JP2006246163A - Image-reading device having noise reduction circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize SN improvement, without increasing lamp luminance or the scale of a lens concerning the SN improvement processing of an image-reading device. <P>SOLUTION: This image reading device is configured to calculate (max-min) from the peripheral pixel values of a pixel under consideration, and to execute LPF processing to a read image signal when the (max-min) value is a predetermined reference level or less, and not to execute the LPF processing, when the (max-min) value is the predetermined reference level or higher. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a noise reduction circuit in an image reading apparatus using an image sensor.

  Conventionally, an image reading apparatus using an image sensor has a configuration shown in FIG.

  In FIG. 8, 801 is a document, 802 is a document table glass, 803 is a document illumination lamp, 804 to 806 are folding mirrors, 807 is a lens, and 808 is a linear image sensor. Also, 811 is an automatic document feeder, 810 is a document conveying belt, and 812 is a document discharge tray.

  In the image reading apparatus shown in FIG. 8, the document illumination lamp 803 and the folding mirrors 804 to 806 scan in the sub-scanning direction, and the document can be read two-dimensionally.

  FIG. 9 shows an example of a read signal processing circuit.

  In FIG. 9, an image read signal output from the linear image sensor 901 is converted into a digital signal by an AD converter 902. The subtraction circuit 903 removes the black offset component by subtracting the black offset correction value. The shading correction circuit 904 corrects the light amount unevenness (shading unevenness) in the main scanning direction caused by the characteristics of the illumination system and the lens.

Moreover, as a prior art example, patent document 1 and patent document 2 can be mention | raise | lifted, for example.
JP-A-6-6591 JP 2001-2111318 A

  In such an image reading apparatus, in recent years, improvement in reading speed is strongly demanded.

  In order to increase the reading speed, it is necessary to increase the driving speed of the linear image sensor, but this also reduces the exposure time for each light receiving pixel to receive light. As a result, the output image signal also becomes smaller, and the signal-to-noise ratio (SN) tends to deteriorate in proportion thereto.

  Conventionally, as a method of preventing the deterioration of the SN, a method of increasing the signal intensity by increasing the amount of illumination light has been used.

  However, the increase in the amount of illumination has the disadvantages of increasing costs, increasing power consumption, increasing the size of the device, etc., and the development of high-intensity lamps in particular has become a major technical hurdle. Therefore, there is a need for a noise reduction circuit that can improve the SN without increasing the amount of light.

  The present invention provides the following noise suppression circuit and solves the conventional problems.

(1) Image reading apparatus having the following: An image sensor for reading an image, a means for converting the output of the image sensor into a digital signal, a filter means for performing band limiting processing on the image reading signal converted into a digital signal -Switching means for switching and outputting the filtered signal and the unfiltered signal for each pixel-For each pixel (pixel of interest) of the image read signal converted into a digital signal Means for calculating maximum value (max) and minimum value (min) of signal values of surrounding pixels and calculating (max-min)-Means for comparing (max-min) with a predetermined reference value-Comparison Based on the result, means for controlling the switching means for each pixel (pixel of interest) By having the above means, noise generation of the read image signal is achieved. To suppress.

  (2) In claim (1), when the value of (max-min) is less than or equal to a predetermined value, the switching means is controlled to output a filtered signal.

(3) An image reading apparatus having the following: an image sensor for reading an image, a means for converting the output of the linear image sensor into a digital signal, a band limiting process for the image reading signal converted into the digital signal 1st filter means ・ Second filter means for performing band limiting processing with a characteristic different from that of the first filter means for the image read signal converted into the digital signal ・ Signal subjected to the first filter processing, Switching means for selecting and outputting, for each pixel, one of the signals subjected to the filtering process 2 and the signal not subjected to the filtering process. Each of the image reading signals converted into digital signals Find the maximum value (max) and minimum value (min) of the signal values of the surrounding pixels for the pixel (target pixel), and (max-min) Calculating to means-the basis of the value of (max-min), to have a means said means for controlling said switching means for each pixel (target pixel), to suppress the noise component of the read image signal.

(4) Image reading apparatus having the following: a color image sensor for reading an image, a means for converting each color output of the color image sensor into a digital signal, a band for each color signal of the image reading signal converted into a digital signal A plurality of filtering means for performing a restriction process. A switching means for switching and outputting the signal subjected to the filtering process and the signal not subjected to the filtering process for each pixel. Each of the image reading signals converted into digital signals. Means for obtaining maximum value (max) and minimum value (min) of signal values of surrounding pixels for each color signal of pixel (pixel of interest) and calculating (max-min) for each color signal A plurality of comparison means for comparing (max-min) of each pixel with a predetermined reference value. Means for controlling said switching means for each eye pixel By having the above means, the noise component of the read image signal is suppressed.

