JP2837170B2 - Auto focus device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an autofocus device, and more particularly, to an autofocus device in a video camera or the like.

2. Description of the Related Art Conventionally, an imaging device such as a video camera includes an autofocus device for automatically focusing on a subject. Generally, such an autofocus apparatus performs focusing by a method called hill-climbing control as described below.

First, a signal in a predetermined high frequency band is extracted from a luminance signal obtained from an image sensor, and its amplitude is detected and converted into a digital signal. The digital signals thus obtained are all integrated for a certain period, such as one field or one frame. The value obtained as a result is called a focus evaluation value. That is,
A focus evaluation value is obtained for each field or each frame.

FIG. 4 is a diagram showing the relationship between the position of the focus lens and the focus evaluation value. In the figure, the horizontal axis indicates the position of the focus lens, and the vertical axis indicates the focus evaluation value. In the figure, P indicates a lens position where the focus is exactly adjusted. As can be seen from the figure, the focus evaluation value shows the maximum value at the lens position P, and monotonically increases or decreases before and after that. Therefore, the focus evaluation value is obtained for each field while moving the focus lens in the positive direction in the figure, and this is compared with the focus evaluation value of the previous field. As a result, if the focus evaluation value of the current field is larger, the focus lens is further moved in the same direction as before. Conversely, if the focus evaluation value of the previous field is larger, it can be determined that the position of the focus lens has exceeded the focus position P. Therefore, the focus lens is moved in the opposite direction, and the focus evaluation value of the previous field is The focus lens is stopped where the same focus evaluation value is obtained. For example, assuming that the focus lens is at the position Q in the figure in the initial state, the focus lens initially moves in the positive direction by comparing the focus evaluation values as described above. Eventually, when the focus lens exceeds the focus position P, the focus evaluation value starts to decrease, so that the focus lens moves in the negative direction and stops at the focus position P. As described above, the focus lens automatically moves to the in-focus position.

Note that, in order to actually focus the image projected on the center of the screen, not the entire screen but a partial area at the center of the screen is targeted for focusing.

By the way, when a single frequency band is extracted from the luminance signal to obtain the focus evaluation value, the following problem occurs.

The frequency band of the obtained luminance signal and its level differ depending on the brightness, shading, etc. of the subject. Therefore, depending on the condition of the subject, there may be a case where the frequency band to be extracted does not exist in the obtained luminance signal, or a case where the frequency band is present has an extremely low level. In such a case, if only one frequency band is extracted from the luminance signal, the focus evaluation value cannot be obtained, or even if it is obtained, the value is not accurate. Further, the higher the extracted frequency band is on the higher frequency side, the smaller the obtained focus evaluation value becomes. In particular, this tendency is more remarkable as the position of the focus lens is far from the in-focus position. Therefore, if the position of the focus lens is significantly deviated from the in-focus position in the initial state,
There may be a case where the focus evaluation value is not obtained and the autofocus device does not function.

FIG. 5 is a diagram showing the relationship between the focus evaluation value and the position of the focus lens for two different frequency bands. In the figure, the horizontal axis indicates the position of the focus lens, and the vertical axis indicates the focus evaluation value. In the figure, a is a curve showing a change in focus evaluation value obtained from a certain high frequency band, and b is a curve showing a change in focus evaluation value obtained from a lower frequency band than in the case of a. As can be seen from these graphs, the higher the frequency band is, the sharper the curve indicating the change in the focus evaluation value is, but the lower the height of the peak is. On the other hand, when the frequency band is low, the curve indicating the change in the focus evaluation value draws a gentle mountain, but the mountain height is high. Therefore, it has been considered that a plurality of different frequency band components are extracted from the luminance signal, a focus evaluation value is obtained for each hidden frequency band component, and these are used properly depending on the subject. For this reason, the conventional autofocus device is provided with a series of circuit units necessary for obtaining the focus evaluation value by the same number as the number of frequency bands to be extracted.

