JPS6313576A - Original reading circuit - Google Patents

Abstract

PURPOSE:To obtain binary data faithful to an original by providing a limit circuit to detect a CCD output signal level except the ON section of an analog switch, control the gate voltage of a variable resistance element by a comparator and limit an input signal to an amplifier. CONSTITUTION:The peak value of an input level to an amplifier 5 is compared with a limiter voltage +V2 with a comparator 14, when the voltage +V2 is exceeded, the output voltage of the comparator 14 is increased, a capacitor 8 is charged through a diode 13, the gate voltage of a variable resistance element 6 is increased, the resistance value is lowered, the input level to the amplifier 5 is lowered and the signal of the black level is controlled so as not to exceed a slice voltage +Vs. Namely, the peak value of the voltage of a picture signal 101 is detected at the place of the voltage +V2 so that a voltage VB of a picture signal 102 cannot exceed a voltage +Vs and the resistance value of the variable resistance element 8 is controlled. Thus, a binary signal 103 can detect that the place of a black level is black. When an original is all white, the voltage of the picture signal 101 will not exceed the voltage +V2.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an original reading circuit with an improved image signal level control method.

[Conventional technology]

FIG. 3 shows an example of a conventional document reading circuit.

In Fig. 3, 2 is a binary data output terminal, 3 is an input terminal for interval signals for background control, l is a CCD, and 4.7
is a resistor, 5 is an amplifier with a constant gain, 6 is a variable resistance element, 8 is a capacitor, 9 is a diode, 10 is an analog switch, 11.12 is a comparator, 10V is the background level voltage at the output terminal, +V5 is a slice voltage for binarizing the image signal.

First, an image signal output from the CCDI is voltage-divided by a resistor 4 and a variable resistance element 6, amplified by an amplifier, and input to a comparator 11. In the comparator 11, if the amplified image signal exceeds +■1, the comparator 1
The output voltage of 1 increases and is applied to the diode 9 through the ON period of the analog switch 10. The diode 9 becomes forward biased, charges the capacitor 8, and increases the gate voltage of the variable resistance element 6. When the gate voltage of the variable resistance element 6 increases, the resistance value of the variable resistance element 6 decreases, and the amplifier 5
If the input voltage is lowered and the image signal is amplified again by the amplifier 5 and input to the comparator 11 and amplified is lower than +■1,
The output voltage of the comparator 11 decreases and is applied to the diode 9 through the ON period of the analog switch 10, but the diode 9 becomes reverse biased and the diode 9 is turned off completely. Then, the voltage charged in the capacitor 8 is slowly discharged through the resistor 7, and the gate voltage of the variable resistance element 6 is lowered. When the gate voltage of the variable resistance element 6 decreases, the resistance value of the variable resistance element 6 increases, thereby lowering the input voltage to the amplifier 5. After repeating the above-described operation, the level of the image signal from the CCD 1 was as close as possible to a voltage of 10V1, and was compared with +V5 by the comparator 12 to obtain binary data.

Next, the state of the image signal will be explained with reference to FIGS. 3, 4, and 51g.

Reference numeral 15 indicates an area where an image of the CCD is transmitted, 16 indicates a reading width of the document, and 17 indicates an area where 1() is turned ON.

Also, 1 and 0 on the left side of the waveform indicate ON and OFF states, and W
indicates the white level, and B indicates the black level. Fourth
The figure shows the waveform when an all-white original is read, and FIG. 5 shows the waveform when an all-black original is read.

Document width 1 of both signals 201 when the document is read by CCDI
In the area 6, the level changes depending on the background of the document, and in order to maintain the level in the area of document width 16 at a constant level, the external control signal 204 is input and the analog switch 10 is turned on. As a result, the area 16 of the signal 202 changes from a certain level to -vl, and is binarized at the level +5 to obtain binarized data 203. However, in conventional circuits, whether the original is completely white or completely black, the binary data is at the white level.

[Problem that the invention seeks to solve]

However, since the conventional document reading circuit is controlled during the ON period of the analog switch, if the CCD output signal is at the black level when the analog switch is ON, the black level becomes the +Vl level. It works like that. As a result, the signal exceeds the level of 10V5, resulting in a bright black signal being mistakenly recognized as white and output as binary data.

[Means for solving problems]

In order to solve the above-mentioned drawbacks, the document reading circuit of the present invention detects the CCD output signal level outside the ON period of the analog switch, controls the gate voltage of the variable resistance element using a newly provided comparator, and controls the gate voltage of the variable resistance element. It has a circuit that puts certain limits on the input signal.

