JPH0419864Y2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Field of Application> The present invention relates to an FM stereo receiver, and particularly to an FM stereo receiver that can improve the S/N ratio in a weak electric field without causing discomfort.

<Prior Art> In an FM stereo receiver, it is known that the S/N ratio in a weak electric field can be improved by switching from stereo to monaural in a weak electric field where the noise level increases. Also, when the electric field is weak, FM
It is also known that the effective S/N ratio can be improved by attenuating the high frequency level of the signal.

For this reason, conventionally, the received electric field strength is monitored, and when the received electric field strength falls below a predetermined first level, switching is performed from stereo to monaural.
When the received electric field strength further decreases to below a second level, the high frequency range is attenuated, and when the received electric field strength further decreases to below a third level, the output signal is cut off.

<Problems that the invention is trying to solve> By using this method, a certain amount of S/
It is possible to improve the S/N ratio, but since all switching is done in a switching manner, the S/N ratio can only be improved in stages, and the change in the output signal state at the switching point is large and may be harmful to the listener (user). There was a problem that it gave a sense of discomfort.

Also, since it switches between stereo and monaural at a predetermined level, if the received electric field strength fluctuates around that level, it will change to stereo or monaural at the fluctuation period, and the sound will change to tremolo. There are also problems that arise.

In view of the above, an object of the present invention is to provide an FM stereo receiver that can improve the S/N ratio in a weak electric field without causing discomfort, and does not cause tremolo in the sound.

<Means for solving the problems> FIG. 1 is a block diagram of an FM stereo receiver according to the present invention.

11 is the antenna, 12 is the front end, 13
is an intermediate frequency amplifier, 14 is an FM detector, 15 is a variable attenuator, 16 is a stereo demodulation circuit, 17 is a pseudo-stereo circuit, 18 is a switching circuit, 19 is a frequency control circuit (high cut circuit), 20 is a level detection circuit, 21 and 22 are comparators, and 23 and 24 are level setting circuits.

<Operation> The stereo demodulation circuit 16 receives the stereo composite signal S _CMP , which is the output of the variable attenuator 15, and outputs the L and R signals.
Outputs stereo signals S _L and S _R of both channels and monaural signal _SM . Further, the pseudo-stereo circuit 17 receives the stereo composite signal S _CMP and outputs pseudo-stereo signals S _L ′, S _R ′ of both the L and R channels.

The switching circuit 18 outputs the stereo signals S L and S R when the received electric field strength detected by the level detection circuit 19 is a first level E ₁ or higher, and outputs the stereo signals S _L and S _R when the received field strength detected by the level detection circuit 19 is lower than the first level and a second level E ₂ or higher. It outputs pseudo-stereo signals S _L ′ and S _R ′ at the second level, and outputs a monaural signal S _M when the level is lower than the second level.

The high-cut circuit 19 operates when the third level E between the first and second levels becomes equal to or lower than ₃ , and continuously changes the frequency characteristics with respect to the switching circuit output. It operates when the fourth level E4 becomes lower than ₄ and continuously changes the switching circuit output level.

<Embodiment> FIG. 1 is a block diagram of an FM stereo receiver according to the present invention.

In the figure, 11 is an antenna, 12 is a front end, 13 is an intermediate frequency amplifier, 14 is an FM detector,
15 is a variable attenuator that continuously changes the level of the output signal (stereo composite signal) S _CMP according to the received electric field strength E when the received electric field strength falls below a predetermined level; 16 is the output from the variable attenuator 15; 17 is a stereo demodulation circuit which inputs the stereo composite signal S _CMP and outputs stereo signals S _L and S _R of both L and R channels and a monaural signal S _M of (L+R); 17 receives the stereo composite signal S _CMP ; This is a pseudo-stereo circuit that receives input and outputs pseudo-stereo signals S _L ′ and S _R ′.

The pseudo stereo circuit 17, as shown in FIG.
An amplifier 17a that amplifies a monaural signal and a comb filter 17 having frequency characteristics shown in FIG.
b, and a differential circuit 17c that outputs the difference between the comb filter output and the amplifier output.The output of the comb filter 17b is used as an L-channel pseudo-stereo signal S _L ', and output R-
The pseudo stereo signal S _R ′ of the channel is
You can get a stereo feel compared to monaural. Note that a monaural signal generated from the stereo composite signal S _CMP is applied to the amplifier 17a by means not shown.

Returning to FIG. 1, 18 indicates stereo signals S _L , S _R and pseudo stereo signals depending on the level of received electric field strength.
S _L ′, S _R ′, a switching circuit that outputs a monaural signal S _M ,
19 is a frequency characteristic control circuit (high-cut circuit) that operates when the received electric field strength falls below a predetermined level and continuously changes the frequency characteristic of the switching circuit output according to the received electric field strength; 20 is a frequency characteristic control circuit for the intermediate frequency amplifier 13; This is a level detection circuit (level meter) which outputs a level meter signal LMO having a value corresponding to the received electric field strength (antenna input level) E, as shown in FIG.

