JPS6214979B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a stereo demodulation circuit, and particularly to a stereo demodulation circuit suitable for use when receiving AM stereo broadcasting.

Normally, when receiving AM monaural broadcasts or AM stereo broadcasts, it is common to experience the influence of beat interference from neighboring stations, and this phenomenon is particularly severe at night.

Therefore, conventional beat elimination methods for AM receivers have been proposed for monaural broadcasting, such as inserting a notch filter into the audio frequency stage, narrowing the intermediate frequency stage, or using a fading method to receive one sideband. However, the former two methods have the disadvantage that not only the beat signal but also the original signal is attenuated, deteriorating the demodulation frequency characteristics. In addition, in the latter single sideband reception method, the upper and lower sidebands have only the same amplitude modulation information, so in monaural broadcasting, such beat interference can be removed without deteriorating the demodulation frequency characteristics as in the former two cases. Although it is possible, when applied to AM stereo broadcasting, this
In AM stereo broadcasting, the difference signal, that is, L-
Since the phase modulation spectrum by the R signal is vertically asymmetric, it has the disadvantage that it cannot be applied.

The present invention has been made in view of these points, and
The present invention provides a stereo demodulation circuit that can remove such beat interference in either monaural broadcasting or AM stereo broadcasting.

Hereinafter, an example of the case where an embodiment of the present invention is applied to a Magnabox type AM stereo receiver that uses amplitude modulation for the sum signal L+R and phase modulation for the difference signal L-R will be explained with reference to FIGS. 1 to 4. A detailed explanation will be given based on the following.

FIG. 1 shows the configuration of this embodiment. In FIG. 1, an AM stereo signal received by an antenna 1 is amplified by a high frequency circuit 2 including a high frequency amplification circuit, a mixing circuit, and a local oscillation circuit (not shown). , which is converted into an intermediate frequency signal and then supplied to the intermediate frequency amplification circuit 3. Typically, high-frequency amplifier circuits maintain channel separation by making the bandwidth of each tuned circuit wider than that of a normal AM receiver, thereby minimizing the loss of PM signal components that are distributed over a wider band than the normal AM signal. It is done like this. In addition, the local oscillator circuit is used in a normal AM receiver to determine the S/N of the reproduced phase modulated signal.
Better short-term stability than you normally have, to reduce phase noise that limits the ratio
For example, it is set to be 1/1000 radian or less at 100Hz or higher. Furthermore, the intermediate frequency amplifier circuit 3 is usually configured to have a passband sufficient to cover sidebands generated by phase modulation and a constant group delay to reduce the possibility of PM-AM conversion. There is. Although not shown, this intermediate frequency amplification circuit 3 is controlled by the AGC voltage like the high frequency circuit 2.

The intermediate frequency signal related to the AM stereo signal obtained at the output side of the intermediate frequency amplifier circuit 3 is supplied to an envelope detector 4 and also to a vibration limiter 5. Then, the envelope detector 4 performs envelope detection on the intermediate frequency signal to extract a sum signal L+R, while the amplitude limiter 5 removes the AM component included in the intermediate frequency signal, and the phase detector 6 performs phase detection to obtain a sum signal L+R. Signal LR is taken out.

These signals are L as shown in Figure 2A and B.
Although the signal components have an in-phase relationship, the R signal component has an anti-phase relationship. Therefore, by adding the sum signal and the difference signal in the adder 7a of the matrix circuit 7, the output terminal 8 receives the main channel signal, that is, the L signal component. The signal is taken out, and by subtracting the sum signal and the difference signal in the subtracter 7b of the matrix circuit 7, a sub-channel signal, that is, an R signal is taken out at the output terminal 9. These configurations are conventional stereo demodulation circuits that are commonly used.

