Noise reduction circuit for video signal recording/reproduction device

Abstract: A reproduction system noise reduction circuit of a VTR is implemented in combination with a recursive type noise reduction circuit and a non-recursive type noise reduction circuit. The recursive type noise reduction circuit has a characteristic with complimentary relation to that of a recording system vertical emphasis circuit to function as a vertical de-emphasis circuit subjecting a reproduced luminance signal to vertical de-emphasis. The non-recursive type noise reduction circuit cancels noise components without line correlation from the reproduced luminance signal subjected to vertical de-emphasis. It is not necessary to increase noise reduction amount in each noise reduction circuit because vertical de-emphasis and noise reduction are carried out individually. Therefore, degradation of vertical resolution and increase in raining-like noise are prevented.

Description: The present invention relates to noise reduction circuits, and more particularly, to a noise reduction circuit for reducing noise included in a reproduced video signal from a video signal recording/reproduction device such as a video tape recorder (referred to as a VTR hereinafter). Video signal recording/reproduction devices such as VTRs are usually provided with noise reduction circuits for reducing noise within reproduced video signals generated at the time of recording video signals onto a recording medium such as a magnetic tape and/or reproducing video signals therefrom. Generally, a video signal comprises a luminance signal (Y signal) and a chrominance signal (C signal) interleaved with each other at the interval of 1/2·f H . In recording, such video signal is Y/C separated and the extracted luminance signal is processed by a luminance signal processing circuit to be recorded. When such luminance signal is recorded, chrominance signal components apart from a luminance signal spectrum by 1/2·f H may possibly remain in the luminance signal, and such chrominance signal components become noise components. Fig. 1 is a block diagram schematically showing a non-recursive type noise reduction circuit which is an example of such conventional noise reduction circuits for reducing noise components. Referring to Fig. 1, a reproduced luminance signal as a reproduced video signal, including noise, provided from a reproduction system circuit (not shown) of a VTR for example, is applied to an 1H delay line 2 implemented with a CCD delay line for example, via an input terminal 1 to be delayed by 1H period (H: horizontal period of a video signal). The delay line is not limited to 1 delay line, and may be any nH (n: natural number) delay line. The reproduced luminance signal delayed by 1H period in 1H delay line 2 is applied to a negative input terminal of a subtractor 3. The current reproduced luminance signal provided to input terminal 1 is applied to a positive input terminal of subtractor 3, as well as to a positive input terminal of a subtractor 6. Subtractor 3 subtracts the reproduced luminance signal 1H-delayed by 1H delay line 2 from the current reproduced luminance signal which is provided from input terminal 1. Because the video information of the reproduced luminance signals having 1H period interval are in close approximity, i.e. have line correlation, noise components not related to video information (no line correlation) are extracted from subtractor 3. The extracted noise component has the amplitude thereof limited via a limiter 4 to be applied to an attenuator 5 where its level is attenuated. The noise component is then applied to a negative input terminal of subtractor 6. The positive input terminal of subtractor 6 is supplied with the current reproduced luminance signal from input terminal 1. Subtractor 6 subtracts the noise component from the current reproduced luminance signal. Thus, a reproduced luminance signal having the noise component reduced is provided via an output terminal 7. With the noise reduction circuit of the non-recursive type of Fig. 1, not only the noise components but also the signal components not having line correlation are canceled from the reproduced luminance signal by subtractor 6. The resolution in the vertical direction of the reproduced picture is therefore degraded with pictures having blurred edges. Because this phenomenon is notable where noise reduction effect is increased, the SN ratio could not be improved sufficiently in the non-recursive type noise reduction circuit of Fig. 1. A VTR has been proposed performing de-emphasis in the vertical direction and noise reduction simultaneously, where the level of a signal component not having line correlation is emphasized, i.e., vertical emphasis is applied to a luminance signal to record a video signal on a magnetic tape, and the reproduced luminance signal is passed through a recursive type noise reduction circuit at the time of reproduction. A recursive type noise reduction circuit employed in such VTRs is disclosed in JP-A-57211885 and 59158684, for example. Fig. 2 is a block diagram