JPH0549137B2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to an FM stereo receiver, and in particular can improve the S/N ratio in a weak electric field or when multipath noise occurs while maintaining a stereo feeling.
Regarding FM stereo receiver.

<Prior Art> In an FM stereo receiver, it is known that the S/N ratio can be improved by changing from stereo to monaural when multipath noise is large or when the noise level is weak in a weak electric field.

For this reason, conventional methods monitor received field strength and multipath noise level, switch from stereo to monaural when the received field strength falls below a predetermined threshold level, and switch from stereo to monaural when it exceeds the threshold level. The system switches from monaural to stereo, and further switches from stereo to monaural when multipath noise is large even if the received electric field strength is above a predetermined level.

<Problems to be Solved by the Invention> As described above, conventional FM stereo receivers switch between stereo and monaural at predetermined threshold levels in terms of received field strength and multipath noise level. Ta.

For this reason, if the received electric field strength or multipath noise level fluctuates around a predetermined threshold level, the sound will become stereo with a wide spread or become monaural with no sound spread at the fluctuation period. There was a problem in that it gave the listener (user) an audible sense of discomfort.

It should be noted that when the threshold level falls below a predetermined threshold level, the degree of separation is weakened depending on the received electric field strength, and the transition from stereo to monaural state is performed sequentially. Although it is possible to eliminate the sense of strangeness, there is a problem that the sound spread is lost during separation control, resulting in a lack of stereo feeling.

From the above, the present invention can improve the S/N ratio when the multipath noise level is large or when the noise level is weak in a weak electric field, and does not give an audible discomfort when switching. The purpose of the present invention is to provide an FM stereo receiver with a wide receiving electric field range and a stereo feel.

<Means for solving the problem> FIG. 1 is a block diagram of an FM stereo receiver according to the present invention, in which 19 is a stereo demodulator, 20 is a block diagram of an FM stereo receiver according to the present invention;
2 is a mixer, 21 is a pseudo-stereo circuit, 22 is a switching circuit, and 23 is a level detection circuit (level meter). As shown in FIG. 2, the stereo demodulator 19 includes a stereo circuit 19a, a separation control circuit 19b, and a high-cut control circuit 19c.

<Operation> The stereo demodulation circuit 19a (Fig. 2) outputs stereo signals L and R from the composite signal S _CMP ,
The separation control circuit 19b receives the stereo signals L and R, controls the degree of separation thereof, and outputs stereo signals S _L and S _R. The pseudo-stereo circuit 21 changes the phase of the input signal and outputs pseudo-stereo signals S _L ′ and S _R ′ of different phases.

The received electric field strength detected by the level detector 23 is at the first threshold level (for example,
55 dBμ) or more, the degree of separation is maximized, and the signals L and R generated from the stereo demodulation circuit 19a are directly used as stereo signals S _R and S _L ,
A stereo state is created using the stereo signal.

On the other hand, when the received electric field strength is below the first threshold level, separation control by the separation control circuit 19b and sound field correction by the pseudo-stereo circuit 21 are operated, and the output of the separation control circuit is transferred to the pseudo-stereo circuit. 21 to the speakers, the separation operation improves the S/N ratio, and the reduction in sound spread due to the separation operation is corrected by the sound field correction operation of the pseudo-stereo circuit. improve the S/N ratio while maintaining

Then, when the received electric field strength is below the first threshold level or below the second threshold level, the monaural signal generated using the stereo output is input to the speaker via the pseudo-stereo circuit 21. to create a pseudo-stereo state and give a stereo feeling.

<Embodiment> 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, and 15 is an intermediate frequency amplifier.
Noise removal circuit that removes pulse noise (ignition noise, etc.) included in FM detection output, 1
6 is included in the composite signal S _CMP (L+R)
17 is a buffer amplifier, and 18 is a first switch that selectively outputs the composite signal S _CMP and the low-pass filter output according to the received electric field strength and multipath noise level. circuit, 1
Reference numeral 9 denotes a stereo demodulation section which receives the signal from the switching circuit and outputs stereo signals S _L and S _R of both the L and R channels, respectively.