FIG. 3 is a schematic block diagram showing an example of a conventional autofocus device. Referring to the figure, this autofocus device includes band filters (band pass filters; hereinafter abbreviated as BPFs) 1 and 2 for passing different frequency bands, digital absolute value conversion circuits 42 and 43, integration circuit 30 and 31, a CPU (central processing unit) 20, a focus motor control circuit 21, a synchronization separation circuit 11, a gate control circuit 12, an oscillation circuit 14, an A / D (analog-digital) conversion pulse generation circuit 22, , Latch pulse generation circuit 23
And The digital absolute value conversion circuit 42 includes the absolute value conversion circuit 3 and the A / D converter 5, and the digital absolute value conversion circuit 43 similarly includes the absolute value conversion circuit 4 and the A / D converter 6. The integrating circuit 30 includes an adding circuit 7 and a latch circuit 8
Similarly, the integrating circuit 31 includes an adding circuit 9 and a latch circuit 10.

The BPFs 1 and 2 extract different frequency band components from a luminance signal Y obtained from an image sensor (not shown).
The first and second frequency band components extracted by the BPFs 1 and 2 are rectified by the absolute value conversion circuits 3 and 4, respectively, and become a DC potential (hereinafter, referred to as an absolute value signal) corresponding to the level, A /. Input to D converters 5 and 6. On the other hand, the synchronization separation circuit 11 separates the horizontal synchronization signal and the vertical synchronization signal from the luminance signal Y. The separated horizontal synchronization signal and vertical synchronization signal are input to the gate control circuit 12. The gate control circuit 12 uses the horizontal synchronization signal and the vertical synchronization signal only when a region (focus control area) on the screen to be focused is scanned.
A signal that opens the gate of the circuit 13 is generated and output to the gate circuit 13. On the other hand, the output of the oscillation circuit 14 is given to the gate circuit 13. Therefore, the gate circuit 13 supplies the output of the oscillation circuit 14 to the next-stage A / D conversion pulse generation circuit 22 and latch pulse generation circuit 23 only when the focus control area is being scanned. The latch pulse generation circuit 23 converts the oscillation output of the oscillation circuit 14 into a latch pulse for operating the latch circuits 8 and 10. Similarly, the A / D conversion pulse generation circuit 22 outputs the oscillation output of the oscillation circuit 14 to the A / D converters 5 and 6.
Is converted into an A / D conversion pulse for operating the A / D converter and supplied to A / D converters 5 and 6. Therefore, latch circuit 8 and
10 and A / D converters 5 and 6 operate only when the focus control area is being scanned, and perform the following processing. That is, the A / D converters 5 and 6 digitize the output signals of the absolute value conversion circuits 3 and 4, that is, the absolute value signals, only when the focus control area is being scanned, and integrates them in the next stage. Input to the circuits 30 and 31 (hereinafter, the digitized absolute value signal is referred to as absolute value data).

In the integrating circuit 30, the adding circuit 7 adds the input absolute value data to the data held in the latch circuit 8. Similarly, in the integrating circuit 31, the adding circuit 9 adds the input absolute value data to the data held in the latch circuit 10. The resulting data is input to latch circuits 8 and 10, respectively. The latch circuits 8 and 10 synchronize with the latch pulse from the latch pulse generation circuit 23,
Keep the new data entered. The latch circuits 8 and 10 are both synchronized with the vertical synchronizing signal, that is,
It is reset for each field and outputs the data held at that time to the CPU 20. Therefore, the above-described series of operations of the adder circuit 7 and the latch circuit 8 and the adder circuit 9 and the latch circuit 10 are repeated for one field period, so that the integrating circuits 30 and 31 output luminance signals of different frequency bands from each other. The obtained focus evaluation value is output and input to CPU 20. The CPU 20 compares the two input focus evaluation values and performs hill-climbing control on a frequency band in which a larger focus evaluation value can be obtained. For example, if the focus evaluation value obtained from the first frequency band is larger, this focus evaluation value is compared with the focus evaluation value of the first frequency band obtained from the previous field. As a result of this comparison, if the change in the focus evaluation value indicates an increase, the CP
U20 gives a command to the focus motor control circuit 21 to further move the position of the lens in the same direction as before. Conversely, if the change in the focus evaluation value indicates a decrease,
0 gives the focus motor control circuit 21 an instruction to move the lens position in the opposite direction. Then CPU2
A value of 0 performs comparison using the value of the focus evaluation value before indicating a decrease as the compared value. As a result, if the change in the focus evaluation value is substantially zero, a command to stop the lens at that position is given to the focus motor control circuit 21. The focus motor control circuit 21 moves or stops the lens in response to a command from the CPU 20. The following is also performed by utilizing the difference in focus evaluation value change due to the difference in frequency band as shown in FIG. That is, when the focus lens is located at a position greatly deviated from the in-focus position, the above-described hill-climbing control is performed using the focus evaluation value in the low-frequency band where a large value is obtained. Then, the hill-climbing control is performed using the focus evaluation value of the band on the high frequency side where the rate of change is large. Of course, if more frequency bands are extracted,
The processing performed by CPU 20 is the same as described above.

