JPH0216076B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a noise removal device that is applied to magnetic recording and reproducing devices, video disk devices, etc., and reduces noise signal components in a specific range included in recorded and reproduced video signals. .

Conventional household magnetic recording and reproducing devices have ample frequency range to use, and even if the reproduced video signal contains noise across the entire frequency band, the noise in the high frequency band is visually noticeable. Ta. For this reason, noise removal has conventionally been carried out with the main purpose of removing noise in the high frequency band, but in recent years, with the miniaturization of household magnetic recording and reproducing devices, the increase in density and the use of short wavelength bands have made it possible to eliminate noise components. However, unlike in the past, it has become necessary to remove noise in a lower frequency band than in the past. However, when attempting to remove noise in the low frequency band with conventional noise removal devices, the width of the edge portion of the signal containing high frequency components where noise is not removed increases in proportion to the low frequency band. Ta. That is, to explain the conventional noise removal device in detail, in FIG. 3 is an amplifier, 4 is an amplitude limiter, 5 is an attenuator, 6 is a mixer, 7 is an output terminal, and 7 is an input terminal. , the video signal input from the input terminal 1 is separated into high frequency components by the differentiating means 2,
The signal is amplified by an amplifier 3, limited to a constant amplitude by an amplitude limiter 4, and attenuated to an appropriate level by an attenuator 5. The mixer 6 mixes the video signal input from the input terminal 1 and the output signal of the attenuator 5 in opposite phases, and outputs a video signal with reduced noise to the output terminal 7. Figure 2 shows a specific circuit of the differentiating means, where 8 is the input terminal of the differentiating means, 9 is the capacitor, and 1
0 is a resistor, 11 is an output terminal of the differentiating means, and the capacitance value of capacitor 9 is C. If the resistance value of resistor 10 is R, the transfer characteristic of the differentiating means is SRC/(SRC+1) (S is Laplace transform variable). Now, differentiation means 2
When the video signal shown in FIG. 3a is input to the input terminal 8 of the differentiating means 2, the third
The signal shown in Figure b is output. In other words, when the input signal changes stepwise by a certain level, the output of the differentiating means 2 changes by the same level at the moment of the change, and then a discharge or charging current flows according to the sign or negative of the change in the input signal, and the output of the differentiating means 2 changes by the same level. The output potential of changes toward the potential of the reference point to which one end of the resistor is coupled. In addition, here, from capacitance 9 to resistance 1
The case where current flows to the reference point via 0 is called discharge, and the case where current flows to the capacitor 9 in the opposite direction is called charge. The charging/discharging time constant in this case is
It is RC. Then, the signal of the differentiating means 2 is amplified,
The waveform when amplitude limitation is applied is as shown in FIG. 3c. Assuming that the level at which the amplitude of the input signal is limited is V _B and the amplification degree of the amplifier 3 is K, the output waveform of the amplitude limiter 4 is amplitude limited for the portion where the output of the differentiating means 2 exceeds V _B. Therefore, it becomes a flat signal with a level of KV _B. The time width of this flat portion is determined by the change level of the input signal and the time constant RC of the differentiator circuit. For example, time t=0
Assuming a step signal that _changes by _level V _{A at}
is, t ₁ = RC ln(V _B /V _A ). That is, the amplitude of the differential signal is limited by this time interval. FIG. 3d shows an output waveform obtained by attenuating the amplitude-limited output to a predetermined level by an attenuator 5 and mixing it by a mixer 6 in a phase opposite to that of the input signal.

Next, considering the case where high frequency noise N is superimposed on the input signal as shown in FIG. 3a, the noise N is also superimposed on the output of the differentiating means 2 according to its pass band. The portion of this noise N that is not limited by the amplitude limiter 4 remains as shown in FIG. 3c and is canceled by the mixer 6.

