JPH0810906B2 - Autofocus video camera - Google Patents

Description

TECHNICAL FIELD The present invention relates to an autofocus circuit of a video camera that automatically adjusts a focus based on a video signal obtained from an image sensor.

(B) Prior art The method of using the video signal itself from the image sensor in the autofocus device of the video camera to evaluate the focus control state is essentially free from parallax and has a shallow depth of field. There are many advantages such as being able to focus accurately on a distant subject. Moreover, a special sensor for autofocus is unnecessary, and the mechanism is extremely simple.

Conventionally, one example of this autofocus method is "NHK
A so-called hill-climbing servo control, which is described as "automatic focus adjustment of a television camera by a hill-climbing servo system" by Ishida et al. This hill-climbing servo control rotates the lens focus ring so that the amount of high-frequency components of the video signal is always maximized, so the lens does not remain stopped in the out-of-focus state, and it follows very well. It is a good method.

However, this method has a major drawback in that the lens is constantly moved.

One of the drawbacks is that the lens does not stop even when the subject is in focus, so that the shooting screen always shakes even after focusing on a stationary object. A lens currently used in a television camera changes a focal length by rotating a focus ring, which changes a field angle of a captured image. For this reason, in this method, in which the focus ring continues to vibrate even after focusing, the screen image becomes very difficult to see because the subject on the screen increases or decreases in a certain cycle.

The second drawback is power consumption. Currently, home video cameras often use a battery as a power source because of their portability, and when the focus motor is constantly driven to repeat forward / reverse rotation, the motor is driven in a certain direction due to inrush current. It consumes more power than when it is rotated, and the recordable time becomes shorter. In addition, since the focus ring is always rotated, problems such as wear of gears occur.

The applicant has proposed a new autofocus circuit system for eliminating these drawbacks in Japanese Patent Application No. 61-273212. The outline of the contents will be described below as a conventional example. Incidentally, in the above-mentioned Japanese Patent Application, an extremely fine control method is adopted in order to improve the focusing accuracy, but here, only the portions related to the present invention will be described.

FIG. 3 is a block diagram of an autofocus circuit according to the above-mentioned prior art.

The image formed by the lens (1) becomes a video signal by the image pickup circuit (4) including an image pickup element and is input to the focus evaluation value generation circuit (5). The focus evaluation value generating circuit (5) is configured, for example, as shown in FIG. The vertical sync signal (VD) and the horizontal sync signal (HD) separated from the video signal by the sync separation circuit (5a) are input to the gate control circuit (5b) to set the sampling area. In the gate control circuit (5b), vertical sync signal (VD),
Based on the horizontal sync signal (HD) and fixed oscillator output,
Set a rectangular sampling area in the center of the screen,
The gate circuit (5c) is supplied with a gate opening / closing signal which permits the passage of the luminance signal only in the range of this sampling area.

Only the luminance signal corresponding to the area within the sampling area is filtered by the gate circuit (5c) and the high pass filter (H.
After passing through PF) (5d), only the high frequency component is separated, and amplitude detection is performed by the detection circuit (5e) at the next stage. This detection output is integrated for each field by the integration circuit (5f) and converted into a digital value by the A / D conversion circuit (5g) to obtain the focus evaluation value of the current field. The focus evaluation value generation circuit (5) configured as described above always outputs the focus evaluation value for one field.

Immediately after the start of the autofocus operation, the first focus evaluation value is held in the maximum value memory (6) and the initial value memory (7). Then, the focus motor control circuit (focus motor control means) (10) rotates the focus motor (3) in a predetermined direction and monitors the output of the second comparator (9). The second comparator (9) compares the focus evaluation value after driving the focus motor with the initial value evaluation value held in the initial value memory (7) and outputs the magnitude.

The focus motor control circuit (10) rotates the focus motor (3) in the first direction until the second comparator (9) outputs a large or small output, and the current focus evaluation value is higher than the initial evaluation value. , If the output is larger than the preset fluctuation range, the rotation direction is kept as it is, and the current evaluation value is smaller than the fluctuation range as compared with the initial evaluation value. When output is made, reverse the rotation direction of the focus motor (3),
The output of the first comparator (8) is monitored.

The first comparator (8) compares the maximum evaluation value up to now held in the maximum value memory (6) with the current evaluation value, and the current focus evaluation value is stored in the maximum value memory (6). Of the two comparison signals (P1) and (P2) which are larger than the first threshold value and which are smaller than the preset first threshold value (second mode). Maximum memory here (6)
Is updated based on the output of the first comparator (8) when the current evaluation value is larger than the content of the maximum value memory (6), and the maximum evaluation value up to the present is always Retained.