As shown in FIG. 2, the stereo demodulator 19 converts the input signal into stereo signals L, L, and L of both left and right channels.
A stereo demodulation circuit 19a that generates R, and a separation circuit 19b that controls the degree of channel separation based on the control signal Sa.
and a stereo signal output from the separation control circuit 19c based on the control signal Sb.
It has a high-cut circuit 19c that attenuates high-frequency components (for example, 7 KHz or higher) of L' and R' and outputs stereo signals S _L and S _R.

20 is a mixer that mixes the stereo signals S _L and S _R and outputs a monaural signal S _M ; 21 is a mixer that changes the phase of the input signal and outputs two pseudo-stereo signals S _L ′ and S _R ′ of different phases; It is a pseudo stereo circuit,
Phase shift network section 21a and addition circuit 21
It has b.

FIG. 3 is a block diagram of the pseudo stereo circuit 21. The phase shift network section 21a is composed of two phase shift network circuits PSN1 and PSN2 for L-channel and R-channel,
The phase of each output signal of each phase shift network circuit is controlled by a control signal Sc. Such a phase shift network circuit
A specific circuit example of PSN1 and PSN2 is shown in FIG. In Figure 4, by changing the DC voltage of the control signal Sc, the varactor diode VD
1. By changing the capacitance C of VD2, the phase difference φ between the input and output can be made as shown in FIG. Therefore, each phase shift network circuit PSN1,
If the characteristics of the varactor diodes VD1 and VD2 of PSN2 are made different, the output signals SL'' and SR'' obtained at the terminals OUT _L and OUT _R can vary from zero to any value depending on the DC level of the control signal Sc. A phase difference θ can be added.

Returning to FIG. 3, the adder circuit 21b in the pseudo-stereo circuit includes first and second low-pass filters LPF1 and LPF2 having the same characteristics, and first and second low-pass filters LPF1 and LPF2 having the same characteristics.
second high-pass filters HPF1, HPF2; first and second attenuators AT1, AT2 having the same characteristics whose attenuation amount is controlled by the control signal Sd;
It includes first and second differential amplifiers DF1 and DF2. first and second low-pass filters LPF1,
The LPF2 passes the low-frequency components of the signals S _L ″ and S _R ″ output from the phase network section 21a, and filters the first and second high-pass filters HPF1,
HPF2 passes the high frequency components of the signals S _L ″, S _R ″,
The first and second attenuators AT1 and AT2 control the control signal Sd
The first differential amplifier DF1 amplifies the difference between the output signal of the low-pass filter LPF1 and the output signal of the attenuator AT1 to generate an L-channel pseudo-stereo signal. S _L ′ and the second differential amplifier DF
2 amplifies the difference between the output signal of the low-pass filter LPF2 and the output signal of the attenuator AT2 and outputs an R-channel pseudo-stereo signal S _R '.

Returning to FIG. 1, 22 is a pseudo-stereo circuit that selectively converts the stereo signals S _L and S _R from the stereo demodulator 19 and the monaural signal S _M from the mixer 20 according to the received electric field strength and multipath noise level. This is a second switching circuit that outputs to 21. A level meter (level detection circuit) 23 is connected to the intermediate frequency amplifier 13 and outputs a level signal LMO having a value corresponding to the received electric field strength (antenna input level) E, as shown in FIG. Furthermore, in Figure 6
NS is the noise characteristic and SG is the signal characteristic.

24 is a multipath noise detection circuit which receives the output from the FM detector 14 (or may be the noise removal circuit 15) and detects multipath noise components of 100 KHz or more, and 25 is a control circuit.

This control circuit 25 has a received electric field strength of a threshold level E _H2 (approximately 30 dBμ, as shown in Figs. 6 and 7).
(See figure difference) In the above case, the first switching circuit 18 selects and outputs the composite signal S _CMP , and the second switching circuit 22 selects and outputs the stereo signals S _L and S _R from the stereo demodulator 19. When E _H2 or lower, the first switching circuit 18 selects and outputs the main signal and pilot signal output from the low-pass filter 16, and the second switching circuit 2
2 to output the monaural signal S _M from the mixer 20. Further, the control circuit 25 switches the first switching circuit 1 when the detection level of multipath noise by the multipath noise detection circuit 24 is high even if the received electric field strength is equal to or higher than the threshold level E _H2 .
8 to output the main signal and pilot signal output from the low-pass filter 16, and
The second switching circuit 22 outputs the monaural signal S _M from the mixer 20.