In the present invention, in addition to the above configuration, as shown in the figure, the amplitude frequency characteristic is constant (gain 1) in the phase demodulation system.
However, regarding the frequency characteristics of the phase, only a specific frequency, for example, a frequency that causes beat disturbance, is 0° to -360.
A so-called all-pass filter (hereinafter referred to as APF) 11 and 12 capable of phase shifting is provided. In other words, when a beat signal is mixed into the received signal, the sum signal L+R and difference signal L-R obtained at the output sides of the envelope detector 4 and phase detector 6, respectively, are as shown in FIG. 2 A and B.
As shown in , since the beat amplitude component B _A and the phase component B _P which have a random phase relationship with each other depending on the beat frequency are superimposed, the difference signal L−R
passes through the APFs 11 and 12, and appears on the output side of the envelope detector 4. For example, for the beat amplitude component B _A shown in FIG. 2C, the beat amplitude component B A shown in FIG. Component B
While extracting the beat phase component B _P ′ which is opposite in phase to _A ,
On the output side of the APF 12, a beat phase component B _P having the same phase as the beat amplitude component B _A as shown in FIG. 2D is extracted. Of course, at this time, APFs 11 and 12 do not phase shift signals other than the beat signal, so the L signal component and the R signal component have a predetermined relationship, that is, as shown in FIG. 2 C and D, the L signal component are taken out with an in-phase relationship, and the R signal is taken out with an anti-phase relationship.

Then, as described above, when the difference signals obtained by adding a predetermined phase relationship to the beat signals in the APFs 11 and 12 are supplied to the adder 7a and the subtracter 7b, respectively,
In the adder 7a, the beat amplitude component B _A and the phase component B _P ', which are in an anti-phase relationship, are canceled out, and the R signal component, which is also in an anti-phase relationship, is also removed, and the output terminal 8 receives the main channel signal. Only the L signal is extracted. In addition, in the subtracter 7b, the beat amplitude component B _A and the phase component B _P , which are in the same phase relationship, are canceled out, and the beat amplitude component B A and the phase component B P, which are in the same phase relationship, are canceled out.
The signal components are also removed, and only the R signal, which is the sub-channel signal, is taken out at the output terminal 9.

Therefore, no beat signal is output to the output terminals 8 and 9. By switching the connection relationship between the APFs 11 and 12 as shown by the broken line in FIG. 1 depending on which of the upper and lower sidebands the beat signal appears in, all the beat signals can be eliminated.

FIG. 3 shows an example of a specific circuit configuration of the APFs 11 and 12 shown in FIG. 1. Third
In the figures, corresponding parts of APFs 11 and 12 will be described with the same suffixes a to h.
The inverting input terminal and non-inverting input terminal of the amplifier 11a are connected to an input terminal 13 connected to the output side of the phase detector 6 (FIG. 1) via resistors 11b and 11c, respectively.
connected to. Capacitors 11d and 11e are connected to both ends of the resistor 11c, and the other ends of these capacitors 11d and 11e are grounded via a resistor 11f and connected to the output terminal of the amplifier 11a via a resistor 11g. Also, the amplifier 11a
The output terminal of is connected to its inverting input terminal via a resistor 11h, and is also connected via an output terminal 14 to an adder 7a (FIG. 1).

Regarding the APF 12, each section indicated by 12a to 12h is similarly connected, and the output terminal of the amplifier 12a is connected via the output terminal 15 to the subtracter 7b (first
(Figure).

And as a condition of APF, resistors 11b and 11
The values of h and 12b and 12h are equal, and the values of resistors 11c and 12c are twice the values of resistors 11g and 12g, respectively, and the values of capacitor 11
The values of d, 11e, 12d and 12e are the same.

So, for example, set the APF11 side to -90°, APF1
If the second side is set as a -270° system and each constant is set as shown below, an APF characteristic as shown in FIG. 4 with a mutually negative phase relationship at a frequency of 10 kHz can be obtained.

C ₀ = 1000PF, R ₁ = 22.0271kΩ, R ₂ = 100.056k
Ω, R ₃ = 11.0135kΩ, R ₁ ′=24.9025kΩ, R ₂ ′=
142.497kΩ, R ₃ ′=12.4513kΩ In the above, C ₀ is the capacitor 11d,
Values of 11e, 12d and 12e, R ₁ , R ₂ and R ₃
are the values of resistors 11c, 11f and 11g, respectively,
R ₁ ', R ₂ ' and R ₃ ' are the values of resistors 12c, 12f and 12g, respectively.