[Problems to be Solved by the Invention] As described above, in the conventional autofocus device, since each of a plurality of different frequency band components extracted from the luminance signal is individually processed, a circuit unit necessary for a series of processes is provided. Multiple copies were required. For example, the autofocus device shown in FIG. 3 is provided with two digital absolute value circuits each including an absolute value circuit and an A / D converter. As a result, not only the cost of the autofocus device has been increased, but also the internal circuit has been complicated.

SUMMARY OF THE INVENTION An object of the present invention is to provide an autofocus apparatus which solves the above-described problems and can obtain a focus evaluation value from each of a plurality of different frequency band components with a lower cost and a simple circuit configuration than in the past. That is.

[Means for Solving the Problems] In order to achieve the above object, an autofocus apparatus according to the present invention includes a means for imaging a subject and extracting a luminance signal, and a frequency component of a plurality of different bands from the luminance signal. A plurality of frequency component extracting means for extracting the output of each of the plurality of frequency component extracting means, and a means for sequentially switching and selecting the extracted outputs from the plurality of frequency component extracting means for every one or two horizontal scanning periods to form one absolute value output; A plurality of focus evaluations for calculating a plurality of focus evaluation values by integrating one absolute value output from the absolute value output forming means for each of a plurality of different bands in synchronization with the switching of the extraction output every two horizontal scanning periods. The value calculating means and the plurality of focus evaluation values from the plurality of focus evaluation value calculating means are compared with each other, and the focus lens is moved in response to the largest focus evaluation value. Calculation and control means.

[Operation] The auto-focusing device according to the present invention is configured as described above, and can switch the output from the plurality of frequency extracting means every predetermined number of horizontal scanning periods to form an absolute value. Therefore, an evaluation value can be obtained from each of the plurality of frequency bands by a single absolute value forming unit.

[Embodiment] FIG. 1 is a schematic block diagram of an autofocus apparatus showing one embodiment of the present invention. Referring to the figure, this autofocus device includes BPFs 1 and 2 that pass different frequency band components, a digital absolute value conversion circuit 41,
Switching circuits 16 and 17, integrating circuits 30 and 31, synchronization separating circuit 11, switching / gate control circuit 15, and gate circuit 13
, An oscillation circuit 14, a CPU 20, a focus motor control circuit 21, an A / D conversion pulse generation circuit 22, and a latch pulse generation circuit 23.

The internal configuration of the integrating circuits 30 and 31 is the same as that of the conventional autofocus device shown in FIG. The digital absolute value conversion circuit 41 is an absolute value conversion circuit as in the prior art.
18 and an A / D converter 19.

As before, BPFs 1 and 2 are image sensors (not shown)
From the luminance signal Y obtained from the above. The signals of the respective frequency band components extracted by the BPFs 1 and 2 are both input to the switching circuit 16.

