Automatic focusing apparatus

Abstract: An automatic focusing apparatus uses an image signal provided by a taking lens system of a camera for focusing the taking lens system. The apparatus comprises: a taking lens system; a pickup element for converting the optical image of an object formed by the taking lens system into a corresponding electric signal; a signal processing circuit which processes the output signal of the pickup element to provide two focus signals corresponding to the respective quantities of horizontal and vertical high-frequency components of the object in an image plane; a lens driving mechanism for driving the taking lens system; and a control unit for controlling the lens driving mechanism according to the two focus signals to focus the taking lens system. The additional use of information about the vertical direction enable focusing the taking lens system for any object including such as a horizontal line.

Description: The present invention relates to an automatic focusing apparatus for a video camera. Various automatic focusing apparatus for video cameras have been proposed and put to practical use. Among those known automatic focusing apparatus, an automatic focusing apparatus of a system which uses the output signals of the image pickup element of the video camera, namely, an automatic focusing apparatus of a so-called "In-focus control servo system" is disclosed, for example, in Ishida et al., "Automatic Focusing of a Television Camera on an In-Focus Control Servo System", NHK Gijutsu Kenkyu Hokoku, Vol. 17, No. 1, p.21. An example of the conventional automatic focusing apparatus which uses image signals for focusing the taking (photographing) lens system of a video camera will be described hereinafter. In this conventional automatic focusing apparatus, the output signal of an image pickup element is transmitted through a high-pass filter to a rectifier. The output signal of the rectifier is applied to a differential hold circuit. The output signal of the differential hold circuit is amplified by a servo amplifier. A motor for driving a focusing mechanism for focusing a taking lens system is driven by the output of the servo amplifier. In operation, the image of an object focused on the image pickup element by a taking lens system is converted into a corresponding electric image signal, and then the electric image signal is applied to the high-pass filter. The rectifier rectifies the high-frequency component of the electric image signal filtered by the high-pass filter, and then provides an output signal (hereinafter referred to as "focus voltage signal") proportional to the high-frequency component of the electric image signal. The differential hold circuit holds the focus signal for a predetermined time period to obtain a difference between the successive focus signals, and then provides a focus voltage variation signal representing the rate of variation of the focus voltage with time. The focus voltage reaches a maximum when the taking lens system is focused and decreases with the deviation of the taking lens system from a position where the image is in focus, and hence the focus voltage curve is a generally upward convex curve having a peak point. When the output signal of the differential hold circuit is positive, the focus voltage is ascending along the focus voltage curve, and hence the motor is driven so that the the focusing ring is turned further in the same direction and the focusing ring is stopped upon the arrival of the focus voltage variation signal at zero. When the output signal of the differential hold circuit is negative, the focus voltage is descending along the focus voltage curve, and hence the motor is driven so as to turn the focusing ring in the reverse direction. Thus, the output of the differential hold circuit is applied after being amplified by the servo amplifier to a motor driving circuit to drive the motor so that the focusing ring is turned accordingly for focusing. The conventional automatic focusing apparatus of such constitution, however, executes a signal processing operation only with respect to the horizontal direction of an image plane, and hence, in some cases, the automatic focusing apparatus is unable to focus the taking lens system on a horizontally invariable object, such as a horizontal line. According to the present invention there is provided an automatic focusing apparatus comprising: a taking lens system including a focusing lens unit; photoelectric conversion means for converting an optical image of an object formed by the taking lens system into a corresponding electric signal; signal processing means for processing an output signal of the photoelectric conversion means to obtain first and second focus signals respectively corresponding to the quantities of horizontal and vertical high-frequency components of an image plane, the signal processing means including a horizontal filter means for extracting a horizontal high-frequency signal component of the output signal of the photoelectric conversion means and a horizontal rectifying means for rectifying an output of the horizontal filter means to obtain the first focus signal; lens driving means for driving the focusing lens unit; and control means responsive to the first and second focus signals for controlling the lens driving means for focusing, characterised in that said horizontal rectifying means comprises: a maximum peak detecting means for detecting a maximum peak level of the output of the horizontal filter means in each horizontal line; and an adding means for accumulatively adding outputs of the maximum peak detecting means in each field to obtain the first focus signal. Preferably, the automatic focusing apparatus of the present invention is able to focus the taking lens system on an object, such as a horizontal line, for which the conventional automatic focusing apparatus are unable to focus the taking lens system. Since an