JP2502277B2 - Noise suppression circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a noise suppression circuit used when a video signal (image signal) is reproduced in a VTR, a video disk (VD) or the like, and particularly to a wide frequency band. The present invention relates to a noise suppression circuit that can effectively suppress noise.

[Conventional technology]

In a VTR, a video disc, etc., a normal noise suppression circuit is provided in the video signal reproduction system.

FIG. 13 shows a conventional example of a noise suppressing circuit (10) provided in such a reproducing system.

In this figure, the input video signal S _I (Fig. 14A) supplied to the terminal (1) is supplied to the primary high-pass filter (2) and is limited to the frequency band as shown in Fig. 15. The high-frequency video signal S _H (FIG. 14B) containing the noise component is extracted, and this is supplied to the limiter (4) via the amplifier (3) to suppress the high-frequency video signal. As shown in FIG. 14C, the limiter output S _L having almost no noise component is obtained. Limiter output S _L is further VOLUME (5)
After being level-adjusted in step S1, it is added in the adder (6) together with the original video signal S _I. At this time, the limiter output S _L is added in an inverted state.

As a result, since the limiter output S _L is added in anti-phase to the noise component contained in the original video signal S _I , the output video signal S _O in which the noise component is suppressed as shown in FIG. ) Is obtained.

[Problems to be solved by the invention]

By the way, such a noise suppression circuit (10) can sufficiently suppress only noise in a specific frequency band, and has a drawback that noise distributed over the entire frequency band cannot be effectively suppressed.

That is, the phase characteristic of the high pass filter (2) is the 16th
The curve a shows the curve, and the phase characteristic due to the propagation delay time of the limiter (4) including the amplifier (3) shows the curve b. Therefore, the total phase from the terminal (1) to the volume (5) is shown. The characteristic is as shown by the curve c.

On the other hand, since the original video signal S _I is supplied to the adder (6) without being given the phase characteristic, the phase characteristic of the S _L component output as the output video signal S _O has a phase opposite to that of the curve c. As a result, the noise can be effectively suppressed only in the angular frequency ω _O where the phase is zero and the vicinity band including the frequency, and the noise component existing in other frequency bands remains. .

That is, as shown in FIG. 17, if the magnitude of the noise component (noise power) included in the input video signal S _I is represented by the curve d, the noise power remaining in the output video signal S _O is the curve e.
Therefore, the noise suppression effect is maximum at the angular frequency ω _O where the phase is zero, and the other noise suppression effects are not sufficient. In addition, the noise power may increase more than the input in the high frequency region. Therefore,
The effect of S / N improvement cannot be expected very much.

Therefore, the present invention solves such a conventional problem, and proposes a noise suppression circuit capable of suppressing a noise component almost uniformly over the entire frequency band.

[Technical means for solving problems]

In order to solve the above problems, according to the present invention, as shown in FIG. 1, a primary phase equalizer (15) is provided in the transmission system of the original video signal S _I and the signal transmission for extracting the noise component is performed. The system is provided with a secondary high pass filter (11).

And those phase characteristics have only a fixed phase difference,
The respective phase constants are selected so that the other characteristics are completely the same.

[Action]

According to this configuration, the phase characteristic of the secondary high-pass filter (11) is as shown by the curve f in FIG. 3, and the phase characteristic of the primary phase equalizer (15) is as shown by the curve g. The phase difference between them is π (= 180 °) over the frequency band.
Therefore, if in-phase addition is performed with the limiter output S _L shown in FIG. 2 and the original video signal S _I that has passed through the primary phase equalizer (15), the noise of the output video signal S _O is spread over the entire frequency band. Oppressed.

Considering this with reference to noise power,
It looks like Figure 4. The curve h in the same figure is the input noise power, and the curve i is the output noise power. As is apparent from this, the output noise power is the high pass filter (1
It is easy to understand that it is sufficiently suppressed in the band above the low cutoff frequency ω _{C of} 1).

〔Example〕

FIG. 1 is a system diagram showing an example of a noise suppression circuit (10) according to the present invention, in which an input video signal S _I supplied to a terminal (1) is supplied to a secondary high-pass filter (11),
Similar to that shown in Fig. 14, the high-frequency video signal S _H containing a noise component
Is extracted (Fig. 2B) and is supplied to the limiter (13) via the amplifier (12) to suppress the high frequency image signal and output the limiter output S _L (Fig. C) of almost only noise components. This is further supplied to the adder (17) via the level adjusting volume (14).

The input video signal S _I is further phase-equalized by the primary phase equalizer (15) and then supplied to the adder (17) via the delay circuit (16). Delay circuit (16) is an amplifier (12)
And the propagation delay time caused by the limiter (13).

The original video signal S _I that has passed through the primary phase equalizer (15)
And the limiter output S _L are added in phase.

By the way, the transfer function of the above-mentioned first-order phase equalizer (15)
H _EQ (s) (where s = jω and ω is the input angular frequency) is And the phase characteristic φ _EQ (s) at that time is If this is illustrated, it becomes like the curve g in FIG.

