Video signal noise reduction circuit

Abstract: A noise canceller circuit (12) is connected in a stage succeeding a picture quality control circuit (11) to which the video signal is input. A noise cancel amount for the video signal can be controlled so as to correspond to the gain in the frequency characteristic at the same time when the adjustment of the frequency characteristic of the video signal is performed in the picture quality control circuit (11). As a result, the noise cancel amount in the noise canceller circuit (12) can be controlled correspondingly to the level of the noise component even if a level of the noise component in the video signal is changed when the gain in a cer­ tain bandwidth of the video signal is adjusted in the picture quality control circuit (11). Therefore, the noise component can be cancelled effectively.

Description: The present invention relates to a video signal noise reduction circuit for use in a video tape recorder and the like. In a video tape recorder, as shown in Fig. 1, a picture quality control circuit 51 is connected to a noise canceller circuit 52 in the succeeding stage. A regenerated video signal is input to the noise canceller circuit 52. The picture quality control circuit 51 adjusts the frequency characteristic of the video signal under the control of an external controller 53, thereby controlling the quality of a picture. A gain for the video signal is usually controlled in a certain band by the external controller 53. The noise canceller circuit 52 regards a video signal at a level lower than a predetermined level (limiter level) as a noise component, and extracts the noise component from the video signal to cancel the noise component. The noise canceller circuit 52 is not ordinarily designed to delete the noise component completely, and a minute noise component is often left. For example, it reduces the noise component to -10dB level. Therefore, if the picture quality control circuit 51 is connected to the noise canceller circuit 52 in the succeeding stage, the minute noise component which has been without being cancelled is amplified by the picture quality control circuit 51 and impairs the picture quality. Furthermore, even if another noise canceller circuit (not shown) similar to the noise canceller circuit 52 is connected to the output of the picture quality control circuit 51, the conventional noise canceller circuit 52, which has above fixed limiter level, performs but incomplete cancellation of the noise component. This is because the level of the noise component included in the video signal should be changed according to the gain control of the picture quality control circuit 51. Accordingly, it is an object of the present invention to provide a video signal noise reduction circuit which can cancel the noise component of the video signal effectively, to thereby improve the picture quality, even when the level of the noise component is controlled in the picture quality control circuit by adjusting the frequency characteristic of the video signal. According to the present invention, there is provided a noise reduction circuit for a video signal comprising: picture quality control means for controlling a signal amplitude in a certain part of the frequency band of the video signal; noise cancelling means for cancelling a noise component in said video signal; and controlling means for controlling said picture quality control means and said noise cancelling means; characterised in that said noise cancelling means is arranged in a stage succeeding said picture quality control means to receive said video signal from said picture quality control means; said controlling means is connected to both said picture quality control means and said noise cancelling means, for controlling the amount of noise which said noise cancelling means is to cancel in correspondence with the signal amplitude to which said picture quality control means controls in said selected frequency band of said video signal. As a result, even if the level of noise component of the video signal is controlled by adjusting the gain in the certain band of the video signal in the picture quality control circuit, the noise component can be effectively cancelled since the cancel amount of the noise in the noise canceller circuit can be controlled in accordance with the level of the noise component. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a block diagram showing a conventional arrangement of a picture quality control circuit 51 and a noise canceller circuit 52; Fig. 2 is a block diagram showing a first embodiment of a video signal noise reduction circuit of the present invention; Fig. 3 is a block diagram showing a second embodiment of a video signal noise reduction circuit of the present invention; Fig. 4 is a block diagram showing a third embodiment of a video signal noise reduction circuit of the present invention; Fig. 5A is a diagram showing the frequency characteristic of the picture quality control circuit 11 in of the present invention; and Fig. 5B is a diagram showing the frequency characteristic of the noise canceller circuit 12 in the present invention. Embodiments of the present invention will be described hereinafter with reference to the accompanying drawings. Fig. 2 is a block diagram showing a first embodiment of a noise reduction circuit for video signal of the invention. This circuit comprises a picture quality control circuit 11 to which a regenerated video signal is input from the video tape recorder etc., a noise canceller circuit 12 connected in a stage succeeding the picture quality control circuit 11, and an external controller 13. A conventional noise canceller circuit (not shown) is usually connected in a stage preceding the picture quality control circuit 11. The external controller 13 comprises, for example, a