NOISE REDUCTION CIRCUIT FOR VIDEO SIGNAL

Abstract: A noise reduction circuit for video signal comprises first through third circuits each for obtaining from an input video signal a high frequency component including a noise compo­ nent and for limiting large amplitude parts thereof, where a filter part (32, 40; 58, 59; 82; 92) of the second circuit has an impulse response characteristic wherein an amplitude in­ creases or decays with time and is complementary to that of a filter part (31, 34; 51, 54; 81; 91) of the first circuit and a filter part (33, 41; 52, 62; 83; 93) of the third circuit has an impulse response characteristic which is essentially a combination of the two impulse response characteristics, and a fourth circuit (35, 38, 39, 43; 53, 56, 57. 61) for carrying out a subtraction between the input video signal and an output signal of the first circuit so as to obtain a first signal which is substantially eliminated of the noise component except for a noise compo­ nent and distortion remaining at parts in vicinities of rises and falls in the video signal caused by the amplitude limitation carried out in the first circuit, carrying out a subtraction be­ tween output signals of the second and third circuits so as to obtain a second signal which includes the noise component and the waveform distortion of the first signal with an inver­ ted phase, and for adding the first and second signals so as to produce an output video signal which has been substantially eliminated of all the noise component.

Description: The present invention generally relates to noise reduction circuits for video signal, and more particularly to a noise reduction circuit which can effectively eliminate a noise component existing in a luminance signal of a reproduced video signal. Generally, a noise component is mixed in a video signal which is reproduced from a recording medium in a video signal recording and reproducing apparatus such as a magnetic recording and reproducing apparatus (so-called video tape recorder) which plays a magnetic tape. However, in a recording system of the magnetic recording and reproducing apparatus, especially a high frequency component of a luminance signal of the video signal which is recorded and reproduced is subjected to a pre-emphasis and then to a frequency modulation. For this reason, the frequency of a frequency modulated signal especially at a part immediately after a part (rise) where the luminance signal rapidly changes from a black level to a white level and at a part immediately after a part (fall) where the luminance signal rapidly changes from a white level to a black level is high because of the pre-emphasis carried out in the recording system. But in a general magnetic recording and reproducing apparatus, a signal-to-noise (S/N) ratio of a recording and reproducing characteristic is poor in a high frequency range. Hence, the S/N ratio is poor at the parts immediately after the rise and immediately after the fall in the luminance signal, and it is desirable to positively eliminate the noise component existing at such parts of the luminance signal. As one example of the conventional noise reduction circuit, there is a first noise reduction circuit comprising a lowpass filter, first and second subtracting circuits and a limiter. An input luminance signal including a noise component is supplied to the lowpass filter and to the first and second subtracting circuits. An output signal of the lowpass filter is subtracted from the input luminance signal in the first subtracting circuit, and an output signal of the first subtracting circuit is supplied to the limiter. An output signal of the limiter is subtracted from the input luminance signal in the second subtracting circuit, and an output signal of the second subtracting circuit is obtained as an output of the noise reduction circuit. However, as will be described later on in the present specification in conjunction with the drawings, the first noise reduction circuit suffers a disadvantage in that it is impossible to eliminate the noise component existing at the parts immediately after the rise and immediately after the fall in the luminance signal. The S/N ratio is poor at the parts immediately after the rise and immediately after the fall in the luminance signal as described before, and it is desirable to positively eliminate the noise component existing at such parts of the luminance signal. On the other hand, as another example of the conventional noise reduction circuit, there is a second noise reduction circuit previously proposed in a European Patent Application No.85309539.6 filed December 30, 1985 in which the applicant is the same as the applicant of the present application. The second noise reduction circuit comprises first and second circuit parts each having a circuit construction similar