the input video signal level is below the level D, the gain limit circuit 7 fixes the 2nd variable gain amplifier circuit 2 to the maximum gain. Thus, if the illuminance of an object is lowered, the sensitivity is increased until the object is confirmed on the screen.

Description

[Detailed description of the invention] U) Industrial application field failure #! A is an automatic gain control circuit (AGC) of an amplifier used in electronic equipment such as video cameras and electronic still cameras.
related to circuits).

(b) Conventional technology In video cameras, an auto gain control circuit (hereinafter referred to as AGC circuit) is used to fix the image output signal at a constant level regardless of the illuminance of the subject. This A
As for the GC circuit, a method is used in which the gain is changed according to the input signal level and the output signal is kept constant. This A
An example of a GC circuit is HO4N5/14 published in '19 JP-A-6I-96876.

The AGC circuit according to the prior art has a feature of fixing the amplification gain when the subject illumination is high (the input signal level exceeds a predetermined level).

As a result, when the input signal level is large, the gain decreases in order to keep the output signal level constant at 7, causing a dip in the low brightness area, and VI'ii! ! The problems of unnatural lil characteristics and conspicuous deterioration of color reproducibility are solved.

By applying the AGC circuit of the prior art, the circuit shown in FIG.
As shown in Figure 3, when the subject illumination 1 becomes small and the input signal level falls below the ffr level, the amplification gain is fixed at the maximum gain, and when the subject illuminance increases and the input signal level exceeds a predetermined level, the gain is set to the minimum. Techniques for fixing prices are also commonly used.

Here, FIG. 2 shows the relationship τ between the AGC input signal level and the AGC output signal level, and FIG. 3 shows the relationship τ between the AGC input signal level and the AGC circuit gain. If the input signal level is greater than or equal to the level, the output signal level is the minimum gain @ytm*, and the output signal level is proportional to the input signal level. If the input signal level is below level 0, the maximum gain (g) is maintained as the gain, and the output No. 1 level is proportional to the input signal level.

When the input signal level is in the range of level 4 to Vpel (b), the output signal level is always fixed at level σJK, and the gain is like a curve. Therefore, when the destructive body illumination is low, the screen will be as bright as possible, and when it is high, the gradation will be q!
It is possible to prevent the unnaturalness of the notes and the noticeable deterioration of the reproducibility.

el to Il! Problems that should be solved by 11
, areas where the screen is dark due to extremely low illumination and the subject is not clear on the screen (for example, when the output signal level is
In other words, the sensitivity should be increased until the subject can be barely seen even if the noise increases to a certain extent. level (input signal level t when the slope decreases to half of the slope t level 0 → set to level 0, input-output signal level and human signal level-gain relationship t.

The dotted lines in FIGS. 2 and 3 must be changed immediately. That is, the threshold t level of the input signal level for changing the maximum gain 1c gain 0-0 and fixing it at this
It is necessary to set it to . In this case, the input signal level is from level 0 to more than half of the output signal level (like q, the output signal level is level σ), and the gain is J up to the range where N recognition of the subject is possible.
'*21111, and in proportion to this, the °C noise component is also excessively amplified, resulting in disadvantages such as poor S/N ratio and unsightly images.

Ninth Means for Solving the Problem The present invention is a #11 variable gain amplifier that controls the amplification gain and fixes the output No. 41 level to the first output level when the input signal level is within the level range of Ml. When the output of the first variable gain amplifying means is within the level range of Kabuto 2, which is lower than the 31g10 level @surroundings, jl! Output signal novel tf to a second output level lower than the first output level! i
and 1!2 variable gain amplification means connected in series with N1 variable gain amplification means to add 1' (r%
It is my honor.

(e) Operation Since the present invention is constructed as described above, when the illuminance of the subject is extremely low, both variable gain amplification means are activated and the fixed region of the output No. 18 level becomes two stages.

(f) Example Hereinafter, one flow side of the present invention will be explained according to lipid blood.

Figure N1 is a circuit block diagram of this embodiment, Figure 4 is an AGC input input idol signal level force determination signal level characteristic diagram, and N5Z
FIG. 2 is an AGC input input, image signal level-AGO circuit characteristic diagram.

The imaged video signal obtained from the COD is supplied to the hexagonal terminal (IN) as an input image signal of the AGO, and is amplified by the first variable gain amplification circuit (1) and the second variable gain amplification circuit (2). After t, the signal is output to the output terminal (OUT) and supplied to a video signal processing system such as a color signal separation circuit in the subsequent stage.

A part of the output of the first variable gain amplifier circuit (1) is supplied to the gain control signal generation circuit (3). This gain control signal generation circuit (3) is such that the output of the first variable gain amplifier circuit (1) is always l.
A gain control signal (GCl) ((()) that specifies the amplification gain so that it is fixed at the level (I) in the K4 diagram is generated, and a gain limiting circuit (
4). The gain limiting circuit (4) is specified by the gain control signal (GCl), and if the gain is 01 or more -
is fixed as this CF)k maximum gain, and when it is less than 7t0, iC is fixed as this t)Ck minimum gain, and when the gain is between (to) and 0, it is released and the gain l
! It functions to cause the first variable gain amplifier circuit (1)' to perform amplification while maintaining the gain specified by the II control signal CGCI).

Similarly, part of the output of the second variable gain amplifier circuit (2) passes through the SW'f circuit (5), which will be described later, to the gain control signal generation circuit (
6). This gain control signal generation circuit (6) generates a gain 1! IJ control signal (GC2) which specifies the second variable gain increase@circuit (2J output is always shown in Fig. 4) and supplies it to the gain limiting circuit (7). .Gain 1111 @ circuit (7) is controlled by gain control IN (GC2) to fix this ■ as the maximum gain, and [
[Stock] Below is the solution @ state and the gain is ll1lfllllI
#1 (GC:2) specified with 'tt of 7t8 profit, #
I2 variable gain amplification circuit M (functions to add 1 amplification to 2J).

