JPS6243380B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a stereo signal receiving system that can maintain a stereo effect with a good S/N ratio even in a weak electric field.

When an FM stereo receiver receives a weak electric field, the S/N of stereo broadcasting is 20 dB worse than that of monophonic broadcasting. Then, when the antenna input signal level decreases and the S/N drops to about 50 dB,
Stereo reception is automatically switched to monophonic reception to improve the S/N ratio. However, since the stereo is switched to monophonic, the three-dimensional effect is suddenly lost. Furthermore, when moving in a car, the input electric field constantly fluctuates significantly, and the stereo
Since monophonic switching is also performed, the situation is extremely unfavorable to the auditory sense. Therefore, as a means to improve this, we improved the stereo demodulator and changed the degree of separation of the left and right demodulated signals in response to the input electric field, resulting in quasi-stereo demodulation, which caused a sudden drop in S/N and a stereoscopic effect. This is an attempt to alleviate the decline in Here, quasi-stereo refers to an intermediate state between mono and stereo in which left and right stereo signals are added to reduce FM stereo noise that occurs as the antenna input becomes lower. In this state, the degree of separation between the left and right sides is not sufficient, so the sound image tends to concentrate in the center. By reducing this damage by using the sound image enlargement effect, the aim is to achieve an effect in which the sound image is sufficiently enlarged while reducing noise.

FIG. 1 shows a system diagram of the conventional method. In the figure, a received signal obtained by an antenna 1 is amplified by a high frequency amplifier circuit 2, mixed with excitation from a local oscillator 3, and outputted from a mixer circuit 4 as an intermediate frequency signal.
Further, after being amplified by an intermediate frequency amplifier 5, the signal is detected by an FM detector 6 to obtain a demodulated output. When receiving stereo broadcasting, the stereo demodulator 7 outputs different left and right stereo signal outputs Lout and Rout. The demodulation characteristics of the stereo demodulator 7 are controlled by the output of the intermediate frequency level detector 8. Its characteristics are shown in Figure 2. FIG. 2 shows the relationship between the antenna input signal level, the demodulated signal, and the noise level. Curve 12 represents the dependence of the demodulated signal level on the antenna input signal level,
Curve 13 is the noise characteristic during stereo reception, curve 14
indicates the noise characteristics during monophonic reception. As shown in the figure, in a weak electric field region, for example, in a region where the antenna input signal level is about 30 μV or less, quasi-stereo demodulation, which is between stereo demodulation and monophonic demodulation, is performed, and the noise characteristic shown by curve 15 is obtained. When the antenna input signal level further decreases, a decrease in the antenna input signal level is detected by the output of the intermediate frequency level detector 8, and the receiver enters a monophonic reception state. In other words, if the antenna input signal has gradually decreased from a sufficiently high level, the second
The noise characteristics are shown by solid lines 13, 15, and 14 in the figure.

At this time, the stereo demodulator is as shown in Figure 3.
Lout and Rout signals can be obtained. That is, if an antenna input signal of approximately 30 μV or more is obtained, a sufficient degree of separation can be obtained between the left and right signal outputs of 16 and 17, and if it is lower than this, a quasi-stereo signal output can be obtained. However, although quasi-stereo can ensure better S/N than stereo, the problem remains that the three-dimensional effect deteriorates. The pilot signal detector 9 detects the presence or absence of a pilot signal, and if there is no pilot signal, the receiver is placed in a monophonic reception state.

An object of the present invention is to provide a stereo signal reception method that improves stereoscopic effect without deteriorating noise characteristics even when receiving a weak electric field.

Therefore, when the antenna input signal level is low, the present invention obtains a quasi-stereo signal to ensure S/N, but at the same time, it adds a sound image enlargement device to improve the stereo feeling, and the electric field is weaker than before. The purpose is to maintain the stereo feeling until input.

