Endoscope output signal control device and endoscope apparatus making use of the same

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an endoscope output signal control device having a means for controlling the quantity of light impinging upon an imaging means which picks up an endoscopic image. The invention also is concerned with an endoscope apparatus which makes use of such an endoscope output signal control device.

2. Related Art Statement

In recent years, electronic endoscopes have been put into practical use and incorporate, as an imaging means, solid-state imaging devices such as CCDs (Charge Coupled Device). This type of endoscope also is referred to as an "electronic scope".

Electronic scopes hitherto proposed, however, suffer a common disadvantage in that the dynamic range is restricted when compared with conventional optical endoscopes.

Under this circumstance, the present inventor has proposed, in Japanese Unexamined Patent Publication 61-61588, an improved electronic endoscope apparatus which expands the dynamic range which is restricted by the solid-state imaging device and which realizes high-speed control with good response to a change in the intensity of the light reflected from an object. This electronic endoscope uses, as a light control means, an electric control means so as to attain high-speed control and expand the dynamic range.

This electronic endoscope will be described in more detail with reference to FIG. 23.

White light from a lamp 3 of a light source unit 2 is made to pass through a light control means 4 such as an iris capable of controlling the light quantity and is then changed into a parallel light beam having a small diameter as it passes through optical lenses 5 and 6. This fine parallel light beam is made to pass through a rotary color filter 8 which is rotatingly driven by a motor 7. The light beam is converged by a condenser lens 9 so as to impinge upon the light incidence end of a light guide 12 of an electronic scope 11. The rotary color filter 8 has a disk-shaped filter frame having three sector-shaped openings to which are attached color filters capable of transmitting light of red, green and blue colors, respectively. As the color filter 8 rotates, red, green and blue color filters are successively brought into the path of the illuminating light so that illuminating light of red, green and blue colors (referred to as sequential light) are successively and sequentially applied to the light incident end of the light guide 12. The operation speed of the motor 7 which drives the rotary color filter 8 is controlled at a predetermined level by means of a servo circuit 14.

The light guide 12 sequentially transmits the sequential light so that the light emanates from the emanating end of an elongated insert section 15 of the endoscope so as to impinge upon the object 17 through a light distribution lens 16.

The sequential light reflected by the illuminated object 17 is focused through an objective lens 18 on the end of the insert section 15 on a solid-state imaging device (referred to as "SID" hereinafter) such as a CCD which is disposed on the focal plane of the objective lens 18. The thus formed image is photoelectrically converted so that an electrical signal corresponding to the optical image is obtained. The electrical signal is read in accordance with a drive signal which is applied from a drive circuit (not shown) to the SID 19 and is amplified by an amplifier 21. The amplified signal is delivered to a signal processing device (referred to as "video processor") 22 having the light source unit 2 and a signal processing means.

The electrical signal input to the video processor 22 is delivered through a first level detection means 23 to one of the input terminals of a first comparator means 24. The first level detection means 23 detects the output level corresponding to the level of the electrical signal. The electrical signal derived from the first level detection means 23, serving as a brightness information signal and applied to one of the input terminals of the first comparator means 24, is compared with a reference signal Vr1 applied to the other input terminal of the comparator means 24, whereby the brightness of the object image represented by the electrical signal is compared with a reference brightness represented by the reference signal Vr1. The output from the first comparator means 24, serving as a light quantity control signal, is delivered to a light control means 4 which is disposed between the lamp 3 and the lens 5 so as to control the opening of the iris thereby controlling the quantity of the light passed to the lens 5. More specifically, when the level of the electrical signal input to the first comparator means 24 is higher than the level of the reference signal Vr1, i.e., when the object image is brighter than the reference brightness, the iris opening is decreased to reduce the quantity of light by an amount corresponding to the difference in the brightness. Conversely, when the object image is darker than the reference brightness, the light control is conducted such as to increase the quantity of the light.

Thus, the light quantity control is executed in three steps: namely, (a) picking up the change in the intensity of the light reflected by the object, (b) detecting the change in the light intensity as a change in the output from the SID 19 and (c) controlling the change in the output as a change in the light quantity. These steps (a), (b) and (c) are executed in a closed loop sequentially and cyclically and the automatic light control (ALC) function is performed upon completion of each cycle in accordance with the change in the light reflected from the object, so as to optimally control the quantity of the illuminating light.