increased quantity of information is available for focusing the taking lens system on an object having information for both the horizontal and vertical directions, the possible focusing range is expanded. Furthermore, the detecting circuit which limits the band, and the adding circuit which eliminates the noise components and adds the signal components preferably reduce the noise components in the focus voltage, which enables stable focusing operation. Figure 1 is a block diagram of an automatic focusing apparatus; Figure 2 is a circuit diagram of a horizontal differentiation circuit incorporated into the automatic focusing apparatus of Fig.1; Figure 3 is a circuit diagram of a horizontal band-pass filter (hereinafter abbreviated as "BPF") incorporated into the automatic focusing apparatus of Fig. 1; Figure 4 is a circuit diagram of a detecting circuit incorporated into the automatic focusing apparatus of Fig. 1, according to the present invention; Figure 5 is a circuit diagram of a vertical BPF incorporated into the automatic focusing apparatus of Fig. 1; Figure 6 is a graph showing the variation of focus voltage with the position of a focusing lens; Figure 7 is a flow chart showing the operation of a control unit incorporated into the automatic focusing apparatus of Fig. 1; Figures 8(a) to 8(g) are waveform charts showing the waveforms of output signals of the components of the automatic focusing apparatus of Fig. 1; Referring to Fig. 1, an automatic focusing apparatus will be described hereinafter in order to facilitate an understanding of the preferred embodiment according to the present invention. A taking lens system 1 (for video camera, the taking lens system in general has a zooming function) comprises a focusing lens 2 capable of focus adjustment, a variator lens 3 capable of varying magnification, a master lens 4 capable of image formation, and a diaphragm 5. Indicated at 6 is a charge-coupled device (hereinafter abbreviated as "CCD"). A signal processing unit 90 provides focus signals corresponding to the respective quantities of the horizontal and vertical high-frequency components of an object. The signal processing unit 90 comprises an A/D converter 7, a horizontal differentiation circuit 8, gate circuit 9, a horizontal BPF 10, a rectifier 11, vertical differentiation circuit 12, a gate circuit 13, a vertical BPF 14, a rectifier 15 and a gate signal generator 16. The output signal of the CCD 6 is given to the A/D converter 7. The output of the A/D converter 7 is transferred to a control unit 17 through the horizontal differentiation circuit 8, the gate circuit 9, the horizontal BPF 10 and the rectifier 11, and also through the vertical differentiation circuit 12, the gate circuit 13, the vertical BPF 14 and the rectifier 15 to the control unit 17. The gate circuits 9 and 13 are connected to the output terminal of the gate signal generator 16. The output of the control unit 17 is applied to a lens driving circuit 18 which in turn drives a lens driving mechanism 19. In operation, the image of an object formed on the CCD 6 by the taking lens system 1 is converted into a corresponding electric signal by the CCD 6, and then the electric signal is applied to the A/D converter 7. Then, the A/D converter 7 converts the electric signal into a corresponding digital signal, and the horizontal differentiation circuit 8 removes the dc component of the digital signal. Referring to Fig. 2, the horizontal differentiation circuit 8 comprises an input terminal 20, an adder 21, a subtractor 22, a unit clock delay element 23, constant multipliers 24 and 25 for multiplying input signals by constants, and an output terminal 26. When the clock rate of a clock applied to the horizontal differentiation circuit 8 is 14.31818 MHz and the constants for the constant multipliers 24 and 25 are 0.875 and 0.9375, respectively, the horizontal differentiation circuit 8 functions as a high-pass digital filter having a cut-off frequency f c of about 300 kHz and a gain of 0 dB. Fig. 8(a) is a horizontal synchronizing signal for the camera, Fig. 8(b) is the output signal of the A/D converter 7 and Fig. 8(c) is an output signal which appears at the output terminal 26. The differential waveforms of Fig. 8(c) corresponding to the leading edge and the trailing edge of the image output are unnecessary for focusing. Therefore, the gate signal generator 16 provides a gate signal for extracting only the central portion of the image plane to remove these unnecessary signals. A horizontal gate signal shown in Fig. 8(d) is applied to the gate circuit 9, and then the gate circuit 9 provides a signal shown in Fig. 8(e). Since the input gate signal applied to the gate circuit 9 is a digital signal, the gate circuit 9 may be a simple AND circuit. The output signal of the gate circuit 9 is applied to the horizontal BPF 10. Referring to Fig. 3, the horizontal BPF 10 is a duplex secondary digital band-pass filter comprising an input terminal 27, an output terminal 37, adders 28, 29 and 30, constant multipliers 33, 34, 35 and 36 for multiplying input signals by constants, and unit clock delay elements 31 and 32. When an input signal shown in Fig. 8(e) is applied to the input terminal 27, an output signal indicated by a continuous line in Fig. 8(f) appears at the output terminal 37. The amplitude of this output signal corresponds to the quantity of a high-frequency signal component contained in a signal representing an object, necessary for focusing. The output signal of the horizontal BPF 10 is applied to the rectifier 11. Referring to Fig. 4, this shows a modification to Fig. 1 to form an embodiment of the present invention in which the rectifier 11 comprises an absolute value circuit 59, a rectifying circuit 38 and an adding circuit 39. The rectifying circuit 38 comprises an input terminal 40, a comparator 41 having one input terminal connected to the input terminal 40, a switching circuit 42 having one