JP2015125294A - Image display device, image processor, and control method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To inhibit a clamp period from overlapping an image validation period during performing clamp processing.SOLUTION: An image processor includes specification means for measuring a signal parameter value of an input analog image signal, comparing the signal parameter value with a signal parameter value of an analog image signal in a predetermined format stored in storage means and specifying a format of the input analog image signal, detection means for detecting a start position of an image validation period of the input analog image signal, and detecting a horizontal back porch period of the input analog image signal on the basis of the detected start position, means for performing clamp processing of the input analog image signal in a shorter horizontal back porch period between the detected horizontal back porch period and a horizontal back porch period regulated in the specified format, and conversion means for converting the input analog image signal into a digital image signal on the basis of a reference level determined by the clamp processing.

Description

  The present invention relates to an image display device, an image processing device, and a control method thereof.

  A display device such as a liquid crystal display device drives display means such as a liquid crystal panel using a signal obtained by converting a digital image signal by A / D conversion with respect to an analog image signal output from an image output device such as a PC. There are various formats for analog image signals output from an image output device such as a PC. Therefore, in the display device, the format is estimated based on the timing of the synchronization signal and the effective period of the image, and A / D conversion and display are performed based on the estimation result.

  When A / D conversion is performed, in order to determine the reference level (black level, pedestal level) of the image signal, the signal level (potential) for a predetermined period is measured, and clamping processing using the average value as the reference level is performed. Is called. Of the image signals, a blanking period without an image, for example, a horizontal back porch period is used as a period for performing the clamping process. Considering that noise is superimposed on the signal in the clamp period, it is desirable that the clamp period is long so that the influence of noise is reduced.

  As a method for determining the clamp period, there is a method in which the input image signal is assumed to be a signal of a specific format, and the horizontal back porch period determined by the format is used as the clamp period. This method is effective when an image signal having a general format in which the display device holds format information is input. However, when an image signal having a format that is out of the general standard is input, there may be a case where the back porch period is different from the assumed one. When the horizontal back porch period of the input signal is shorter than the assumed horizontal back porch period, the clamp period takes an effective period. Since the signal level may exceed the reference level in the effective period, if the reference level is determined based on the average value of the signal level measured during the clamp period, the reference level may not be an appropriate level. In this case, the range of the digital image signal obtained by A / D conversion is narrowed, and there are cases where black crushing and black floating occur.

  On the other hand, Patent Document 1 discloses a method of detecting a start position of an effective period of an image signal and determining a clamp period based on the detection result.

JP 2002-23726 A

In Patent Document 1, the timing at which a signal of the pedestal level or higher is received for the input image signal is detected as the start position of the effective period (that is, the end position of the horizontal back porch period). However, in this method, when an image signal whose start position of the effective period is a pedestal level (an image signal having a black region at a specific position) is input, a position after the start position of the effective period defined in the format Is detected as the start position of the effective period. When the clamp period is set based on the detection result, the clamp period becomes longer than the actual horizontal back porch period and takes an effective period. Therefore, when the start position of the effective period is switched to an image signal having a pedestal level or higher, a signal having a pedestal level or higher is used for the clamping process. In this case, as in the case described above, if the reference level is determined based on the average value of the signal levels measured during the clamp period, the appropriate reference level cannot be obtained. Therefore, there is a possibility that image quality deterioration such as black crushing or black floating occurs in the digital image signal obtained by A / D conversion.

  The present invention has been made in view of such a problem, and the clamp period takes an effective image period when performing clamp processing in the horizontal back porch period of the analog image signal in order to perform A / D conversion. The purpose is to suppress.

The present invention provides a specifying means for measuring the signal parameter value of an input analog image signal and comparing the signal parameter value of the analog image signal of a predetermined format stored in the storage means with the format of the input analog image signal. When,
Detecting means for detecting a start position of an image effective period of the input analog image signal, and detecting a horizontal back porch period of the input analog image signal based on the detected start position;
Processing means for performing clamp processing on the input analog image signal in the shorter horizontal back porch period of the detected horizontal back porch period and the horizontal back porch period defined in the specified format;
Conversion means for converting the input analog image signal into a digital image signal based on a reference level determined by the clamping process;
Is an image processing apparatus.