Thus, the level of the output from the amplifier 21 is controlled by the ALC function and is input to a signal processing circuit 27 through a variable gain amplifier 26. The signal processing circuit 27 is capable of temporarily storing the sequential red, green and blue signals and reading these signals simultaneously so as to form simultaneous red, green and blue signals. Then, suitable correcting operations such as gamma correction are executed on the simultaneous signals and the thus processed simultaneous signals are delivered to a TV monitor 28, whereby a color image of the object is formed on the TV monitor 28.

The output from the variable gain amplifier 26 is delivered to one of the input terminals of a second comparator means 30 after passing through a second level detection means 29 so as to be compared with a reference level Vr2 which is received by the other input terminal of the second comparator means 30, so that the second comparator means 30 delivers a signal corresponding to the result of the comparison.

For instance, when the level of the output from the variable gain amplifier 26 received by the second comparator means 30 is greater than reference level Vr2, the second comparator means 30 produces a signal for controlling the variable gain amplifier 26 so as to reduce the level of the output therefrom. Thus, the second comparator means performs a cyclic operation similar to that performed by the first comparator means 24, by an electrical gain correction means. With this arrangement, it is possible to elevate the output signal level so as to enable the observer to easily observe an image brightness level of which is still below a predetermined acceptable level even when the light control means 4 has been operated by the ALC such as to fully open the iris. In general, the light control means 4 controlled by the ALC circuit is composed of a mechanical iris motor and iris blades and generally exhibits a low response speed, but is capable of reducing any fluctuation on the TV monitor 28 by virtue of subsequent electrical correcting operation. In fact, the electrical correction means provides a remarkable effect in improving the quality of the image.

The known arrangement shown in FIG. 23 is so designed that the ALC function is always active and operative. A more practical arrangement therefore has been proposed as shown in FIG. 24.

In the arrangement shown in FIG. 24, the reference voltage Vr1 applied to the first comparator means 24 of the circuit shown in FIG. 23 is forcibly changed in accordance with light quantity control step so as to allow the light quantity control of the light control means 4 to be set manually.

In general, endoscopic diagnosis encounters a large change in the intensity of light reflected by the object (upper digestive organs and upper digestive system) depending on the portion of the object under diagnosis and other factors. It is difficult to optimize the light quantity over the entire part of the object with the known SID having a restricted dynamic range. A doctor as the user therefore has to delicately control the light quantity applied to the portion of the object which requires a minute check. The manual setting function afforded by the arrangement shown in FIG. 24 copes well with such a demand. More specifically, referring to FIG. 24, a light control switch SW 31 is disposed on, for example, a front panel which accommodates a signal processing device 22. The light control switch SW 31 comprises a light quantity up switch 32a and a light quantity down switch SW 32b. These switches SW 32a and SW 32b are connected to a CPU 34 via an input port 33. Each of these switches is set such that it delivers a signal of "H" level by a resistor R when it is in an off state. However, as the switch is turned on, the output level is changed to "L" which is delivered to the CPU 34 through the input port 33.

The CPU 34 is connected through a data BUS and an address BUS to a ROM 35 which stores program contents and information necessary for execution of the programs and is connected to a RAM 36 which provides a working area for the execution of the program. The CPU 34 also is connected to a programmable timer 37 so as to output frequency data corresponding to the operation of the light control switch SW 31 input through the input port 33. The output of the programmable interval timer 37 is input to a frequency/voltage (F/V) conversion circuit 38 so as to be converted into a voltage proportional to the frequency. This voltage is input to a differential amplifier 39 which computes the difference between this voltage and the reference level Vr1. The difference is input to the other input terminal of the first comparator means 24. Other portions of the arrangement shown in FIG. 24 are materially the same as those of the circuit shown in FIG. 23.

Thus, the circuit shown in FIG. 24 allows the reference level into the first comparator means 24 to be freely set at any desired level, thus providing an ALC which enables the light quantity to be set at any desired level.

For instance, if the output level of the F/V converter 38 is set at "0", the circuit shown in FIG. 24 operates in the same manner as the arrangement shown in FIG. 23.

Conversely, if the output of the F/V converter 38 is set at a level higher than "0", the differential amplifier 39 produces an output of a level lower than the reference level Vr1, so that the ALC functions such as to attain a light quantity corresponding to this low level output. Namely, the iris opening is reduced to decrease the light quantity.

