JPH04346563A - Image reader - Google Patents

Abstract

PURPOSE:To obtain a high SN signal by always keeping a fixed potential before the image signal preceding by one line is read to eliminate the residual images and obtaining the difference between the optical signal and noise signal after reading these signals out of a phototransistor. CONSTITUTION:When an output switch 3-1 is closed, an optical signal is outputted from a phototransistor PT 1-1. In this case, the preceding image information, i.e., a residual image is eliminated. When the switch 3-1 is closed again, the PT 1-1 is reset. Then only a noise signal is outputted from the PT 1-1 with the next closing of the switch 3-1. When a base switch 4-1 is closed, the PT 1-1 is reset and then PT 1-2 is selected. Thus both optical and noise signals free from the residual images are read out in the same way. In such a way, the image signals of all phototransistors PT 1-1, 1-2... are read out. Then the difference is obtained between the read optical and noise signals and a high SN image signal containing no noise component is obtained.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image reading device used in facsimiles, copying machines, etc.

0002

2. Description of the Related Art In recent years, facsimile machines and copying machines are required to produce clear images, and image reading devices that read the images are required to have a high signal-to-noise ratio and low afterimage.

An example of a conventional image reading device will be described below with reference to the drawings. FIG. 3 shows the configuration of a conventional image reading device. The phototransistors 11-1, 11-2, . . . detect light at their positions, and are provided in a desired number at regular intervals to form a phototransistor array. Phototransistor arrays 12-1, 12-2 for noise cancellation,
... provide a light-shielding layer on the surface of the light-receiving section to prevent light from being irradiated, thereby outputting noise corresponding to the light-detecting phototransistors 11-1, 11-2, and so on. Thyristors 13-1, 13-2, ... are NP
It is composed of an N transistor and a PNP transistor. The input coupling transistor 14-1 transmits an external start signal to the first stage thyristor 13-1, and the interstage coupling transistors 14-2, 14-3, . . . detect the conduction state of the main stage. The signal is transmitted to the thyristor in the next stage, and a thyristor shift register is constituted by thyristors 13-1, 13-2, . . . and interstage coupling transistors 14-2, 14-3, . control transistor 1
5-1, 15-2, . . . are output transistors 16-
1, 16-2, . . . are light detection phototransistors 11-1, 11- selected by the shift register.
The optical signals of 2, . . . and the noise signals of the phototransistors 12-1, 12-2, . . . for noise cancellation are read out. The bias circuit 17 sets the operating voltage of the thyristor shift register. The optical signal output line 18 connects each photodetector phototransistor 1
The emitters of the phototransistors 1-1, 11-2, . . .
An optical signal from ... is output. Noise signal output line 1
9 is a phototransistor 12 for each noise cancellation.
-1, 12-2, ... emitters are commonly connected,
Each noise canceling phototransistor 12-1,
Noise signals from 12-2, . . . are output. Vcc is a power supply for the image reading device, ST is a start signal, and φ1 and φ2 are shift clocks for shifting the thyristor shift register, which are two-phase signals. So indicates the optical signal output, and No indicates the noise signal output.

FIG. 4 is a timing chart of the conventional image reading device shown in FIG. 3, and FIG. 5 is a circuit diagram of the light receiving section. In FIG. 5, Cbc is the collector-base capacitance of the photodetection phototransistors 11-1, 11-2, . . . , and Ia is the photodetection phototransistor 11-1, 11-.
2, the photocurrent generated in . . . , Vb is the base potential of the photodetection phototransistors 11-1, 11-2, .
Cbc' is phototransistor 1 for noise cancellation
2-1, 12-2, . . . Vb' is the base potential of the phototransistors 12-1, 12-2, . . . for noise cancellation.

The operation of the image reading apparatus configured as described above will be explained below with reference to FIGS. 3, 4, and 5.