The present invention provides a specifying step for measuring a signal parameter value of an input analog image signal and specifying the format of the input analog image signal by comparing with a signal parameter value of an analog image signal of a predetermined format stored in a storage means. When,
A detection step of detecting a start position of an image effective period of the input analog image signal and detecting a horizontal back porch period of the input analog image signal based on the detected start position;
A processing step of performing clamp processing on the input analog image signal in the shorter horizontal back porch period of the detected horizontal back porch period and the horizontal back porch period defined in the specified format;
A conversion step of converting the input analog image signal into a digital image signal based on a reference level determined by the clamping process;
Is a control method of an image processing apparatus.

  ADVANTAGE OF THE INVENTION According to this invention, when performing a clamp process in the horizontal back porch period of an analog image signal in order to perform A / D conversion, it can suppress that a clamp period takes an image effective period.

The figure which shows the internal structure of a projector. The figure which shows the clamp period determination flow in Example 1. FIG. The figure which shows the example of a corresponding | compatible format table. The figure which shows a signal format specific flow. The figure which shows the example of an image in 1 frame.

  Hereinafter, the present invention will be described in detail with reference to examples. The embodiment described below is an example for realizing the present invention, and can be appropriately changed according to the configuration of the apparatus to which the present invention is applied and various conditions. The present invention is limited to the following embodiment. Is not to be done. In addition, the present invention can be implemented by appropriately combining a part of each embodiment described later.

Example 1
In the first embodiment, an example in which the present invention is applied to a projector incorporating a liquid crystal panel will be described. <Overview of projector>
FIG. 1 is a diagram illustrating a main configuration of the projector 100 according to the first embodiment.
The control unit 101 controls each block of the projector 100. The control unit 101 is connected to each block by a bus as shown in FIG. 1, and accesses each block via the bus to perform control instruction, data transmission / reception, and the like.
The operation unit 102 receives an operation from the user.
The power supply unit 103 controls power supply to each block of the projector 100.
The liquid crystal unit 104 includes one liquid crystal panel or three liquid crystal panels, and an image is formed on the liquid crystal panel.
The liquid crystal driving unit 105 forms an image on the liquid crystal panel of the liquid crystal unit 104 based on an image signal input from an image processing unit 117 described later.
The light source 106 irradiates the liquid crystal unit 104 with light.

The projection optical system 107 projects an optical image obtained by irradiating the liquid crystal unit 104 with light emitted from the light source 106 and transmitting the light through the liquid crystal unit 104 on a screen (not shown).
The light source control unit 108 controls the amount of light of the light source 106 and the like.
The optical system control unit 109 controls operations of the zoom lens, the focus lens, and the like of the projection optical system 107, and performs zoom magnification, focus adjustment, and the like.
The analog input unit 110 receives an analog image signal output from a PC, a DVD player, a TV tuner, or the like. The analog input unit 110 includes, for example, an RGB terminal and an S terminal.

  The A / D conversion unit 111 converts the analog image signal input from the analog input unit 110 into a digital image signal. Examples of the A / D conversion method include a double integration type that integrates an analog input voltage for a certain period of time, a parallel comparison type that uses a comparator, and a successive approximation type that compares with a D / A conversion value. The system is not limited in the practice of the invention. The A / D converter 111 further measures the signal parameter value of the input analog image signal. For example, the A / D converter 111 measures the frequency of the synchronization signal, the number of scanning lines, the signal polarity, etc. as signal parameter values.

