JP4239588B2 - Filter cutoff frequency change circuit and autofocus system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a filter cutoff frequency changing circuit and an autofocus system, and more particularly to a filter cutoff frequency changing circuit and an autofocus system applied to a contrast type autofocus that detects an image contrast from a video signal and performs focusing. About.
[0002]
[Prior art]
In general, autofocus for a video camera or the like is based on a contrast method. This contrast method generates a focus evaluation value by integrating the high frequency components of a video signal within a certain range (focus area) of the video signal obtained from the image sensor, and the focus evaluation value is the maximum (maximum). The focus adjustment is automatically performed so that As a result, it is possible to obtain the best focus (focusing) that maximizes the sharpness (contrast) of the image captured by the image sensor.
[0003]
Conventionally, a high-pass filter (HPF) or a band-pass filter (BPF) is used to extract a high-frequency component from a video signal as described above. However, HPF or BPF (hereinafter referred to as HPF) will be described. It is desirable that the cut-off frequency (fc) can be changed according to the state of the subject to be photographed (see, for example, Patent Documents 1 and 2).
[0004]
[Patent Document 1]
JP 63-74273 A
[Patent Document 2]
Japanese Patent Laid-Open No. 3-297282 [0006]
[Problems to be solved by the invention]
However, when the HPF is configured with a digital filter, the lower the fc, the more the order (number of taps) needs to be increased in order to obtain the same characteristics, and the circuit size increases.
[0007]
In addition, when the HPF is configured by a custom IC such as FPGA or PLD, a particularly low frequency HPF requires many gates, and the chip size increases, making it difficult to reduce cost and size.
[0008]
Therefore, it is not desirable to change the coefficient of the digital filter or use a plurality of HPFs in order to change the cutoff frequency of the HPF.
[0009]
The present invention has been made in view of such circumstances, and it is possible to filter the frequency components of the target signal without changing the cutoff frequency by changing the frequency characteristics of the filter itself, and without using a plurality of filters. An object of the present invention is to provide a filter cut-off frequency changing circuit and an autofocus system that can change the frequency component cut off by the above-described operation.
[0010]
[Means for Solving the Problems]
In order to achieve the object, the cutoff frequency changing circuit of the filter according to claim 1 includes a filter that blocks a frequency component equal to or lower than a predetermined cutoff frequency among frequency components of an input signal, and an input signal. SRAM that is a storage circuit that temporarily stores the clock, a clock generation circuit that generates a read clock for reading the signal stored in the SRAM to output to the filter, and a read clock that is generated by the clock generation circuit And a CPU for changing the frequency of the read signal, changing the frequency of the read clock to change the transmission speed of the input signal and outputting it to the filter, whereby the frequency component of the input signal is The frequency component cut off by the filter is changed .
[0012]
Also, auto-focus system according to claim 2, the high-pass filter from the video signal obtained by the photoelectric conversion of an image formed by the optical system removed the high frequency components, based on the high frequency component claim in autofocus system for focusing of the optical system automatically, the high-pass filter as the filter of claim 1, changes the frequency component cut off by the high-pass filter to the frequency components of the video signal Te 1 A cutoff frequency changing circuit for the filter is provided.
[0013]
According to the present invention, the same effect as changing the cutoff frequency of the filter is obtained by changing the transmission rate of the target signal input to the filter to the filter without changing the frequency characteristics of the filter. Even if you want to change the frequency component to be cut off (frequency component to be transmitted) with respect to the frequency component of the target signal, there is no need to change the frequency characteristic of the filter, and multiple filters with different frequency characteristics (cutting frequency) are used There is no need to do this. Therefore, the cost of the filter and its peripheral circuits can be reduced and the circuit size can be reduced.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, preferred embodiments of a filter cutoff frequency changing circuit and an autofocus system according to the present invention will be described in detail with reference to the accompanying drawings.
[0015]
FIG. 1 is a block diagram showing the overall configuration of an autofocus system to which the present invention is applied. The autofocus system shown in the figure is generally applied to a television camera. For example, a focus lens (group) FL, a variator lens (group) VL, and a compensator lens (group) CL are used as photographing lenses (shooting optical systems). A zoom lens (group) ZL, an iris I, a relay lens (group) RL, and the like are arranged.
[0016]
The subject image formed by the photographic lens is photoelectrically converted by the image pickup device (CCD) 10 of the TV camera body, and subjected to predetermined signal processing by a predetermined signal processing circuit 12. Thereby, a video signal of a predetermined standard (for example, NTSC system) is output from the signal processing circuit 12 for video display.