The document reading circuit of the present invention includes a photoelectric conversion element that converts the image information of the document from light into an electrical signal, a variable resistance element that changes the level of the signal output from the photoelectric conversion element, and a variable resistance element that amplifies the signal. an amplifier with a constant gain; a first comparator that compares the level of the amplified signal with a background level voltage;
a switch that opens and closes the output signal from the first comparator in response to an external signal; a second comparator that compares the limiter voltage with the level of the input signal of the amplifier; It is characterized by comprising means for changing the resistance value of the variable resistance element depending on the higher voltage, and a third comparator for comparing the slice voltage and the level of the output signal of the amplifier.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a circuit diagram of an embodiment of the present invention. 1-12.
+V+ and +Vs are the same as the same symbols in FIG. 13 is a diode, 14 is a comparator, and 12 is a limiter voltage.

FIG. 2 shows waveforms at various parts of the circuit diagram in FIG. 1 when an all-black document is read.

-6= The signal output from the CCD 1 is voltage-divided by the resistor 4 and variable resistance element 6, input to the amplifier 5, amplified, and input to the comparator 11. The comparator 11 compares the voltage of the +■ beak, and if it is lower than 10 Vl, the output voltage of the comparator 11 becomes low, passes through the ON section of the analog switch 10, cuts off the diode 9, and transfers the voltage that was being charged to the capacitor 8 to the resistor 7. The gate voltage of the variable resistance element 6 is lowered, and the resistance value of the variable resistance element 6 is increased. As the resistance value of the variable resistance element 6 increases, the input level to the amplifier 5 increases. Therefore, the peak value of the input level of the amplifier 5 is compared with +2 by the comparator 14, and if it exceeds +2, the output voltage of the comparator 14 is increased, the capacitor 8 is charged through the diode 13, and the gate of the variable resistance element 6 is The voltage is increased and the resistance value of the variable resistance element 6 is lowered, and the input level to the amplifier 5 is lowered to control the black level signal so as not to exceed +5. That is, the voltage of the image signal 101 in FIG. 2 increases as the resistance value of the variable resistance element 6 increases, and is amplified by the amplifier 5 and becomes
．． It becomes 02. At that time, the voltage VA of the image signal 101 becomes the voltage VB of the image signal 102, and the peak value of the voltage of the image signal lot is changed to the point where the voltage voltage ■2 becomes voltage ■2 so that the voltage ■8 does not exceed the voltage → -■5. The resistance value of the variable resistance element 6 is controlled by the detection. As a result, the binarized signal 103 can be detected as black at the black level. Also, if the original is completely white, image signal 1
Since the voltage of 01 never exceeds voltage terminal 2, the output voltage of comparator 14 is always low, and diode 13 is cut off, so the operation is the same as that of the conventional circuit.

〔Effect of the invention〕

As explained above, the present invention has the effect of obtaining binary data faithful to the original by newly providing a certain limiting circuit.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing one embodiment of the present invention, and Fig. 2 is a circuit diagram showing an embodiment of the present invention.
Figure 3 is a circuit diagram of the conventional example, Figure 4 is a waveform diagram showing the operation of
5 are waveform diagrams showing the operation of FIG. 3. 1...CCD, 2...Binarized data output terminal, 3...
...Background control section signal input terminal, 4.7...Resistor,
5... Amplifier with constant gain, 6... Variable resistance element, 8... Capacitor, 9.13... Diode,
10... Analog switch, 11... Comparator, 12... Comparator, 14... Comparator,
15... COD effective image signal area, 16... Original width, 17... Analog switch ON section, 10 ■ l...
- Background level voltage at the output terminal, +■2... limiter voltage, 1VS... slice voltage for binarization, G... ground. Figure 1, Figure 2b, Mustard + Mouth, Figure 5

Claims (1)

[Claims] A photoelectric conversion element that converts the image information of the original from light to an electrical signal, a variable resistance element that changes the level of the signal output from this photoelectric conversion element, and an amplifier with a constant gain to amplify the signal. a first comparator that compares the level of the amplified signal with a background level voltage; a switch that opens and closes the output signal from the first comparator according to an external signal; a second comparator for comparing the level of the output signal of the amplifier; A document reading circuit comprising: a third comparator for comparing the level with the third comparator.