  Further, the A / D conversion unit 111 performs a clamping process in the pedestal period (horizontal back porch period) of the analog image signal in order to determine a reference level for A / D conversion. Specifically, in the clamping process, the signal level is measured in the horizontal back porch period, an average value of the measured values is obtained, and this is used as the reference level. The clamp processing function of the A / D conversion unit 111 is realized by a clamp pulse generation circuit and a clamp circuit, and the clamp circuit includes an adder circuit and an average value circuit (not shown). The clamp pulse generation circuit generates a clamp pulse to the clamp circuit based on an instruction from the control unit 101. The clamp circuit performs addition on the received analog image signal in a period during which the clamp pulse is at a high level (for example, 32 clock periods). The clamp circuit further outputs the addition result to the average value circuit and averages the addition result. Since 32 clock additions are performed here, the addition results are averaged by shifting by 5 bits (1/32). The A / D converter 111 performs A / D conversion for each line using the averaged result as a reference level.

The digital input unit 112 receives a digital image signal from a PC, a DVD player, or the like. The digital input unit 112 includes, for example, an HDMI (registered trademark) terminal. In the case of an HDMI (registered trademark) terminal, a control signal may be transmitted from the outside at the same time, whereby image control or the like may be performed. The image signal input from the digital input unit 112 is directly transmitted to the image processing unit 117.
The USB interface 113 is an interface that receives a file storing image data and various types of information data from an external device or writes the file to the external device. A pointing device, a keyboard, a flash memory, and the like are connected to the USB interface 113.

  The card interface 114 is an interface for reading and writing a file in which image data and various types of information data are stored on a card-type recording medium, and an SD card, a compact flash (registered trademark), or the like can be inserted therein. The document file input from the card interface 114 is played back by the file playback unit 132. The file playback unit 132 generates an image signal to be presented to the user from the document file and outputs the image signal to the image processing unit 117.

The communication unit 115 is an interface that transmits and receives a file storing image data and various types of information data, a control signal, and the like from an intranet or the Internet, and includes, for example, a wired LAN or a wireless LAN.
The internal memory 116 is a storage device that stores a file in which image data and various types of information data are stored, and includes a semiconductor memory, a hard disk, and the like.

  The image processing unit 117 analyzes the image signals input from these various interfaces and the file playback unit 132 and the image signals input from the control unit 101, and performs corrections suitable for display on the liquid crystal unit 104. Or do. For example, the image processing unit 117 converts the number of pixels according to the number of pixels of the liquid crystal panel or the number of frames (frame rate) with respect to the image signal. The liquid crystal panel performs AC driving for displaying an image by alternately switching the direction of the voltage applied to the liquid crystal. This driving method utilizes the property that the liquid crystal panel can form an image regardless of whether the direction of the voltage applied to the liquid crystal is the forward direction or the reverse direction. In this driving method, since it is necessary to input an image for the forward direction and an image for the backward direction to the liquid crystal driving unit 105, processing for doubling the number of frames with respect to the image signal is performed.

The image processing unit 117 can also perform various types of analysis on the digital image signal input from the A / D conversion unit 111. For example, there may be no DE (Data Enable) signal indicating the display start position in the analog image signal input to the analog input unit 110. In such a case, the image processing unit 117 measures the display start position by analysis based on the horizontal / vertical synchronization signal and the image signal. The control unit 101 performs control related to image processing and A / D conversion by the A / D conversion unit 111 based on the analysis result.

  The image processing unit 117 performs keystone correction. In the keystone correction, for example, when the projection optical system 107 is not directly facing the screen, the image is deformed so as to correct the trapezoidal distortion generated in the projection image. In the keystone correction, the image displayed on the liquid crystal panel is enlarged or reduced in the horizontal direction and / or the vertical direction. As a result, the trapezoidal distortion generated in the projected image due to the positional relationship between the projection optical system 107 and the screen is canceled out by the distortion generated in the image formed on the liquid crystal panel by the shape deformation correction, and the image projected onto the screen. Can be approximated to an image with a normal aspect ratio. The keystone correction may be automatically performed based on an inclination angle obtained by an inclination sensor 118 described later, or may be manually performed by the user operating the operation unit 102 or the like.