to that of the first noise reduction circuit described before. The first and second circuit parts are coupled in series so that the output signal of the first circuit part (that is, the output signal of the second subtracting circuit in the first circuit part) is supplied to the second circuit part (that is, the lowpass filter in the second circuit part), and the output signal of the second subtracting circuit in the second circuit part is obtained as an output of the second noise reduction circuit. The lowpass filter in the second circuit part has an impulse response characteristic complementary to that of the lowpass filter in the first circuit part. As will be described later in conjunction with the drawings, the second noise reduction circuit can eliminate the noise component existing at the parts immediately after the rise and immediately after the fall in the luminance signal. However, although the noise component is satisfactorily eliminated, there is a problem in that the waveform of the luminance signal becomes distorted in the vicinities of the rise and fall thereof when the noise reduction is carried out in the second noise reduction circuit According to a first aspect of the invention, there is provided a noise reduction circuit as defined in the appended Claim 1. According to a second aspect of the invention, there is provided a noise reduction circuit as defined in the appended Claim 3. Preferred embodiments of the invention are defined in the other appended claims. It is thus possible to eliminate the noise component included in the input video signal without generating waveform distortions at parts immediately before and after rises and falls in the input video signal. The present invention will be further described, by way of example, with reference to the accompanying drawings, in which: FIGS. 1 and 2 are system block diagrams respectively showing first and second examples of the conventional noise reduction circuit; FIGS. 3(A) through 3(F) show signal waveforms for explaining the operation of the circuits shown in FIGS. 1 and 2; FIG. 4 is a system block diagram showing a first embodiment of the noise reduction circuit according to the present invention; FIGS. 5(A) through 5(O) show signal waveforms for explaining the operation of the circuit shown in FIG. 4; FIGS. 6A through 6C respectively show impulse response characteristics of lowpass filters in the circuit shown in FIG. 4; FIG. 7 is a system block diagram showing a second embodiment of the noise reduction circuit according to the present invention; FIGS. 8(A) through 8(N) show signal waveforms for explaining the operation of the circuit shown in FIG. 7; FIG. 9 is a system circuit diagram showing an embodiment of the lowpass filter in the circuits shown in FIGS. 4 and 7; FIG. 10 is a diagram for explaining the constants in the circuit system shown in FIG.9; FIG.11 is a system block diagram showing a third embodiment of the noise reduction circuit according to the present invention; FIGS.12A through 12C respectively show impulse response characteristics of highpass filters in the circuit shown in FIG.11; and FIG.13 is a system block diagram showing a fourth embodiment of the noise reduction circuit according to the present invention. FIG.1 shows a first example of the conventional noise reduction circuit. An input luminance signal a1 including a noise component as shown in FIG.3(A) is applied to an input terminal 11. For example, the input luminance signal a1 is obtained by reproducing a frequency modulated signal from a recording medium and frequency-demodulating the reproduced frequency modulated signal. A lowpass filter 12 produces a signal b1 shown in FIG.3(B) from the input luminance signal a1, and this signal b1 is subtracted from the input luminance signal a1 in a first subtracting circuit 13. An output signal of the first subtracting circuit 13 is passed through a limiter 14 which limits the amplitude thereof, and an output signal of the limiter 14 is subtracted from the input luminance signal a1 in a second subtracting circuit 15. As a result, a luminance signal c1 shown in FIG.3(C) is produced from the second subtracting circuit 15 and is obtained via an output terminal 16. But according to the circuit shown in FIG.1, it is impossible to eliminate the noise component from the luminance signal c1 in a time period in which the limiter 14 carries out the amplitude limitation, and the noise component remains at parts immediately after the rise and immediately after the fall in the luminance signal c1 as shown in FIG.3(C). As described before, the S/N ratio is poor at the parts immediately after the rise and immediately after the fall in the luminance signal, and it is desirable to positively eliminate the noise component existing at such parts of the luminance signal. FIG.2 shows a second example