[0017]
On the other hand, the signal processing circuit 12 also outputs a video signal for autofocus (AF) control, and the AF control video signal is input to the focus evaluation value generation unit 14. The video signal for AF control is, for example, a luminance signal, and a voltage signal corresponding to the luminance value of each pixel constituting the screen is sequentially transmitted from the upper stage to the lower stage along the horizontal scanning line. It should be noted that the focus evaluation value generation unit 14 and the CPU 16 to be described later may be incorporated into the camera body, or may be incorporated into a driving device that drives each operation unit (focus lens FL, etc.) of the photographing lens. It may be arranged in any part of the apparatus constituting the system.
[0018]
The focus evaluation value generation unit 14, which will be described in detail later, generates a focus evaluation value indicating the level of contrast (sharpness) of the captured image based on the video signal given from the signal processing circuit 12. FIG. 2 is a diagram illustrating the magnitude of the focus evaluation value obtained by changing the position of the focus lens FL (focus position) when shooting a subject that does not change. As shown in the figure, the focus evaluation value generally indicates a mountain-shaped curve, and the focus position F0 when the focus evaluation value at the peak point P0 is obtained is in the best focus state.
[0019]
The CPU 16 acquires the focus evaluation value generated by the focus evaluation value generation unit 14, and performs autofocus control based on the acquired focus evaluation value. The focus of the photographing lens is changed by driving the focus lens FL in the optical axis direction by the focus motor FM, and the focus motor FM is driven according to a control signal given from the CPU 16 to the focus motor drive circuit 18. The CPU 16 is provided with a position signal indicating the current position of the focus lens FL from the potentiometer FP, and the CPU 16 drives the focus motor FM while referring to the position signal to bring the focus lens FL to a desired position. Move.
[0020]
In the autofocus control, the CPU 16 performs a wobbling operation for changing the focal position of the photographing lens back and forth, and the focus evaluation value of the focus lens FL whose focus evaluation value is increased by the focus evaluation value obtained from the focus evaluation value generation unit 14 during that time. Detect the moving direction. For wobbling, the focus lens FL may be moved back and forth, or when there is a dedicated wobbling lens, the lens may be moved back and forth. When the direction in which the focus evaluation value increases is detected, the focus lens FL is subsequently moved in that direction. When it is detected that the focus evaluation value has decreased while moving the focus lens FL, the peak point of the focus evaluation value is recognized, and the focus lens FL is stopped at that position. Thereby, the focus of the photographing lens is set to the best focus state.
[0021]
Next, the configuration of the focus evaluation value generation unit 14 and its peripheral circuits is shown in FIG. The focus evaluation value generation unit 14 mainly includes an A / D converter 20, an SRAM 22, a high pass filter (HPF) 24, and an adder circuit 26. As shown in FIG. 1, the video signal (luminance signal) output from the signal processing circuit 12 of the camera body is input to the focus evaluation value generation unit 14. First, the digital signal is output from the A / D converter 20 for each pixel. Converted into (pixel data). Of the pixel data sequentially output from the A / D converter 20, pixel data within a predetermined period is temporarily stored in the SRAM 22. Note that the SRAM 22 stores, for example, only pixel data within a predetermined range (focus area) on the screen to be autofocused. Further, the storage capacity of the SRAM 22 is, for example, a capacity capable of storing pixel data in the focus area for one screen. The SRAM 22 may use a storage circuit of a type other than the SRAM, and the storage capacity may be different from that described above.
[0022]
The pixel data once stored in the SRAM 22 is sequentially read out to a high-pass filter (HPF) 24 composed of a digital filter. The pixel data read out to the HPF 24 is pixel data in a case where a low-frequency component is cut off from a predetermined cut-off frequency fc and only a high-frequency component is extracted from the frequency components of the video signal in the focus area. Is converted to
[0023]
The pixel data obtained by the HPF 24 is sequentially output to the adder circuit 26 and integrated by the adder circuit 26 for each focus area range for one screen. Thereby, focus evaluation value data is generated and read by the CPU 16.
[0024]
On the other hand, as a peripheral circuit of the focus evaluation value generation unit 14, a synchronization separation circuit 28 and a clock generation circuit 30 that generate a clock signal and the like for synchronizing the processing operation of each circuit are disposed. The video signal output from the signal processing circuit 12 of the camera body is branched to the synchronization separation circuit 28 in addition to being input to the focus evaluation value generation unit 14. Then, the horizontal synchronizing signal is separated from the video signal by the synchronizing separation circuit 28, and the horizontal synchronizing signal is given to the clock generating circuit 30.
