JPH0131829B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic gain control circuit, and more particularly to an automatic gain control circuit that controls the background level of an image to a constant level.

Generally, in facsimile machines and the like, a method is known in which an image sensor is used to scan a document and convert it into an electrical signal.

This type of image sensor also converts the part corresponding to the background of the document into an electrical signal, so as shown in Figure 1, there are cases where the background is white (Figure 1A) and cases where it is gray (Figure 1B). ), the base levels of output signals a and b (amplitudes C and C' in A and B in Figure 1) are different, and when they are passed through a binarization circuit whose threshold voltage is Y in A and B in Figure 1, The output signals a and b have the waveforms A and B in FIG. 2, and especially the waveform in FIG. 2B, which is the output signal b, has the disadvantage that information such as characters on the original is lost. In order to avoid the influence of this difference in base level, conventional image sensors have an automatic gain control circuit inserted on their output side to perform gain control so that the base level remains constant.

As shown in FIG. 3, the conventional facsimile device converts the image of the document 1 formed by the lens 2 into an electrical signal 9 by the image sensor 3, and further amplifies it by the variable gain amplifier 4 and outputs it. , the output signal 10 of the variable gain amplifier 4 and the reference voltage 6 are compared by the comparator 5, and when the output signal 10 of the variable gain amplifier 4 exceeds the reference voltage 6, the gain of the variable gain amplifier 4 is reduced by the output signal 11. Furthermore, when the output signal 10 of the variable gain amplifier 4 falls below the reference voltage 6, the gain controller 8 is actuated by the output signal 11 to output a control signal 13 so as to increase the gain of the variable gain amplifier 4. Furthermore, this conventional facsimile apparatus has a base detector 7, which determines that the base level of the original has changed if the output signal 11 of the comparator 5 does not change within a certain period of time. The output signal 12 is provided to output the output signal 12 so as to reduce the time constant of the gain controller 8, and to rapidly follow the base level of the original. As shown in the detailed circuit in FIG. 4, this gain controller converts the level of the output signal 11 of the comparator 5 with IC1, charges the capacitor C1 with diode D1 and resistor R1, and charges capacitor C1 with diode D2 and resistor R3 , and outputs a signal 13 that increases or decreases the gain of the variable gain amplifier 4. Here, it is assumed that when the capacitor C1 is charged, the gain of the variable gain amplifier 4 decreases, and when the capacitor C1 is discharged, the gain increases. In order to fix the base level of the original at a constant voltage (reference voltage 6), the charging time constants R1, C1 of the capacitor C1 are usually made sufficiently smaller than the discharging time constants R3, C1. This is to prevent the base level from fluctuating due to characters, graphics, etc. in the original. Here, if the background density of the original changes from white to gray, the variable gain amplifier 4
The output signal 10 of will continue to have a voltage on the black side,
The output signal 11 of the comparator 5 causes the gain controller 8 to
The capacitor C1 is discharged by the IC1, and the gain of the variable gain amplifier 4 is operated in the direction of increasing it. As mentioned above, since the discharge time constants C1 and R3 are set to sufficiently large values, it takes a long time to return to the normal base level. Therefore, the base detector 7 detects that the output signal 11 of the comparator 5 does not change for a certain period of time, and the output signal 12 of the base detector 7 operates the IC2 shown in FIG. /R1+R2, C1R3R4/R3+R4 and quickly return to the normal base level. Further explaining the conventional base detector 7, FIG. 5 shows a block diagram of the base detector 7, and FIG. 6 shows its timing chart. This base detector 7 is a counter that counts clock signals. , a change point detector 23 that detects the change point of the output signal from the comparator, and a flip-flop 24 that holds the state.
1 measures the passage of time by counting the clock signal of the clock oscillator 22 (FIG. 6, 1). The change point detector 23 receives the output signal 11 of the comparator 5.
(Fig. 6 2) detects a change (L→H or H→L) and outputs an output signal 25 (Fig. 6 3),
Clear counter 21 and flip-flop 24.

When the output signal 11 of the comparator 5 changes, the counter 21 and flip-flop 24 are cleared, and the counter 21 starts counting the clock signal of the clock oscillator 22, and as shown in 5 in FIG. 6, the count value changes over time. The count is increased accordingly, and when the set value is reached, the output signal 26 is output and the flip-flop 24 is set. Flip-flop 24 outputs output signal 12 (FIG. 6, 4) to control gain controller 8 shown in FIG.

As described above, when the conventional automatic gain control circuit is used in a facsimile apparatus having a variable sub-scanning speed, the following drawbacks occur. In other words, if there is a horizontal black line (a line parallel to the main scanning direction) in the document, if the sub-scanning speed decreases while scanning this horizontal line, the time it takes to scan the horizontal line becomes longer and variable. The output signal 10 of the gain amplifier 4 will continue to have a black voltage, and the output signal 11 of the comparator 5 will not change, and the base detector 7 will operate and the voltage shown in FIG.
Activate IC2, reduce the charging/discharging time constant, and quickly return to the normal base level. As a result, even though there were horizontal black lines on the original, the output of the automatic gain control circuit was white (base level). This drawback can be solved to some extent by lengthening the time it takes for the base detector 7 to detect that the output signal 11 of the comparator 5 does not change, but the following drawback arises. That is, a document whose base level is changing (for example, a white mount 30 as shown in FIG.
(when the diazo copy 31 is pasted on the document), the automatic gain control circuit operates so that the base level of the document becomes white in the area shown in FIG. 7A. Next, when the area shown in FIG. 7B is scanned, the output signal 10 of the variable gain amplifier 4 continues to be at a black level until the base detector 7 operates, and in extreme cases, the area shown in FIG. Everything will be black and the information will be lost.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an automatic gain control circuit that eliminates the drawbacks of automatic gain control circuits used in conventional facsimile apparatuses and that causes less deterioration in image quality even when there is a change in the background density of a document.