schematically showing such a conventional recursive type noise reduction circuit. Referring to Fig. 2, a reproduced luminance signal, including noise, provided from a reproduction system circuit (not shown) of a VTR is applied to a positive input terminal of a subtractor 9 and a positive input terminal of a subtractor 13, via an input terminal 8. Subtractor 9 subtracts the reproduced luminance signal supplied from a 1H delay line 10 from the reproduced luminance signal to extract a component not having line correlation (mainly the noise component). The extracted component is provided to an attenuator 12 having an arbitrary attenuation coefficient via a limiter 11, where the level is attenuated. The attenuated component is then applied to the negative input terminal of subtractor 13. The positive input terminal of subtractor 13 is supplied with the current reproduced luminance signal from input terminal 8, whereby subtractor 13 subtracts the component without line correlation from the current reproduced luminance signal. Thus, the reproduced luminance signal having the noise component reduced and the signal component without line correlation emphasized at the time of recording de-emphasized is applied to an output terminal 14, and to a 1H delay line 10. A series of the above mentioned operation is repeated thereafter. The recursive type noise reduction circuit of Fig. 2 implements a closed-loop in which the reproduced luminance signal provided from output terminal 14 is fed-back to 1H delay line 10. The noise components are reduced according to the attenuation coefficient of attenuator 12, and the signal components without line correlation are de-emphasized to improve the SN ratio significantly. The noise reduction rate in the recursive type noise reduction circuit of Fig. 2 is great because de-emphasis in the vertical direction and reduction of the noise components are carried out simultaneously with one circuit. This results in degradation of the resolution of the reproduced picture in the vertical direction as mentioned before. Also, the noise components having line correlation which were not reduced as described above is particularly emphasized, leading to a problem of raining like trails in the vertical direction on the reproduced picture called "raining noise". US-A-4,750,037 discloses a noise reduction circuit according to the preamble of claim 1 including an emphasis means and first and second noise reduction means. Patents abstracts of Japan, Vol. 6, No. 238 (E-144) 26.11.1982 and JP-A-57138276 disclose a non-recursive emphasis circuit. An object of the present invention is to provide a noise reduction circuit of a video signal recording/reproduction device preventing degradation in vertical resolution in reproduced pictures. Another object of the present invention is to provide a noise reduction circuit of a video signal recording/reproduction device suppressing the increase of "raining noise" in reproduced pictures. A further object of the present invention is to simplify the structure of a noise reduction circuit of a video signal recording/reproduction device. These objects are achieved by a noise reduction circuit as defined in claim 1; the dependent claims are related to further developments of the invention. The main advantage of the present invention lies in that it is not necessary to increase the noise reduction amount of the noise reduction circuits to allow suppression of vertical resolution degradation and raining-like noise increase, by subjecting the reproduced video signal to vertical de-emphasis with the recursive type noise reduction circuit, as well as reducing noise components with the non-recursive type noise reduction circuit in reproducing a video signal subjected to vertical emphasis at the time of recording. The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. Fig. 1 is a block diagram schematically showing a conventional non-recursive type noise reduction circuit. Fig. 2 is a block diagram schematically showing a conventional recursive type noise reduction circuit. Fig. 3 is a block diagram schematically showing a noise reduction circuit according to a first embodiment of the present invention. Fig. 4 is a graph for explaining the operational characteristics of the first embodiment of Fig. 3. Fig. 5 is a block diagram schematically showing a noise reduction circuit of a second embodiment of the present invention. Fig. 3 is a block diagram schematically showing a noise reduction circuit of a VTR according to a first embodiment of the present invention. Referring to Fig. 3, a luminance signal to be recorded provided from a recording system circuit (not shown) of a VTR for example, is applied to a 1H delay line 22 implemented with a CCD delay line or the like through an input terminal 21 to be delayed by 1H. The 1H-delayed luminance signal is supplied to the negative input terminal of a subtractor 23. The current luminance signal provided to input terminal 21 is applied to the positive input terminal of subtractor 23, as well as to one input terminal of an adder 26. Subtractor 23 subtracts