of the conventional noise reduction circuit previously proposed in the European Patent Application No.85309539.6 referred to before. In FIG.2, those parts which are the same as those corresponding parts in FIG.1 are designated by the same reference numerals, and description thereof will be omitted. The circuit shown in FIG.2 comprises first and second circuit parts each having a circuit construction similar to that of the noise reduction circuit shown in FIG.1. The first and second circuit parts are coupled in series so that the output signal of the first circuit part (that is, the output signal of the second subtracting circuit 15 in the first circuit part) is supplied to the second circuit part (that is, a lowpass filter 17 in the second. circuit part), and an output signal of the second subtracting circuit 20 in the second circuit part is obtained as an output of the second noise reduction circuit via an output terminal 21. The lowpass filter 17 in the second circuit part has an impulse response characteristic complementary to that of the lowpass filter 12 in the first circuit part. Hence, the lowpass filter 17 produces a luminance signal d1 shown in FIG.3(D) by delaying the incoming luminance signal c1 by a predetermined delay time and supplies this luminance signal d1 to subtracting circuits 18 and 20. Furthermore, the lowpass filter 17 produces a signal e1 shown in FIG.3(E) which is supplied to the subtracting circuit 18. The subtracting circuit 18 subtracts the signal e1 from the luminance signal d1 and supplies a resulting signal to a limiter 19. An output signal of the limiter 19 is subtracted from the luminance signal d1 in the subtracting circuit 20. Hence, a luminance signal f1 shown in FIG.3(F) having the noise components immediately after the rise and fall of the luminance signal effectively eliminated is produced from the subtracting circuit 20 and is obtained via the output terminal 21. However, although the noise component is satisfactorily eliminated, there is a problem in that the waveform of the luminance signal becomes distorted in the vicinities of the rise and fall thereof as shown in FIG.3(F) when the noise reduction is carried out in the noise reduction circuit shown in FIG.2. The present invention eliminates the problems of the above described conventional circuits by eliminating the noise component without introducing distortion in the signal waveform. FIG. 4 shows a first embodiment of the noise reduction circuit according to the present invention. An input luminance signal a2 which includes a noise component and rises at a time t0 as shown in FIG. 5(A) is applied to an input terminal 20. As described before, the input luminance signal a2 is obtained by reproducing a frequency modulated signal from a recording medium and frequency-demodulating the reproduced frequency modulated signal, for example. The input luminance signal a2 is supplied to lowpass filters 31, 32 and 33, subtracting circuits 34 and 35, and a delay circuit 36. The lowpass filter 31 has an impulse response characteristic G1(t) shown in FIG. 6A in which the amplitude decays with time, and produces a signal b2 shown in FIG. 5(B) which is supplied to the subtracting circuit 34. As can be seen from Fig. 5(B), the gradient of the luminance signal wavefront passing through the lowpass filter 31 decreases as the amplitude of the luminance signal a2 increases. This is due to the impulse response characteristic of the lowpass filter 31 which is shown in Fig. 6A. The subtracting circuit 34 subtracts the signal b2 from the input luminance signal a2 and produces a signal c2 shown in FIG. 5(C). A limiter 37 limits the amplitude of the signal c2 and supplies to the subtracting circuit 35 a signal d2 shown in FIG. 5(D). The lowpass filter 31, the subtracting circuit 34 and the limiter 37 constitute a first circuit. The subtracting circuit 35 subtracts the signal d2 from the input luminance signal a2 and produces a signal e2 shown in FIG. 5(E). The signal e2 is delayed by a predetermined delay time in a delay circuit 38, and a signal n2 shown in FIG. 5(N) from the delay circuit 38 is supplied to an adder 39. The delay time of the delay circuit 38 is identical to a delay time dt of the lowpass filter 31. The lowpass filter 32 has an impulse response characteristic G2(t) shown in FIG. 6B in which the amplitude increases with time, and produces a signal f2 shown in FIG. 5(F) which is supplied to a subtracting circuit 40. As can be seen from FIG. 5(F), the gradient of the luminance signal wavefront passing through the lowpass filter 32 increases as the amplitude of the luminance signal a2 increases. This is due to the impulse response characteristic of the lowpass filter 32 which is shown in FIG. 6B. This impulse response characteristic G2(t) is complementary to the impulse response characteristic