JPS6016067A - Automatic gain control amplifier - Google Patents

Abstract

PURPOSE:To obtain a picture signal having stable contrast information by setting a gain every time a light quantity change being a prescribed value or over is detected by a light quantity change detecting means. CONSTITUTION:When a light quantity fluctuation being a prescribed value or over is caused during one period of a pulse signal (e) is caused, an outut signal (g) of a comparator 18c of a light quantity change detecting circuit 18 is turned on. An AND signal between this signal (g) and a disable signal (d) is outputted from an AND gate 18d as a detection signal (h) and given to a gain control circuit 19. The circuit 19 consists of a latch 19a and a decoder 19b. The latch 19a fetches an output signal of a peak detector 14b at the leading of the signal (h) and holds it. The peak value held in the latch is decoded by the decoder 19b and converted into a gain control signal (c). The signal (c) adjusts the gain of a variable gain amplifier circuit 13 to the peak value at that time of an optimum value. Thus, even if the light quantity of the light source 16 is fluctuated during reading of an original, it is compensated and a prescribed level of picture signal (b) is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an automatic gain control amplifier for amplifying an image signal output from a document reading device of a facsimile transmitting device.

2/- Configuration of conventional example and its problems FIG. 1 is a schematic block diagram showing a conventional example of an automatic gain control amplifier used in a facsimile transmitter. In this figure, a variable gain amplifier circuit 1 amplifies an image signal input to an input terminal 2 from a document reading device (not shown), and outputs the amplified image signal to an output terminal 3. A peak detection circuit 4 detects the peak value of the image signal output from the variable gain amplifier circuit 1 for each scanning line.

Reference numeral 5 denotes a signal holding circuit, which takes in and holds the output signal (peak value) of the peak detection circuit 4 at the timing of the latch pulse sent out from the pulse generation circuit 6. The latch pulse is sent when the document reading device starts reading one page of the document. A control circuit 7 controls the gain of the variable gain amplifier circuit 1, and controls the gain so as to be inversely proportional to the peak value held in the signal holding circuit 5.

The above peak value is the image signal value corresponding to the white background of the original.
There is a proportional relationship with the light amount of the light source for document illumination.

3. Therefore, by changing the gain of the variable gain amplifier circuit 1 in inverse proportion to the peak value, an image signal can be obtained at the output terminal 3 that compensates for level changes due to variations in the amount of light from the light source.

However, as described above, after the gain is set according to the peak value at the start of document reading, the gain is fixed at ti until the next document reading starts, so compensation for changes in light intensity during document reading is impossible. However, there is a problem in that the image signal level fluctuates. For example, immediately after the light source is turned on, the light amount increases over time toward a steady light amount, but the light amount change curve is extremely steep. Therefore, immediately after the light is turned on, the amount of light changes significantly while the document is being read, resulting in a large change in the image level.
This results in the inconvenience that an image with unnecessary shading changes is transmitted.

In order to solve this problem, it is conceivable to repeatedly reset the gain at regular intervals according to the peak value of the image signal during the document reading period.

However, even if the light intensity is constant, the image signal peak value changes depending on the density of the background of the document. A new problem arises in that the image signal is likely to become unstable due to changes in the image signal.

Purpose of the Invention The present invention solves the above-mentioned conventional problems, and is capable of compensating for fluctuations in the image signal level due to changes in the amount of light during reading of an original, and is also stable even when reading originals with many changes in the gray level of the background. An object of the present invention is to provide an automatic gain control amplifier capable of obtaining an image signal having grayscale information.

Structure of the Invention The present invention provides a light amount detection means for detecting the light amount of a light source for document illumination, and a light amount change detection means for detecting a change of more than a certain value in the detected light amount by this means. The above object is achieved by configuring a structure in which gain setting is performed in the same way as in the conventional art each time a change in the amount of light is detected.

Description of examples 5/・-1 Embodiments of the present invention will be described below with reference to the drawings.

FIG. 2 is a block diagram of an automatic gain control amplifier according to an embodiment of the present invention. In this figure, reference numeral 11 denotes an input terminal to which an image signal a output from a document reading device (not shown) consisting of a COD image sensor or the like is input;
2 is an output terminal for image signals. Reference numeral 13 denotes a variable gain amplifier circuit, which includes an amplifier 13a having a constant gain, a resistor 13b and a resistor array 13c forming a variable attenuator, and a selector 13d that selectively conducts one resistor in the resistor array 13c. . The image signal a input to the input terminal 11 is attenuated by a variable attenuator at a certain attenuation ratio, and then amplified by the amplifier 13a to become an output image signal. Therefore, the gain of this variable gain amplifier circuit 13 is variable by changing the attenuation ratio of the variable attenuator. The attenuation ratio is determined by the value of the resistor selected by the selector 13d in the resistor array 13C, and the selected resistor is determined by the value of the gain control signal C6/- (digital signal) input to the selector 13d. Therefore, the gain of the variable gain amplifier circuit 13 is determined according to the gain control signal C.

