JPH0490271A - Noise eliminating circuit - Google Patents

Abstract

PURPOSE:To prevent fog of an edge of a picture at a dark pattern and to sufficiently eliminate noise at a dark pattern by providing a level detection means detecting a level of an input composite video signal and a level control means controlling a level of an output signal of a limiter in response to a detection level of the level detection means to the noise reduction circuit. CONSTITUTION:A noise reduction circuit 6 controls a gain G of a variable gain amplifier (VCA) 64 in response to a video signal level so that the gain G is almost O in the vicinity of,e.g. a white peak level and almost 1 in the vicinity of the pedestal level. That is, when the video signal level is high, the gain G of the VCA 64 is decreased to reduce a subtraction component from an original signal. Thus, even when a BPF 61 extracts a noise component and a high frequency component of the video signal, since the high frequency component is not reduced from the original signal, fog of an edge of the picture is prevented. On the other hand, when the video signal level is low, the gain G of the VCA 64 is increased to increase the reduction component from the original signal. Thus, since the noise component is sufficiently cancelled, the noise component included in the original signal is sufficiently eliminated.

Description

TECHNICAL FIELD The present invention relates to a noise removal circuit that removes noise components contained in a composite video signal.

BACKGROUND TECHNOLOGY As shown in FIG. 10, a conventional circuit of this type includes a BPF (band pass filter) 6 from an input composite video signal.
1, the extracted noise component is passed through a limiter 63 to limit its amplitude, and the subtracter 62 subtracts this amplitude-limited noise component from the input composite video signal, which is the original signal. A configuration that removes noise components included in a composite video signal is known. FIG. 11 shows waveforms at various parts in a circuit having such a configuration.

Furthermore, although the basic configuration is the same, there is also a configuration in which a notch filter for attenuating the color signal acid content is placed before or after the BPF61 (Utility Model Application No. 60-153061). (see publication).

In this way, the noise component (
Noise component (c) obtained by extracting b) and limiting the amplitude
is subtracted from the input composite video signal (a). In the conventional circuit configured as shown in FIG. As shown in the figure, the high frequency components (edge portions) of the composite video signal are crushed, which has the disadvantage of blurring the edges of the image and deteriorating the representation of details.

SUMMARY OF THE INVENTION [Object of the Invention] Accordingly, an object of the present invention is to provide a noise removal circuit that effectively removes noise while preventing blurring of the edges of an image, thereby making it possible to improve the image quality.

[Structure of the Invention] A noise removal circuit according to the present invention includes an extraction means for extracting a noise component from an input composite video signal, and a limiter that limits the amplitude of the extracted output of the extraction means, and the output signal of the limiter A noise removal circuit configured to remove noise components by subtracting from the input composite video signal, the circuit comprising: level detection means for detecting the level of the input composite video signal; and level control means for controlling the level of the output signal of the limiter accordingly.

[Operation of the Invention] In the noise removal circuit according to the present invention, the signal level of the input composite video signal is detected, and the output signal of the limiter that limits the amplitude of the noise component extracted from the input composite video signal is adjusted according to the detected level. Control levels.

Example Hereinafter, embodiments of the present invention will be described in detail based on the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention applied to, for example, a video signal processing system of a recording medium performance device.

In the figure, a recording medium such as a video disc (not shown)
RF multiplied signal R read from HP via F amplifier 1
Only the video band component is extracted by F (bypass filter) 2, the amplitude is limited by a limiter 3, and the resulting signal is supplied to a demodulation circuit 4. The demodulation circuit 4 has, for example, an FM detector configuration based on the same principle as a well-known quadrature detector, and instead of a 90' phase shifter in the quadrature detector, a D
elayed-MMV (mono multivibrator) 4
1 is used. Then, the positive phase output of the multiplier 42 passes through an LPF (low pass filter) 43 and is derived as a composite video signal. This composite video signal is subjected to time axis correction by a time axis correction circuit and then supplied to a noise removal circuit 6 according to the present invention.

In the noise removal circuit 6 according to the present invention, the input composite video signal is supplied to the BPF 62 and becomes the subtracted input of the subtracter 61. The BPF 62 extracts noise components contained in the input composite video signal. The extracted noise component is amplitude limited by a limiter 63 having amplitude limiting characteristics as shown in FIG. The amplitude-limited noise component is level-controlled by a VCA 64, which is a variable gain amplifier, and then becomes a subtraction input of a subtracter 62, where it is subtracted from the input composite video signal, which is the original signal. Gain G of VCA64 is 0<G<1
It is variable within the range of . As is clear from Figure 3, VC
Even if the gain G of A64 is changed, the limiter 63 and V
The limit width for noise components that have passed through the CA 64 remains unchanged.

Incidentally, when an image with a high video signal level, ie, a bright image, has edges, the blur becomes more noticeable, but on the other hand, there is a visual characteristic that small noises are less noticeable in a bright image. That is, as shown in FIG. 4, in the case of a dark image (a) with a low video signal level, the noise level cannot be ignored relative to the video signal level, but in the case of a bright image (b) with a high video signal level, In this case, the noise level can be ignored compared to the video signal level.

Focusing on this visual characteristic, in the noise removal circuit 6 according to the present invention, the gain G of the VCA 64 is changed from G to 0 in the vicinity of the white peak level according to the video signal level.
, G41 is controlled near the pedestal level. That is, when the video signal level is high, the gain G of the VCA 64 is lowered to reduce the subtracted component from the original signal. As a result, even if the BPF 61 extracts the high-frequency components of the video signal along with the noise components, the high-frequency components are not subtracted from the original signal, thereby making it possible to prevent the edges of the image from being blurred. In addition,
At this time, although the noise removal accuracy decreases, the S/N ratio is good in bright images, so the noise is not noticeable. on the other hand,
When the video signal level is low, the gain G of VCA64
Increase the subtraction component from the original signal. As a result, the noise components can be sufficiently canceled out, so that the noise components included in the original signal can be sufficiently removed.