G1(t) of the lowpass filter 31. The delay circuit 36 delays the input luminance signal a2 by the delay time dt and produces a signal g2 shown in FIG. 5(G) which is supplied to subtracting circuits 40 and 41. The subtracting circuit 40 subtracts the signal f2 from the signal g2 and produces a signal h2 shown in FIG. 5(H). A limiter 42 limits the amplitude of the signal h2 and produces a signal i2 shown in FIG. 5(1) which is supplied to a subtracting circuit 43. A threshold level of the amplitude limitation in the limiter 42 is the same as that of the limiter 37. The lowpass filter 32, the delay circuit 36, the subtracting circuit 40 and the limiter 42 constitute a second circuit. The lowpass filter 33 has an impulse response characteristic G3(t) shown in FIG. 6C in which the amplitude increases with time similarly to the impulse response characteristic G2(t) of the lowpass filter 32 and the amplitude thereafter decays similarly to the impulse response characteristic G1(t) of the lowpass filter 31. The lowpass filter 33 produces a signal j2 shown in FIG. 5 (J) from the input luminance signal a2 and supplies the signal j2 to the subtracting circuit 41. As can be seen from FIG. 5(J), the gradient of the luminance signal wavefront passing through the lowpass filter 33 increases first and then deceases as the amplitude of the luminance signal a2 increases. This is due to the impulse response characteristic of the lowpass filter 33 which is shown in FIG. 6C. The subtracting circuit 41 subtracts the signal j2 from the signal g2 and produces a signal k2 shown in FIG. 5(K). A limiter 44 limits the amplitude of the signal k2 and produces a signal ℓ 2 shown in FIG. 5(L) which is supplied to the subtracting circuit 43. A threshold level of the amplitude limitation in the limiter 44 is the same as that of the limiter 37. The lowpass filter 33, the decay circuit 36, the subtracting circuit 41 and the limiter 44 constitute a third circuit. The subtracting circuit 43 subtracts the signal i2 from the signal ℓ 2 and produces a signal m2 shown in FIG. 5(M). The signal m2 is added to the signal n2 in the adder 39, and a luminance signal o2 which has been eliminated of the noise component and has no waveform distortion as shown in FIG. 5(O) is produced from the adder 39 and is obtained via an output terminal 45. The subtracting circuits 35 and 43, the adder 39 and the delay circuit 38 constitute a fourth circuit. Therefore, according to the present embodiment, the output signal d2 of the first circuit and the input luminance signal a2 are subjected to a subtraction and to a delay in the fourth circuit so as to obtain the signal n2. The signal n2 includes at a part immediately after the edges thereof the noise component and the waveform distortion caused by the limiter 37. However, the output signal i2 of the second circuit is subtracted from the output signal ℓ2 of the third circuit in the fourth circuit so as to obtain the signal m2 which includes the noise component and the waveform distortion of the signal n2 with an inverted phase. Furthermore, the fourth circuit adds the signal n2 and the signal m2 so as to obtain the luminance signal o2 which has been eliminated of the noise component and has no waveform distortion. In other words, the fourth circuit carries out a subtraction between the input luminance signal and the output signal of the first circuit so as to obtain the signal n2 which is substantially eliminated of the noise component except for the noise component and distortion remaining at the parts immediately after the rise and immediately after the fall in the luminance signal caused by the limiter 37, carries out a subtraction between the output signals of the second and third circuits so as to obtain the signal m2 which includes the noise component and the waveform distortion of the signal n2 with the inverted phase, and adds the signals n2 and m2 so as to produce the luminance signal o2 which has been eliminated of the noise component and has no waveform distortion. FIG.7 shows a second embodiment of the noise reduction circuit according to the present invention. An input luminance signal a3 which includes a noise component and rises at a time t10 as shown in FIG.8(A) is applied to an input terminal 50. As described before, the input luminance signal a3 is obtained by reproducing a frequency modulated signal from a recording medium and frequency-demodulating the reproduced frequency modulated signal, for example. The input luminance signal a3 is supplied to lowpass filters 51 and 52 and a delay circuit 53. The lowpass filter 51 has the impulse response characteristic G2(t) shown in FIG.6B and produces a signal b3 shown in FIG.8(B). A signal c3 shown in FIG.8(C) is obtained from the delay circuit 53 which delays the input luminance signal a3 by a delay time dt. A subtracting circuit 