On the other hand, the transfer function of the second-order high-pass filter (11)
H _HPF (s) is generally However, since ω _C is given at a cutoff angular frequency of about 1 MHz, Q _C (= 0. Of the second-order high-pass filter (11).
When (5) is selected, equation (3) is as follows.

The phase characteristic φ _HPF (s) when Q _C is selected as described above is Becomes This is shown in FIG. 3 as curve f. Therefore, among the (2) phase characteristic phi _EQ of formula (s), omega _O = when selected omega to _C ... (8), the phase characteristics phi _EQ primary phase equalizer (15) (S) is Becomes As a result, as is clear from the equations (7) and (9), the phase of the primary phase equalizer (15) and the phase of the secondary high-pass filter (11) are completely different except for the fixed phase value of π. Match. That is, the curves f and g are over the entire frequency band,
Even if the phase is shifted by π, the amount of phase at each frequency completely matches 180 ° except for the fixed phase of π.

Therefore, as shown in FIG. 1, the limiter output S _L including the phase characteristics due to the propagation delay time and the original video signal S _I also delayed by the time corresponding to the propagation delay time are added to the adder (17). In-phase addition allows noise components mixed in the original video signal S _I due to the limiter output S _L to be effectively suppressed over the entire frequency band.

That is, as shown in FIG. 4, the noise power existing in the original video signal, that is, the input video signal S _I , is distributed with a uniform intensity over the entire frequency band as shown by the curve h. When this is supplied to the noise suppression circuit (10), the cutoff angular frequency ω
_In the frequency band above _C , the noise power is suppressed uniformly as shown by the curve i.

Although the noise power is not sufficiently suppressed in the frequency band near the cutoff angular frequency ω _C , this is because the noise suppression circuit (10) itself has a relatively high frequency range among the noise components included in the input video signal S _I. This is for the purpose of suppressing the noise component existing on the side, and for suppressing the noise component on the low frequency side.

By the way, the above-mentioned secondary high-pass filter (11) is the fifth
A high pass filter having an R, C, L configuration as shown in the figure can be used. On the other hand, as the primary phase equalizer (15), an equalizer using a transformer T and a capacitor C as shown in FIG. 9 can be used.

When using the secondary high-pass filter (11) shown in Fig. 5, If each phase constant is selected, the transfer function H _HPF (s) shown in equation (6) and the phase characteristic φ shown in equation (7)
_HPF (s) can be obtained.

6 to 8 show another example of the secondary high-pass filter (11) usable in the present invention. That is, as shown in FIG. 6, the primary high-pass filter (1
If two 1A) and (11B) are used and these are cascade-connected via a buffer amplifier (11C), a secondary high-pass filter (11) can be constructed, and as shown in FIG. Primary high-pass filter (12A), (12
A second-order high-pass filter (11) can be configured by using two B) and connecting them in series via a buffer amplifier (11C). Furthermore, as shown in Fig. 8, C, R
A high-pass filter (13A) of the primary configuration and a high-pass filter (13B) of the R, L configuration are used, and these are connected in cascade via a buffer amplifier (11C). Can be configured.

FIG. 10 shows another embodiment of the noise suppressing circuit (10) according to the present invention. In this example, the second-order high-pass filter and 2
This is a case where the following low-pass filters are combined to obtain the phase characteristics of the curve f in FIG.

In this case, a secondary Lag-Lead low pass filter is used as the low pass filter (21), which is connected to the next stage of the limiter (13), and an amplifier ( The limiter (23) is further connected via 22).

In this configuration, the cutoff frequency of the secondary high-pass filter (11) is set to a higher cutoff frequency than that shown in FIG. 2-3M in this example
Set to Hz.

In this way, when the cutoff frequency of the secondary high-pass filter (11) is selected to be high, the high frequency components of the input video signal S _I extracted by this high-pass filter (11) are reduced and the output video signal is reduced. This is because the high frequency characteristics of S _O are improved.

Now, in the configuration of FIG. 10, the transfer function H _LPF (s) of the quadratic low-pass filter (21) is Given in. Where K is a constant and ω _H and ω _L are parameters that determine the shape of the secondary low-pass filter characteristic.
ω _H is the angular frequency of the shoulder portion on the high frequency side, ω _L is the angular frequency of the shoulder portion on the low frequency side, and ω _H > ω _L.

Now, in this configuration, the transfer characteristics that the noise component receives by the filters (11) and (21) are: Becomes Therefore, If the phase constant of the second-order low-pass filter (21) is selected so that, the transfer characteristic shown in equation (12) becomes Therefore, since the transfer characteristics are equivalent to those when the secondary high-pass filter (11) shown in FIG. 1 is used, the angular frequency ω _{O of the} primary phase equalizer (15) is also in the case of FIG. hand, so that the _{ω L = ω O ... (15} ), if selected omega _L of the secondary low-pass filter (21), the phase characteristic as shown in FIG. 3 in this configuration is obtained, noise The component can be effectively suppressed over the entire frequency band.