the luminance signal delayed by 1H at 1H delay line 22 from the current luminance signal from input terminal 21 to provide a signal component without line correlation. The extracted signal component is applied to a multiplier 25 having a coefficient of K1 via a limiter 24 to be attenuated in level. The signal component having the level attenuated is then applied to the other input terminal of adder 26. Adder 26 adds the signal component without line correlation to the current luminance signal. Hence, a luminance signal having a signal component without line correlation emphasized is provided from adder 26. Input terminal 21, 1H delay line 22, subtractor 23, limiter 24, multiplier 25 and adder 26 constitute a non-recursive type vertical emphasis circuit which applies a vertical emphasis to a luminance signal in a luminance signal recording system of a VTR. The vertical-emphasized luminance signal provided from adder 26 is applied to a recording/reproduction system 27 inclusively showing a recording system modulation circuit, a recording system magnetic head, a magnetic tape, a magnetic head for reproduction, a reproduction system demodulation circuit and the like, and is recorded on the magnetic tape. The luminance signal vertically emphasized and including noise generated at the time of recording/reproduction, which is reproduced from the magnetic tape at the time of reproduction, is applied to the positive input terminal of a subtractor 28 as well as to the positive input terminal of a subtractor 31. Subtractor 28 subtracts the reproduced luminance signal provided from a 1H delay line 32 from the reproduced luminance signal to extract a component without line correlation (mainly noise component). The extractive component is applied to a multiplier 30 of a coefficient of K2 via a limiter 29 to have its level attenuated. The attenuated component is then applied to the negative input terminal of a subtractor 31. The positive input terminal of subtractor 31 is supplied with the current reproduced luminance signal, whereby the component without line correlation is subtracted from the current reproduced luminance signal. Thus, a reproduced luminance signal having the component without line correlation reduced is applied to positive input terminals of subtractors 33 and 36, as well as to 1H delay line 32. Thereafter, the above described series of operation is repeated. Subtractor 28, limiter 29, multiplier 30, subtractor 31 and 1H delay line 32 constitute a recursive type noise reduction circuit. If coefficient K2 of multiplier 30 of the recursive type noise reduction circuit of the reproduction side is set to have a characteristic complementary to that of coefficient K1 of the non-recursive type vertical emphasis circuit of the recording side, the recursive type noise reduction circuit of the reproduction side functions substantially as a vertical de-emphasis circuit subjecting a luminance signal to de-emphasis in the vertical direction. The reproduced luminance signal provided from subtractor 31 subjected to vertical de-emphasis is delayed by 1H in 1H delay line 32 and applied to the negative input terminal of subtractor 33. Subtractor 33 subtracts the reproduced luminance signal delayed by 1H from the -reproduced luminance signal supplied from subtractor 31 to extract a noise component without line correlation. The extracted noise component is applied to a multiplier 35 of a coefficient K3 via a limiter 34 to be attenuated in level. The noise component is then applied to the negative input terminal of a subtractor 36. The positive input terminal of subtractor 36 is supplied with the reproduced luminance signal from subtractor 31, whereby subtractor 36 further subtracts the noise component from the reproduced luminance signal subjected to vertical de-emphasis. The reproduced luminance signal having the noise component reduced is provided from an output terminal 37. 1H delay line 32, subtractor 33, limiter 34, multiplier 35 and subtractor 36 constitute a non-recursive type noise reduction circuit. In accordance with the first embodiment, the recursive type noise reduction circuit of the preceding stage of the reproduction system performs mainly de-emphasis in the vertical direction to eliminate the need to increase the noise reduction amount of this recursive type noise reduction circuit, in comparison with the conventional example of Fig. 2 where vertical de-emphasis and noise reduction are carried out simultaneously. Therefore degradation of vertical resolution and increase of raining-like noise arising from the noise reduction circuit can be suppressed. The non-recursive type noise reduction circuit of the succeeding stage performs mainly reduction of noise components, allowing noise component reduction without particularly increasing the noise reduction amount of this non-recursive type noise reduction circuit. Therefore, noise having line correlation is not emphasized, and raining-like noise arising from this noise reduction circuit does not occur. Fig. 4 is a graph explaining the operation characteristic of the first