input terminal connected to the input terminal 40, a low-pass filter 48 comprising adders 43 and 44, a unit clock delay element 45 and constant multipliers 46 and 47 for multiplying input signals by constants, and an output terminal 49. The output terminal of the constant multiplier 47 is connected to one of the input terminals of the comparator 41 and to the other input terminal of the switching circuit 42. The switching circuit 42 changes the input to the adder 43 according to the output of the comparator 41. The output of the rectifying circuit 38 is applied via the output terminal 49 to the adding circuit 39. The adding circuit 39 comprises a switching circuit 50, a comparator 51, an input terminal 52, a D-flip-flop 53, a terminal 54, an integrator 57 comprising an adder 55 and a unit clock delay element 56, and an output terminal 58. The output of the detecting circuit 38 is transferred through the output terminal 49 to the switching circuit 50 and the comparator 51 of the adding circuit 39. A signal of a predetermined level is applied to the input terminal 52. The signal applied to the input terminal 52 is compared with the output of the rectifying circuit 38 by the comparator 51, and the switching circuit 50 is controlled by the output of the comparator 51. Actually, a plurality of D-flip-flops are provided and the number of the D-flip-flops corresponds to the number of bits of the signal to be processed. The data holding timing of the D-flip-flop 53 for holding the output data of the switching circuit 50 is determined by a horizontal synchronizing signal applied to the terminal 54. An adder 55 and a unit clock delay element 56 which is a recursive delay element the output of which is fed back to the input constitute an integrator 57. The frequency of the clock applied to the delay element 56 is synchronous with the horizontal synchronizing signal. The operation of the rectifier 11 thus constituted will be described hereinafter. An absolute value circuit 59 applies the absolute value of an input signal applied to the rectifier 11 to the input terminal 40. Since the contact of the switching circuit 40 connected to the input terminal 40 is closed, the input signal is transferred to the low-pass filter 48. The output of the low-pass filter 48 is compared with the input signal by the comparator 41. When the output signal is greater than the input signal, the comparator 41 provides an output to close the contact of the switching circuit 42 connected to the output terminal of the low-pass filter 48, so that the output of the low-pass filter 48 is held. This operation is continued during a horizontal scanning period. Consequently, a peak value of a band in one line of the image signal limited by the low-pass filter 48 is held. The low-pass filter 48 removes high-frequency noise, which is different from the function of a simple peak value holding circuit. The rectifying circuit 38 is reset by a horizontal synchronizing signal and starts the rectification cycle for the next line. The waveform of the output signal of the rectifying circuit 38 is shown in Fig. 8(g). The peak value in one line of the image signal thus held is applied to the adding circuit 39. A value corresponding to an output level of the rectifying circuit 38 when only a noise component (hereinafter referred to as "noise level") is applied to the same is applied to the input terminal 52 of the comparator 51. The comparator 51 and the switching circuit 50 inhibit the application of the input signal applied to the adding circuit 39 to the integrator 57 when the level of the input signal is lower than the noise level. When the level of the input signal to the adding circuit is higher than the noise level, the D-flip-flop 53 makes the integrator 57 execute integration for each line to obtain an integral of the input signals for one field period. The integral appears at the output terminal 58. The adding circuit 39 is reset by a vertical synchronizing signal to start operation for the next field. Since signals of a level below the noise level are removed even if the object has high-frequency signal components only in several lines on an image plane, such as a fly on a white wall, only the signals for lines including high-frequency signal components are added and an output can be obtained without being removed together with noise. The level of a signal spanning n lines is multiplied by n because the signal is added n times by the integrator 57, whereas the noise is multiplied merely by n . Accordingly, the SN ratio of the output signal of the adding circuit 39 is improved. In the above description digital signals corresponding to the image signals are processed, and hence peak values are held without fail. Furthermore, the line signals can be achieved by means of a very simple construction. Thus, the rectifier 11 provides the horizontal high-frequency signal components of the image plane. Operation for obtaining the vertical high-frequency signal components will be described hereinafter. Basically, the mode of operation for obtaining the vertical high-frequency signal components is the same as that for obtaining the horizontal high-frequency signal components, except that a filter circuit is used for obtaining the vertical high-frequency signal components. Referring to Fig. 5, the vertical BPF 14 comprises an input terminal 60, an adder 61, subtractors 62 and 63, a one-scanning-period delay element 64, such as a first-in first-out circuit (hereinafter abbreviated as "FIFO"), a constant multiplier 65 and an output terminal 66. The constant multiplier 65 multiplies an input signal by a constant. Suppose that the constant k for the constant multiplier 65 is -0.75, Then, vertical cut-off frequency f c = 12 TV line resolution for a television signal of the NTSC system. Thus, the vertical BPF 14, similarly to the horizontal differentiation circuit 8, functions as a filter for removing the vertical dc components. When