(13) is a motor position memory that stores the focus ring position by receiving a focus ring position signal that supports the position of the focus ring (2) that supports the lens (1), similar to the maximum value memory (6). Based on the output of the first comparator (8), the focus ring position when the maximum evaluation value is reached is updated so as to be always held.

The focus motor control circuit (10) monitors the output of the first comparator (8) while rotating the focus motor (3) in the direction determined based on the output of the second comparator (9), and In order to prevent malfunction due to noise,
The second evaluation that the current evaluation value at the output of the comparator (8) is smaller than the preset first threshold value as compared with the maximum evaluation value.
At the same time when the mode is designated, the focus motor (3) is rotated in the reverse direction. After this reverse rotation, the contents of the motor position memory (13) and the current focus ring position signal are compared by the third comparator (14) and when they match, that is, the focus ring (2) has the maximum focus evaluation value. The focus motor control circuit (10) functions so as to stop the focus motor (3) when returning to the position. At the same time, the focus motor control circuit (10) outputs a lens stop signal (LS).

(11) is a fourth memory that holds the focus evaluation value at that point when the lens stop signal (LS) is issued after the autofocus operation by the focus motor control circuit (10) is finished. The content held in the fourth memory (11) is compared with the current focus evaluation value by the fourth comparator (12), and if the value becomes larger than the first threshold value for restarting, the subject is The subject change signal is output to the focus motor control circuit (10) because of the change. Upon receiving this signal, the focus motor control circuit (10) restarts the autofocus operation again and follows the change of the subject.

This method has extremely high tracking ability and high focusing accuracy. However, if the subject illuminance changes at a constant frequency that is different from the frequency of one field of the video signal, a malfunction may occur due to the change in illuminance. It may happen. This situation will be described in some detail below.

For example, in an NTSC type video camera, the frequency of one field is 60 Hz, but when this is used under the illumination of a discharge lamp such as a fluorescent lamp that lights at 50 Hz, such a malfunction occurs. Since the brightness of the discharge lamp lit at 50 Hz changes at a frequency of 100 Hz as shown in FIG. 5, the illuminance of the subject also changes at a frequency of 100 Hz. Since one field of the video signal is 60Hz, the ripple component of 20Hz corresponding to one half of the difference between the two frequencies is based on the sampling theorem as shown in Fig. 6 (dotted line is normal focus evaluation value). , Is superimposed on the output of the focus evaluation value. If the magnitude of this ripple is much smaller than the preset threshold value, there is no problem, but if it has a magnitude above a certain level, the peak of this ripple is mistaken for the maximum value of the focus evaluation value. During the focusing operation, the focus motor stopped at this position, or in the focused state, a malfunction occurred such that the maximum evaluation value changed and the system was restarted.

As one means for dealing with such a malfunction, a means for increasing the first and second threshold values is conceivable, but if the threshold value is set too large, there is a disadvantage that the focusing accuracy is lowered.

(C) Problems to be Solved by the Invention According to the conventional technology, when the field frequency of the video signal of the video camera and the frequency of the discharge lamp that is a light source illuminating the subject are different, the illuminance of the subject varies. , The ripple component generated by the difference between the field frequency and the fluctuation frequency of the illuminance of the subject is superimposed on the focus evaluation value, and the peak value of this ripple is the maximum focus evaluation value when the autofocus mechanism is performing focusing operation. The focus motor is stopped at this point as the in-focus position, and when it is in the in-focus state, an erroneous operation such as restarting due to a change in the focus evaluation value occurs.

(D) Means for Solving the Problem The present invention is to drive the focus motor by the focus motor control means so that the maximum value of the focus evaluation values obtained up to that time is equal to or more than the first threshold value set in advance. When the focus evaluation value has decreased, the focus operation is performed by fixing the lens to the maximum point, and after the focus is completed, the focus evaluation value fluctuates above the preset second threshold value. In this case, in the autofocus video camera that restarts the focusing operation, a determination circuit that determines at least the change in the focus evaluation value that is equal to or larger than the first threshold is provided. When the ripple frequency of the focus evaluation value caused by the difference between the fluctuation frequency fp of f and the field frequency f ₀ of the video signal is fr, M = f ₀ / fr is determined by M
It is characterized in that it is configured to determine that there is a change only when it changes at least continuously more than once.

(E) Operation Since the present invention is configured as described above, when detecting the fluctuation of the focus evaluation value, the ripple for one cycle is always included. The output of the ripple component is superimposed on the original focus evaluation value and oscillates above and below it, so the output of the focus evaluation value from the current focus evaluation value continues for a preset number of times, When it increases or decreases beyond the first and second thresholds, it always indicates that the original focus evaluation value has changed beyond the above thresholds. On the other hand, if there is no change that exceeds the threshold value continuously for a preset number of times, it is affected by the fluctuation of the ripple component and causes a malfunction.