21 and 22 are comparison circuits, and the comparison circuit 21 becomes high level (“1”) when the level meter signal LMO becomes below the first level L ₁ (corresponding to the received electric field strength E ₁ , see Fig. 4). Outputs the switching signal SW1 of
The comparator circuit 22 outputs a high level (“1”) switching signal SW2 when the level meter signal LMO exceeds the second level L ₂ (corresponding to the received electric field strength E ₂ ).

23 and 24 are level setting circuits, and the level setting circuit 23 sets a third level _E3 between _E1 and _E2 , and when the received electric field strength E becomes equal to or less than the third level _E3 , the high cut circuit 19 The high-cut operation is started, and when the high-cut operation decreases further, the high-frequency attenuation degree of the high-cut circuit is continuously changed. Level setting circuit 2
4 sets a fourth level _E4 near the second level _E2 , and starts attenuation operation by the variable attenuator ₁₅ when the received electric field strength E becomes equal to or less than the fourth level E4;
When the level decreases further, the degree of level attenuation of the variable attenuator 15 is continuously changed.

Hereinafter, the operation of the circuit shown in FIG. 1 will be explained with reference to FIG. 4.

Now, the received electric field strength E is the first level E ₁ (approximately
When there is a strong electric field of 45 dBμ) or more, both the first and second switching signals SW1 and SW2 are at a high level.
Therefore, in this case, the switching circuit 18
S _L and S _R are output to the high cut circuit 19 at the next stage.

However, when the received electric field strength E becomes weaker and reaches a level between the first level _E1 and the second level ₂ (approximately 15 dBμ) ( _E2 <E< _E1 ), the first switching signal SW1 is inverted to low level. do. As a result, SW1="0",
SW2=“1”, and the switching circuit 18 switches the pseudo stereo signals S _{L ′} , S L ′, S _R to the stereo signals S _L , S R
S _R ' is output to the high cut circuit 19.

Then, the received electric field strength E becomes even weaker and the second
When the level E becomes less than ₂ (E<E ₂ ), the second switching signal
SW2 is also inverted to low level. As a result, SW
1=“0”, SW2=“0”, and the switching circuit 18
outputs a monaural signal S _M to the high-cut circuit 19 instead of the pseudo stereo signals S _L ′ and S _R ′.

On the other hand, in parallel with the switching operation by the switching circuit 18, attenuation control by the high cut circuit 19 and the variable attenuator 15 is performed. That is, the high-cut circuit 19 starts high-cut operation when the received electric field strength E becomes equal to or lower than the third level E _{3 (} approximately 35 dBμ) set between _the first and second levels, The frequency characteristics for the output of the switching circuit 18 are continuously changed within the range according to the magnitude of the received electric field strength E. For example, the degree of attenuation of high-frequency components of 10 KHz or higher is increased as the received electric field strength E becomes smaller.

Further, the variable attenuator 15 has a received electric field strength E of the second
When the output level becomes lower than the fourth level _E4 near the level _E2 , the output level is continuously attenuated.

<Effects of the invention> As described above, according to the invention, the stereo state, pseudo-stereo state, and monaural state are created depending on the received electric field strength, and the pseudo-stereo state is interposed between the stereo and monaural states, and high-cut operation and level attenuation operation are performed. Since it is configured to be continuously controlled according to the strength of the received electric field, it is possible to improve the S/N ratio in the case of a weak electric field, and there is no tremolo in the sound, so you can enjoy good listening without any discomfort. Now I can do it.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of the FM stereo receiver according to the present invention, Fig. 2 is a block diagram of the pseudo-stereo circuit in Fig. 1, Fig. 3 is a frequency characteristic diagram of the comb-shaped filter in the pseudo-stereo circuit, and Fig. 4 is a block diagram of the pseudo-stereo circuit in Fig. 1.
The figure is a level meter output, signal, and noise characteristic diagram illustrating each operating point of state switching. 15... Variable attenuator, 16... Stereo demodulation circuit, 17... Pseudo stereo circuit, 18... Switching circuit, 19... High cut circuit, 20... Level detection circuit, 21, 22... Comparator, 23, 24...
Level setting circuit.

Claims (1)

[Claims for Utility Model Registration] A level detection circuit that detects received electric field strength, a stereo demodulation circuit that outputs stereo signals S _L and S _R of both L and R channels, and a stereo demodulation circuit that outputs stereo signals S L and S _R of both L and R channels; , R, _a pseudo-stereo circuit that outputs pseudo-stereo signals S _{L ′} , S _{R ′} of both channels; a switching circuit that outputs pseudo-stereo signals S _L ′, S _R ′ when the second level E 2 is below the second level E ₂ and outputs a monaural signal when the second level E 2 is below the second level; A frequency control circuit that operates when the third level E between the levels falls below ₃ and continuously changes the frequency characteristics for the switching circuit output, and a frequency control circuit that operates when the frequency falls below the fourth level near the second level. An FM stereo receiver comprising variable attenuation means for continuously changing the switching circuit output level.