JPS6314517Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a noise removal circuit that removes noise from a luminance signal or a color difference signal.

As shown in the block diagram and circuit diagram of FIGS. 1a and 1b, the conventional noise removal circuit includes a primary high-pass filter 1, a limiter 2 that limits the output of the high-pass filter 1, and a limiter 2 that limits the output of the limiter 2 to the high-pass filter 1. and a subtraction means 3 for subtracting from the input.

Now, as shown in FIG. 2a, the input signal 9 contains noise.
is applied, the high-pass filter 1 outputs only high-frequency components as the output of the high-pass filter 1, as shown in FIG. 2b. The amplitude of this high-frequency component is limited by limiter 2, and the limiter 2
The output signal waveform becomes a signal with the signal component removed as shown in FIG. 2c, and noise can be removed by subtracting it from the input signal shown in FIG. 2a.

However, when the conventional noise elimination circuit is used as described above, noise is not eliminated when the limiter 2 is in operation, that is, at the contour portion of the signal component, and the output signal has the drawback of being as shown in FIG. 2d.

Therefore, in the case of a signal with a narrow band such as a color difference signal, the contour part spreads, so the part where noise cannot be removed becomes large, and although it can be used for a luminance signal, it cannot be used for a color difference signal. The disadvantage is that it cannot be done.

The present invention has been made in view of the above, and aims to provide a noise removal circuit which can eliminate the above-mentioned drawbacks and can also effectively work on color difference signals to remove noise.

The present invention will be explained below with reference to examples.

FIG. 3 is a circuit diagram showing the basic circuit of the present invention.

The basic circuit of the present invention forms a push-pull emitter follower circuit with transistors Tr ₁ and Tr ₂ ,
A resistor _R1 is connected between the bases of transistors Tr ₁ and Tr ₂ .
A series circuit with R ₂ is connected, and the input signal is applied to a common connection point between resistors R ₁ and R ₂ . Note that in the circuit shown in Fig. 3, the transistor Tr ₃
The input signal is applied to the common connection point of the resistors _R1 and _R2 via a buffer composed of an emitter follower. On the other hand, a capacitor C ₁ is connected to the output point of the push-pull emitter follower consisting of transistors Tr ₁ and Tr ₂ .
A current path consisting of a resistor _R3 is formed between the output point and the common connection of resistors _R1 and _R2 .

In the basic circuit shown in Figure 3, the output of the push-pull emitter follower is a transistor.
It is configured to output via a buffer amplifier consisting of Tr ₄ .

Note that I ₁ and I ₂ are constant current sources for base bias of transistors Tr ₁ and Tr ₂ , and their current values are equal to I.

In the basic circuit of the present invention configured as described above, the voltage between the bases of transistors Tr ₁ and Tr ₂ is V BE (Tr ₁ ) of transistor Tr ₁ + V _BE (Tr 1 ) of transistor Tr _2.
It is assumed that V _BE is set smaller than (Tr ₂ ).

First, when the level of the input signal is low, transistors Tr ₁ and Tr ₂ are in an off state, and the input signal passes through resistor R ₃ , is integrated by capacitor C ₁ , and is output. That is, when the level of the input signal is low, it passes through a low-pass filter consisting of resistor _R3 and capacitor _C1 . In this state, the input signal level is (V _BE (Tr ₁ ) − R ₁ I) + (V _BE (Tr ₂ )
−R ₂ I). Usually Tr ₁ and
Since Tr ₂ is set to V _BE (Tr ₁ ) = V _BE (Tr ₂ ) and R ₁ = R ₂ , the threshold level is 2V _BE (Tr ₁ ) - 2RI.

Next, at the rising edge of the input signal, when the voltage at the common connection point e between resistors R ₁ and R ₂ becomes higher than the voltage at the output point f of the push-pull emitter follower by more than (V _BE - RI), the transistor Tr ₁ turns on. Therefore, the potential difference between the common connection point e and the output point f is maintained at (V _BE −RI). When the input signal rises, the transistor _Tr1 is turned off, and thereafter the input signal is outputted through the resistor _R3 . Conversely, when the input signal falls and the voltage at the common connection point e becomes lower than the voltage at the output point f by (V _BE - RI), the transistor Tr ₂ turns on, and the common connection point e and the output point f The potential difference between (V _BE −UI)
When the input signal falls, the transistor Tr ₂ is turned off. After that, the input signal is outputted through the resistor R ₃ . In this way, minute noise contained in the input signal is removed by passing through a low-pass filter consisting of resistor R ₃ and capacitor C ₁ , and large amplitude signals are removed by transistors Tr ₁ and Tr _2. Almost flat frequency characteristics can be obtained. This state is shown in Figure 4 for the rising portion of the input signal.
The dashed line is the threshold level, which is the constant current source I ₁
and the current value I of _I2 .

