JPH0326949B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a noise reduction circuit, and is used, for example, in a circuit for reducing noise in a reproduced luminance signal of a VTR, etc., in which the amplitude of a limiter is reduced only when there is a correlation between input signals. It is an object of the present invention to provide a circuit that can sufficiently subtract noise by canceling the limited operation and improve the signal-to-noise ratio.

BACKGROUND ART In magnetic recording and reproducing devices, noise reduction circuits are known for reducing noise caused by recording and reproducing included in video signals reproduced from magnetic recording media. FIG. 7 shows a block system diagram of an example of a conventional noise reduction circuit.
In the figure, the reproduced luminance signal containing noise that has entered input terminal 1 is delayed by one horizontal scanning period (1H) by 1H delay circuit 2, and is subtracted from the reproduced luminance signal that has entered input terminal 1 in subtracter 3. be done.

Here, if there is an H correlation in the reproduced luminance signal, the video information in the 1H time interval is extremely similar, and therefore, the subtracter 3 extracts noise that is not related to the video information. This noise is supplied to a subtracter 5 through a limiter 4, where its level is attenuated, and then supplied to a subtracter 6, where it is subtracted from the reproduced luminance signal input to the input terminal 1. This subtraction gives us
A reproduced luminance signal with reduced noise is taken out from the output terminal 7. This conventional noise reduction circuit can improve the SN ratio by about 3 dB.

Problems to be Solved by the Invention In general, noise with extremely large amplitude is generated due to crosstalk etc. in the portion (edge) where the luminance signal rises from the black level to the white level or falls from the white level to the black level. As shown in the figure, the signal taken out from the subtracter 3 includes an extremely small pulse p, which is extremely large and exceeds the limiter level l (generally about 10% of the peak-to-peak value of the signal component).

Generally, when there is no line correlation, the signal input to terminal 1 and the output signal of 1H delay circuit 2 are different, so the subtracter 3 extracts the signal component and noise of the difference between the two signals, and there is no limiter 4. and subtractor 6
The limiter 4 is necessary because the signal received at the terminal 1 is subtracted by the signal component taken out from the subtracter 3 in the subtracter 3.

In the conventional circuit, the limiter 4 operates regardless of the presence or absence of line correlation, so when a pulse p is generated especially when there is a line correlation, the entire pulse p can be extracted by limiting the amplitude of the pulse p by the limiter level l. First, in the subtracter 6, this pulse p
cannot be subtracted, so there are problems in that the noise reduction effect at the edges is small and the image quality at the edges is degraded.

Means for Solving the Problems and Their Effects The present invention detects whether or not there is a correlation between the low-frequency components of the input signal, and cancels the amplitude limiting operation of the limiter only when it is detected that there is a correlation. The above problem is solved as a configuration with a circuit,
Each embodiment will be described below with reference to the drawings.

Embodiment FIG. 1 shows a block system diagram of a first embodiment of the circuit of the present invention. In the figure, the same components as those in FIG. 7 are given the same numbers and their explanations will be omitted. In the figure, the reproduced luminance signal input to terminal 1 is a (second
A) in the figure, and a signal delayed by 1H in the 1H delay circuit 2 is denoted by b (B in the figure). Here, since the signals a and b are extremely similar signals, the signal components are canceled by the subtracter 3 to form the signal c (C in the figure). The signal c has large amplitude parts c ₁ and c ₂ that are not canceled because there is no line correlation.

Signal c is amplified by amplifier 8, and limiter 9
A and 9b (circuits in which transistors x ₁ and x ₂ are omitted in the limiter 11 to be described later) limit their amplitudes to the limiter levels indicated by broken lines in C in the figure, respectively, and then a Schmitt inverter (not shown) The waveform is shaped into a control signal (D in the same figure) that is at L level during the period of part c ₁ and at H level during the other periods, while the waveform is shaped by a Schmitt inverter (not shown) and becomes the control signal during the period of part c ₂ . The control signal e (E in the figure) is at L level and at H level for other periods, and the signal e is supplied as is while the signal d is inverted in polarity by the inverter 10 and supplied to the limiter 11, respectively.

Note that a low-pass filter 15 (cutoff frequency is, for example, 1 MHz) is provided between the subtracter 3 and the amplifier 8, so that the pulse p is not extracted.

The limiter 11 has the configuration shown in FIG. 3, in which the transistor _x1 is turned off during the H level period of the control signal d input to the terminal 11a, and limits the amplitude of the portion _c1 of the signal c. During the L level period, the transistor x ₁ is turned on and the amplitude limiting operation is canceled, while during the L level period of the control signal e input to the terminal 11b, the transistor x ₂ is turned off and the part c ₂ of the signal c is turned off. While limiting the amplitude, transistor x ₂ is turned on during the H level period, and the amplitude limiting operation is canceled.

As a result, the amplitude of the signal c is limited during the portions c ₁ and c ₂ where there is no line correlation, while the amplitude of the signal c is not limited during periods other than the portions c ₁ and c ₂ where there is line correlation, and therefore, If there is a line correlation and noise with a pulse p exceeding the limiter level l of the limiter 11 comes in, the pulse p will be
The output of the limiter 11 is taken as a signal f (F in the figure). A subtracter 6 subtracts a signal obtained by subtracting a signal f from a signal a by a subtracter 5, and noise including the pulse p is sufficiently reduced to obtain a signal g (G in the figure).

