JPH0628391B2 - Noise elimination circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention extracts a relatively high frequency component from a video signal through a filter and then limits a signal of a predetermined level or higher to reverse the video signal. The present invention relates to a noise removing circuit used when removing noise from this video signal by adding in phases.

[Conventional technology]

As a noise removing method for recording and reproducing a video signal, first, as shown in FIGS. 2 (a) and 2 (b), a high-pass component F ₁ and a limiter circuit L ₁ of the video signal on the recording side are used. forming a waveform S ₂ was extracted, recording the waveform S ₃ to a high-frequency component is emphasized by adding the waveform S ₁ of the original video signal by adder circuit a _1.

Then, on the reproducing side, as shown in FIGS. 3A and 3B, the waveform S ₄ of the reproduced video signal containing noise is converted into a high-pass component including a noise component by the high-pass filter F ₂ and the limiter circuit L ₂ . Waveform S ₅ is formed and the level is adjusted through the equalizer E _1, and then the waveform S ₄ is added to the delay circuit D 5.
_{1 to} delay the predetermined time to match the phases, and adder circuit A
_{In 2} there is known a technique of adding both in opposite phases to obtain a waveform S ₆ from which a noise component is removed.

Such a noise removing method is disclosed in, for example, Japanese Patent Laid-Open No. 55-567.
As disclosed in Japanese Patent Laid-Open No. 75-75, it is a known technical means as a noise elimination method for a video signal. However, since the video signal passes through the high pass filter twice during recording and reproduction,
There is a problem that the error noise HHS _{1 which} is double differentiated remains as seen in the waveform S ₆ .

This double differential error noise HHS ₁ is a noise of a very small level, but such noise also poses a problem as image quality deterioration that cannot be ignored when dubbing is repeated.

FIG. 4 (a) shows such double differential type error noise HH.
A noise eliminating circuit on the reproducing side proposed so that S can be eliminated is also shown, and its operation will be mathematically described below.

First, the transfer function of the signal circuit of the high-pass filter F ₁ provided on the recording side of FIG. 2 (a) and FIG. 3 (a),
When the transfer functions of the signal circuits of the high-pass filters F ₂ and F ₃ of FIG. 4 (a) are represented by equivalent values H, S ₂ = HS ₁ S ₃ = S ₁ + S in FIG. 2 (a). The signal of ₂ = S ₁ + HS ₁ is obtained.

When the signal of the waveform S ₃ is recorded and then reproduced, the noise component n is added, so that S ₄ = S ₃ + n = S ₁ + HS ₁ + n.

Figure 3 (a) in the conventional example _{S 5 = HS 4 = H (} S 1 + HS 1 + n) S 6 = S 4 -S 5 = S 1 + HS 1 + n -H (S 1 + HS 1 + n) = S 1 + N (1−H) + HHS ₁ = S ₁ + HHS ₁ and the double differential error noise of HHS ₁ remains as described above.

(Note that the noise component n is limited by H
It can be regarded as _n ≒ n) Incidentally, in the noise reduction circuit of FIG. 4 (a) is 4 (as shown in the waveform diagram of _{b) S 8 = S 4 -S} 7 = S 1 + HS 1 + n- Since it is possible to make HS ₈ = S ₁ + HS ₁ + n −H (S ₁ + HS ₁ + n −HS ₈ ) = S ₁ + n (1-H) −HHS ₁ + HHS ₈ n (1-H) ≈0, S than _{8 (1-HH) = S} 1 (1-HH), becomes _S 1 = S _8, the noise component n can be removed.

However, this noise removing circuit cannot match the signal phase of the delayed waveform S ₇ obtained from the high pass filter F ₃ and the limiter circuit L _{3 with} the signal phase of the waveform S ₄ .

Therefore, in an actual circuit, it is difficult to obtain a signal from which noise is removed as seen in the waveform S ₈ , and the canceling effect is unavoidable.

[Problems to be solved by the invention]

The noise elimination circuit of the present invention is made in view of the above points, and is intended to eliminate the error noise due to double differentiation even in an actual circuit.

[Means for solving problems]

FIG. 1 (a) is a block diagram showing an embodiment of the present invention. A ₄ and A ₅ are anti-phase adder circuits, F ₄ is a high pass filter, L ₄ is a limiter circuit, E ₄ is an equalizer, and D _{4 is} Indicates a delay circuit.

In addition, RV ₁ and RV ₂ are variable resistors for level adjustment.

[Action]

The operation of this circuit will be described below with reference to the waveform chart of FIG.

The waveform S ₄ of the reproduced video signal input from the terminal T ₁ contains the noise component n, and as described above, assuming that the transfer function of the circuit passing through S ₄ = S ₁ + S ₂ + n high-pass filter F ₄ is H, the adder circuit A The output of ₄ is S ₉ = S ₄ −S ₁₀ and S ₁₀ = HS ₉ = H (S ₄ −S ₁₀ ) = H (S ₁ + S ₂ + n−S ₁₀ ) S ₁₀ = H {(S ₁ + S _{2 + n) -H (S 1 +} S 2 + n-S 10)} = HS 1 + HS 2 + Hn-HHS 1 -HH ( since it is _{S 2 + n-S 10)} HS 1 = S 2, Hn = n, S 10 = S ₂ + n-HH (S ₂ + n-S ₁₀ ) ∴ (1-HH) S ₁₀ = (1-HH) (S ₂ + n) S ₁₀ = S ₂ + n On the other hand, in the addition circuit A ₅ , S ₁₁ = S ₄ -S _{'10 (where, S' 10 = S 10)} = S 1 + S 2 + n- (S 2 + n) = S 1 Therefore, Video signal to remove the same noise component n and the waveform S ₁ of the recording time of the video signal can be extracted.

The signal actually calculated by the adder circuit A ₅ can be a signal (S ₄ ′) that has passed through the delay circuit D ₄ and a signal (S ′ ₁₀ ) whose frequency characteristic has been improved by the equalizer E ₄ . Delay and waveform distortion due to signal processing can be corrected by the delay circuit D ₄ and the equalizer E ₄ .

[Effect of the Invention] As described above, the noise removing circuit (noise canceller) of the present invention can effectively remove the double differential type error noise generated in the signal conversion circuit including the high-pass filter of the recording-reproducing system. The effect is that you can do it.

[Brief description of drawings]

1 (a) and 1 (b) are block diagrams and waveform diagrams of the noise elimination circuit of the present invention, and FIGS. 2 (a) and 2 (b) are circuit diagrams and waveforms thereof for emphasizing a high frequency of a video signal. FIGS. 3 (a) and 3 (b) are a general circuit diagram for removing a noise component from a reproduced video signal and its waveform diagram, FIG. 4 (a),
FIG. 6B is a block diagram of a noise removing circuit that can remove double differential type error noise. In the figure, F ₁ , F ₂ , F ₃ , and F ₄ are high-pass filters, L
₁ , L ₂ , L ₃ , and L ₄ are limiter circuits, D ₄ is a delay circuit, E ₄ is an equalizer, and A ₄ and A ₅ are adder circuits.

Claims (1)

[Claims] An input terminal to which a reproduced video signal is input, a signal conversion circuit including a high-pass filter and a limiter circuit for extracting a high frequency component of the reproduced video signal, and a reproduced video supplied from the input terminal. The output of the signal conversion circuit is subtracted from the signal, the subtraction output is supplied to the input side of the signal conversion circuit, and the output of the signal conversion circuit is subtracted from the reproduced video signal. A noise removal circuit comprising a second subtraction circuit for obtaining a removed output signal.