26, when the received electric field strength becomes lower than the first threshold level E _H1 (approximately 55 dBμ, see Figure 7), the separation circuit 19b (see Figure 2) operates as the received electric field strength decreases. A level setting circuit 27 outputs a control signal Sa that weakens the _degree of separation;
A level setting circuit 28 outputs a control signal Sb that increases the high-frequency attenuation amount by the high-cut circuit 19c as the received electric field strength becomes smaller when the threshold level E _H3 (approximately 25 dBμ) or less is reached.
Signal output from the phase shift network section 21a when E _H2 <E < E _H1 (in sound field correction state)
A level control circuit 29 outputs a control signal Sc that increases the phase difference between S _L ″ and S _R ″ as the received electric field strength decreases, and 29 is an addition circuit 21 in the sound field correction state.
This is a level control circuit that outputs a control signal Sd that increases the phase difference (particularly the phase difference in the high frequency range) between the pseudo stereo signals S _L ′ and S _R ′ outputted from the stereo signals S L ′ and S R ′ outputted from the stereo signals S L ′ and S R ′ as the received electric field strength becomes smaller.

That is, the level control circuit 29
The amount of attenuation by the attenuators AT1 and AT2 in 1b is decreased as the received electric field strength becomes smaller, thereby increasing the high frequency components included in the pseudo stereo signals S _L ′ and S _R ′. As is clear from FIG. 5, the phase shift amount of the high frequency component is larger than that of the low frequency component. Therefore, as the received electric field strength decreases, more high-frequency components are included in the pseudo-stereo signals S _L ′, S _R ′, and the phase difference between the pseudo-stereo signals S _L ′, S _R ′ increases.

The control signals Sc and Sd are both controlled so that in the sound field correction state (E _H2 <E < E _H1 ), the pseudo stereo effect (sound field correction effect) becomes larger as the received electric field strength becomes smaller. Figure 7 shows pseudo stereo effect (sound field correction effect), separation,
Degree of high cut and each threshold level
The relationship between E _H1 to E _H3 is shown.

In the pseudo-stereo circuit 21, the capacitances of the varactor diodes VD1 and VD2 (see FIG. 4) of the phase shift network circuits PSN1 and PSN2 are controlled by the control signal Sc output from the level control circuit 28, thereby controlling the output. Signal S _L ″,
A phase difference θ occurs between the output signals S L ″ and S _R ″.The output signals S _L ″ and S _{R ″ can be used as pseudo-stereo signals, and can be input to a speaker (not shown) to create a phase difference θ between the output signals S L ″ and S R} ″. This pseudo-stereo signal can give a wider sound.
In order to further increase the sound spread by S _L ″ and S _R ″, the signal S _L ″ is passed through a low-pass filter LPF1 (see Figure 3) and applied to the positive input terminal of the differential amplifier DF1, and the signal S _R ″ high pass filter
It is applied to the negative terminal of differential amplifier DF1 through HPF1. Here, if the value of the control signal Sd output from the level control circuit 29 is determined so that the amount of attenuation of the attenuator AT1 is zero, a pseudo stereo signal can be obtained.
S _L ′ is output as a component with the opposite phase of the low frequency component of the signal S _L ″ and the high frequency component of the signal _SR ″. Similarly, the pseudo-stereo signal S _R ′, which is the output of the differential amplifier DF2, is output as a component having a phase opposite to that of the low-frequency component of the signal S _R ″ and the high-frequency component of the signal _SL ″. As a result, the pseudo-stereo signals S _L ′ and S _R ′ have a large phase difference, especially in the high frequency components, and thereby an even larger sound spread is obtained.

