JPS59135972A - Automatic background control circuit - Google Patents

Abstract

PURPOSE:To obtain a received picture with fidelity to an original by switching a time constant in response to the scanning speed in a facsimile device. CONSTITUTION:When the scanning speed is low, switches 18, 27 are switched to capacitors 14, 23 for low scanning speed having large capacitance so as to increase the time constant of a peak hold circuit. On the other hand, when the scanning speed is high, the switches 18, 27 are switched to capacitors 15, 24 for high scanning speed with small capacitance so as to decrease the time constant of the peak hold circuit. Thus, the facsimile is operated suitably at all times and a signal with fidelity to the original is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an automatic background control circuit for eliminating the effects of changes in the amount of light from an illumination light source or changes in the density of the background of a document in a facsimile machine or the like.

Conventional configurations and their problems In general, facsimile machines are sensitive to the effects of changes in the amount of light due to the rise characteristics and aging of an illumination light source such as a fluorescent lamp that illuminates the document, and changes in the density of the background of the document. In order to eliminate this, the level of the image signal obtained by reading the original is corrected by an automatic background control circuit according to the change in the amount of IJ'J light recording and the change in the density of the background of the original.

However, in conventional automatic background control circuits of this type, the time constant of the time constant circuit that defines the response time of the circuit to the level fluctuation of the image signal is fixed to one type, so when the scanning speed is switched However, it was not possible to adequately accommodate all scanning speeds.

That is, by adapting the time constant to a fast scanning speed,
When switching to a slow scanning speed, the output level fluctuates greatly during a very short scanning period, causing density unevenness in the received image. On the other hand, if the time constant is adapted to the slow scanning speed, There is a drawback that when switching to a high scanning speed, it is not possible to follow sudden changes in the roughness of the document surface.

OBJECTS OF THE INVENTION The present invention was made in order to eliminate the above-mentioned drawbacks of the conventional art, and it is an object of the present invention to provide an automatic background control circuit that can always operate properly even when the scanning speed is changed.

Structure of the Invention The automatic background control circuit according to the present invention is capable of switching the time constant to a plurality of values depending on the scanning speed.

DESCRIPTION OF EMBODIMENTS The present invention will be explained in more detail below based on embodiments shown in the drawings.

Figure 1 shows an automatic background control circuit according to this example.
1 is an input terminal for inputting an analog image signal S1 obtained by forcing a reading element such as an image sensor to read a document; 2 is an image signal amplifier for inputting the analog image signal S1 from the input terminal 1; 3.4 .5 is a differential amplifier.

The output terminal of the picture signal amplifier 2 is connected to the inverting input terminal of the differential amplifier 3 via a resistor 6.7 connected in series. Further, a resistor 8 is inserted between the inverting input terminal and the output terminal of the differential amplifier 3. moreover,
The output terminal of the differential amplifier 3 is the output terminal 9. It is connected to an inverting input terminal of differential amplifier 4 and a non-inverting input terminal of differential amplifier 5.

The non-inverting input terminal of the differential amplifier 4 is connected to a connection point between resistors 10 and 11 inserted between the positive power supply and the ground line. The output terminal of this differential amplifier 4 is connected via a resistor 12 and a diode 13 to
It is connected to a low scanning speed capacitor 14, a high scanning speed capacitor 15 having a smaller capacity than this capacitor 14, one end of the bottom resistor 16, and a control terminal of a control element 17 such as a P-channel field effect transistor. There is.

The other ends of the capacitors 14 and 15 are selectively grounded by a switch 18. The other end of the resistor 16, together with one main electrode of the control element 17, is connected to a positive power source. Here, the resistors 12, 16, the diode 13, and the capacitor 14, 15 selected by the switch 18 constitute a peak hold circuit.

The other main electrode of the control element 17 is connected to the non-inverting input terminal of the differential amplifier 3 via a -resistor 19,
Furthermore, this terminal is grounded via a resistor 20.

The differential amplifier 5 has its inverting input terminal grounded, and its output terminal connected to a resistor 21 and a diode 2.
2 to a low scanning speed capacitor 23, a high scanning speed capacitor 24 having a smaller capacity than this capacitor 23, one end of a resistor 25, and a control electrode of a control element 26 such as a P-channel field effect transistor. ing.゛The other end of the capacitor 23, 24 is connected to the switch 18.
The grounding is selectively performed by a switch 27 interlocked with the grounding. Further, the other end of the resistor 25 is connected to a negative power source. Here, the resistors 21, 25, the diode 22, and the capacitor 23.
The one selected as the switch 27 among the 24 constitutes a peak hold circuit.