The tilt sensor 118 detects the tilt of the projector 100.
The timer 119 detects the operation time of the projector 100, the operation time of each block, and the like.
The thermometer 120 measures the temperature of the light source 106 of the projector 100, the temperature of the liquid crystal unit 104, the outside air temperature, and the like.
Infrared receiving units 121 and 122 receive an infrared signal from a remote controller or other device that operates projector 100 and sends a signal corresponding to the received infrared signal to control unit 101. In the first embodiment, a plurality of infrared receiving units are installed. For example, the infrared receiving unit 122 is installed in front of the projector 100 (the direction in which the projection optical system 107 is provided), and the infrared receiving unit 121 is installed in the rear of the projector 100.

The focus detection unit 123 detects the distance between the projector 100 and a screen (not shown), and detects the focus distance.
The imaging unit 124 images a screen direction (not shown).
The screen photometry unit 125 measures the amount of light and the luminance of light reflected by the screen.
The light source photometry unit 126 measures the light quantity and luminance of light emitted from the light source 106.
The display unit 133 is disposed in the main body of the projector 100 and displays the status of the projector 100, a warning, and the like.
The display control unit 128 controls the display unit 133.
The battery 129 supplies power to the projector 100 when the projector 100 main body is used in an environment without an external power source.

The power input unit 130 receives AC power from the outside, rectifies it into a predetermined voltage, and supplies it to the power source unit 103.
The cooling unit 131 is a device that cools the projector 100 by, for example, releasing heat inside the projector 100 to the outside, and includes, for example, a heat sink and a fan.
The RAM 134 is used for development of a program stored in the internal memory 116, a frame memory for projected images, and the like.

<Description of normal operation of projector>
Here, the normal operation of the projector 100 will be described.
When the operation unit 102 gives an instruction to turn on the power, the control unit 101 of the projector 100 according to the first embodiment instructs the power supply unit 103 to supply power to each block, and puts each block in a standby state. After the power is turned on, the control unit 101 instructs the light source control unit 108 to start light emission from the light source 106. Next, the control unit 101 instructs the optical system control unit 109 to adjust the projection optical system 107 from the information on the focus distance obtained by the focus detection unit 123 and the like. The optical system control unit 109 operates the zoom lens and the focus lens of the projection optical system 107 to control the projection light to form an image on the screen. Now you are ready for projection.

  Next, for example, the image signal input to the digital input unit 112 is converted by the image processing unit 117 to a resolution suitable for the liquid crystal unit 104, gamma correction, luminance unevenness countermeasure correction, keystone correction, and the like. Is called. Based on the image signal corrected by the image processing unit 117, the liquid crystal driving unit 105 drives the liquid crystal unit 104 to form an image on the liquid crystal panel. When the light emitted from the light source 106 passes through the liquid crystal unit 104, the image formed on the liquid crystal panel of the liquid crystal unit 104 is projected onto a screen (not shown) by the projection optical system 107, and the image is displayed on the screen. .

During projection, the control unit 101 detects the temperature of the light source 106 and the like with the thermometer 120, and, for example, operates the cooling unit 131 to cool down when the temperature of the light source 106 reaches 40 degrees or more.
When the operation unit 102 instructs to turn off the power, the control unit 101 instructs each block to perform a termination process. When preparation for power supply stop is completed, the power supply unit 103 sequentially stops power supply to each block. The cooling unit 131 operates for a while after power supply to each block is stopped, and cools the projector 100.
Here, the case of displaying an image based on the image signal input from the digital input unit 112 has been described, but the same processing is performed when displaying the image signal and image data input from the various interfaces.