Now, if we increase the DC voltage of the control signal Sc and make the capacitances of varactor diodes VD1 and VD2 both zero, each phase shift network circuit PSN
1, the phase difference φ between the input and output of PSN2 both becomes zero, and the phase difference θ between the output signals S _L ″ and S _R ″ also becomes zero. Then, if a control signal Sd is applied so that the amount of attenuation in the adder circuit 21b is further increased, the input of the differential amplifier DF1 becomes only the signal obtained by passing the signal S _L '' through the low-pass filter LPF1, and the input of the differential amplifier DF2 becomes Low pass filter the signal S _R ″
Only the signal passed through the LPF 2 is output, and the outputs of the differential amplifiers S _L ′ and S _R ′ are output as the same signal with no phase difference, resulting in a monaural signal with no sound spread, with the sound image localized at the center of the speaker. In this case, high-frequency components are cut by each low-pass filter, but by widening the frequency characteristics of the low-pass filters, it is possible to make the deterioration of the frequency characteristics accompanying the cutting of high-frequency components audibly negligible. .

On the other hand, when the DC voltage of the control signal Sc is gradually lowered, the phase shift network circuit PSN1,
As the phase difference θ between the output signals S _L ″ and S _R ″ of the PSN 2 gradually increases, and the amount of attenuation is gradually reduced by the control signal Sd in the adder circuit 21b, the phase difference in the high frequency component becomes particularly large. Become. Therefore, when such pseudo-stereo signals S _L ′, S _R ′ are input to the speakers, the sense of localization of the sound image becomes weaker, but the spread of the sound is expanded (pseudo-stereo effect).

Next, the overall operation of FIG. 1 will be explained.

(A) When the received electric field strength is greater than the first threshold level E _H1 (see Figure 7) and the multipath noise level is small. In this case, the first switching circuit 18 is replaced by the noise removal circuit 15. Composite signal output from
S _CMP is output to the stereo demodulation section 19. The stereo demodulator 19 uses the composite signal S _CMP to generate stereo signals S _L and S _R of both left and right channels, and the second switching circuit 22 generates stereo signals S L and S R of both left and right channels.
S _L and S _R are outputted to a speaker (not shown) via a pseudo-stereo circuit 21 to output a sound with a rich stereo feel that has a spacious sound and localizes the sound image depending on the sound source. ...stereo state In this case, the stereo demodulation circuit 19a (second
Control signals Sa, 27 of the level setting circuits 26, 27 are set so that the stereo signals _L , _R output from
Sb is set, and the stereo signals S _L , S _R
The control signals Sc and Sd are controlled so that the signal is input as is to a speaker (not shown), and the sound field correction effect (pseudo-stereo effect) is made zero.

(B) When the received electric field strength is below the first threshold level E _H1 The received electric field strength is at the first threshold level
Since the noise starts to increase when it becomes lower than E _H1 , the control signal Sa is controlled, and the separation control circuit 19b is operated to weaken the degree of separation as the received electric field strength decreases as shown in Fig. 7. Improve.

By the way, when the degree of separation is weakened, the sound tends to be monaural, the sound spread narrows, and the stereo effect disappears. Therefore, at the same time as weakening the degree of separation, the control signal Sc,
By controlling Sd, the pseudo-stereo effect is strengthened as shown in Figure 7, and the reduction in sound spread due to separation control is compensated for.
Spreading the sound, in other words, maintaining a stereo feeling. ...Separation/sound field correction state (C) When the received electric field strength is less than the second threshold level E _H2 When the received electric field strength becomes less than the second threshold level E _H2 , the control circuit 25 sets the level The setting circuit 26 generates a control signal Sc that makes the separation zero, and a switch signal SW is generated to cause the first switching circuit 18 to switch and output the signal from the low-pass filter 16 to eliminate noise. The monaural signal S _M from the mixer 20 is
is input to the pseudo stereo circuit 21. As a result, a pseudo-stereo state is created by a pseudo-stereo signal created using the monaural signal S _M. …
Pseudo-stereo state By the way, in a stereo state, the sound is spread out, and the sense of localization of the sound source, that is, the sense of the position of the sound source, is clear. However, in a pseudo-stereo state, although the sound is spread out, there is no sense of localization of the sound image.
Therefore, directly switching from a stereo state to a pseudo-stereo state creates an audible sense of discomfort. In other words, unless the stereo state and pseudo-stereo state are switched in a manner that reduces the change in sound spread and localization of the sound source, the change in sound will create a sense of discomfort. However, in the present invention, the sound field correction state (B) is interposed, and the received electric field strength is the first.
Below the threshold level E _H1 , the magnitude of the control signals Sc and Sd is controlled so that as the received electric field strength decreases, the pseudo-stereo effect increases as shown in Figure 7. It is possible to shift to a pseudo-stereo state without any noise, and moreover, the pseudo-stereo allows a stereo sound field to be formed in a weak electric field.