A system diagram of an embodiment of this system is shown in FIG. Fourth
In the figure, 1 to 9 represent the same functions as those with the same numbers in FIG. Reference numerals 18 and 19 denote variable sound image enlarging circuits, which in this embodiment are comprised of spatial transfer characteristic adding circuits 20 and 21 and linear switches 22 and 23, respectively. Note that the variable sound image enlarging circuit may vary the spatial transmission frequency characteristic only by the spatial transmission characteristic adding circuit. Output of stereo demodulator 7 whose separation characteristic changes depending on the antenna input signal level
L'out and R'out are each distributed into three parts.
First, L'out is directly connected to the linear switch 22 (connect a buffer amplifier if necessary)
In addition, two spatial transfer characteristic adding circuits 20 and 21
connected to. Next, R'out is directly connected to the linear switch 23, but similarly to L'out, it is connected to two spatial transfer characteristic adding circuits 20 and 21. In addition to L'out, the linear switch 22 receives as its input the output signal of one 20 of the spatial transfer characteristic adding circuits. Similarly, the linear switch 23
In addition to R'out, the output signal of the spatial transfer characteristic addition circuit 21 is obtained and used as its input. Linear switch 2
The outputs of 2 and 23 are connected to terminals 10 and 1, respectively.
1 as stereo signals Lout and Rout.

Here, we will discuss the role of the spatial transfer characteristic adding circuit. What determines the sense of stereo (or the sense of direction and distance of a sound source) are the spatial transmission characteristics, that is, the amplitude frequency characteristics and delay time characteristics (phase characteristics) when sound waves reach the listener's ears from the sound source. be. These are mainly determined by head diffraction effects and are closely related to stereo perception. Therefore, a spatial transfer characteristic adding circuit is a circuit that replaces these physical characteristic quantities with an electric circuit. The spatial transfer characteristic addition circuit is provided to obtain a sound image enlargement effect, and performs addition/subtraction processing on the left and right stereo signals by giving them desired frequency characteristics. For example, as shown in Japanese Patent Application Laid-Open No. 116203/1983, in a stereo sound field using two general speakers, it is not possible to obtain a sound image localized outside the plane surrounded by the speakers. A sound image localization setting circuit part (SX ₁ of JP-A-52-116203) and a crosstalk removal circuit part (JP-A-116-116203) that removes crosstalk that should occur in the reproduction sound field from the output from the sound image localization setting circuit part (JP-A-52-116203). SX ₂ ) of No. 116203/1972), it is possible to obtain a sound image localized beyond the position of the two speakers. FIG. 5 shows an example of the frequency characteristics of the spatial transfer characteristic adding circuit. This feature is characterized by a peak at 2-3kHz and a sharp cut-off from 7-8kHz. This characteristic seems to be related to the difference in characteristics between the direct component and the cross component from the sound source for both left and right ears, or to the resonance phenomenon of the auditory canal.

In order to obtain the sound image enlargement effect, a signal L'out passes through the spatial transfer characteristic adding circuit 20 and reaches the linear switch 22, and a signal R'out passes through the spatial transfer characteristic adding circuit 20 and reaches the linear switch 22. The frequency characteristics are different. Unexamined Japanese Patent Publication 1977-
As shown in Japanese Patent No. 116203, the sound image is enlarged by adding left and right stereo signals with different frequency characteristics, that is, desired frequency characteristics. By changing this amount of addition, the amount of expansion of the sound image is changed. Here, the amount of expansion of the sound image is changed by inputting to a linear switch and changing the amount of addition. It is necessary to appropriately select the level difference between peak values and the overall level difference. The frequency characteristics when the output of the stereo demodulation circuit 7 is sent to the linear switch 22 or 23 are assumed to be generally flat as shown in FIG. 6.

By controlling these two types of signal systems with different frequency characteristics and continuously changing the frequency characteristics, it is possible to continuously change the sound image enlargement effect.

Next, an embodiment of the linear switch circuit is shown in FIG. Connecting the collectors of different differential amplifier circuits of the two sets of differential amplifier circuits 24,
One is directly connected to the power supply terminal 26, and the other is connected to the power supply terminal 26 via a resistor 25. A switch output is obtained from the connection point between the resistor 25 and the differential amplifier circuit pair 24 to the output terminal 28 via the capacitor 27.