54 subtracts the signal b3 from the signal c3 and produces a signal d3 shown in FIG.8(D). A limiter 55 which has the same threshold level as the limiter 37 described before limits the amplitude of the signal d3 and produces a signal e3 shown in FIG.8(E). The lowpass filter 51, the delay circuit 53, the subtracting circuit 54 and the limiter 55 constitute a first circuit. A subtracting circuit 56 subtracts the signal e3 from the signal c3 and produces a signal f3 shown in FIG.8(F) which is supplied to an adder 57. A lowpass filter 58 has the impulse response characteristic G1(t) shown in FIG.6A, and produces a signal g3 shown in FIG.8(G) from the signal c3. A subtracting circuit 59 subtracts the signal g3 from the signal c3 and produces a signal h3 shown in FIG.8(H). A limiter 60 which has the same threshold level as the limiter 37 described before limits the amplitude of the signal h3 and produces a signal i3 shown in FIG.8(I). The signal i3 is supplied to a subtracting circuit 61. The lowpass filter 58, the delay circuit 53, the subtracting circuit 59 and the limiter 60 constitute a second circuit. The lowpass filter 52 has the impulse response characteristic G3(t) shown in FIG.6C and produces a signal j3 shown in FIG.8(J) from the input luminance signal a3. A subtracting circuit 62 subtracts the signal j3 from the signal c3 and produces a signal k3 shown in FIG.8(K). A limiter 63 which has the same threshold level as the limiter 37 described before limits the amplitude of the signal k3 and produces a signal ℓ3 shown in FIG.8(L). The lowpass filter 52, the delay circuit 53, the subtracting circuit 62 and the limiter 63 constitute a third circuit. The subtracting circuit 61 subtracts the signal i3 from the signal ℓ3 and produces a signal m3 shown in FIG.8(M). The adder 57 adds the signal m3 and the signal f3 described before, and a luminance signal n3 shown in FIG.8(N) which has been eliminated of the noise component and has no waveform distortion is produced from the adder 57 and is obtained via an output terminal 64. The delay circuit 53, the subtracting circuits 56 and 61, and the adder 57 constitute a fourth circuit. Therefore, according to the present embodiment, the output signal e3 of the first circuit is subtracted from the delayed luminance signal c3 in the fourth circuit so as to obtain the signal f3. The signal f3 includes at the parts immediately before the edges thereof the noise component and the waveform distortion caused by the limiter 55. The second circuit produces the signal i3 from the delayed luminance signal c3, and this signal i3 is subtracted from the output signal ℓ3 which is obtained from the third circuit so as to obtain the signal m3. The signals f3 and m3 are added so as to obtain the luminance signal n3 which has been eliminated of the noise component and has no waveform distortion. In other words, the fourth circuit carries out a subtraction between the input luminance signal and the output signal of the first circuit so as to obtain the signal f3 which is substantially eliminated of the noise component except for the noise component and distortion remaining at the parts immediately before the rise and immediately before the fall in the luminance signal caused by the limiter 51, carries out a subtraction between the output signals of the second and third circuits so as to obtain the signal m3 which includes the noise component and the waveform distortion of the signal f3 with the inverted phase, and adds the signals f3 and m3 so as to produce the luminance signal n3 which has been eliminated of the noise component and has no waveform distortion. Thus, the operation of the present embodiment is basically the same as that of the first embodiment described before. The lowpass filters 31, 32 and 33 of the first embodiment and the lowpass filters 51, 52 and 58 of the second embodiment may respectively be constituted by a transversal type filter shown in FIG.9. In FIG.9, the lowpass filter comprises n-1 delay circuits 71₁ through 71 n-1 coupled in series and each having a delay time DL, n coefficient multipliers 72₁ through 72 n respectively for multiplying coefficients K₁ through K n , and an adder 73. An input signal to the lowpass filter is applied to a terminal 70 and is supplied to the delay circuit 71₁ and to the coefficient multiplier 72₁. An output signal of an arbitrary delay circuit 71 i is supplied to a coefficient multiplier 72 i+ ₁, where i = 1, 2, ..., n-1. Output signals of the coefficient multipliers 72₁ through 72 n are supplied to the adder 73 which adds these signals, and an output signal of the adder 73 is obtained via a terminal 74 as an output signal of the lowpass filter. An output signal of the delay circuit 71 n-1 is also obtained via a terminal 75. The coefficients K₁ through K n are set to appropriate values depending on the