Note that the received electric field strength is at the threshold level.
The reason why only the main signal (L+R) is output instead of the composite signal S _CMP when E _H2 or less is as follows. That is, if the distortion component of the composite signal increases significantly due to noise, the operation of the stereo demodulator 19 becomes unstable, and when the monaural signal _SM is produced by the mixer 20 using the stereo signals S _L and S _R , noise increases. As a result, noise is mixed into the pseudo stereo signals S _L ′ and S _R ′ as well. This is because FM triangular noise has a high level of noise, especially in the sub-signal (L-R) band, which makes the operation of the stereo demodulation circuit 19a unstable. By removing the noise, the stereo demodulation circuit can maintain stable operation.
Therefore, when the received electric field strength falls below the second threshold level E _H2 and the noise increases,
Alternatively, as will be described later, when multipath noise becomes large, the first switching circuit 18 switches the sub-signal (L-R) removed signal to the stereo demodulator 1.
9.

(D) When the received electric field strength is below the third threshold level E _H3 The received electric field strength is below the third threshold level E _H3
(Fig. 7) When the following occurs, the high-cut circuit 19c
(FIG. 2) starts operating, and as the received electric field strength decreases, the amount of high-frequency component cut is increased by the control signal Sd, making suppression of noise in a weak electric field even more effective. ...Pseudo-stereo high-cut state (E) When the multipath noise level is above the predetermined threshold level (A) to (D) are cases where the multipath noise is below the predetermined level, but the received electric field strength Regardless, when the multipath noise becomes large and exceeds a predetermined threshold level, the control circuit 25 causes the first switching circuit 18 to select the main signal (L+R) from the low-pass filter 16.
At the same time, the second switching circuit 22 causes the monaural signal S _M to be output to the pseudo-stereo circuit 21, and further controls the signals Sa, Sb, Sc, and Sd to create a pseudo-stereo state, thereby reducing noise. Plan.

The above description assumes that the pseudo-stereo effect (sound field correction effect) is zero at the first threshold level E _H1 or higher, as shown by the solid line in Figure 7.
As shown by the dotted line in the figure, the control signals Sc and Sd can be controlled above the first threshold level so that the pseudo stereo effect increases as the received electric field strength increases. By increasing the pseudo-stereo effect as the received field strength increases as shown by the dotted line, the normal stereo condition can be changed to a wide stereo condition in which the sound spreads further, thereby increasing the sense of presence.

Incidentally, in the above description, one pseudo-stereo circuit 21 is provided after the second switching circuit 22 (see FIG. 1), but the stereo circuit 19 and the mixer 20
It is also possible to configure one pseudo stereo circuit to be provided in each subsequent stage.

<Effects of the Invention> As explained above, according to the present invention, when the electric field is weak below the first threshold level E _H1 , the degree of separation is weakened according to the received electric field strength to improve the S/N ratio. At the same time, the sound field correction effect (pseudo-stereo effect) by the pseudo-stereo circuit is increased in accordance with the received electric field strength to compensate for the narrowing of the sound due to the separation control, so that the received electric field strength reaches the first threshold E. _H1
From the stereo condition above, the received electric field strength is at the second level.
When transitioning to a pseudo-stereo state when the threshold is below E _H2 (< E _H1 ), the S/N ratio can be improved and the stereo feeling can be maintained, and the transition from a stereo state to a pseudo-stereo state or a pseudo-stereo state can be made without any discomfort. You can switch to stereo mode.