k = +0.75, the vertical cut-off frequency f c = 120 TV line resolution, and the vertical BPF 14 functions, similarly to the horizontal BPF 10, as a filter for extracting the high-frequency signal components of the object, necessary for focusing. The vertical filter thus constituted processes vertical signals. The constant k for the constant multiplier 65 (Fig. 5) of the vertical differentiation circuit 12 (Fig. 1) is -0.75. High-frequency signal components included in the starting and ending portions of an effective vertical scanning period in a signal produced by removing dc components by the vertical differentiation circuit 12 are not the high-frequency signal components of the object. Therefore, the gate circuit 13 executes AND operation on the basis of the output signal of the vertical differentiation circuit 12 and the output signal of the gate signal generator 16 to extract only signal components corresponding to the central portion of the image plane and to eliminate signal components corresponding to unnecessary portions of the image plane. Desired frequency components are extracted from the output signal of the gate circuit 13 by the vertical BPF 14 having the constant multiplier 65 which multiplies the constant k = +0.75, the frequency component extracted by the vertical BPF is rectified by the rectifier 15. The rectifier 15 is similar to the rectifier 11 in construction. The rectifier 15 extracts the vertical high-frequency signal components of the object in each field period. The outputs of the rectifiers 11 and 15 are applied to the control unit 17, and then the control unit 17 provides a control signal to control the lens driving circuit 18 which in turn drives the lens driving mechanism 19. Practically, the lens driving mechanism 19 is a motor for driving the focusing lens 2. The control unit 17, for example, is a microcomputer. The operation of the control unit 17 will be described with reference to Figs. 6 and 7. The outputs of the rectifiers 11 and 15 are added by a microcomputer to obtain a focus voltage. Fig. 6 is a curve showing the variation of the focus voltage with the deviation of the position of the focusing lens 2 of the taking lens system 1, in which the position of the focusing lens 2 is measured on the horizontal axis, and the focus voltage, i.e., the sum of the outputs of the rectifiers 11 and 15 on the vertical axis. When the taking lens system 1 is focused, the focus voltage reaches the maximum level. The level of the focus voltage decreases as the degree of unfocused state of the taking lens system 1 increases, hence, the curve has a generally upward convex shape having a peak point. In Fig. 7, Dn represents a focus voltage for the nth field, and Dn-1 represents a focus voltage for the (n-1)th field. Upon the start of the focusing operation, first the motor is driven for rotation in an optional direction, and then Dn and Dn-1 are compared with each other. When Dn > Dn-1, the focus voltage is ascending along the curve of Fig. 6. In this state, the motor is driven continuously. When Dn < Dn-1, the motor is reversed. Then, Dn is compared with a predetermined level Vth. When an inequality Dn > Vth is satisfied, the focus voltage is near the peak point of the curve of Fig. 6. When Dn < Dn-1, the focus voltage has moved past the peak point, and hence the motor is stopped. Thus, the taking lens system 1 is brought into focus through the focusing operation. Since the automatic focusing apparatus in the above description comprises the horizontal BPF for extracting the horizontal high-frequency components from the output of the pickup element, the vertical BPF for extracting the vertical high-frequency components from the output of the pickup element, the rectifiers for rectifying the outputs of the horizontal and vertical BPFs, the control unit which provides control signals to control the lens driving mechanism for driving the taking lens system or the focusing lens unit of the taking lens system, and the driving circuit for driving the lens driving mechanism according to the control signals provided by the control unit, the automatic focusing apparatus carries out the focusing operation on the basis of the focus voltage signal including the vertical data of the image plane in addition to the horizontal data of the same. Accordingly, the automatic focusing apparatus of the present invention is able to focus the taking lens system on an object, such as a horizontal line, for which the conventional automatic focusing apparatus are unable to focus the taking lens system. Since an increased quantity of information is available for focusing the taking lens system on an object having information for both the horizontal and vertical directions, the possible focusing range is expanded. Furthermore, the detecting circuit which limits the band, and the adding circuit which eliminates the noise components and adds the signal components reduce the noise components in the focus voltage, which enables stable focusing operation. Although digital signals are processed for focusing operation in the preferred embodiments, an improved automatic focusing apparatus can be constructed by additionally providing the conventional automatic focusing apparatus with vertical filter capable of extracting the vertical frequency component of an analog signal. Furthermore, although the outputs of the rectifiers 11 and 15 are added by the control unit 17 in the preferred embodiment, the sum of the outputs of the horizontal and vertical BPFs may be given to a single rectifier to omit one of the rectifiers 11 and 15. Still further, the control unit 17 may be designed so as to use the outputs of the rectifiers 11 and 15 alternately. Still further, although the constants for the constant multipliers are varied selectively to vary the cut-off frequencies of the BPFs in the preferred embodiment, the constitution of the BPFs may be changed to vary the cut-off frequencies.

