JPH0339965Y2 - - Google Patents

Description

[Detailed description of the invention] It is preferable for the audibility to have a higher noise detection sensitivity for noise removal during stereo reception than in monaural reception.

Figure 1 is a block diagram showing the configuration of a conventional pulse noise removal device for FM receivers. In the figure, 1 is the antenna, 2 is the tuner, 3 is the pulse noise removal device, and 4 is the pulse noise up to the demodulation circuit. The output of the noise removal device, 5 is a noise detection section, 6 is a high-pass filter for detecting pulsed noise, 7 is a noise amplifier,
8 is a switching pulse generation circuit, 9 is a noise amplifier AGC, 10 is a delay low-pass filter,
11 is a gate for noise removal.

The FM signal from tuner 2 is applied to point Q in FIG. The noise detection section 5 extracts pulse noise from this signal using a high-pass filter 6,
After being amplified by the amplifier 7, when the amplitude exceeds a certain predetermined threshold, the switching
A pulse generating circuit 8 generates a pulse for closing the noise removal gate 11. noise amplifier
The AGC 9 is provided to prevent the amplifier 7 from being saturated and to prevent the gate 11 from being turned off continuously. Low pass filter 10
is the gate 11, which performs a delay function to prevent pulsed noise from passing through the gate before it is turned off.

Figure 2 is a diagram showing the output level of pulse noise relative to the electric field without a noise removal device. In the figure, curve A is the output level of the audio signal relative to the electric field, and curve B is the FM detection output when FM modulation is not applied. This is a change in the internal audio noise that occurs. Since there is no modulation, it is naturally received in monaural. Curve C is the pulse noise level in the audio band that occurs in the detection output when pulse noise is input to the antenna in an unmodulated state.
Compared to curve B, the S/
The deterioration state of N can be determined. Curve 2 is the pulse noise level in the audio band where only the 19kHz stereo pilot signal is modulated, FM stereo demodulated, and output after stereo operation. A comparison with curve C shows that the influence of pulse noise occurs even in high electric fields. Curve E represents the high-frequency component of the pulsed noise of the detection output, and represents the signal level passed through the high-pass filter of FIG.

In FIG. 2, the following is determined.

(i) Compared to monaural reception, stereo reception is more susceptible to interference even at small pulse noise levels.

(ii) The point on curve B where the audio internal noise begins to be interfered with by pulsed noise is P ₁ for monaural reception and P ₂ for stereo reception. At this time, the level of the high-frequency component of the pulse noise is P′ ₁ for monaural reception and P′ ₂ for stereo reception, and stereo reception is more susceptible to interference even with small pulse noise. This means that during stereo reception,
This is because the bandwidth used by the detection output signal expands from 15kHz during monaural reception to 53kHz.

Therefore, in the case of monaural reception, pulsed noise can only be detected up to the noise level of P′ ₁ ;
There is no need to increase the detection sensitivity any further. When the detection sensitivity is set too high, noise removal is performed even when no noise is perceptible to the auditory sense, and signal distortion occurs due to switching of the gate 11. However, in stereo reception, it is said _that increasing the noise detection sensitivity to near P′ ₂ is better for audibility;
The sensitivity is set to detect the level. For this reason, during monaural reception, noise removal devices are used in such a way that the sound quality deteriorates in an electric field of P ₁ (P′ ₁ ) or more.

In this way, conventionally, the noise detection sensitivity was fixed regardless of whether it was stereo or monaural, and if the detection sensitivity was adjusted to stereo reception, the sensitivity would be too high for monaural reception, and when noise was removed, distortion would increase. If the sensitivity is adjusted to monaural reception, there will be a drawback that noise removal performance will be insufficient in stereo reception.

An object of the present invention is to provide a pulse noise removal device for an FM receiver that can automatically set noise detection sensitivity to an appropriate state regardless of stereo reception or monaural reception.

In order to achieve the above purpose, the FM according to the present invention
The pulse noise removal device for the receiver includes a noise detection circuit that can vary the gain of the noise amplifier or the noise detection threshold, a differential amplifier that amplifies the difference between the outputs of the left and right channels of the stereo output, and is connected to the output of the differential amplifier. The present invention has a rectifying and smoothing circuit, and when an output appears in the rectifying and smoothing circuit and stereo reception is detected, the pulse noise detection sensitivity of the noise detection circuit is adjusted to increase.

Hereinafter, the present invention will be explained in more detail using examples with reference to the drawings, but these are merely illustrative and it is understood that various modifications and improvements may be made without going beyond the scope of the present invention. Of course.

FIG. 3 is a block diagram showing the configuration of a pulse noise removing device for an FM receiver according to the present invention. In the figure, reference numbers common to those in FIG. 1 represent the same ones as in FIG. Signal demodulation circuit, 13 is stereo output, 14 is differential amplifier, 1
5 is a rectifier circuit, 16 is a smoothing circuit, and 17 is an attenuator for adjusting sensitivity.