Further, according to the present invention, when the received electric field strength falls below the second threshold E _H2 or when multipath occurs, a pseudo-stereo state is created and a high-cut state is created, so that the S/N ratio can be further improved. Moreover, the stereo feeling can still be maintained.

Furthermore, according to the present invention, the signal input to the speakers is always extracted through the pseudo-stereo circuit regardless of the received field strength, and the magnitude of the pseudo-stereo effect is controlled even in a stereo state where the received field strength is high. By doing so, it is possible to create a wide stereo state when a strong electric field is applied, further widening the sound field and increasing the sense of presence.

[Brief explanation of the drawing]

Figure 1 is a block diagram of an FM stereo receiver according to the present invention, Figure 2 is a block diagram of a stereo demodulator, Figure 3 is a block diagram of a pseudo stereo circuit, and Figure 4 is a circuit diagram of a phase shift network circuit. , Fig. 5 is a diagram of the relationship between the capacitance of the varactor diode and the input/output phase difference φ in the phase shift network circuit, Fig. 6 is a diagram of the output characteristics of the reception level detection circuit, and Fig. 7 is a diagram of the pseudo stereo with respect to the reception electric field strength. It is a characteristic diagram showing the degree of effect, separation, and high cut. 19... Stereo demodulation section, 19a... Stereo demodulation circuit, 19b... Separation control circuit, 19c... High cut control circuit, 20... Mixer, 21... Pseudo stereo circuit, 22... Switching circuit, 23 ... Level detection circuit (level meter), 24 ... Multipath noise detection circuit.

Claims (1)

[Claims] 1. 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; a separation control circuit that changes the degree of separation of the stereo output according to the output of the level detection circuit; and a separation control circuit for the L channel; Equipped with a phase shift means for the R channel, it has a pseudo-stereo circuit that shifts the phase of each input signal and changes the difference in the amount of phase shift according to the received electric field strength to change the degree of the pseudo-stereo effect. However, when the received electric field strength is above the first threshold level, the stereo state is established, and when the received electric field strength is below the first threshold level and above the second threshold level, In this method, separation control by the separation control circuit and sound field correction by the pseudo-stereo circuit are operated, and the pseudo-stereo effect by the pseudo-stereo circuit is increased as the received electric field strength decreases, and the output of the separation control circuit is A monaural signal is input to the speaker via a stereo circuit, and when the received electric field strength is below the second threshold level, a monaural signal generated using the stereo output is input to the speaker via a pseudo-stereo circuit to generate pseudo-stereo. FM stereo receiver characterized by state. 2. A high-cut control section is provided that removes components of a predetermined frequency or higher when the received electric field strength is a weak electric field below a third threshold level near the second threshold level. FM stereo receiver described in Range 1. 3. The pseudo-stereo circuit includes two low-pass filters that pass the low frequency side of the output of each phase shift means for the L channel and the R channel, and the high frequency side of the output of each phase shift means for the L channel and the R channel. Two attenuators to which each high-pass filter output signal is input and which reduce the amount of attenuation as the received electric field strength decreases, the low-pass filter output of the L channel, and the high-pass signal of the R channel. a first means for outputting a difference signal between the output of the attenuator into which the filter output is input as a pseudo stereo signal for the L channel; and an attenuator output into which the output of the R channel low-pass filter and the high-pass filter output of the L channel are input. and second means for outputting a difference signal between the two as a pseudo stereo signal for the R channel.
FM stereo receiver as described in section. 4. The FM stereo receiver according to claim 3, wherein the FM stereo receiver is configured to output a stereo signal to a speaker through a pseudo-stereo circuit even in a stereo state. 5. When the received electric field strength is above the first threshold level, the pseudo-stereo effect by the pseudo-stereo circuit is increased as the received electric field strength increases, and the state changes from a stereo state to a wide stereo state according to the received electric field strength. An FM stereo receiver according to claim 1. 6. Claim 1, characterized in that a multipath noise detection circuit for detecting multipath noise is provided, and switching between a stereo state and a pseudo-stereo state is performed according to the multipath noise level regardless of the received electric field strength. FM stereo receiver.