On the other hand, the sync separation circuit 11 separates the vertical sync signal and the horizontal sync signal from the luminance signal Y as in the conventional case. The separated horizontal synchronization signal and vertical synchronization signal are both input to the switching / gate control circuit 15. The switching / gate control circuit 15 generates a signal for opening the gate circuit 13 only while the focus control area is being scanned, based on the input horizontal synchronization signal and vertical synchronization signal, and inputs the signal to the gate circuit 13. I do. At the same time, the switching / gate control circuit 15
A signal (switching signal) for operating 16 and 17 every one horizontal scanning period is created and input to switching circuits 16 and 17.
Here, the switching circuit 16 switches the connection so that the output from the BPF1 and the output from the BPF2 are alternately input to the absolute value conversion circuit 3 in the next stage. Therefore, the switching circuit 16 synchronizes with the switching signal from the switching / gate control circuit 15, that is, alternately outputs the outputs of the BPFs 1 and 2 for one horizontal scanning period to the absolute value conversion circuit 18 every one horizontal scanning period. input.

Absolute value conversion circuit 18 and A / D converter 19 at the next stage perform the same operation as the conventional one. That is, the output of the BPF 1 or 2 is rectified by the absolute value conversion circuit 18 to become a DC potential,
The signal becomes an absolute value signal and is input to the A / D converter 19. A / D converter
19, the A / D conversion pulse generation circuit 22 sends the A / D
It operates by receiving a conversion pulse. The A / D conversion pulse is generated by the A / D conversion pulse generation circuit 22 from the oscillation output of the oscillation circuit 14 that is input only when the gate of the gate circuit 13 is open, as in the related art. . Therefore, the A / D converter 19 digitizes only signals extracted from the luminance signal obtained from the focus control area among the signals input from the absolute value conversion circuit 18.

Next, the output of the A / D converter 19 is input to the switching circuit 17. The switching circuit 17 switches the connection so that the output of the A / D converter 19 is alternately input to the integrating circuits 30 and 31. Here, since the same signal as the switching signal supplied to the switching circuit 16 is supplied to the switching circuit 17, the switching circuit 17
It operates at the same timing as. Therefore, in the focus control area, the absolute value data obtained from the frequency band component extracted by BPF1 and the absolute value data obtained from the frequency band component extracted by BPF2 are integrated by the integrating circuit.
Entered separately for 30 and 31.

The operations of integrating circuits 30 and 31 are the same as in the prior art. That is, in the integrating circuit 30, the adding circuit 7 adds the input absolute value data to the data held in the latch circuit 8, and inputs the resulting data to the latch circuit 8. The latch circuit 8 holds the input data as new holding data. The operation of the integrating circuit 31 is the same as the operation of the integrating circuit 30. Of course, the latch circuits 8 and 10 operate by receiving the latch pulse from the latch pulse generating circuit 23 as in the conventional case. As in the conventional case, the latch pulse is output from the gate circuit 13 only when the latch pulse generation circuit 23 is scanning the focus control area.
It is created from the oscillation output of the oscillation circuit 14 input through the. Therefore, latch circuits 8 and 10 operate only when the focus control area is being scanned.
The latch circuits 8 and 10 reset the held data in synchronization with the vertical synchronization signal, that is, after inputting the data held for each field to the CPU 20 as in the conventional case. As a result, the integrating circuit 30 and the
Signals input to each of the 31, that is, signals for フ ィ ー ル ド fields are all added, and the added data, that is, the focus evaluation value of each frequency band is given to the CPU 20.

FIG. 2 is a diagram showing the relationship between the scanning lines for one field on the screen and the processing in this embodiment for the luminance signal obtained from each scanning line. Referring to the figure, focus control area 51 is set on screen 50. First and second frequency band components extracted from a luminance signal obtained by scanning a scanning line (shown by a broken line in the drawing) outside the focus control area 51 are converted into absolute value data by a digital absolute value switching circuit 41. It is not processed. However, regarding the luminance signal obtained by scanning the scanning lines in the focus control area 51, the first and second frequency band components are alternately extracted for each scanning line, and the digital absolute value switching circuit 41
Are converted to absolute value data and input to the integrating circuits 30 and 31. Therefore, a focus evaluation value of a different frequency band is obtained from a luminance signal obtained from a scanning line indicated by a solid line in the drawing and a luminance signal obtained from a scanning line indicated by a dashed line in the drawing. That is, it is possible to obtain focus evaluation values of two different frequency bands for each field as in the related art.