The thyristors 13-1, 13-2, . . . are N-gate type, and are turned on by applying a negative voltage to the gates compared to the anodes. When the ST terminal becomes "H", the input coupling transistor 14-1 becomes "on", and the gate of the thyristor 13-1 turns "on".
"L", and the first stage thyristor becomes "on".After that, even if the gate of thyristor 13-1 becomes "H", the cathode of thyristor 13-1 remains "L", that is, φ1.
Since it is "L", the "on" state is maintained. Thereafter, the thyristor 13-1 becomes "off" when φ1 becomes "H". This state is shown at 13-1 in FIG. Next, the thyristor 13-1 is in the "on" state and the emitter of the interstage coupling transistor 14-2 is "L", that is, φ
1 becomes "L", the interstage coupling transistor 14-2 turns "on", and the gate of the thyristor 13-2 becomes "L".
Therefore, the thyristor 13-2 becomes "on". In this state, the cathode of thyristor 13-2, that is, φ2
When φ2 becomes "L", the thyristor 13-2 maintains the "on" state, and when φ2 becomes "H", it becomes the "off" state. This state is shown at 13-2 in FIG. As described above, the thyristors 13-1, 13-2, . . . are sequentially shifted. On the other hand, the control transistor 15-1 is "on" only while the thyristor 13-1 is "on" and φ1 is "L", and the base of the output transistor 16-1 is "L". This state is shown at the base 16-1 in FIG. Output transistor 16-1 is output transistor 16
When the base of -1 becomes "L", it becomes "on", accessing both the phototransistor 11-1 for light detection and the phototransistor 12-1 for noise cancellation, and the optical signal output line 18 and noise signal output. A signal is output on line 19. This state is shown as optical signal output So and noise signal output No in FIG.

Next, the process of reading out the optical signal output from the phototransistor for light detection and the noise signal output from the phototransistor for noise cancellation will be explained using FIG. First, the output transistor 16-1 receives an access signal from the control transistor and becomes "on" to charge the collector-base capacitance Cbc of the photodetecting phototransistor 11-1. Thereafter, the collector and base of the phototransistor 11-1 for light detection are opened, and during the accumulation time, the charge accumulated in the collector-base capacitance is discharged by the photocurrent Ia due to light reception in the phototransistor 11-1, and the phototransistor The base potential Vb of 11-1 rises. After the accumulation time ends, the output transistor 16-1 receives an access signal from the control transistor again and becomes "on", and the phototransistor 1
The collector-base capacitance Cbc of 1-1 is charged, and this charging current is amplified by the current amplification factor of phototransistor 11-1 and output to optical signal output line 18. on the other hand,
When the output transistor 16-1 receives an access signal from the control transistor and turns on, it simultaneously charges the collector-base capacitance Cbc' of the noise canceling phototransistor 12-1. After that, the collector of the phototransistor 12-1 for noise cancellation,
Although the base is open, it does not receive light during the storage time, so no photocurrent is generated and the charge stored in the collector-base capacitance is not discharged. Therefore, the base potential Vb' of the noise canceling phototransistor does not change, and after the accumulation time ends, the output transistor 16
Even if -1 receives an access signal from the control transistor and turns on, the collector-base capacitance Cbc of the noise canceling phototransistor 12-1 is not charged, and only the noise component appears on the noise signal output line 19. is output. In actual image signal output, a subtracter is used to subtract the noise signal from the noise canceling phototransistor from the optical signal from the photodetecting phototransistor in order to improve the S/N ratio.

Furthermore, since the phototransistor for light detection is of a floating base type, the base potential Vb of the phototransistor changes depending on the amount of light received during the storage time, that is, the magnitude of the photocurrent. The charging current that flows through the phototransistor for light detection depends on the base potential, and when the base potential is low, only a small amount of charging current flows, so when there is continuous black image information, there is no continuous photocurrent. The base potential decreases little by little with each access, and only noise components are output to the optical signal output line. When white image information is received in this state, the base potential rises, and the optical signal is read out by charging during the next access, and the base potential decreases due to charging. At this time, the base potential Vb after the accumulation time of the first line of white image information is in a state where black image information continues, that is, the base potential Vb
When the amount of discharge due to light has decreased sufficiently, the amount of discharge due to light increases, and in this state, the image information is accessed and read out. Therefore, the base potential after reading out decreases only to a level higher than the potential when black image information is continuous.

Furthermore, the base potential after the accumulation time of the second line of white image information is increased by the amount of discharge caused by light from the base potential of the first line of white image information, which has not decreased sufficiently. The state is accessed and image information is read. The base potential when reading the second line of white image information is higher than the base potential of the first line of white image information, so the signal of the second line of white image information is whiter. is larger than the image signal of the first line of image information. On the other hand, when white image information continues and changes to black image information, the opposite phenomenon occurs, and the signal of the first line of black image information is higher than the signal of the second line of black image information. also becomes larger.

[0010] (For example, "Television Society Journal," Vol. 41, No. 11, pages 998-1003).

(Japanese Unexamined Patent Publication No. 106387/1983).

