JPS6325751B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a focusing voltage generation circuit in an automatic focusing device for a video camera.

As an automatic focusing device for video cameras, it detects the definition of the screen using high frequency components in the video signal, and adjusts the lens so that the definition is maximized, that is, the high frequency components are maximized. So-called hill-climbing control, which controls the rotation of a range ring (hereinafter referred to as a helicoid), is known (see NHK Technical Research Report, 1965, Vol. 17, No. 1, Volume 86, p. 21). This will be briefly explained below using FIGS. 1 and 2.

FIG. 1 is a block diagram showing the structure of an automatic focusing device using a hill climbing method. In the same figure,
1 is a lens, 2 is a camera circuit, 3 is a focal voltage (V _F ) generation circuit, and 4 is a high-pass filter. Reference numeral 5 denotes a double-wave detection circuit, which is capable of double-wave detection of a portion exceeding a predetermined threshold value. Reference numeral 6 denotes an integrating circuit which integrates an input signal for a certain period of time until a reset pulse signal is applied from an integral reset circuit 7 that generates a reset signal (this integrated output voltage is called a focal voltage). 8 is a differential voltage detection hold circuit, 9 is a motor drive circuit, and 10 is a motor for rotating the helicoid of the lens 1.

The operation of the configuration shown in FIG. 1 will be explained below using FIG. 2. Light from a subject entering the lens 1 forms an image, becomes an electric signal in the camera circuit 2, and is output as a video signal. For example, assuming a simple object as shown in FIG. 2A, where the center of the screen is dark and both ends are bright, a video signal such as a is output from the camera circuit 2. When only the high frequency component of this video signal a is extracted by the high pass filter 4, the horizontal contour component of the image is obtained as shown in b. As the focus shifts, the high frequency components of the imaged image decrease, so the contour signal components also decrease as shown by the broken line. These contour signal components are aligned to one side as shown in c by a double-wave rectifier circuit 5 that detects only signals exceeding a predetermined threshold value _VTH . If the contour signal components aligned in one direction are integrated by the integrating circuit 6 for a certain period of time (for example, 1 field = 1/60 sec), then
As its output, a voltage corresponding to the high frequency component of the video signal, a so-called focal voltage, is obtained. This focal voltage corresponds to the definition of the captured image, so the second
As shown in FIG. d, if the position of the helicoid of the lens 1 (referred to as A) matches the distance between the lens 1 and the subject, it becomes maximum, and as it deviates from this position, it decreases. Therefore, if the helicoid position is controlled by some means so that it climbs the peak of the focal voltage, and the helicoid is guided to the top of the mountain where the focal voltage is maximum, an automatic focusing device can be formed.

This means is achieved by the differential hold circuit 8 to motor 10 shown in FIG. That is, motor 10
While moving the helicoid position, the differential hold circuit 8 samples and holds the focal voltage appearing in the output of the integrating circuit 6 at regular time intervals, and if the focal voltage is increasing over time, it is a positive voltage, and it is decreasing. If so, it is a circuit that generates a negative voltage, so if the output voltage of the differential hold circuit 8 is positive, the motor drive circuit 9 keeps the rotational direction of the motor 10 as it is and continues climbing the mountain, and the same output voltage If is negative, the motor 10 is reversed to move the helicoid back down the mountain. In this way, the helicoid position control closed loop having the configuration shown in FIG. Focusing can be done automatically by reaching a steady state.

The reason for obtaining a double-wave rectified wave as shown in FIG. 2c by setting an appropriate threshold value V _TH during double-wave detection or after double-wave detection as shown in FIG. 2b is as follows. That is, obtained in response to image blur,
Focal voltage characteristics with respect to helicoid position (mountain shape)
is the frequency distribution of the high frequency component of the subject itself,
Alternatively, the same helicoid position changes depending on the aperture value F, focal length f, so-called depth of field, etc. of the lens 1, resulting in various variations. For example, when the depth of field is deep, as shown in A, the mountain becomes flat (a direction in which it is difficult to detect the differential voltage), and as the depth of field becomes shallower, the slope of the mountain becomes larger. In order to perform correct and stable focusing operation, the height and shape (slope) of the peak of the focal voltage should be as independent as possible from the lens condition f, F, subject, etc., and the differential hold circuit should have a sufficient differential voltage. It is preferable that the amount of inclination is large enough to be detected. If the threshold voltage V _TH is set appropriately here, it is possible to obtain good focal voltage characteristics on average for all lens conditions and subjects as shown in (B), and a sufficient differential voltage can always be detected. . By setting such characteristics, it is possible to perform a good hill-climbing determination and to perform a focusing operation without malfunction. Setting the threshold value V _TH in this way is an important means for creating a good mountain shape.