The control electrode of the control element 26 is grounded via a resistor 28, one main electrode of the element 26 is directly grounded, and the other main electrode of the element 26 is connected to the resistor 6. It is connected to the connection point with 7.

Next, the operation of this embodiment will be explained.

In this automatic background tljlJ control circuit, when the white level of the output S2 of the image signal amplifier 2 is at the reference level as shown in FIG. 2(a), the peak value of the output S3 of the differential amplifier 3 (
The white level) is also set to the reference level as shown in FIG. 3(a).

Furthermore, if the switches 18 and 27 are now switched to the low scanning speed capacitors 14 and 23, the peak hold circuit whose components include these capacitors 14 and 23 causes the capacitors 14 and 27 to be switched to the low scanning speed capacitors 14 and 23, respectively.
23 holds a voltage at a level corresponding to the peak value of the output S3 of the differential amplifier 3.

Then, due to an increase in the light intensity of the illumination light source and a decrease in the density of the background of the original, the output S20 white level of the differential amplifier 3 becomes negative from the reference level in FIG. 2(a) as shown in FIG. 2(b). If the peak value of the output S3 of the differential amplifier 3 is on the positive side than the reference level as shown in FIG. 3(b), the input of the inverting input terminal of the differential amplifier 4 changes to the positive side with respect to the reference voltage of the non-inverting input terminal of the amplifier 4 determined by the resistor 10.11.

Then, the output of the differential amplifier 4 changes to the negative side, making the diode 13 conductive through the resistor 12, discharging the charge stored in the capacitor 14, and causing the control element 17 to become conductive.
The voltage of the control electrode 14 is lowered, and the impedance of the control element 14 is increased. As a result, the voltage at the non-inverting input terminal of the differential amplifier 3 decreases, and the peak value of the output S3 of the differential amplifier 3, which once changed to the positive side as described above, becomes
It changes to the negative side and is pulled back to the base 6g level shown in FIG. 3(a).

Further, as described above, when the differential changes to the positive side as shown in FIG. 3(b), the output of the differential amplifier 5 also changes to the positive side, causing the diode 22 to conduct through the resistor 21,
The electric charge stored in the capacitor 23 is discharged. Therefore, the voltage at the open side l electrode of the control element 26 decreases,
Since the impedance of the control element 26 increases and the input of the non-inverting input terminal of the differential amplifier 3 changes to the negative side, this also causes the output S3 of the differential amplifier 3 to change to the positive side as described above. The peak value changes to the negative side, and is returned to the reference level shown in FIG. 3(a), resulting in a stable state.

On the other hand, due to a decrease in the light meter of the light source and an increase in the density of the background of the original, the white level of the signal S2 of the image signal amplifier 2 is lowered as shown in Fig. 2 (C
), if it changes to the positive side, the differential amplifier 3
The peak value of the output S3 is as shown in FIG. 3(C).
It changes to the negative side from the reference level shown in Figure (a).

Then, the input of the inverting input terminal of the differential amplifier 4 changes to the negative side, so the output of the differential amplifier 4 changes to the positive side, the diode 13 becomes cut-off, and the capacitor 14 changes to the positive side through the resistor 16. Charged from the power source.

As a result, the voltage at the 'tMU control electrode of the control cable 17 changes to the positive side, and the impedance of the control element 17 decreases, so the voltage at the non-inverting input terminal of the differential amplifier 3 changes to the glass side. . As a result, the peak value of the output S3 of the differential amplifier 3, which once changed to the negative side as described above, changes to the positive side and is pulled back to the reference level shown in FIG. 3(,).

1, as mentioned above, the differential amplifier 3 as shown in Figure 3(C)
When the peak value of the output S3 changes to the negative side from the reference level shown in FIG. It is charged to negative from the negative power supply via.