Note that the gain G of the VCA 64 does not necessarily need to be changed linearly according to the video signal level, and the gain may be changed stepwise in about two or three steps.

Further, in the above embodiment, the VCA 6 is provided after the limiter 63.
Although the output level of the limiter 63 is controlled by controlling the gain of the VCA 64, the configuration is not limited to this. As shown in FIG. 5, an amplifier 66 with a fixed gain (for example, about 0.7 times) is installed after the limiter 63.
The limit width of the limiter 63 may be controlled according to the output of the LPF 65;
It is also possible to control the limit width of the VCA 64 and the gain of the VCA 64 at the same time. FIG. 6 shows input/output characteristics when controlling the limit width of the limiter 63.

Next, detection of the video signal level for gain control of the VCA 64 will be described. When a video disk with digital audio is used as a recording medium, the frequency spectrum of the recorded signal is as shown in FIG. A voltage corresponding to the video signal level is derived as the negative phase output of the multiplier 42 by performing FM detection on the RF multiplied signal read from the video disc by the demodulation circuit 4 which has the same principle as a quadrature detector. There is an inversely proportional relationship between the video signal level and the carrier frequency as shown in FIG. Therefore, by using the DC component of the negative phase output of the multiplier 42 as the control voltage of the VCA 64 via the LPF 65, the VCA
This means that 64 gains G can be controlled.

In this embodiment, as a control voltage for gain control of the VCA 64, an F
Multiplier 42 in demodulation circuit 4 with M detector configuration
Although a case has been described in which a negative phase output is used, the present invention is not limited to this, and in short, any voltage corresponding to the video signal level may be used. For example, since there is a direct proportional relationship between the video carrier frequency and the video signal level in the read RF multiplied signal from a video disc as shown in FIG. By performing F-V conversion on the voltage, a voltage corresponding to the video signal level can be obtained. Even more simply, it is also possible to obtain the DC component of the composite video signal, which is the FM demodulated output, using an LPF and use this as the control voltage for the VCA 64.

Note that when the noise removal circuit according to the present invention is applied to, for example, a video signal processing system of a video disc player, the carrier frequency changes depending on the rotational speed of the video disc when the spindle servo is not locked. Although there is a concern that problems may occur with this configuration, video disc players usually squelch the video output when the spindle servo is unlocked, so there is no practical problem even if the carrier frequency fluctuates. However, in the squelch state, VCA64
■ Set the gain G to the gain corresponding to the gray level.

■ Hold the gain just before the squelch is applied.

By adopting such methods, the following effects can be obtained. That is, in the case of method (2), there is no sudden gain change even if the video signal level after the squelch is canceled is a level corresponding to white or black. In the case of method (2), especially when the squelch is for a short time, there is a high possibility that the image immediately before the squelch is similar to the image after the squelch is canceled, so that fluctuations in gain can be suppressed. Alternatively, method (2) may be used for a short-time squelch, and method (2) may be used for a long-time squelch.

As described in detail, the noise removal circuit according to the present invention is configured to detect the signal level of the input composite video signal and control the output level of the limiter according to this detection level. On the screen, it is possible to prevent the edges of the image from being blurred and achieve high image quality, while on the other hand, it is possible to perform sufficient noise removal on a dark screen.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the present invention applied to a video signal processing system of a recording medium performance device, FIG. 2 is a diagram showing the amplitude limiting characteristics of the limiter, and FIG. 3 is a diagram showing the gain control of the VCA. FIG. 4 is a waveform diagram showing the relationship between the video signal level and the noise level.
(a) shows the case of a dark image, (b) shows the case of a bright image, FIG. 5 is a block diagram showing another embodiment of the present invention, and FIG. 6 shows the case of controlling the amplitude width of the limiter. Figure 7 is a diagram showing the input/output characteristics; Figure 7 is a diagram showing the frequency spectrum of the recording signal of a video disc with digital audio; Figure 8 is a diagram showing the relationship between the carrier frequency and the video signal level at the reverse phase output of the multiplexer; Figure 9 is a diagram showing the relationship between the carrier frequency and the video signal level when the RF multiplied signal carrier frequency is directly F-V converted.
The figure is a block diagram showing a conventional circuit, and FIG. 11 is a waveform diagram of each part in the circuit of FIG. Explanation of symbols of main parts 3.63...Limiter 6...Noise removal circuit 42...Multiplexer 4...Demodulation circuit 64... VCA

Claims (3)

[Claims] (1) An extraction means for extracting a noise component from an input composite video signal, and a limiter for limiting the amplitude of the extracted output of the extraction means, and by subtracting the output signal of the limiter from the input composite video signal. A noise removal circuit configured to remove noise components, the circuit comprising: level detection means for detecting the level of the input composite video signal; and control of the level of the output signal of the limiter according to the detection level of the level detection means. What is claimed is: 1. A noise removal circuit comprising: level control means for controlling the noise level; (2) A noise removal circuit characterized in that the level control means includes a variable gain amplifier provided after the limiter and whose gain changes according to the detection level. (3) The noise removal circuit, wherein the level control means includes the limiter, and the limit width of the limiter changes depending on the detection level.