JPH05304484A - Fm receiver - Google Patents

Abstract

PURPOSE:To reduce the distortion of a modulation signal at the time of monaural reception when multi-pass distortion is generated by providing a high frequency interruptio filter for passing only the low frequnecy signal of the modulation signals and a selective circuit for selecting and outputting either one of the modulation signal and the low frequency signal. CONSTITUTION:An FM detected modulation signal F is inputted to one of a signal selecting circuit 4, passes through a high frequnecy interruption filter 3 and a modulated signal LF of the low frequency components is inputted to the other side of the circuit 4. Also, an AC voltage signal V goes to an AM component detecting signal VD through an AM component detecting circuit 6 and controls the circuit 4. The circuit 4 is switched so as to select and output the signal F when no multi-pass distortion is present and to select the signal LF for which the signal VD is increased and reaches a set value when the multi-pass distortion is generated. Then, the filter 3 appropriately sets interruption characteristics in order to suppress high frequency distortion components due to the multi-pass strain at the time of the interruption frequency and monaural reception. Thus, as the distortion part of the signal F, the multi-pass distortion at the time of the monaural reception can be substantially improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an FM receiver, and more particularly to an FM receiver used for a car radio, an FM tuner and the like.

[0002]

2. Description of the Related Art As shown in FIG. 3, a conventional FM receiver includes an intermediate frequency amplifier circuit 1 for amplifying an FM modulated wave of an input intermediate frequency signal, and an FM modulator amplified by the intermediate frequency amplifier circuit 1. An FM detection circuit 2 that FM-detects a signal and outputs a demodulation signal, an input level detection circuit 5 that detects the level of the FM-modulated signal and outputs a level detection signal, and an AM component of the level detection signal. An AM component detection circuit 6 that outputs a DC voltage control signal proportional to the AM component,
Hearing correction circuit 7 that receives the demodulated signal and performs hearing correction
And a stereo demodulation circuit 8 for demodulating the output signal of the audibility correction circuit 7 into a stereo signal.

As shown in FIG. 4, the audibility correction circuit 7 includes a mute circuit 71 and a high cut control circuit 72.
And a stereo noise control circuit 73 and a control circuit 74 for controlling them.

Next, the operation of the conventional FM receiver will be described.

An input signal I, which is an FM modulated wave of an intermediate frequency signal, is amplified by an intermediate frequency amplifier circuit 1 and an FM detection circuit 2
Then, FM detection is performed and the demodulated signal F is input to the audibility correction circuit 7 and audibility correction is performed. The outputs H and N of the hearing correction circuit 7 are
The stereo demodulation circuit 8 demodulates to an LR stereo signal,
These are output as outputs OL and OR, respectively.

The auditory sense correction circuit 7 is a circuit for performing auditory sense correction for reducing noise. In general, the noise generated during FM demodulation is triangular noise proportional to the frequency, so noise is generated more in stereo using a high frequency sub-channel signal than in monaural using only the low frequency main channel. There are many. Further, when the input level of the FM modulation signal becomes low and the limiter operation of the limiter circuit of the intermediate frequency amplifier circuit 1 becomes insufficient, impulse noise occurs,
S / N deteriorates. Therefore, as the input level of the FM-modulated signal decreases from a sufficiently high state, stereo noise control, high-cut control, and mute are controlled in this order to reduce noise and perform auditory sense correction.

Demodulated signal F input to the auditory sense correction circuit 7
Are muted by the mute control signal M from the control circuit 74 in the mute circuit 71. Mute circuit 71
Is divided into two and is input to the high cut control circuit 72 and the stereo noise control circuit 73, respectively. The high cut control circuit 72 is
Controlled by the high cut control signal CH from the control circuit 74, the high frequency component of the input signal is cut by the high frequency cutoff filter to suppress the noise generated in the high frequency component and output the stereo composite signal H. The stereo noise control circuit 73 is controlled by the stereo noise control signal CN from the control circuit 74,
Normally, the stereo composite signal which is an input signal is output as it is as the output N, but when the noise increases, the output N is cut off. In this state, the output of the audibility correction circuit 7 is only the stereo composite signal H from the high cut control circuit 72, which is equivalent to a monaural state. The control circuit 74 operates in response to the auditory sense correction control signal C from the arithmetic circuit described later, and the mute control signal M.
, A high-cut control signal CH and a stereo noise control signal CN are generated.

On the other hand, the intermediate frequency signal A, which is another output of the intermediate frequency amplifier circuit 1, is level-detected by the input level detection circuit 5, and the level detection signal D which is a DC voltage corresponding to the level of the intermediate frequency signal A is detected. The continuous or excessive fluctuation of the intermediate frequency signal A is output as an AC voltage signal V. The AC voltage signal V is supplied to the AM component detection circuit 6
And is output as an AM component detection signal VD which is a DC voltage corresponding to the level of the AC voltage signal V. The level detection signal D and the AM component detection signal VD are input to the arithmetic circuit 9, are subjected to a predetermined arithmetic operation, and are output as the control signal C to the auditory sense correction circuit 7.

In FIG. 2, when distortion due to multipath occurs in the received signal, (A) is the FM modulation signal I, (B) is the level detection signal D, (C) is the demodulation signal F, and (D). 8A and 8B are waveform diagrams showing the waveforms of the demodulated signal F that has passed through the high-frequency cutoff filter. The multipath distortion occurrence point W of the FM modulation signal I is detected as the AC component X of the level detection signal D, and the demodulation signal F periodically shows distortion increase points Y due to multipath corresponding to the modulation frequency. .. Further, this increase in distortion increases as the modulation frequency increases, so the AC component X of the level detection signal D is detected in the AM component detection time 5, and the auditory sense correction circuit 7 is controlled via the arithmetic circuit 9 to force it. I switched from stereo to monaural to make a audible correction. Further, the arithmetic circuit 9 performs the arithmetic operation for using the control signal C for both the level detection of the FM modulation signal and the multipath detection.

