JPH0543564Y2 - - Google Patents

Description

[Detailed description of the invention] (Industrial application field) The present invention obtains binary image information in a device that reads images using sensors arranged in a line in the scanning direction, such as a CCD line sensor or a contact image sensor. The present invention relates to a black and white level discrimination circuit for use in, for example, electronic blackboards.

(Prior Art) FIG. 4 shows a black and white level discrimination circuit in a conventional image reading device.

This discrimination circuit is constituted by a white peak hold circuit constituted by an operational amplifier 1 and the like, and a comparison circuit constituted by a comparator 2. The original signal _S1 obtained from the CCD line sensor (not shown) is a signal that outputs a high voltage value in a bright area and a low voltage value in a dark area. This original signal _S1 is input to the operational amplifier 1 and comparator 2, and the operational amplifier 1 receives the original signal S1.
Compare the levels of S ₁ and white peak hold signal S ₂ ,
If the original signal _S1 is higher, the transistor 3 connected to the output stage is turned on, and the capacitor 6 is charged from the emitter of the transistor 3 through the diode 4 and the resistor 5 at a constant charging time constant. On the other hand, the level of the original signal S ₁ is the white peak hold signal S ₂
If the voltage is lower than , the transistor 3 is turned off and discharged at a constant discharge time constant determined by the capacitor 6 and the resistor 7 connected in parallel to the capacitor 6. In the charging/discharging circuit formed by the resistors 5, 7 and the capacitor 6, normally,
The value of the resistor 7 is set to be greater than the value of the resistor 5, and the discharging time constant is set to be greater than the charging time constant. The discharge current from the capacitor 6 becomes the base input of a transistor 8, and the emitter output of this transistor 8 becomes the white peak hold signal _S2 . Furthermore, a divided potential (so-called slice level signal C ₃ ) obtained by dividing the emitter potential of transistor 8 by resistors 9 and 10
is input to the inverting terminal of comparator 2, and the original signal is input to the non-inverting terminal of comparator 2.
A comparison with S ₁ is made. The result of this comparison is the binary image information signal _S4 . That is, the original signal S ₁
When the voltage is higher than the voltage divided by the resistors 9 and 10, the output is at a white level, and when it is lower, the output is at a black level (see FIG. 5).

(Problem to be solved by the invention) However, in the conventional black and white level discrimination circuit, if a gray part exists for a long time in the original signal _S1 ,
If the white fluctuation is large, a problem will occur. In other words, if there is a long gray area where the black area is relatively shallow compared to the white level, it is necessary to determine this area as black.
It is necessary to make the discharge time constant determined by the capacitor 6 and resistor 7 sufficiently large [Fig. 6 a, b
reference〕. That is, if the discharge time constant is small, the white peak hold signal is
Since S ₂ and slice level signal S ₃ each fall quickly, they are determined to be white in the middle of the gray area, and correct output cannot be obtained. Additionally, when a signal with large white fluctuations is input, if the discharge time constant is large, the white peak hold signal _S2 will not intersect with the original signal _S1 , and all subsequent data will be determined as black. See Figures 7a and b]. However, if the discharge time constant is small, subsequent white levels can be determined appropriately [see FIGS. 7c and 7d]. As mentioned above,
In the case where the gray portion exists for a long time and in the case where the white variation is large, the value of the discharge time constant for obtaining a correct output becomes a trade-off, and it was not possible to obtain a correct output in both cases.

(Means for Solving the Problem) The black and white level discrimination circuit in the image reading device of the present invention detects the white peak value level of the image information signal in the device that reads the image using a line sensor arranged in a line in the scanning direction. and a first slice level lower than the white peak value level. a first comparison means for comparing the signal and the image information signal; and a first comparison means for comparing the image information signal with the first comparison means,
When the image information signal is higher than the first slice level signal, the time constant of the discharge circuit is switched so as to rapidly lower the slice level, and when the image information signal is lower than the first slice level signal, the slice level is gradually lowered. A switching means for switching the time constant of the discharge circuit and a comparison between a second slice level signal lower than the first slice level signal and the image information signal, when the image information signal is higher than the second slice level signal; The second comparison means sets the output as a white level, and sets the output as a black level when the output is low.

(Operation) The first slice level signal lower than the white peak value level is compared with the image information signal (original signal) obtained from the line sensor, and the discharge time constant is switched.

That is, when the original signal is higher than the first slice level signal, the discharge time constant of the discharge circuit is switched to a small value so as to rapidly reduce the slice level. Further, when the original signal is lower than the first slice level signal, the discharge time constant of the discharge circuit is switched so that the slice level is gradually lowered.

On the other hand, a second slice level signal set lower than the first slice level signal is compared with the original signal to determine the black and white level.

That is, when the original signal is higher than the second slice level signal, it is determined to be a white level, and when it is lower than the second slice level signal, it is determined to be a black level.

As a result, for example, if a gray part exists for a long time in the original signal, the discharge time constant will already be large before the second slice level signal becomes a lower level than the original signal (before it is inverted), so it will be inverted. The level moves backward, making it possible to avoid the phenomenon of reversal.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of a black and white level discrimination circuit in an image reading apparatus according to the present invention.