Conversely, the light quantity is increased when the output from the F/V converter 38 is smaller than 0.

The level of the output of the F/V converter 38 can be set by means of the light quantity control switch SW 31.

For instance, when the up switch SW 32a is pressed, the resulting information is input to the CPU 34 through the input port 33. The CPU 34 then executes a routine as shown in FIG. 25. Namely, the CPU 34 writes a predetermined set frequency value in the programmable interval timer 37 such that the timer frequency is lowered by one stage. The set frequency value is stored in the ROM 35. In consequence, the programmable interval timer 37 in which the set frequency value is written produces a frequency signal which is one stage lower than the preceding frequency signal. The frequency signal now output from the programmable interval timer 37 is converted into voltage by the F/V converter 38 and the thus obtained voltage is input to a differential amplifier 39 which produces the difference between the input voltage signal and the reference level Vr1. The differential output from the differential amplifier 39 constitutes the reference level to be input to the first comparator means 24. The reference level in this state is one stage higher than the preceding reference level and the ALC operates to raise the light quantity level to this higher reference level. Thus, the operation is the same as that obtained when the command level is changed in one cycle of operation of the ALC circuit. In other words, it is possible to vary the state of the light control means 4 even when the level of the output from the first level detection means 23 is the same. For instance, the light control means 4 operates to increase the light quantity when the up switch SW 32a is pressed.

As will be understood from the foregoing description, the electronic endoscope apparatus shown in FIG. 24 can expand the detactable range between bright and dark objects having small and large reflectivity levels and can operate with a high response speed, by virtue of the provision of the ALC circuit and the electrical automatic gain control means (referred to as "AGC" hereafter). The endoscope apparatus shown in FIG. 24, however, suffers from the following problem. In some cases, a doctor as the observer wishes to lower the brightness of the object portion under examination for the purpose of minute examination when the image of the object portion is too bright to observe. In such a case, the doctor operates the light control switch SW 31 so as to reduce the light quantity set by the ALC. Unfortunately, however, the AGC operates so as to maintain the output therefrom at a constant level insofar as the output level set by the ALC falls within the range controllable by the AGC. It is therefore impossible to lower the level of brightness of the image on the monitor TV. This problem is not so serious when the brightness level is around the center step of the light control but is critical when the brightness is near the brightest or the darkest step, as will be understood from the following description taken in conjunction with FIGS. 26A and 26B. FIG. 26A illustrates the relationship between the light quantity steps and the illumination light quantity or the amount of exposure of the SID 19 controlled by the light control means 4. It will be seen that the relationship is substantially linear.

FIG. 26B shows the output level obtained through the AGC circuit as obtained when the AGC circuit is operated while the light quantity control step is changed. As will be seen from these Figures, the output level is changed along the solid-line curve due to the operation of the AGC circuit even when the light quantity is changed by the light control means. Thus, the signal level is fixed despite a change in the light quantity control step, in the insensitive range in which the AGC operates.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide an endoscope output signal control device which can provide both an enlarged dynamic range and higher speed of response to change in the light quantity, without causing any unfavorable effect on the light quantity controlling function, as well as an endoscope apparatus incorporating such an endoscope output signal control device.

Another object of the present invention is to provide an endoscope output signal control device which is capable of maintaining the brightness of the image at a level near each control limit of the automatic light control means substantially at a constant level even when the set level of the brightness is changed, as well as an endoscope apparatus incorporating such an endoscope output signal control device.

To these ends, according to one aspect of the present invention, there is provided an endoscope output signal control device comprising: a light quantity varying device capable of varying the quantity of light incident to an imaging device for forming an endoscopic image;

light quantity setting device for variably setting the quantity of the incident light by driving the light quantity varying device; an automatic gain control device for conducting control so as to maintain the signal from the imaging device substantially at a constant level; and; automatic gain control eliminating device capable of eliminating, at least when the level of the signal from the imaging device is within a range controllable by the light quantity varying device, any influence of the automatic gain control device on the change in the level of the signal caused by a change in the level of light quantity set by the light quantity setting device. Preferably, the endoscope output signal control device of the invention further comprises automatic light control device capable of driving the light quantity varying device so as to control the quantity of the incident light in such a manner as to maintain the signal from the imaging device substantially at a constant level.