However, since the limited portion of this noise component is clipped and the noise cannot be left behind, the output of the mixer 6 does not cancel the noise, resulting in a signal N' in FIG. 3d. Moreover, since this limited portion has a constant DC level, the DC component at the output of the mixer 6 is reduced, or in other words, the signal component is reduced. In short, this noise removal device removes noise from flat parts of the input signal, but has no noise removal effect from parts where the DC level changes or parts with AC components, and conversely the signal components decrease. There is a drawback.
Furthermore, the time width t ₁ of the portion where the output signal of the differentiating means 2 is limited and becomes flat is proportional to the time constant RC of the differentiating means 2, and this time constant represents the pass band of the differentiating means 2 and suppresses low frequency noise. In order to allow the signal to pass, RC must be increased, which increases the time width _t1 .

Therefore, it is an object of the present invention to promote the reduction of residual noise, especially immediately after a sharp level change included in a video signal, that is, at the edge portion of the video signal, and to compensate for the loss of signal components at the edge portion with a small amount. The object of the present invention is to provide a simple noise removal device.

An embodiment of this invention is shown in FIGS. 4 to 7. First, in FIG. 4, parts that operate in the same way as in FIG. 1 are given the same reference numerals. That is, 1 is an input terminal, 2 is a differentiating means, 3 is an amplifier,
4 is an amplitude limiter (limiter means); 5 is an attenuator;
6 is a mixer, 7 is an output terminal, 12 is an amplitude limiter 4
This is a rapid charging/discharging circuit that increases the charging/discharging current of the differentiating means 2 by detecting the operation of the differential means 2. This operation will be explained using FIG. 5. Here, for simplicity, we will use the input signal as the level at t=0 as shown in Figure 5a.
Using a step signal that changes to V _A , differentiating means 2
The one shown in Figure 2 is used. Fifth
Figure b shows the output of the differentiating means 2, which instantaneously changes to level V _A at t=0, and after that, if there is no rapid discharge circuit 12 according to this embodiment, the output changes over time as shown in Figure 3 above. Discharge occurs at a constant RC (dashed line in Figure 5b), and the period during which the amplitude is limited by the amplitude limiter 4 is t ₁ = -RC ln (V _B /V _A ); In the case of the embodiment, since the rapid charge/discharge circuit 12 is provided, the absolute value of the output level of the differentiating means 2 is the level at which the amplitude limiter 4 limits the amplitude.
When the output of the _{differentiating} means 2 is positive, the quick charging/discharging circuit 12 is operated, and when the output of the differentiating means 2 is positive, the discharging circuit is operated, and when it is negative, the charging circuit is operated.
As shown by the solid line in Figure b, the discharge rate becomes faster. As a result, level V _B is reached in time t ₂ which is shorter than time t ₁ . When the output of differentiating means 2 reaches level V _B ,
The discharge has a time constant RC determined by the differentiating means 2, and a signal whose amplitude is limited by time _t2 is taken out at the output of the amplitude limiter 4 (FIG. 5c). Here, it goes without saying that if the charging/discharging current is made sufficiently large, the time t ₂ can be made sufficiently small. Therefore, after _t2 , it is possible to extract noise according to the passband of the differentiating means. FIG. 5d shows the output waveform of the mixer. In this way, by selecting the current of the rapid charge/discharge circuit 12 to an appropriate value, it is possible to separate the noise immediately after the level fluctuation of the input signal, and since the flat portion in the output of the amplitude limiter 4 is extremely reduced, Loss of signal components due to this portion is also reduced. Furthermore, when the time t ₂ is made sufficiently small, the output of this noise canceling device, which is obtained by subtracting the output of the amplitude limiter from the input signal, has a characteristic in which the input signal is slightly de-emphasized, as shown in FIG. 5d. Therefore, this can be corrected by emphasis.

Note that the current of the rapid charge/discharge circuit 12 may be a constant current or may be proportional to the output level of the differentiating means. The latter case is essentially equivalent to reducing the time constant by reducing the value of the resistance constituting the differentiating means.