14 is a peak detection circuit that detects the peak value of the image signal for each scanning line, and an analog/digital converter 14a
and a peak detector 14b. Peak detector 1
4b detects and holds the peak value of the image signal digitized by the analog/digital converter 14a while the disable signal d is off. Disable signal d
When turned on and then turned off again, the peak value held until then is discarded and the newly detected peak value is held. The disable signal d is a signal that is turned on during the blanking period of the image signal and turned off during the valid period,
It is sent out from the timing generator 15. The timing generator 15 also sends out a pulse signal e at a period shorter than the time required to read one page of the original, for example, at a period of several tens of lines, at the beginning of reading the original.

Reference numeral 16 denotes a light source for document illumination 7, - provided in the document reading device. 17 is a light amount detector, which includes an optical sensor 17a that detects the amount of light from the light source 16;
A constant gain amplifier 17b whose input is the output signal of
A light amount signal f proportional to the light amount of the light source 16 is output from this amplifier 1γb.

Reference numeral 18 denotes a light amount change detection circuit, which includes an analog/digital converter 18a, a latch 18b, a comparator 18C, and an AND gate 18d. The light quantity signal f is converted into a digital signal by the analog/digital converter 18a, and is supplied to the input of the latch 18b and one input of the comparator 18C.

The latch 18b captures and holds an input signal (the amount of light from the light source 16) at the falling edge of the pulse signal e.

The output signal (light amount) of this latch 18b is applied to the other input of the comparator 18C. Comparator 18C receives pulse signal e
The output signal q is activated during the on period of the output signal q, and turns on the output signal q when the difference between the signals applied to the two inputs is greater than or equal to a certain value. In other words, when the light amount fluctuates by a certain value or more in one cycle of the pulse signal e, the output signal q of the comparator 18c is turned on.

The AND signal of this output signal q and the disable signal d is
It is output from the AND gate 18d as a detection signal.

Therefore, the detection signal does not turn on during the image signal valid period.

19 is a gain control circuit, which includes a latch 19a and a decoder 1.
Consisting of 9b. The latch 19a captures and holds the output signal (peak value) of the peak detector 14b at the rising edge of the detection signal. The peak value held here is decoder 1
9b and converted into a gain control signal C. This gain control signal C is a variable signal composed of a resistor 13b and a resistor array 13c so that the gain of the variable gain amplifier circuit 13, or the gain of this automatic gain control amplifier, is optimized for the peak value at that time. Selector 13d selects a resistor (in resistor array 13c) that optimizes the attenuation ratio of the attenuator.
It allows you to choose. That is, when the peak value is large, a resistor that provides a large damping ratio is selected, and when the peak value is small, a resistor that provides a small damping ratio is selected by the selector 13d.

As is clear from the above explanation, in this automatic gain control amplifier, the gain is repeatedly set at the cycle of the pulse signal e from the start of document reading, so even if the light intensity of the light source 16 changes during document reading. This can be compensated for and an image signal of a constant level can be obtained. Moreover, the gain setting operation is performed only when the light intensity fluctuates by a certain value or more, and the gain is fixed when the light intensity is stable, so the gain setting is unconditionally repeated at a fixed period while the document is being read. An image signal with stable gradation information can be obtained without causing a change in the gradation level as in the case of returning the image.

Furthermore, if the gain changes during the image signal effective period, there is a risk that the image signal will be disturbed. In order to avoid such inconveniences, this automatic gain control amplifier uses an AND gate 18d to suppress the detection signal and prohibit gain setting operations during the image signal valid period.

It should be noted that although the above embodiment is mainly composed of digital circuits, it may be changed to analog circuits as appropriate10.

Of course it is possible.

Effects of the Invention As described above, the present invention includes means for detecting the light intensity of a light source for document illumination, and means for detecting a change in the detected light intensity by this means, which detects a change in the light intensity by more than a certain level. Since the gain is set according to the peak value detected by the peak detection means when the peak value is detected, there are no level fluctuations due to changes in the amount of light during document reading, and the effect is that an image signal with a stable constant level of density information can be obtained. There is.

[Brief explanation of the drawing]

FIG. 1 is a schematic block diagram showing a conventional automatic gain control amplifier, and FIG. 2 is a block diagram of an automatic gain control amplifier according to an embodiment of the present invention. 11...Input terminal, 12...Output terminal, 13...Variable gain amplifier circuit, 14...
・Peak detection circuit, 16...Light source, 17...
... light amount detector, 18 ... light amount change detection circuit, 19 ... gain control circuit.

Claims (1)

[Claims] A variable gain signal amplification means that receives the image signal output from the document reading device, a peak detection device that detects the peak value of the image signal output from the signal amplification device, and a document illumination device in the document reading device. a light amount detecting means for detecting the light amount of the light source; a light amount change detecting means for detecting a change in the amount of light detected by the light amount detecting means that exceeds a certain value; and a light amount change detecting means for sending out a detection signal; and gain control means for controlling the gain of the signal amplification means according to the peak value detected by the detection means.