The invention also provides an endoscope apparatus incorporating the output signal control device, the apparatus comprising, in addition to the features of the output signal control device, an endoscope unit including an elongated insert section having an observation window at its end and a focusing optical system for receiving the light reflected from the object through the observation window so as to focus the light from the object, an imaging device for forming the image focused by the focusing optical system, a signal processing device for processing signals from the imaging device, and an illuminating device for supplying illuminating light to the field of vision of the image focusing system.

These and other objects, features and advantages of the present invention will become clear from the following description of the preferred embodiments when the same is read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a first embodiment of the endoscope output signal control device in accordance with the present invention;

FIG. 2 is a diagram showing the relationship between a control voltage applied to a gain control terminal of a variable gain amplifier used in the embodiment shown in FIG. 1 and the level of the output signal from the variable gain amplifier;

FIG. 3 is a diagram showing the relationship between the light quantity control steps and level of output from an F/V converter;

FIG. 4 is an illustration of a closed loop transmission function of an AGC circuit incorporated in the embodiment shown in FIG. 1;

FIGS. 5(A) to 5(C) are diagram showing the relationship between the AGC output characteristic and a change in the distance as realized by the embodiment shown in FIG. 1;

FIGS. 6(A) to 6(C) are diagrams showing, for the purpose of comparison with the operation characteristic of the embodiment shown in FIG. 1, the relationship between the AGC output characteristic and the change in distance as observed when the command of the AGC is fixed;

FIG. 7 is a block diagram showing an AGC circuit and elements around this circuit in a second embodiment of the present invention;

FIG. 8 is a block diagram showing an AGC circuit and elements around this circuit in a third embodiment of the present invention;

FIG. 9 is a block diagram showing an AGC circuit and elements around this circuit in a fourth embodiment of the present invention;

FIG. 10 is an illustration of a light control device of the type which incorporates iris blades used in the embodiment shown in FIG. 9;

FIG. 11 is a diagram showing the relationship between a link angle in the light control device of FIG. 10 and the level of output from a photo-coupler;

FIG. 12 is a circuit diagram illustrating the photo-coupler and elements around the photo-coupler;

FIG. 13 is an illustration of an iris mechanism used in a modification of the embodiment shown in FIG. 9;

FIG. 14 is an illustration of an iris mechanism used in another modification of the embodiment shown in FIG. 9;

FIGS. 15 and 16 are illustrations of methods for detecting rotational position in the iris mechanisms shown in FIGS. 13 and 14, respectively;

FIG. 17 is an illustration of an essential portion of an endoscope apparatus as a fifth embodiment of the present invention;

FIG. 18 is a diagram showing the control characteristic of a liquid crystal device as a light control used in the embodiment shown in FIG. 17;

FIG. 19 is an illustration of an essential portion of an endoscope apparatus as a sixth embodiment of the present invention;

FIG. 20 is an illustration of an essential portion of an endoscope apparatus as a seventh embodiment of the present invention;

FIG. 21 is an illustration of the whole of an endoscope apparatus as an eighth embodiment of the present invention;

FIG. 22 is an illustration of an essential portion of the endoscope apparatus shown in FIG. 21;

FIG. 23 is a block diagram showing the construction of a known endoscope apparatus;

FIG. 24 is a block diagram showing the construction of another known endoscope apparatus which is an improvement in the apparatus shown in FIG. 23;

FIG. 25 is a flow chart showing a processing routine for conducting light control in the device shown in FIG. 24;

FIG. 26(A) is a chart showing the relationship between the light control steps and the light quantity; and

FIG. 26(B) is a chart showing the relationship between light control steps and signal level.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first embodiment of the present invention will be described hereinunder with reference to FIGS. 1 to 6.

An electronic endoscope apparatus 41 has an electronic scope 11 including an elongated insert section 15, a signal processing device (video processor) detachably connected to the electronic scope and having an endoscope output signal control device, and a TV monitor 28 connected to the signal processing device 22.

The signal processing device 22 includes a light source section 2.