According to the present invention, in a facsimile apparatus including an image sensor that converts an original into an electrical signal and a variable gain amplifier that amplifies the output signal of the image sensor and whose sub-scanning speed is variable, the output signal of the variable gain amplifier and a reference A comparator that compares the voltage, a gain controller that controls the gain of the variable gain amplifier according to the output signal of the comparator, and that can change the operating time constant. An automatic gain control circuit is obtained, which includes a base detector that detects that the output signal of the gain controller does not change and controls the gain controller to shorten its operating time constant.

An embodiment of the present invention will be described in detail below with reference to the drawings.

FIG. 8 shows a facsimile machine including an automatic gain control circuit according to one embodiment of the present invention. In FIG. 8, this facsimile apparatus converts an image of a document 51 formed by a lens 52 into an electrical signal 59 by an image sensor 53, amplifies it by a variable gain amplifier 54, and sends out an output signal 60. On the other hand, the output signal 60 of the variable gain amplifier 54 is compared with the reference voltage 56 by the comparator 55, and the output signal 61 thereof is supplied to the gain controller 58. The gain controller 58 is configured to adjust the output signal 60 of the variable gain amplifier 54 to a reference voltage 5.
When the voltage exceeds 6, the gain of the variable gain amplifier 54 is decreased, and when the output signal 60 of the variable gain amplifier 54 falls below the reference voltage 56, the control signal 63 is outputted so as to increase the gain of the variable gain amplifier 54. . A distance pulse 64 generated every time the document 51 moves a unit distance and an output signal 61 of the comparator 55 are input to the base detector 57, and while the document moves a certain distance, the output signal of the comparator 55 61 does not change, the gain controller 58 is controlled by the output signal 62 of the base detector 57 so as to shorten its operating time constant.

Therefore, if there is a black horizontal line (line parallel to the main scanning direction) in the document, and the length of the black horizontal line in the sub-scanning direction is within the operating distance of the base detector 57,
The output signal 60 of the variable gain amplifier 54 will maintain the black level.

In addition, in the case of a document like that shown in FIG. 7, the automatic gain control circuit operates so that the base level of the document becomes white in the area shown in FIG. 7A, and when scanning the area shown in FIG. 7B, the base level becomes white. Up to the distance at which the detector 57 operates,
The output signal 60 of the variable gain amplifier 54 continues to be at a black level, and then when the base detector 57 is activated, the automatic gain control circuit is activated to set the base level of the document in area B in FIG. 7 to white. do.
The area in FIG. 7B becomes black up to the distance where the base detector 57 is activated, but after activation it follows the base level and becomes white, so the area (distance) where information is lost is limited. There is an advantage.

To further explain the details of the base detector 57 according to this embodiment, FIG. 9 shows a block diagram of the base detector 57, and FIG. 10 shows its timing chart. In FIGS. 9 and 10, a distance pulse 64 (FIG. 10 1) is generated each time the document or optical platform moves a unit sub-scanning distance. The counter 71 measures the moving distance of the original or the optical bench by counting the distance pulses 64. The change detector 73 detects the output signal 61 of the comparator 55 (first
0 (Fig. 10) changes (from L to H or H to L), and outputs an output signal 75 (Fig. 10, 3). The counter 71 and flip-flop 74 are cleared by the output signal 75 of the change point detector 73.

When the output signal 61 of the comparator 55 changes, the counter 71 and the flip-flop 74 are cleared, and the counter 71 starts counting distance pulses 64, and the original or the optical bench moves a unit distance as shown in FIG. The count value increases every time,
When the set value is reached, an output signal 76 is output and the flip-flop 74 is set. Flip-flop 74 provides an output signal 62 (FIG. 10, 4) to control gain controller 58, shown in FIG. Thereafter, the gain controller 58 sets the gain of the variable gain amplifier 54 as in the conventional case.

As described above, the present invention has the advantage of being able to suppress information deterioration due to the contents of the document (characters, graphics, changes in background density) within a certain distance, and realizing a good gain control operation.

[Brief explanation of drawings]

Figure 1 shows the image sensor output signals of A when the background of the document is white and B when the background is gray. FIG. 3 is a diagram showing a facsimile device using a conventional automatic gain control circuit; FIG. 4 is a diagram showing the gain controller in FIG. 3; FIG. 5 is a diagram showing the gain controller in FIG. Figure 6 is a diagram showing the time chart of each part of the base detector, Figure 7 is a diagram showing a manuscript with a diazo copy pasted on a white mount, and Figure 8 is a diagram showing the base detector of the present invention. A diagram showing a facsimile apparatus using an automatic gain control circuit as an embodiment, FIG. 9 is a diagram showing a base detector used in an embodiment of the present invention, and FIG. 10 is a diagram showing a base detector used in an embodiment of the present invention. It is a diagram showing a time chart of each part. 51... Original document, 52... Condensing lens, 53... Image sensor, 54... Variable gain amplifier, 55... Comparator, 56... Reference voltage, 57... Base detector, 58
...gain controller, 71...counter, 73...change point detector, 74...flip-flop.

Claims (1)

[Claims] 1. In a facsimile apparatus having a variable sub-scanning speed and comprising an image sensor that converts a document into an electrical signal and a variable gain amplifier that amplifies the output signal of the image sensor, the output signal of the variable gain amplifier and a reference voltage are a comparator for comparison; a gain controller capable of controlling the gain of the variable gain amplifier according to the output signal of the comparator and changing an operating time constant;
A base detector detects that the output signal of the comparator does not change during a certain distance of sub-scanning of the original, and controls the gain controller to shorten the operating time constant. automatic gain control circuit.