As a result, according to the present invention, the focus motor stops at the wrong position during the focusing operation due to the ripple of the focus evaluation value caused by the difference between the frequency of the discharge lamp that illuminates the subject and the field frequency of the video signal. , There is no malfunction such that the focus motor is accidentally restarted in the in-focus state.

(F) Example An example of the present invention will be described below with reference to the drawings. Incidentally, the same parts as those of the conventional example (FIG. 3) are designated by the same reference numerals and their explanations are omitted.

FIG. 1 is a circuit block diagram of this embodiment. The image formed by the lens (1) becomes a luminance signal by the image pickup circuit (4) having an image pickup element and is input to the focus evaluation value generation circuit (focus evaluation value detection means) (5). The focus evaluation value generation circuit (5) has the same configuration as that shown in FIG. 3 and outputs the focus evaluation value for one field.

The output of the focus evaluation value generation circuit (5) is input to the first comparator (8), and if the value is larger than the value held in the maximum value memory (6), the output (S1) becomes H. The level is reached and the maximum value memory (6) and motor position memory (13) are rewritten. If the output of the focus evaluation value generation circuit (5) is smaller than the value stored in the maximum value memory (6) by exceeding a preset first threshold value, the first continuous number detection circuit (15) A pulsed output (S2) is emitted.
As shown in FIG. 2, the first continuous number detecting circuit (15) includes a shift register (19a), which has a predetermined number of times,
(19b), (19c), (60Hz field frequency and 50
In the proof of Hz, a 20 Hz ripple component is generated in the focus evaluation value, so if the detection of the focus evaluation value change that is performed for each field is 60/20 = 3, that is, three or more consecutive times, the effect of this ripple component Since the change in the focus evaluation value can be correctly detected without being affected, it is composed of three shift registers) and an AND circuit (20) for connecting these outputs. As a result, the signal is sent to the focus motor control circuit (10) only when the predetermined number of times M is continuously counted, the focus motor (3) is rotated in the reverse direction, and the maximum value of the held maximum value is retained. It stops at the position and the focusing operation is completed. This operation will be described in some detail below.

In FIG. 7, the vertical axis represents the focus evaluation value, the horizontal axis represents time, and the like represent the timing at which signals are sent from the focus evaluation value generation circuit (5). The solid curve is the original focus evaluation value, the dotted line is the focus evaluation value with ripples superimposed, and point A is the evaluation value currently stored in the maximum value memory (6). A vertical line mark written on the original focus evaluation value curve is an example of a timing at which a ripple peak occurs. The width ΔX indicated by the broken line is the magnitude of the first threshold value set in advance.

When the signal is taken in now, the output (S2) of the first comparator (8) is not emitted to the value at this time and the L level is maintained, but this value is larger than A, The values of the maximum value memory (6) and the motor position memory (13) are changed, and thereafter, the width ΔX is compared with reference to the point A '. After that, the output (S2) of the first comparator (8) becomes ... L, ... H, ... H, ... H, ... H, so the focus motor control circuit (10) is the first time at the time. The H signal is issued (count number M = 3), and the focus motor rotates in the reverse direction and is fixed at the position where A'is recognized. Although this position is slightly different from the point A, which is the original maximum value, this error is in the range of ± 1.5 fields and is practically safe.

The output of the focus evaluation value generation circuit (5) is also input to the second comparator (9) and compared with the value held in the initial value memory (7) to be larger than the preset third threshold value ΔZ. If so, the H level output (S3) is input to the second consecutive number detection circuit (16). The second consecutive number detecting circuit (16) has the same configuration as the above-mentioned first consecutive number detecting circuit (15). The operation here is shown in FIG. The type of the content curve in FIG. 8 is exactly the same as that in FIG. 7, and the description thereof will be omitted. Starting from the first value B, check H and L ... L, H, H, L, H, H,
Since it becomes H, the second continuous number detection circuit (16) supplies an H-level output signal to the focus motor control circuit (10) at the time of, and keeps rotating the motor in the current rotation direction. , The output of the first continuous number detecting circuit (15) is monitored, and the operation returns to the maximum value determining operation.

If the output of the focus evaluation value generation circuit (5) is smaller than the preset third threshold value ΔZ, the second comparator (9) outputs the first
An H level output (S4) is issued to a third consecutive number detecting circuit (17) having the same configuration as the consecutive number detecting circuit (15). As shown in FIG. 9, at this time, for example, L / L / H / L /
L, H, H, H, and an H level signal is output from the third consecutive number detecting circuit (17), and the focus motor control circuit (10) rotates the motor in the reverse direction and at the same time, the first consecutive number detecting circuit. The output of (15) is monitored, the operation returns to the above maximum value determination operation, and hill climbing servo control is continued.

