JP2007214973A - Video signal processing circuit and video image display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To detect automatically a Nyquist frequency of an AD converter internally or externally wire connected in or to a circuit, and compensate the cut-off frequency of a low-pass filter through the Nyquist frequency. <P>SOLUTION: A video image signal processing circuit includes: an LPF1 for varying cut-off frequency; means (resistor 4 and switch 5) for bringing a specified portion of a back porch or a front porch of an video image signal passed through LPF1 to a high impedance state; an HPF 7 for receiving feed-back of the video image signal passed through video image output impedance 6 and extracting a high-pass portion of the feed-back signal; an amplifier 8 for amplifying the feed-back signal passed through the HPF7; and a frequency setting means 9. In the frequency setting means 9, a sampling carrier factor from ADC11 included in the high impedance portion of a feed-back signal, which was amplified by the amplifier 8, is detected so that the Nyquist frequency is sought. Based on the Nyquist frequency, a cut-off frequency of the LPF1 is set. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention can automatically detect a Nyquist frequency of a video signal processing circuit and a video display device, more specifically, an analog-to-digital converter (AD converter), and correct a cutoff frequency of a low-pass filter by the Nyquist frequency. The present invention relates to a video signal processing circuit and a video display device including the circuit.

  Conventionally, in a video display device such as a liquid crystal display device, a variable filter that can arbitrarily set a cutoff frequency for cutting a high frequency component (noise component) of a video signal is used. In such a liquid crystal display device, for example, a video based on a video signal output from a personal computer (PC) is displayed. Since this video signal has a different resolution depending on the model of the PC, the frequency band is also different. Filtering is performed by a variable (low-pass) filter capable of setting a cutoff frequency corresponding to the frequency band of the signal.

  FIG. 5 is a circuit diagram showing an example of a conventional video signal processing circuit. In FIG. 5, 101 is a low-pass filter (LPF) capable of changing or switching the cut-off frequency, 102 is an amplifier of 0 dB or 6 dB, and 103 is 75Ω. A driver 104 for driving, a video output impedance of 75Ω, and an AD converter (ADC) 105 for performing analog-digital conversion.

  The input video signal is input to the LPF 101, and the high frequency component is blocked by the LPF 101. The input video signal after passing through the LPF 101 is amplified by the amplifier 102, passes through the driver 103 and the video output impedance 104, and is output to the ADC 105. The ADC 105 performs processing for converting each channel of the input analog signal into a digital signal in synchronization with the rising edge of the start clock (sampling clock) of the AD conversion sequence.

  Here, there are a plurality of frequency bands of the video signal depending on the type of the video display device, and a noise component exists in a band higher than the frequency band. Therefore, noise components are removed by providing an LPF corresponding to the frequency band of each video signal. However, since it is difficult to provide an optimal LPF for all frequency bands, it is limited. Several types of LPFs are provided. In this case, the cutoff frequency of the LPF may not match the upper limit frequency of the video signal frequency band.

  When the cut-off frequency of the LPF is lower than the upper limit frequency of the frequency band of the video signal, a signal in a band higher than the cut-off frequency of the video signal is cut (cut off), so that the video deteriorates and the LPF Is higher than the upper limit frequency of the frequency band of the video signal, there is a problem that noise is generated in the video without removing the noise component in the band higher than the upper limit frequency of the video signal.

In order to solve the above problem, a variable frequency filter that can set an optimum high-frequency cutoff (cut-off) frequency for the frequency band of the input signal is disclosed (for example, Patent Document 1 and Patent Document 1). 2). For example, in the variable frequency filter described in Patent Document 1, the first and second bias voltages applied to the first and second variable capacitance elements (variable capacitance diodes) are controlled by an external control voltage, and the input signal Filtering is performed with an optimum cutoff frequency for the frequency band.
JP 2003-168946 A JP 2005-79876 A

  However, in the conventional circuit, the cut-off frequency of the low-pass filter is varied depending on the sampling signal format. Therefore, when the sampling frequency of the AD converter is low, sampling is performed up to a band exceeding the Nyquist frequency, and aliasing distortion occurs. Will occur. If the sampling frequency is known in advance, the cutoff frequency can be controlled. However, when outputting a signal to the outside, there is no means to know the sampling frequency, and if aliasing distortion occurs, a low-pass signal is sent in the middle of the transmission path. There was no workaround other than inserting a filter.

  The present invention has been made in view of the above-described circumstances, and automatically detects the Nyquist frequency of an analog-digital conversion unit (AD converter) connected inside or outside the circuit, and the low-pass filter is detected based on the Nyquist frequency. It is an object of the present invention to provide a video signal processing circuit capable of correcting a cutoff frequency and a video display device including the circuit.

  In order to solve the above-described problem, a first technical means of the present invention includes a low-pass filter that varies a cut-off frequency in a video signal processing circuit connected to an AD converter that performs analog-digital conversion, and the low-pass filter. High impedance means for setting a specific part of the back porch or front porch of the video signal that has passed through to a high impedance state, output impedance provided at a subsequent stage of the high impedance means, and feedback of the video signal that has passed through the output impedance And a high-pass filter for extracting a high-frequency portion of the feedback signal, means for amplifying the feedback signal that has passed through the high-pass filter, and the AD converter included in the high-impedance portion of the amplified feedback signal Frequency setting means for detecting the sampling carrier component, obtaining a Nyquist frequency from the detected sampling carrier component, and setting a cutoff frequency of the low-pass filter based on the obtained Nyquist frequency. It is what.

  The second technical means includes detection means for detecting the type of the video signal that has passed through the low-pass filter in the first technical means, and the frequency setting means is configured to determine a frequency range from the type of the video signal by the detection means. The sampling carrier component from the AD converter is detected based on the set frequency range.

  According to a third technical means, in the first or second technical means, the frequency setting means compares a phase difference between the video signal and the feedback signal of the video signal to generate a difference signal; A loop filter that cuts off an alternating current component of the difference signal and a voltage controlled oscillator that changes an oscillation frequency in accordance with an output voltage from the loop filter are provided.

  A fourth technical means compares the synchronizing signal level of the video signal that has passed through the low-pass filter with the synchronizing signal level of the feedback signal of the video signal in any one of the first to third technical means, A means for maintaining two synchronization signal levels at the same level is provided.

  A fifth technical means is characterized by a video display device including the video signal processing circuit in any one of the first to fourth technical means.

  According to the present invention, it is possible to automatically detect a sampling frequency (Nyquist frequency) of an AD converter, which has been impossible until now, and to provide an optimum low-pass filter (anti-alias filter) following the sampling frequency. Therefore, it is possible to always ensure a good band and obtain a high-quality video.

(First embodiment)
FIG. 1 is a circuit diagram showing a configuration example of a video signal processing circuit according to the first embodiment of the present invention. In the figure, the video signal processing circuit drives a variable low-pass filter (LPF) 1 that can vary the cutoff frequency, a signal format detection means 2 that detects the type (format) of the video signal that has passed through the LPF 1, and 75Ω. The driver 3, the resistor 4 for determining the impedance when a specific portion of the video signal is set to the high impedance state, the switch 5 that is turned on during the high impedance period, the video output impedance 6 of 75Ω, A high-pass filter (HPF) 7 that receives feedback after passing through the video output impedance 6 and extracts only high-frequency components from the feedback signal, an amplifier 8 that amplifies the feedback signal that passes through the HPF 7, and a cutoff frequency of the LPF 1 Frequency setting means 9 to be set, and each of the above And a control means 10 for controlling the means, and is connected to an AD converter (ADC) 11 by an internal or external connection.

  The resistor 4 and the switch 5 constitute high impedance means for bringing a specific part of the back porch or front porch of the video signal that has passed through the LPF 1 into a high impedance state. The switch 5 is a means that is turned OFF during a high impedance period and is switched ON during a period other than the high impedance period.

  The frequency setting means 9 detects the sampling carrier component generated by sampling by the ADC 11 from the feedback signal, obtains the Nyquist frequency (that is, 1/2 of the sampling frequency) from the detected sampling carrier component, and determines the LPF 1 by the Nyquist frequency. Set the cutoff frequency. The frequency setting means 9 automatically sets the cut-off frequency of the LPF 1 so that a sufficient attenuation can be secured at the Nyquist frequency.

  Here, various formats such as 480i (interlace), 480p (progressive), 1080i, 1080p, and 720p exist in the video signal input to the video signal processing circuit of the present invention. It is a signal format for providing video. Since the frequency band differs depending on each signal format, if this signal format can be detected in advance, the frequency band of the video signal can be narrowed down, so that the setting process of the cutoff frequency by the frequency setting means 9 can be performed faster. it can.

In FIG. 1, a case where the cutoff frequency of LPF 1 is 27 MHz and the sampling frequency of ADC 11 is 30 MHz will be described as an example.
The band of the video signal is limited by passing through the LPF 1. If this band limitation is not sufficiently attenuated at 1/2 of the sampling frequency of the ADC 11 (Nyquist frequency), aliasing distortion occurs.

  The type (format) of the video signal that has passed through the LPF 1 is detected by the signal format detection means 2 and output to the driver 3. Further, the signal format detection unit 2 inputs the detected video signal type to the frequency setting unit 9. The video signal after passing through the signal format detection means 2 is output from the 75Ω driver 3 via the video output impedance 6.

  Immediately before the video signal from the driver 3 is output from the video output impedance 6, the resistor 4 and the switch 5 bring the specific part of the back porch or front porch of the video signal into a high impedance state. The sampling carrier component of the ADC 11 is superimposed on the high impedance portion, and this signal is fed back from the ADC 11.

  For example, in the case of a horizontal sync signal used for an NTSC signal, the period (1.5 μsec) from the start of the horizontal blanking period (10.9 μsec) to the start of the horizontal sync signal is called the front porch. A period from the end of the horizontal synchronizing signal to the start of the video (4.7 μsec) after a signal period (4.7 μsec) is referred to as a back porch. A color burst signal or the like is superimposed on the back porch. The specific portion that is brought into a high impedance state by the resistor 4 and the switch 5 may be either a front porch or a back porch.

  Since the sampling carrier component described above is a high frequency, the amplification frequency is amplified by the amplifier 8 after the feedback signal has passed through the HPF 7, so that the sampling frequency of the ADC 11 can be easily identified. The frequency setting means 9 automatically controls the cut-off frequency of the LPF 1 so that it can be sufficiently attenuated at this frequency because 1/2 of the determined sampling frequency becomes the Nyquist frequency.

  FIG. 2 is a block diagram showing a configuration example of a main part of the frequency setting means 9 shown in FIG. The frequency setting means 9 constitutes a PLL (Phase Locked Loop) circuit, compares the phase difference between the video signal and the feedback signal of the video signal and generates a difference signal, and an AC component of the difference signal And a voltage controlled oscillator (VCO) 9c that changes the oscillation frequency in accordance with the output voltage from the loop filter 9b. The counter 12 is means for counting the frequency of the feedback signal amplified by the amplifier 8, and outputs the counted frequency to the signal format detection means 2.

  The frequency setting process according to the present invention will be specifically described based on the configuration shown in FIG. The signal format detection means 2 detects the signal format of the video signal and sets the frequency range of the VCO 9c. The frequency range of the VCO 9c is set by the signal format detection means 2 and the counter 12. A feedback signal from the ADC 11 is input to the HPF 7, amplified by the amplifier 8, and then input to the phase comparator 9 a. On the other hand, the video signal that has passed through the signal format detection means 2 is input to the phase comparator 9a through the VCO 9c.

  The phase comparator 9a compares the phase difference between the feedback signal (reference signal) from the amplifier 8 and the video signal (comparison signal) from the VCO 9c to generate a difference signal. The loop filter 9b cuts the AC component of the difference signal input from the phase comparator 9a and generates a control voltage for controlling the VCO 9c. The VCO 9c locks to the sampling carrier component included in the feedback signal according to the control voltage from the loop filter 9b.

  In this way, the frequency setting unit 9 detects the sampling carrier component included in the feedback signal from the ADC 11 to obtain the Nyquist frequency, and sets the cutoff frequency of the LPF 1.

(Second Embodiment)
FIG. 3 is a circuit diagram showing a configuration example of a video signal processing circuit according to the second embodiment of the present invention. The video signal processing circuit of the present embodiment further includes a variable gain amplifier 13 and a synchronization level detection means 14 in addition to the configuration shown in FIG. 1, whereby the synchronization signal level of the video signal after passing through the LPF 1 And the synchronization signal level of the feedback signal of the video signal are compared, and the two synchronization signal levels are maintained at the same level.

  The sync level detecting means 14 feeds back the video signal that has passed through the video output impedance 6, detects the sync signal level of the feedback signal, and also detects the sync signal level from the video signal output from the LPF 1. The two sync signal levels are compared, and the gain of the variable gain amplifier 13 is automatically controlled so as to be the same level. For example, the variable gain amplifier 13 can be variably set in a range of −2 dB to 8 dB.

  In the present embodiment, in addition to the circuit configuration shown in the first embodiment, by providing the variable gain amplifier 13 and the synchronization level detection means 14, the synchronization signal level is always maintained in an appropriate state, and the circuit is stabilized. I try to plan.

In FIG. 3 described above, an example will be described in which the cutoff frequency of the LPF 1 is 27 MHz and the sampling frequency of the ADC 11 is 30 MHz.
The video signal that has passed through the LPF 1 and the signal format detection means 2 is applied to the variable gain amplifier 13. This video signal passes through the variable gain amplifier 13, is output from the 75Ω driver 3, and the video signal that has passed through the video output impedance 6 is fed back. The synchronization level detection means 14 detects the synchronization signal level of the feedback signal, detects the synchronization signal level from the video signal output from the LPF 1 and compares these two synchronization signal levels so that they become the same level. The gain of the variable gain amplifier 13 is automatically controlled.

  Also, the video signal from the driver 3 is set to a high impedance state by the resistor 4 and the switch 5 immediately before being output from the video output impedance 6 by the resistor 4 and the switch 5. The sampling carrier component of the ADC 11 is superimposed on the high impedance portion, and this signal is fed back from the ADC 11.

  Since the sampling carrier component has a high frequency, the sampling frequency of the ADC 11 can be easily determined by amplifying the feedback signal through the HPF 7 and then the amplifier 8. The frequency setting means 9 automatically controls the cut-off frequency of the LPF 1 so that it can be sufficiently attenuated at this frequency because 1/2 of the determined sampling frequency becomes the Nyquist frequency.

  FIG. 4 is a block diagram showing a configuration example of a video display device provided with the video signal processing circuit of the present invention, in which 20 is a video display device, and the video display device 20 includes a video signal processing circuit 21, AD A converter (ADC) 22, a resolution conversion unit 23, a liquid crystal drive unit 24, a liquid crystal panel 25, and a control unit 26 are provided. In the video display device 20 of this example, the video signal processing circuit of the present invention is applied to a liquid crystal display device.

  The video signal processing circuit 21 corresponds to the video signal processing circuit of the present invention shown in FIG. 1 or 3, the ADC 22 corresponds to the ADC 11, and the control unit 26 corresponds to the control means 10.

  In FIG. 4, the video signal processing circuit 21 to which a video signal is input from a tuner (not shown) detects the sampling frequency (Nyquist frequency) of the ADC 22 and follows the sampling frequency as shown in FIG. Thus, the cutoff frequency of the LPF (not shown) is set. The ADC 22 performs analog-digital conversion on the output signal from the video signal processing circuit 21 to generate a digital signal. The resolution conversion unit 23 converts the resolution of the digital signal generated by the ADC 22 into a resolution suitable for the liquid crystal panel 25.