impulse response characteristic the lowpass filter is to have. For example, in the case of the lowpass filter 31 shown in FIG.4, the coefficients K₁ through K n are set to appropriate values so that the lowpass filter 31 has the impulse response characteristic G1(t) shown in FIG.6A. Hence, when n= 6 in this case, the coefficients K₁ through K₆ of the coefficient multipliers 72₁ through 72₆ can be obtained by sampling the impulse response characteristic G1(t) shown in FIG.6A with a sampling time DL as shown in FIG.10. The signal from the terminal 75 has been delayed by a time (n-1)·DL by the delay circuits 71₁ through 71 n-1 . Since this time (n-1)·DL is equal to the delay time dt of the lowpass filter 32 in the case where the lowpass filter shown in FIG.9 is used as the lowpass filter 32, it is possible to supply the signal from the terminal 75 to the subtracting circuits 40 and 41 in FIG.4 instead of the output signal g2 of the delay circuit 36. Similarly, since the delay time of the lowpass filter 33 is equal to 2dt, an output signal of a delay circuit 71 (n-1)/2 may be used instead of the signal g2 in the case where the lowpass filter shown in FIG.9 is used as the lowpass filter 33. By taking such measures, it is possible to omit the delay circuit 36. In the case where the lowpass filter shown in FIG.9 is used as the lowpass filter 51 shown in FIG.7, the signal from the terminal 75 may be used instead of the signal c3. In the first and second embodiments described heretofore, the high frequency component of the input luminance signal a2 (or a3) is extracted in the first circuit by use of the lowpass filter 31 (or 58) and the subtracting circuit 34 (or 59). However, it is possible to extract the high frequency component of the input luminance signal in the first circuit by use of a highpass filter. In this case, a highpass filter may be used in each of the first through third circuits and the times in which the limiters in the first through third circuits carry out the amplitude limitation should be set identical to each other. FIG.11 shows a third embodiment of the noise reduction circuit according to the present invention in which a highpass filter is used to extract the high frequency component of the input luminance signal. In FIG.11, those parts which are the same as those corresponding parts in FIG.4 are designated by the same reference numerals, and description thereof will be omitted. In the present embodiment, a highpass filter 81 having an impulse response characteristic Ga(t) shown in FIG. 12A is used instead of the lowpass filter 31 and the subtracting circuit 34 shown in FIG. 4. A highpass filter 82 having an impulse response characteristic Gb(t) shown in FIG. 12B is used instead of the lowpass filter 32, the subtracting circuit 40 and the delay circuit 36 shown in FIG. 4. A highpass filter 83 having an impulse response characteristic Gc(t) shown in FIG. 12C is used instead of the lowpass filter 33, the subtracting circuit 41 and the delay circuit 36 shown in FIG. 4. Accordingly, the highpass filters 81, 82 and 83 respectively produce the signals c2, h2 and k2 shown in FIGS. 5(C), 5(H) and 5(K) from the input luminance signal a2. As can be seen from FIG. 5(C), a high frequency component of the luminance signal including the noise component is outputted from the highpass filter 81, the wavefront of such output component has a fast-rise slow-decay characteristic corresponding to the amplitude change of the inputted luminance signal. This is due to the impulse response characteristic of the filter 81 which is shown in FIG. 12A. From FIG. 5(H), the wavefront of the output of the highpass filter 82, has a slow-rise fast-decay characteristic due to the impulse response characteristic of the filter 82 which is shown in FIG. 12B. From FIG. 5(K), the wavefront of the output of the highpass filter 83, has a characteristic of slow-rise preshoot immediately followed by a slow-decay overshoot of opposite polarity due to the impulse response characteristic of the filter 83 which is shown in FIG. 12C. In the present embodiment, the highpass filter 81 and the limiter 37 constitute a first circuit, the highpass filter 82 and the limiter 42 constitute a second circuit, and the highpass filter 83 and the limiter 44 constitute a third circuit. FIG. 13 shows a fourth embodiment of the noise reduction circuit according to the present invention in which a highpass filter is used to extract the high frequency component of the input luminance signal. In FIG. 13, those parts which are the same as those corresponding parts in FIG. 7 are designated by the same reference numerals, and description thereof will be omitted. In the present embodiment, a highpass filter 91 having the impulse response characteristic Gb(t) shown in FIG. 12B is used instead of the lowpass filter 51 and the