variable resistor for controlling a voltage, and is connected to both the picture quality control circuit 11 and the noise canceller circuit 12. The picture quality control circuit 11 controls the picture quality in accordance with the frequency characteristic of the video signal controlled by the external controller 13. The picture quality control circuit 11 controls the gain in a certain frequency band of the video signal. The noise canceller circuit 12 regards the video signal, which is input from the picture quality control circuit 11 and which is at a level lower than a predetermined level (limiter level), as a noise component. The circuit 12 extracts the noise component from the video signal to cancel the noise component. More specifically, the circuit 12 subtracts the noise component from the original video signal, i.e., the video signal supplied from the picture quality control circuit 11. The amount of noise which the noise canceller circuit 12 cancels is controlled by the external controller 13 in accordance with the gain of the frequency characteristic of the video signal at the same time when the frequency characteristic of the video signal is adjusted in the picture quality control circuit 11. Fig. 3 is a block diagram showing a second embodiment of a noise reduction circuit for video signal of the present invention. This noise reduction circuit comprises a picture quality control circuit 11 and an external controller 13 which are identical to those of the first embodiment shown in Fig. 2. The noise canceller circuit 12 comprises an input terminal 20 which receives a video signal from the picture quality control circuit 11, an adder/subtractor 21 one input terminal of which receives the video signal from the input terminal 20, a high-pass filter 22 which passes high-frequency component of the video signal, a limiter 23 which extracts any video signal at a level lower than a predetermined level, and a low-pass filter 24 which passes low-frequency component of the video signal. The high-pass filter 22, the limiter 23 and the low-pass filter 24 are sequentially connected between the input terminal 20 and the other input terminal of the adder/substractor 21. The limiter level of the limiter 23 is controlled in accordance with the gain of the frequency characteristic of the video signal in the picture quality control circuit 11, which is adjusted by the external controller 13. Accordingly, the amount of noise to be cancelled from the input video signal can be controlled in the noise canceller circuit 12, by subtracting the noise component extracted on the basis of the level-controlled limiter level from the video signal supplied from the picture quality control circuit 11 through the input terminal 20. Fig. 4 is a block diagram showing a third embodiment of a noise reduction circuit of the present invention. The third embodiment also comprises a picture quality control circuit 11 and an external controller 13 which are identical to those of the first embodiment shown in Fig. 2. In a noise canceller circuit 12 in Fig. 4, a variable gain amplifier 31 is inserted in a stage succeeding the low pass filter 24. A gain of the variable gain amplifier 31 can be controlled by the external controller 13 in accordance with the gain in the frequency characteristic of the video signal, which is controlled by the picture quality control circuit 11. Accordingly, the level of the noise, which is extracted on the basis of a predetermined limiter level from the video signal supplied from the picture quality control circuit 11, can be controlled by the variable gain amplifier 31 in accordance with the controlled gain in the frequency characteristic of the video signal. Therefore, the cancel amount of the noise can be controlled by subtracting the level-controlled noise from the video signal supplied from the picture quality control circuit 11. The operation of the noise reduction circuit for video signal of the present invention will be described hereinafter with reference to the frequency characteristics in Figs. 5A and 5B. Fig. 5A shows the frequency-gain characteristic of the picture quality control circuit 11, and Fig. 5B shows that of the noise canceller circuit 12. Upon control of the picture quality, the frequency characteristic of the picture quality control circuit 11 can be controlled by the external controller 13. In this case, the gain in a certain band of the video signal can be usually controlled as is shown in Fig. 5A. Three cases are shown in Fig. 5A. The gain A shows that the video signal level in the certain band is increased to the maximum level a (20log₁₀a = A). The gain B shows that the video signal level in such bandwidth is decreased to the minimum level b (20log₁₀b = B). The flat gain as an intermediate level shows that the video signal level in such band is not adjusted. The gain of the certain band of the video signal can be controlled by the picture quality control circuit 11 to change the level of the noise component mixed in the video signal. In the present invention, the noise canceller circuit 12 can be controlled by means of the external controller 13 at the same time when the picture quality is controlled by the picture quality control circuit 11. As a result, the noise cancel level in the noise canceller circuit 12 can be changed correspondingly to the level of the noise component of the video signal. In particular, the limiter levels set in the noise canceller circuit 12 can be controlled to the levels a and b , respectively, when the gains in the frequency characteristic of the video signal are controlled to the maximum gain A and the minium gain B by the picture quality control circuit 11. The noise canceller circuit 12 can therefore cancel the noise component effectively, as is shown in Fig. 5B, to thereby improve the picture quality.