embodiment of Fig. 3 in comparison with the conventional example of Fig. 2. The operation of the first embodiment will be explained in details with reference to Figs. 3 and 4. The transfer function of the non-recursive type recording system vertical emphasis circuit implemented with elements 21 to 26 of Fig. 3 is derived as in the following manner. Firstly, the following equation is established between input luminance signal V 21 at input terminal 21 and output V 26 of adder 26. V 26 (t) = V 21 (t) + K1 (V 21 (t) - V 21 (t-T)) where T represents 1H period. The transfer function H 26 (ω) of the recording system vertical emphasis circuit is expressed as below. H 26 (ω) = V 26 (ω) / V 21 (ω) = 1 + K1 - K1 · exp (-sT) The transfer function of the recursive type reproduction system vertical de-emphasis circuit implemented with elements 28 to 32 is derived as in the following manner. Firstly, the following equation is established between reproduced luminance signal V 27 provided from recording/reproduction system 27 and output V 31 of subtractor 31. V31 (t) = V 27 (t) - K2 (V 27 (t) - V 31 (t - T)) The transfer function H 31 (ω) of the reproduction system vertical de-emphasis circuit is expressed as below. H 31 (ω) = V 31 (ω) / V 27 (ω) = (1 - K2) / (1 - K2 · exp (-sT)) The transfer function of the non-recursive type reproduction system noise reduction circuit implemented with elements 32 to 37 is derived as in the following manner. Firstly, the following equation is established between reproduced luminance signal V 31 provided from subtractor 31 subjected to vertical de-emphasis and output luminance signal V 31 at output terminal 37. V 37 (t) = V 31 (t) - K3 (V 31 (t) - V 31 (t-T)) The transfer function H 37 (ω) of the reproduction system noise reduction circuit is expressed as below. H 37 (ω) = V 37 (ω) / V 31 (ω) = 1 - K3 + K3 · exp (-sT) In view of the foregoing, K1 and K2 should be determined to establish the relationship of: H 26 (ω) x H 31 (ω) = 1 for the characteristic of the recording system vertical emphasis circuit to have complimentary relation to that of the reproduction system vertical de-emphasis circuit. That is to say, from the above equations of (2), (4) and (7), K1 and K2 should be determined to establish: K1 ≃ K2 / (1 - K2) By determining K1 and K2 as in the above equation, degradation of vertical resolution and increase of raining-like noise arising from the recursive type vertical de-emphasis circuit can be suppressed while carrying out vertical de-emphasis. Fig. 4 is a graph showing the frequency-amplitude characteristics where K1 = 0.5, K2 = 0.33, and K3 = 0.164 in the structure of Fig. 3, in comparison with the conventional example of Fig. 2. The recursive type noise reduction circuit shown in the conventional example of Fig. 2 does not include the elements corresponding to elements 33 to 36 of Fig. 3. Also, it is assumed that the attenuation coefficients K1 and K2 of the recording system and the reproduction system are both specified as 0.5. In the convention example, K1 ≦ K2 / (1 - K2). Accordingly, the de-emphasis amount of the vertical direction in the reproduction system is greater than the emphasis amount of the vertical direction in the recording system. Referring to broken line a representing the first embodiment and solid line b representing the conventional example in Fig. 4, the pass-band of a reproduced luminance signal of the first embodiment is wider than that of the conventional example to allow passage of original reproduced luminance signals in a wider range, when the attenuation amounts of the amplitudes between frequencies apart from each other by (1/2) f H (f H : horizontal frequency) are the same. This means that the noise component having a frequency of (1/2) f H deviated from the reproduced luminance signal is canceled by the non-recursive type noise reduction circuit of the succeeding stage to eliminate the need to increase the noise reduction amount of the reproduction system vertical de-emphasis circuit which is a recursive type noise reduction circuit as great as the recursive type noise reduction circuit (Fig. 2) of the conventional example. Therefore, degradation of vertical resolution and increase of raining-like noise arising from the reproduction system vertical de-emphasis circuit are suppressed. The entire circuit structure of the embodiment of Fig. 3 is simplified by feature of the recursive type vertical de-emphasis circuit and the non-recursive type noise reduction circuit sharing a common 1H delay line 32. Fig. 5 is a block diagram schematically showing a noise reduction circuit of a second embodiment of the present invention. Depiction of the recording system emphasis circuit (elements 21 through 26 of Fig. 3) of the preceding stage of recording/reproduction system 27 is omitted in Fig. 5, and the description thereof is not repeated. The embodiment of Fig. 5 is basically formed by a recursive type noise reduction circuit of the preceding stage and a non-recursive type noise reduction circuit of the succeeding stage in a manner formed by similar to the aforementioned