JPH0258833B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic focusing device for a video camera or the like.

As an automatic focusing device that takes advantage of the characteristics of a video camera, it detects the definition of the screen using the high-frequency components of the video signal being shot, and adjusts the distance ring (hereinafter referred to as helicoid) of the lens to maximize the definition. So-called hill-climbing control, which controls the rotation of the This method was published in NHK Technical Research Report, 1966, No. 17.
Volume, No. 1, Volume 86, page 21, it is described in detail by Ishida et al. as ``Automatic focus adjustment of a television camera using a mountain-climbing servo method.'' This method will be explained below using Figures 1 and 2. Explain briefly.

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 photoelectric conversion circuit, 3 is a video signal output terminal, 4 is a high-bass filter, 5 is a detector, 6
7 is a differential hold circuit, 7 is a motor drive circuit, and 8 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 the characteristic diagram shown in FIG. 2.

The light from the subject that enters the lens 1 forms an image,
The photoelectric conversion circuit 2 converts the signal into an electric signal, which is output to the terminal 3 as a video signal. Only the high-frequency components of this video signal are extracted by a high-pass filter 4, detected by a detector 5, and then appear at a terminal 51. Voltage corresponding to the high frequency component of the video signal appearing at terminal 51 (curve A in Figure 2) (hereinafter referred to as focal voltage)
corresponds to the definition of the photographed image, so lens 1
If the position of the helicoid (denoted as A) matches the distance between the lens 1 and the subject, it will be maximum, and as it deviates from this position, it will decrease. In FIG. 2, curve B reaches a maximum when the helicoid position (designated A) matches the distance between the lens 1 and the subject, and decreases as it deviates from this position. In FIG. 2, curve B shows the output voltage waveform of the output terminal 61 of the differential hold circuit 6 when the helicoid position is moved from close to ∞, and curve C shows the waveform of the output voltage at the output terminal 61 of the differential hold circuit 6 when the helicoid position is moved from ∞ to near.

What can be determined from Figure 2 is that 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 6 to motor 8 shown in FIG. That is, while moving the helicoid position by the motor 8, the differential hold circuit samples and holds the focal voltage appearing at the terminal 51 at fixed time intervals, and if the focal voltage is increasing over time, it will be a positive voltage, and if the focal voltage is decreasing, it will be a positive voltage. If the output voltage of the differential hold circuit 5 connected to the terminal 61 is positive, the motor drive circuit 7 keeps the rotational direction of the motor 8 as it is and continues climbing the mountain. If the output voltage is negative, the motor 8 is reversed to return the helicoid to the uphill direction.

In this way, the helicoid position control closed loop with the configuration shown in FIG. 1 climbs the mountain created by the focal voltage by referring to the output voltage of the differential hold circuit 6, and finally vibrates in small increments at the top of this mountain. However, by reaching a steady state, the focus can be adjusted automatically.

The above is an automatic focusing device for a video camera using the mountain climbing method.

Since this method performs automatic focusing using the photographed image itself, it has a simpler structure, requires fewer initial adjustments, and is cheaper than a method that uses open-loop control of the helicoid position based on the distance measured by an independent distance measuring mechanism. Although an accurate automatic focusing device can be constructed, there are points to be improved as described below.

The point to be improved is the dynamic range of the focal voltage. That is, even if the amplitude of the video signal that is the output signal of the photoelectric conversion circuit 2 such as a camera circuit is suppressed to a constant value by the automatic or manual gain adjustment circuit of the photoelectric conversion circuit 2, the output signal of the detector 5 is the video signal. To correspond to the amount of high frequency components contained in When a large number of vertical line components are included, the peak of the focal voltage becomes high. This difference is very large in actual images. On the other hand, the dynamic range of the detector is mainly limited by the power supply voltage of the circuit. For images that do not contain many vertical line components, the output voltage of the detector 5 becomes low and may not reach a level sufficient for the differential hold circuit 6 to operate. Unable to obtain output voltage. On the other hand, if the height of the focal voltage is increased so that the output voltage of the detector 5 is sufficient to perform a hill-climbing operation for an image that does not contain many strong vertical line components, the result will be very clear. For images containing many vertical line components, the output voltage of the detector 5 is saturated and the peak of the focal voltage shown in FIG. 2 is crushed, so even in this case, the desired signal is supplied to the differential hold circuit 6. Therefore, accurate mountain climbing movements cannot be performed.