Claims: A noise reduction circuit for a video signal comprising : picture quality control means video (11) for controlling a signal amplitude in a certain part of the frequency band of the video signal; noise cancelling means (12) for cancelling a noise component in said video signal; and controlling means for controlling said picture quality control means (11) and said noise cancelling means (12): chracterized in that said noise cancelling means (12) is arranged in a stage succeeding said picture quality control means (11) to receive said video signal from said picture quality control means (11); said controlling means (13) is connected to both said picture quality control means (11) and said noise cancelling means (12), for controlling the amount of noise which said noise cancelling means (12) is to cancel in correspondence with the signal amplitude to which said picture quality control means (11) controls in said selected frequency band of said video signal. A noise reduction circuit according to claim 1, characterized in that said noise cancelling means (12) comprises: a video signal input terminal (20) connected in a stage succeeding said picture quality control means (11), for receiving a video signal supplied from said picture quality control means (11); a high-pass filter (22) connected to said video signal input terminal (20), for passing high-frequency components of said video signal; a limiter (23) connected to an output terminal of said high-pass filter (22), for extracting, as a noise component, a signal from said video signal, which is at a level lower than a limiter level, said limiter being controlled by the same control signal supplied from said controlling means (13) as that used in the control carried out in said picture quality control means (11); a low-pass filter (24) connected to an output terminal of said limiter (23), for passing low-frequency components of the output signal of said limiter ; and a subtracter (21) one input terminal of which is connected to said video signal input terminal and the other input terminal of which is connected to an output terminal of said low-pass filter (24), for subtracting said noise component extracted on the basis of said limiter level, from said video signal supplied from said video signal input terminal (20). A noise reduction circuit according to claim 1, characterized in that said noise cancelling means (12) comprises: a video signal input terminal (20) connected in a stage succeeding said picture quality control means (11), for receiving a video signal from said picture quality control means (11); a high-pass filter (22) connected to said video signal input terminal (20), for passing high-frequency components of said video signal; a limiter (23) connected to an output terminal of said high-pass filter (22), for extracting, as a noise component, a signal from said video signal, which is at a level lower than a limiter level, said limiter being controlled by the same control signal supplied from said controlling means (13) as that used in the control carried out in said picture quality control means (11); a low-pass filter (24) connected to an output terminal of said limiter (23), for passing low-frequency components of the output signal of said limiter; a variable gain amplifier (31) having a gain controlled by the same control signal supplied from said controlling means (13) as that used in the control carried out in said picture quality control means (11), for controlling the noise level is extracted based on said limiter level; and a subtracter (21) one input terminal of which is connected to said video signal input terminal (20) and the other input terminal of which is connected to an output terminal of said variable gain amplifier (31), for subtracting said noise component extracted on the basis of said limiter level controlled by said controlling means (13), from said video signal supplied from said video signal input terminal (20).

Inventors: OKADA TERUO C O INTELLECTUAL P, SHIMIZU MASATO C O INTELLECTUA, OKADA, TERUO, C/O INTELLECTUAL PROPERTY DIVISION, SHIMIZU, MASATO, C/O INTELLECTUAL PROPERTY DIV.
Applicants: TOSHIBA AVE KK, TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA, TOSHIBA AVE CO., LTD
Application year: 1990
Main classification: H04N5
IPC codes: H04N5