JPS5836025A - Noise eliminator - Google Patents

Abstract

PURPOSE:To prevent the generation of a noise accompanying holding operation by detecting a noise and holding an audio signal, and reducing level variation when the holding operation ends. CONSTITUTION:If an audio signal outputted from an amplifying and detecting circuit 2 contains a noise, a noise detecting circuit 4 outputs a control signal. This control signal is supplied to a control waveform generating circuit 7 and converted into a control waveform of a predetermined function, which is supplied to a variable impedance low-pass filter 8. The low-pass filter 8 has a variable impedance part 8c equipped with a field effect transistor 8a, and a holding capacitor 8b for holding an audio signal level during a rise of the control signal from the noise detecting circuit 4, and when a cutoff frequency is controlled by the control waveform signal to eliminate the noise, the control waveform signal is increased gradually to increase the cutoff frequency gradually, obtaining smooth level transition from a held value to the audio signal.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is designed to smooth the level transition from the hold value to the audio signal when detecting noise and holding the audio signal to prevent the generation of secondary noise associated with noise removal operations. This invention relates to a noise removal device.

As a conventional noise removal device, for example, there is one shown in FIG. 1, which includes an intermediate frequency circuit 1 that outputs an intermediate frequency signal, an amplification/detection circuit 2 that amplifies and detects the intermediate frequency signal and outputs an audio signal, and an audio a noise detection circuit 4 that outputs a control signal when noise is detected in the signal; a delay circuit 3 that delays the audio signal by the time from when the noise detection circuit 4 detects the noise until it outputs the control signal; A stone holding circuit 5 which inputs a signal and holds the level of an audio signal when a control signal is output, and an audio section 6 which inputs the audio signal output from the holding circuit 5 and emits a sound, The circuit 5 includes a gate switch 5a that is turned off when the control signal is input, and a holding capacitor 5b that holds the level of the audio signal when the gate switch is turned off.

In the above configuration, the operation will be explained with reference to FIGS. 2(a), 2(b), and 2(c). stomach))
is input, and when noise N is detected, the control signal (Figure 2 (b)
) is output. Since the switch 5a of the holding circuit 5 is turned off when the control signal is output, the holding circuit 5 maintains the level at the rise of the control signal of the audio signal S output by the delay circuit 3 and delayed by the certain time. capacitor 5
Level L of b. When the control signal falls, the switch 5a is turned on and the delay circuit 3 outputs the audio signal (FIG. 2(c)).

The audio section 6 generates audio based on the output of the holding circuit 5.

However, according to the conventional noise removal device, as shown in FIG. 2(C), when noise is detected when the level change of the audio signal S is relatively large, the audio signal S If the level fluctuation becomes relatively large when moving to the level of , there is a risk that a harsh or lick noise may occur.

In view of the above, the present invention detects noise and holds the audio signal at L/L, and reduces the level fluctuation at the end of the hold to prevent noise from occurring due to the hold operation. To provide a noise removal device equipped with a variable impedance low-pass filter circuit that gradually lowers the rough tracing frequency to smooth the level transition from a hold value to an audio signal when the frequency is reduced and noise disappears.

The following is a noise removal device according to the present invention? , will be explained in detail in 1NII.

FIG. 3 shows an embodiment of the present invention. The same parts as in FIG. 1 are indicated by the same reference numerals, so the explanation will be repeated. When output (output when noise is detected in the audio signal)
, a control waveform generation circuit 7 is added that outputs a control waveform signal of a predetermined function, and the cutoff frequency is controlled by the control waveform signal, and when the noise disappears, the cutoff frequency is increased by gradually increasing the control waveform signal. The holding circuit 5 is replaced by a variable impedance low-pass filter circuit 8 that gradually increases in size to smooth the level transition from the hold value to the audio signal. The variable impedance low-pass filter circuit 8 includes field effect transistors 8a (8...
It has a variable impedance section 8C with a source, D...drain, G...gate) and a holding capacitor 8b that holds the level of the audio signal at the rise of the control signal output by the noise detection circuit 4. CL R,
and Ryo are bias resistances). FIG. 4 shows the pass characteristics of the variable impedance low-pass filter 8. As the gate voltage applied to the gate electrode G of the field effect transistor 8a increases, the cutoff frequency of the input signal f increases and the characteristics change from a to b. It shows that it changes as →C→d.

In the above configuration, the operation thereof will be explained with reference to FIGS. 5(a) to 5(a). If the audio signal outputted by the amplification detection circuit 2 contains noise N (FIG. 5(A)), the noise detection circuit 4 outputs a control signal (FIG. 5C).
mouth)). When the control signal is no longer output, the control waveform generation circuit 7 generates a signal from time t□ to 1. It is output as a function waveform F' that has a constant level at (1, when the noise disappears) and linearly increases from time t2 to t. When this function waveform F is applied to the gate electrode G of the field effect transistor 8a of the variable impedance low-pass filter 8, the gate voltage is low from time t8 to tyo, so that the low-pass filter 8 has a low Since it exhibits a characteristic with a cutoff frequency, passage of the audio signal is blocked. Due to this cutoff, the holding capacitor 8b of the low-pass filter circuit 8 holds the level of the audio signal before the cutoff (the hold value is assumed to be the same). Next, at time t, ~t, as the level of the function waveform F increases, the low-pass filter circuit 8 changes from the curve in FIG. The frequency gradually increases, and the band of the audio signal S passing through becomes very busy, so that the output of the low-pass filter circuit 8 approximates the audio signal S. Finally, at time t, the impedance of the variable impedance section 80 becomes the minimum and exhibits the characteristic of curve change, so that the audio signal passes through the sweet low-pass filter circuit 8. Through the above operations, the level transition shown in FIG. 5) is performed, and large fluctuations in the level of the noise N can be removed. #, function waveform F
is not limited to the waveform shown in FIG. 501, and other waveforms may be used, for example, the −th linear portion may be changed exponentially.

- As explained above, according to the noise removal device according to the present invention, when noise is detected, the cutoff frequency is decreased to hold the audio signal, and then the cutoff frequency is gradually increased to convert the hold value to the audio signal. Equipped with a variable impedance low-pass filter circuit that smoothes level transitions,
Mouth that can prevent noise caused by hold operation

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing a conventional noise removal device. Figures 2 (A), (B), and (C) are time charts of Figure 1. FIG. 3 is an explanatory diagram showing one embodiment of the present invention. Figure 4 is the third
The time chart of the variable impedance low pass 743 shown in the figure. Explanation of symbols 1...Intermediate frequency circuit 2...Amplification detection circuit 3...Delay circuit 4...Noise detection circuit 5...Holding Circuit 6...
Audio section 7...Control waveform generation circuit 8...Variable impedance low-pass filter circuit Patent applicant Nissan Motor Co., Ltd.

Claims (1)

[Claims] (1) In a noise removal device that allows the audio signal to pass through as it is when no noise is detected, and blocks the audio signal from passing when noise is detected and maintains the level of the audio signal immediately before the interruption, the audio signal is blocked from passing through. a control waveform generation circuit that outputs a control waveform signal that decreases the cutoff frequency when the audio signal is transmitted, and gradually increases the cutoff frequency when the passage of the audio signal is resumed; A noise removal device characterized by comprising a variable impedance low-pass filter circuit.