An object of the present invention is to eliminate the above-mentioned drawbacks of the prior art and to expand the dynamic range of the detection circuit so as to perform a good hill-climbing operation regardless of whether there are many or few vertical line components in the photographed image. An object of the present invention is to provide an automatic focusing device that eliminates malfunctions caused by insufficient dynamic range of a detector.

The drawback of the conventional automatic focusing device explained using FIG. 1 is that the dynamic range of the detection voltage of the detector 5 is insufficient. A way to compensate for the lack of dynamic range is to use some kind of automatic gain control circuit to prevent the detection voltage of the detector 5 from becoming saturated, but in this case a normal automatic gain control loop cannot be used. That is, in a normal automatic gain control loop, closed loop control is performed so that the detected voltage itself remains at a constant value, so the peak of the focal voltage itself is flattened, and the hill climbing circuit becomes inoperable.

Therefore, in the present invention, when the focal voltage becomes larger than a certain threshold value and there is a risk of saturation, the sensitivity of the detector is lowered by one step, and conversely, when the focal voltage becomes smaller than a certain threshold value, the output voltage of the differential hold circuit appears. If there is a possibility that the detector is not detected, increase the sensitivity of the detector by one step and adjust the sensitivity to maintain an appropriate detector sensitivity by changing the sensitivity in stages. However, at the moment the sensitivity is switched, an erroneous output is output from the differential hold circuit due to the change in the detector output caused by the sensitivity switch. Although this erroneous output is instantaneous, since the mountain-climbing autofocus relies only on the output of the differential hold circuit to control the motor, this instantaneous erroneous output can cause the motor to reverse, which can be fatal. This may lead to malfunction. To prevent this, when switching the sensitivity, the focusing loop is opened for at least the period in which the detection output before switching is held in the differential hold circuit, and the focusing operation is stopped, and then the focusing operation is restarted. . At this time, the distance ring is rotated in the same direction as before the sensitivity switch, so that it will climb the mountain correctly even if the focusing operation is started again.

FIG. 3 shows a block diagram of an embodiment of the present invention. 1 to 9 in the figure are blocks having the same functions as the blocks with the same numbers shown in FIG. In FIG. 3, 10 is a stepped variable gain amplifier, 11 is a latch circuit, 12 is an excessive signal detection circuit, 13 is an under-signal detection circuit, and 14 is a latch pulse generation circuit.

The operation of the circuit shown in Fig. 3 will be explained below using Fig. 4. Fig. 4 shows the distance versus focal voltage characteristic when imaging a certain subject, with the vertical axis representing the focal voltage and the horizontal axis representing the distance ring position. It's set aside.

It is assumed that the stepped variable gain amplifier 10 has four stages of gain, and based on the shape of the peak in the case of a gain of 1, the shape of the peak of the focal voltage at each gain is as shown in FIG. Here, gain 1 has the maximum gain, and the gains become lower in order. In the stepped variable gain amplifier 10, the gain is decreased by one step in response to the signal from the excessive output detection circuit 12, and conversely, the gain is increased by one step in response to the signal from the under-output detection circuit 13.

The excessive output detection circuit generates an excessive output detection signal to the stepped variable gain amplifier 10 when the output signal from the detector 5 exceeds a certain threshold value and there is a risk of saturation. The under-output detection circuit 13 is the detector 5
If the output signal from the differential hold circuit 6 becomes smaller than a certain threshold and there is a possibility that a differential signal cannot be obtained from the differential hold circuit 6, an undersized output signal is generated to the stepped variable gain amplifier 10. If the initial position of the range ring is at point W and the gain is 1 immediately after the start of the focusing operation, the differential hold circuit 6 obtains the focal voltage at point a in FIG. 4. Since the slope at point a is large and the direction of rotation of the distance ring is quickly determined, the distance ring immediately begins to rotate toward point b. At point b, at which the excessive output detection threshold is reached, the excessive output detection circuit 12 generates an excessive output detection signal, reducing the gain of the stepwise variable gain amplifier 10 from gain 1 to gain 2. This gain change generates a focal voltage at point c in FIG.

