JP3221280B2 - Multipath noise reduction device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a portable or in-vehicle mobile receiver for use in FM broadcast receivers, TV receivers, AM broadcast receivers and the like capable of receiving stereo broadcasts. The present invention relates to a multipath noise reduction device.

[0002]

2. Description of the Related Art A mobile receiver, especially an on-vehicle FM stereo broadcast receiver, often suffers from noise or distortion due to multipath, and is very unpleasant to hear. It is known that these noises and distortions are remarkably improved by stopping the stereo demodulation operation in the state of receiving multipath and performing monaural demodulation. For this reason, even in the prior art, whether or not multipath has been detected is detected, and if so, the demodulation operation of the stereo decoder is controlled to make it monaural or close to monaural, thereby reducing multipath noise and reducing audibility. An improved receiver is provided.

FIG. 4 shows a configuration of a conventional FM broadcast receiver for a mobile body of this kind. In FIG. 4, 1 is an antenna for receiving a broadcast wave, 2 is a front end for extracting a desired wave from a radio wave input from the antenna 1 and outputting an intermediate frequency,
3 is an intermediate frequency amplifier circuit for amplifying the output intermediate frequency,
Reference numeral 4 denotes an FM detection circuit for extracting an audio signal from the amplified intermediate frequency, and reference numeral 5 denotes a stereo decoder for distributing the audio signal to a left channel and a right channel.
And the right channel output Rch. Reference numeral 6 denotes a signal meter output circuit, which is an intermediate amplifier circuit 3 that indicates electric field strength.
In addition to the output of a DC voltage proportional to the amplitude of the received signal, an FM voltage is amplitude-modulated when multipath interference occurs, so that an AC voltage is also output. Reference numeral 7 denotes a band-pass filter for band-limiting the output of the signal meter output circuit 6, reference numeral 8 denotes a level detection circuit for converting the output of the band-pass filter 7 into a DC voltage, and reference numeral 9 denotes a level detection circuit 8.
Is a separation control circuit that controls the separation (degree of separation) of the stereo decoder 5 according to the output of the stereo decoder 5 to reduce multipath interference noise.

Next, multipath noise reduction control in the above conventional example will be described. Among the outputs of the signal meter output circuit 6, the level of the high-frequency noise component increases due to the harmonic distortion component of the noise + low-frequency modulation component when subjected to multipath. This high-frequency AC component, for example, 15k
All or a part of the frequency of about Hz to 100 kHz is extracted by the band pass filter 7. Next, the level of the extracted noise is converted into a DC voltage through a level detection circuit 8, and the output voltage is applied to a separation control circuit 9 to be applied with a threshold value corresponding to the amount of multipath received. From the stereo demodulation state to the monaural demodulation state.

FIG. 5 shows the relationship between the multipath intensity (the magnitude of the multipath noise) and the separation (degree of separation) in the conventional multipath noise reduction control. As is apparent from this figure, conventionally, the multipath noise is reduced by controlling the separation from stereo to monaural as the multipath intensity increases.

[0006]

However, since the conventional multipath noise reduction control operates irrespective of the modulation rate of the radio wave source, there is a problem that the noise reduction effect on audibility varies depending on the modulation rate. there were.

SUMMARY OF THE INVENTION The present invention solves such a conventional problem, and makes the noise detection sensitivity variable according to the modulation rate so that the noise reduction effect can be prevented from changing on the audibility. It is intended to provide a device.

[0008]

According to the present invention, in order to achieve the above object, means for detecting a modulation factor of a radio wave source is provided, and a threshold value of a separation control circuit is varied by an output voltage thereof. Things.

[0009]

Therefore, according to the present invention, by detecting the modulation factor of the radio source and varying the threshold value of the separation control circuit, a constant noise reduction effect can be obtained irrespective of the modulation factor of the radio source. Can be obtained at

[0010]

FIG. 1 shows a configuration of an embodiment of the present invention. Elements similar to those of the conventional configuration shown in FIG. 4 are denoted by the same reference numerals. In FIG. 1, 1 is an antenna, 2 is a front end, 3 is an intermediate frequency amplification circuit, 4 is an FM detection circuit, 5 is a stereo decoder, 6 is a signal meter output circuit, 7 is a band pass filter, and 8 is a first level. A detection circuit 9 is a separation control circuit. 10 is FM
A low-pass filter to which the output of the detection circuit 4 is input, 11
Receives the output of the low-pass filter 10 and
A second level detection circuit connected to the separation control circuit 9 together with the first level detection circuit 8; these low-pass filter 10 and second level detection circuit 11
Means for detecting the modulation factor of the radio wave source.