The subsequent processing for the two types of focus evaluation values in the CPU 20 and the operation of the focus motor control circuit 21 are the same as those in the related art.

In this embodiment, two types of frequency bands are extracted from the luminance signal to obtain the focus evaluation value.
Further, if the number of integrating circuits is further increased, it becomes possible to obtain focus evaluation values of more frequency bands using a single digital absolute value conversion circuit. Again,
The connection may be switched every horizontal scanning period so that the output signals from each BPF are alternately supplied to the digital absolute value conversion circuit.

In the present embodiment and the above-described modified example, a plurality of frequency bands extracted from the luminance signal are switched every horizontal scanning period and input to the digital absolute value conversion circuit, and the focus evaluation value is obtained for each field. However, the switching period of each frequency band and the period for obtaining one focus evaluation value are not limited to the above-described periods, and may be any given periods. For example, the frequency range input to the digital absolute value conversion circuit may be switched every two horizontal scanning periods, and the focus evaluation value may be obtained by integrating the absolute value data of each frequency band for each frame. However, it is desirable that all the focus evaluation values obtained from the respective frequency bands are obtained uniformly at the same ratio from the entire focus control area.

Further, in the above embodiment, the BPF is used as a means for extracting a predetermined frequency band, but other frequency band extracting means may be used.

[Effects of the Invention] The autofocus device according to the present invention is configured as described above, and provides the following effects.

A single absolute value forming means for one field or one
Focus evaluation values for a plurality of different frequency bands can be obtained from a luminance signal for a certain period such as a frame. For this reason, a single circuit unit such as an absolute value conversion circuit, which is conventionally required in plural, may be used. Therefore, the internal circuit is simplified, and the manufacturing cost of the device is reduced accordingly.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing a correspondence relationship between scanning lines on a screen and processing for a luminance signal obtained from the scanning lines in the embodiment shown in FIG. FIG. 3 is a diagram showing an example of the prior art, and FIGS. 4 and 5 are diagrams showing a general relationship between a lens position and a focus evaluation value. In the figure, 1 and 2 are BPFs, 3, 4, and 18 are absolute value conversion circuits, 5, 6, and 19 are A / D converters, 7 and 9 are addition circuits, 8 and 10 are latch circuits, and 11 is a synchronous circuit. Separation circuit, 15 is switching / gate control circuit, 16 and 17 are switching circuits, 20 is CP
U and 21 are focus motor control circuits, 30 and 31 are integrating circuits, 41, 42, and 43 are digital absolute value conversion circuits, 50
Is a screen, and 51 is a focus control area. In the drawings, the same reference numerals indicate the same or corresponding parts.

Claims (1)

(57) [Claims] 1. An auto-focus apparatus for performing focusing by automatically moving said focus lens in a video camera or the like including a focus lens, comprising: means for taking an image of a subject and extracting a luminance signal; A plurality of frequency component extraction means for extracting frequency components of a plurality of different bands; and an extraction output from the plurality of frequency component extraction means sequentially switched and selected every one or two horizontal scanning periods, and one absolute value output. Means for synthesizing, and synchronizing with the switching of the extraction output for each of the one or two horizontal scanning periods, integrating one absolute value output from the absolute value output forming means for each of the plurality of different bands. A plurality of focus evaluation value calculation means for calculating a plurality of focus evaluation values; and a plurality of focus evaluation value calculation means from the plurality of focus evaluation value calculation means. Autofocus device comprising an arithmetic and control means responsive to move the focus lens to the largest focus evaluation value by comparing the dregs evaluation value to each other.