JP3098532B2 - Image reading device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image reading apparatus for reading a document image, such as a copying machine, a facsimile, and an image scanner.

[Prior Art] The number of corrections for black level correction in this type of conventional image reading apparatus is configured as shown in FIG. 7, for example. 71 and 74 are amplifiers respectively, 72 is a capacitor for AC coupling, 73 is an analog switch for clamping the voltage of the capacitor 72 to a predetermined voltage, and 75 is an A / A which converts an analog signal from the amplifier 74 into a digital signal. It is a D (analog / digital) converter. In the above configuration,
An image signal from a photoelectric conversion unit (here, a line image sensor, not shown) is input to an input terminal of the amplifier 71, an analog switch 73 is closed at a black level position of the signal, and an A / D converter 75 I was getting voltage.

[Problems to be Solved by the Invention] However, in the conventional example as described above, if there is a slight waveform distortion in the input signal, the distortion is averaged to generate a level difference, and the ON resistance of the analog switch element (73) is increased. However, since there is a difference, there is a drawback that the black level easily causes an error with respect to a desired voltage.

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and its object is to perform accurate black level correction with a simple configuration and also cancel offset and drift of the entire analog circuit up to an A / D converter. It is another object of the present invention to provide an image reading apparatus that can perform the above-described operations.

Means for Solving the Problems In order to achieve the above object, an image reading apparatus according to the first aspect of the present invention provides a photoelectric converter that converts light from a document into an analog signal, and converts the analog signal into a digital signal. A /
A D converter, a charge / discharge circuit charged / discharged in accordance with the output of the A / D converter, and an addition circuit for adding the output of the charge / discharge circuit to the analog signal; Causing the charge / discharge circuit to charge when the reference voltage of the A / D converter does not exceed, and causing the charge / discharge circuit to discharge when the voltage of the optical black region exceeds the reference voltage. It is characterized by.

Here, the charging / discharging circuit may include an analog switch, a capacitor, and a charging / discharging resistor.

[Operation] In the present invention, light from a document is converted into an analog signal by a photoelectric converter, the analog signal is converted into a digital signal by an A / D converter, and the output of the charge / discharge circuit is converted into an analog signal by an addition circuit. When the voltage of the optical black area does not exceed the reference voltage of the A / D converter, the charge / discharge circuit is charged.When the voltage of the optical black area exceeds the reference voltage, the charge / discharge circuit is discharged. Thus, accurate black level correction can be performed with a simple configuration, and offset and drift of the entire analog circuit up to the A / D converter can be canceled.

[Example] Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

A. Basic Configuration FIG. 1 shows a basic configuration of an embodiment of the present invention. In FIG. 1, reference numeral A denotes a photoelectric converter which receives reflected light from a document illuminated by a light source through an imaging optical system, and converts the reflected light into an analog electric signal. B is an A / D converter that converts this analog electric signal into a digital signal. C is black level difference detecting means for detecting a difference between the black level of the analog electric signal and the predetermined voltage because the black level of the analog electric signal depends on a predetermined voltage. D is a black level adjusting means for averaging the black level difference obtained from the detecting means C to obtain a correction value for the input analog electric signal. E is a correction operation means for obtaining the image information voltage of the input analog electric signal from a predetermined voltage based on the correction value obtained from the adjustment means D.

As an example, the black level difference detecting means C uses the comparison signal of the A / D converter B, the black level difference adjusting means D is composed of a capacitor, a charge / discharge resistor and an analog switch, and the correction calculating means E is an operational amplifier. An adder circuit is used.

B. Internal Configuration FIG. 3 shows a mechanical internal configuration of the image reading apparatus according to one embodiment of the present invention.

The image reading device 3 includes a pulse motor 31 that drives the reading head 3, a white reference plate 3 that generates reference light of a light source 35.
2. It has a document table glass 34 that supports a document 33 with the surface (surface on which image information is present) to which light from the light source 35 shines. here,
The light source 35 that illuminates the document 33 is, for example, a fluorescent lamp. The reading head 36 scans the document surface. Further, the image reading device 3
Is a control circuit unit 37, a lens 38 for imaging the reflected light from a certain surface of the document 33 with image information on the photoelectric converter 313, mirrors 39, 310, 311, which guide the reflected light, and the image reading device and an external data processing device. It has an interface connector 312 for coupling by communication. A photoelectric converter 313 converts light of the document image information formed by the lens 38 into a voltage. Here, it is a line image sensor. Reference numeral 314 denotes a wire for driving the read head 36.

FIG. 4 shows the details of the platen glass 34 of FIG. 3 as viewed from above.

As shown in FIG. 4, a white reference plate 32 is provided at an end of the document table glass 34. 4a is the width of the white reference plate 32, and 4b is the document scanning direction.

FIG. 5 shows a detailed circuit configuration example of the control circuit unit 37 of FIG.

The AGC circuit 51 performs gain control on the level of the signal from the adding circuit 58. The A / D converter 52 converts the signal from the AGC circuit 51 from an analog signal to a digital signal. The timing generation circuit 53 controls the entire timing of the control circuit unit 37. The interface circuit 54 exchanges data with an external data processing device. The MPU 55 is a central processing unit that controls the entire control unit. ROM
The (read-on memory) 56 has a program for operating the MPU 55 therein. RAM (random access memory) 5
Reference numeral 7 denotes a storage device that can be written and read for the operation of the MPU 55. The addition circuit 58 adds the adjustment voltage from the black level adjustment circuit 59 and the output voltage of the photoelectric converter. The black level adjusting circuit 59 generates a voltage to be applied to the adding circuit 58.