  (5) In claim (4), when all the (max-min) values of the respective color signals are equal to or smaller than a predetermined value, the switching means is controlled to output a filtered signal.

  As described above, the present invention makes it possible to effectively suppress the noise component of the read image signal without increasing the amount of illumination light.

Example 1
FIG. 1 shows a noise reduction circuit according to the first embodiment of the present invention.

  In FIG. 1, reference numeral 101 denotes a pixel having a maximum signal level and a minimum signal level selected from a plurality of peripheral pixels centered on the target pixel, and a difference (max) between the maximum signal level max and the minimum signal level min. -Min), 102 is a comparator that compares the value of (max-min) with a predetermined value, 103 is an LPF that limits the bandwidth of the input image signal, and 104 is according to the output of the comparator, The selector outputs the signal after the LPF processing and the signal before the LPF processing by switching for each pixel.

  The operation of FIG. 1 will be described. The value of (max-min) indicates whether there is a large signal change among surrounding pixels. The value of (max-min) is less than or equal to a predetermined value. In this case, it is considered that there is no large signal change around the pixel of interest and there is only a small noise component that can be suppressed, and LPF processing is performed on the image signal to output a signal with a reduced noise component. Conversely, when the value of (max-min) is larger than the predetermined value, there is a large signal change, so it is considered that the image should not be blurred by the LPF, and a signal that is not subjected to LPF processing is switched and output.

  By doing so, it is possible to suppress the noise component in the density flat portion where the noise component is visually noticeable. Although the side pressure of the noise component is not applied to the portion where the signal change is large, the noise component is not so noticeable visually at the portion where the signal change is large.

  Further, the relationship between the pixel of interest and the peripheral pixels is shown in FIGS.

  FIG. 2A shows a case where a total of 9 pixels in the area of 3 × 3 pixels around the pixel of interest is a peripheral pixel, and FIG. 2B shows a total of 5 pixels in the area of 1 × 5 pixels around the pixel of interest. This is a case where peripheral pixels are used.

(Example 2)
FIG. 3 shows a second embodiment of the present invention.

  The difference from the first embodiment is that the reference level generation circuit 201 changes the comparison reference level input to the comparator 102 in accordance with the LPF output of the image signal.

  Thereby, the reference level can be reduced in the dark part of the image, and the reference level can be increased in the bright part of the image. This is because, as a characteristic of the image sensor, the light shot noise component is larger in the bright portion than in the dark portion, and as a result, the size of the noise component of the read image signal is increased. Thereby, an appropriate noise suppression effect can be obtained with respect to the dark part to the bright part.

(Example 3)
FIG. 4 shows a third embodiment of the present invention.

  The difference from FIG. 1 is that it has 403, 404 and a plurality of LPFs, and 401, 402 and a plurality of comparators, so that LPFs having different characteristics can be selected according to the value of (max-min). This is the point. This is also intended to provide an appropriate noise suppression characteristic for the dark part to the bright part of the read image signal.

Example 4
FIG. 5 shows a fourth embodiment of the present invention.

  This is a combination of the second embodiment and the third embodiment. This is also intended to provide an appropriate noise suppression characteristic for the dark part to the bright part of the read image signal.

(Example 5)
FIG. 6 shows a fifth embodiment of the present invention.

  FIG. 6 shows an embodiment in which the present invention is applied to RGB color reading signals.

  As an operation, the (max-min) value of the peripheral pixel is calculated for each color of RGB. Then, each (max-min) value is compared with a predetermined reference value level (RGB), and each comparison result is input to a determination circuit 615. The determination circuit 615 performs a logical operation of logical product or logical sum-etc on each comparison result NR, NG, NB, and outputs a control signal for selectors 610-612. This prevents RGB signals from being controlled independently as LPF ants / pears. The purpose of this is to prevent a sense of incongruity on an image from being generated by subjecting each RGB signal to LPF processing independently. Of course, if no problem occurs on the image, the noise suppression processing shown in the first to fourth embodiments may be performed for each color signal.

(Example 6)
FIG. 7 shows an example in which the concept of the embodiment of the present invention shown in FIG. 1 is further developed.

1 is different from the calculation of (max-min) in FIG.
(1) Calculation of standard deviation of peripheral pixels (2) Calculation of variance value of peripheral pixels (3) Calculation of Δmax of peripheral pixels (Δmax = | peripheral pixel value−maximum value of target pixel value |)
(4) In addition, it represents the variation amount of the peripheral pixels.

(Example 7)
Further, FIG. 12 shows an example in which the noise suppression circuit described so far is applied to an image reading apparatus using a linear image sensor.