In FIG. 4, the characteristic of the APF 11 is shown by a broken line α, and the characteristic of the APF 12 is shown by a solid line β. From this figure 4, in order to ensure stereo separation, the phase deviation is zero up to the desired frequency, e.g. 5 kHz, and both outputs are approximately in phase, and at the specific frequency to be removed, e.g. 10 kHz, the phase deviation becomes 180°. It can be seen that both outputs have a substantially antiphase relationship.

In FIG. 4, the constants of APF 11 and 12 are set on the assumption that the frequency of the beat signal is 10 kHz, but it can be made to correspond to any frequency by varying the values of resistors 11f and 12f. That is, assuming that the transfer functions of the APFs 11 and 12 are F〓(s) and F〓(s), respectively, these can be expressed as in the following equations.

F〓(s)= ^As2 -Bs+1/ ^As2 +Bs+1...
...(1) F〓(s)= ^A's2 -B's+1/ ^A's2 +B'
s+1……(2) If s=jω in the above equations (1) and (2),
Phases of APF11 and 12 φ〓(ω) and φ〓
(ω) are respectively expressed by the following equations.

φ〓(ω)=-2tan ^-1 ωB/1-ω ² A......(3) φ〓(ω)=-2tan ^-1 ωB'/1-ω ² A'......(4) Ta In the above formulas (3) and (4), A=C〓×R ₁・R ₂・R ₃ /R ₁ +R ₂ ………(5) B=C ₀ (2R ₂・R ₃ −R ₁・R ₂ - _R3・_R1 )/
R ₂ +R ₃ (6). In the above equations (5) and (6), C ₀ and R ₁ to _{R 3} correspond to the values of the capacitor and resistor described above. Therefore, as described above, C ₀ , R ₁ and R ₃ are set in a predetermined relationship in advance as conditions for configuring the APF, so R ₂ is the only variable that can be varied in (5) and (6) above. Therefore, it can be seen that by varying the values of the corresponding resistors 11f and 12f in FIG. 3, it is possible to construct an APF compatible with any frequency.

As described above, according to the present invention, a plurality of APFs are provided on the output side of the phase demodulation system so that the phases of their outputs are opposite to each other with respect to a specific frequency such as a beat frequency.
The output of each of these APFs is combined with the output of the other demodulation system, such as an amplitude demodulation system, to remove signals related to specific frequencies, that is, beat signals. Even if beat interference occurs in any of the channels, it can be eliminated and good reception can be achieved without impairing the frequency characteristics.

Further, the present invention can be similarly applied to the case of monaural broadcasting, that is, it is sufficient to consider that the difference signal L−R=0, and a versatile stereo demodulation circuit can be provided.

In the above-mentioned embodiments, the present invention is applied to a Magnabox type AM stereo receiver, but the present invention is not limited to this, and it may be applied to a system in which at least one of the demodulation systems is a phase demodulation system. ,
Needless to say, the present invention can be similarly applied to, for example, a Motorola AM stereo receiver.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing one embodiment of the present invention, and FIG.
The figure is a diagram for explaining the operation of Figure 1.
The figure is a circuit diagram showing a specific example of the main part of the present invention,
FIG. 4 is a characteristic diagram showing APF characteristics related to FIG. 3. 4 is an envelope detector, 5 is an amplitude limiter, 6
is a phase detector, 7 is a matrix circuit, 11, 1
2 is an all-pass filter (APF).

Claims (1)

[Claims] 1. In a stereo demodulation circuit having at least a phase demodulation system on one side of the demodulation system, a plurality of all-pass wave generators are provided on the output side of the phase demodulation system, the outputs of which are in an anti-phase relationship with respect to a specific frequency. established,
A stereo demodulation circuit characterized in that the signals related to the specific frequency are removed by combining each output of the all-pass waveform with the output of the other demodulation system.