Transistors 29 and 30 are connected to the constant current circuits of the differential amplifier circuit, respectively, and the output of the stereo demodulator and the spatial transfer characteristic addition circuit 20 are connected to the bases of the transistors 29 and 30 via capacitors 31 and 32, respectively.
Alternatively, the output of 21 is given as an input.
One connection point 35 of the bases of the transistors of the differential amplifier circuit is biased to a voltage E ₀ provided by resistors 33, 34. The other base connection point is connected to terminal 36 and is used to control switching of the input signal of the linear switch. If the voltage at the control terminal 36 is changed around E ₀ as shown in FIG. 8, the signals at the input terminals 31 and 32 can be continuously switched to the output terminal 28. Therefore, if the voltage at the terminal 36 and the stereo demodulator 7 are appropriately controlled by the output of the level detector 8, the degree of separation of the stereo demodulation and the sound image expansion effect can be controlled.

Note that the resistors 39 and 40 are bias resistors of constant current transistors, and the capacitors 37 and 38 are bias capacitors.

In the system of the present invention shown in FIG. 4 described above, in the example shown in FIG. 9, the antenna input signal level S is controlled in the range of about 30 μV to 100 μV to about 2 to 3 μV so that the stereo demodulator performs quasi-stereo demodulation. , and the sound image enlargement effect in this range is shown in Figure 9. When the antenna input signal level is high, the sound image enlargement ratio is decreased, and as the level becomes low, the ratio increases up to the maximum q/p. If you do this, the dotted line will give a three-dimensional effect.
L′out and R′out are expanded to Lout and Rout shown by broken lines. As a result of the above, the three-dimensional effect can be maintained even at antenna input signal levels that are 10 to 15 dB lower than before. At this time, since the stereo demodulation is performed as quasi-stereo demodulation as in the past, the S/N ratio does not decrease. Even during stereo broadcasting, if monophonic reception is to be performed, there is no problem in forcibly switching the pilot detection circuit for monophonic reception.

As described above, according to the present invention, it is possible to obtain a better three-dimensional effect than in the past without lowering the S/N ratio to a region where the antenna input signal level is 10 to 15 dB lower than in the past.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a conventional stereo receiver, Fig. 2 is a diagram showing the antenna input signal level dependence of demodulated signal output and noise output in each reception state, and Fig. 3 is a diagram showing changes in conventional stereo output. , FIG. 4 is a block diagram of an embodiment of the present invention, and FIG. 5 is a block diagram of an embodiment of the present invention.
The figure is a frequency characteristic diagram of the spatial transfer characteristic addition circuit, No. 6.
The figure is a frequency characteristic diagram when the antenna input signal level is sufficiently high and no sound image expansion effect is applied. Figure 7 is a circuit diagram showing an example of a linear switch. Figure 8 is a control voltage dependence of the output of the linear switch. The characteristic diagram, FIG. 9, is a diagram showing the effect of implementing the present invention. 1...Receiving antenna, 2...High frequency amplification circuit, 3...
Local oscillation circuit, 4...Mixer circuit, 5...Intermediate frequency amplifier circuit, 6...FM detector, 7...Stereo demodulator, 8...Level detector, 9...Pilot signal detector, 10...Left output, 11...Right output , 18... Variable sound image enlarging circuit, 19... Variable sound image enlarging circuit, 20... Spatial transmission characteristic adding circuit, 21... Spatial transmission characteristic adding circuit, 22... Linear switch, 23... Linear switch.

Claims (1)

[Claims] 1. A stereo demodulator that obtains a stereo signal, a signal between stereo and monophonic, or a monophonic signal as a demodulated signal, a pair of variable sound image enlargement circuits connected to the output of the stereo demodulator, and a level detector that detects the antenna input signal level. A stereo demodulator and a pair of variable sound image enlargement circuits are controlled by the output of the antenna input signal level detector, and the demodulated output of the stereo demodulator is converted from a stereo signal to a signal intermediate between stereo and monophonic, and further to a monophonic signal. 1. A stereo signal receiving method characterized in that the stereo sound is improved when the antenna input signal level is low by changing the sound image expansion ratio of the variable sound image expansion circuit from small to large.