FIG. 6 shows the differentiating means 2. A concrete circuit of the amplitude limiter 4 and the rapid charging/discharging circuit 12, in which 8 is an input terminal of a differentiating means, a capacitor 9 and a resistor 10
constitutes a differentiating means, and resistor 13. transistor 1
4, 15 and voltage source 16 constitute an amplitude limiter, resistor 17 and transistor 18 constitute a charging circuit, and further resistor 19 and transistor 2
0 constitutes a discharge circuit. The voltage source 21 is for appropriately biasing the output of the rapid charge/discharge circuit, and can also serve as the input bias power supply for the amplifier 3. The limiter characteristics of the amplitude limiter are determined by the conduction and cutoff characteristics of the resistor 13 and the transistors 14 and 15, and when the voltage value of the voltage source 16 is _E16 and the base-emitter voltage value of the transistors 14 and 15 is _VBE , When the output voltage of the amplifier 3 is more than E ₁₆ −V _BE and less than E ₁₆ +V _BE , the transistors 14 and 15 are cut off, and the input signal is directly outputted to the attenuator 5 as the amplitude limiter output, and E ₁₆ −
Below V _BE , transistor 14 is conductive, E ₁₆ +V _BE
In the above case, the transistor 16 becomes conductive, and its output becomes approximately E ₁₆ −V _BE and E ₁₆ +V _BE , respectively. Further, when the transistors 14 and 15 are cut off, no current flows through the resistors 17 and 19, but when either of them is turned on, current flows through the resistors 17 and 19, and when the generated voltage reaches a certain value, a rapid charge/discharge circuit is formed. Either transistor 18 or 20 operates.
That is, when the output of the differentiating means 2 exceeds a certain value, the rapid charging/discharging circuit is operated to increase the charging/discharging current of the differentiating means to speed up charging and discharging. Note that the limiter characteristics can also be changed by applying an appropriate potential between the bases of the transistors 14 and 15.

Note that various modifications of the configuration of the rapid charge/discharge circuit are possible; for example, the resistor 17 may be replaced with the transistor 1.
8, connect the collector of transistor 18 to the base of transistor 20, insert a resistor with an appropriate value into the collector of transistor 20, connect the emitter of transistor 20 to the ground side of resistor 19, and connect the collector of transistor 18 to the base of transistor 20. 10 and the voltage source 21, and insert a resistor of an appropriate value in parallel with the resistor 10 constituting the differentiating means 2 when the transistors 14 and 15 constituting the amplitude limiter become conductive. It is also possible to reduce the time constant of the differentiating means 2.