subtracting circuit 54 shown in FIG.7. A highpass filter 92 having the impulse response characteristic Ga(t) shown in FIG. 12A is used instead of the lowpass filter 58 and the subtracting circuit 59 shown in FIG. 7. A highpass filter 93 having the impulse response characteristic Gc(t) shown in FIG. 12C is used instead of the lowpass filter 52 and the subtracting circuit 62 shown in FIG. 7. Accordingly, the highpass filters 91 and 93 respectively produce the signals d3 and k3 shown in FIGS. 8(D) and 8(K) from the input luminance signal a3, and the highpass filter 92 produces the signal h3 shown in FIG. 8(H) from the delayed luminance signal c3 obtained from the delay circuit 53. As can be seen from FIG. 8(H), a high frequency component of the luminance signal including the noise component is outputted from the highpass filter 92, the wavefront of such output component having a fast-rise slow-decay characteristic corresponding to the amplitude change of the inputted luminance signal. This is due to the impulse response characteristic of the filter 92 which is shown in FIG. 12A. From FIG. 8(D), the wavefront of the output of the highpass filter 91 has a slow-rise fast-decay characteristic due to the impulse response characteristic of the filter 91 which is shown in FIG. 12B. From FIG. 8(K), the wavefront of the output of the highpass filter 93 has a characteristic of slow-rise preshoot immediately followed by a slow-decay overshoot of opposite polarity due to the impulse response characteristic of the filter 93 which is shown in FIG. 12C. In the present embodiment, the highpass filter 91, the delay circuit 53 and the limiter 55 constitute a first circuit, the highpass filter 92, the delay circuit 53 and the limiter 60 constitute a second circuit, and the highpass filter 93, the delay circuit 53 and the limiter 63 constitute a third circuit. Therefore, according to the embodiments described heretofore, it is possible to eliminate the noise component included in the input luminance signal by the first through fourth circuits, without generating waveform distortions at parts immediately before and after rises and falls in the input luminance signal. In each of the embodiments described heretofore, the noise reduction circuit may be designed to carry out an analog signal processing or a digital signal processing on the input luminance signal. When carrying out the digital signal processing on the input luminance signal, an analog-to-digital converter is provided in a stage before the input terminal 30 or 50. When carrying out the signal processing on the input luminance signal in the noise reduction circuit digitally, the circuit can be mass produced with ease, and the signal processing is virtually unaffected by temperature changes and age.

Claims: 1. A noise reduction circuit for eliminating noise components from a video signal, comprising: input means (30, 50) supplied with an input video signal for inputting the video signal into said noise reduction circuit; output means (45, 64); delay means (38, 53) supplied with the video signal inputted to the input means for delaying, by a fraction of time which is identical to the delay time (dt) of a first lowpass filter (31, 58), said video signal; said first lowpass filter (31, 58) being connected directly or via said delay means (53) to said input means for receiving the input video signal and having an impulse response (Fig. 6A) in which the impulse amplitude decreases with time causing a waveform change in the video signal passing therethrough; a second lowpass filter (32, 51) connected to said input means for receiving the input video signal and having an impulse response (Fig. 6B) in which the impulse amplitude increases with time causing a waveform change in the video signal passing therethrough; a third lowpass filter (33, 52) connected to said input means for receiving the input video signal and having an impulse response (Fig. 6C) in which the impulse amplitude increases and then decreases with time causing a waveform change in the video signal passing therethrough; first circuit means comprising: said first lowpass filter (31, 58), first subtracting means (34, 59) supplied directly or via said delay means (53) with the input video signal and an output signal of the first lowpass filter for subtracting the output signal of the first lowpass filter from the input video signal or the delayed input video signal respectively, and a first limiter (37, 60) supplied with an output signal of the first subtracting means for limiting the amplitude thereof, said first circuit means extracting a high frequency component including noise from said input video signal supplied to said input means; second circuit means comprising said second lowpass filter (32, 51), second subtracting means (40, 54) supplied with an output signal of the second