0012

[Problems to be Solved by the Invention] However, in the above configuration, the phototransistor is a floating base type phototransistor, and the base potential before the storage time is not constant. When the image changes to white, it overlaps with the previous image information and becomes an optical signal output. This causes an afterimage. In addition, the noise is canceled by the difference between the two phototransistors, the phototransistor for light detection and the phototransistor for noise cancellation, but since different phototransistors are used, if the characteristics differ even slightly, the noise component cannot be completely canceled. As described above, the conventional example has problems in that afterimages occur, noise components cannot be completely canceled, and a high SN ratio cannot be obtained.

[0013] In view of the above problems, the present invention has been developed to provide a method that does not cause afterimages.
The present invention provides an image reading device that can obtain a high SN ratio.

[0014]

[Means for Solving the Problems] In order to solve the above problems, the image reading device of the present invention includes a phototransistor array for light detection, a switch connected to the base of each phototransistor of the phototransistor array. , a shift register for sequentially selecting each of the phototransistors, an output switch for accessing each of the phototransistors, and a switch connected to a signal output line of the phototransistor array.

[0015]

[Operation] With the above-described structure, the present invention eliminates afterimages by always keeping the base potential of the phototransistor at a constant potential before the accumulation time, that is, before reading the image signal of one line before, and from one phototransistor. By reading out the optical signal and the noise signal and taking the difference between them, it is possible to obtain an image signal with a high signal-to-noise ratio and no noise components.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An image reading apparatus according to an embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows the configuration of an image reading device according to an embodiment of the present invention. Phototransistor 1
-1, 1-2, . . . perform optical detection at the reading position, and shift registers 2-1, 2-2, . . . detect phototransistors 1-1, 1-2, . select. The output switches 3-1, 3-2, . . . are switches for reading out optical signals and noise signals from the phototransistors 1-1, 1-2, . Base switch 4-1, 4
-2, . . . are phototransistors 1-1, 1-2, .
The phototransistor 1-1 is connected to the base of the phototransistor 1-1,
After reading out the signals of phototransistors 1-1, 1-2, . . . , the bases of phototransistors 1-1, 1-2, . . . are set to a certain potential. The AND circuits 5-1, 5-2, . . . create the timing to close the base switches 5-1, 5-2, .
The NOT circuit 6 generates an inverted signal of the gate pulse GP. The read switch 7 connects the phototransistors 1-1, 1-
This is a switch that outputs signals from 2, . . . to the outside. Vcc is a power supply for the image reading device, ST is a start signal for starting image reading, and CK is a clock to control scanning of the shift register. Vbb is a power supply for setting the base potential, GP is a read signal and is a signal for outputting the read signal to the outside, and sig is a signal output and is a read signal from each phototransistor 1-1, 1-2, . . .

The operation of the image reading apparatus configured as described above will be explained below with reference to FIGS. 1 and 2.

First, FIG. 2 is a timing chart of the image reading device shown in FIG. First, in the storage type image reading device, image information before the start of image reading, that is, one line before, is stored, so when reading an image, it is first necessary to reset the phototransistor.

Hereinafter, resetting of the phototransistor will be explained. First, the shift register 2-1 is turned on by the start signal ST and the clock CK, and the phototransistor 1-1 is selected. , that is, the image information s1 of one line before is read. Next, open switch 3-1 and base switch 4-
1 is closed and the base potential of phototransistor 1-1 is set to V.
bb, close the output switch 3-1 again, and read out the noise signal d1 without image information. Furthermore, the switch 3-1 is opened, the base switch 4-1 is closed again, and the base potential of the phototransistor 1-1 is set to Vbb again, and the collector-base capacitance of the phototransistor 1-1 is charged. 1 reset is completed.

When the clock CK is input again, the next shift register 2-2 is turned on, and the phototransistor 1-2 is turned on.
2 is selected, the base potential of the phototransistor 1-2 is set to Vbb by opening and closing the output switch and the base switch, the collector-base capacitance of the phototransistor 1-2 is charged, and the phototransistor 1-2 is reset. .

In this way, the base potentials of all the phototransistors 1-1, 1-2, . . . are set to Vbb, the collector-base capacitance is charged, and the reset is completed. In this state, when the image information during the accumulation time is irradiated as light to the phototransistors 1-1, 1-2,..., a photocurrent is generated, and this photocurrent causes each phototransistor 1-1, 1-2, . The charge stored in the collector-base capacitance of -2,... is lost, and each phototransistor 1
-1, 1-2, . . . base potential increases.