FIGS. 3A and 3B are circuit diagrams showing a conventional double-wave rectifier circuit and an integrating circuit, respectively, which can be considered relatively easily. In the same figure, high-frequency components in the positive direction and negative direction of the video signal applied to the base of the transistor Q ₁ are connected to the collector by setting the resistor R ₁ = R ₂ .
Since they appear at the emitters at the same level and with opposite polarities, they are detected in the same direction by diodes D ₁ and D ₂ . Select R ₃ = R ₅ , R ₅ = R ₆ , R ₃ / R ₄
= Appropriate value in the direction higher than the voltage determined by R ₅ / R ₆
By setting R ₇ /R ₈ , the above-mentioned detection threshold voltage V _TH
can be determined. This detected signal is applied to the base of transistor _Q2 of integrating circuit B. Now reset switch RS by reset pulse
At the moment when is closed, the charge across the integrating capacitor Ci is charged to the voltage Ei. Next, when the reset switch RS is opened, the charge stored in the capacitor Ci is discharged through the collector of the transistor _Q2 , low resistor _R9 .
It is controlled by the voltage applied to the base of Q ₂ , i.e. the emitter current. Therefore, an integrated voltage of the high frequency component input to the base is obtained across the capacitor Ci. However, in this case, an integrated voltage is also generated due to the steady-state DC bias current component. Therefore, this stationary component impairs the dynamic range, resulting in serious obstacles and restrictions when converting the circuit into an IC, and also makes it difficult to obtain the above-mentioned sufficient focal voltage, resulting in poor control characteristics. It has the disadvantage that it is no longer in good condition.

An object of the present invention is to provide a focus voltage generation circuit that can obtain good focus voltage characteristics for any subject or lens condition as described above, and is suitable for IC implementation.

The key point of the present invention is to prevent the generation of an integrated voltage due to the DC bias component, and to perform integration only for the level exceeding a predetermined threshold voltage of the double-wave rectified waveform consisting of the high-frequency components of the image. It has a sufficient dynamic range and good focal voltage characteristics.More specifically, an integrating capacitor and an integrating capacitor are connected to the collector of the first transistor of the differential pair transistor. A switch element that is charged to a predetermined voltage value and reset is connected, and a predetermined DC voltage E _C higher than the base DC voltage E _B of the first transistor is applied to the base of the second transistor. Therefore, the double-wave rectified wave superimposed on the base DC voltage of the first transistor is integrated only for the level exceeding the predetermined threshold voltage _VTH , and within a certain period (for example, 1/60
The integration and reset are performed every sec).

Next, an embodiment of the present invention will be described with reference to the drawings.

FIG. 4 is a circuit diagram showing one embodiment of the present invention. In the figure, Q with subscripts indicates a transistor, R indicates a low resistor, C indicates a capacitor, E indicates a DC voltage source, and I indicates a DC current source. In FIG. 4, transistors Q ₃ and Q ₄ are differentially connected, and a DC voltage E _C is applied to the base of transistor Q ₄ . The DC voltage E _C is the predetermined threshold voltage of the double-wave rectified wave consisting of the image signal high-frequency components superimposed on the DC bias voltage E _B input to the base of Q ₃ .
Settings are made so that emitter current begins to flow through transistor Q ₃ only when V _TH is exceeded. Transistors Q ₅ and Q ₆ , low resistance R ₁₀ , and DC voltage source _ED constitute an integral reset circuit. When an integral reset pulse is applied to the base of transistor _Q6 as shown in the figure, the transistor is
Q ₆ is turned off and transistor Q ₅ is turned on to connect the integrating capacitor to the collector of transistor Q ₃ .
Ci is charged to approximately the DC voltage source voltage E _D (ignoring the base-to-emitter voltage of _Q5 ). Next, when the integral reset pulse becomes high level, transistor _Q6 turns on and the collector of transistor _Q6 becomes
When the transistor Q ₅ is turned off, the electric charge stored in the integrating capacitor Ci is discharged through the transistor Q ₃ (at this time, the discharge current exceeds the threshold voltage V _TH applied to the base of the transistor Q ₃ ). The voltage across the integrating capacitor Ci (which is a current controlled by a double-wave rectified waveform) drops, but the amount of the drop is equal to the value obtained by integrating the collector current of transistor _Q3 from the time the integral reset is released until the next reset, i.e. It is proportional to the total discharge charge during that time. Therefore, the amount of voltage drop across the integrating capacitor Ci can be used as the focusing voltage.