FIG. 7 shows another embodiment of the present invention, which serves as both an amplitude limiter and a rapid charge/discharge circuit.
In the figure, 8 is an input terminal of the differentiating means, and a capacitor 9 and a resistor 10 constitute the differentiating means, which is biased by a voltage source 21 at its voltage value E ₂₁ .
Amplifier 22 has appropriate amplification and polarity.
The diodes 23 and 24 constitute an amplitude limiter and are coupled with opposite polarities. Now, the amplifier 22 has a sufficiently large input impedance and a sufficiently small output impedance, and when its amplification degree K is positive, the output is in phase with the input. When negative, the phase is reversed. Further, it is assumed that the input and output DC potentials of the amplifier 22 are equal. As shown in the figure, the input terminal of an amplifier 22 is directly coupled to a capacitor 9 and a resistor 10 which constitute a differentiating means, and a diode 23, which constitutes an amplitude limiter,
One end of the parallel combination of 24 is coupled to the input terminal of amplifier 22, and the other end is coupled to the output terminal. This results in
An output signal from the differentiating means is applied to one end of the diodes 23 and 24, and an AC signal obtained by multiplying this by K is applied to the other end. Therefore, a signal obtained by multiplying the AC output signal of the differentiating means by (1-K) is applied to both ends of the diodes 23 and 24. Now, when the absolute value of the signal obtained by multiplying the output signal of this differentiating means by (1-K) does not exceed the conduction potential V _D of the diode, it is taken out as an output, and when it exceeds V _D , the diode becomes conductive and the conduction potential It becomes V _D. At this time, a voltage V _D /1-K is generated at the output of the differentiating means. Here, the amplification degree K is a value of 1 or less, and the limiter characteristic is that when the amplification degree K is less than 1 or more than zero and is greater than the conduction potential V _D , when the amplification degree K is negative, that is, the input and output of the amplifier 22 is When the polarity is reversed, the value becomes smaller than the conduction potential V _D. Therefore, by appropriately selecting the value of the amplification degree K, arbitrary limiter characteristics can be obtained. This is particularly useful when the amplification factor K is negative; when it is zero, it is equivalent to the absence of the amplifier 22, and the diode 2
The other ends of 3 and 24 are equivalent to being coupled to voltage source 21. In addition, the diodes 23 and 24 also serve as a rapid charging/discharging circuit, and the output of the differentiating means is E ₂₁ −
When it tries to become smaller than V _D /1-K, it is charged by the diode 23, and when it tries to become larger than E ₂₁ +V _D /1-K, it is discharged by the diode 24. When the output impedance of the amplifier 22 is sufficiently small, the charging and discharging resistance at this time becomes the conduction resistance of the diodes 23 and 24, which is sufficiently small, and the charging and discharging becomes very fast. Note that when the charging/discharging time constant is set to an appropriate value, the output of the differentiating means and the diodes 23, 2
4. An appropriate resistor may be inserted between the connection points of the input terminals of the amplifier 22.

As described above, in the noise eliminating device of the present invention, the charging/discharging current flows through the differentiating means by the rapid charging/discharging circuit during the period when the amplitude of the high frequency component separated by the differentiating circuit exceeds the amplitude limit level of the limiter means. As a result, the period during which the amplitude of the high frequency component separated by the differentiating circuit exceeds the amplitude limit level of the limiter means in the edge portion of the input signal can be shortened, and the low frequency band of the input signal can be increased. Even if the noise of the edge part of the input signal is removed, there is very little residual noise in the edge part of the input signal, which is a drawback of the conventional method, and there is little deletion of the signal waveform, so the waveform of the deleted part has a natural shape. In order to correct this, it is only necessary to add a slight emphasis characteristic to the video signal, or to slightly reduce the de-emphasis characteristic of the reproduction circuit in the magnetic recording/reproducing device, and the effect is that an extremely faithful reproduction signal can be obtained.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a conventional noise removing device, FIG. 2 is a circuit diagram of its differentiating means, FIG. 3 is an operating waveform diagram of a conventional noise removing device, and FIG. 4 is a block diagram of an embodiment of the present invention. Figure 5 is the operating waveform diagram,
FIG. 6 is a specific circuit diagram of the main part of FIG. 4, and FIG. 7 is a circuit diagram of another embodiment of the present invention. 1...Input terminal, 2...Differentiating means, 3...Amplifier, 4
... Amplitude limiter, 5... Attenuator, 6... Mixer, 7... Output terminal, 9... Capacitance, 10... Resistor, 12... Rapid charge/discharge circuit, 13, 17, 19... Resistor, 14, 15,
18, 20... Transistor, 16, 21... Voltage source, 22... Amplifier, 23, 24... Diode.

Claims (1)

[Claims] 1 Differentiating means for separating high frequency components in an input signal; limiter means for limiting the amplitude of the high frequency components separated by the differentiating means; and limiter means for limiting the amplitude of the high frequency components separated by the differentiating circuit. a rapid charging/discharging circuit that increases the charging/discharging current flowing through the differentiating means during a period in which the amplitude limit level of the limiter means is exceeded; and a high frequency component obtained by the limiter means and the input signal that are in opposite phases to each other. A noise removal device equipped with a mixer for synthesizing at a high level and with an appropriate level relationship.