first embodiment. The difference from the first embodiment of Fig. 3 is that the second embodiment is implemented to share one adder as adders 28 and 33 of Fig. 3. Referring to Fig. 5, a luminance signal, subjected to vertical emphasis and including noise, provided from recording/reproduction system 27 at the time of reproduction is applied to the positive input terminal of a subtractor 41, as well as to the positive input terminal of a subtractor 45 via a delay equalizer EQ. Subtractor 41 subtracts the output of a 1H delay line 46 from the reproduced luminance signal to extract a component not having line correlation. A chrominance signal resulting from the interleaving at the time of recording may possibly remain in the extracted component. Therefore, at the time of reproducing, there is a possibility that the remaining chrominance signal may be emphasized by the non-recursive type and recursive type noise reduction circuits at the succeeding stage due to phase shift of the 1H delay line or the like. As a result, at the time of reproducing, the extracted component has the chrominance carrier removed by a trap circuit 42 which traps a signal of the subcarrier frequency f sc (3.58MHz) of a chrominance signal. This prevents leakage of chrominance signals and flicker at edge portions on the color picture arising from non-recursive type noise reduction circuits. The aforementioned delay equalizer EQ 47 is provided for the purpose of adjusting the time lag due to trap circuit 42. The component without line correlation provided from trap circuit 42 is applied to a multiplier 44 of a coefficient K2' via a limiter 43 to be attenuated in level. The attenuated component is then applied to a negative input terminal of subtractor 45. The positive input terminal of subtractor 45 is supplied with a reproduced luminance signal from recording/reproduction system 27. Subtractor 45 subtracts the component without line correlation from the current reproduced luminance signal. Thus, a reproduced luminance signal having the component without line correlation reduced is applied to the positive input terminal of subtractor 51, as well as to a 1H delay line 46. The above-mentioned series of operation is repeated thereafter. As set in the foregoing, subtractor 41, trap circuit 42, limiter 43, multiplier 44, subtractor 45, 1H delay line 46 and delay equalizer EQ 47 constitute a recursive type noise reduction circuit (vertical de-emphasis circuit). The noise component without line correlation provided from trap circuit 42 is applied to a multiplier 49 of a coefficient K3' via a limiter 48 to be attenuated in level. The attenuated component is then applied to the negative input terminal of subtractor 51. The positive input terminal of subtractor 51 is supplied with the reproduced luminance signal from subtractor 45. Subtractor 51 subtracts the noise component from the reproduced luminance signal subjected to vertical de-emphasis. Thus, lH delay line 46, subtractor 41, trap circuit 42, limiter 48, multiplier 49 and subtractor 51 constitute a non-recursive type noise reduction circuit. In the second embodiment, the coefficient K2' of multiplier 44 is preferably K2' < 1.0. Particularly, K2' < 0.5 is normally set. Coefficient K3' of multiplier 49 is set to K3' ≤ 0.5. The multiplier 49 of the second embodiment is different from the multiplier 35 of the first embodiment in that the multiplier 49 receives as an input the noise component extracted from the recursive type noise reduction circuit. As a result, according to the second embodiment, the amount of the noise component supplied to the noise reduction circuit at the succeeding stage is increased in comparison with the first embodiment. Accordingly, the coefficient K3' of the multiplier 49 of the second embodiment is set to be smaller than the coefficient K3 of the multiplier 35 of the first embodiment. For example, when the coefficient K3 of the multiplier 35 of the first embodiment is set to 0.164, the coefficient K3' of the multiplier 49 of the second embodiment is set to approximately 0.15. In the above-mentioned first and second embodiments, each of the recursive type noise reduction circuit and the non-recursive type noise reduction circuit comprises a limiter and a multiplier. By adjusting the characteristics thereof, it is possible to implement the order of the recursive type noise reduction circuit and the non-recursive type noise reduction circuit in reverse. Furthermore, the delay line in each embodiment is not limited to a 1H delay line and may be nH (n : natural number) delay line. As set forth in the foregoing, reproduced luminance signals are subjected to vertical de-emphasis with a recursive type noise reduction circuit, as well as noise components being reduced with a non-recursive type noise reduction circuit, in reproducing luminance signals subjected to vertical emphasis at the time of recording, to eliminate the need to increase the noise reduction amount of each noise reduction circuit, and suppressing degradation of vertical resolution and increase of raining-like noise.