As described above, according to the circuit configuration of the present invention, an integrated voltage due to the fixed DC bias component is not generated at all, and only an integrated voltage due to the high frequency component is obtained.
Therefore, all the dynamic range of the collector voltage from the collector voltage E _D of transistor _Q3 when the integrating capacitor Ci is reset until the transistor _Q3 is saturated is allotted to the integration of the high-frequency components of the video signal. Therefore, it is possible to obtain a focal voltage that is effective over a wide range from small contrast to large contrast. In addition, the DC voltage E _C applied to the base of transistor Q ₄
Since the threshold voltage V _TH can be arbitrarily determined by setting , the slope of the peak of the focal voltage characteristic corresponding to the helicoid position (image blur) can be freely changed. Further differential pair transistors Q ₃ and Q ₄
The threshold value V _TH setting and integral operation can be performed in the same way and easily, making it suitable for IC implementation.

FIG. 5 is a circuit diagram showing another embodiment of the present invention, in which a reset switch circuit is constructed by a transistor _Q7 and a DC voltage source _ED . An integral reset pulse is applied to the base of transistor _Q7 , and when the integral reset pulse is at a low level, transistor _Q7 is turned on and the voltage of integral capacitor Ci is reset to approximately _ED . When the integral reset pulse is high level, the transistor
Q ₇ is turned off, and during this period, the high frequency component applied to the base of transistor Q ₃ is integrated, as in the explanation in FIG. 4, and an integrated voltage is generated in the integrating capacitor Ci to obtain a focal voltage.

As explained above, according to the present invention, the focal voltage can correspond to high-frequency components from low contrast to large contrast (for example, from a brightness change of about 5% to a brightness change of 100%) by effectively securing a dynamic range. Furthermore, from a comprehensive perspective such as the type of subject and depth of field (aperture value F, focal length zoom value f), the slope of the mountain is good enough to detect a sufficient differential voltage. By selecting the threshold voltage V _TH in this manner, it is possible to obtain good focus voltage characteristics in all imaging situations, and to provide a focus voltage generation circuit suitable for IC implementation.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of a conventional automatic focusing device, FIG. 2 A is an explanatory diagram showing an example of a subject, and FIGS. 2 a to c are waveform diagrams showing the process until the focal voltage is obtained. FIG. 2d is a characteristic diagram of the focus voltage, FIG. 3 is a circuit diagram showing an example of a conventional focus voltage generation circuit, and FIGS. 4 and 5 are circuit diagrams each showing an embodiment of the present invention. . Explanation of symbols 1... Lens system, 2... Camera circuit, 3
...Focus voltage generation circuit, 4...High pass filter, 5
...Double wave detection and detection threshold setting circuit, 6... Integral circuit, 7... Integral reset circuit, 8... Difference detection circuit, 9... Motor drive circuit, 10... Motor.

Claims (1)

[Claims] 1 A pair of differentially connected transistors, an integrating capacitor connected to the collector side of the first transistor of the first and second transistors constituting the pair of transistors, and an integral capacitor that is periodically turned on and off. It consists of a switch element that charges the capacitor from the power supply to a predetermined voltage when it is switched on, and a voltage source of a DC voltage for threshold setting that is applied to the base electrode of the second transistor; Of the detected output of the high frequency component of the video signal input to the base electrode of the second transistor, the output is determined by the magnitude of the DC voltage applied from the DC voltage source to the base electrode of the second transistor. An output component exceeding a threshold level causes the capacitor to be discharged through a first transistor when the switch element is off, and the resulting voltage change at the terminals of the capacitor is used as a focusing voltage. A focal voltage generation circuit for an automatic focusing device, characterized in that: