Abstract:
A video signal improving circuit comprises an input section which receives a video signal; a delay circuit which delays the video signal to generate a plurality of delayed video signals which have delayed times different from one another; an improving signal producing circuit which generates, as an improving signal, an improving amount for performing a transient improvement on the video signal based upon an original video signal before delay and the plurality of delayed video signals; a band-limiting filter which performs a frequency band limitation on the improving signal; a timing correcting circuit which causes timings of any one of the plurality of delayed video signals and the improving signal which has passed through the band-limiting filter to match with each other; and an adder which adds the improving signal which has passed through the band-limiting filter to the delayed video signal which has passed through the timing correcting circuit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2004-8646, filed on Jan. 16, 2004, the entire contents of which are incorporated herein by reference. 
   BACKGOUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a video signal improving circuit. 
   2. Background Art 
   In order to clarify an edge of an image and display the image sharply, a transient improvement is performed on a video signal such as a luminance signal or a color-difference signal.  FIG. 14  is a diagram showing a rising portion of a video signal before and after transient improvement is performed. A video signal X is a video signal before a transient improvement and a video signal Y is a video signal after the transient improvement. When a gradual-rising signal, such as the video signal X, is used, an edge of an image is blurred to become unclear, as shown in  FIG. 15 . On the other hand, when a sharp-rising signal, such as the video signal Y, is used, an edge of an image becomes clear, as shown in  FIG. 16 . 
   The transient improvement includes a luminance transient improvement to a luminance signal and a color-difference transient improvement to a color-difference signal. The luminance transient improvement is called LTI (Luminance Transient Improver), SRT (Super Real Transient), or the like. The color-difference transient improvement is called CTI (Color Transient Improver), color SRT (Color Super Real Transient) or the like. 
   In general, a frequency band of a video signal improved by the transient improvement depends on characteristics of a transient improving circuit. For this reason, the transient improving circuit performs transient improvement on even signals in a frequency band which does not require transient improvement. At this time, for example, when a jitter occurs in a low frequency band which does not require transient improvement, an edge of the jitter is also made clear due to the transient improvement, which results in such a problem that the jitter becomes visible easily (refer to Japanese Paten Application Laid-Open (JP-A) No. 10-150582). 
   SUMMARY OF THE INVENTION 
   A video signal improving circuit according to an embodiment of the invention comprises an input section which receives a video signal; a delay circuit which delays the video signal to generate a plurality of delayed video signals which have delayed times different from one another; an improving signal producing circuit which generates, as an improving signal, an improving amount for performing a transient improvement on the video signal based upon an original video signal before delay and the plurality of delayed video signals; a band-limiting filter which performs a frequency band limitation on the improving signal; a timing correcting circuit which causes timings of any one of the plurality of delayed video signals and the improving signal which has passed through the band-limiting filter to match with each other; and an adder which adds the improving signal which has passed through the band-limiting filter to the delayed video signal which has passed through the timing correcting circuit. 
   A video signal improving circuit according to another embodiment of the invention comprises an input section which receives a video signal; a delay circuit which delays the video signal to generate a plurality of delayed video signals having delayed times different from one another; a band-limiting filter which performs a frequency band limitation on an original video signal before delay and the plurality of delayed video signals; an improving signal producing circuit which generates, as an improving signal, an improving amount for performing a transient improvement on the video signal based upon the original video signal and the plurality of delayed video signals which have passed through the band-limiting filter; a timing correcting circuit which causes timings of any one of the plurality of delayed video signals and the improving signal to match with each other; and an adder which adds the improving signal to the delayed video signal which has passed through the timing correcting circuit. 
   A video signal improving circuit according to further embodiment of the invention comprises an input section which receives a video signal; a first delay circuit which delays the video signal to generate a plurality of delayed video signals having delayed times different from one another; an improving signal producing circuit which generates, as an improving signal, an improving amount for performing a transient improvement on the video signal based upon an original video signal before delay and the plurality of delayed video signals; an amplifier circuit amplifying the improving amount; a frequency detecting circuit which detects a frequency of the plurality of delayed video signals to reduce a gain of the amplifier circuit in the case that the detected frequency belongs to a frequency band which does not require the transient improvement; and an adder which adds the improving signal amplified by the amplifier circuit to one of the plurality of video signals. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a video signal improving circuit  100  according to a first embodiment of the present invention. 
       FIG. 2  is a graph showing an improving amount of an improving signal to a frequency of a video signal. 
       FIG. 3  is a graph showing a gain characteristic of the band-limiting filter  3 . 
       FIG. 4  is a graph showing an improving amount of an improving signal which has passed through the band-limiting filter  3 . 
       FIG. 5  is a block diagram of a video signal improving circuit  200  according to a second embodiment of the present invention. 
       FIG. 6  is a block diagram of a video signal improving circuit  300  according to a third embodiment of the present invention. 
       FIG. 7  is a graph showing an improving amount of an improving signal outputted from the gain control circuit  26 . 
       FIG. 8  is a graph showing an improving amount obtained when the gain of the gain control circuit  26  is reduced in both of a low frequency band lower than the peak frequency and a high frequency band exceeding the peak frequency. 
       FIG. 9  is a block diagram of a video signal improving circuit  400  according to a fourth embodiment of the present invention. 
       FIG. 10  is a timing chart of an output signal S 1  to S 6  when the video signal is a high frequency signal. 
       FIG. 11  is a timing chart of an output signal S 1  to S 6  when the video signal is a low frequency signal. 
       FIG. 12  is a block diagram of a video signal improving circuit  500  according to a fifth embodiment of the present invention. 
       FIG. 13  is a block diagram of a video signal improving circuit  600  according to a sixth embodiment of the present invention. 
       FIG. 14  is a diagram showing a rising portion of a video signal before and after transient improvement is performed. 
       FIG. 15  is a diagram showing an image before transient improvement is performed. 
       FIG. 16  is a diagram showing an image after transient improvement is performed. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments according to the present invention will be explained below with reference to the drawings. The present invention is not limited to the embodiments. Each of video signal improving circuits according to the following embodiments modifies a gain of an improving amount of a transient improvement based upon a frequency of an input video signal. Thereby, the transient improvement is performed on a video signal requiring improvement and an improving amount of transient improvement can be reduced to other video signals. The video signal improving circuit is embedded in an analog television receiver, a digital television receiver or the like, and can be used for a video signal processing in the television receiver, a monitor device therein or the like. In the drawings, same reference numerals denote same or similar elements or members. 
     FIG. 1  is a block diagram of a video signal improving circuit  100  according to a first embodiment of the present invention. The video signal improving circuit  100  comprises a transient improving circuit  2 , a band-limiting filter (hereinafter, also called “BPF (Band Pass Filter)”)  3 , and a timing correcting filter  4 . The transient improving circuit  2  includes delay lines  21  to  24 , and an improving signal producing circuit  25 . As shown in  FIG. 14 , the transient improving circuit  2  improvs a video signal so as to make a slope of a rising or falling of the video signal steeper. 
   The delay lines  21  to  24  are connected from an input terminal  1  in series. The delay line  21  delays an original video signal inputted from the input terminal  1  to output a first delayed video signal. The delay line  22  further delays the first delayed video signal from the delay line  21  to output a second delayed video signal. The delay line  23  further delays the second delayed video signal from the delay line  22  to output a third delayed video signal. The delay line  24  further delays the third delayed video signal from the delay line  23  to output a fourth delayed video signal. In order to perform transient improvement easily, it is preferable that the delay lines  21  to  24  delay the video signals by the same delay amount, respectively. The video signal is constituted of, for example, a color-difference signal and a luminance signal, or, RGB signals. 
   A node between the input terminal  1  and the delay line  21  is defined as N 1 , a node between the delay line  21  and the delay line  22  is defined as N 2 , a node between the delay line  22  and the delay line  23  is defined as N 3 , a node between the delay line  23  and the delay line  24  is defined as N 4 , and a node between the delay line  24  and the improving signal producing circuit  25  is defined as N 5 . The improving signal producing circuit  25  is connected to the node N 1  to N 5 , respectively. Thereby, the improving signal producing circuit  25  can be inputted with the original video signal, and the first to fourth delayed video signals. The improving signal producing circuit  25  performs arithmetic operation on these signals using the original video signal and the first to fourth delayed video signals to generate an improving signal indicating an improving amount based upon a slope of a rising or falling of the video signal. 
   In general, a frequency of a video signal to be subjected to transient improvement is determined according to a difference of a delay amount. In detail, when improvement is performed on a video signal with a low frequency, the improving signal producing circuit  25  generates an improving signal by using a plurality of video signals forming a large difference(s) in delay amount among them. In this case, for example, the improving signal producing circuit  25  uses the original video signal (N 1 ), the second delayed video signal (N 3 ), and the fourth delayed video signal (N 5 ) to calculate a difference between the original video signal and the second delayed video signal and a difference between the second delayed video signal and the fourth delayed video signal. Further, the improving signal producing circuit  25  generates an improving signal by performing such an operation as differentiating these differences. 
   On the other hand, when improvement is performed on a video signal with a high frequency, the improving signal producing circuit  25  generates an improving signal using a plurality of video signals forming a small difference(s) in delay amount. The improving signal producing circuit  25  uses the first delayed video signal (N 2 ), the second delayed video signal (N 3 ), and the third delayed video signal (N 4 ) to calculate a difference between the first delayed video signal and the second delayed video signal and a difference between the second delayed video signal and the third delayed video signal. Further, the improving signal producing circuit  25  obtains an improving signal by performing such a processing as differentiating these differences. 
     FIG. 2  is a graph showing an improving amount of an improving signal to a frequency of a video signal. A peak frequency is a frequency of a video signal to be subjected to transient improvement. As shown in  FIG. 2 , improving amount occurs even in frequencies other than the peak frequency. That is, when a video signal is subjected to a transient improvement by using an improving signal outputted from the improving signal producing circuit  25 , video signals with frequencies other than the peak frequency are also subjected to a transient improvement. 
   Therefore, a band-limiting filter  3  connected to the improving signal producing circuit  25  is inputted with the improving signal from the improving signal producing circuit  25  to convert a gain of the improving signal dependent on a frequency. A gain characteristic of the band-limiting filter  3  is shown in  FIG. 3 . In this embodiment, the band-limiting filter  3  is a high pass filter. Therefore, the improving signal is limited regarding a low frequency band lower than the peak frequency. 
     FIG. 4  is a graph showing an improving amount of an improving signal which has passed through the band-limiting filter  3 . A broken line S shows an improving signal before it passes through the band-limiting filter  3 , and a solid line T shows an improving signal after it passes through the band-limiting filter  3 . Since the band-limiting filter  3  is the high pass filter, the improving amount in the low frequency band is reduced in this manner. 
   The timing correcting filter  4  is connected between the node N 3  and an adder  5 . The timing correcting filter  4  is inputted with the second delayed video signal to output the video signal to the adder  5 . At this time, the timing correcting filter  4  delays the second delayed video signal. This delay is performed for synchronizing the second delayed video signal with the improving signal which has passed through the band-limiting filter  3 , because the improving signal is delayed by passing of the improving signal through the band-limiting filter  3 . The timing correcting filter  4  may be constituted of, for example, a delay line(s). 
   The adder  5  is connected to the band-limiting filter  3  and the timing correcting filter  4 , respectively, and is inputted with the improving signal from the band-limiting filter  3  and with the second delayed video signal from the timing correcting filter  4 . Further, the adder  5  adds the improving signal and the second delayed video signal to output the added result from an output terminal  6 . Thereby, a video signal improved such as a video signal Y shown in  FIG. 14  is outputted from the output terminal  6 . 
   A reason for inputting the second delayed video signal into the timing correcting circuit  4  will be explained below. As described above, when a video signal is a signal with a low frequency, the improving signal producing circuit  25  generates an improving signal by using the original video signal, the second delayed video signal, and the fourth delayed video signal. When a video signal is a signal with a high frequency, the improving signal producing circuit  25  generates an improving signal by using the first delayed video signal, the second delayed video signal, and the third delayed video signal. In either case, the second delayed video signal is an intermediate signal among these delayed video signals which has an intermediate value between the original video signal and a delayed video signal delayed in the maximum. That is, the both of the cases use a signal delayed from the second delayed video signal by a fixed time and a signal advanced from the second delayed video signal by the same fixed time. As shown in FIG.  14 , therefore, it is necessary to use the second delayed video signal in order to improve a slope of a rising or a falling of a video signal about a center thereof in an arrow direction. 
   According to this embodiment, as shown in  FIG. 4 , an improving amount to video signals belonging to a frequency band which does not require transient improvement can be reduced. Thereby, for example, even when a jitter occurs in a video signal belonging to a low frequency band, the jitter is not enhanced on a screen. 
   In this embodiment, though the high pass filter is used as the band-limiting filter  3 , a band pass filter or a low pass filter may be used as the band-limiting filter  3 . When the low pass filter is used as the band-limiting filter  3 , an improving amount to a video signal belonging to a high frequency band exceeding a peak frequency thereof can be reduced. When the band pass filter is used as the band-limiting filter  3 , an improving amount to a video signal belonging to a lower frequency band lower than the peak frequency and belonging to a high frequency band exceeding the peak frequency can be reduced. Thereby, even when noises or the like occur in a high frequency band, or both of a high frequency band and a low frequency band, the video signal improving circuit  100  according to the embodiment can improve transient of a video signal with an intended frequency to display an image sharply without enhancing the noises. 
   (Second Embodiment) 
     FIG. 5  is a block diagram of a video signal improving circuit  200  according to a second embodiment of the present invention. The video signal improving circuit  200  is different from the first embodiment in that the former is provided with band-limiting filters  31  to  35  instead of the band-limiting filter  3 . The band-limiting filters  31  to  35  are provided inside a transient improving circuit  210 , and the transient improving circuit  210  is different from the transient improving circuit  2  shown in  FIG. 1  in this point. 
   The band-limiting filters  31  to  35  are connected between the node N 1  and the improving signal producing circuit  25 , between the node N 2  and the improving signal producing circuit  25 , between the node N 3  and the improving signal producing circuit  25 , between the node N 4  and the improving signal producing circuit  25 , and between the node N 5  and the improving signal producing circuit  25 , respectively. The other constituent elements of the second embodiment may be similar to those of the first embodiment. Each of the band-limiting filters  31  to  35  may be either of a high pass filter, a band pass filter, and a low pass filter like the band-limiting filter  3 . 
   In this embodiment, the band-limiting filters  31  to  35  perform band-limitation on the original video signal and the first to fourth delayed video signals before producing an improving signal. Thereby, this embodiment can achieve an advantage or merit similar to the first embodiment. 
   (Third Embodiment) 
     FIG. 6  is a block diagram of a video signal improving circuit  300  according to a third embodiment of the present invention. The video signal improving circuit  300  is different from the video signal improving circuit  100  of the first embodiment in that the former is provided with a gain control circuit  26  and a frequency detecting circuit  50 . The gain control circuit  26  is provided inside a transient improving circuit  310 , and the transient improving circuit  310  is different from the transient improving circuit  2  shown in  FIG. 1  in this point. 
   The gain control circuit  26  is connected between the improving signal producing circuit  25  and the adder  5 . The frequency detecting circuit  50  is connected between the node N 3  and the gain control circuit  26 . A gain (an improving amount) of the gain control circuit  26  is fixed or controlled by I 2 CBUS or the like. When control is performed by I 2 CBUS, image evaluation is performed and a gain is set based upon the evaluation result. The gain of the gain control circuit  26  can be modified based upon a frequency detected by the frequency detecting circuit  50 . For example, the frequency detecting circuit  50  detects a frequency of the second delayed video signal, so that, when the detected frequency is lower than a preset threshold frequency, the gain of the gain control circuit  26  is reduced. 
     FIG. 7  is a graph showing an improving amount of an improving signal outputted from the gain control circuit  26 . A broken line S indicates an improving amount obtained when the frequency detecting circuit  50  is not provided, and a solid line T and U indicates an improving amount obtained when the gain of the gain control circuit  26  is reduced. The frequency detecting circuit  50  may gradually reduce the gain of the gain control circuit  26  according to lowering of the frequency of the second delayed video signal, such as the solid line T. The frequency detecting circuit  50  may reduce the gain of the gain control circuit  26  down to almost zero when the frequency of the second delayed video signal is lower than the threshold, such as the solid line U.  FIG. 7  shows the gain of the gain control circuit  26  in a low frequency band lower than the peak frequency, but such a constitution may be employed that the gain of the gain control circuit  26  in a high frequency band exceeding the peak frequency is reduced. 
     FIG. 8  is a graph showing an improving amount obtained when the gain of the gain control circuit  26  is reduced in both of a low frequency band lower than the peak frequency and a high frequency band exceeding the peak frequency. As shown with a solid line V in  FIG. 8 , the frequency detecting circuit  50  can reduce the gain of the gain control circuit  26  down to almost zero when the frequency of the second delayed video signal becomes lower than a first threshold and when the frequency of the second delayed video signal becomes higher than a second threshold. 
   This embodiment can achieve an advantage similar to that of the first embodiment. Further, since the video signal improving circuit  300  of this embodiment can reduce the gain of the gain control circuit  26  to almost zero on reaching the first threshold and the second threshold, jitters or noises present in a frequency band lower than the first threshold or a frequency band higher than the second threshold can be completely prevented from being enhanced. 
   Incidentally, the gain control circuit  26  may be constituted as a circuit for controlling an improving amount inside the improving signal producing circuit  25 . In this case, the circuit may be used instead of the gain control circuit  26 . 
   (Fourth Embodiment) 
     FIG. 9  is a block diagram of a video signal improving circuit  400  according to a fourth embodiment of the present invention.  FIG. 10  is a timing chart of signals S 1  to S 6  obtained when a video signal has a relatively high frequency.  FIG. 11  is a timing chart of signals S 1  to S 6  obtained when a video signal has a relatively low frequency. 
   The video signal improving circuit  400  is different from the video signal improving circuit  300  according to the third embodiment in that the former is provided with delay lines  420 ,  430 , and  440 , a comparing circuit  450 , a double side-band rectifier  460 , and a level detector  470 . However, the video signal improving circuit  400  is similar to the video signal improving circuit  300  according to the third embodiment in that the former detects the frequency of a video signal, thereby controlling the gain of the gain control circuit  26 . The frequency detecting circuit  51  includes therein the delay line  440 , the comparing circuit  450 , the double side-band rectifier  460 , and the level detector  470 . 
   The delay lines  420  and  430  are provided for adjusting a delay time in a signal line producing an improving signal and a delay time in signal line for detecting a frequency of a video signal. The delay line  440  is connected between nodes N 6  and N 7  to delay the video signal. A video signal at the node N 6  is represented as S 1  and a video signal at the node N 7  is represented as S 2 . As shown in  FIG. 10 , the signal S 2  is delayed from the signal S 1  by an amount corresponding to passing of the signal S 2  through the delay line  440 . 
   Two inputs of the comparing circuit  450  are connected to the nodes N 6  and N 7  which are inputted with the signals S 1  and S 2 . The comparing circuit  450  calculates a difference between the signals S 1  and S 2 . In detail, the comparing circuit  450  calculates (S 1 -S 2 ) to output a non-inverted signal S 3  and an inverted signal S 4 . The waveforms of the signals S 3  and S 4  are shown in  FIG. 10 . 
   The double side-band rectifier  460  connected to outputs of the comparing circuit  450  to be inputted with the signals S 3  and  54 . The double side-band rectifier  460  detects the maximum value in the signals S 3  and  54  to generate a signal S 5 . The waveform of the signal S 5  is shown in  FIG. 10 . 
   The level detector  470  is connected between the double side-band rectifier  460  and the gain control circuit  26  to be inputted with the signal S 5 . The level detector  470  stores therein a predetermined detection level L in advance to compare the signal S 5  and the detection level L with each other. When the signal S 5  exceeds the detection level L, the level detector  470  outputs H (high), and when the signal S 5  is lower than the detection level L, the level detector  470  outputs L (low). At this time, the waveform of an output signal S 6  of the level detector  470  is shown in  FIG. 10 . 
   As shown in  FIG. 11 , when the frequency of the video signal is low, the wavelengths of the signals S 1  and S 2  become long. The peak of the signal S 1  corresponds to an ascending slope portion of the signal S 2 , and the peak of the signal S 2  corresponds to a descending slope portion of the signal S 1 , respectively. Therefore, the peaks of the signals S 1  and S 2  are cancelled by taking a difference between the signals S 1  and S 2 , as shown with the signals S 3  and S 4 . Accordingly, the amplitude value is made small as compared with the case that the video signal has a high frequency. As a result, when the video signal has a low frequency, the signal S 5  does not exceed the detection level L and the signal S 6  maintains L (low). 
   Thus, the video signal improving circuit  400  of this embodiment can detect a frequency of a video signal. For example, when the signal S 5  is lower than the detection level L, the gain of the gain control circuit  26  is made zero and when the signal S 5  exceeds the detection level L, the gain of the gain control circuit  26  is increased. Thereby, this embodiment can provide a characteristic as such shown by a curve U in  FIG. 7 . On the contrary, such a constitution may be employed that, when the signal S 5  exceeds the detection level L, the gain of the gain control circuit  26  is made zero, and when the signal S 5  is lower than the detection level L, the gain of the gain control circuit  26  is increased. With such a constitution, the video signal improving circuit  400  of this embodiment can cut a video signal belonging to a frequency band higher than the peak frequency. 
   Further, the frequency may be determined depending on periods T 1  and T 3  where a signal S 6  shown in  FIG. 10  is H (high), or a period T 2  between the periods T 1  and T 3  where the signal S 6  is L (low). In this case, since the frequency of a video signal can be detected in an analog manner, the video signal improving circuit  400  of this embodiment can provide a characteristic as such shown by a curve T in  FIG. 7 . 
   By changing the delay amount in the delay line  440  or the detection level L in the level detector  470 , the frequency (the threshold in  FIG. 7 ) which reduces the gain of the gain control circuit  26  can be changed. 
   (Fifth Embodiment) 
     FIG. 12  is a block diagram of a video signal improving circuit  500  according to a fifth embodiment of the present invention. This embodiment is different from the video signal improving circuit  400  of the fourth embodiment in that the former further comprises a delay line  441 , a comparing circuit  451 , a double side-band rectifier  461 , and a level detector  471  inside a frequency detecting circuit shown by a broken line. The frequency detecting circuit  51  further includes therein the delay line  441 , the comparing circuit  451 , the double side-band rectifier  461 , and the level detector  471 . For example, the delay line  441 , the comparing circuit  451 , the double side-band rectifier  461 , and the level detector  471  are used for lowering a video signal belonging to a high frequency band, while the delay line  440 , the comparing circuit  450 , the double side-band rectifier  460 , and the level detector  470  are used for lowering a video signal belonging to a low frequency band. 
   For this end, delay amounts in the delay lines  440  and  441  are made different from each other, or the detection levels L in the level detectors  470  and  471  are made different from each other. Thereby, an output signal S 6  of the level detector  470  and an output signal S 7  of the level detector  471  become H (high) at their frequencies different from each other. The signal S 6  and an inverted signal of the signal S 7  are inputted into an AND gate AND an output signal S 8  of the AND gate AND is supplied to the gain control circuit  26 . 
   This embodiment can provide a characteristic such as shown by a curve V in  FIG. 8  by setting the delay line  440 ,  441  or the detection level L properly. For example, the delay line  440 ,  441 , or the detection level L is set such that, when the frequency of a video signal is lower than the first threshold, both the signals S 6  and S 7  are made L (low). At this time, since the signal S 8  is L (low), the gain of the gain control circuit  26  is decreased. The delay line  440 ,  441 , or the detection level L is set such that, when the frequency of a video signal is higher than the first threshold and lower than the second threshold, the signal S 6  is made H (high) and the signal S 7  is made L (low). At this time, since the signal S 8  is H (high), the gain of the gain control circuit  26  is increased. Further, the delay line  440 ,  441 , or the detection level L is set such that, when the frequency of a video signal is higher than the second threshold, both the signals S 6  and S 7  is made H (high). At this time, since the signal S 8  is L (low), the gain of the gain control circuit  26  is decreased. Thus, this embodiment can provide a characteristic such as shown by a curve V in  FIG. 8 . 
   (Sixth Embodiment) 
     FIG. 13  a block diagram of a video signal improving circuit  600  according to a sixth embodiment of the present invention. The video signal improving circuit  600  is different from the video signal improving circuit  500  of the fifth embodiment that the former is not provided with the delay line  441 , the comparing circuit  451 , and the double side-band rectifier  461  but with the level detector  471 . The frequency detecting circuit  51  includes therein the level detector  471 , but does not include the delay line  441 , the comparing circuit  451 , and the double side-band rectifier  461 . 
   Since the video signal improving circuit  600  does not have the delay line  441 , the signals S 6  and S 7  can not be controlled by the delay amount of a video signal based upon the frequency of the video signal. However, since the video signal improving circuit  600  is provided with the level detectors  470  and  471 , the signals S 6  and S 7  can be controlled by making the detection levels L in these level detectors different from each other based upon the frequency of the video signal. 
   This embodiment has an advantage or merit similar to that in the fifth embodiment. Further, this embodiment can be reduced in circuit scale as compared with the fifth embodiment.