Next, a case where there is noise in the rising edge of the input signal will be explained. When an input signal containing noise at the rising edge, such as the input signal shown in FIG. 5, is applied, the noise is removed during period _t1 by a low-pass filter consisting of resistor _R3 and capacitor _C1 . During period t ₂ , transistor Tr ₁ is turned on,
The output signal rises while maintaining the difference between the input signal and (V _BE - RI), and in period _t3 , there is no noise, so the input signal falls, so transistor _Tr1 is turned off, and the signal is output through the low-pass filter. This state is maintained until the input/output potential difference exceeds (V _BE - RI). In period t ₄ , transistor Tr ₁ is again in the on state, and the output signal rises while maintaining the difference between the input signal and (V _BE - RI), and in period t ₅ , transistor Tr ₁ is in the off state and the input signal is is output through a low-pass filter. Therefore, the noise in the rising portion is also reduced. Although the output signal waveforms are shown as angular in FIG. 5, it goes without saying that in reality they are connected by smooth curves.
Furthermore, even if noise is present in the rising edge of the input signal, the transistor Tr ₂ acts in the same manner, reducing the noise.

FIG. 6 is a specific circuit diagram of an embodiment using the basic circuit of the present invention.

In this specific circuit, the input and output DC levels are completely equal and the gain is approximately 1.

Input signals are diodes D ₁ , D ₂ and transistors
A push-pull emitter follower consisting of Tr ₅ and Tr ₆ and constant current source circuits I ₃ and I ₄ receives the input signal and converts it to low impedance. The low current source circuit _I1 is composed of a transistor _Tr7 and a diode _D3 , and the low current source circuit _I2 is composed of a transistor _Tr8 and diodes _D4 and _D5 , and resistors _R1 and R2 _. The threshold level is set by the current flowing through. diode
_D5 is a diode for temperature compensation at the threshold level. The output points of transistors Tr ₁ and Tr ₂ are grounded through capacitor C ₁ , and transistors Tr ₁ and Tr ₂ can sufficiently drive capacitor C ₁ within the used band, and the rising and falling edges of the signal will not be dulled. do not have. Also transistors Tr ₂ and Tr ₂
The voltage at the output point of is applied to the base of one transistor Tr ₉ constituting the differential amplifier, and the voltage at the common connection point of resistors R ₁ and R ₂ is applied to the base of the other transistor Tr ₁₀ constituting the differential amplifier. Then, feedback is applied by a differential amplifier so that the common connection point of resistors R ₁ and R ₂ and the DC level of the output points of transistors Tr ₁ and Tr ₂ are made equal. In addition, diodes D ₆ and D ₇ are connected to the collector of transistor Tr ₉ and the collector of transistor Tr ₁₀ , and transistor Tr ₁₁ , which forms a current mirror circuit together with diode D ₇ , is connected to the output point of transistors Tr ₁ and Tr ₂ and the power supply +V. _cc and connect the current flowing through the diode _D6 to the diode
The current flowing through transistor Tr ₁₂ is transferred to transistor Tr 12 which forms a current mirror circuit with D ₆ , and the current flowing through transistor Tr ₁₂ is passed through diode D ₈ to form a current mirror circuit together with diode D ₈ .
The configuration is such that the transistor Tr ₁₃ is connected between the output point of Tr ₁ and Tr ₂ and the ground.

Therefore, the low-pass filter characteristics at a minute level are determined by the capacitor C ₁ and the current flowing through the transistors Tr ₁₁ and Tr ₁₃ .

The operation of this specific circuit configured as described above is similar to that of the basic circuit described above, and it can be easily integrated into an integrated circuit.

Having explained the noise removal of color difference signals above,
If the capacitance of the capacitor _C1 is made small and the band is set from DC to 4.2MHz, it can also be used for a luminance signal system.

As explained above, according to the present invention, noise can be removed not only in the flat portion of the input signal but also in the rising and falling portions, and has almost no effect on the rising and falling portions of the signal. Therefore, it is very effective in removing noise not only from luminance signals but also from color difference signals, and does not change the DC level between input and output. In addition, only one capacitor is required, making it suitable for integrated circuit implementation. Furthermore, the threshold level can be freely set by changing the output current of the constant current source circuit.

[Brief explanation of drawings]

1A and 1B are block diagrams and circuit diagrams of a conventional noise removal circuit, FIG. 2 is a waveform diagram for explaining the operation of the noise removal circuit of FIG. 1, and FIG. 3 is a circuit diagram of the basic circuit of the present invention. Figure 4 is an input/output characteristic diagram of the basic circuit shown in Figure 3, Figure 5 is an explanatory diagram for explaining the operation of the basic circuit shown in Figure 3, and Figure 6 is an implementation of the present invention. FIG. 3 is a specific circuit diagram of an example. Tr ₁ and Tr ₂ ...transistor, _C1 ...capacitor, _R3 ...resistance.

Claims (1)

[Scope of utility model registration request] first and second transistors constituting a push-pull type emitter follower circuit;
the first and second resistors connected in series between the base of the transistor and the base of the second transistor, and a capacitor connected to the output point of the emitter follower circuit, the first and second resistors 1. A noise removal circuit comprising a current path for applying an input signal to a common connection point of the transistors and charging and discharging the capacitor by the input signal when the first and second transistors are off.