  Here, the reason why the noise reduction circuit 1201 is arranged before the subtraction circuit 903 for black offset correction will be described.

  FIG. 10 is a diagram illustrating the input range of the AD conversion circuit.

  As shown in the figure, the AD conversion circuit has an input D range defined by an upper reference voltage and a lower reference voltage, and converts a signal level therebetween into a digital signal. For this reason, as shown in FIG. 10, the image signal input to the AD conversion circuit is input so that the black level to white level signals are within the AD input D range. For this reason, the black signal converted into the digital signal has a black offset component of Δ as shown in FIG. The black offset correction subtracting circuit described above is for removing the offset by subtracting the black offset component. When this is done, as shown in FIG. Are all clipped to “0”. If the signal is input to the noise suppression circuit in such a state, the black signal is slightly displaced to the white side by LPF processing.

  In order to prevent this, in the seventh embodiment of the present invention shown in FIG. 12, the noise suppression circuit is arranged before the subtraction circuit for black offset correction.

(Example 8)
FIG. 13 shows an eighth embodiment of the present invention.

  This is an example when the present invention is applied to a reading circuit of an RGB three-line image sensor. Here, the noise reduction circuit 806 is the one shown in FIG. Reference numeral 801 denotes a three-line color image sensor, which converts it into a digital signal by AD conversion circuits 802 to 804, and corrects an RGB reading line shift by an RGB line delay correction circuit 805. As described in the explanation of FIG. 6, the noise reduction circuit 806 is intended to avoid turning ON / OFF the RGB LPF processing for each color signal independently. This is described as an example because it is necessary to be in the subsequent stage of the RGB line delay correction circuit.

It is a figure which shows the noise reduction circuit of 1st Example of this invention. It is a figure which shows the relationship between a focused pixel and a surrounding pixel. It is a figure which shows the 2nd Example of this invention. It is a figure which shows the 3rd Example of this invention. It is a figure which shows the 4th Example of this invention. It is a figure which shows the 5th Example of this invention. It is a figure which shows the 6th Example of this invention. It is a figure which shows the example of an image reading apparatus. It is a figure which shows the example of a read signal processing circuit. It is a figure explaining the input range of the AD converter circuit. It is a figure for demonstrating black offset correction. It is a figure which shows the 7th Example of this invention. It is a figure which shows the 8th Example of this invention.

Claims (5)

An image reading apparatus having the following: An image sensor for reading an image, a means for converting the output of the image sensor into a digital signal, a filter means for performing a band limiting process on the image reading signal converted into a digital signal, and the filter Switching means for switching and outputting the processed signal and the signal that has not been filtered for each pixel. For each pixel (target pixel) of the image read signal converted to a digital signal, Means for calculating maximum value (max) and minimum value (min) of pixel signal value and calculating (max-min)-Means for comparing (max-min) with a predetermined reference value-Based on the comparison result Means for controlling the switching means for each pixel (pixel of interest) By having the above means, the noise component of the read image signal is suppressed. To.   In claim 1, when the value of (max-min) is less than or equal to a predetermined value, the switching means is controlled to output a filtered signal. An image reading apparatus having the following: An image sensor for reading an image, a means for converting the output of the linear image sensor into a digital signal, and a first filter for performing band limitation processing on the image reading signal converted into the digital signal Means: Second filter means for subjecting the image read signal converted into a digital signal to a band limiting process having characteristics different from those of the first filter means. Signal subjected to the first filter process, second filter Switching means for selecting and outputting one of the processed signal and the signal that has not been filtered for each pixel. Each pixel of the image reading signal converted into a digital signal (attention For (pixel), calculate the maximum value (max) and minimum value (min) of the signal values of the surrounding pixels and calculate (max-min) Based on the value of means-the (max-min) that, to have a means said means for controlling said switching means for each pixel (target pixel), to suppress the noise component of the read image signal. An image reading apparatus having the following: a color image sensor for reading an image, a means for converting each color output of the color image sensor into a digital signal, a band limiting process for each color signal of the image reading signal converted into a digital signal A plurality of filter means for switching a switching means for switching and outputting the filtered signal and the unfiltered signal for each pixel, and each pixel of the image read signal converted into a digital signal (attention Means for calculating the maximum value (max) and the minimum value (min) of the signal values of surrounding pixels for each color signal of pixel) and calculating (max-min) for each color signal; -Min), a plurality of comparison means for comparing each with a predetermined reference value. Each pixel (target pixel) based on the plurality of comparison results ) Means for controlling the switching means every time By having the above means, the noise component of the read image signal is suppressed.   According to a fourth aspect of the present invention, when all the (max-min) values of the respective color signals are equal to or smaller than a predetermined value, the switching means is controlled so as to output a filtered signal.

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