[0025]
The clock generation circuit 30 provides a clock signal having a constant frequency to the A / D converter 20, the HPF 24, the adder circuit 26, and the CPU 16 in synchronization with the horizontal synchronization signal provided from the synchronization separation circuit 28, and outputs them. Synchronize circuit operation.
[0026]
In contrast, the SRAM 22 is provided with a write clock that instructs data writing in accordance with the timing at which each pixel data is output only during the period in which the pixel data in the focus area is output from the A / D converter 20. . As a result, only the pixel data in the focus area is stored in the SRAM 22. The SRAM 22 is supplied with a read clock for instructing reading of data stored in the SRAM 22.
[0027]
Here, the frequency of the read clock given from the clock generation circuit 30 to the SRAM 22 is not constant, but is changed by a command from the CPU 16. For example, when the write clock frequency and the read clock frequency are the same, the pixel data is read from the SRAM 22 at the same speed as the pixel data is written to the SRAM 22. In this case, the transmission speed of the video signal output from the signal processing circuit 12 to the HPF 24 is not changed in the SRAM 22. That is, the speed at which the voltage signal corresponding to each pixel is sequentially output as a video signal from the signal processing circuit 12 matches the speed at which each pixel data is sequentially input to the HPF 24. Therefore, the upper limit of the frequency component cut off by the HPF 24 among the frequency components of the video signal output from the signal processing circuit 12 (the lower limit of the frequency component passing through the HPF 24), that is, the cut-off frequency is defined as the frequency characteristic of the HPF 24. Is the cutoff frequency fc.
[0028]
On the other hand, when the frequency of the write clock and the read clock are different due to the change in the frequency of the read clock, the pixel data is read from the SRAM 22 at a speed different from the speed at which the pixel data is written to the SRAM 22. In this case, the transmission speed of the video signal output from the signal processing circuit 12 to the HPF 24 is changed in the SRAM 22. That is, the speed at which the voltage signal corresponding to each pixel is sequentially output as a video signal from the signal processing circuit 12 is different from the speed at which each pixel data is sequentially input to the HPF 24. Therefore, when each frequency component of the video signal output from the signal processing circuit 12 is input to the HPF 24, it is converted to a component having a different frequency. For this reason, the upper limit of the frequency component cut off by the HPF 24 among the frequency components of the video signal output from the signal processing circuit 12 changes from the cut-off frequency fc.
[0029]
For example, assume that the frequency of the read clock is increased by n times with respect to the write clock, and the read speed is increased by n times with respect to the write speed of the pixel data to the SRAM 22. At this time, the transmission speed of the video signal is converted to n times by the SRAM 22, and when each frequency component of the video signal output from the signal processing circuit 12 is input to the HPF 24, the frequency component of n times Converted to. On the other hand, since the cutoff frequency fc of the HPF 24 is constant, this is equivalent to changing the cutoff frequency of the HPF 24 for the video signal output from the signal processing circuit 12 to fc / n.
[0030]
As a specific numerical example, the prescribed cutoff frequency fc of the HPF 24 is 4 MHz, for example, and the frequency of the read clock is 1, 2 or 4 times the frequency of the write clock, that is, the value of n described above. Is changed to 1, 2, and 4. At this time, the cutoff frequency of the HPF 24 is equivalently changed to 4 MHz, 2 MHz, or 1 MHz.
[0031]
Note that it is more preferable that the specified cutoff frequency fc of the HPF 24 be as high as possible because the order (the number of taps) can be reduced and the circuit size of the HPF 24 can be reduced. Further, it is not necessary to set any one of the cutoff frequencies of the HPF 24 to be equivalently changed to the prescribed cutoff frequency fc of the HPF 24.
[0032]
Further, the frequency of the read clock is changed by a command from the CPU 16, but the change condition is not limited to a specific condition. For example, when the frequency of the readout clock is changed in accordance with a user instruction, the case where the frequency of the readout clock is automatically changed according to the state of the subject to be photographed, the photographing conditions, and the like can be considered. In general, when high-precision focusing is required, priority is given to lowering the read clock frequency to increase the equivalent cutoff frequency of the HPF 24 and increasing the focusing speed over accuracy. In this case, a change condition is considered in which the frequency of the read clock is increased and the equivalent cutoff frequency of the HPF 24 is decreased.