In the configuration shown in FIG. 10, the delay time of the delay circuit (16) is equal to that of the amplifiers (12), (22) and the limiter (13),
It is set to the total propagation delay time including (23).

 FIG. 11 shows still another embodiment of the present invention.

In the video signal reproducing apparatus as described above, a low-pass filter is generally provided in front of the noise suppressing circuit (10),
The band of the reproduced video signal is limited and then supplied to this noise suppression circuit (10) to execute noise suppression processing. In this case, a phase equalizer (31) is provided between the low-pass filter (30) and the noise suppression circuit (10) in order to compensate for the delay characteristic generated by the low-pass filter (30) shown in FIG. It is provided.

That is, the delay characteristic of the low-pass filter (30) is
In the case of the curve j shown in the figure, the phase characteristic of the phase equalizer (31) is added so as to compensate for this delay characteristic,
The characteristic is shown by the curve m in FIG. In this case, in the conventional example as shown in FIG. 13, since the input video signal S _I is directly supplied to the adder (6), the phase equalizer (31) at this time is It is necessary to use the primary phase equalizer indicated by the curve k shown in FIG. 12 and the secondary phase equalizer indicated by the curve l.

Therefore, the conventional structure has a drawback that the structure of the phase equalizer (31) is complicated.

However, in the case of the configuration according to the present invention, since the primary phase equalizer (15) is already provided in the transmission system of the original video signal, the primary phase equalizer (15) is used. It can also be used as a phase equalizer for delay characteristic compensation in the low-pass filter (30). Therefore, the phase equalizer (31) can be configured by only the secondary phase equalizer, and the configuration can be simplified.

If the delay characteristic of the low-pass filter (30) is not the curve j in FIG. 12 but the delay amount increases on the higher frequency side than this, then the primary and secondary phase equalizers Must be cascade-connected in multiple stages, but in such a case, according to the present invention, the structure of the phase equalizer (31) is simplified by the amount of the primary phase equalizer (15). It is possible.

〔The invention's effect〕

As described above, according to the present invention, the primary phase equalizer is provided in the transmission system of the original video signal S _I , and the secondary high-pass filter is provided in the signal transmission system for extracting the noise component so that their phase characteristics are The phase constants of the two are selected so that they have only a fixed phase difference and the other characteristics completely match.

According to this configuration, the phase difference between them becomes π (= 180 °) over the entire frequency band, so that the noise component of the output video signal S _O can be suppressed over the entire frequency band.

Therefore, the noise suppression effect superior to the conventional one is exhibited, and the reproduced image quality can be significantly improved.

Furthermore, in the conventional noise suppression circuit, the limiter output S _L
By the high-frequency component of the video signal contained in connexion, the original image signal S _I
Since the high frequency component of is subtracted, the high frequency characteristic is deteriorated. On the other hand, in the present invention,
Since the S / N improvement effect can be obtained, if the volume (14) is narrowed down to obtain the same S / N improvement effect as the conventional one, the high frequency component of the video signal included in the limiter output S _L ( The level of the positive and negative contour pulse components generated by the secondary high-pass filter (11) is also extremely small, so that the high frequency component of the original video signal S _I is subtracted by this. Also less.

Therefore, in this case, in addition to the effects described above, the high frequency characteristics of the output video signal S _O can be improved as compared with the conventional case.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a system diagram showing an example of a noise suppressing circuit according to the present invention, FIG. 2 is a waveform diagram for explaining its operation, and FIGS. 3 and 4 are also for explaining its operation. FIG. 5 to FIG. 8 are characteristic curve diagrams to be provided, FIG. 5 to FIG. 8 are configuration diagrams showing an example of a secondary low-pass filter, FIG. 9 is a configuration diagram showing an example of a primary phase equalizer, and FIG.
11 and 12 are system diagrams showing another example of the present invention, FIG. 12 is a characteristic curve diagram for explaining the operation of FIG. 11, FIG. 13 is a system diagram of a conventional noise suppression circuit, and FIG. 14 is Waveform diagrams for explaining the operation, and FIGS. 15 to 17 are characteristic curve diagrams for similarly explaining the operation. (10) is a noise suppression circuit, (11) is a secondary high-pass filter, (13) and (23) are limiters, (15) is a primary phase equalizer, (21) is a secondary low-pass filter, (17). Is an adder,
S _I is the original (input) video signal, S _H is the high frequency video signal, S _L is the limiter output, and S _O is the output video signal.

Claims (1)

(57) [Claims] 1. An input video signal is supplied to a primary phase equalizer and a secondary high pass filter, and a high band video signal containing a noise component output from the secondary high pass filter is level-limited by a limiter. , The level-limited high-frequency video signal is added to the original video signal output from the primary phase equalizer, and the phase characteristics of the secondary high-pass filter and the phase of the primary phase equalizer are added. A noise suppression circuit in which each phase constant is selected so that the characteristic has a phase of 180 degrees over the entire frequency band.