JPH01277032A - Fm stereo receiver - Google Patents

Abstract

PURPOSE:To listen without the change of stereo feeling even when there is a switch from stereo to pseudo-stereo based on a multipulse noise level by changing almost equally the degree of stereo separation and the degree of pseudo-stereo separation when a receiving electric field strength is at a first level or below. CONSTITUTION:A level meter 20 of an FM stereo receiver detects the receiving electric field strength and a multipulse noise detecting circuit 21 detects a pulse noise. A stereo demodulating circuit 16 to input the outputs of an FM detector 14 and a noise removing circuit 15 outputs stereo signals SL and SR of both L and R channels. A separation control circuit 17 lowers the degree of the separation of the stereo signals as the receiving electric field strength becomes smaller, pseudo-stereo signals S'L and S'R are outputted from a pseudo-stereo circuit 18, pseudo-stereo effect is increased when the receiving electric field strength becomes smaller, and stereo signals L and R are outputted from a switching circuit 19 in accordance with the level of the receiving electric field strength.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to an FM stereo receiver, and in particular uses a combination of stereo and pseudo-stereo to improve the S/N ratio and stereo feeling in weak electric fields and when multipath noise occurs. FM that can improve
Regarding radio receivers.

<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 when multipath noise is large or when the electric field is weak. For this reason, in the past, the received field strength (antenna input level) and multipath noise level were monitored, and when the received field strength became below a predetermined threshold level, it was switched from stereo to monaural, and when the received field strength became equal to or higher than the threshold level. When this happens, the system switches from monaural to stereo, and further switches from stereo to monaural when multipath noise is large even if the received field strength is above a predetermined level.

However, if the received electric field strength or multipath noise level fluctuates around a predetermined level, the frequency of the fluctuations may cause the sound to become stereo with a wide spread, or become monaural with no sound spread, which may cause problems for the listener (user). Gives an audible sense of discomfort.

For this reason, when a predetermined threshold level is reached, the degree of stereo separation is weakened in accordance with the received electric field strength, and the state is gradually shifted from stereo to monaural. According to this method, it is possible to eliminate the sense of discomfort compared to the case of directly converting to a monaural state. However, it is not possible to improve the lack of stereo feeling in a monaural state.

Therefore, we have created a pseudo-stereo circuit, which switches from stereo to pseudo-stereo when the separation deteriorates due to an electric field or when the multipath noise level exceeds a certain level, expanding the sound spread in weak electric fields. A method has been proposed to maintain the stereo effect.

<Problems to be Solved by the Invention> However, in this proposed method, when switching between stereo and pseudo-stereo state due to multipath noise, the stereo separation value in switching 4 India is different between stereo and pseudo-stereo state, There is a problem in that the change in state is unfavorable to the auditory sense of the ear and the ear.

In light of the above, an object of the present invention is to provide an FM stereo receiver that can make changes in the state between stereo and pseudo-stereo due to multipath noise inaudible.

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

12 is a front end, 16 is a stereo demodulation circuit, 17
18 is a separation control circuit, 18 is a pseudo stereo circuit, 19 is a switching circuit, 20 is a level meter, 21 is a multipath noise detection circuit, and 22 is an OR circuit.

Effect> As the received electric field strength becomes smaller, the stereo separation circuit 17 weakens the degree of separation 1 of the stereo signal, and the pseudo-stereo circuit 18 increases the pseudo-stereo effect.

When the multipath noise detected by the multipath noise detection circuit R121 exceeds a predetermined level, the stereo signal S
Instead of L, S, pseudo stereo signals SL/, S, / are output.

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

In the figure, 1.1 is the antenna, 12 is the front end, and 13
14 is an intermediate frequency amplifier, 14 is an FM detector, 15 is a noise removal circuit that removes pulsed noise (such as Ibunirasian noise) contained in the FM detection output, and 16 is a stereo decoded signal SCMP output from the noise removal circuit. has been entered,
A stereo demodulation circuit outputs stereo signals SL and SR of both R channels, and 17 is a separation control section (blend circuit) that controls the degree of stereo separation according to the received electric field strength (antenna input). This separation control section 17
As shown in Fig. 2, when the received electric field strength EA becomes below the first level E, (approximately 55 dBμ), the degree of separation is weakened as the received electric field strength decreases, and S/
Improve the N ratio.

18 is a pseudo-stereo circuit, which outputs the stereo composite signal Sc
Mp is input and a pseudo stereo signal SL' is generated.

8.1 and, as shown in FIG. 2, when the received electric field strength EA becomes less than E1, the pseudo stereo effect increases as the received electric field strength becomes smaller.