The signal received by the antenna 1 and passed through the tuner 2 is delayed by the delay low-pass filter 10, passes through the noise removal gate 11, and then input to the stereo signal demodulation circuit 12, where it is demodulated into a stereo signal. . Here, the signal strengths of the left and right channels are equal during monaural reception. Therefore, the output of the differential amplifier 14 becomes zero. During stereo reception, the signal strengths of the left and right channels are not the same, and the differential amplifier 14 amplifies the signal difference. The AC component of the amplified signal is transferred to the current circuit 1.
5, and further smoothed by a smoothing circuit 16 to control a sensitivity adjustment attenuator 17. At this time, the attenuator 17 is controlled so that there is no gain loss, and is set so that the gain loss increases when the output of the differential amplifier 10 is 0.

Even in stereo reception, this problem naturally occurs even when the signal strengths of the left and right channels are equal. In this case,
The output of the differential amplifier 14 is 0, the gain loss increases, and the noise detection sensitivity decreases. but,
At this time, if pulse noise is mixed in, the remaining part of the noise will be output to the left and right channels, but the stereo signal demodulation circuit 12
The left and right channels are switched over time in Hz, and the audio output of pulse noise is rarely at the same level for the left and right channels at the same time. The state is shown in FIG.

Figure 4 shows the stereo composite signal waveform (dashed line) when the signal strengths of the left and right channels are the same, the signal waveform with the pilot signal added to it (solid line), and the pulse noise (shaded area). shows. The time periods marked with ○ belong to the left channel, and the time periods marked with × belong to the right channel.
They alternate at a frequency of 1/38kHz. On the other hand, as shown in FIG. 4, pulse noise almost never mixes into the left and right channels in the same state. This has a detection output band that extends to 53kHz, which is necessary for stereo, and 38kHz.
This is because the 53kHz component is larger than the 38kHz component due to the characteristic called triangular noise of FM demodulation in pulse noise for the Hz composite sub-signal.

Therefore, when pulse noise is generated in the stereo demodulated output, the differential amplifier 14 detects the level difference, reduces the gain loss of the attenuator 17, and operates to increase the detection sensitivity. .

In this way, the noise detection sensitivity can be lowered during monaural reception, and the detection sensitivity can be increased during stereo reception, and the noise removal operation is performed in a manner suitable for each reception condition.

In FIG. 3, the sensitivity is switched by the attenuator 16, but the invention is not limited to this, and it goes without saying that the trigger level of the switching pulse generating circuit 8 may be changed.

As explained above, according to the present invention, the audibly optimal noise detection sensitivity suitable for monaural reception and stereo reception is automatically set,
An advantage is obtained that the noise detection sensitivity is not too high during monaural reception and does not have a disadvantageous effect.

Furthermore, the pulse noise removal device for FM receivers according to the present invention can also be used in a stereo demodulator that has a function of automatically blending from stereo to monaural depending on the level of electric field strength.
It also has the advantage of being able to freely and continuously control the noise detection sensitivity according to the amount of blending.

[Brief explanation of the drawing]

Figure 1 is a block diagram showing the configuration of a conventional pulse noise removal device for FM receivers, Figure 2 is a diagram showing the output level of pulse noise with respect to the electric field without a noise removal device, and Figure 3 is a diagram showing the output level of pulse noise with respect to the electric field without a noise removal device. A block diagram showing the configuration of the pulse noise removal device for an FM receiver according to the present invention, Fig. 4 shows a stereo composite signal when the signal strengths of the left and right channels are the same, and a signal with a pilot signal added to it. and a waveform diagram of pulse noise. DESCRIPTION OF SYMBOLS 1...Antenna, 2...Tuner, 3...Pulse noise removal device, 4...Output of the pulse noise removal device leading to the demodulation circuit, 5...Noise detection section, 6...Pulse noise detection height Pass filter, 7... Noise amplifier, 8... Switching
Pulse generation circuit, 9...Noise amplifier AGC, 1
0...Delay low-pass filter, 11...Noise removal gate, 12...Stereo signal demodulation circuit,
13...Stereo output, 14...Differential amplifier, 1
5... Rectifier circuit, 16... Smoothing circuit, 17... Variable attenuator for sensitivity adjustment.

Claims (1)

[Scope of utility model registration request] It has a noise detection circuit with variable noise amplifier gain or noise detection threshold, a differential amplifier that amplifies the difference between the outputs of the left and right channels of the stereo output, and a rectification and smoothing circuit connected to the output of the differential amplifier. A pulse noise removing device for an FM receiver, characterized in that when an output appears in a rectifying and smoothing circuit and stereo reception is detected, the pulse noise detection sensitivity of the noise detection circuit is adjusted to increase.