C. Description of Operation Next, the operation of this embodiment will be described.

As shown in FIGS. 3 and 4, a glass supporting the original
While illuminating the lower surface of the document 33 placed on 34 with the light source 35,
The read head 36 moves at a constant speed in a predetermined direction 4b.
At this time, the reflected light 3a from the lower surface of the document 33 is imaged at the level 38, converted into an image voltage signal 3b by the photoelectric converter 313, and guided to the control circuit unit 37. At this time, the image signal 3b enters the adding circuit 58 in FIG.

FIG. 2 shows details of the adding circuit 58 and the black level adjusting circuit 59 in FIG.

The adding circuit 58 includes the operational amplifier 11. 1b and 1c are differential inputs, and 1a is an addition input. Further, the output voltage of the black level adjustment circuit 59 becomes the addition input 1a. The black level adjustment circuit 59 has analog switches 21 and 22, an operational amplifier 23 as a buffer, a charging resistor R3, a discharging resistor R4, a charging capacitor C, and the like.

The output signal 3b of the photoelectric converter 313, which is a line image sensor, generally includes an OS signal and a DOS signal. OS
Is the image output and DOS is its noise canceller output. FIG. 6 is obtained from the operational amplifier 11 of FIG.
4 shows a waveform example of a difference output signal between a DOS signal and an OS signal. This waveform has 1 to n waveforms for each pixel, and represents one main scanning waveform. In the image waveform, an OB (optical black) area has a mask applied to a light receiving portion. The voltage at that portion is used as a black reference voltage.

The clamp signal shown in FIGS. 2 and 6 is a control signal for the OB area of the black level adjustment circuit 59. Also,
OVF signal is output from the A / D converter 52 of FIG. 5, a V _a voltage A / D input is applied to the + Vref of the A / D converter 52 +
When the voltage exceeds the voltage direction, the signal is generated from the A / D converter 52 every pixel clock.

Then, when the OVF signal is “0” in the OB area, the analog switch 21 is turned on by the logic circuits 24, 25, and 26 in FIG.
Then, the charging capacitor C is charged through the charging resistor R3. On the other hand, when the OVF signal is “1” in the OB area, the analog switch 22 is turned on, and the capacitor C is discharged through the discharge resistor R4. At this time, by increasing the time constants of C, R3, and R4, the conversion accuracy of the A / D converter 52 can be prevented. In this way, the black level is correctly adjusted for the input of the A / D converter 52. As a result, the image signal is correctly digitally converted by the A / D converter 52.

D. Other Embodiments When the black level of the output of the photoelectric converter 313 is not uniform for each pixel, an adder (not shown) is added to the output terminal of the black level adjusting circuit 59, and the black level is adjusted by this adder. By adding the non-uniform voltage, the non-uniform black level can be further eliminated by the black level correction.

[Effects of the Invention] As described above, according to the present invention, accurate black level correction can be performed with a simple configuration, and offset and drift of the entire analog circuit up to the A / D converter can be canceled. .

[Brief description of the drawings]

FIG. 1 is a block diagram showing a basic configuration of an embodiment of the present invention, FIG. 2 is a circuit diagram showing details of an adder circuit and a black level adjusting circuit which are main parts of an embodiment of the present invention, and FIG. FIG. 4 is a schematic sectional view showing a mechanical internal configuration of an image reading apparatus according to an embodiment of the invention, FIG. 4 is a plan view showing a configuration of a platen glass part of the apparatus shown in FIG. 3, and FIG. 5 is a control circuit shown in FIG. FIG. 6 is a block diagram showing details of the unit, FIG. 6 is a waveform diagram showing output characteristics of the adder circuit of FIG. 2, and FIG. 7 is a circuit diagram showing an example of a circuit configuration of a correction circuit of a conventional device. 11 ... operational amplifier, 23 ... operational amplifier, 21,22 ... analog switch, 24, 27 ... AND gate, 25 ... inverter, 33 ... original, 35 ... light source, 37 ... control circuit unit, 38,39,310,311 ... imaging optical system, 51 ... AGC circuit, 52 ... A / D converter, 58 ... addition circuit, 59 ... black level adjustment circuit, 313 ... photoelectric converter.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) H04N 1/40-1/409 H04N 1/46 H04N 1/60

Claims (2)

(57) [Claims] A photoelectric converter that converts light from a document into an analog signal; an A / D converter that converts the analog signal into a digital signal; and a charge / discharge device that is charged / discharged according to the output of the A / D converter. A charge / discharge circuit, and an addition circuit for adding an output of the charge / discharge circuit to the analog signal, wherein the charge / discharge circuit is provided when a voltage in an optical black region does not exceed a reference voltage of the A / D converter. An image reading device for charging the battery when the voltage of the optical black region exceeds the reference voltage. 2. The image reading apparatus according to claim 1, wherein said charge / discharge circuit comprises an analog switch, a capacitor, and a charge / discharge resistor.