19 is a switching circuit, 20 is a level meter (level detection circuit) connected to the intermediate frequency amplifier 13 and outputs the received field strength (antenna input level) EA, 21 is a multipath noise detection circuit, and 22 is an OR circuit.

FIG. 3 shows an embodiment of a multipath noise detection circuit and a level meter. The level meter 20 is connected to the level meter terminal TL of the intermediate amplifier 13, and the level shift LED
20a, a common emitter type transistor 20b that controls the degree of conductivity according to the received electric field strength, and an output port! $2
It is composed of 0 c.

The multipath noise detection circuit 21 also includes a high-pass filter 21a that extracts and outputs multipath noise components of 100 KHz or more included in the level meter terminal TL output.
A noise amplifier 21b, a detector (low-pass filter) 21c that converts a multipath noise signal into direct current, and an output level Erl when the multipath noise level becomes small.
It has a time constant Js21d that reduces the time constant Js21d with a predetermined time constant. In addition, the time constant is determined by capacitor C and resistors R15 and R1.
6.

FIG. 4 is a block diagram of the pseudo-stereo circuit 18, and shows the stereo decoded signal S. A comb-shaped filter 18a receives the PM and has the frequency characteristic F1 in FIG.
and a differential circuit 18b that outputs the difference between the comb filter output and the input signal, and the output of the comb filter 18a is an L-channel pseudo stereo signal SL';
The output of the differential circuit 18b is an R-channel pseudo-stereo signal S, /, and when these pseudo-stereo signals are used, a stereo feeling can be obtained compared to monaural signals.

Incidentally, the frequency characteristic F2 in FIG. 5 is the frequency characteristic between the input and the output of the differential circuit. That is, the pseudo stereo signal SL' can be regarded as the output of the band eliminator filter,
The pseudo-stereo signal SR' can be regarded as the output of a bandpass filter.

FIG. 6 shows a circuit example of the pseudo-stereo circuit 18, and by changing the value of the variable resistor R4 in the comb-shaped filter 18a, the Q of the band eliminator filter and the band pass filter can be controlled.

Figure 7 shows the frequency characteristics when changing the value of variable resistor a4, and the dotted line indicates R4 = 56Ω,
- The dashed line is Ω for R4 = 100, and the solid line is Ω for R4 = 160.
In this case, as the resistance value increases, the Q increases and the characteristics become steeper, separation errors become clearer, and the stereo effect increases.

FIG. 8 shows an example of a variable resistor @R4, in which the resistance value is increased as the output (received electric field strength) EA of the level meter 20 becomes larger. In the figure, ICI to IC4 are comparators, TRI to TR4 are switching transistors, and level meter output EA is set to level E.
, (see Figure 2), only transistor TR4 is turned on and R4=R14.
As the level meter output becomes smaller, (
ii) At the second level, transistors TR3 to TR4 are turned on, and the resistance value of R4 becomes parallel to R13 and R14;
turns on and the resistance value of R4 becomes the parallel of R12 to R14.
At the fourth level, which is close to the Gψ level E2 (see FIG. 2), all the transistors TRI to TR4 are turned on, and the resistance value of R4 becomes the parallel connection of R11 to R14. That is, the resistance value of R4 decreases as the received electric field strength decreases.

The overall operation of FIG. 1 will be explained below. It is assumed that the multipath noise level is initially smaller than a predetermined level.

(When the al received electric field strength is above the first level E, a) In this case, the separation control circuit 17 does not control separation, and the OR circuit 22 outputs a low level switching signal SW.

As a result, the switching port #I22 receives the stereo signals SL, S1 . I is output to a speaker (not shown), and a rich stereo sound is output in which the sound image is localized depending on the sound source and the sound spread is wide or narrow.

(b) When the received electric field strength is less than E1 If EA<E, the separation control circuit 17 controls the degree of separation as shown in FIG. 2 according to the level meter output (received electric field strength) EA. . However, the OR circuit 22 still outputs the low level switching signal SW.

As a result, the switching circuit 22 outputs the stereo signal SL output from the stereo demodulator 16 and subjected to separation control.
, SF1 are output to a speaker (not shown) to improve the S/N ratio and eliminate the audible discomfort.

(When the C1 received field strength is below the second level E2, the degree of separation becomes zero, and when the level near the received field strength at which the pseudo stereo effect is maximum is E2 (see Figure 2), the received field strength When EA is less than or equal to E2, a high level switching signal SW is output from the OR circuit 22. When the switching signal SW becomes high level, the switching circuit 19 outputs a pseudo stereo signal S instead of the stereo signals SL and SR. L.I.

Outputs S, /. As a result, a stereo sound with a wider sound range than monaural sound can be obtained.