White light from a lamp 3 of a light source unit 2 is made to pass through a light control means 4 such as an iris capable of controlling the light quantity and is then changed into a parallel light beam having a small diameter as it passes through optical lenses 5 and 6. This fine parallel light beam is made to pass through a rotary color filter 8 which is rotatingly driven by a motor 7 and is converged by a condenser lens 9 so as to impinge upon the light incidence end of a light guide 12 of an electronic scope 11. The rotary color filter 8 has a disk-shaped filter frame having three sector-shaped openings to which are attached color filters capable of transmitting light of red, green and blue colors, respectively. As the color filter 8 rotates, red, green and blue color filters are successively brought into the path of the illuminating light so that illuminating light of red, green and blue colors (referred to as sequential light) are successively and sequentially applied to the light incident end of the light guide 12. The operation speed of the motor 7 which drives the rotary color filter 8 is controlled at a predetermined level by means of a servo circuit 14.

The light guide 12 sequentially transmits the sequential light so that the light emanates from the emanating end of an elongated insert section 15 of the endoscope so as to impinge upon the object 17 through a light distribution lens 16.

The sequential light reflected by the illuminated object 17 is focused through an objective lens 18 on the end of the insert section 15 on a solid-state imaging device (referred to as "SID" hereinafter) 19 such as a CCD which is disposed on the focal plane of the objective lens 18. The thus formed image is photoelectrically converted so that an electrical signal corresponding to the optical image is obtained. The electrical signal is read in accordance with a drive signal which is applied from a drive circuit (not shown) to the SID 19 and is amplified by an amplifier 21. The amplified signal is delivered to a signal processing device (referred to as "video processor") 22 having the light source unit 2 and a signal processing means.

The electrical signal input to the video processor 22 is delivered through a first level detection means 23 to one of input terminals of a first comparator means 24. The first level detection means 23 detects the output level corresponding to the level of the electrical signal. The electrical signal derived from the first level detection means, serving as a brightness information signal and applied to one of the input terminals of the first comparator means 24 is compared with a reference signal Vr1 applied to the other input terminal of the comparator means 24, whereby the brightness of the object image represented by the electrical signal is compared with a reference brightness represented by the reference signal Vr1. The output from the first comparator means 24, serving as a light quantity control signal, is delivered to a light control means 4 which is disposed between the lamp 3 and the lens 5 so as to control the opening of the iris thereby controlling the quantity of the light passed to the lens 5. More specifically, when the level of the electrical signal input to the first comparator means 24 is higher than the level of the reference signal Vr1, i.e., when the object image is brighter than the reference brightness, the iris opening is decreased to reduce the quantity of light by an amount corresponding to the difference in the brightness. Conversely, when the object image is darker than the reference brightness, the light control is conducted such as to increase the quantity of the light.

Thus, the light quantity control is executed in three steps: namely, (a) picking up the change in the intensity of the light reflected by the object, (b) detecting the change in the light intensity as a change in the output from the SID 19 and (c) controlling the change in the output as a change in the light quantity. These steps (a), (b) and (c) are executed in a closed loop sequentially and cyclically and the automatic light control (ALC) function is performed upon completion of each cycle in accordance with the change in the light reflected from the object, so as to optimally control the brightness of the illuminating light.

Thus, the level of the output from the amplifier 21 is controlled by the ALC function and is input to a signal processing circuit 27 through a variable gain amplifier 26. The signal processing circuit 27 is capable of temporarily storing the sequential red, green and blue signals and reading these signals simultaneously so as to form simultaneous red, green and blue signals. Then, suitable correcting operations such as gamma correction are executed on the simultaneous signals and the thus processed simultaneous signals are delivered to a TV monitor 28, whereby a color image of the object is formed on the TV monitor 28.

The output from the variable gain amplifier 26 is delivered to one of the input terminals of a second comparator means 30 after passing through a second level detection means 29 so as to be compared with a reference level Vr2 which is received by the other input terminal of the second comparator means 30, so that the second comparator means 30 delivers a signal corresponding to the result of the comparison.

For instance, when the level of the output from the variable gain amplifier 26 received by the second comparator means 30 is greater than reference level Vr2, the second comparator means 30 produces a signal for controlling the variable gain amplifier 26 so as to reduce the level of the output therefrom. Thus, the second comparator means performs a cyclic operation similar to that performed by the first comparator means 24, by an electrical gain correction means. With this arrangement, it is possible to elevate the output signal level so as to enable the observer to easily observe an image the brightness level of which is still below a predetermined acceptable level even when the light control means 4 has been operated by the ALC such as to fully open the iris. In general, the light control means 4 controlled by the ALC circuit is composed of a mechanical iris motor and iris blades and generally exhibits a low response speed, but is capable of reducing any fluctuation on the TV monitor 28 by virtue of subsequent electrical correcting operation. In fact, the electrical correction means provides a remarkable effect in improving the quality of the image.