For this reason, the voltage of the control electrode of the control element 26 changes to the negative side, and the impedance of the control element 26 decreases, so that -H plus The voltage at the inverting input terminal of the differential amplifier 3 that has changed to the side is shown in Figure 2 (a) of IJff.
), this also causes
The peak value of the output S3 of the differential amplifier 3 changes to the positive side and is pulled back to the reference level shown in FIG. 3(a), resulting in a stable state.

In addition, when the switches 1, 8, and 27 are switched to the high running differential speed capacitors 15 and 24i141, respectively (as shown in C11 [when the peak value of the output S3 of the differential y-so-1 converter 3 fluctuates) For low scanning speed capacitor 1
4.23 necessarily high scanning speed capacitor 15.24
The same operation as described above is performed by charging and discharging the battery.

Here (in C), the response time of this automatic background side leg circuit to the fluctuation of the white level of the image signal S1 is as follows:
the time constant of said peak hold circuit of which these capacitors 14.23 are components, when the output is switched to the low scanning speed capacitors 14.231111;
This in turn depends on the capacitance of these capacitors 14, 23.

Also, switch 18, 27 or high scanning speed capacitor 1
When it is switched to 5,24ijllll.

The response time depends on the time constant of the peak hold circuit of which these capacitors 15 and 24 are components, and thus on the capacitance of these capacitors 15 and 24.

However, conventionally, two sets of condensers f1 in this device
Instead of 4 and 23 and 15 and 24, only one set of capacitors was provided. Therefore, if the capacitance of these capacitors and the time constant of the peak hold circuit constituted by these capacitors are too small compared to the fixed feeding speed (d1, as mentioned above, the differential There is a drawback that the peak value of the output S3 of the amplifier 3 fluctuates, resulting in density unevenness in the received image obtained based on the output S3.

In addition, if the capacitance of the capacitors in the set of capacitors and the time constant of the peak hold circuit constituted by these capacitors are too large compared to the scanning speed, for example, as shown in Fig. 4 (-), the initial Mainly scans a document in which characters or figures 3o are present on a light-colored background part 29 spanning the entire width, and then light-colored characters or figures 32 are present on a white background 31 spanning the entire width in the direction of arrow P. When this happens, the output S3 of the differential amplifier 3 becomes as shown in Fig. 4(b), which corresponds to the character or figure 32 on the white background 31 that should originally appear as A in Fig. 4(C). Parts were no longer visible. That is, as mentioned above, there is a drawback that it is not possible to follow sudden changes in the density of the background of a document.

However, in this automatic background control circuit, when the scanning speed is low, the switches 18 and 27 are switched to the large capacitance low scanning speed reaction capacitors 14 and 23 to increase the time constant of the peak hold circuit. When the scanning speed is fast, by switching the switch 18° 27 to the small capacitance capacitor 15° 24 for high scanning speed and reducing the time constant of the peak hold circuit, even when the scanning speed is slow, Even if it is early, it is possible to always operate properly and obtain a signal faithful to the original as shown in FIG. 4(C), and a received image faithful to the original.

Effects of the Invention As described above, the automatic background control circuit according to the present invention has an excellent effect in that it can always operate properly even when the scanning speed is changed.

[Brief explanation of drawings]

FIG. 1 is a circuit configuration diagram of an automatic background control circuit according to an embodiment of the present invention, FIGS. 2 and 3 are signal waveform diagrams in the same circuit, FIG. 4(a) is an example of a ttri manuscript, and FIG. b)
4 is a signal waveform diagram showing a line of outputs of the conventional automatic background control circuit when the original shown in FIG. 4 (-) is read;
Figure (c) is a signal waveform diagram showing a desirable output of the automatic background control circuit when the i'ji manuscript is read. 1...Input terminal, 3,4,5...Differential amplifier, 9...Output terminal, 12...Resistor, 13...・Diode)", 14... Capacitor for low scanning speed, 15... Capacitor for high scanning speed, 18... Switch, 21...
Resistor, 22... diode, 23... capacitor for low scanning speed, 24... capacitor for high scanning speed, 25... resistor, 27...・・・
switch.

Claims (1)

[Scope of Claims] It has an amplifier that amplifies an image signal obtained by scanning a document, and a peak hold circuit that receives the output of this amplifier as an input, and controls the output of the amplifier based on the output of the peak hold circuit. 8. An automatic background control circuit for controlling an output level, wherein the time constant of the peak hold circuit can be switched to a plurality of values according to a scanning speed.