<Description of clamping period determination processing>
Hereinafter, a clamp period determination process during A / D conversion in the first embodiment will be described with reference to FIGS.
FIG. 2 is a flowchart illustrating a clamp period determination process according to the first embodiment. The start trigger of the clamp period determination process is detection of an analog image signal input to the analog input unit 110, a change in signal timing (signal parameter value) of the analog image signal input to the analog input unit 110, and the like. The change in signal timing occurs, for example, when the output resolution setting is changed in an image signal output device that outputs an analog image signal. However, the start trigger of the clamp period determination process is not limited to these.

  When the A / D conversion unit 111 detects that an analog image signal is input from the analog input unit 110, in step S101, the control unit 101 performs a signal format specifying process. The control unit 101 is notified from the A / D conversion unit 111 that an analog image signal input has been detected by an interrupt notification. Or the control part 101 grasps | ascertains that the input of the analog image signal was detected by polling the A / D conversion part 111 for the presence or absence of the input of a synchronizing signal. However, the method for the control unit 101 to grasp that the input of the analog image signal is detected is not limited to these.

The signal format specifying process will be described with reference to FIGS.
FIG. 3 is a table storing signal parameter value information in a predetermined format of an analog image signal that can be supported by the projector 100 according to the first embodiment. This table is held in the internal memory 116 and is expanded in the RAM 134 based on an instruction from the control unit 101 when the projector 100 is activated. The table has a resolution 201, a scanning method 202, a horizontal synchronization frequency 203, a vertical synchronization frequency 204, a horizontal total number of dots 205, a total number of vertical lines 206, a horizontal synchronization signal width 207, a horizontal back porch period 208, a quantization clock for each format. 209 information. As shown in FIG. 3, it is assumed that the projector 100 according to the first embodiment is compatible with five formats.

The flow of the signal format specifying process will be described with reference to FIG.
In step S <b> 201, the control unit 101 acquires the signal parameter value of the input analog image signal measured by the A / D conversion unit 111 and loads it into the RAM 134. The signal parameter values acquired by the control unit 101 are the horizontal synchronization frequency (MEAS_hFreq) and the total number of vertical lines (MEAS_vTotal))
Suppose that However, the signal parameter value acquired by the control unit 101 is not limited to this.

In S202 to S203, the control unit 101 performs collation processing to determine whether or not a format that matches the signal parameter value loaded in the RAM 134 exists in the table. The control unit 101 includes the horizontal synchronization frequency and the total number of vertical lines in the format table, the horizontal synchronization frequency (MEAS_hFreq) and the total number of vertical lines (MEAS_vTotal) acquired from the A / D conversion unit 111,
And search for a format that satisfies the matching conditions.

The coincidence condition is that for the horizontal synchronization frequency, the difference between the horizontal synchronization frequency (MEAS_hFreq) of the input signal and the horizontal synchronization frequency 203 (TBL_hFreq) of the format table is within a predetermined threshold. Here, the threshold value is set to 0.5 (KHz). Regarding the total number of vertical lines, the total number of vertical lines (MEAS_vTotal) of the input signal is equal to the total number of vertical lines 206 (TBL_vTotal) of the format table. The control unit 101 performs a collation process on the acquired signal parameter value to determine whether there is a format in the table that satisfies both the horizontal synchronization frequency and the total number of vertical lines that satisfy the matching condition.

For example, the horizontal synchronization frequency (MEAS_hFreq) of the analog image signal measured by the A / D conversion unit 111 is 74.038 (KHz), and the total number of vertical lines (MEAS_vTotal) is 1235 (L
ine). The control unit 101 acquires these signal parameter values from the A / D conversion unit 111, loads them into the RAM 134, and performs comparison / collation processing with the format table held in the projector 100 (S202 to S203). In the above example, the control unit 101 specifies a format with a resolution of 1920 × 1200 from the format table as a format that matches the input signal (S204), and ends the signal format specifying process of S101.

  In the first embodiment, the example in which the format is specified based on the horizontal synchronization frequency and the total number of vertical lines has been described. For example, the format may be specified using a vertical synchronization frequency, a vertical / horizontal effective period, or the like.