Claims: An automatic focusing apparatus comprising: a taking lens system (1) including a focusing lens unit (2); photoelectric conversion means (6) for converting an optical image of an object formed by the taking lens system into a corresponding electric signal; signal processing means (90) for processing an output signal of the photoelectric conversion means to obtain first and second focus signals respectively corresponding to the quantities of horizontal and vertical high-frequency components of an image plane, the signal processing means including a horizontal filter means (8, 9, 10) for extracting a horizontal high-frequency signal component of the output signal of the photoelectric conversion means and a horizontal rectifying means (11) for rectifying an output of the horizontal filter means to obtain the first focus signal; lens driving means (18, 19) for driving the focusing lens unit; and control means (17) responsive to the first and second focus signals for controlling the lens driving means for focusing, characterised in that said horizontal rectifying means (11) comprises: a maximum peak detecting means (59, 38) for detecting a maximum peak level of the output of the horizontal filter means in each horizontal line: and an adding means (39) for accumulatively adding outputs of the maximum peak detecting means in each field to obtain the first focus signal. An automatic focusing apparatus according to claim 1, wherein said maximum peak detecting means (59, 38) comprises: an absolute value circuit (59) for providing an absolute value of the output of the horizontal filter means; a low-pass filter (48); a comparator (41) for comparing an output signal of the low-pass filter and an output signal of the absolute value circuit; and a switching means (42) responsive to an output signal of the comparator for selectively applying either the output signal of the low-pass filter or the output signal of the absolute value circuit to the low-pass filter. An automatic focusing apparatus according to claim 1 or 2, wherein said adding means (39) comprises: a comparator (51) for comparing a level of an output of the maximum peak detecting means (59, 38) with a predetermined level; a switching means (50) responsive to an output of the comparator for selectively providing either the input signal or a signal of zero level; and an integrater (57) for integrating output signals of the switching means. An automatic focusing apparatus according to any of claims 1, 2 or 3, wherein the signal processing means (90) comprises: a gate signal generating means (16) for generating a gate signal indicative of a central portion of the image plane; a horizontal differentiation means (8) for removing a dc component of a horizontal signal included in the output signal of the photoelectric conversion means; a first gate means (9) responsive to the gate signal for sampling signals on a time axis from an output signal of the horizontal differentiation means; a horizontal band-pass filter (10) for extracting a horizontal high-frequency signal component from an output signal of the first gate means; the horizontal rectifying means (11) for rectifying an output of the horizontal band-pass filter to generate the first focus signal; a vertical differentiation means (12) for removing a dc component of a vertical signal included in the output signal of the photoelectric conversion means; a second gate means (13) responsive to the gate signal for sampling signals on a time axis from an output signal of the vertical differentiation means; a vertical band-pass filter (14) for extracting a vertical high-frequency signal component from an output signal of the second gate means; a vertical rectifying means (15) for rectifying an output of the vertical band-pass filter to generate the second focus signal.

Inventors: HIRAO YOSHIAKI, KUGA RYUICHIRO, MIURA TERUYOSHI, YONEYAMA MASAYUKI, HIRAO, YOSHIAKI, KUGA, RYUICHIRO, MIURA, TERUYOSHI, YONEYAMA, MASAYUKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1988
Main classification: H04N5
IPC codes: H04N5, G02B7