lowpass filter (32, 51) and the video signal delayed by said fraction of time after having been supplied to the input means (30, 50) for subtracting an output signal of the second lowpass filter from the delayed input video signal, and a second limiter (42, 55) supplied with an output signal of the second subtracting means for limiting the amplitude thereof, said second circuit means extracting a high frequency component including noise from said input video signal supplied to said input means; third circuit means comprising: said third lowpass filter (33, 52), third subtracting means (41, 62) supplied with the input video signal delayed by said fraction of time after having been supplied to the input means (30, 50) and an output signal of the third lowpass filter for subtracting the output signal of the third lowpass filter from the delayed input video signal, and a third limiter (44, 63) supplied with an output signal of the third subtracting means for limiting the amplitude thereof, said third circuit means extracting a high frequency component including noise from said input video signal supplied to said input means; and means (35, 39, 43, 56, 57, 61) for adding output signals of said first to third circuit means and said delay means to each other and outputting an added result to the output means (45, 64). 2. A noise reduction circuit as claimed in Claim 1, wherein the noise reduction circuit further comprises an additional delay means (36) for delaying, by said fraction of time, the video signal supplied to the input means and for supplying the delayed video signal to said second subtracting means (40) and to said third subtracting means (41). 3. A noise reduction circuit for eliminating noise components from a video signal, comprising: input means (30, 50) supplied with an input video signal for inputting the video signal into said noise reduction circuit; output means (45, 64); delay means (38, 53) supplied with the input video signal inputted to the input means for delaying, by a fraction of time which is identical to the delay time (dt) of a first highpass filter (81, 92) the input video signal; said first highpass filter (81, 92) being connected directly or via said delay means (53) to said input means for receiving the input video signal and having an impulse response (Fig. 12A) which possesses a fast-rise and slow-decay characteristic causing a waveform change in the video signal passing therethrough; a second highpass filter (82, 91) connected to said input means for receiving the input video signal and having an impulse response (Fig. 12B) which possesses a slow-rise and fast-decay characteristic causing a waveform change in the video signal passing therethrough; a third highpass filter (83, 93) connected to said input means for receiving the input video signal and having an impulse response (Fig. 12C) which possesses a slow-rise and slow-decay characteristic causing a waveform change in the video signal passing therethrough; first circuit means comprising said first highpass filter (81, 92) and a first limiter (37, 60) supplied with an output signal of the highpass filter, for extracting high frequency and noise components from said input video signal supplied to the input means (30, 50); second circuit means comprising said second highpass filter (82, 91) and a second limiter (42, 55) for extracting high frequency and noise components from said input video signal supplied to the input means (30, 50); third circuit means comprising said third highpass filter (83, 93) and a third limiter (44, 63) for extracting high frequency and noise components from said input video signal inputted to said input means (30, 50); and fourth circuit means (35, 39, 43, 56, 57, 61) supplied with output signals of the first to third circuit means and further with an output signal of the delay means (38, 53) for adding the output signals of the first to third circuit means and the output signal of the delay means to produce an output signal indicative of a sum of the output signals, said fourth circuit means further supplying the output signal indicative of the sum to the output means (45, 64). 4. A noise reduction circuit as claimed in Claim 3, wherein said delay means (38, 53), said first circuit means (81, 37; 92, 60) and said fourth circuit means (35, 39; 61, 57) are connected in series between said input means and said output means. 5. A noise reduction circuit as claimed in Claim 4, wherein said delay means (38) further delays the output signal of said first circuit means. 6. A noise reduction circuit as claimed in Claim 4, wherein said delay means (53) is connected between said input means and an input terminal of the first circuit means.

Inventors: OZAKI, HIDETOSHI
Applicants: VICTOR COMPANY OF JAPAN, LIMITED
Application year: 1987
Main classification: H04N5
IPC codes: H04N5