At this time, a focal voltage smaller than that before the gain change is input from the differential hold circuit 6, so an output is generated that indicates that the voltage is descending from the mountain. Since this is a misjudgment, it is necessary to prevent misfocusing operations caused by this.

To prevent this, a latch circuit 11 and a latch pulse generating circuit 14 shown in FIG. 3 are provided, and their operations will be explained.

Normally, the latch circuit 11 simply transmits the output of the differential hold circuit 6 to the motor drive circuit 7, but when a latch pulse is input, it latches the output of the differential hold circuit 6 at the rising edge of the pulse and transfers the output to the motor drive circuit. 7 and releases the latch at the falling edge to return to the original state. The latch pulse generating circuit 14 is a circuit that generates a pulse with a constant time width when an excessive output detection signal or an insufficient output detection signal is input. Therefore, when an over-output detection signal or an under-output detection signal is generated, the latch pulse generation circuit 14 outputs a latch pulse with a fixed time width, latches the correct differential hold circuit output before making an erroneous judgment, and transmits it to the motor drive circuit 7 to prevent an erroneous judgment. Prevents malfunctions caused by differential hold circuit output. After the latch pulse width has elapsed for the differential hold circuit to malfunction due to the gain change and return to normal differential hold circuit operation due to the detection signal at the new gain, the latch is released and normal focusing operation is resumed.
Through this series of operations, the circuit stops the focusing point direction judgment operation at the same time as the gain change occurs, maintains the distance ring rotation direction in the same direction as before the gain change, and then performs the focusing operation under the new gain. let Therefore, after moving from point b to point c in FIG. 4, the distance ring rotates toward point d. When it reaches point d, it moves to point e and the distance ring rotates toward the summit f in the same way as when it reaches point b. Near point f, a conventional hill-climbing operation is performed and the range ring stops at the in-focus position. In order to show the traces of this series of operations, solid lines are used in FIG. 4. Conversely, even if the gain is 4, which is a low gain, and the initial position W is the initial value, the detection signal from the under-output detection circuit 13 immediately moves from point G to point H in FIG. 4, following the focal voltage curve of gain 4. The focus reaches point F and the focusing operation ends.

As described above, in this embodiment, a signal representing the focused state is output from the detector 5 according to the focusing condition of the lens 1, and the detection level of this signal representing the focused state is set in advance. It is supplied to excessive and under output detection circuits 12 and 13. Therefore, if the signal representing the in-focus state exceeds the tolerance range determined by the detection levels of the over- and under-output detection circuits 12 and 13 during the focusing operation, an over- or under-output detection signal is generated, and the variable gain The gain of amplifier 10 is changed in steps. That is, when an overdetection signal is generated, the gain of the variable gain amplifier 10 is reduced by one step, and as a result, the signal representing the focusing operation is returned from the upper limit value of the allowable range to the intermediate value, and conversely, when an underdetection signal is generated, the gain of the variable gain amplifier 10 is reduced by one step. is generated and the gain of the variable gain amplifier 10 is increased by one step, and as a result, the signal representing the in-focus state is returned from the lower limit value of the allowable range to the intermediate value.

When the signal representing the focusing operation exceeds the permissible range determined by the detection levels of the excessive and under output detection circuits 12 and 13, the gain of the variable gain amplifier 10 is changed stepwise, so that the detector 5 is dynamically activated. This can prevent the microwave from running out. As a result, a normal focus voltage characteristic is obtained, and sufficient signal amounts are obtained for various objects, so that stable automatic focusing control operations can be performed. Moreover, the gain of the variable gain amplifier 10 is constant from the switching point (points b, c) to the next switching point (points d, e), so it depends on the focal voltage corresponding to the focusing state and the movement of the lens 1. It will be obtained linearly depending on the situation. Therefore, a characteristic of the focus voltage always increasing as the focus approaches the focus point can be obtained, so that a good focusing operation that always reaches the focus point can be performed. Of course, at the switching point, a discontinuous change in the focus voltage, that is, a sudden decrease or increase, may occur, so there is a risk of misjudgment of the direction of the focus point. is sustained, so
Malfunctions during gain switching can be prevented, and malfunctions that previously occurred due to insufficient dynamic range have been eliminated.