(8) This is a level comparison circuit that compares the output level of GI quality gain amplification repair (1) with 1/t of the level, and when the cutting edge level is below J, the switch circuit (5J'1i
l-close the circuit. In addition, if the switch circuit 111 t5J is open, the gain control signal (GC2) will not be generated,
Second variable gain stage I11&circuit (2J is the first variable gain amplifier circuit (2J is the 7-band compensation circuit for correcting the aperture blur of the camera, which outputs the υ output as it is without amplifying it at all) Deruri, aperture control circuit (93
Since K, the amount of correction is normally held at a constant value. The aperture control circuit (9) is a gain control circuit (73 outputs and 12 outputs are also controlled as follows. Switch circuit (5)
When the gain control signal generating circuit (6) does not generate the gain control signal (GC2) due to open circuit, the gain limiting circuit (
7) also does not produce any output, and the aperture control circuit functions as described above to maintain the aperture correction circuit (correction ft of Q at a constant value ■).

The switch circuit (5) is closed and the gain control signal (GC2
) is generated, the aperture correction circuit (lI is controlled in inverse proportion to the specified gain, that is, if the gain is specified to be large, the correction amount is decreased. If the gain specified by the signal (GC2) is greater than or equal to Q, it is defined as the maximum gain hook, so the correction amount during this period is fixed at a constant value (n) (m) n).

Next, the output video signal level applied to each region of the input video signal level and the AGC circuit originality will be explained.

If the input video signal level is above the level limit, gain control @
The minimum gain is fixed at 0 in circuit (4), and the output of variable gain amplifier circuit (1) changes immediately on the straight line (L1) in Figure 4, and its level is always greater than υ. , the switch circuit (5) is opened by the output of the level comparison circuit (8), the second variable gain amplification circuit (2) does not perform amplification, and the first variable gain amplification circuit (13) is connected to the output terminal (OUT). The output is output as is. Therefore, when the subject illuminance is high, the output level remains high.

If the input video signal level is equal to the level factor ~@, the output of the variable gain width circuit (1) always maintains the level (I) by the gain control signal (GCl). At this time, the switch circuit (5) is also open. Therefore, in this area, Atsushi 1
Normal AGO operation of variable gain increase@circuit (1) is achieved.

If the input video signal level is level - 0, the gain limiting circuit (4) sets the first variable gain amplifier circuit (1) to the maximum gain.
) is fixed, and its output is directly Jml! in Figure 4. (I, 2
), the switch circuit (5) is opened at this time as well. Therefore, if the input video signal 't is higher than both bells, which is a problem in the prior art, the first variable gain amplifier circuit (1 )
Amplification with a gain larger than the maximum gain (2) is avoided.

If the input video signal level is level (Q-0), the gain of the first variable gain width circuit (1) is fixed to a maximum gain of 0,
The output changes as shown at point M (L, 5) of 2g411, but in this region it is always below the level (J), the switch circuit +5J is closed, and the gain control signal (GC
2), the output of the second variable gain amplifier circuit (2) is set to level 0.
Fixed.

If the input narrow image signal level is below the level Ω, the gain of the first variable gain amplifier circuit (1) is fixed at the maximum gain [shares], and its output is at the point l! of the fourth factor. As shown by A (L5) (it changes, but it is always below the level (J) even in this region, the switch circuit (5) is closed, and the gain control @ circuit (7)
Increases the second variable gain! Since the tgl path (2) is fixed at a trivial gain, its output is M! in Figure 4. ! ! (L
4) Changes as shown below. Therefore, if the input video signal level is below level υ and the subject illuminance is extremely low, the sensitivity may be increased until the subject can be seen on the screen even at the cost of S/N' (k). It becomes possible.

As mentioned above, in the region where the input video signal level is level 0 or less, the switch circuit (5) is closed, so the gain control signal (ec2) is supplied to the average control circuit (9), and the gain control signal (ec2) is supplied to the second control circuit (9). Increase @ circuit (as the gain of 2J increases, the amount of aperture distortion correction becomes smaller, that is, it is weakened by aperture correction, and works to increase 8/N. Note that the area above level 0 In this case, the aperture correction amount is maintained constant.

In the main flow side circuit, even if two stages of the variable gain increase #X circuit are connected in series, or eight stages are connected in series to make the operating range of the AGC circuit QN stages, the same effect will not be obtained. .

(g) Effects of the Invention As described above, according to the present invention, by setting the operating range of the AGC circuit to 72 levels, priority is given to S/N when the brightness of the screen is not difficult to see, and conversely, when the screen brightness is not difficult to see, priority is given to S/N. When it becomes dark, the gain can be increased to make the FiiJ surface brighter.

[Brief explanation of the drawing]

1, 4, and M5 relate to one 5Aj11 example of the present invention, in which FIG. 1 is a circuit block diagram, FIG. 4 is an input-output characteristic diagram, and FIG. 5 is an input-gain characteristic diagram. FIG. 2 is an input-output characteristic diagram of the conventional example, and FIG. 3 is an input-gain characteristic diagram of the conventional example. (1)...First variable gain amplifier circuit, ■...Second variable gain amplifier circuit.

Claims (1)

[Claims] (1) a first variable gain amplification means that controls the amplification gain to fix the output signal level at the first output level when the input signal level is within a first level range; and an output of the first variable gain amplification means. is within a second level range lower than the first level range, the first variable gain amplification means fixes the output signal level to a second output level lower than the first output level. and second variable gain amplification means connected in series to the automatic gain control circuit.