Next, signal readout after the accumulation time has ended will be explained. When the start signal and clock are input again,
Shift register 2-1 turns "on" and phototransistor 1-1 is selected. In this state, output switch 3
-1 is closed, the base potential has risen above Vbb due to the photocurrent, so the base current flows to charge the collector-base capacitance of the phototransistor 1-1, and from the emitter of the phototransistor 1-1 to the phototransistor An optical signal amplified by a current amplification factor of 1-1 is output. At this time, since the base potential of the phototransistor 1-1 is always set to the constant voltage Vbb at the time of reset, previous image information, that is, no afterimage occurs. Next, when the base switch 3-1 is closed, the phototransistor 1-
The base potential of phototransistor 1-1 is set to Vbb, the collector-base capacitance of phototransistor 1-1 is charged, and phototransistor 1-1 is placed in a reset state. Then, when the base switch 3-1 is closed, the phototransistor 1-1
Since the base potential of the phototransistor 1-1 is hardly irradiated with light, the base potential of the phototransistor 1-1 remains at Vbb, and the charge stored in the collector-base capacitance of the phototransistor 1-1 also hardly changes. As a result, the next time the output switch 3-1 is turned on, there is no optical signal from the emitter of the phototransistor 1-1.
Only the noise signal is output. Therefore, by subtracting the noise signal from the optical signal, a noise-free image signal is obtained. Thereafter, the phototransistor 1-1 is reset by closing the base switch 4-1 again. Next, when the clock is turned on again, the next shift register 2-2 is turned on, the phototransistor 1-2 is selected, and by opening and closing the output switch and base switch, an optical signal and a noise signal without afterimage are generated. It is read and then reset.

In the manner described above, the optical signals and noise signals of all the phototransistors 1-1, 1-2, . . . are read out, making it possible to read out image signals free of afterimages and noise.

As described above, according to this embodiment, there is a phototransistor array for light detection, a base switch connected to the base of each phototransistor in this phototransistor array, and a shift switch for sequentially selecting the phototransistor array. By providing a resistor, an output switch for accessing each phototransistor, and a read switch connected to the signal output line of the phototransistor array, the base potential of the phototransistor is always kept at a constant potential before the accumulation time. Because of this, there is no afterimage, and by reading out the optical signal and the noise signal from one phototransistor and taking the difference between them, it is possible to obtain an image signal with a high SN ratio and no noise component.

[0026]

As described above, the present invention provides a phototransistor array for light detection, a base switch connected to the base of each phototransistor of this phototransistor array, and a shift register for sequentially selecting the phototransistor array. , an output switch for accessing each of the phototransistors, and a read switch connected to the signal output line of the phototransistor array to keep the base potential of the phototransistor at a constant potential before the accumulation time. This eliminates afterimages, and by reading out the optical signal and the noise signal from one phototransistor and taking the difference between them, it is possible to obtain an image signal with no noise component and a high SN ratio.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of an image reading device according to an embodiment of the present invention.

[Fig. 2] Timing chart for explaining the operation in this embodiment

[Figure 3] Circuit diagram of a conventional image reading device

[Figure 4] Timing chart of conventional image reading device

[Figure 5] Circuit diagram of the light receiving section of a conventional image reading device

[Explanation of symbols]

1-1 Phototransistor 1-2 Phototransistor 1-3 Phototransistor 1-4 Phototransistor 2-1 Shift register 2-2 Shift register 2-3 Shift register 2-4 Shift register 3-1 Output switch 3-2 Output switch 3-3 Output switch 3-4 Output switch 4-1 Base switch 4-2 Base switch 4-3 Base switch 4-4 Base switch 5-1 AND circuit 5-2 AND circuit 5-3 AND circuit 5-4 AND circuit 6 NOT circuit 7 Read switch

Claims (3)

[Claims] 1. A phototransistor array for light detection, a base switch connected to the base of each phototransistor in the phototransistor array, a shift register for sequentially selecting the phototransistor array, and a shift register for accessing each of the phototransistors. An image reading device comprising: an output switch for outputting a phototransistor array; and a reading switch connected to a signal output line of the phototransistor array. 2. The image reading device according to claim 1, wherein the base switch is closed twice during a period of reading an image signal of one pixel of each phototransistor. 3. The image reading device according to claim 2, wherein the readout switch is opened while the base switch is closed.