In this embodiment, the light control also can be done manually. Namely, the reference voltage Vr1 applied to the first comparator means 24 of ALC circuit and the reference voltage Vr2 applied to the second comparator means 30 in the AGC circuit are forcibly changed in accordance with light quantity control step so as to allow the light quantity control to be done manually.

In general, endoscopic diagnosis encounters a large change in the intensity of light reflected by the object (upper digestive organs and upper digestive system) depending on the portion of the object under diagnosis and other factors. It is difficult to optimize the light quantity over the entire part of the object with the known SID 19 having a restricted dynamic range. A doctor as the user therefore has to delicately control the light quantity applied to the portion of the object which requires a minute check. The manual setting function easily complies with such a demand. More specifically, referring to FIG. 1, a light control switch SW 31 is disposed on, for example, a front panel which accommodates a signal processing device 22. The light control switch SW 31 comprises a light quantity up switch 32a and a light quantity down switch SW 32b. These switches SW 32a and SW 32b are connected to a CPU 34 via an input port 33. Each of these switches is set such that it delivers a signal of "H" level by a resistor R when it is in an off state. However, as the switch is turned on, the output level is changed to "L" which is delivered to the CPU 34 through the input port 33.

The CPU 34 is connected through a data BUS and an address BUS to a ROM 35 which stores program contents and information necessary for execution of the programs and is connected to a RAM 36 which provides a working area for the execution of the program. The CPU 34 also is connected to a programmable interval timer 37 so as to output frequency data corresponding to the operation of the light control switch SW 31 input through the input port 33. The output of the programmable interval timer 37 is input to a frequency/voltage (F/V) conversion circuit 38 so as to be converted into a voltage proportional to the frequency. This voltage is input to a differential amplifier 39 which computes the difference between this voltage and the reference level Vr1. The difference is input to the other input terminal of the first comparator means 24.

Thus, the described circuit allows the reference level into the first comparator means 24 to be freely set at any desired level, thus providing an ALC which enables the light quantity to be set at any desired level.

The change in the output from the F/V converter 38 is adjustable by a light control switch 31. FIG. 3 shows the output characteristic of the F/V converter in relation to the light quantity control steps. As will be seen from this Figure, the F/V output characteristic of + (plus) potential is obtained when the light control switch SW 31 is turned to the UP side. For the purpose of simplifying the explanation, it is assumed here that this polarity and conversion gain are determined by restrictions posed by the ALC circuit. This assumption, however, is only illustrative. For instance, it is not always necessary that the output of the F/V converter changes its polarity between + (plus) and - (minus).

The output from the F/V converter 38 is made to pass through a reference voltage generating circuit 42 and is then applied as an AGC reference voltage to the reference input terminal of the second comparator means 30.

FIG. 2 shows the control characteristic of the variable gain amplifier 26 used in this embodiment.

The reference voltage generating circuit 42 has an amplifier 43 for amplifying the output signal from the F/V converter 38 and a subtractor 44 having an input terminal (non-inversion input terminal) for receiving the output from the amplifier 43 and another input terminal (inversion input terminal) for receiving the reference level Vr2.

The gain and the offset of the amplifier 43 are adjusted such that the output signal from the F/V converter 38 matches for the control system of the AGC loop. For instance, the amplifier 43 amplifies the output signal from the F/V converter 38 by a gain G.sub.1 and provides an offset OFS.sub.1 so as to produce an output F/V OUT.times.G.sub.1 +OFS.sub.1. This signal is delivered to the subtractor 44 so as to be subtracted from the reference level Vr2 so that the reference input terminal of the second comparator means 30 receives a signal V44 which is represented by V44=Vr2-(F/V OUT.times.G.sub.1 +OFS.sub.1).

The output from the F/V converter 38, having the + (plus) polarity, is supplied through the amplifier 43 to the subtractor 44 so as to have a polarity which serves to reduce the control voltage of the variable gain amplifier 26. Since the variable gain amplifier 26 has a characteristic that the output level increases as the control voltage becomes lower, the + (plus) output from the F/V converter 38 serves to increase the output leve