Note that noise can be reduced by the same operation as in the above case also when there is line correlation and when there is no line correlation. That is, when there is no line correlation, a control signal of a level that enables the amplitude limiting operation of the limiter 11 is obtained from the large amplitude signal component taken out from the subtracter 3, and when there is line correlation, it is taken out from the subtracter 3. A control signal having a level to cancel the amplitude limiting operation of the limiter 11 is obtained from the noise component.

As a result, when there is no line correlation, a signal with reduced noise components is extracted as in the conventional circuit, and when there is line correlation, a signal with reduced noise components including pulse p is extracted.

FIG. 4 shows a block system diagram of a second embodiment of the circuit of the present invention, in which the same components as in FIG. 1 are given the same numbers and their explanations will be omitted. In the figure, a reproduced luminance signal a input to a terminal 1 is extracted by a differentiating circuit 12 in the high frequency range (edge) of 1.5 MHz or more into positive polarity and negative polarity, and is waveform-shaped by a shaping circuit 13. Signals i (FIG. 5B) are taken out for each polarity, and are passed through the AND gates 14a and 14a, respectively.
14b.

On the other hand, the limiters 9a and 9b take out a signal h (FIG. 5A) which is at H level when there is line correlation and at L level when there is no line correlation, as in the first embodiment, and the AND gates 14a and 14b is supplied to

The signal h and the signal i corresponding to the positive polarity are ANDed by the AND gate 14a, and the result is a signal j (C in the same figure).
Further, the polarity is inverted by the inverter 10, and the control signal is set to L level when there is line correlation, and set to H level when there is no line correlation. On the other hand, the signal h and the signal i corresponding to the negative polarity are connected to the AND gate 14.
If the AND is taken at b and there is a line correlation, then H
If there is no level or line correlation, the control signal j is set to L level.

The control signals j and are respectively connected to the terminals 11 of the limiters 11.
a, 11b, thereby limiter 1
1 has a line correlation, and the amplitude limiting operation is canceled only during the edge generation period, so that noise including the pulse p can be sufficiently reduced.

FIG. 6 shows a block system diagram of a third embodiment of the circuit of the present invention, in which the same components as those in FIG. 4 are given the same numbers and their explanations will be omitted. This is a cyclic type, in which a signal obtained by delaying the output of the subtracter 6 in the 1H delay circuit 2 is subtracted from the reproduced luminance signal inputted to the terminal 1 in the subtracter 3. The other operations are the same as those of the first and second embodiments, so their explanation will be omitted.

Although each of the above embodiments has a circuit configuration related to H correlation, the present invention is not limited to this, and a circuit configuration related to field correlation or frame correlation may be used.

4. Effect of Brightness As mentioned above, the noise reduction circuit according to the present invention is equipped with a circuit that detects the presence or absence of correlation in the low frequency components of the input signal and cancels the amplitude limiting operation of the limiter only when there is a correlation. If there is a correlation and there is incoming noise with pulses exceeding the limiter level, the pulses are taken out of the limiter as is, which can sufficiently reduce the noise, especially in the high frequency range (edge) of, for example, 1.5 MHz or higher.
It has features such as being able to improve the signal-to-noise ratio. and,
A circuit is installed that detects the presence or absence of correlation in the low-frequency components of the input signal, as well as the presence or absence of high-frequency components in the input signal, and releases the limiter's amplitude limiting operation only when there is both correlation and high-frequency components. In some cases, the noise at the edge portion can also be sufficiently reduced.

[Brief explanation of the drawing]

1 and 2 are a block system diagram and a signal waveform diagram for explaining the operation of the first embodiment of the circuit of the present invention, respectively;
FIG. 3 is a specific circuit diagram of a part of the circuit of the present invention, and FIG.
5 and 5 are respectively a block system diagram and a signal waveform diagram for explaining the operation of the second embodiment of the circuit of the present invention, FIG. 6 is a block system diagram of the third embodiment of the circuit of the present invention, and FIG. 7 is a conventional circuit. FIG. 8, which is an example block system diagram, is a signal waveform diagram of noise including pulses. 1... Reproduction brightness signal input terminal, 2... 1H delay circuit,
3, 6...Subtractor, 7...Output terminal, 8...Amplifier, 9
a, 9b, 11...limiter, 10...inverter,
12... Differential circuit, 13... Shaping circuit, 14a, 14
b...And gate, 15...Low pass filter.

Claims (1)

[Claims] 1. A delay circuit that delays an input signal or an output signal; a first subtraction circuit that subtracts the output signal of this delay circuit from the input signal; and a signal output from the first subtraction circuit. A noise reduction circuit having a limiter that limits the amplitude of the noise reduction circuit, and a second subtraction circuit that subtracts the signal output from the limiter from the input signal, the output signal of the first subtraction circuit being supplied, A low-pass filter that outputs only high-frequency components, and a signal output from this low-pass filter detects whether there is a correlation in the low-frequency components of the input signal, and only when there is a correlation, the limiter operates to limit the amplitude. A noise reduction circuit comprising: a circuit for canceling the noise reduction circuit; 2. A delay circuit that delays an input signal or an output signal, a first subtraction circuit that subtracts the output signal of this delay circuit from the input signal, and a limiter that limits the amplitude of the signal output from this first subtraction circuit. and a second subtraction circuit that subtracts the signal output from the limiter from the input signal, the noise reduction circuit is supplied with the output signal of the first subtraction circuit and outputs only low-frequency components. A low-pass filter, and a signal output from the low-pass filter to detect the presence or absence of correlation in the low-frequency components of the input signal, and to detect the presence or absence of high-frequency components in the input signal, and to detect the correlation and high frequency components. and a circuit for canceling the amplitude limiting operation of the limiter only when both of the range components are present.