Note that when the received electric field strength EA becomes lower than the first level El, the variable resistor WR4 (sixth
The resistance value (see FIG. 8 and FIG. 8) decreases in accordance with the received electric field strength EA, and the degree of separation of the pseudo-stereo signal decreases, but a stereo sound with a wider sound can be obtained than in monaural.

The above is a case where the multipath noise level is below a predetermined level. Next, a case where the multipath noise level is above a predetermined level will be described.

(d) When the received electric field strength is E or more When EA>El, when the multipath noise level reaches a predetermined level or more, the OR circuit 22 outputs a high-level switching signal SW. As a result, the switching circuit 22 switches the pseudo stereo signals SL', SR output from the pseudo stereo circuit 18 instead of the stereo signals SL, S.
′ is output. As a result, the sound spread is maintained.
N ratio can be improved. Note that the resistance value of the variable resistor R4 of the pseudo-stereo circuit 18 is the maximum, and therefore the degree of separation of the pseudo-stereo signal is the maximum, so even if the state changes, there is no big change in the stereo feeling. There is no audible unnaturalness.

(, 1 When the received electric field strength is less than or equal to E1 When the multipath noise level exceeds a predetermined level in the case where EA<El, a high level switching signal SW is output from the OR circuit Hs22. As a result, the switching circuit 22
are the pseudo-stereo signals BLI, S output from the pseudo-stereo circuit 18 instead of the stereo signals SL, S.
, prints #.

By the way, in the case of EA, separation control is performed by the separation control circuit $17, and the degree of separation changes as shown in FIG. 2 in accordance with the received electric field strength EA. Therefore, if you switch from a stereo signal to a pseudo-stereo signal or vice versa while keeping the pseudo-stereo effect constant, the sound spread between stereo and pseudo-stereo at the switching point will change.
(Stereo effect) is different, creating a strange feeling. - However, in the present invention, the resistance value of the variable resistor R4 in the pseudo-stereo circuit 18 is made smaller as the received electric field strength EA becomes smaller, thereby restricting the degree of separation of the pseudo-stereo output.

That is, when the received electric field strength is less than E1, the degree of separation in pseudo stereo is changed almost the same as the degree of separation in stereo. As a result, no matter what level the switching between stereo and pseudo-stereo occurs, there is no significant change in the stereo feel, and there is no unnaturalness in the auditory sense.

In this case, even if the multipath noise level exceeds a predetermined level, there is no change in the state since the pseudo stereo state has already been achieved.

<Effects of the Invention> According to the present invention, since the degree of stereo separation 1 and the degree of pseudo-stereo separation are changed in substantially the same way when the received electric field strength becomes lower than the first level, the multi-channel Even if there is switching between stereo and pseudo-stereo based on the pass noise level, the stereo feeling does not change, which is preferable for the auditory sense.

[Brief Description of the Drawings] Fig. 1 is a block diagram of an FM stereo receiver according to the present invention; Fig. 2 is an explanatory diagram of stereo separation characteristics and pseudo-stereo effects; Figure 3 is an example circuit diagram of a level meter and multipath noise level detection circuit, Figure 4 is a block diagram of a pseudo-stereo circuit, Figure 5 is a frequency characteristic diagram of a pseudo-stereo circuit, and Figure 6 is a diagram of a pseudo-stereo circuit. FIG. 7 is a diagram showing the relationship between the resistance value and frequency characteristics of a variable resistor in a pseudo-stereo circuit, and FIG. 8 is a circuit diagram of an embodiment of the variable resistor. 12... Front end, 16... Stereo demodulation circuit, 17... Separation control circuit, 18... Pseudo stereo circuit, 19... Switching circuit', 20... Level meter, 21... Multipath noise detection circuit patent application Person Alpine Co., Ltd. Agent Patent Attorney Chiki Saito - Niguchi [3-18m+] Figure 5 0.1 1.0 KHz K)lz, '9CHZ Hot water Figure 6

Claims (1)

[Claims] A level detection circuit that detects received electric field strength, a multipath noise detection circuit, a stereo demodulation circuit that outputs stereo signals S_L and S_R of both L and R channels, and the received electric field strength is small. a separation control circuit that weakens the degree of separation of the stereo signal as the received field strength decreases; a pseudo-stereo circuit that outputs the pseudo-stereo signals S_L', S_R' and increases the pseudo-stereo effect as the received electric field strength decreases; When the level near the received electric field strength at which the signal becomes zero and the pseudo-stereo effect is maximum is E_2, when the received electric field strength is E_2 or more and no multipath noise occurs, the stereo signal S_L,
S_R and outputs pseudo stereo signals S_L' and S_R when multipath noise occurs regardless of the received field strength or when the received field strength is below level E_2.
An FM stereo receiver equipped with a switching circuit that outputs '.