[0010]

In the conventional FM receiver described above, the control operation for detecting the level of the FM modulation signal and the control operation for detecting the multipath are optimized for the stereo signal and used for both. Although it is effective against the multipath distortion, it has a drawback that it has almost no effect in the case of a monaural signal.

[0011]

The FM receiver of the present invention comprises:
An FM detection circuit that FM-detects the intermediate frequency-amplified FM modulation signal and outputs a demodulation signal; an input level detection circuit that detects the level of the FM modulation signal and outputs a level detection signal; AM component is detected and this A
An AM component detection circuit that outputs a control signal of a DC voltage proportional to the M component, a high-frequency cutoff filter that passes only a low-frequency signal of a predetermined frequency or less of the demodulation signal, and the demodulation controlled by the control signal. And a signal selection circuit for selecting either one of the signal and the low-frequency signal.

[0012]

Embodiments of the present invention will now be described with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of an FM receiver of the present invention.

As shown in FIG. 1, the FM receiver of this embodiment includes an intermediate frequency amplifier circuit 1, an FM detector circuit 2, an input level detector circuit 5, and an AM component detector circuit 6 as in the conventional case. In addition, a high-frequency cutoff filter 3 for extracting only low-frequency signals below the set frequency of the demodulated signal from the FM detection circuit 2,
A signal selection circuit 4 which is controlled by an AM component detection signal from an AM component detection circuit 6 and which selects and outputs one of the demodulated signal and the low-frequency signal is configured.

Next, the operation of this embodiment will be described.

An input signal I, which is an FM modulated wave of an intermediate frequency signal, is amplified by an intermediate frequency amplifier circuit 1 and then an FM detection circuit 2
Then, FM detection is performed and the demodulated signal F is input to one of the signal selection circuits 4. Further, the demodulated signal F passes through the high-frequency cutoff filter 3, and the low-frequency demodulated signal LF, which is its low-frequency component, is input to the other one of the signal selection circuits 4.

On the other hand, the intermediate frequency signal A, which is the other output of the intermediate frequency amplifier circuit 1, is level-detected by the input level detection circuit 5, and the continuous or excessive fluctuation of the intermediate frequency signal A is detected by the AC voltage signal V. Is output as. The AC voltage signal V is input to the AM component detection circuit 6 and is output as an AM component detection signal VD which is a DC voltage corresponding to the level of the AC voltage signal V. This AM component detection signal VD is
The signal selection circuit 4 is controlled.

The signal selection circuit 4 selects the demodulated signal F and outputs it as it is in the state where there is no multipath distortion. When multipath distortion occurs, the AM component detection signal VD increases, and when it reaches a set value, the low frequency demodulation signal LF is switched to be selected. The high-frequency cutoff filter 3 appropriately sets the cutoff frequency and its cutoff characteristics so as to significantly suppress the distortion component in the high frequency range due to multipath distortion in the case of monaural. As a result, like the distorted portion Z of the demodulated signal F shown in FIG.
It is possible to significantly improve multipath distortion during monaural reception.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and various modifications can be made. For example, it is also possible to obtain optimum multipath distortion suppression characteristics for stereo and monaural by adding the same audibility correction circuit and stereo demodulation circuit as in the conventional example without departing from the gist of the present invention. Of course.

It is also possible to provide a high-frequency cutoff filter and a signal selection circuit at the outputs of the LRs of the stereo demodulation circuit, and control them by AM component detection signals to obtain the same effect without departing from the gist of the present invention. Of course, it can be applied.

[0021]

As described above, the FM receiver of the present invention has a high-frequency cutoff filter that passes only a low-frequency signal below a set frequency, and either a demodulated signal or the low-frequency signal. It has a signal selection circuit for selecting and outputting, and when a multipath distortion occurs, by selecting the low-frequency demodulated signal that has passed through a high-frequency cutoff filter as the output, it is possible to effectively suppress the multipath distortion even in the case of monaural. There is an effect.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of an FM receiver of the present invention.

FIG. 2 is a waveform diagram showing an example of an operation when distortion due to multipath occurs in a received signal.

FIG. 3 is a block diagram showing an example of a conventional FM receiver.

FIG. 4 is a block diagram showing an example of an auditory sense correction circuit;

[Explanation of symbols]

 1 intermediate frequency amplification circuit 2 FM detection circuit 3 high cutoff filter 4 signal selection circuit 5 input level detection circuit 6 AM component detection circuit 7 auditory sense correction circuit 8 stereo demodulation circuit 9 arithmetic circuit 71 mute circuit 72 high-cut control circuit 73 stereo noise control Circuit 74 Control circuit

Claims (1)

[Claims] 1. An FM-modulated signal that has undergone intermediate frequency amplification is FM-converted.
An FM detection circuit that detects and outputs a demodulation signal, an input level detection circuit that detects the level of the FM modulation signal and outputs a level detection signal, and an AM component of the level detection signal that is proportional to the AM component An AM component detection circuit that outputs a control signal of the DC voltage, a high-frequency cutoff filter that passes only a low-frequency signal having a frequency equal to or lower than a predetermined frequency of the demodulation signal, the demodulation signal and the low-frequency filter controlled by the control signal. An FM receiver comprising: a signal selection circuit that selects either one of a range signal and a range signal.