Although the distance ring continues to rotate during gain switching, it goes without saying that the distance ring may be stopped by, for example, stopping the motor during the gain switching.

Further, although the gain is switched in the embodiment, the constant of the high-pass filter may be changed, or the base clip amount of the high-pass filter output signal may be changed.

Further, in the embodiment, the output of the differential hold circuit is latched, but the value held by the differential hold circuit is reset, and the focal voltage under the new gain is not entered into the differential hold circuit. Further, in the description of the embodiment, the gain is switched in four stages, but the number of stages may be any number according to necessity.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of a conventional automatic focusing device, FIG. 2 is a characteristic diagram for explaining the operation of the configuration in FIG. 1, and FIG. 3 is an implementation of the automatic focusing device according to the present invention. FIG. 4, a block diagram showing an example, is a characteristic diagram for explaining the operation of the configuration of FIG. 3. 1: Lens, 2: Photoelectric conversion circuit, 7: Motor drive circuit, 8: Lens drive motor, 9: Hill climbing circuit, 10: Stepped variable gain amplifier, 11: Latch circuit, 12: Excessive output detection circuit, 13: Underestimation Output detection circuit, 14: Latch pulse generation circuit.

Claims (1)

[Scope of Claims] 1. An optical lens system with a variable focal length, a photoelectric conversion circuit that converts an image formed by the optical lens system into a video signal, and a stepwise variable gain that converts the video signal into a video signal. a variable gain amplifier circuit that amplifies the video signal, a high-pass filter that extracts the high-frequency component of the video signal, and a detector that detects the output signal of the high-pass filter, and the high-frequency component signal increases in accordance with the detection output. a focus control circuit that controls the focal length of the optical lens system in the direction of a focus state detection means for extracting a signal representing a focus state that changes according to a change in a focal length of the feedback loop; When detecting deviation from the allowable range determined by the focus state and the lower limit value, if the signal representing the focused state increases more than the upper limit value, the gain of the variable gain amplifier circuit is decreased in stages, and the lower limit value is detected. an automatic focusing device comprising: gain control means for increasing the gain of the variable gain amplifier circuit in steps when the gain decreases by more than . 2 The focusing control system has a holding circuit that holds the temporal difference of the detected output for a certain period of time, and the focusing control system has a holding circuit that holds the temporal difference of the detected output for a certain period of time, and the gain of the variable gain amplifier circuit is kept constant immediately after the gain of the variable gain amplifier circuit is changed stepwise by the gain control means. At the time, an automatic focusing operation is performed according to the output of the holding circuit, and after the above-mentioned certain period of time has elapsed since the gain of the variable gain amplifier circuit is changed stepwise, an automatic focusing operation according to the detection output is performed. The automatic focusing device according to claim 1, characterized in that the automatic focusing device restarts. 3 The gain control means includes an overdetection circuit that reduces the gain of the variable gain amplifier circuit stepwise when the signal representing the focused state exceeds an upper limit; and an under/under detection circuit that increases the gain of the variable gain amplification circuit in stages when the gain exceeds the value, and the focus state detection means generates a signal in the focus control circuit as a signal representing the focus state. The automatic focusing device according to claim 1, wherein the detected output is supplied to the over/under detecting circuit. 4. The variable gain amplifier circuit is characterized in that when the detected output increases above an upper limit value, the gain of the variable gain amplifier circuit is decreased by one step, and when it decreases below a lower limit value, the gain of the variable gain amplifier circuit is increased by one step. An automatic focusing device according to claim 3.