In Figure 1, the circuit configuration for holding the white peak value level from the original signal _S1 is the same as the conventional circuit configuration, and consists of operational amplifier 1, etc., and the white peak hold signal _S5 is output to the inverting terminal of operational amplifier 1. has been done. The difference from the conventional circuit is that the resistor 7 that determines the discharge time constant is divided into two resistors 7a and 7b, and the emitter side resistor of the transistor 8 is formed by connecting three resistors 11 to 13 in series. The difference is that two slice level signals S ₇ and S ₆ used for determining the black and white level of the original signal are extracted from connection points A and B between these resistors.
The higher slice level signal S ₇ of these two slice level signals S ₇ and S ₆ is taken out from the connection point A between the resistors 11 and 12, and this slice level signal S ₇ is connected to the inverting terminal of the comparator 14. has been entered. The original signal _S1 is input to the non-inverting terminal of this comparator 14, and the original signal
If S ₁ is higher than this first slice level signal S ₇ , transistor 15 is turned on. The collector side of this transistor 15 is connected to the connection point C between the resistors 7a and 7b that determines the discharge time constant, and when the transistor 15 is turned on, the resistors 7a and 7b
Since the connection point C of 7b is grounded, the discharge time constant is determined by the value of resistor 7a and the capacitance of capacitor 6. On the other hand, the first slice level signal S ₇
If is larger than the original signal S ₁ , transistor 1
5 maintains the OFF state, and at this time, discharge is performed with a time constant determined by the sum of the values of the resistors 7a and 7b and the capacitance of the capacitor 6.

On the other hand, the emitter side resistor 1 of the transistor 8
A second slice level signal is generated at the connection point B of 2 and 13.
S ₆ (lower than the first slice level signal S ₇ ) is determined, and this second slice level signal S ₆ is input to the non-inverting terminal of the comparator 2 .

Therefore, when the original signal _S1 is higher than the first slice level signal _S7 , the transistor 15 is turned on, so when the waveform of the original signal _S1 falls, the discharge time constant becomes small and the slice level suddenly changes. descend. On the other hand, when the original signal _S1 is lower than the first slice level signal _S7 , the transistor 15 is turned off, and the discharge time constant becomes longer at this point. That is, if the gray portion 17 exists for a long time in the original signal S ₁ [see Figure 2 a], before the second slice level signal S ₆ becomes a lower level than the original signal S ₁ (before it is inverted).
Since the discharge time constant has already become large, the reversal level moves backward and the reversal phenomenon can be avoided [see Figures 2 a and b]. Further, when the white fluctuation is large, the discharge time constant becomes small, so that followability is good and correct output can be obtained [see FIGS. 3a and 3b].

(Effects of the invention) The black and white level discrimination circuit of the invention comprises a first comparing means for comparing a first slice level signal lower than the white peak value level and an image information signal;
a switching means for switching the time constant of the discharge circuit according to the comparison result of the comparing means; and a second switching means for determining the monochrome level by comparing the image information signal with a second slice level signal lower than the first slice level signal. Since the configuration includes a comparing means, even if the gray level continues for a long time or the white fluctuation is large, the second slice level signal is set at a lower level than the original signal (before inversion). Since the constant becomes large and the reversal phenomenon can be avoided, the black and white level can be determined correctly.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a black and white level discriminating circuit in an image reading device according to the present invention, FIGS. Figures 3a and 3b are waveform diagrams illustrating changes in the discharge time constant when white level fluctuations are large, Figure 4 is a circuit diagram showing a conventional black and white level discrimination circuit, and Figures 5a, b, and c are waveform diagrams illustrating changes in the discharge time constant when white level fluctuations are large. 4 is a waveform diagram illustrating the signal waveform of each part in FIG.
Figures a to d are graphs illustrating changes in output due to the magnitude of the discharge time constant when the white fluctuation is large. 1... operational amplifier, 2... comparator (second
(comparison means), 5, 7...resistor, 6...capacitor, 14...comparator (first comparison means).

Claims (1)

[Claims for Utility Model Registration] A device for reading an image using a line sensor arranged in a line in the scanning direction, which maintains a white peak value level of an image information signal and adjusts the white peak value level according to a decrease in the image information signal. white peak level holding means comprising a discharge circuit that reduces the peak value level with a predetermined time constant; and first comparison means that compares the image information signal with a first slice level signal lower than the white peak value level. , based on the comparison result of the first comparing means, when the image information signal is higher than the first slice level signal, the time constant of the discharge circuit is switched so as to rapidly reduce the slice level, and the image information signal is a switching means for switching a time constant of the discharge circuit so as to gradually lower the slice level when the signal is lower than the first slice level signal; and a second slice level signal lower than the first slice level signal and an image information signal. In an image reading device, the image reading device is characterized in that the second comparison means sets the output to a white level when the image information signal is higher than the second slice level signal, and sets the output to a black level when the image information signal is lower than the second slice level signal. Black and white level discrimination circuit.