Claims: A noise reduction circuit for a video signal recording/reproduction device, comprising non-recursive type vertical emphasis means (22, 23, 24, 25, 26) subjecting a video signal to vertical emphasis at the time of recording, said noise reduction circuit comprising: first noise reduction means (28, 29, 30, 31, 32) of a recursive type for subjecting a reproduced video signal to vertical de-emphasis with a characteristic having complementary relation to that of said vertical emphasis means, and second noise reduction means (32, 33, 34, 35, 36) of a non-recursive type for cancelling noise components not having line correlation from the reproduced video signal, characterized in that said non-recursive type vertical emphasis means comprises first delay means (22) for delaying a video signal to be recorded by nH, n: natural number, H: horizontal period of video signal, first subtracting means (23) for subtracting the output of said first delay means from said video signal to be recorded, first limiter means (24) for limiting the amplitude of the output of said first subtracting means, first multiplying means (25) for attenuating the output of said first limiter means by a first coefficient (K1), and adding means (26) for adding the output of said first multiplying means to said video signal to be recorded, said first noise reduction means of the recursive type comprises second subtracting means (28, 41) for receiving said reproduced video signal as a positive input, third subtracting means (31, 45) for receiving said reproduced video signal as a positive input, second delay means (32, 46) for delaying the output of said third subtracting means by a period of nH, said second subtracting means receiving the output of said second delay means as a negative input for subtracting the same from said reproduced video signal, second limiter means (29, 43) for limiting the amplitude of the output of said second subtracting means, and second multiplying means (30, 44) for attenuating the output of said second limiter means by a second coefficient, said third subtracting means receiving the output of said second multiplying means as a negative input for subtracting the same from said reproduced video signal, and said second noise reduction means of the non-recursive type comprises said second delay means (32) for delaying the output of said second subtracting means by a period of nH. The noise reduction circuit according to claim 1, wherein said second noise reduction means of the non-recursive type further comprises fourth subtracting means (33) for subtracting the output of said second delay means form the output of said third subtracting means, third limiter means (34) for controlling the amplitude of the output of said fourth subtracting means, third multiplying means (35) for attenuating the output of said third limiter means by a third coefficient (K3), fifth subtracting means (36) for subtracting the output of said third multiplying means form the output of said third subtracting means, and means (37) for providing the output of said fifth subtracting means as a reproduced video signal. The noise reduction circuit according to claim 1, wherein first and second coefficient K1 and K2 are selected so that the relationship of K1 ≃ K2 / (1 - K2) is established between coefficient K1 of said first multiplying means and coefficient K2 of said second multiplying means. The noise reduction circuit according to claim 1, wherein said second noise reductions means of the non-recursive type further comprises said second subtracting means (41) for subtracting the output of said second delay means from said reproduced video signal, fourth limiter means (48) for controlling the amplitude of the output of said second subtracting means, fourth mulitplying means (49) for attenuating the output of said fourth limiter means by a fourth coefficient (K3'), sixth subtracting means (51) for subtracting the output of said fourth multiplying means from the output of said third subtracting means, and means (52) for providing the output of said sixth subtracting means as a reproduced video signal. The noise reduction circuit according to claim 1, wherein said first and second delay means have delay periods of 1H.

Inventors: IIZUKA HIROSHI, SHIRAI MITUZOU, IIZUKA, HIROSHI, SHIRAI, MITUZOU
Applicants: SANYO ELECTRIC CO, SANYO ELECTRIC CO., LTD.
Application year: 1991
Main classification: H04N5
IPC codes: H04N5