  Next, in S102, the horizontal effective period start position (start position of the image effective period) of the analog image signal, that is, the end position of the horizontal back porch period is measured. The measurement of the horizontal effective period start position is performed by the image processing unit 117. A method for measuring the effective period in one frame will be described with reference to FIG.

FIG. 5 is a diagram showing an image of 7 Pixel × 7 Line. The white pixels indicate effective pixels 301 that can be detected as an image, and the other pixels indicate reference level pixels 302 that are black pixels that cannot be detected as an image. The effective pixel is a pixel other than black in the digital image signal A / D converted by the A / D converter 111. In other words, for example, when the A / D conversion range is 0 to 255, any of the RGB values of the digital image signal is a pixel other than 0. For the effective pixel, the image processing unit 117 can detect that an image signal has been input.
When the image processing unit 117 detects the input of the vertical synchronization signal 303 and the input of the horizontal synchronization signal 304, the image processing unit 117 starts counting the number of times of input of the quantization clock 305 and the horizontal synchronization signal 304.

  The image processing unit 117 enters an operation of measuring the digital image signal A / D converted by the A / D conversion unit 111 at the timing when the quantization clock 305 is input. When detecting the effective pixel 301, the image processing unit 117 sets the count value of the quantization clock 305 at that time as the start position of the horizontal image signal. Further, the image processing unit 117 sets the count value of the horizontal synchronization signal 304 as the start position of the image signal in the vertical direction. The image processing unit 117 holds information on the start position of the image signal in the horizontal direction and the vertical direction in a register provided in the image processing unit 117. The operation of holding information in the register is performed only when the effective pixel of the image signal is first detected in each of the horizontal direction and the vertical direction. When detecting an effective pixel, the image processing unit 117 clears the count of the quantization clock started by detecting the input of the horizontal synchronization signal 304. The image processing unit 117 repeats the measurement of the start position of the horizontal effective period until the input of the next vertical synchronization signal 303 is detected.

When the image processing unit 117 detects the input of the next vertical synchronization signal 303, the quantization clock and the maximum value of the horizontal synchronization signal counted up to that point are terminated for the horizontal and vertical image signals (valid period), respectively. The position is held in a register provided in the image processing unit 117.
The image processing unit 117 detects the start position and the end position of the effective period of the image signal of each frame by performing the above operation every time the vertical synchronization signal 303 is input. Thereby, the image processing unit 117 measures the horizontal effective period start position, that is, the horizontal back porch period.

After the above measurement processing, the control unit 101 reads the horizontal effective period start position (MEAS_hStart) held in the image processing unit 117 and loads this value into the RAM 134.
Next, in S103, the control unit 101 loads the horizontal back porch period 208 (TBL_hStart) defined in the format specified in S101 into the RAM 134.
In S104, the control unit 101 measures the measured horizontal effective period start position (MEAS_hStart).
That is, the horizontal back porch period of the input signal is compared with the horizontal back porch period 208 (TBL_hStart) corresponding to the specified format.

In S104, when MEAS_hStart exceeds TBL_hStart, the control unit 101
Advances to S105, and the clamp period is set within a range within TBL_hStart.
In S104, when MEAS_hStart is equal to or less than TBL_hStart, the control unit 101 proceeds to S106, and sets the clamp period within a range within MEAS_hStart.

With the above processing, even if the format specification of the input signal is incorrect and the horizontal back porch period of the specified format is longer than the actual horizontal back porch period, the horizontal back porch period of the actual input signal is measured. , Can set an appropriate back porch period. Thereby, it can suppress that a clamp period takes on an effective period.
On the other hand, the presence of a black level (reference level) signal in the horizontal back porch period of the input image causes the clamp period to be longer than the horizontal back porch period of the specified format even in the case where the effective period start position is erroneously detected. Is not set. Thereby, it can suppress that a clamp period takes on an effective period.