Next, the operation of the above embodiment will be described. Of the modulation components obtained from the detection output from the FM detection circuit 4, for example, in the case of FM broadcasting, 50 Hz to 15 k
Hz is the modulation component of the left channel and the right channel (L + R), and the main component of the modulation signal exists.
In order to eliminate the influence of the 9 kHz pilot signal (modulation rate of 10%), the output is passed through a low-pass filter 10 and its output is level-detected by a second level detection circuit 11 to obtain a DC voltage corresponding to the modulation rate. Then, the DC voltage is applied to the separation control circuit 9 to vary the control threshold value according to the modulation rate, thereby controlling the separation in the stereo decoder 5 from the stereo demodulation state to the monaural demodulation state.

As described above, according to the above embodiment, the modulation component is detected from the output of the FM detection circuit 4 by the low-pass filter 10 and the second level detection circuit 11, and the threshold of the separation control circuit 9 is determined according to the output. Since the value is variable, multipath noise is conspicuous When the modulation rate is low, even a small number of multipaths are likely to be in a monaural demodulation state, reducing noise, and even if the same multipath appears, it is not conspicuous When the modulation rate is high, the effect of reducing noise on the audibility can be made constant by making it difficult to cause a monaural demodulation state and not to impair the stereo feeling.

FIG. 2 shows the relationship between the multipath intensity and the separation in the multipath noise reduction control of the above embodiment. As is clear from this figure, according to the present embodiment, it is understood that the multipath intensity changes depending on the magnitude of the modulation factor.

The control of the separation based on the multipath strength may be a control such that the switching is suddenly changed from a stereo demodulation state to a monaural demodulation state as shown in FIG.

[0015]

According to the present invention, as is apparent from the above embodiment, means for detecting the modulation factor of the radio wave source is provided, and the threshold value of the separation control circuit is varied by the output voltage. Regardless of the modulation factor of the source, a certain noise reduction effect can be obtained on the audibility, and when the audible noise is conspicuous, a deeper separation control is performed to balance the sound quality (stereo feeling) and the noise reduction. It has the effect that it can be taken.

[Brief description of the drawings]

FIG. 1 is a schematic block diagram illustrating a configuration of an FM broadcast stereo receiver according to an embodiment of the present invention.

FIG. 2 is a characteristic diagram showing an example of separation control in the embodiment.

FIG. 3 is a characteristic diagram showing another example of separation control in the embodiment.

FIG. 4 is a schematic block diagram showing a configuration of a conventional FM broadcast stereo receiver.

FIG. 5 is a characteristic diagram showing an example of separation control in a conventional example.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 antenna 2 front end 3 intermediate frequency amplification circuit 4 FM detection circuit 5 stereo decoder 6 signal meter output circuit 7 bandpass filter 8 first level detection circuit 9 separation control circuit 10 low-pass filter 11 second level detection circuit

Claims (2)

(57) [Claims] 1. A multipath noise reduction device comprising means for detecting a modulation factor of a radio wave source, wherein a threshold value of a separation control circuit is varied by an output voltage of the device. 2. A signal meter output circuit for outputting an amplitude modulation component of an antenna input as an AC voltage, a band-pass filter for band-limiting the frequency of the AC voltage, and converting the band-limited AC voltage to a DC voltage. A first level detection circuit, a separation control circuit for changing a stereo separation by the converted DC voltage, a low-pass filter for mainly extracting left channel and right channel modulation components from an output obtained by detecting an antenna input, A second level detector for detecting a modulation component of a channel and a right channel and converting the modulated component into a DC voltage, wherein a control threshold of the stereo separation control circuit is changed by an output of the second level detector. A multipath noise reduction device characterized by the above-mentioned.