  The liquid crystal driving unit 24 generates an analog signal by digital-analog (DA) conversion of the digital signal whose resolution has been converted by the resolution converting unit 23, and generates a driving signal for driving the liquid crystal panel 25 from the analog signal. . The liquid crystal panel 25 displays an image according to a drive signal from the liquid crystal drive unit 24.

  The video signal processing circuit according to the present invention is not limited to the liquid crystal display device described above, and can be applied to all video display devices to which video signals of a plurality of types of frequency bands are input. The specific circuit configuration of the video signal processing circuit is not limited to the above-described embodiments, and can be changed without departing from the gist of the present invention.

  As described above, according to the present invention, it is possible to automatically detect a sampling frequency (Nyquist frequency) of an AD converter, which has been impossible until now, and an optimum low-pass filter (anti-alias filter) that follows the sampling frequency. Therefore, it is possible to always ensure a good band and obtain a high-quality video.

1 is a circuit diagram showing a configuration example of a video signal processing circuit according to a first embodiment of the present invention. It is a block diagram which shows the principal part structural example of the frequency setting means shown in FIG. It is a circuit diagram which shows the structural example of the video signal processing circuit which concerns on the 2nd Embodiment of this invention. It is a block diagram which shows the structural example of the video display apparatus provided with the video signal processing circuit of this invention. It is a circuit diagram which shows an example of the conventional video signal processing circuit.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1,101 ... Variable low-pass filter (LPF), 2 ... Signal format detection means, 3,103 ... Driver, 4 ... Resistance, 5 ... Switch, 6,104 ... Image output impedance, 7 ... High pass filter (HPF), 8, DESCRIPTION OF SYMBOLS 102 ... Amplifier, 9 ... Frequency setting means, 9a ... Phase comparator, 9b ... Loop filter, 9c ... Voltage controlled oscillator (VCO), 10 ... Control means, 11, 22, 105 ... AD converter (ADC), 12 ... Counter DESCRIPTION OF SYMBOLS 13 ... Variable gain amplifier, 14 ... Synchronization level detection means, 20 ... Video display apparatus, 21 ... Video signal processing circuit, 23 ... Resolution conversion part, 24 ... Liquid crystal drive part, 25 ... Liquid crystal panel, 26 ... Control part.

Claims (5)

In a video signal processing circuit connected to an AD converter that performs analog-digital conversion,
A low-pass filter that varies the cutoff frequency;
High impedance means for setting a specific portion of the back porch or front porch of the video signal that has passed through the low pass filter to a high impedance state;
An output impedance provided at a subsequent stage of the high impedance means;
A high-pass filter that receives feedback of the video signal that has passed through the output impedance and extracts a high-frequency portion of the feedback signal;
Means for amplifying the feedback signal passed through the high pass filter;
A sampling carrier component from the AD converter included in a high impedance portion of the amplified feedback signal is detected, a Nyquist frequency is obtained from the detected sampling carrier component, and the low-pass filter is cut based on the obtained Nyquist frequency. A video signal processing circuit comprising: frequency setting means for setting an off frequency.   2. The video signal processing circuit according to claim 1, further comprising detection means for detecting a type of the video signal that has passed through the low-pass filter, wherein the frequency setting means sets a frequency range from the type of the video signal by the detection means. A video signal processing circuit for detecting a sampling carrier component from the AD converter based on the set frequency range.   3. The video signal processing circuit according to claim 1, wherein the frequency setting means compares a phase difference between the video signal and a feedback signal of the video signal to generate a difference signal, and the difference A video signal processing circuit comprising: a loop filter that cuts an alternating current component of a signal; and a voltage-controlled oscillator that changes an oscillation frequency in accordance with an output voltage from the loop filter.   4. The video signal processing circuit according to claim 1, wherein the synchronization signal level of the video signal that has passed through the low-pass filter is compared with the synchronization signal level of the feedback signal of the video signal, and two synchronizations are performed. A video signal processing circuit comprising means for maintaining a signal level at the same level.   A video display device comprising the video signal processing circuit according to claim 1.

Images