From the above description, it is clear that according to the present invention, a malfunction such as a motor stop due to a ripple component does not occur during a hill climbing servo.

Next, the case of restarting from the state of reaching the focus will be described.

When the series of hill climbing servos are completed, the lens stop signal (LS) is output from the focus motor control circuit (10) to stop the lens and the fourth memory (11) stores
The evaluation value of A'is held. This value and the current evaluation value output from the focus evaluation value generation circuit (5) are compared by the fourth comparator (12). The fourth comparator (12) is composed of a window comparator, and if the output of the focus evaluation value generation circuit (5) is larger or smaller than the preset second threshold value ΔY, the output of H level. (S5) is output to the fourth consecutive number detection circuit (18).

By the way, since the lens is stopped in this state, the output of the focus evaluation value generation circuit (5) fluctuates around a constant value D as shown by the solid line in FIG. 10 unless the subject changes. . As described above, the "preset number" completely includes one cycle of this ripple, and in this embodiment, as described in the description of the conventional example, the value of the second threshold value ΔY is made sufficiently large. Therefore, it is possible to include the L signal at least once during the series of “preset times”. For example, in FIG. 10, H, L, H, L, L, H, L, L, H
・ It becomes like L ・ L. As a result, there is no malfunction due to ripple unless the subject changes.

It is assumed that the output of the focus evaluation value generation circuit (5) changes in addition to the ripple amount when the subject changes, for example, when the subject (a) in FIG. 11 changes to the subject (b). (The horizontal axis of FIG. 11 shows the lens position. Therefore, FIG. 11 shows the change of the focus evaluation value with respect to the object distance.) At this time, the focus evaluation value generating circuit (5) as shown in FIG.
The output of changes in a discontinuous manner. Then, the fourth comparator (1
The output of 2) is always L, and the fourth continuous count detection circuit (1
8) changes the subject change signal to the focus motor control circuit (1
It will be output to 0) and a reboot will be performed. Here again, it is clear that the malfunction due to the ripple does not occur.
In the above description, the first to fourth consecutive number detecting circuits are described by using hardware, but it is extremely easy to process by software using a microprocessor.

(G) Effect of the Invention As described above, according to the present invention, when the hill-climbing servo control is performed, it depends on the power supply frequency of the discharge lamp that illuminates the object during the hill-climbing servo and when restarting. The beat frequency generated between the fluctuating frequency of the subject illuminance and the field frequency of the video signal, and a fixed number of times determined by the field frequency of the video signal continuously,
Unless the focus evaluation value increases or decreases, the extreme value of the focus evaluation value generated during that time causes the hill-climbing servo mechanism to stop,
Alternatively, since it is not restarted, a malfunction does not occur due to the ripple between the illuminance of the subject and the field frequency as in the conventional case.

[Brief description of drawings]

1, 2, and 5 to 12 relate to an embodiment of the present invention. FIG. 1 is a circuit block diagram, FIG. 2 is a main circuit block diagram, FIG. 5, and FIG. Figure, Figure 7, Figure 8, and Figure 9
FIG. 10, FIG. 11, FIG. 11 and FIG. 12 are operation explanatory diagrams. Further, FIGS. 3 and 4 are circuit block diagrams of a conventional example. (1) …… Lens, (3) …… Focus motor,
(4) ... Imaging circuit (imaging element), (5) ... Focus evaluation value generation circuit (evaluation value detection means), (10) ... Focus motor control circuit, (15) ... First continuous number detection circuit (First detecting means), (16) ... Second continuous number detecting circuit, (1
7) ... Third consecutive number detecting circuit, (18) ... Fourth consecutive number detecting circuit (second detecting means).

Claims (1)

[Claims] 1. A focus evaluation value detecting means for detecting, as a focus evaluation value, an integrated output of a high frequency component level of a video signal obtained from an image pickup device for each predetermined field period, and a movement of a focus lens driven by a focus motor. When the focus evaluation value, which changes along with the focus evaluation value, once exceeds the maximum value and then decreases by a preset first threshold value or more,
In an autofocus video camera comprising a motor control unit that stops the focus motor at the position of the maximum value, further, a comparison unit that detects a change in the focus evaluation value that is equal to or more than the first threshold value; The detection by the comparison means determines the fluctuation frequency fp of the illuminance of the subject.
And fr is the ripple frequency of the focus evaluation value caused by the difference between the field frequency f ₀ of the video signal and
= F ₀ / fr and a continuous number detecting means for detecting when performed at least contiguous M or more times determined by the change the focus evaluation value is equal to or larger than the first threshold value by the output of the continuous number detecting means An autofocus video camera characterized in that it is determined that