  In the first embodiment, the example in which the clamp period is determined when the input of the image signal is detected or when the signal timing of the image signal is changed is described. However, the timing for determining the clamp period is not limited to this. For example, the control unit 101 may periodically acquire the horizontal effective period start position information from the image processing unit 117 and determine the clamp period again based on the clamp period determination flow when a change is detected.

  As mentioned above, although preferable embodiment of this invention was described, this invention is not limited to these embodiment, A various deformation | transformation and change are possible within the range of the summary. For example, the present invention is not limited to a projector but can be applied to a liquid crystal display device, a CRT (Cathode Ray Tube), a MEMS (Micro Electro Mechanical Systems) display, and the like. The present invention is not limited to an image display device, and can be applied to a clamp process for an analog input signal and an image processing apparatus that performs the clamp process.

101: Control unit, 111: A / D conversion unit, 117: Image processing unit

Claims (13)

Specifying means for measuring a signal parameter value of the input analog image signal and specifying the format of the input analog image signal by comparing with a signal parameter value of the analog image signal of a predetermined format stored in the storage means;
Detecting means for detecting a start position of an image effective period of the input analog image signal, and detecting a horizontal back porch period of the input analog image signal based on the detected start position;
Processing means for performing clamp processing on the input analog image signal in the shorter horizontal back porch period of the detected horizontal back porch period and the horizontal back porch period defined in the specified format;
Conversion means for converting the input analog image signal into a digital image signal based on a reference level determined by the clamping process;
An image processing apparatus comprising:   The image processing apparatus according to claim 1, wherein the signal parameter values are a horizontal synchronization frequency and a total number of vertical lines.   The specifying means includes a difference between a horizontal synchronization frequency of the predetermined format and the measured horizontal synchronization frequency within a threshold value, and the total number of vertical lines is the total number of vertical lines measured. The image processing apparatus according to claim 2, wherein an equal format is specified as a format of the input analog image signal.   The image processing apparatus according to claim 1, wherein the processing unit performs the clamping process when detecting an input of an analog image signal.   The image processing apparatus according to claim 1, wherein the processing unit performs the clamping process when a format of an input analog image signal is changed. The detection means periodically detects the start position of the image valid period,
The image processing apparatus according to claim 1, wherein the processing unit performs the clamping process when there is a change in a start position of an image valid period detected by the detection unit. The image processing apparatus according to any one of claims 1 to 6,
Display means for displaying an image based on the digital image signal converted by the conversion means. A specifying step of measuring a signal parameter value of the input analog image signal and specifying the format of the input analog image signal by comparing with a signal parameter value of the analog image signal of a predetermined format stored in the storage unit;
A detection step of detecting a start position of an image effective period of the input analog image signal and detecting a horizontal back porch period of the input analog image signal based on the detected start position;
A processing step of performing clamp processing on the input analog image signal in the shorter horizontal back porch period of the detected horizontal back porch period and the horizontal back porch period defined in the specified format;
A conversion step of converting the input analog image signal into a digital image signal based on a reference level determined by the clamping process;
A method for controlling an image processing apparatus.   The method according to claim 8, wherein the signal parameter values are a horizontal synchronization frequency and a total number of vertical lines.   In the specifying step, a difference between the horizontal synchronization frequency and the measured horizontal synchronization frequency is within a threshold value in the predetermined format, and the total number of vertical lines is the total number of vertical lines measured. The method for controlling an image processing apparatus according to claim 9, wherein an equal format is specified as a format of the input analog image signal.   The method for controlling an image processing apparatus according to claim 8, wherein, in the processing step, the clamp processing is performed when an input of an analog image signal is detected.   The method of controlling an image processing apparatus according to any one of claims 8 to 10, wherein, in the processing step, the clamp processing is performed when a format of an input analog image signal is changed. In the detection step, periodically detecting the start position of the image effective period,
The method of controlling an image processing apparatus according to any one of claims 8 to 10, wherein, in the processing step, the clamp processing is performed when a start position of the image effective period detected by the detection step is changed.

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