[0033]
Next, the variable range of the read clock frequency will be described. When the writing clock has a predetermined frequency, the time (writing time) required to write all the pixel data in the focus area to the SRAM 22 in one horizontal scanning period (1H) is 1H * (1 / m). In this case, the time required to read out the pixel data from the SRAM 22 is at least 1H * ((m−1) / m) which is the remaining time of one horizontal scanning period 1H, that is, (m−1) of the writing time. ) Can be slowed down to twice. When the clock frequency is set, the read clock frequency can be reduced to 1 / (m-1) times the write clock frequency.
[0034]
If the SRAM 22 can operate simultaneously with a write clock and a read clock having different frequencies, the read time can be delayed up to 1H, which is m times as long as the write time 1H * (1 / m). In the case of the reading time (reading speed), it is possible to deal with the case where the processing target is every predetermined number of horizontal scanning lines.
[0035]
Basically, the direction of increasing the frequency of the read clock is not limited in time by the standard of the video signal, and depends on the processing capability of the HPF 24.
[0036]
Thus, without changing the frequency characteristic (cutoff frequency) of the HPF 24, and without using a plurality of HPFs having different cut-off frequencies, the cut-off frequency of one HPF 24 is equivalent by changing the transmission speed of the video signal. Thus, the cost can be reduced and the circuit size can be reduced. By reducing the circuit size, it is possible to distribute the improvement in performance such as improving the frequency characteristics such as the cutoff frequency of the HPF in the vacant space. Also, under certain conditions, such as processing only a part of the screen, the high-frequency signal stored in the SRAM 22 can be read and processed slowly, and high-speed processing can be applied to circuits that do not support high-speed processing. is there.
[0037]
In the above embodiment, the case where the present invention is applied to an autofocus system has been described. However, the present invention can also be applied to other apparatuses and systems.
[0038]
In the above-described embodiment, the case where the cutoff frequency of the HPF 24 can be changed has been described. However, the present invention is a filter other than a high-pass filter, and a frequency characteristic corresponding to the cutoff frequency of a low-pass filter, a band-pass filter, or the like. It can be applied to a filter having
[0039]
Further, the present invention is not limited to being applied to an autofocus system.
[0040]
【The invention's effect】
As described above, according to the filter cutoff frequency changing circuit and the autofocus system according to the present invention, by changing the transmission speed of the target signal input to the filter to the filter without changing the frequency characteristics of the filter. Since the same effect as that obtained by changing the cutoff frequency of the filter is obtained, the frequency characteristic of the filter is changed even when the frequency component to be blocked by the filter (frequency component to be transmitted) is changed with respect to the frequency component of the target signal. There is no need to use a plurality of filters having different frequency characteristics (cutoff frequencies). Therefore, the cost of the filter and its peripheral circuits can be reduced and the circuit size can be reduced.
[Brief description of the drawings]
FIG. 1 is a block diagram showing an overall configuration of an autofocus system to which the present invention is applied.
FIG. 2 is a diagram illustrating an example of a relationship between a focus position and a focus evaluation value.
FIG. 3 is a block diagram illustrating a configuration of a focus evaluation value generation unit and its peripheral circuits.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 ... Image sensor, 12 ... Signal processing circuit, 14 ... Focus evaluation value production | generation part, 16 ... CPU, 18 ... Focus motor drive circuit, 20 ... A / D converter, 22 ... SRAM, 24 ... High pass filter, 26 ... Addition Circuit, 28 ... Sync separation circuit, 30 ... Clock generation circuit, FL ... Focus lens, ZL ... Zoom lens, I ... Iris, RL ... Relay lens, FM ... Focus motor, FP ... Potentiometer

Claims (2)

Among the frequency components of the input signal, a filter that cuts off a frequency component equal to or lower than a predetermined cutoff frequency, and
SRAM which is a storage circuit for temporarily storing the input signal ;
A clock generation circuit for generating a read clock for reading the signal stored in the SRAM for output to the filter;
A CPU for changing the frequency of a read clock generated by the clock generation circuit, and changing the frequency of the read clock to change the transmission speed of the input signal and outputting it to the filter. A cutoff frequency changing circuit for a filter, wherein a frequency component cut off by the filter is changed with respect to a frequency component of the received signal. An autofocus system that extracts a high-frequency component from a video signal obtained by photoelectrically converting an image formed by an optical system by a high-pass filter, and automatically focuses the optical system based on the high-frequency component In
An auto circuit comprising the filter cut-off frequency changing circuit according to claim 1, wherein the high-pass filter is a filter according to claim 1, and a frequency component cut off by the high-pass filter is changed with respect to a frequency component of the video signal. Focus system.

Images